EP3369164A1

EP3369164A1 - Sensing failure of a capacitor in a converter

Info

Publication number: EP3369164A1
Application number: EP15788378.6A
Authority: EP
Inventors: Arman HASSANPOOR; Stefan Valdemarsson
Original assignee: ABB Schweiz AG
Current assignee: Hitachi Energy Ltd
Priority date: 2015-10-29
Filing date: 2015-10-29
Publication date: 2018-09-05
Also published as: CN108352779A; WO2017071754A1

Abstract

A processing module (50) for use in conjunction with at least one capacitor (30) included in a converter (100) for conversion of alternating current power to direct current power, or vice versa, is disclosed. The processing module (50) is configured to receive values of sensed current (I_cap) flowing into the at least one capacitor (30) and possibly values of sensed voltage (I_cap) over the at least one capacitor (30). According to embodiments of the present invention, sensed current (I_cap) flowing into the at least one capacitor (30) and possibly voltage (I_cap) over the at least one capacitor (30) may be utilized in order to provide or generate an indication of any possible failure or onset of failure of the at least one capacitor (30). A method (300) in the processing module (50) and system (200) comprising the processing module (50) are also disclosed.

Description

SENSING FAILURE OF A CAPACITOR IN A CONVERTER

TECHNICAL FIELD

The present invention generally relates to the field of power transmission equipment. Specifically, the present invention relates to a processing module and a method in a processing module for use in conjunction with a capacitor included in a converter for conversion of alternating current (AC) power to direct current (DC) power, or vice versa, for sensing failure or onset of failure of the capacitor. The converter may for example comprise a High Voltage Direct Current (HVDC) converter.

BACKGROUND

HVDC power transmission has become increasingly important due to increasing need for power supply or delivery and interconnected power transmission and distribution systems. In a HVDC power system there is generally included an interface arrangement including or constituting a HVDC converter station, which is a type of station configured to convert high voltage DC to AC, or vice versa. A HVDC converter station may comprise a plurality of elements such as the converter itself (or a plurality of converters connected in series or in parallel), one or more transformers, capacitors, filters, and/or other auxiliary elements. Converters, which often are referred to as 'converter valves', or simply 'valves', may comprise a plurality of solid-state based devices such as semiconductor devices and may be categorized as line-commutated converters (LCCs) or voltage source converters (VSCs), e.g. depending on the type of switches (or switching devices) which are employed in the converter. A plurality of solid-state semiconductor devices such as IGBTs may be connected together, for instance in series, to form a building block, or cell, of a HVDC converter, or HVDC converter valve. HVDC technology may be classified as Current Source Converter (CSC) based HVDC and VSC based HVDC. While CSC based HVDC converters employ thyristors as switches or switching elements (and/or other switches or switching elements that are not self-commutated), VSC based HVDC converters employ IGBTs as switches or switching elements (and/or other switches or switching elements that are self- commutated). A failure in one of the semiconductor based switches or switching elements may be sensed with relative ease. The semiconductor based switches or switching elements may for example comprise IGBTs. A failure in an IGBT can be sensed for example by way of a gate unit. Capacitors which are employed in HVDC converter cells may be vulnerable to internal failures. And at the end of the capacitor's lifetime, a violent failure of the capacitor may occur.

SUMMARY

VSC based HVDC converters may comprise a plurality of cascaded, electrically connected, converter cells. Each cell may for example comprise a half-bridge cell, having two semiconductor based switches or switching elements connected in series across an electrical energy storage element, e.g. a capacitor, and a bypass switch for controllably and selectively bypassing the semiconductor based switches or switching elements and the electrical energy storage element. That is to say, by way of the bypass switch, current in the cell may controllably and selectively be made to go through the semiconductor based switches or switching elements and the electrical energy storage element, or be made to bypass the semiconductor based switches or switching elements and the electrical energy storage element. In case of a failure in the cell, the bypass switch may be switched so as to bypass, or redirect, the current so as to not pass through the semiconductor based switches or switching elements and the electrical energy storage element.

In HVDC applications there are used different types of capacitors. Capacitors which are used in HVDC applications may be classified as segmented capacitors and non- segmented capacitors.

Segmented capacitors generally include a large number of relatively small capacitor elements or capacitor segments, each of which may be relatively easily be disconnected from the rest of the capacitor elements or capacitor segments in case of a failure occurring in one of capacitor elements or capacitor segments.

Failure of a capacitor, or a capacitor element or capacitor segment, may occur due to gradual degradation of the functionality of the capacitor, or capacitor element or capacitor segment. Such gradual degradation may for example be attributed to one or more of high voltages, transient currents, mechanical disturbances (e.g., strong vibrations), etc.

Degradation of the capacitor, or capacitor element or capacitor segment, over a period of time may result in failure or breakdown of the capacitor, causing total or partial loss of

functionality thereof. In the context of the present application, failure of a capacitor, or a capacitor element or capacitor segment, may mean that that the capacitor, or capacitor element or capacitor segment, has lost functionality by degradation thereof so that the functionality is insufficient for meeting the requirements (e.g., power transfer or conversion requirements), of the particular application in which the capacitor, or capacitor element or capacitor segment, is employed. Failure of a capacitor, or a capacitor element or capacitor segment, may also occur due to catastrophic failure of the capacitor, leading to a complete, or substantially complete, and often relatively rapid, loss of functionality of the capacitor, or capacitor element or capacitor segment, due to, e.g., a short or open circuit.

Sensing of failure of a segmented capacitor can be carried out by means of continuously measuring the capacitance of the segmented capacitor. In case the capacitance of the segmented capacitor falls below a certain threshold value, this may be taken as an indication of a failure of the segmented capacitor, and the cell in which the segmented capacitor is included may then be bypassed. Measurement of capacitance of segmented capacitors in cells may be implemented in VSC based HVDC power transmission systems.

Non-segmented capacitors generally include several relatively small capacitor elements electrically connected in series and/or in parallel. The cost of a non-segmented capacitor is in general (much) lower than the cost of a segmented capacitor. However, non- segmented capacitors may cause destructive damage at their end of life. In contrast to segmented capacitors, it may be difficult to sense failure of a non-segmented capacitor by means of continuously measuring the capacitance of the non-segmented capacitor. This is due to that the capacitance of a non-segmented capacitor in general may not change much before failure of the non-segmented capacitor. For a non-segmented capacitor there may be a very rapid change in capacitance only at the very end of its lifetime which may be difficult to sense with a sufficient period of time remaining before the failure of the capacitor, which may cause destructive damage, e.g., in case of build-up of pressure in the capacitor and possibly even explosion thereof. In order to sense onset of failure of non-segmented capacitors pressure sensors may be used. A pressure sensor may for example be arranged or mounted on a housing of the non-segmented capacitor in order to sense any increase in pressure within the housing which may occur at the onset of failure of the non-segmented capacitor. However, such pressure sensors may be relatively expensive, and their reliability may be relatively low. Therefore, employing such pressure sensors to sense onset of failure of non-segmented capacitors might increase the cost of the HVDC power transmission system, and the operational reliability of the HVDC power transmission system may decrease.

In view of the above, a concern of the present invention is to provide means for sensing failure, or onset of failure or imminent failure, of a capacitor of a converter, which may be relatively inexpensive.

A further concern of the present invention is to provide means for sensing failure, or onset of failure or imminent failure, of a capacitor of a converter, which may only have a small or no detrimental effect on the operational reliability of the overall system in which the converter is included.

The converter may for example comprise a non-segmented capacitor.

To address at least one of these concerns and other concerns, a processing module, a method, a converter and a system in accordance with the independent claims are provided. Preferred embodiments are defined by the dependent claims. Upon a failure of the at least one capacitor, or at the onset of failure, or imminent failure, of the at least one capacitor, the resistance over the at least one capacitor will decrease from its 'normal' or nominal value or 'normal' or nominal range of values. Hence, in the context of the present application, a "condition" of the at least one capacitor should be understood as a characterization of the at least one capacitor with respect to its

'normal' or nominal functionality. The measure of a condition of the at least one capacitor can thus be considered, in accordance with one or more embodiments of the present invention, as a score indicative of the state of functionality of the at least one capacitor. If the at least one capacitor is not able to operate according to its 'normal' or nominal functionality it may get a lower score whereas if the at least one capacitor is able to operate according to its 'normal' or nominal functionality it may get a higher score. As resistance of any resistive elements of the at least one capacitor are reduced, the current through them will increase, which may result in an increase in energy losses in the at least one capacitor. For sensing failure of the at least one capacitor, or onset of failure or imminent failure, of the at least one capacitor, embodiments of the present invention are based on sensing or monitoring changes in at least one of quantities such as, for example, resistance of any resistive elements of the at least one capacitor (that is, internal resistances of the at least one capacitor), power of the at least one capacitor, current into, and possibly through, the at least one capacitor, etc. Such changes in at least one quantity may be utilized to assess whether there is a failure, or onset of failure or imminent failure, of the at least one capacitor. The quantities may be used to determine a measure of a condition of the at least one capacitor. The determined measure is compared with a predefined threshold value. If the determined measure exceeds the predefined threshold value, there may then be indicated or determined that there is a failure, or onset of failure or imminent failure, of the at least one capacitor. For example in case there are relatively large changes in at least one of the above-mentioned quantities the determined measure may exceed the predefined threshold value, and it may subsequently be indicated or determined that there is a failure, or onset of failure or imminent failure, of the at least one capacitor. As will be described further in the following, changes in resistance of any resistive elements of the at least one capacitor and power of the at least one capacitor may be determined based on current flowing into the at least one capacitor, and possibly voltage over the at least one capacitor. Thus, according to embodiments of the present invention, sensed current flowing into the at least one capacitor and possibly voltage over the at least one capacitor are utilized in order to sense failure or onset of failure (or imminent failure) of the at least one capacitor.

According to a first aspect there is provided a processing module for use with, or in conjunction with, at least one capacitor, which at least one capacitor is included in a converter for conversion of alternating current power to direct current power, or vice versa. At least during part of a power conversion operation of the converter, current in the converter flows into the at least one capacitor. The processing module is configured to receive and/or retrieve values of sensed current flowing into the at least one capacitor. The processing module is configured to determine, on basis of at least the values of the sensed current flowing into the at least one capacitor, a measure of a condition of the at least one capacitor. The processing module is configured to compare the determined measure with a predefined threshold value. The processing module is configured to, on a condition that the determined measure exceeds the threshold value, generate an indication of failure, or onset of failure, or imminent failure, of the at least one capacitor.

In a control and monitoring system for a power transmission system that includes a converter for conversion of alternating current power to direct current power, or vice versa, there is generally included current sensing functionality and/or voltage sensing functionality for sensing current into and possibly through various elements included in the power transmission system and/or sensing voltage over the various elements. In particular, such a control and monitoring system for a power transmission system including a converter may include existing current sensing functionality and/or voltage sensing functionality for sensing current into and possibly through at least one capacitor of the converter and/or sensing voltage over the at least one capacitor. And if not, such current sensing functionality and/or voltage sensing functionality may be relatively easily implemented. Therefore, a processing module according to the first aspect may be relatively easily implemented in existing power transmission system equipment. Thereby, by way of a processing module according to the first aspect, a relatively inexpensive means for sensing failure, or onset of failure or imminent failure, of a capacitor of a converter, may be provided. Further, by way of a processing module according to the first aspect, there may be less or even no need of any additional sensors such as pressure sensors as described in the foregoing for sensing any increase in pressure within a housing in which the at least one capacitor is accommodated, in order to sense failure of the at least one capacitor, or at the onset of failure, or imminent failure, of the at least one capacitor. Thereby, by way of a processing module according to the first aspect, the capability to sense failure of the at least one capacitor, or the onset of failure or imminent failure, of the at least one capacitor may be implemented without any possible detrimental effect on the operational reliability of the overall system in which the converter is included (as may be the case if instead employing sensors such as pressure sensors for achieving such capability).

The indication may for example comprise or be constituted by a message, data, and/or a signal or signaling indicative of failure or onset of failure, or imminent failure, of the at least one capacitor.

The capacitor of the converter may be comprised in a cell of the converter (i.e. a converter cell). The converter may comprise a plurality of cells. Any one or each of the converter cells may comprise a plurality of switching elements electrically connected to the capacitor. The converter may for example comprise a VSC, which for example may comprise at least one multilevel converter cell. Each multilevel converter cell may for example comprise a half-bridge cell, or a full-bridge cell. The converter may be arranged in or constitute an interface arrangement for coupling an AC power system with a DC power system. The converter may for example comprise a plurality of multilevel converter cells arranged in one or more phase arms in respective ones of one or more phase legs, where there may be one phase leg per AC phase of the AC power system. The phase legs may for example be connected in parallel between terminals of the DC power system. In case the converter comprises a half-bridge cell, there may be two switches or switching elements connected in series across the capacitor. A midpoint connection between the switches or switching elements and one of capacitor terminals may act as an external connection. Each of the switches or switching elements may for example comprise a power semiconductor switch with turn-on and turn-off capability such as, for example, an IGBT, and a diode, which may be connected anti-parallel with respect to the switch or switching element.

The capacitor may comprise at least one capacitor module, which at least one capacitor module comprises a plurality of capacitor elements. The capacitor elements may for example be arranged in N capacitor units which each includes n, e.g., two or more, capacitor elements electrically connected in series, with the different capacitor units being electrically connected in parallel with each other. There may be tens, or even hundreds, of capacitor units included in the capacitor module. For example, the number N of capacitor units may for example be 10, 20, 40, or 50, or 100, 200, 400, or 500 or even more. In case each capacitor element would have a capacitance 2C_e, and there for example would be two capacitor elements electrically connected in series in each capacitor element (i.e., n = 2), the total theoretical capacitance of the at least one capacitor module would then become Ctotai = NC_e, as known in the art, according to the principles that the equivalent capacitance of several capacitor elements connected in parallel is the sum of the capacitances of the individual capacitor elements, and that the reciprocal of the equivalent capacitance of several capacitor elements connected in series is the sum of the reciprocals of the capacitance of the individual capacitor elements. According to one or more embodiments of the present invention, current flowing into the capacitor or into the at least one capacitor module, and/or voltage over the capacitor or over the at least one capacitor module, may be utilized in order to sense failure of the capacitor or the at least one capacitor module, or the onset of failure, or imminent failure, of the capacitor or the at least one capacitor module. In order to sense failure of the capacitor or the at least one capacitor module, or the onset of failure, or imminent failure, of the capacitor or the at least one capacitor module, the current flowing into the capacitor or into the at least one capacitor module, and/or voltage over the capacitor or over the at least one capacitor module, may be continually sensed, in order to integrate the current or voltage, respectively, over time. Possibly, the sensed current and/or voltage may be used to determine, for example, any power loss in the capacitor, any voltage level mismatch between the sensed voltage over the capacitor and a voltage over the capacitor determined based on current flowing into the capacitor or into the at least one capacitor module, or a level of a charge which flows through any resistive elements of the capacitor or the at least one capacitor module. As will be described further in the following, such determined quantities may be used in order to determine if there is a failure of the capacitor or the at least one capacitor module, or if there is an onset of failure, or imminent failure, of the capacitor or the at least one capacitor module.

The processing module may be directly or indirectly (e.g., via one or more intermediate components) communicatively coupled with at least one current sensor configured to sense current flowing into the at least one capacitor. The processing module may be configured to receive and/or retrieve values of sensed current flowing into the at least one capacitor from the at least one current sensor. The at least one current sensor may in principle be constituted by or comprise any appropriate current sensor, e.g., a current sensor as known in the art.

The processing module may be configured to receive and/or retrieve values of sensed voltage over the at least one capacitor. The processing module may be configured to determine the measure of the condition of the at least one capacitor further on basis of the values of sensed voltage over the at least one capacitor.

The processing module may be directly or indirectly (e.g., via one or more intermediate components) communicatively coupled with at least one voltage sensor configured to sense voltage over the at least one capacitor. The processing module may be configured to receive and/or retrieve values of sensed voltage over the at least one capacitor from the at least one voltage sensor. The at least one voltage sensor may in principle be constituted by or comprise any appropriate voltage sensor, e.g., a voltage sensor as known in the art.

The processing module may be configured to receive and/or retrieve a plurality of values of sensed current flowing into the at least one capacitor sensed continually, or periodically with some predefined periodicity, during a first selected period of time. The processing module may be configured to determine the measure of the condition of the at least one capacitor further on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time. The processing module may be configured to determine the measure of the condition of the at least one capacitor by means of (approximately) integrating the sensed current flowing into the at least one capacitor over time. This may for example be done by approximating the integration based on a summation involving the plurality of values of sensed current flowing into the at least one capacitor or a function thereof and the time intervals between instants when the current flowing into the at least one capacitor were sensed, which as such is known in the art. The processing module may be configured to receive and/or retrieve a plurality of values of sensed voltage over the at least one capacitor sensed continually, or periodically with some predefined periodicity, during a second selected period of time. The processing module may be configured to determine the measure of the condition of the at least one capacitor further on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time. The processing module may be configured to determine the measure of the condition of the at least one capacitor further by means of (approximately) integrating the sensed voltage over the at least one capacitor over time. This may for example be done by approximating the integration based on a summation involving the plurality of values of sensed voltage over the at least one capacitor or a function thereof and the time intervals between instants when the voltage over the at least one capacitor were sensed, which as such is known in the art.

The first selected period of time and the second selected period of time may be the same, or they may be overlapping.

For example, according to one or more embodiments of the present invention, the processing module may be configured to receive and/or retrieve values of sensed voltage over the at least one capacitor. The processing module may be configured to determine, on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time and the values of sensed voltage over the at least one capacitor, a charge which flows through any resistive elements of the at least one capacitor. The processing module may be configured to determine the measure of the condition of the at least one capacitor further on basis of the charge.

In alternative or in addition, according to one or more embodiments of the present invention, the processing module may be configured to receive and/or retrieve a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time, and receive and/or retrieve a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time. The processing module may be configured to determine a power of the at least one capacitor on basis of the plurality of values of sensed voltage, and determine a power input into the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time, and the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time. The processing module may be configured to determine the measure of the condition of the at least one capacitor further on basis of a difference between the determined power of the at least one capacitor and the determined power input into the at least one capacitor.

In alternative or in addition, according to one or more embodiments of the present invention, the processing module may be configured to receive and/or retrieve a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time, and receive and/or retrieve a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time. The processing module may be configured to determine a first voltage over the at least one capacitor on basis of the plurality of values of sensed current sensed continually during the first selected period of time, and determine a second voltage over the at least one capacitor on basis of the plurality of values of sensed voltage sensed continually during the second selected period of time. The processing module may be configured to determine the measure of the condition of the at least one capacitor further on basis of a difference between the determined first voltage and the determined second voltage.

The capacitor may comprise at least one capacitor module (which in some applications may be referred to as a 'capacitor can'). In accordance with one or more embodiments of the present invention, the capacitor may comprise at least two capacitor modules which are electrically connected with each other, for example in parallel and/or in series.

According to one or more embodiments of the present invention, the at least one capacitor comprises at least two capacitor modules electrically connected in parallel, wherein at least during part of operation of the converter current in the converter flows into each of the at least two capacitor modules. The processing module may be configured to receive and/or retrieve values of sensed current flowing into each of the at least two capacitor modules. The processing module may be configured to determine the measure of the condition of the at least one capacitor further on basis of a difference between the values of sensed current flowing into respective ones of the at least two capacitor modules.

In the context of the present application, by a half-bridge cell it is meant a circuit comprising two switches or switching elements connected in series across an electrical energy storage element, e.g. a capacitor, with a midpoint connection between the switches or switching elements and one of the electrical energy storage element terminals as external connections. Each of the switches or switching elements may for example comprise a power semiconductor switch with turn-on and turn-off capability such as, for example, an IGBT, and a diode, which may be connected anti-parallel with respect to the switch or switching element.

In the context of the present application, by a full-bridge cell it is meant a circuit similar to the half-bridge cell, but comprising four switches or switching elements connected in an H bridge arrangement, instead of two as in the half-bridge cell. Each of the switches or switching elements may for example comprise a power semiconductor switch with turn-on and turn-off capability such as, for example, an IGBT, and a diode, which may be connected anti-parallel with respect to the switch or switching element.

According to a second aspect there is provided a method in a processing module for use with, or in conjunction with, at least one capacitor. The at least one capacitor is included in a converter for conversion of alternating current power to direct current power, or vice versa, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor. The method comprises receiving values of sensed current flowing into the at least one capacitor. On basis of at least the values of the sensed current flowing into the at least one capacitor, a measure of a condition of the at least one capacitor is determined. The determined measure is compared with a predefined threshold value. On a condition that the determined measure exceeds the threshold value, an indication of failure or onset of failure of the at least one capacitor is generated.

A plurality of values of sensed current flowing into the at least one capacitor, sensed continually during a first selected period of time, may be received. The measure of the condition of the at least one capacitor may be determined on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time.

Values of sensed voltage over the at least one capacitor may be received. The measure of the condition of the at least one capacitor may be determined further on basis of the values of sensed voltage over the at least one capacitor.

A charge which flows through any resistive elements of the at least one capacitor may be determined on basis of the plurality of values of sensed current and the values of sensed voltage. The measure of the condition of the at least one capacitor may be determined further based on the charge.

A plurality of values of sensed voltage over the at least one capacitor, sensed continually during a second selected period of time, may be received. The measure of the condition of the at least one capacitor may be determined on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time.

As described in the foregoing, the processing module may be directly or indirectly (e.g., via one or more intermediate components) communicatively coupled with at least one current sensor, which is configured to sense current flowing into the at least one capacitor, and/or at least one voltage sensor, which is configured to sense voltage over the at least one capacitor. The at least one current sensor and the at least one voltage sensor may in principle be constituted by or comprise any appropriate current sensor or voltage sensor, respectively, e.g., a current sensor or a voltage sensor, respectively, as known in the art. Current sensing capability or functionality and/or voltage sensing capability or functionality for sensing current flowing into the at least one capacitor and voltage over the at least one capacitor, respectively, may in alternative or in addition be provided by a control and monitoring system for a power transmission system in which the converter is included.

According to one or more embodiments of the present invention, a plurality of values of sensed voltage over the at least one capacitor, sensed continually during a second selected period of time, are received, and a plurality of values of sensed voltage over the at least one capacitor, sensed continually during a second selected period of time, are received. The first selected period of time and the second selected period of time may be the same, or they may overlap with respect to each other.

According to an embodiment of the present invention, a power of the at least one capacitor may be determined on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time. A power input into the at least one capacitor may be determined on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time, and the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time. The measure of the condition of the at least one capacitor may be determined further based on a difference between the determined power of the at least one capacitor and the determined power input into the at least one capacitor.

In alternative or in addition, according to another embodiment of the present invention, a first voltage over the at least one capacitor may be determined on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time. A second voltage over the at least one capacitor may be determined on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time. The measure of the condition of the at least one capacitor may be determined further based on a difference between the determined first voltage and the determined second voltage.

The at least one capacitor may comprise at least two capacitor modules electrically connected in parallel, wherein at least during part of operation of the converter current in the converter flows into each of the at least two capacitor modules. The method may comprise receiving values of sensed current flowing into each of the at least two capacitor modules. The measure of the condition of the at least one capacitor may be determined based on a difference between the values sensed currents flowing into respective ones of the at least two capacitor modules.

According to a third aspect there is provided a converter for conversion of alternating current power to direct current power, or vice versa. The converter comprises at least one capacitor, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor. The converter comprises a processing module configured to, on basis of at least sensed current flowing into the at least one capacitor, determine a measure of a condition of the at least one capacitor. The processing module is configured to compare the determined measure with a predefined threshold value, and, on a condition that the determined measure exceeds the threshold value, generate an indication of failure or onset of failure of the at least one capacitor. According to a fourth aspect there is provided a system comprising a converter for conversion of alternating current power to direct current power, or vice versa, the converter comprising at least one capacitor wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor. The system comprises a processing module configured to, on basis of at least sensed current flowing into the at least one capacitor, determine a measure of a condition of the at least one capacitor. The processing module is configured to compare the determined measure with a predefined threshold value. The processing module is configured to, on a condition that the determined measure exceeds the threshold value, generate an indication of failure or onset of failure of the at least one capacitor.

As described in the foregoing, the capacitor may comprise at least one capacitor module, which at least one capacitor module comprises a plurality of capacitor elements, with the capacitor elements for example being arranged in N capacitor units which each includes n capacitor elements electrically connected in series, with the different capacitor units being electrically connected in parallel with each other. There may for example be two capacitor elements electrically connected in series in each capacitor element. In case of a failure of one of the capacitor elements, or onset of failure of one of the capacitor elements, in a capacitor unit, the resistance over the capacitor element will decrease significantly as compared to the resistance over the other capacitor element connected in series with the capacitor element that has failed (or is about to fail). A 'healthy' capacitor element will generally have a relatively high resistance (in case of a HVDC converter application, e.g. in the range of ΜΩ). Upon the onset of a failure of the capacitor element resistance over the capacitor element will gradually, and possibly rapidly, decrease (in case of a HVDC converter application, to a resistance e.g. in the range of kQ). The other capacitor element connected in series with the capacitor element that has failed (or is about to fail) may then be exposed to the full capacitor voltage. Such a situation may lead to a critical stage where the full capacitor energy may be discharged in a single element, possibly very rapidly, which may lead to a rupture and/or explosion of a housing in which the capacitor is accommodated, an outbreak of fire, or another hazardous situation. For avoiding such a situation, the converter may comprise a bypass switch, which may be configured so as to be controllably and selectively switchable between at least a first mode, in which, at least during part of the power conversion operation of the converter, current in the converter flows into the at least one capacitor, and a second mode, in which the current in the converter is conveyed so as to (electrically) bypass the at least one capacitor. The system may comprise a control module, which may be

communicatively coupled with the processing module and with the converter, and at least with the bypass switch of the converter. The control module may be configured to, upon having received the indication from the processing module, control the bypass switch (e.g., by transmitting one or more control signals or messages to the bypass switch) so as to switch the bypass switch into the second mode. This may facilitate or allow for actuating or switching the bypass switch such that in case of a failure or onset of failure of the at least one capacitor, the current is bypassed the at least one capacitor, and possibly also one or more other components of the converter, such as, for example, a plurality of switching elements electrically connected to the at least one capacitor.

The processing module and/or the control module may for example include or be constituted by any suitable central processing unit (CPU), microcontroller, digital signal processor (DSP), Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA), etc., or any combination thereof. The processing module and/or the control module may optionally be capable of executing software instructions stored in a computer program product e.g. in the form of a memory. The memory may for example be any combination of read and write memory (RAM) and read only memory (ROM). The memory may comprise persistent storage, which for example can be a magnetic memory, an optical memory, a solid state memory or a remotely mounted memory, or any combination thereof.

According to a fifth aspect there is provided a computer program product configured to, when executed in a processing module according to the first aspect, perform a method according to the second aspect.

According to a sixth aspect there is provided a computer-readable storage medium on which there is stored a computer program product configured to, when executed in a processing module according to the first aspect, perform a method according to the second-aspect.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments. It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the description herein. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 is a schematic circuit diagram of a cell of a converter in accordance with an embodiment of the present invention.

Figure 2 is a schematic block diagram of a system according to an embodiment of the present invention.

Figures 3-6 are schematic flowcharts of methods according to an embodiment of the present invention. Figure 7 is a schematic view of computer-readable means carrying computer program code according to embodiments of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein;

rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

Figure 1 is schematic circuit diagram of a cell or converter cell 100 of a converter in accordance with an embodiment of the present invention. The converter is configured to convert alternating current power to direct current power, or vice versa. In principle the converter may include only one cell, such as the cell 100 illustrated in Figure 1. However, the converter will in general include several electrically connected cells, which may be identical or substantially identical to the cell 100 illustrated in Figure 1. The cells of the converter may for example include several cascaded, electrically connected cells.

In accordance with the embodiment of the present invention illustrated in Figure 1 the converter is a VSC based HVDC converter, and the cell 100 comprises a half- bridge cell, which comprises two semiconductor based switches or switching elements 10, 20, such as, for example, IGBTs, which are connected in series across a capacitor 30. Current in the converter, or the cell 100, flows into the capacitor 30 at least during part of a power conversion operation of the converter or cell 100. In Figure 1 the current flowing into the converter, or the cell 100, is indicated by I_ceii, and the current flowing into the capacitor 30 is indicated by I_cap.

In accordance with the embodiment of the present invention illustrated in

Figure 1, the cell 100 comprises a bypass switch 40 for selectively bypassing the

semiconductor based switches or switching elements 10, 20 and the capacitor 30. The bypass switch 40 is configured so as to selectively switchable between at least a first mode and a second mode. In the first mode, current in the converter flows into the capacitor 30 - at least during part of the power conversion operation of the converter or cell 100. In the second mode, the current in the converter or the cell 100 is conveyed so as to (electrically) bypass the capacitor 30. In accordance with the embodiment of the present invention illustrated in Figure 1, the capacitor 30 of the converter or the cell 100 comprises two capacitor modules 31, 32, which are electrically connected in parallel. However, it is to be understood that this is exemplifying and in accordance with one or more embodiments of the present invention. According to embodiments of the present invention, the capacitor 30 may include a single capacitor module 31 or 32, or more than two electrically connected capacitor modules, which for example may be connected in parallel relatively to each other.

In accordance with the embodiment of the present invention illustrated in Figure 1, each capacitor module 31, 32 of the capacitor 30 comprises a plurality of capacitor elements. The capacitor module 31 comprises several capacitor elements, two of which are indicated by reference numerals in Figure 1 at 33 and 34. The capacitor module 32 also comprises several capacitor elements, which however are not indicated by reference numerals in Figure 1. The capacitor elements of the capacitor 30 may in the following be referred to collectively as the capacitor elements 33, 34.

Further in accordance with the embodiment of the present invention illustrated in Figure 1, the capacitor elements included in the capacitor 30 may be arranged in N capacitor units, where the number N of capacitor units for example may be 10, 20, 40, or 50, or 100, 200, 400, or 500 or even more. In Figure 1 two capacitor units of the capacitor module 31 are indicated by reference numerals at 35 and 36, and two capacitor units of the capacitor module 32 are indicated by reference numerals at 37 and 38. It is to be understood that each of the capacitor modules 31, 32 includes more than two capacitor units, as indicated by the dots between the capacitor units 35 and 36 and between the capacitor units 37 and 38, respectively. The capacitor units of the capacitor 30 will in the following be referred to collectively as the capacitor units 35-38. Each of the capacitor units 35-38 includes two capacitor elements electrically connected in series. It is to be understood that any or each of each of the capacitor units 35-38 may include more than two electrically connected capacitor elements, or even a single capacitor element. The different capacitor units 35-38 are electrically connected in parallel with each other. If each capacitor element has a capacitance 2C_e, and there are Ni capacitor units in the capacitor module 31 and N₂ capacitor units in the capacitor module 32 (with N + N₂), the total theoretical capacitance of the capacitor module 31 becomes Ci, total = NiCe, and the total theoretical capacitance of the capacitor module 32 becomes

C₂, total = N₂C_e.

The resistances indicated at Ri and R₂ in Figure 1 represent resistive elements of the capacitor elements 33 and 34, respectively. Ri and R₂ do not necessarily have to be separate, dedicated resistors which are physically installed in the circuit, but they may be constituted by way of the (parasitic) resistances of the capacitor elements 33 and 34, respectively, themselves. Figure 2 is a schematic block diagram of a system 200 according to an embodiment of the present invention. The system 200 comprises a cell, or converter cell, 100 of a converter, such as, for example, the cell 100 illustrated in Figure 1. The system 200 comprises a processing module 50. The processing module 50 may be operatively and/or communicatively coupled (for example using any appropriate wired and/or wireless communication technique or communication link as known in the art) to the cell 100 so as to permit transmission of one or more messages, data, and/or one or more signals or signaling between the cell 100 and the processing module 50. For example, the processing module 50 may be configured to receive values of sensed current flowing into the capacitor 30 and/or sensed voltage over the capacitor 30. Sensing of current flowing into the capacitor 30 and/or voltage over the capacitor 30 may be carried out for example by means of a control and monitoring system for a power transmission system in which the converter, or the cell 100, is included. In a control and monitoring system for a power transmission system there is generally included current sensing functionality and/or voltage sensing functionality for sensing current into and possibly through various elements included in the power transmission system and/or sensing voltage over the various elements. Such control and monitoring systems are as such known in the art. In alternative or in addition, sensing of current flowing into the capacitor 30 and/or voltage over the capacitor 30 may be carried out by some other entity or entities, which possibly may be arranged in the converter or the cell 100, and which for example may employ an appropriate current sensor and/or voltage sensor as known in the art.

In alternative to an arrangement illustrated in Figure 2, wherein the processing module 50 is operatively and/or communicatively coupled to the cell 100, the processing module 50 may instead be comprised in the converter or the cell 100.

In accordance with the embodiment of the present invention illustrated in

Figure 2, the system 200 comprises a control module 60. The control module 60 is communicatively coupled with the processing module 50, and with the converter or the cell 100. The control module 60 may at least be communicatively coupled with the bypass switch 40 of the converter or the cell 100. The control module 60 may be configured to transmit control signals to the cell 100, e.g., to the bypass switch 40 in the cell 100 or to some other component(s) in the cell 100, for controlling operation thereof. According to an embodiment of the present invention, the processing module 50 and the control module 60 may be implemented in and/or constituted by a single entity or unit.

Thus, the processing module 50 may be operatively and/or communicatively coupled, for example using any appropriate wired and/or wireless communication technique or communication link as known in the art, to an entity other than the cell 100 (that is, the processing module 50 may not be operatively and/or communicatively coupled to the cell 100), which other entity is not shown in Figure 2, that is capable of sensing current flowing into the capacitor 30 or into the respective capacitor modules 31 and 32, and possibly voltage over the at least one capacitor 30 or over the respective capacitor modules 31 and 32, whereby the processing module 50 may receive sensed current and/or voltage from the entity. As indicated in the foregoing such an entity may for example comprise a control and monitoring system for a power transmission system in which the converter or the cell 100 is included.

With further reference to Figure 1, the processing module 50 is configured to process at least (values of) sensed current I_cap flowing into the capacitor 30, and/or possibly voltage over the capacitor 30, which is indicated in Figure 1 by V_cap, and based thereon provide an indication if the capacitor 30 has failed, or is about to fail (if that should the case).

In accordance with the embodiment of the present invention illustrated in

Figure 1, the processing module 50 is configured to receive values of sensed current I_cap flowing into the capacitor 30. The processing module 50 is configured to determine, on basis of at least the values of the sensed current I_cap flowing into the capacitor 30, a measure of a condition of the capacitor 30. The processing module 50 is configured to compare the determined measure with a predefined threshold value, and, on a condition that the determined measure exceeds the threshold value, generate an indication of failure or onset of failure of the capacitor 30. The indication may for example comprise or be constituted by a message, data, and/or a signal or signaling indicative of failure or onset of failure, or imminent failure, of the capacitor 30. The processing module 50 may transmit the indication to the control module 60. The control module 60 may be configured to, upon having received the indication from the processing module 50, control the bypass switch 40 so as to switch the bypass switch into the second mode. As indicated in Figure 1, current in the cell 100 may in this manner be bypassed the capacitor 30, and possibly also one or more other components of the cell 100 such as the switches or switching elements 10, 20.

The system 200 may comprise more than one converter or cell 100. The processing module 50 and possibly the control module 60 may be operatively and/or communicatively coupled with each converter or cell, in order to sense whether there is a failure or onset of failure of a capacitor of the respective converters or cells, and control operation of the respective converters or cells, respectively.

A method in a processing module in accordance with the above is illustrated in

Figure 3. Figure 3 is a schematic flowchart of a method 300 in a processing module according to an embodiment of the present invention. The method 300 is for use with, or in conjunction with, at least one capacitor included in a converter for conversion of alternating current power to direct current power, or vice versa, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor. The converter and the processing module may for example comprise a converter or cell 100 and a processing module 50, respectively, as described herein with reference to Figures 1 and 2. The method 300 comprises receiving values of sensed current flowing into the at least one capacitor, 301. Optionally, values of sensed voltage over the at least one capacitor may be received, 302. A measure of a condition of the at least one capacitor is determined on basis of at least the values of the sensed current flowing into the at least one capacitor, and possibly the values of sensed voltage over the at least one capacitor, 303. As indicated above, the step 302 is optional, and the step 301 may be immediately followed by the step 303, as indicated in Figure 3. The determined measure is compared with a predefined threshold value, 304. An indication of failure or onset of failure of the at least one capacitor is generated on a condition that the determined measure exceeds the threshold value, 305. The step 301 may comprise receiving a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time. The step 302 may comprise receiving a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time. The step 303 may comprise determining the measure of the condition of the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time, and/or on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time.

With further reference to Figures 1 and 2, and as will be described further in the following, values of sensed current I_cap flowing into the capacitor 30 and/or possibly voltage Vcap over the capacitor 30 may for example be used to determine any power loss in the capacitor 30, any voltage level mismatch between the sensed voltage V_cap over the capacitor 30 and a voltage over the capacitor 30 as determined based on current I_cap flowing into the capacitor 30, or a level of a charge which flows through any resistive elements Ri, R₂ of the capacitor 30. Such quantities may, alone or in combination, be used in order to determine if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30.

The determination if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30 may for example be based on a differential current measurement based on sensing of currents flowing into the respective ones of the capacitor modules 31 and 32, which currents are indicated in Figure 1 by Ii and I₂, respectively. As long as the capacitor modules 31 and 32 are 'healthy', the current I_cap flowing into the capacitor 30 is evenly, or substantially evenly, shared between the capacitor modules 31 and 32, That is, as long as the capacitor modules 31 and 32 are 'healthy', the currents Ii and I₂ are equal, or substantially equal. In the case illustrated in Figure 1 where there are capacitor modules 31 and 32, the currents Ii and I₂ will be equal, or substantially equal, to half of the current I_cap as long as the capacitor modules 31 and 32 are 'healthy'. While the capacitor modules 31 and 32 are 'healthy', a differential current determined from a difference between the currents Ii and I₂ may deviate from zero by a relatively low current ±Idiff, Idiff may for example be about 5 % of I_cap while the capacitor modules 31 and 32 are 'healthy'. It is to be understood that the case where Idiff is about 5 % of I_cap is according to a non- limiting example and that variations are possible. Idiff, or the fraction of I_cap to which Idiff may equal, may be dependent for example on the current sensing accuracy of the sensors used to sense the current I_cap flowing into the capacitor 30, and/or timing of a control operation carried out by the control module 60, such as, for example, the frequency with which the control module 60 is configured to transmit control signals to the cell 100, e.g., to the bypass switch 40 in the cell 100 or to some other component(s) in the cell 100, for controlling operation thereof. However, if one of the capacitor modules 31 and 32 fails, or at the onset of failure of one of the capacitor modules 31 and 32, the deviation of the differential current may start to increase so as to exceed Idiff. This increase may be utilized as a measure of a condition of the capacitor 30, for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30. The same or a similar principle applies if the capacitor 30 comprises more than two capacitor modules connected in parallel with respect to each other.

Hence, the processing module 50 may be configured to receive values of sensed current Ii, h flowing into each of the capacitor modules 31, 32, and determine the measure of the condition of the capacitor 30 on basis of a difference between the sensed currents Ii, h flowing into respective ones of the capacitor modules 31 and 32. Such as mentioned in the foregoing, the processing module 50 may be configured to then compare the determined measure with a predefined threshold value, and, on a condition that the determined measure exceeds the threshold value - e.g., in case Idiff exceeds some predefined current difference threshold value, generate an indication of failure or onset of failure of the capacitor 30.

With respect to the embodiment of the present invention described herein based on a differential current measurement and with further reference to Figure 3, the method 300 may be for use in conjunction with at least one capacitor included in a converter, wherein the at least one capacitor comprises at least two capacitor modules electrically connected in parallel, and wherein at least during part of operation of the converter current in the converter flows into each of the at least two capacitor modules. The step 301 may comprise receiving values of sensed current flowing into each of the at least two capacitor modules. The measure of the condition of the at least one capacitor may be determined in step 304 further based on a difference between the values sensed currents flowing into respective ones of the at least two capacitor modules.

The cell 100 may be included in a converter, which may be arranged in or constitute an interface arrangement for coupling an AC power system with a DC power system (not shown in Figure 1 or Figure 2). The converter may, such as mentioned in the foregoing, for example comprise a plurality of multilevel converter cells arranged in one or more phase arms in respective ones of one or more phase legs, where there may be one phase leg per AC phase of the AC power system. The phase legs may for example be connected in parallel between terminals of the DC power system. The current I_cap flowing into the capacitor 30 may for example be obtained by multiplying the cell firing pulse and the sensed or measured current of a phase arm in which the cell 100 is included.

The processing module 50 may be configured to receive and/or retrieve a plurality of values of sensed current I_cap flowing into the capacitor 30, wherein the current I_cap has been or is sensed continually, or periodically with some predefined periodicity, during a first selected period of time. In alternative or in addition, the processing module 50 may be configured to receive and/or retrieve a plurality of values of sensed voltage V_cap over the capacitor 30, wherein the voltage V_cap has been or is sensed continually, or periodically with some predefined periodicity, during a second selected period of time. The first selected period of time and the second selected period of time may be the same, or substantially the same

(e.g., to the most part, for example overlapping by 80-90 %). The processing module 50 may be configured to determine the measure of the condition of the capacitor 30 on basis of the plurality of values of sensed current I_cap flowing into the capacitor 30 and/or on basis of the plurality of values of sensed voltage V_cap over the capacitor 30. The processing module 50 may be configured to (approximately) integrate the sensed current I_cap flowing into the capacitor 30 over time, and/or (approximately) integrate the sensed voltage V_cap over the capacitor 30 over time. This may for example be done by approximating the integration based on a summation involving the plurality of values of sensed current I_cap and the plurality of values of sensed voltage V_cap, respectively, or a function thereof and the time intervals between instants when the current I_cap or the voltage V_cap, respectively, are sensed. Such approximate integration is as such known in the art.

In alternative or in addition, a measure of a condition of the capacitor 30 for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30 can be based on a charge which flows through any resistive elements of the capacitor 30. With further reference to Figure 1, an expression for the current I_cap can be derived as follows:

I_Cap = (C 1 , total + C₂, total)(dVcap/ dt) + I_r, ( 1 )

(Ci, total + C₂, totai)(dVcap/dt) can be considered as the capacitive current, and I_r can be considered as the resistive current. If the resistive elements of the capacitor 30 (e.g., Ri and R₂ as schematically indicated in Figure 1) are large enough, the current I_cap will be purely or substantially purely capacitive. Sensing of the resistive current I_r, which will increase rapidly at a failure or onset of a failure of the capacitor 30, can therefore provide a measure of the condition of the capacitor 30, for determining whether there is a failure or onset of failure of the capacitor 30. However, the resistance of the resistive elements of the capacitor elements 33, 34 of the capacitor 30 may not be known or difficult to determine. By subtracting the capacitive current, which is determined based on the sensed voltage V_cap, from the sensed current I_cap, the resistive current I_r can be estimated as follows: Ir' - leap - (Cl , total + C₂, total)(dV_Cap/dt). (2)

The estimation of the resistive current in equation (2) is denoted by I_r', since there may be some deviation between the estimated I_r' and the 'true' I_r that is not possible to sense or measure. Equation (2) can be integrated over a selected period of time in order to get rid of any noise in the time derivative of V_cap: ir' dt = leap dt - (Cl , total + C₂, total) Vcap. (3)

The integration of I_r' represents the charge Q_r which flows through the (any) resistive elements of the capacitor 30, i.e. Q_r = V dt.

Upon a failure of the capacitor 30, or at the onset of failure or imminent failure of the capacitor 30, the resistances of the resistive elements of the capacitor 30 (e.g., Ri and R₂ as schematically indicated in Figure 1 ) will start to rapidly decrease from their 'normal' or nominal values or 'normal' or nominal ranges of values. While the resistances of the resistive elements of the capacitor 30 are large enough (i.e. when the capacitor 30 has not failed or is not about to fail) and if a relatively accurate value of the total capacitance (Ci , total + C₂, total) of the capacitor 30 is used, Q_r should theoretically become zero, or substantially zero. Upon a failure of the capacitor 30, or at the onset of failure or imminent failure of the capacitor 30, there may be a rapid change in Q_r over time. Thus, Q_r may be used as a measure of a condition of the capacitor 30, for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30.

Thus, in view of the above, the processing module 50 may be configured to receive and/or retrieve a plurality of values of sensed current I_cap flowing into the capacitor 30, wherein the current I_cap has been or is sensed continually, or periodically with some predefined periodicity, during a first selected period of time. The processing module 50 may be configured to receive and/or retrieve values of sensed voltage V_cap over the capacitor 30. The processing module 50 may be configured to determine, on basis of the plurality of values of sensed current I_cap flowing into the capacitor 30 and the values of sensed voltage V_cap over the capacitor 30, a charge (e.g., Q_r) which flows through any resistive elements (e.g., Ri , R₂) of the capacitor 30. The processing module 50 may be configured to determine the measure of the condition of the capacitor 30 further on basis of the charge. Such as mentioned in the foregoing, the processing module 50 may be configured to then compare the determined measure with a predefined threshold value, and, on a condition that the determined measure exceeds the threshold value - e.g., in case the charge deviates from zero by some relatively small value AQ_r, e.g., by (about) 1 C, generate an indication of failure or onset of failure of the capacitor 30. The processing module 50 may be configured to determine the measure of the condition on basis of the charge (e.g., Q_r) by determining a rate of change (e.g., rise) over time of the charge. Thus, processing module 50 may be configured to determine the measure of the condition of the capacitor 30 on basis of the charge by determining a time derivative of the charge. The rate of change or time derivative of the charge may thus constitute the measure of the condition of the capacitor 30, which subsequently can be compared with a predefined threshold value. Prior to the comparison, a filter may be applied to the rate of change or time derivative of the charge.

A method in a processing module in accordance with the embodiment of the present invention described above with reference to equations (l)-(3) is illustrated in Figure 4. Figure 4 is a schematic flowchart of a method 300 in a processing module according to an embodiment of the present invention. The method 300 illustrated in Figure 4 is for use with, or in conjunction with, at least one capacitor included in a converter for conversion of alternating current power to direct current power, or vice versa, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor. The converter and the processing module may for example comprise a converter or cell 100 and a processing module 50, respectively, as described herein with reference to Figures 1 and 2.

The steps 302, 304 and 305 in Figure 4 are the same as the steps 302, 304 and 305, respectively, in the method 300 described with reference to Figure 3.

The step 301 in Figure 4 is the same as the step 301 in the method 300 described with reference to Figure 3, except for that the step 301 in Figure 4 comprises receiving a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time.

The method 300 illustrated in Figure 4 comprises determining, 310, on basis of the plurality of values of sensed current, as obtained in step 301, and the values of sensed voltage, as obtained in step 302, a charge which flows through any resistive elements of the at least one capacitor. According to the method 300 illustrated in Figure 4, the measure of the condition of the at least one capacitor is determined in step 303 based on the charge as determined in step 310.

In alternative or in addition, a measure of a condition of the capacitor 30 for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30 may be based on the principle that the energy of the cell 100 will be stored in the capacitor 30 of the cell 100. The instantaneous power loss Pi_0Ss of the capacitor 30 at a time instant t may be determined by sensing a difference between a power P_cap of the capacitor 30 at the time instant t and a power Pi_n input into the capacitor 30 at the time instant t. This can be formulated as follows in equations (4)-(6):

Ploss(t) = Pin(t) - Pcap(t), (4) where:

Pin(t) = (l/2)[Vcap(t+At)-Icap(t+At) + Vcap(t Icap(t)], (5) and

Pcap(t) = (2At)^"1(Cl, total + C₂, total) [Vcap(t+At)² - Vcap(t)²], (6) where V_cap(t+ At) and I_cap(t+At) represent the voltage V_cap and the current I_cap, respectively, at a time instant t+Δί. At may for example be a time interval between

consecutive or successive sensings of the voltage V_cap and the current I_cap, respectively.

Equations (5) and (6) may be integrated so as to obtain a function of Pi_0Ss of time. While the capacitor 30 is 'healthy', Pi_oss should be zero, or substantially zero. Upon a failure of the capacitor 30, or at the onset of failure or imminent failure of the capacitor 30, there may be a rapid change in Pi_oss over time. Thus, Pi_oss may be used as a measure of a condition of the capacitor 30, for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30.

Thus, in view of the above, the processing module 50 may be configured to receive or retrieve a plurality of values of sensed current I_cap flowing into the capacitor 30, which have been or are sensed continually during a first selected period of time, and receive or retrieve a plurality of values of sensed voltage V_cap over the capacitor 30, which have been or are sensed continually during a second selected period of time. As mentioned in the foregoing, the first selected period of time and the second selected period of time may be the same, or substantially the same (e.g., to the most part overlapping). The processing module 50 may be configured to determine a power P_cap of the capacitor 30 on basis of the plurality of values of sensed voltage V_cap, and determine a power Pi_n input into the capacitor 30 on basis of the plurality of values of sensed current I_cap and the plurality of values of sensed voltage Vcap. The processing module 50 may be configured to determine the measure of the condition of the capacitor 30 on basis of a difference between P_cap and Pin, i.e. Pi_oss. The determined Pi_oss should preferably be filtered, e.g. utilizing a digital filter, so as to remove any noise in Pi_oss caused by any measurement or sensing errors and any calculation errors. Such filtering may use a filter time constant which depends on the sensing time interval (that is, the period of time during which the voltage V_cap and the current I_cap are sensed), power balancing methods, etc. Such as mentioned in the foregoing, the processing module 50 may be configured to then compare the determined measure with a predefined threshold value, and, on a condition that the determined measure exceeds the threshold value - e.g., in case Pi_oss deviates from zero by some relatively small value ΔΡ, e.g. tens of kW, such as, for example, (about) 50 kW, generate an indication of failure or onset of failure of the capacitor 30. The processing module 50 may be configured to determine the measure of the condition on basis of Pi_oss by determining a rate of change (e.g., rise) over time of Pi_oss. Thus, processing module 50 may be configured to determine the measure of the condition of the capacitor 30 on basis of Pi_oss by determining a time derivative of Pioss. The rate of change or time derivative of Pioss may thus constitute the measure of the condition of the capacitor 30, which subsequently can be compared with a predefined threshold value. Prior to the comparison, a filter may be applied to the rate of change or time derivative of Pioss.

A method in a processing module in accordance with the embodiment of the present invention described above with reference to equations (4)-(6) is illustrated in Figure 5. Figure 5 is a schematic flowchart of a method 300 in a processing module according to an embodiment of the present invention. The method 300 illustrated in Figure 5 is for use with, or in conjunction with, at least one capacitor included in a converter for conversion of alternating current power to direct current power, or vice versa, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor. The converter and the processing module may for example comprise a converter or cell 100 and a processing module 50, respectively, as described herein with reference to Figures 1 and 2.

The steps 304 and 305 in Figure 5 are the same as the steps 304 and 305, respectively, in the method 300 described with reference to Figure 3.

The steps 301 and 302 in Figure 5 are the same as the steps 301 and 302, respectively, in the method 300 described with reference to Figure 3, except for that the step 301 in Figure 5 comprises receiving a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time, and the step 302 in Figure 5 comprises receiving a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time.

The method 300 illustrated in Figure 5 comprises determining, 306, a power of the at least one capacitor on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time, as obtained in step 302. The method 300 illustrated in Figure 5 comprises determining, 307, a power input into the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time, as obtained in step 301, and the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time, as obtained in step 302. According to the method 300 illustrated in Figure 5, the measure of the condition of the at least one capacitor is determined in step 303 based on a difference between the determined power of the at least one capacitor and the determined power input into the at least one capacitor, as determined in steps 306 and 307, respectively. In alternative or in addition, a measure of a condition of the capacitor 30 for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30 may be based on the principle that the measured or sensed current I_cap flowing into the capacitor 30 should theoretically change the voltage V_cap over the capacitor 30 according to equation (7) below:

Vcap = (l/QJIcap dt, (7) where C = Ci , total + C₂, total is the total capacitance of the capacitor 30. The integration of I_cap is carried out over a selected period of time. However, in case of a failure of the capacitor 30 or at the onset of a failure of the capacitor 30, the current I_cap flowing into the capacitor 30 cannot change the voltage V_cap over the capacitor 30 according to equation (7). Therefore, a measure of a condition of the capacitor 30 for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30 may be based on a difference between sensed voltage over the capacitor 30 and voltage over the capacitor 30 that has been determined (or calculated) based (inter alia) on the current I_cap flowing into the capacitor 30. This may be formulated as follows in equations (8)-( 10):

Vdiff(t) = Vca_P, calc(t) - Vca_P, sensed(t), (8) where:

Vcap, calc(t) = (l/[2C])[I_Cap(t+At) + Icap(t)]At, (9) and

Vcap, sensed(t) = Vcap(t+ At) - Vcap(t), ( 10) where V_cap(t+ At) and I_cap(t+At) represent the voltage V_cap and the current I_cap, respectively, at a time instant t+At. At may for example be a time interval between

Hence, Vdiff(t) in equation (8) is a difference at a time instant t between a voltage V_cap, caic(t) at the time instant t over the capacitor 30, which voltage V_cap, caic(t) is determined or calculated based (inter alia) on the current I_cap flowing into the capacitor 30, and a voltage V_cap, sensed(t) at the time instant t over the capacitor 30, derived based on a difference between voltage V_cap over the capacitor 30 sensed at time instants t and t+At. Equations (8)-( 10) may be integrated so as to obtain a function of Vdiff of time. As indicated in the foregoing, while the capacitor 30 is 'healthy' Vdiff should be zero, or substantially zero. Upon a failure of the capacitor 30, or at the onset of failure or imminent failure of the capacitor 30, there may be a rapid change in Vdiff over time, and hence Vdiff may be used as a measure of a condition of the capacitor 30, for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30.

Thus, in view of the above, the processing module 50 may be configured to receive or retrieve a plurality of values of sensed current I_cap flowing into the capacitor 30, which have been or are sensed continually during a first selected period of time, and receive or retrieve a plurality of values of sensed voltage V_cap over the capacitor 30, which have been or are sensed continually during a second selected period of time. As mentioned in the foregoing, the first selected period of time and the second selected period of time may be the same, or substantially the same (e.g., to the most part overlapping). The processing module 50 may be configured to determine a first voltage over the capacitor 30 on basis of the plurality of values of sensed current I_cap. The first voltage may for example correspond to or be based on Vcap, caic according to equation (9). The processing module 50 may be configured to determine a second voltage over the capacitor 30 on basis of the plurality of values of sensed voltage Vcap. The second voltage may for example correspond to or be based on V_cap, sensed according to equation (10). The processing module 50 may be configured to determine the measure of the condition of the capacitor 30 on basis of a difference between the determined first voltage and the determined second voltage. The difference between the determined first voltage and the determined second voltage should preferably be filtered, e.g. utilizing a digital filter, so as to remove any noise in the difference between the determined first voltage and the determined second voltage caused by any measurement or sensing errors and any calculation errors. Such filtering may use a filter time constant which depends on the sensing time interval (that is, the period of time during which the voltage V_cap and the current I_cap are sensed), power balancing methods, etc. Such as mentioned in the foregoing, the processing module 50 may be configured to then compare the determined measure with a predefined threshold value, and, on a condition that the determined measure exceeds the threshold value - e.g., in case the difference between the determined first voltage and the determined second voltage deviates from zero by some relatively small value Ad, e.g., (about) 50 V, generate an indication of failure or onset of failure of the capacitor 30. The processing module 50 may be configured to determine the measure of the condition on basis of the difference between the determined first voltage and the determined second voltage by determining a rate of change (e.g., rise) over time of the difference. Thus, processing module 50 may be configured to determine the measure of the condition of the capacitor 30 on basis of the difference between the determined first voltage and the determined second voltage by determining a time derivative of the difference. The rate of change or time derivative of the difference may thus constitute the measure of the condition of the capacitor 30, which subsequently can be compared with a predefined threshold value. Prior to the comparison, a filter may be applied to the rate of change or time derivative of the difference.

A method in a processing module in accordance with the embodiment of the present invention described above with reference to equations (8)-(10) is illustrated in Figure 6. Figure 6 is a schematic flowchart of a method 300 in a processing module according to an embodiment of the present invention. The method 300 illustrated in Figure 6 is for use with, or in conjunction with, at least one capacitor included in a converter for conversion of alternating current power to direct current power, or vice versa, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor. The converter and the processing module may for example comprise a converter or cell 100 and a processing module 50, respectively, as described herein with reference to Figures 1 and 2.

The steps 304 and 305 in Figure 6 are the same as the steps 304 and 305, respectively, in the method 300 described with reference to Figure 3.

The steps 301 and 302 in Figure 6 are the same as the steps 301 and 302, respectively, in the method 300 described with reference to Figure 3, except for that the step 301 in Figure 6 comprises receiving a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time, and the step 302 in Figure 6 comprises receiving a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time.

The method 300 illustrated in Figure 6 comprises determining, 308, a first voltage over the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time, as obtained in step 301.

The method 300 illustrated in Figure 6 comprises determining, 309, a second voltage over the at least one capacitor on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time, as obtained in step 302.

According to the method 300 illustrated in Figure 6, the measure of the condition of the at least one capacitor is determined in step 303 based on a difference between the determined first voltage and the determined second voltage, as determined in steps 308 and 309, respectively.

The ways described in the foregoing of determining a measure of a condition of the capacitor 30 for determining if there is a failure of the capacitor 30 or an onset of failure of the capacitor 30 with reference to equations (l)-(3), equations (4)-(6), and equations (7)-(10), respectively, and based on a differential current measurement have all been simulated and validated using simulations employing PSCAD, also known as PSCAD/EMTDC, developed by Manitoba HVDC Research Centre, 211 Commerce Drive, Winnipeg, Manitoba, R3P 1A3, Canada.

Referring now to Figure 7 there is shown a schematic view of computer- readable means 401, 402 carrying computer program code according to embodiments of the present invention. The computer-readable means 401, 402 or computer program code is configured to execute or run in a processing module according to an embodiment of the present invention, e.g. a processing module 50 as described above with reference to Figures 1 and 2. The computer-readable means 401, 402 or computer program code is configured to, when executed in the processing module, perform a method according to an embodiment of the present invention, e.g. as described above with reference to Figure 7. The computer- readable means 401, 402 or computer readable storage mediums, shown in Figure 7 include a Digital Versatile Disc (DVD) 401 and a floppy disk 402. Although only two different types of computer-readable means 401, 402 are depicted in Figure 8, the present invention

encompasses embodiments employing any other suitable type of computer-readable means or computer-readable digital storage medium, such as, but not limited to, a nonvolatile memory, a hard disk drive, a CD, a Flash memory, magnetic tape, a USB memory device, a Zip drive, etc.

While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A processing module (50) for use in conjunction with at least one capacitor (30) included in a converter (100) for conversion of alternating current power to direct current power, or vice versa, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor, the processing module being configured to:

receive values of sensed current (I_cap) flowing into the at least one capacitor; determine, on basis of at least the values of the sensed current flowing into the at least one capacitor, a measure of a condition of the at least one capacitor;

compare the determined measure with a predefined threshold value; and on a condition that the determined measure exceeds the threshold value, generate an indication of failure or onset of failure of the at least one capacitor.

2. A processing module according to claim 1, further configured to:

receive a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time; and

determine the measure of the condition of the at least one capacitor further on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time.

3. A processing module according to claim 2, further comprising:

receive values of sensed voltage (V_cap) over the at least one capacitor; and determine, on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time and the values of sensed voltage over the at least one capacitor, a charge which flows through any resistive elements of the at least one capacitor; and

determine the measure of the condition of the at least one capacitor further on basis of the charge.

4. A processing module according to claim 1 or 2, further configured to:

receive values of sensed voltage over the at least one capacitor; and determine the measure of the condition of the at least one capacitor further on basis of the values of sensed voltage over the at least one capacitor.

5. A processing module according to claim 4, further configured to: receive a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time; and

determine the measure of the condition of the at least one capacitor further on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time.

6. A processing module according to claim 1, further configured to:

receive a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time;

receive a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time;

determine a power of the at least one capacitor on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time;

determine a power input into the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time, and the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time; and

determine the measure of the condition of the at least one capacitor further on basis of a difference between the determined power of the at least one capacitor and the determined power input into the at least one capacitor.

7. A processing module according to claim 1, further configured to:

determine a first voltage over the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time;

determine a second voltage over the at least one capacitor on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time; and

determine the measure of the condition of the at least one capacitor further on basis of a difference between the determined first voltage and the determined second voltage.

8. A processing module according to any one of claims 5-7, wherein the first selected period of time and the second selected period of time are the same.

9. A processing module according to any one of claims 5-7, wherein the first selected period of time and the second selected period of time are overlapping.

10. A processing module according to claim 1 , wherein the at least one capacitor comprises at least two capacitor modules electrically connected in parallel, wherein at least during part of operation of the converter current in the converter flows into each of the at least two capacitor modules, the processing module being further configured to:

receive values of sensed current flowing into each of the at least two capacitor modules; and

determine the measure of the condition of the at least one capacitor further on basis of a difference between the values of sensed current flowing into respective ones of the at least two capacitor modules.

11. A method (300) in a processing module (50) for use in conjunction with at least one capacitor (30) included in a converter (100) for conversion of alternating current power to direct current power, or vice versa, wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor, the method comprising:

generating (305) an indication of failure or onset of failure of the at least one capacitor.

12. A method according to claim 11 , further comprising receiving a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time, and on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time, determining the measure of the condition of the at least one capacitor.

13. A method according to claim 12, further comprising: receiving (302) values of sensed voltage (V_cap) over the at least one capacitor; and

determining (310), on basis of the plurality of values of sensed current and the values of sensed voltage, a charge which flows through any resistive elements of the at least one capacitor, wherein the measure of the condition of the at least one capacitor is determined further based on the charge.

14. A method according to claim 11 or 12, further comprising receiving (302) values of sensed voltage over the at least one capacitor, wherein the measure of the condition of the at least one capacitor is determined further on basis of the values of sensed voltage over the at least one capacitor.

15. A method according to claim 14, further comprising receiving a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time, wherein the measure of the condition of the at least one capacitor is determined further on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time.

16. A method according to claim 11, the method further comprising:

receiving a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time;

receiving (302) a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time;

determining (306) a power of the at least one capacitor on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time; and

determining (307) a power input into the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time, and the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time;

wherein the measure of the condition of the at least one capacitor is determined further based on a difference between the determined power of the at least one capacitor and the determined power input into the at least one capacitor.

17. A method according to claim 11, the method further comprising:

receiving a plurality of values of sensed current flowing into the at least one capacitor sensed continually during a first selected period of time; receiving (302) a plurality of values of sensed voltage over the at least one capacitor sensed continually during a second selected period of time;

determining (308) a first voltage over the at least one capacitor on basis of the plurality of values of sensed current flowing into the at least one capacitor sensed continually during the first selected period of time; and

determining (309) a second voltage over the at least one capacitor on basis of the plurality of values of sensed voltage over the at least one capacitor sensed continually during the second selected period of time;

wherein the measure of the condition of the at least one capacitor is determined further based on a difference between the determined first voltage and the determined second voltage.

18. A method according to claim 11, wherein the at least one capacitor comprises at least two capacitor modules electrically connected in parallel, wherein at least during part of operation of the converter current in the converter flows into each of the at least two capacitor modules, the method further comprising:

receiving values of sensed current flowing into each of the at least two capacitor modules;

wherein the measure of the condition of the at least one capacitor is determined further based on a difference between the values sensed currents flowing into respective ones of the at least two capacitor modules.

19. A converter (100) for conversion of alternating current power to direct current power, or vice versa, the converter comprising:

at least one capacitor (30), wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor; and a processing module (50) configured to:

on basis of at least sensed current flowing into the at least one capacitor, determine a measure of a condition of the at least one capacitor;

20. A system (200) comprising:

a converter (100) for conversion of alternating current power to direct current power, or vice versa, the converter comprising at least one capacitor (30) wherein at least during part of a power conversion operation of the converter current in the converter flows into the at least one capacitor; and a processing module (50) configured to:

21. A system according to claim 20, wherein the converter further comprises a bypass switch (40) configured so as to be controllably and selectively switchable between at least a first mode, in which, at least during part of the power conversion operation of the converter, current in the converter flows into the at least one capacitor, and a second mode, in which the current in the converter is conveyed so as to bypass the at least one capacitor, the system further comprising:

a control module (60) communicatively coupled with the processing module and with the bypass switch, the control module being configured to, upon having received the indication from the processing module, control the bypass switch so as to switch the bypass switch into the second mode.

22. A computer program product configured to, when executed in a processing module (50) according to any one of claims 1-10, perform a method (300) according to any one of claims 11-18.

23. A computer-readable storage medium (401; 402) on which there is stored a computer program product configured to, when executed in a processing module (50) according to any one of claims 1-10, perform a method (300) according to any one of claims 11-18.