ES2527704B2 - System and method for measuring the voltages of the capacitive arrangements of the sub-modules of a multilevel power converter with distributed energy storage (MMC) and MMC converter - Google Patents

Abstract

System and method for measuring the voltages of the capacitive arrangements of the sub-modules of a multilevel power converter with distributed energy storage (MMC) and MMC converter # The system comprises: # - voltage sensors (SEN (sub, s1), SEN (sub, s2) SEN (sub, i1), SEN (sub, i2), each of which is configured to measure the voltage between the terminals of a serial arrangement of sub-modules (SMs (1 )… SMs (n); SMi (1)… SMi (n); and # - a means of processing to: # - determine that the voltage measurement performed is a real voltage measurement in the capacitor of an activated sub-module and estimate the capacitor voltages of the deactivated sub-modules; or # - estimate the capacitor voltages of the deactivated and activated sub-modules. # The MMC converter includes the measurement system of the first aspect. # The method comprises combining actual measurements with estimates, depending on the activation / deactivation status of MMC converter sub-modules.

Description

DESCRIPTION

System and method for measuring the voltages of the capacitive arrangements of the sub-modules of a multilevel power converter with distributed energy storage (MMC) and MMC converter. 5

Technical sector

The present invention concerns, in general and in a first aspect, a system for measuring the voltages of the capacitive arrangements, in general formed by a capacitor (C), of the sub-modules (SMs) of a multilevel power converter 10 with distributed energy storage, commonly known as MMC converter (MMC: Modular Multilevel Converter), and more particularly to a system comprising a number of voltage sensors less than the number of MMC converter sub-modules.

A second aspect of the invention concerns an MMC converter that includes the measurement system of the first aspect.

A third aspect of the invention concerns a method of measuring the voltages of the capacitive arrangements of the sub-modules of an MMC converter, which comprises combining real measurements with estimated measurements, taking advantage of the knowledge about the state of activation / deactivation of the sub-modules of the 20 MMC converter.

The present invention relates, in general, to power electronics, and more particularly, in reducing the number of voltage sensors used to measure the voltages of the capacitive arrangements of each SM in an MMC converter. These types of converters are mainly used in 25 high voltage applications, especially in direct current power transmission (HVDC: High Voltage Direct Current), and more recently in high power motor drive.

PRIOR ART 30

The present invention relates to a multilevel converter with distributed energy storage, that is, a device for converting direct current (DC) to alternating current (AC), or vice versa, commonly known as MMC.

The basic topology of this converter was patented in DE10103031B4, although other variants have been published in WO2007023064A1, WO2009149743A1, US8599591B2, and DE102011086087A1. This converter is formed by two branches or semi-phases connected between one of the terminals of the continuous bus and the exit point. Each of these branches consists of the series union of N identical cells or sub-modules 5 plus an inductance [1]. The SMs are formed by a capacitive arrangement, generally formed by a capacitor (C), and by a static converter, usually in the form of half-bridge or full-bridge. These SMs work as a voltage source, providing the tensions of the capacitive arrangements to the branch when they are activated or providing a practically zero voltage when they are deactivated. Thus, the branch tension consists of the sum of the tensions contributed by each one of the activated SMs minus the voltage drops of all the SMs, generally insignificant. The number of SMs that must be active at any time can be defined by various modulation techniques [2-4].

Modulation techniques generally define the number of SMs to be activated, 15 but not which specific SMs should be activated. For this, a voltage balancing algorithm [5] is normally used, which decides the specific SM that will be activated in order to maintain the same voltage in all the SMs. In order to apply this algorithm it is necessary to know all the tensions of the capacitive dispositions of the SMs. twenty

The voltages of the capacitive arrangements of the SMs are currently measured by sensors between the terminals of the relevant capacitive arrangements. In real applications of this type of converter, the number of SMs per branch can amount to hundreds [6]. For this reason, the high amount of voltage measurements and their adaptation for further processing complicates the implementation and control of this converter, while compromising its reliability. So far, some technique has been developed to reduce the processing time of the voltages [7] but not to reduce the number of sensors needed. There are also some control techniques without feedback in which the voltages are not measured [8], but their stability and reliability is very low. 30

References:

[1] A. Lesnicar and R. Marquardt, “An innovative modular multilevel converter topology suitable for a wide power range,” in Power Tech Conference Proceedings, 2003 IEEE Bologna, 2003. 5

[2] L. G. Franquelo, J. Rodríguez, J. I. Leon, S. Kouro, R. Portillo and M. A. M. Prats, “The age of multilevel converters arrives,” Industrial Electronics Magazine, IEEE, vol. 2, pp. 28-39, 2008.

[3] L. Yapeng, H. Pengfei, G. Jie and J. Daozhuo, “A review of module multi-level converters,” in Natural Computation (ICNC), 2011 Seventh International 10 Conference on, 2011, pp. 1934-1940.

[4] M. Hagiwara and H. Akagi, "Control and Experiment of Pulsewidth-Modulated Modular Multilevel Converters," Power Electronics, IEEE Transactions on, vol. 24, pp. 1737-1746, 2009.

[5] M. Saeedifard and R. Iravani, “Dynamic Performance of a Modular Multilevel 15 Back-to-Back HVDC System,” Power Delivery, IEEE Transactions on, vol. 25, pp. 2903-2912, 2010.

[6] K. Friedrich, “Modern HVDC PLUS application of VSC in Modular Multilevel Converter topology,” in Industrial Electronics (ISIE), 2010 IEEE International Symposium on, 2010, pp. 3807-3810. twenty

[7] J. Mei, K. Shen and B. Xiao, “A New Selective Loop Bias Mapping Phase Disposition PWM with Dynamic Voltage Balance Capability for Modular Multilevel Converter,” Industrial Electronics, IEEE Transactions on, vol. PP, pp. 1-1, 2013.

[8] G. S. Konstantinou and V. G. Agelidis, “Performance evaluation of half-bridge 25 cascaded multilevel converters operated with multicarrier sinusoidal PWM techniques,” in Industrial Electronics and Applications, 2009. ICIEA 2009. 4th IEEE Conference on, 2009, pp. 3399-3404.

Explanation of the invention 30

The present invention concerns, in a first aspect, a system for measuring the voltages of the capacitive arrangements of the sub-modules of a multilevel power converter with distributed energy storage (MMC), wherein said power converter comprises at least a phase formed by two semi-phases, each

one of which comprises two or more sub-modules connected in series, and where each sub-module comprises a capacitive arrangement comprising one or more capacitors, two output terminals and switching means that alternatively connect to the two output terminals with the ends of the capacitive arrangement, for an activated state of the sub-module, or short-circuited each other, for a deactivated state of the sub-module.

Unlike the measurement systems for MMC converters of the prior art, the measurement system of the present invention typically comprises:

- two or more voltage sensors, at least one per semi-phase, each of 10 which is configured to measure the voltage between the end terminals of a series arrangement of two or more sub-modules; Y

- processing means with inputs arranged to receive the voltage values of the measurements made by said two or more voltage sensors, and configured for, from at least information received on the status 15 on / off of each sub- module:

- determining that the voltage measurement performed by at least one of said two or more voltage sensors substantially corresponds to a real voltage measurement in the capacitive arrangement of an activated sub-module of its respective serial arrangement of sub-modules; and estimate the tensions in the 20 capacitive provisions of the deactivated sub-modules; or

- estimate the tensions in the capacitive provisions of the deactivated and activated sub-modules.

In general, each of said capacitive arrangements comprises a single capacitor, but the system is valid for measuring the voltage of any possible configuration of capacitive arrangements, such as that formed by several capacitors connected in series and / or parallel.

According to a preferred embodiment, the processing means are configured to make said determination that the voltage measurement performed by at least one of said two or more voltage sensors corresponds substantially to a real voltage measurement in the capacitive arrangement of an activated sub-module of its respective serial arrangement of sub-modules, if only said sub-module is activated.

Preferably, the measurement system comprises two or more semi-phase voltage sensors, each of which is configured to measure the voltage between the end terminals of a series arrangement of two or more sub-modules.

The processing means are configured, according to an embodiment example, to record the measured / estimated voltage values for each capacitive arrangement and to update them when they receive a measure determined as real for a capacitive arrangement, and the processing means comprise an output to send the measured, estimated and updated voltage values to a means for balancing the voltages in the capacitive arrangements.

For an exemplary embodiment, the measuring system comprises at least 10 a semi-phase current sensor configured to measure the current flowing through its respective semi-phase, the processing means comprising an input to receive the measured current values and being configured to carry out said estimation of the voltages in the capacitive arrangements from the received current values, the capacity values of the capacitive arrangements and information on the on / off status of the respective sub-module. Alternatively and / or complementary, other ways of carrying out such estimates of tensions in the capacitive arrangements are also possible.

According to another embodiment, the processing means comprise counters that count the time that the 20 voltage measurements have not been updated for each capacitive arrangement, and are configured to, if the time counted by at least one of said counters is higher than a limit value and higher than the rest of the counters, force the activation of the associated sub-module.

Although the said forced activation can be implemented in different ways, such as acting directly on the sub-modules (for example, through a logic circuitry arranged at the output of the voltage balancing control means in the capacitive arrangements) , in a preferred manner, this is carried out by means of the voltage balancing control means in the capacitive arrangements, for which the processing means are configured to carry out said forced activation by varying the voltage values sent to such means 30 balancing control.

For an exemplary embodiment, the processing means comprise means for updating and estimating stresses, responsible for the previously described estimation of stresses for each capacitive arrangement and for updating

of their values with the real measures when these are available, and means of forced activation, which, as their name indicates, are responsible for carrying out the forced activation of the sub-modules as described in the previous paragraphs.

According to an exemplary embodiment, the measurement system is a redundant system comprising additional voltage sensors, each of which 5 is configured to measure the voltage of a respective semi-phase of the converter, the processing means comprising inputs arranged to receive information on the voltage values of the measurements made by said additional voltage sensors and being configured to perform a redundancy check comparing each of said voltage values with the sum of the 10 voltage values of each semi - phase measured by the voltage sensors, and act accordingly based on the result of said comparison. Optionally, the redundant system also comprises other voltage sensors arranged in parallel to the voltage sensors described above, thereby increasing the redundancy in the measurements. fifteen

Depending on the exemplary embodiment, the measurement system is applied to a single-phase converter or a three-phase converter, in the latter case comprising at least one voltage sensor for each of the six semi-phases of the converter.

A second aspect of the invention concerns a multilevel power converter with distributed energy storage (MMC), wherein said power converter 20 comprises at least one phase formed by two semi-phases, each of which comprises two or more sub - modules connected in series, and where each sub-module comprises a capacitive arrangement comprising one or more capacitors, two output terminals and switching means that alternatively connect the two output terminals with the ends of the capacitive arrangement 25, for an activated state of the sub-module, or short-circuits each other, for a deactivated state of the sub-module.

Unlike the known MMC converters, the one proposed by the second aspect of the present invention comprises, in a characteristic way, the measuring system of the first aspect, with each of said voltage sensors 30 connected between the end terminals of respective ones Serial arrangements of two or more sub-modules.

According to an exemplary embodiment, the converter comprises modulation means that generate a modulation signal, and control means

of balancing that receive said modulation signal, for one input, and the measured voltage values, estimated and updated by the processing means, for another input, and, based on the signal and the values received and a criterion of balancing of the voltages in the capacitive arrangements, generates and sends, by a respective output, to the switching means of the sub-modules and to the processing means an activation / deactivation signals.

A third aspect of the present invention concerns a method of measuring the voltages of the capacitive arrangements of the sub-modules of a multilevel power converter with distributed energy storage, wherein said power converter comprises at least one phase formed by two semi-phases, each of which comprises two or more sub-modules connected in series, and where each sub-module comprises a capacitive arrangement comprising one or more capacitors, two output terminals and switching means that connect, alternatively, to the two output terminals with the ends of the capacitive arrangement, for an activated state of the sub-module, or short-circuited each other, for a deactivated state of the sub-module.

Unlike the known measurement methods, the one proposed by the third aspect of the present invention comprises performing, automatically, the following steps:

- make two or more measurements, at least one per semi-phase, of the voltage between the end terminals of a series arrangement of at least two sub-modules;

 - carry out a validation process of at least one of said two or more voltage measurements, based on at least information on the activated / deactivated state of each sub-module, and in the event that said validation offers a positive result determine that the validated measurement corresponds substantially to a real voltage measurement in the capacitive arrangement of an activated sub-module of its respective serial arrangement of sub-modules; Y

- estimate the voltages in the capacitive arrangements of the deactivated sub-modules and, if the voltage measurement has not been validated, also of the activated sub-modules. 30

For an exemplary embodiment, the method comprises establishing that said validation process offers a positive result if the voltage measurement has been performed when only a sub-module of the respective sub-series serial arrangement

modules were activated and this is the one that includes said capacitive arrangement with respect to which to determine said actual voltage measurement.

According to an exemplary embodiment, the method of the third aspect of the present invention comprises, to measure the voltage of the capacitive arrangement of a sub-module of interest, perform the following steps:

a) performing said measurement of the voltage between the end terminals of a series arrangement of two or more sub-modules that include said sub-module of interest, and a measure of at least the current passing through the semi-phase that includes it;

b) check if only one of said two or more sub-modules is activated, and if so, check if the sub-module that is activated is said sub-module of interest;

b1) if either of these two checks offers a negative result, estimate the voltage in the capacitive arrangement of the sub-module of interest from the measured current value of the semi-phase, the value of the capacitive arrangement and information on the state 15 on / off of the sub-module of interest; Y

b2) if the two checks offer a positive result, consider the voltage measurement as valid and determine that it corresponds substantially to a real voltage measurement of the capacitive arrangement of the sub-module of interest, update a possible previous estimate 20, if it is the case, and send the voltage value of said real measurement to said voltage balancing control means in the capacitive arrangements of the converter.

In general, said step b1) comprises:

b1a) send the estimated voltage value to control means for balancing the voltages in the capacitive arrangements of the converter; or

b1b) count the time that the voltage measurements for the capacitive arrangement of the sub-module of interest have not been updated, and if this is greater than a limit value and greater than the one for the voltage measurements for the 30 capacitive arrangements of the rest of sub-modules, modify the estimated voltage value and send said modified value to a voltage balancing control means in the capacitive arrangements of the converter to force the activation of the sub-module of interest.

According to an embodiment, the method comprises using the measurement system of the first aspect for measuring the voltages of the capacitive arrangements of the sub-modules of a multilevel power converter with distributed energy storage.

The present invention allows, in all its aspects, to measure the voltages of the 5 capacitors (or capacitor clusters, if applicable) of all the SMs of a semi-phase or branch without having to use an independent sensor for each SM. The strategy is to connect the sensors between the output of multiple SMs connected in series and take measurements when only one of the SMs associated with a specific voltage sensor is activated. Thus, the voltage measured by the sensors corresponds to 10 the capacitor voltage of the activated SM minus the voltage drops in the same SM and in the other deactivated SMs (small and that can be easily estimated). Although, for an exemplary embodiment, the present invention can be used with a single sensor for each semi-phase of the converter, to be able to be used for any modulation index and number of SMs, the minimum number of voltage sensors 15 required per semi - Phase are two. You can also use more sensors to increase the measurement frequency. The measurements taken are used to update the values of voltage estimators, which calculate the voltage of the capacitors when actual measurements are not available.

In a later section an implementation of the proposed method 20 by the third aspect of the invention is presented, in the form of an algorithm whose purpose is to ensure the update of all the tensions of each SM, at least once for each period of the modulation signal.

One of the main applications of this proposed measurement method is its use as an alternative to using an individual voltage sensor for each SM. In this way, a drastic reduction in the number of sensors used is achieved, reducing the costs of the converter and its control system. Another application is its use as a redundant system to individual sensors, allowing to detect sensor failures and replacing the wrong sensor measurements.

 30

Brief description of the drawings

The foregoing and other advantages and features will be more fully understood from the following detailed description of some examples of

realization with reference to the attached drawings, which should be taken by way of illustration and not limitation, in which:

Figure 1 shows by way of explanatory example, the block diagram of the MMC converter proposed by the second aspect of the invention comprising a control system that includes the voltage measurement system proposed by the first aspect of the invention, for An example of realization.

Figure 2 shows the MMC converter of Figure 1 when the control system is complemented by a forced measurement update algorithm.

Figure 3 shows a flow chart of the measurement method proposed by the third aspect of the invention, or method of estimating and updating the capacitor voltages, for an exemplary embodiment. The algorithm corresponding to this flowchart is performed independently for each SM. For generalization reasons the subscripts j and n are used, which indicate respectively the upper (s) and lower (i) half-phase and the number of SM.

Figure 4 shows the flow chart of Figure 3 complemented by an update force algorithm.

Figure 5 shows the scheme of an SM of the MMC converter, where the output thereof (indicated as OSM) is specified, where the switching means SW are formed by a semi-bridge structure.

Figure 6 shows in detail a single-phase MMC converter with four 20 SMs per semi-phase (N = 4) with independent sensors located at the terminals of each capacitor. This figure represents the technique used in the prior art to measure the tensions of the different SMs.

Figure 7 shows in detail an embodiment of the MMC converter of the second aspect of the invention, when this is a single phase MMC converter 25 with N = 4 and incorporates the measured system proposed by the first aspect of the invention, for an example of embodiment for which this includes a possible distribution of two voltage sensors per semi-phase.

Figure 8 shows the example of a situation suitable for measuring the capacitor voltages by means of the proposed strategy and using two sensors per semi-30 phase. With the solid line, the activated SMs are shown, which connect the capacitors in series to the semi-phase, and with the broken line the deactivated SMs are shown, which act as short circuits.

Figure 9 shows an example of a three-phase MMC converter, according to the second aspect of the invention (phases a, b and c), with the measurement system of the first aspect comprising two semi-phase voltage sensors.

Figure 10 shows a variation of the measurement topology, using three sensors per semi-phase. Although the example is performed with three sensors, the measurement system is expandable to any number of sensors up to a maximum of N per semi-phase (using one sensor per SM).

Figure 11 shows an exemplary embodiment of the measuring system of the first aspect of the invention, for which it has an improved topology with redundancy, in which a third sensor measures all the semi-phase voltage. 10

Figure 12 shows the topology of Figure 11 with increased redundancy, using sensors in parallel.

Figure 13 shows an example of the application of the measurement system, using it as a redundant system when the capacitor voltage of each SM is already measured individually. fifteen

Figure 14 shows the simulation results of the voltages in the capacitors when the measurement system proposed by the present invention (a) and (b) is used, for the topology illustrated in Figure 1 (both as regards the MMC converter as to the measurement system), and when an individual sensor is used for each SM (c) and (d), for the same MMC converter topology. twenty

Figure 15 shows the experimental results of the capacitor voltages when the measurement system proposed by the present invention (a) and (b) is used, for the topology illustrated in Figure 1, and when an individual sensor is used for each SM (c) and (d), for the same MMC converter topology.

 25

Detailed description of some embodiments

In this section various examples of realization of the different aspects of the present invention will be described, for measuring the voltage in the capacitive arrangements of the sub-modules of an MMC converter, simplifying the description for the case where each of such capacitive arrangements include a single capacitor, although obviously, as claimed and also described in a previous section, the voltage measurement is performed equally on capacitive arrangements that include more than one capacitor

As indicated in the prior art section of the prior art, it is known that in order to obtain a proper operation of the conventional MMC converter, the use of a balance control of the capacitor voltages of the SMs is required. Once the modulator has defined the amount of SMs that must be activated in each semi-phase, the balance control selects the most appropriate SMs in order to keep all the capacitor voltages as close as possible. This control is normally based on ordering all the capacitor voltages of each semi-phase in ascending or descending order in order to select the activation of the SMs with the most appropriate voltages. If the current in the semi-phase is negative (discharge the capacitors), the SMs with the highest 10 voltages must be activated. On the other hand, if the current is positive (charges the capacitors), the SMs must be activated with the lowest voltages. This balancing technique, like other alternative techniques, requires knowing in each switching cycle all the voltages in the capacitors. In applications with a high number of SMs, the measurement of these voltages entails a high cost. This is due to the large number of sensors required, with their respective wiring in a hostile environment (electromagnetic noise), and the need for a large number of input ports in the control system.

The present invention proposes a system / method to measure the capacitor voltages without using an independent sensor located between the 20 terminals of each capacitor. The system is based on dividing the semi-phases of the converter into blocks, placing the sensors at the output of each SM or between the output of several of them, and thus measuring the voltage that each block brings to the branch, that is , the sum of the output voltages of all the SMs in the block. Preferably, the voltage of the measuring block will be acquired when only one of its SMs is activated, so that the voltage measured by the sensor is approximately equal to the voltage of the capacitive arrangement (generally formed by a capacitor) of the activated SM. The difference between the measured and the actual voltage corresponds mainly to the voltage drops in the activated SM and in the other deactivated SMs, a value that can be considered negligible or easily calculated. 30

The proposed measurement system and method is complemented, for an exemplary embodiment, with a voltage estimation system / method in the capacitors of the sub-modules (or in general in the capacitive arrangements). Knowing the duty cycle and the current flowing through each semi-phase, the values of the voltages in the

Capacitors are estimated by mathematical equations that relate voltages and currents in capacitors, which include integral functions. The estimated voltages are updated every time a real measurement is produced in the corresponding SM. In this way, the error accumulated by the estimator is corrected periodically every few switching cycles. 5

The system / method of the present invention can be applied using only one sensor for each semi-phase, but then, its application range is limited, since the voltage could only be measured when only one SM in the entire semi-phase was activated, a situation that is not produced by operating with low modulation rates.

For this reason, it is presented as an improved option to use two sensors per 10 semi-phase, each of which would measure the voltage contributed by half of SMs of the branch. In order to perform measurements in this configuration, a maximum of half plus one (N / 2 + 1) of SMs must be activated in the semi-phase. These may appear so that in one measuring block (half of the semi-phase) only one activated SM appears, which can be measured, and the remaining SMs are activated in the other half of the semi-phase. This situation occurs for any modulation index.

Although the ideal configuration consists of two sensors, the number can be expanded in order to increase the reliability and accuracy in the measurement of the voltage of each capacitor. Increasing the number of sensors increases the probability that there is only one active SM per sensor, and therefore, increases the measurement frequency and updating of the estimators.

For an exemplary embodiment, the system / method of the present invention does not force the updating of the voltage measurement in the capacitors, but takes advantage of the favorable occasions that appear. In order to ensure a periodic update of the voltage measurements, for another embodiment, the measurement system / method of the present invention is complemented by an update force algorithm. If after a preventive time the voltages of all SMs have not been updated naturally, this algorithm modifies the update priority of the SMs to facilitate their activation alone and allow their voltage to be measured. 30

Figure 1 shows both the measurement system proposed by the first aspect of the invention, and the converter of the second aspect that includes it, for an embodiment for which the aforementioned as processing means comprise an estimating block ( 1) (or update block and

voltage estimator) that calculates the capacitor voltages and updates the estimated value each time there is a value available in the voltage measurement sensors (8), which in this case comprise two sensors (9), SENs1 and SENs2, in the upper semi-phase and two sensors (10), SENi1 and SENi2, in the lower semi-phase of the single-phase converter (11). This estimate is made, for the illustrated example, from the branch current, measured by the current sensors SENC1 and SENC2 (7) and the switching state (ss (n) or if (n)) of each SM (4). The estimated voltages (2) are sent to the balancing control (3), which will decide which specific SMs must be activated, complying with the number of SMs (6) defined by the modulator (5).

The flow chart followed by the estimating block (1) in a repetitive manner 10 for each SM is shown in Figure 3, according to an example of embodiment of the method proposed by the third aspect of the invention. At the beginning of each program cycle, in A1, the system voltages and currents are measured. In box A2 it is checked if there is only one SM activated in the corresponding measurement block. If this is the case and if this is the one of interest, that is, the one that includes the capacitor whose voltage you want to measure, which is checked in box A4, this SM will update the value of its estimator with the value measured by the sensor, as indicated in box A5. In contrast, for SMs that are not activated or if there is more than one SM activated in the block, the capacitor voltage will be estimated from the semi-phase current and the state of the SM (4) (on or off ), as indicated in box A3. 20 Finally, the estimated values and the actual measurements are sent to the balanced control, as indicated in box A6.

Figure 2 shows both the measurement system proposed by the first aspect of the invention, and the converter of the second aspect that includes it, for an example of embodiment for which the processing means comprise, in addition to the estimating block ( 1), a forced activation block, referred to as "Force algorithm" (14), which complements the measurement strategy. The block diagram illustrated is like the one in Figure 1 but modified by the inclusion of the forcing algorithm block (14) between the output (2) of the block (1) and an input of the balancing control block (3). This algorithm acts when the time that an SM has been 30 without updating in the estimator exceeds the allowed limit, indicated by the signal (13) of a cycle counter. Then, the forcing algorithm (14) modifies the input voltages to the balancing control (15) in order to modify the activation priority of the SM and force the voltage measurement update of its capacitor.

The flow chart of the strategy complemented by the strain-force algorithm is shown in Figure 4. Boxes A1 to A6 are equivalent to those in the diagram in Figure 3. Unlike the diagram in Figure 3, after performing The estimation of the voltage because the actual measurement is not available increases the value of a counter (13), as indicated in box F1. When this meter exceeds the limit set 5, as shown in box F2, the process of stress forcing will begin, provided that an affirmative answer to the questions raised in the circuit breakers of boxes F3 and F4 is obtained. This forcing will only be carried out in the appropriate semi-period (positive for the upper semi-phase and negative for the lower semi-phase), as indicated in box F3, that is when there are less than half of activated SMs 10, and for Therefore, there is the possibility of updating. In addition, the forcing is only performed for one SM at a time, applying it to the SM with a higher counter value, that is, it takes longer without updating, which is checked in F4. If all conditions are met, the estimated voltages of the capacitors (15), in F5, to be sent (in F6) to the balance control will be modified / falsified, thereby increasing the priority of the SM to be measured and decreasing 15 that of the other SMs of the block, thus facilitating it to be activated alone, since the balancing control block will interpret, from such modified voltages, that the higher priority SM must be activated. Once the actual measurement is made, the counter will restart, as indicated in box F6. Both the modified values and those of 20 real measurements are sent to the balancing control, as indicated in box A6.

The single-phase MMC converter shown in Figures 1 and 2 (11) together with the measurement system proposed by the first aspect of the present invention, forms the converter proposed by the second aspect, and is shown in detail in Figure 7, for an example of realization. This Figure shows the distribution of the 25 voltage sensors using the proposed strategy: two sensors (9), SENs1 and SENs2, in the upper semi-phase and two sensors (10), SENi1 and SENi2, in the semi- lower phase Unlike systems with an independent sensor for each SM (SENSMs (n) or SENSMi (n)) as shown in Figure 6, in the proposed method each of the sensors measures the voltage provided by more than one SM. 30

A variation of the measurement topology is shown in Figure 10, with the following distribution of the voltage sensors: three sensors, SENs1, SENs2, SENs3, in the upper semi-phase and three sensors, SENi1, SENi2, SENi3, in the lower semi-phase, each of which disposed between the end terminals of respective arrangements

in series of SMs: SMs s (1) as (N / 3), s (N / 3 + 1) as (2N / 3), s (2N / 3 + 1) as (N) for the upper semi-phase and SMs i (1) ai (N / 3), i (N / 3 + 1) ai (2N / 3), i (2N / 3 + 1) ai (N) for the lower semi-phase.

Although in Figure 7 and the rest of the figures illustrating the interior of each SM, a schematic of an SM of the MMC converter with a semi-bridge topology 5, illustrated in detail in Figure 5, is shown, the present invention is also applicable for another class of topologies, in particular for a complete bridge topology.

As indicated above, in order to improve the reliability of the proposed measurement system, it can be complemented with redundant sensors, as illustrated in Figures 11 and 12. Adding a sensor to measure the voltage of the entire semi - Phase, it could detect a fault in one of the other sensors and replace it. In addition, sensors can be added in parallel to each of them to detect the failure of more than one sensor.

In the topology of the redundant system shown in Figure 11, the main measurement blocks (9, 10) are complemented with blocks for measuring the total tensions of the upper (16) and lower (17) semi-phases, each one of them formed, in this case, by a voltage sensor (SENsTOTAL, SENiTOTAL).

Figure 12 shows the topology of Figure 11 with increased redundancy, using sensors in parallel (SENs1-R, SENs2-R, SENi1-R, SENi2-R, 20 SENsTOTAL-R, SENiTOTAL-R), both at the main sensors (SENs (n), SENi (n)) and the semi-phase sensors (SENsTOTAL, SENiTOTAL).

Although the measurement system proposed by the first aspect of the present invention has as its main application that of replacing the conventional measurement system based on the inclusion of a voltage sensor by SM, it can also be used in a complementary manner, that is, as It is illustrated in Figure 13, using it as a redundant system when the capacitor voltage of each SM is already measured individually.

 30

Enumeration of the elements represented in the Figures:

(1) Stress estimator and system for updating estimates based on the measurements taken.

(2) Estimated tensions.

 (3) Voltage balance control.

(4) Activation signals of the SMs.

(5) Modulator.

(6) Number of SMs to be activated.

(7) Semi-phase current sensors. 5

(8) Voltage sensors for the proposed measurement method.

(9) Measurement blocks of the upper semi-phase.

(10) Measurement blocks of the lower semi-phase.

(11) Single phase MMC converter without measuring system.

(12) Single phase load or electrical network. 10

(13) Number of switching cycles without updating of each estimated voltage.

(14) Update force algorithm.

(15) Estimated stresses modified by the forcing algorithm.

(16) Block for measuring the total voltage of the upper semi-phase in a redundant system. fifteen

(17) Block for measuring the total voltage of the lower semi-phase in a redundant system.

Example of application of the proposed measurement system:

An example of a particular application of the measurement system of the present invention is presented by way of example in the MMC converter shown in Figure 1. In this case, the converter data are as follows: number of SMs per semi-phase N = 10, DC bus voltage Vdc = 750V, capacitor capacity of SMs C = 1500 µF, semi-phase coil inductance L = 1.5 mH, effective value of output current Io = 10A with a lag of φ = 30º and carrier signal frequency fs = 5 kHz. 25

In this particular application, 10 SMs are available per semi-phase and two voltage sensors per semi-phase (SENs1 and SENs2 in the upper semi-phase, SENi1 and SENi2 in the lower semi-phase), so that each of they will measure the voltage of 5 SMs connected in series. The measurement of each of the sensors will be considered valid when only one SM of the corresponding measurement block is activated. This measurement will be transmitted to the stress estimation block (1), which will correct its estimate with the actual measured value.

The estimation is made in each switching cycle based on the values of the current of the upper (SENCs) and lower (SENCi) semi-phases and of the cycles of

SMs work. The equations used to make this estimate are the following, where C is the capacitor capacity, the subscripts s (n) and i (n) indicate the number of SMs of the upper and lower semi-phases respectively, is and ii represent the Semi-phase currents and s indicates the status of the SM, being 1 when it is activated and 0 when it is deactivated (ss (n), if (n) = {0,1}). 5

 Upper semi-phase

 Lower half phase

When a strain forcing algorithm is used (Figure 2), in case the voltage update of an SM is delayed longer than desired (with the possibility of accumulating a greater error in the estimate), the forcing algorithm will be operational. This block modifies the voltage value that is sent to the voltage balance block, reducing the value of the SM that is to be updated (or increasing it if the current is negative), in order to facilitate its activation, and increasing the value of the other SMs of the set measured by the same sensor (reducing it if the current is negative) facilitating its deactivation. In this way, when the total number of activated SMs 15 allows it, the SM to be updated will be the only one activated in the measurement block.

If a variation of the topology is used with a redundant sensor, such as that shown in Figure 11, a redundancy check block 20 must be included before the estimation block. This redundancy block must verify the coincidence between the sum in each semi-phase of the measurements of the main sensors SENs (n) or SENi (n) (where n indicates the sensor number) with the measurement of the semi-total sensor SENsTOTAL or SENiTOTAL phase. If it does not match, by means of a simple algorithm, the wrong sensor is detected and disabled. The voltage of the disabled block 25 can be calculated as the difference between the total semi-phase sensor and the sum of the other sensors. As shown in Figure 12, as indicated above, the redundant topology can be extended by adding sensors in parallel, both to the main sensors (SENs (n), SENi (n)) and to the semi-sensors. phase (SENsTOTAL, SENiTOTAL). 30

Figures 14 (a) and 14 (b) show the simulation results of the voltages in the capacitors of the upper semi-phase when the proposed measurement system is used with the conditions of this example. As can be seen, its result is very similar to that of Figures 14 (c) and 14 (d), where the simulation results of the voltages in the capacitors in the upper semi-phase 5 are shown when a sensor is used. individual voltage for the capacitor of each SM.

The experimental results are shown in Figure 15 comparing the proposed measurement system / method and the conventional measurement system / method. The converter data used for the experimental results are the following: number of SMs per semi-phase N = 4, DC bus voltage Vdc = 160V, capacity of 10 capacitors of SMs C = 1500 µF, coil inductance of semi-phase L = 6mH, effective value of the output current Io = 3A with a lag of φ = 0º and carrier signal frequency fs = 2777 Hz. Figures 15 (a) and 15 (b) show the experimental results of the voltages in the capacitors of the upper semi-phase when the proposed measurement system is used. Figures 15 (c) and (d) show the 15 experimental results of the capacitor voltages in the upper semi-phase when an individual sensor is used for each SM.

A person skilled in the art could introduce changes and modifications in the described embodiments without departing from the scope of the invention as defined in the appended claims. twenty

Claims (17)

1.- System for measuring the voltages of the capacitive arrangements of the sub-modules of a multilevel power converter with distributed energy storage, where said power converter comprises at least one phase formed by two two semi-phases, each of which it comprises two or more sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)) connected in series, and where each sub-module comprises a capacitive arrangement comprising one or more capacitors (C), two output terminals (X1, X2) and switching means (SW) that alternately connect the two output terminals (X1, X2) with the ends of the capacitive arrangement, for an activated state of the sub-module, or short-circuited each other, for a deactivated state of the sub-module, the measuring system being characterized in that it comprises: - two or more voltage sensors (SENs1, SENs2; SENi1, SENi2), at least one per semi-phase, each of which is configured to measure the voltage between the 15 end terminals of a series arrangement of two or more sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)); Y - processing means with inputs arranged to receive the voltage values of the measurements made by said two or more voltage sensors (SENs1, SENs2; SENi1, SENi2), and configured for, from at least 20 information received on the on / off status of each sub-module (SMs (1)… SMs (n); SMi (1)… SMi (n)): - determining that the voltage measurement performed by at least one of said two or more voltage sensors (SENs1, SENs2; SENi1, SENi2) substantially corresponds to a real voltage measurement in the capacitive arrangement of an activated sub-module of their respective serial arrangement of sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)); and estimate the tensions in the capacitive arrangements of the deactivated sub-modules; or - estimate the tensions in the capacitive arrangements of deactivated and activated sub-modules 30. 2. System according to claim 1, characterized in that the processing means are configured to make said determination that the measurement of voltage performed by at least one of said two or more voltage sensors (SENs1, SENs2; SENi1, SENi2) substantially corresponds to a real voltage measurement in the capacitive arrangement of an activated sub-module of its respective sub-series serial arrangement -modules (SMs (1)… SMs (n); SMi (1)… SMi (n)), if only said sub-module is activated. 5 3. System according to claim 1 or 2, characterized in that it comprises two or more voltage sensors (SENs1, SENs2; SENi1, SENi2) per semi-phase, each of which is configured to measure the voltage between the end terminals of a serial arrangement of two or more sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)). 4. System according to claim 1, 2 or 3, characterized in that said processing means 10 are configured to record the measured / estimated voltage values for each capacitive arrangement and to update them when they receive a determined measurement as real for a capacitive arrangement, and because the processing means comprise an output (2) to send the measured, estimated and updated voltage values to a means for controlling voltage balancing in the 15 capacitive arrangements (3). 5. System according to any one of the preceding claims, characterized in that it comprises at least one current sensor (SENC1, SENC2) per semi-phase configured to measure the current flowing through its respective semi-phase, the processing means comprising a input to receive the measured 20 current values and being configured to carry out said estimation of the voltages in the capacitive arrangements from the received current values, the capacity values of the capacitive arrangements and information on the activated state / deactivated from the respective sub-module (SMs (1)… SMs (n); SMi (1)… SMi (n)). 6. System according to any one of the preceding claims, characterized in that the processing means comprise counters that count the time taken without updating the voltage measurements for each capacitive arrangement, and because they are configured to, if the time counted by At least one of these counters is greater than a limit value and higher than the rest of the counters, forcing the activation of the associated sub-module. 30 7. System according to claim 6, characterized in that the processing means are configured to carry out said forced activation by varying the voltage values sent to said voltage balancing control means in the capacitive arrangements. 8. System according to claim 6 or 7, characterized in that said processing means comprise means for updating and estimating stresses (1) and means for forced activation (14). 5 9. System according to any one of the preceding claims, characterized in that it is a redundant system comprising additional voltage sensors (SENsTOTAL; SENiTOTAL), each of which is configured to measure the voltage of a respective semi-phase of the converter , the processing means comprising inputs arranged to receive information on the voltage values of the 10 measurements made by said additional voltage sensors (SENsTOTAL; SENiTOTAL) and being configured to perform a redundancy check comparing each of said voltage values with the sum of the voltage values of each semi-phase measured by the voltage sensors (SENs1, SENs2; SENi1, SENi2), and act accordingly based on the result of said comparison. fifteen 10. System according to any one of the preceding claims, characterized in that it is applied to a three-phase converter, comprising at least one voltage sensor (SENas1, SENas2, SENbs1, SENbs2, SENcs1, SENcs2; SENai1, SENai2, SENbi1, SENbi2, SENci1 , SENci2), for each of the six semi-phases of the converter. 11.- Multilevel power converter with distributed energy storage, where said power converter comprises at least one phase formed by two semi-phases, each of which comprises two or more sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)) connected in series, and where each sub-module comprises a capacitive arrangement comprising one or more capacitors (C), two output terminals (X1, X2) and means switching (SW) that connect, alternatively, to the two output terminals (X1, X2) with the ends of the capacitive arrangement, for an activated state of the sub-module, or short-circuit each other, for a state deactivated of the sub-module, the converter being characterized in that it comprises the measuring system according to any one of the preceding claims, with each of said voltage sensors (SENs1, SENs2; SENi1, SENi2) 30 connected between the end terminals of respective ones provisions in s erie of two or more sub-modules (SMs (1)… SMs (n); SMi (1) ... SMi (n)). 12. - Converter according to claim 11, characterized in that it comprises modulation means (5) that generate a modulation signal, and balancing control means (3) that receive said modulation signal, through an input (6), and the voltage values measured, estimated and updated by the processing means, by another input (2), and, based on the signal and the values received and a criteria for balancing the tensions in the capacitive arrangements, generates and sends, by a respective output (4), to the switching means (SW) of the sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)) and to the processing some activation / deactivation signals. 13.- Method of measuring the voltages of the capacitive arrangements of the 10 sub-modules of a multilevel power converter with distributed energy storage, where said power converter comprises at least one phase formed by two semi-phases, each of which it comprises two or more sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)) connected in series, and where each sub-module comprises a capacitive arrangement comprising one or more capacitors (C), two output terminals (X1, X2) and switching means (SW) that alternatively connect the two output terminals (X1, X2) with the ends of the capacitive arrangement, for an activated state of the sub-module, or short-circuits each other, for a deactivated state of the sub-module, the measurement method being characterized in that it comprises performing, automatically, the following steps: - make two or more measurements, at least one per semi-phase, of the voltage between the end terminals of a serial arrangement of at least two sub-modules (SMs (1)… SMs (n); SMi (1) ... SMi (n));  - carry out a validation process of at least one of said two or more voltage measurements, based on at least information on the on / off status of each sub-module (SMs (1)… SMs (n); SMi ( 1)… SMi (n)), and if such validation offers a positive result, determine that the validated measure substantially corresponds to a real voltage measurement in the capacitive arrangement of an activated sub-module of its respective arrangement in series of sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)); and 30 - estimate the voltages in the capacitive arrangements of the deactivated sub-modules and, if the voltage measurement has not been validated, also of the activated sub-modules. 14. Method according to claim 13, characterized in that it comprises establishing that said validation process offers a positive result if the voltage measurement has been performed when only a sub-module of the respective serial arrangement of sub-modules (SMs (1 )… SMs (n); SMi (1)… SMi (n)) was activated and this is the one that includes said capacitive arrangement with respect to which to determine said real voltage measurement. 15. Method according to claim 14, characterized in that it comprises the following steps to measure the voltage of the capacitive arrangement of a sub-module of interest: a) performing said measurement of the voltage between the end terminals of 10 a serial arrangement of two or more sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)) that include a said sub-module of interest, and a measure of at least the current that passes through the semi-phase that includes it; b) check if only one of said two or more sub-modules (SMs (1)… SMs (n); SMi (1)… SMi (n)) is activated, and if so, check if the sub-module that is activated is said sub-module of interest; b1) if either of these two checks offers a negative result, estimate the voltage in the capacitive arrangement of the sub-module of interest from the measured current value of the semi-phase, the value of the capacitive arrangement and information on the on / off state 20 of the sub-module of interest; Y b2) if the two checks offer a positive result, consider the voltage measurement as valid and determine that it corresponds substantially to a real voltage measurement of the capacitive arrangement of the sub-module of interest, update a possible previous estimate, if it is the 25 case, and send the voltage value of said real measurement to said voltage balancing control means in the capacitive arrangements of the converter. 16. Method according to claim 15, characterized in that said step b1) comprises: b1a) send the estimated voltage value to a voltage balancing control means in the capacitive arrangements of the converter; or b1b) count the time taken without updating the voltage measurements for the capacitive arrangement of the sub-module of interest, and if it is greater than a limit value and greater than the one for the voltage measurements for the capacitive arrangements of the rest of the sub -modules, modify the estimated voltage value and send said modified value to control means 5 for balancing the voltages in the capacitive arrangements of the converter to force the activation of the sub-module of interest. 17. Method according to any one of claims 13 to 16, characterized in that it comprises using the measuring system according to any one of claims 1 to 10 for measuring the voltages of the capacitive arrangements 10 of the sub-modules of a converter Multilevel power with distributed energy storage.