EP3780038A1

EP3780038A1 - Static on-load tap changer for transformers with discontinuous regulation windings

Info

Publication number: EP3780038A1
Application number: EP19814374.5A
Authority: EP
Inventors: Antonio GÓMEZ EXPÓSITO; Manuel BARRAGÁN VILLAREJO; Francisco de Paula GARCÍA LÓPEZ; José María MAZA ORTEGA
Original assignee: Universidad de Sevilla
Current assignee: Universidad de Sevilla
Priority date: 2018-06-06
Filing date: 2019-05-30
Publication date: 2021-02-17
Anticipated expiration: 2039-05-30
Also published as: ES2734601A1; EP3780038B1; EP3780038C0; ES2734601B2; WO2019234271A1; ES2964632T3; EP3780038A4

Abstract

This document describes an on-load tap-changer device based on solid-state switches for applications that require dynamic voltage regulation. The tap changer of the invention is provided with a central control unit that determines the optimal operating setpoint as a function of the information supplied by the local control units associated with each of the phases. For this purpose, the local control units are linked to a series of voltage and current sensors that determine the operating status of the tap changer. In addition, these local control units actuate the control signals of the solid-state switches to select the optimal tap determined by the central controller.

Description

OBJECT OF THE INVENTION

The object of the invention is framed within the technical field of electricity, more specifically the proposed invention is applied in the area of alternating current electrical power systems and, particularly, in the activities to generate, transport and distribute electrical energy.
The proposed invention is intended for transformers, specifically a specific topology of an on-load tap-changer based on solid-state switches for applications that require dynamic voltage regulation, such as:

Transformers of grids for generating, transporting and distributing electrical energy.
Photovoltaic inverters connected to the power grid.
Inverters associated with energy storage devices connected to the power grid.
Wind turbines.

BACKGROUND OF THE INVENTION

Undoubtedly, the transformer is one of the key pieces of current electrical systems as it makes it possible to adapt the voltage levels between the different portions of the electrical system in an efficient and safe way. Most of the transformers in the power grid are equipped with tap changers which, by modifying the number of turns of the primary winding, enable the value of the secondary voltage to be adjusted within a range around the nominal voltage. In this sense, there are two tap changer technologies: off-load and on-load technology. The former can only be actuated with the transformer disconnected from the grid. However, the latter can be operated when the transformer is operating, enabling a dynamic regulation of the secondary voltage in the event of variations in the primary voltage and/or the load. The proposed invention consists of a specific topology of on-load tap-changer based on solid-state switches for applications that require dynamic voltage regulation among which the following are notable:

The current operation of electrical power systems requires the voltages of the grid nodes to remain within the regulatory limits regardless of the load state. For this purpose, there are different regulating mechanisms, one of them being the tap-changer transformer. Basically, a transformer has several electrically insulated windings that are magnetically coupled such that the relationship between the voltages of the windings, known as the transformation ratio, is a function of the turns ratio thereof. The objective of the tap changer is to maintain the voltage of one of the windings regardless of the voltage of the other and the load state of the transformer. For this purpose, the winding wherein the tap changer is installed is divided into two: the main winding and the regulation winding. The former contains the bulk of the winding turns while the latter is made up of a set of coils with a fractional number of turns in relation to the main one and that are electrically accessible. In this manner, depending on the electrical connection selected in the regulation winding, the total number of turns of the winding can be modified and, therefore, the transformation ratio of the transformer.
The process of modifying the number of turns is called switching, and there are two basic ways to carry it out: on load and off load. In the first case, the change in the number of turns is carried out with the transformer in service, for which purpose an electromechanical switch is normally used, characterized by being a bulky, complex and expensive device. Due to these features, on-load tap-changers are used in applications wherein it is not possible to interrupt the service to regulate the voltage, for example in high-voltage (HV) to medium-voltage (MV) transformers. In the second case, the change in turns is carried out with the transformer out of service, that is, off load. Technologically, the off-load tap-changer is a very simple and low-cost device, which is why it is usually installed in medium-voltage (MV) to low-voltage (LV) transformers. The high number of this type of transformers in the distribution grids makes it impossible invest in on-load tap changers, opting for the option of an off-load tap changer. These make it possible to select the most suitable tap as a function of the average primary voltage, with the objective of keeping the LV voltage within regulatory limits. However, this technology does not enable the LV voltage to be dynamically controlled as a function of the MV voltage variations or the transformer load.
At present, with the massive incorporation of generation from renewable sources, the problem of voltage regulation in electrical grids is more complex. On the one hand, voltage variations in the grid increase because the primary source of energy in renewable generation (wind and photovoltaic for example) is not controlled. On the other hand, and also associated with the variability of primary energy resources, the dynamics associated with voltage variations become faster and faster. As a consequence, the electric power system needs a greater number of resources for controlling the voltages that has a very fast actuation. In this sense, current tap-changer technologies, whether off-load or on-load based on electromechanical switches, do not respond to the current existing requirements of the electrical system.
A technological option to solve this problem is to replace the traditional electromechanical changer with a static device based on power electronics which have very low actuation times. However, the use of tap changers based on power electronics is not novel, with previous patents existing in this regard:

US 5969511. P.G.J.M. Asselman et al . 19/10/1999. Method and device for continuous adjustment and regulation of transformer turns ratio and transformer provided with such device.
ES 2274684. D. Monroy . 16/5/2007. Tap changer for medium-/low-voltage transformers.
US 3195038(A). J. Fry . 13/7/1965. Voltage or current regulator apparatus.
US 3700925(A). P. Wood . 24/10/1972. Thyristor tap changer for electrical inductive apparatus.
US 4220411. J. Rosa. 2/9/1980. Thyristor tap changer for electrical inductive apparatus.

DESCRIPTION OF THE INVENTION

The object of the present invention is intended for an on-load tap-changer based on power electronics with a symmetrical arrangement of solid-state switches, sensors for the monitoring and control thereof, as well as a passive trigger device which ensures the operation thereof in the event of control signal failures; being applicable to transformers that have at least one regulation winding, in addition to the main windings, that is electrically discontinuous, in other words, open at least at one point. Thereby, the regulation winding is divided into at least two electrically insulated half-windings made up of a set of coils the terminals of which are electrically accessible.
A terminal of a solid-state switch based on a power electronics device is connected to each of the terminals of the regulating half-windings, which are electrically accessible. Therefore, there is the same number of solid-state switches as accessible terminals of the regulating half-winding and equal to the number of regulating coils of the half-winding plus one. The other terminals of the solid-state switches are connected to each other forming at least two common points, one for each one of the electrically insulated half-windings. These common points are connected with each other by means of an electrical connection.
The set is completed with a series of voltage and current sensors, required for the monitoring and control of the device, arranged in specific locations. Thus, the current measurement is carried out at the electrical connection existing between the aforementioned common points. Similarly, a voltage sensor is located in each regulating half-winding between an electrically accessible terminal and the common point. With the voltage information provided by the voltage sensors, it is possible to confirm the tap that is in operation, as well as estimate the secondary voltage of the transformer.
The solid-state switch used in the invention can be made up of two anti-series IGBTs with the respective anti-parallel diodes thereof, or an IGBT with four diodes or two anti-parallel thyristors. If necessary, a snubber can be included to reduce overvoltage when switching the solid-state switches.
Said solid-state switches are controlled by means of the corresponding trigger signals calculated by means of a control system, which determines which is the most suitable tap at each moment depending on the operating conditions and the established setpoints. In the case of polyphasic applications, this control system has the following hierarchical structure:

Local controller associated with each of the transformer phases. It is in charge of providing the suitable trigger signals to the solid-state switches depending on the setpoint of the central controller. Additionally, it measures the operation variables of the tap changer (voltage, current, temperature and/or any other parameter) required by the central controller.
Central controller in charge of determining which is the suitable regulating tap in each one of the phases as a function of the operation setpoint selected by the user and the different operation variables sent by each of the local controllers. The central controller, having all the information about the phases, can determine the most suitable operating setpoints at all times in order to optimize the overall performance of the application.

The existing communication between the local controllers and the central control is carried out through optical or wireless means in order to provide adequate galvanic isolation. In the case of single-phase applications, the local and central controllers are embedded within a single control device.
Finally, and to prevent the circuit from remaining open in the event of a failure of the control signals that actuate on the solid-state switches, a passive trigger device is incorporated into one of the solid-state switches associated with each half-winding such that it is guaranteed that, even in the event of control signal failure, the circuit remains closed at all times.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and for the purpose of helping to better understand the features of the invention according to a preferred practical exemplary embodiment thereof, a set of drawings is attached as an integral part of said description in which the following is depicted in an illustrative and non-limiting manner:

Figures 1a and 1b.- Show a view of respective transformers provided with electrically discontinuous regulation windings, in other words with at least one opening point; wherein figure 1a shows a single-phase transformer or a phase of a three-phase star connection while figure 1b shows a phase of a transformer with the three-phase delta connection.
Figure 2.- Shows a topology of a MV/LV tap changer applying the state of the art.
Figure 3.- Shows a diagram wherein the proposed topology of a MV/LV tap changer is represented.
Figures 4a-4c.- Show respective diagrams that represent (4a) Equivalent diagram of the closed solid-state switch (correct control signals). (4b) Equivalent diagram of the open solid-state switch (failure in the control signals). (4c) Voltage and current of the solid-state switch in operation with correct control signals and with failure in the control signals.
Figures 5a-5b.- Show: (5a) A diagram of components of the passive trigger device. (5b) A graph showing the operation of the passive trigger device.

PREFERRED EMBODIMENT OF THE INVENTION

The invention is applicable to transformers equipped with electrically discontinuous regulation windings, in other words, with at least one opening point such as those shown in figure 1. Figure 1a represents a single-phase transformer or one phase of a three-phase star connection, while figure 1b represents a phase of a transformer with the three-phase delta connection. Said transformers comprise a main winding (30 in figure 1a; 30a and 30b in figure 1b) and an electrically discontinuous regulation winding open at a point such that it is divided into an upper half-winding formed by the coils (32, 34) and a lower half-winding formed by the coils (36, 38). Each one of the regulating half-windings is made up of n=2 regulating coils. The terminals of the upper half-winding (42, 43 and 44) and of the lower half-winding (45, 46 and 47) are electrically accessible, both being insulated from each other since the terminals 44 and 45 are not electrically connected.
Figure 3 shows the embodiment of a tap changer based on solid-state switches object of this invention on the transformer shown in figure 1 b. The nominal voltage of the main winding (30a, 30b) is Up-Ur while the nominal voltage of the regulation windings (32, 34, 36, 38) is Ur. A first set of solid-state switches (61a, 62a, 63a) is connected to the accessible terminals of the upper half-winding (42, 43, 44) of the regulation winding. The free terminals of said first set of solid-state switches (61a, 62a, 63a) are in turn connected at a first common point (48a). Similarly, a second set of solid-state switches (61b, 62b, 63b) is connected to the accessible terminals of the lower half-winding (45, 46, 47) of the regulation winding, and the free terminals thereof are connected to a second common point (48b). The common points (48a, 48b) are electrically connected to each other and the current that circulates in said electrical connection is measured by means of the corresponding current sensor (90). The number of solid-state switches (61a, 62a, 63a, 61b, 62b, 63b) in this preferred embodiment is equal to 2(n+1)=6.

Proper operation of the solid-state switches makes it possible to select between five primary voltage levels: closed solid-state switches (61a, 63b) Up-2Ur; closed solid-state switches (62a, 63b) Up-Ur; closed solid-state switches (62a, 62b) Up; closed solid-state switches (63a, 62b) Up+Ur; the closed solid-state switches (63a, 61b) Up+2Ur. Table 1 shows the effective voltages to which the different solid-state switches are subjected as a function of the selected tap. By way of example, if the tap corresponding to the voltage level Up-Ur is selected, the solid-state switches (62a, 63b) must be closed, the voltage of said switches being zero. The voltages of the other switches are easily calculated by applying Kirchhoff's second law and taking into account that the regulating coils are identical to each other. Thus, it is possible to determine the maximum effective voltage to which each switch could be exposed, which is decisive for the purpose of sizing it. Note that the maximum voltages of the solid-state switches are in the 2Ur (switches (61a, 63a, 61b, 63b)) and Ur (switches (62a, 62b)) range. Moreover, and taking into account that the maximum current without overload that can circulate through the solid-state switches is the nominal current of the winding, In, in this specific case the maximum power associated with the tap changer, calculated as the sum of the power of each one of the switches, is: 10Urln.

Table 1. Static tap changer object of the invention: Effective voltages as a function of the selected tap.

Switch	Umax Switch	Primary voltage
Switch	Umax Switch	U_p-2U_r	U_p-U_r	U_p	U_p+U_r	U_p+2U_r
61a	2U_r	0	U_r	U_r	2U_r	2U_r
62a	U_r	U_r	0	0	U_r	U_r
63a	2U_r	2U_r	U_r	U_r	0	0
61b	2U_r	2U_r	2U_r	U_r	U_r	0
62b	U_r	U_r	U_r	0	0	U _r
63b	2U_r	0	0	U_r	U_r	2U_r

This result can be extrapolated to the case of a transformer with a regulation winding with an opening point such that it is divided into two half-windings of n regulating coils, each one of them with an effective nominal voltage Ur. In this case, the maximum effective voltage of the solid-state switches would be nUr and the total maximum power associated with the tap changer would be (0.75n²+n)UrIn if n is even and (0.75n²+n+0.25)UrIn if n is odd.
The proposed invention contributes advantages compared to the state of the art of the tap changer presented in figure 2. Again, it is possible to select five possible primary voltage levels depending on the solid-state switch that is closed: (101) closed Up-2Ur; (103) closed Up-Ur; (105) closed Up; (107) closed Up+Ur; (109) closed Up+2Ur.

The number of solid-state switches (101, 103, 105, 107, 109) in this configuration is equal to the number of regulating coils plus one (2n+1, five in the configuration shown in figure 2). Table 2 shows the effective voltages to which the different solid-state switches are subjected as a function of the selected tap. By way of example, if the tap corresponding to the voltage level Up-Ur is selected, the solid-state switch (103) must be closed, the voltage of said switch being zero. The voltages of the other switches are easily calculated by applying Kirchhoff's second law and taking into account that the regulating coils are identical to each other. Thus, it is possible to determine the maximum effective voltage to which each switch could be exposed during normal operation, which is decisive for the purpose of sizing it. Note that the maximum voltages of the solid-state switches (101, 103, 105, 107, 109) are in the 4Ur (switches (101) and (109)) and 2Ur (switch (105)) range. Moreover, and taking into account that the maximum current without overload that can circulate through the solid-state switches (101, 103, 105, 107, 109) is the nominal current of the winding, In, in this specific case the maximum power associated with the tap changer, calculated as the sum of the power of each one of the solid-state switches (61a, 62a, 63a, 61b, 62b, 63b), is: 16UrIn.

Table 2. Static tap changer according to the state of the art: Effective voltages as a function of the selected tap.

Switch	Umax Switch	Primary voltage
Switch	Umax Switch	U_p-2U_r	U_p-U_r	U_p	U_p+U_r	U_p+2U_r
101	4U_r	0	U_r	2U_r	3U_r	4U_r
103	3U_r	U_r	0	U_r	2U_r	3U_r
105	2U_r	2U_r	U_r	0	U_r	2U_r
107	3U_r	3U_r	2U_r	U_r	0	U _r
109	4U_r	4U_r	3U_r	2U_r	U_r	0

This result can be extrapolated to the case of two regulating half-windings with n coils, each one of them with an effective nominal voltage Ur. In this case, the maximum effective voltage of the solid-state switches (101, 103, 105, 107, 109) would be 2nUr and the total maximum power associated with the tap changer would be (3n²+2n)UrIn.
Therefore, by comparing the topology presented in the invention with that corresponding to the state of the art by means of tables 1 and 2, it is possible to affirm that, although the invention uses a greater number of solid-state switches, both the maximum voltage and the total maximum power associated with these switches are lower. As a consequence, the proposed invention exhibits advantages in relation to the state of the art as it reduces costs, dimensions and weight. Additionally, it is important to highlight that certain voltage levels can be obtained with different combinations of solid-state switches (61a, 62a, 63a, 61b, 62b, 63b). This redundancy provides the system with a certain degree of tolerance to failures.
In relation to the current sensor systems necessary for the monitoring and control of the device, in this assembly only one current sensor (90) is required, represented in figure 3. This fact exhibits advantages with respect to the state of the art, since as seen in figure 2, at least three current sensors (90, 91, 92) would be necessary to monitor the current that circulates through the device.
Additionally, the invention incorporates voltage sensors (80a, 80b) that measure the voltage of solid-state switches that occupy the same relative position in each one of the half-windings, for example, and in light of figure 3 in the solid-state switches (63a, 63b). In this sense, the voltage measurement could also be located by measuring the voltage of the solid-state switches (61a, 61b). The information provided by the sensors (90, 80a, 80b) is measured by a local control unit (70) which, in turn, sends it to the central controller (100). Said central controller (100) receives this information from the local controllers (70) associated with the different phases and determines the most suitable tap for each one of them as a function of the operating parameters and the operating setpoint. Once the taps for each one of the phases have been calculated by the central controller (100), this is sent to the respective local controllers (70) that determine the solid-state switches (61a, 62a, 63a, 61b, 62b, 63b) that must be operated by actuating the respective control signals (71a, 72a, 73a, 71b, 72b, 73b).
Communication between the local control unit (70) and the central controller (100) is preferably carried out by means of optical or wireless means (95) in order to guarantee the galvanic isolation of the tap changer. With the voltage information provided by the voltage sensors (80a, 80b) it is possible to:

Confirm the tap that is selected taking into account Table 1.
Determine the secondary voltage by means of an indirect measurement of the voltage of the regulation winding and the tap that is selected.

The object of the invention is complemented by a passive trigger device (66a and 66b) for each one of the half-windings. The purpose of said device is to close one of the solid-state switches (61a, 62a, 63a, 61b, 62b, 63b) for each one of the regulating half-windings in the event of a failure of the respective control signals (71a, 72a, 73a, 71b, 72b, 73b). One of the solid-state switches (61a, 62a, 63a, 61b, 62b, 63b) in operation and under normal operating conditions practically behaves like a short circuit (zero impedance), as seen in figure 4a, for which reason the voltage and current thereof are as shown in the left portion of figure 4c. However, if for some reason the switch does not close, the equivalent impedance thereof is that of the associated snubber, as shown in figure 5b, for which reason the circulation of a current will cause an overvoltage as shown in the right portion of figure 5b. For this reason, the passive trigger device (66a, 66b) acts when a certain instantaneous voltage value of the solid-state switches is reached, for which reason it is essential to take into account the effective voltage values under normal operating conditions shown in Table 1. The setting of the actuation of the device must be greater than the peak value corresponding to the maximum effective values shown in Table 2 plus a certain safety margin that takes into account the possible voltage variations of the grid.
The physical embodiment of the passive trigger device (66a, 66b) is based on the diagram shown in figure 5a. Said passive trigger device (66a, 66b) is made up of a trigger circuit (68b) that is powered from a non-linear impedance (67b) the value of which is variable depending on the voltage: infinite if the voltage is under a threshold value and zero, if otherwise. Thus, if the trigger signal (72b) coming from the control system (70) is correct, the voltage in the solid-state switch is zero and the passive trigger device has no power supply, for which reason the trigger signal (74b) is not generated. However, if for any circumstance the trigger signal (72b) is interrupted, the voltage in the solid-state switch would begin to rise due to the circulation of current through the snubber, as shown on the right portion of figure 4c. As soon as said voltage exceeds the threshold value of the non-linear impedance (67b), the trigger circuit is powered and, consequently, the trigger signal (74b) is generated which activates the solid-state switch. This operation is explained in detail in figure 5b.
It is necessary to have a passive trigger device for each set of solid-state switches associated with each regulating half-winding. The optimal location of the passive device is in that solid-state switch of the half-winding that is subjected to the lowest maximum effective voltage during normal operation. Thus, in figure 3 said passive trigger devices (66a, 66b) are installed on the specific solid-state switches (62a, 62b). The location on these specific solid-state switches (62a, 62b) achieves:

Detecting the overvoltage in the shortest time possible, since the voltage that must be reached in order for the passive method to actuate is lower compared to the location in others of the remaining solid-state switches (61a, 63a, 61b, 63b).
Improving the current waveform when the control signal of the solid-state switches fails, since the percentage of conduction thereof in the grid cycle is higher as the actuation voltage of the passive trigger method is reached in less time.
Preventing oversizing by voltage of the remaining solid-state switches (61a, 63a, 61b, 63b) because when the passive trigger acts, the voltage to which these solid-state switches (61a, 63a, 61b, 63b) are subjected is lower than the maximum which they support under normal operating conditions. The installation in any other solid-state switch would be associated with the oversizing of the switches with a lower maximum effective voltage, because these would be subjected to a higher voltage than during normal operation in the event of failure of the control signals.

Claims

A tap changer device for a transformer comprising:
• an electrically discontinuous regulation winding open at least at one point such that the regulation winding is divided into at least one electrically insulated upper half-winding and lower half-winding, respectively comprising n regulating coils each of them provided with electrically accessible electric terminals, and

• a main winding
the tap changer device being characterized in that it comprises, for each of the transformer phases:
• a set of first solid-state switches (61a, 62a, 63a) respectively connected to the accessible terminals of the upper half-winding (42, 43, 44) of the regulation winding, free terminals of said first solid-state switches (61a, 62a, 63a) being connected to a first common point (48a),

• a set of second solid-state switches (61b, 62b, 63b) respectively connected to the accessible terminals of the lower half-winding (45, 46, 47) of the regulation winding, free terminals of said first solid-state switches (61 b, 62b, 63b) being connected to a second common point (48b),

• an electrical connection of the common points (48a, 48b) with measurement of the current that circulates therethrough by means of the corresponding current sensor (90),

• a local control unit (70) for each one of the transformer phases that provides the trigger signals (71a, 72a, 73a, 71 b, 72b, 73b) for the solid-state switches (61a, 62a, 63a, 61b, 62b, 63b), and

• a single central control unit (100) that supervises and controls the set of local control units (70) associated with the phases, which determines for each one of them the most suitable tap as a function of operating parameters and the operating setpoint,
wherein the number of solid-state switches (61a, 62a, 63a, 61b, 62b, 63b) is equal to the number of electrical terminals of the regulation winding which, in turn, is equal to twice the sum of the regulation coils plus one 2(n+1).
The tap changer device for a transformer according to claim 1, characterized in that it additionally comprises at least two voltage sensors (80a, 80b) located on corresponding solid-state switches which occupy the same relative position in each one of the regulating half-windings.
The tap changer device for a transformer according to claim 1 or 2, characterized in that it additionally comprises at least one current sensor (90) located between the common points (48a, 48b).
The tap changer device for a transformer according to any one of the preceding claims, characterized in that it additionally comprises passive trigger devices (66a, 66b) that actuate respectively in the event of failure of the control signals (71a, 72a, 73a) which operate the solid-state switches (61a, 62a, 63a) and (71b, 72b, 73b) that operate the solid-state switches (61b, 62b, 63b).
The tap changer device for a transformer according to claim 4, characterized in that the solid-state switch wherein the passive trigger device is installed is that which exhibits a lower maximum effective voltage during normal operation.
The tap changer device for a transformer according to any one of claims 2 to 5, characterized in that it additionally comprises a control unit (70) configured to collect the information captured by the current sensor (90) and the voltage sensors (80a, 80b).