ES2734601B2 - STATIC CHANGING DEVICE FOR CHARGING INPUTS FOR TRANSFORMERS WITH DISCONTINUOUS REGULATION WINDINGS - Google Patents

Description

D E S C R I P T I O N

STATIC ON-LOAD SOCKET CHANGER DEVICE FOR

TRANSFORMERS WITH DISCONTINUOUS REGULATION WINDINGS

OBJECT OF THE INVENTION

The object of the invention is framed in the technical field of electricity, more specifically the proposed invention is applied in the area of alternating current power electrical systems and, particularly, in the activities of generation, transport and distribution of electrical energy .

The proposed invention is directed to transformers, specifically to a specific topology of an on-load tap-changer based on static switches for applications that require dynamic voltage regulation, such as:

• Transformers of electricity generation, transmission and distribution networks.

• Photovoltaic inverters connected to the electricity grid.

• Inverters associated with energy storage devices connected to the electricity grid.

• Wind turbines.

BACKGROUND OF THE INVENTION

Undoubtedly, the transformer is one of the key pieces of current electrical systems as it allows adapting the voltage levels between the different parts of the electrical system in an efficient and safe way. Most of the transformers in the electrical network are equipped with tap changers which, by modifying the number of turns of the primary winding, allow 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: vacuum and load. The former can only be operated with the transformer disconnected from the grid. However, the seconds can be operated when the transformer is operating, allowing 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 on-load tap-changer topology based on static switches for applications that require dynamic voltage regulation, among which the following stand out:

• Transformers of electricity generation, transmission and distribution networks.

• Photovoltaic inverters connected to the electricity grid.

• Inverters associated with energy storage devices connected to the electricity grid.

• Wind turbines.

The current operation of the electrical power systems requires that the voltages of the nodes of the network remain within the regulatory limits regardless of the state of charge. To this end, there are different regulation mechanisms, one of them being the tap-changer transformer. Basically, a transformer has several electrically insulated windings that are magnetically coupled in such a way that the relationship between the voltages of the windings, known as the transformation ratio, is a function of their turns ratio. The purpose of the tap changer is to maintain the voltage of one of the windings regardless of the voltage of the other and the state of charge of the transformer. To do this, the winding in which the changer is installed is divided into two: main winding and regulation winding. The first contains the bulk of the winding turns while the second 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 way, 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 of carrying it out: under load and under no load. In the first case, the change in the number of turns is carried out with the transformer in service, for which an electromechanical switch is normally used, characterized by being a bulky, complex and expensive device. Due to these characteristics, on-load tap-changers are used in applications where 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 of turns is carried out with the transformer out of service, that is, no charge. Technologically, the vacuum 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 distribution networks makes it impossible to make investments in on-load changers, opting for the option of a vacuum changer. These make it possible to select the most suitable tap based on the average primary voltage, with the aim of keeping the LV voltage within regulatory limits. However, this technology does not allow to dynamically control the LV voltage 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 networks is more complex. On the one hand, voltage variations in the grid increase because the primary source of energy from 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 are increasingly rapid. As a consequence, the electrical power system needs a greater number of resources to control voltages whose action is very fast. In this sense, current tap-changer technologies, whether in vacuum 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 whose actuation times are very short. However, the use of tap-changers based on power electronics is not new, there are previous patents in this regard:

• US 5969511. PGJM Asselman et al. 10/19/1999. Method and device for continuous adjustment and regulation of transformer turns ratio and transformer provided with such device.

• ES 2274684. D. Monroy. 5/16/2007. Tap changer for medium / low voltage transformers.

• US 3195038 (A). J. Fry. 7/13/1965. Voltage or current regulator apparatus.

• US 3700925 (A). P. Wood. 10/24/1972. Thyristor tap changer for electrical inductive apparatus.

• US 4220411. J. Rosa. 9/2/1980. Thyristor tap changer for electrical inductive apparatus.

DESCRIPTION OF THE INVENTION

The object of the present invention is directed to a load tap-changer based on power electronics with a symmetrical arrangement of static switches, sensors for monitoring and control, as well as a passive tripping device that ensures its operation in the event of power failures. control signals; being applicable to transformers that have at least one regulation winding, in addition to the main windings, electrically discontinuous, that is, open at least at one point. In this way, the regulating winding is divided into at least two electrically insulated half-windings formed by a set of coils whose terminals are electrically accessible.

To each of the terminals of the regulation half-windings, which are electrically accessible, a terminal of a static switch based on a power electronics device is connected. There are therefore the same number of static 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 static switches are connected to each other forming at least two common points, one for each of the electrically isolated half-windings. These common points are joined together through an electrical connection.

The set is completed with a series of voltage and intensity sensors, required for the monitoring and control of the device, arranged in specific locations. Thus, the intensity measurement is carried out at the electrical connection between the aforementioned common points. Similarly, a voltage sensor is located on 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 to estimate the secondary voltage of the transformer.

The static switch used in the invention can be formed by two antiseries IGBTs with their respective antiparallel diodes, or an IGBT with four diodes or two antiparallel thyristors. If necessary, a snubber can be included to reduce overvoltages at the switching times of the static switches.

Said static switches are controlled by means of the corresponding trip signals calculated by 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 polyphase 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 appropriate trigger signals to the static switches depending on the setpoint of the central controller. Additionally, it measures the operating variables of the changer (voltage, current, temperature and / or any other parameter) required by the centralized controller.

• Centralized controller in charge of determining which is the appropriate regulation tap in each of the phases based on the operating setpoint selected by the user and the different operating variables sent by each of the local controllers. The central controller, having all the information on the phases, can determine the most appropriate operating instructions at all times to optimize the overall operation of the application.

Communication between local controllers and centralized control is done through optical or wireless means to provide adequate galvanic isolation. In the case of single-phase applications, local and centralized controllers are embedded within a single control device.

Finally, and to prevent the circuit from being open in the event of a failure of the control signals that act on the static switches, a passive tripping device is incorporated in one of the static switches associated with each half-winding in such a way that it is guaranteed that, at all times, even in the event of failure of the control signals, the circuit remains closed.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a preferred example of a practical embodiment thereof, a set of drawings is attached as an integral part of said description. where, for illustrative and non-limiting purposes, the following has been represented:

Figures 1a and 1b.- Shows a view of respective transformers provided with electrically discontinuous regulation windings, that is to say with at least one opening point; where in figure 1a a single-phase transformer or a phase of a three-phase star connection is represented, while in figure 1b a phase of a transformer with the three-phase connection in delta is represented.

Figure 2.- Shows a topology of a MV / LV tap changer applying the state of the art.

Figure 3.- Shows a diagram where the proposed topology of a MV / LV tap changer is represented.

Figures 4a-4c.- They show respective diagrams that represent (4a) Equivalent diagram of the closed static switch (correct control signals). (4b) Equivalent diagram of the open static switch (failure in control signals). (4c) Voltage and current of the static switch in operation with correct control signals and with failure in control signals.

Figures 5a-5b.- Show: (5a) A component diagram of the passive tripping device. (5b) A graph showing the operation of the passive tripping device.

PREFERRED EMBODIMENT OF THE INVENTION

The invention is applicable to transformers equipped with electrically discontinuous regulating windings, that is to say with at least one opening point such as those shown in figure 1. Figure 1a represents a single-phase transformer or a phase of a three-phase star connection while in figure 1b a phase of a transformer with the three-phase delta connection is represented. 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 in such a way that it is divided into an upper half winding formed by the coils (32, 34 ) and lower half-winding formed by the coils (36, 38). Each of the regulating half-windings is formed by 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 isolated from each other since the terminals 44 and 45 are not electrically connected.

Figure 3 shows the embodiment of a changer based on static switches object of this invention on the transformer shown in figure 1b. 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 static switches (61a, 62a, 63a ) are connected to the accessible terminals of the upper half winding (42, 43, 44) of the regulating winding. The free terminals of said first set of static switches (61a, 62a, 63a) are in turn connected at a first common point (48a). Similarly, a second set of static switches (61b, 62b, 63b) are connected to the accessible terminals of the lower half-winding (45, 46, 47) of the regulation winding, and their free terminals are connected to a second common point ( 48b). The common points (48a, 48b) are electrically connected to each other and the current flowing in said electrical connection is measured through the corresponding intensity sensor (90). The number of static switches (61a, 62a, 63a, 61b, 62b, 63b) in this preferred embodiment is equal to 2 (n + 1) = 6.

Proper operation of the static switches allows selection between five primary voltage levels: static switches (61a, 63b) closed Up-2Ur; static switches (62a, 63b) closed Up-Ur; static switches (62a, 62b) closed Up; static switches (63a, 62b) closed Up + Ur; the static switches (63a, 61b) closed Up + 2Ur. Table 1 shows the effective voltages to which the different static switches are subjected depending on the selected tap. By way of example, if the tap corresponding to the voltage level Up-Ur is selected, the static 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 regulation coils are identical to each other. In this way, 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 static switches are in the range 2Ur (switches (61a, 63a, 61b, 63b)) and Ur (switches (62a, 62b)). On the other hand, and taking into account that the maximum current without overload that can flow through the static switches is the nominal current of the winding, In, the maximum power associated with the changer, calculated as the sum of the power of each of the switches, is for this specific case: 10UrIn.

Table 1. Static exchanger object of the invention: Effective voltages depending on the selected tap.

This result can be extrapolated to the case of a transformer with a regulation winding with an opening point so that it is divided into two half-windings of n regulation coils, each with an effective nominal voltage Ur. In this case, the maximum voltage effective of the static switches would be nUr and the total maximum power associated with the changer would be (0.75n2 + n) UrIn in case of even n and (0.75n2 + n + 0.25) UrIn for odd n.

The proposed invention provides advantages compared to the state of the art of the tap changer presented in Figure 2. Again, it is possible to select five possible levels of primary voltage depending on the static 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 static switches (101, 103, 105, 107, 109) in this configuration is equal to the number of regulation coils plus one (2n + 1, five in the configuration shown in Figure 2). Table 2 shows the effective voltages to which the different static switches are subjected depending on the selected tap. By way of example, if the tap corresponding to the Up-Ur voltage level is selected, the static 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 regulation coils are identical to each other. In this way, it is possible to determine the maximum effective voltage to which each switch could be exposed in normal operation, which is decisive for the purpose of sizing it. Note that the maximum voltages of the static switches (101, 103, 105, 107, 109) are in the range 4Ur (switches (101) and (109)) and 2Ur (switch (105)). On the other hand, and taking into account that the maximum current without overload that can flow through the static switches (101, 103, 105, 107, 109) is the nominal current of the winding, In, the maximum power associated with the changer, calculated as sum of the power of each of the static switches (61a, 62a, 63a, 61b, 62b, 63b), is for this specific case: 16UrIn.

Table 2. Static exchanger according to the state of the art: Effective voltages depending on the selected tap.

This result can be extrapolated to the case of two regulation half-curves with n coils, each with an effective nominal voltage Ur. In this case, the maximum effective voltage of the static switches (101, 103, 105, 107, 109) would be 2nUr and the total maximum power associated with the exchanger (3n2 + 2n) UrIn.

Therefore, by comparing the topology presented in the invention with that corresponding to the state of the art through Tables 1 and 2, it is possible to affirm that, although the invention uses a greater number of static switches, both the maximum voltage as the maximum total power associated with these switches are lower. As a consequence, the proposed invention presents advantages in relation to the state of the art since it reduces cost, dimensions and weight. Additionally, it is important to highlight that certain voltage levels can be obtained with different combinations of static switches (61a, 62a, 63a, 61b, 62b, 63b). This redundancy gives the system a degree of fault tolerance.

In relation to the intensity sensorisation necessary for the monitoring and control of the device, in this assembly only one intensity sensor (90) is necessary represented in Figure 3. This fact presents advantages with respect to the state of the art, since as can be seen in Figure 2, at least three intensity sensors (90, 91, 92) would be necessary to monitor the intensity that circulates through the device.

Additionally, the invention incorporates voltage sensors (80a, 80b) that measure the voltage of static switches that occupy the same relative position in each of the half-windings, for example, in view of Figure 3 in the static switches (63a, 63b ). In this sense, the voltage measurement could also be located by measuring the voltage of the static 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 based on the operating parameters and the operating instructions. Once the taps for each of the phases have been calculated by the central controller (100), it is sent to the respective local controllers (70) that determine the static 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 optical or wireless means (95) 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 socket that is selected taking into account Table 1.

• Determine the secondary voltage through an indirect measurement of the regulating winding voltage and the tap that is selected.

The object of the invention is complemented with a passive trip device (66a and 66b) for each of the half-windings. The purpose of said device is to close one of the static switches (61a, 62a, 63a, 61b, 62b, 63b) for each of the regulation half windings in the event of a failure of the respective control signals (71a, 72a , 73a, 71b, 72b, 73b). One of the static switches (61a, 62a, 63a, 61b, 62b, 63b) in operation and under normal operating conditions behaves practically like a short circuit (zero impedance), as can be seen in Figure 4a, therefore that its voltage and intensity are as shown in the left part of Figure 4c. However, if for some reason the switch does not close, its equivalent impedance is that of the associated snubber , as shown in Figure 5b, so the circulation of a current will cause a surge as shown in the right part of Figure 5b. For this reason, the passive tripping device (66a, 66b) acts when a certain instantaneous voltage value of the static switches is reached, for which it is essential to take into account the effective voltage values in normal operating conditions shown in the Table 1. The setting of the device's performance must be higher than the peak value corresponding to the maximum effective values shown in Table 2 plus a certain safety margin that takes into account possible variations in the network voltage.

The physical embodiment of the passive firing device (66a, 66b) is based on the scheme shown in Figure 5a. Said passive trip device (66a, 66b) is formed by a trip circuit (68b) that is fed from a non-linear impedance (67b) whose value is variable depending on the voltage: infinite in case the voltage is lower to a threshold value and zero otherwise. Thus, if the trip signal (72b) from the control system (70) is correct, the voltage in the static switch is zero and the passive trip device does not have power, so the trip signal (74b) does not is generated. However, if for any circumstance the trip signal (72b) is interrupted, the voltage in the static switch would start to increase due to the current flow through the snubber, as shown in the right part of Figure 4c. As soon as said voltage exceeds the threshold value of the non-linear impedance (67b), the trip circuit is supplied and, consequently, the trip signal (74b) is generated that activates the static switch. This operation is explained in detail in Figure 5b.

It is necessary to have a passive tripping device for each set of static switches associated with each regulating half winding. The optimal location of the passive device is in that static switch of the half winding that is subjected to the lowest maximum effective voltage in normal operation. Thus, in Figure 3, said passive tripping devices (66a, 66b) are installed on specific static switches (62a, 62b). The location on these specific static switches (62a, 62b) achieves:

• Detect the overvoltage in the shortest time possible, since the voltage that must be reached for the passive method to act is lower compared to the location in other of the remaining static switches (61a, 63a, 61b, 63b).

• Improve the waveform of the intensity when the control signal of the static switches fails, since the percentage of conduction of the same in the cycle power supply is higher as the actuation voltage of the passive tripping method is reached in less time.

Avoid oversizing by voltage of the remaining static switches (61a, 63a, 61b, 63b) because when the passive trip acts, the voltage to which these static switches (61a, 63a, 61b, 63b) are subjected is lower than the maximum they withstand under normal operating conditions. The installation in any other static switch would have associated the oversizing of the switches with a lower maximum effective voltage, because these would be subjected to a higher voltage than in normal operation in the event of failure of the control signals.

Claims (6)

1. Tap changer device for a transformer comprising: • an electrically discontinuous regulating winding open at at least one point in such a way that the regulating winding is divided into at least one electrically insulated upper half-winding and lower half-winding respectively comprising n regulating coils, each one equipped with electrically accessible electrical 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 static switches (61a, 62a, 63a) respectively connected to the accessible terminals of the upper half winding (42, 43, 44) of the regulation winding, some terminals being free of said first static switches (61a, 62a, 63a ) connected to a first common point (48a), • a set of second static switches (61b, 62b, 63b) respectively connected to the accessible terminals of the lower half winding (45, 46, 47) of the regulation winding, some terminals being free of said first static switches (61b, 62b, 63b ) connected to a second common point (48b), • an electrical connection of the common points (48a, 48b) with measurement of the intensity that flows through it through the corresponding intensity sensor (90), • a local control unit (70) for each of the phases of the transformer that provides the trip signals (71a, 72a, 73a, 71b, 72b, 73b) for the static 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 of them the most appropriate tap based on operating parameters and operating setpoint . where the number of static 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). 2. 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 respective static switches that occupy the same relative position in each of the regulation half-windings. 3. Tap changer device for a transformer according to claim 1 or 2, characterized in that it additionally comprises at least one intensity sensor (90) located between the common points (48a, 48b). Tap changer device for a transformer according to any one of the preceding claims, characterized in that it additionally comprises passive tripping devices (66a, 66b) that act respectively in the event of failure of the control signals (71a, 72a, 73a) which operate the static switches (61a, 62a, 63a) and (71b, 72b, 73b) which operate the static switches (61b, 62b, 63b). 5. Tap changer device for a transformer according to claim 4, characterized in that the static switch in which the passive tripping device is installed is the one that has a lower maximum effective voltage in normal operation. 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).