WO2022144165A1

WO2022144165A1 - Determination of residual flux in a power transformer

Info

Publication number: WO2022144165A1
Application number: PCT/EP2021/085440
Authority: WO
Inventors: Michael Stanek; Urmil PARIKH; Mehulbhai SONAGRA
Original assignee: Hitachi Energy Switzerland Ag
Priority date: 2020-12-29
Filing date: 2021-12-13
Publication date: 2022-07-07

Abstract

Determination of residual flux and the validity of the residual flux determined is described. A parameter associated with a voltage of a transformer is monitored. Based on a comparison of the parameter associated with the voltage of the transformer against a preset threshold value integration of the voltage of the transformer is started. Further, the voltage of the transformer is integrated to compute a residual flux.

Description

DETERMINATION OF RESIDUAL FLUX IN A POWER TRANSFORMER

TECHNICAL FIELD

[0001] The present subject matter relates, in general, to determination of residual flux. In particular, the present subject matter relates to determination of residual flux in a power transformer.

BACKGROUND

[0002] Transformers are commonly used in power systems for voltage conversion. During operation of power systems, transformers may be switched on and off, referred to as energization and de-energization, depending on various factors, such as load variation, fault detection, etc. During energization, a transformer may initially draw a large inrush current leading to a voltage drop in the power system. The inrush current generally depends on the residual flux in the transformer core. In order to minimize inrush currents and voltage drops during energization, a transformer is to be energized at those phase angles where residual flux matches prospective flux in each phase. This is also referred to as controlled switching. Generally, the prospective flux is obtained as an integral of the source voltage, while the residual flux is obtained as an integral of the transformer voltage measured during de-energization. These tasks are often performed by a controlled switching device (CSD), which may use the information on prospective fluxes and residual fluxes for optimal energization of a power transformer.

SUMMARY

[0003] Embodiments of the present invention provide a method for determining residual flux in a power transformer, a device for determining residual flux in a power transformer, and a computer readable storage medium for determining residual flux in a power transformer. Objectives of embodiments of the invention include accurate computation of residual flux and determining the validity of the residual flux computed. [0004] According to a first aspect, a method for determining a residual flux for a power transformer is provided. The method comprises monitoring a parameter associated with a voltage of a transformer. Based on a comparison of the parameter associated with the voltage of the transformer against a preset threshold value, the integration of the voltage of the transformer is started. The voltage of the transformer is integrated to compute a residual flux. [0005] According to a second aspect, a device comprising a processor to execute the method for computing a residual flux in a power transformer is provided.

[0006] According to a third aspect a non-transitory computer readable medium containing program instruction that, when executed, causes the processor to perform the method for computing a residual flux in a power transformer, is provided.

[0007] According to one implementation, the preset threshold value is at least one of a first threshold value corresponding to an ON-voltage threshold and a second threshold value corresponding to an OFF-voltage threshold.

[0008] According to an implementation, the integration of the voltage of the transformer is started on detecting a drop in the parameter associated with the voltage of the transformer to a value below the first threshold value.

[0009] According to an implementation, samples of the voltage are buffered during monitoring and at least one cycle of the voltage prior to the detection of the drop in the parameter associated with the voltage of the transformer is included in the integration to compute the residual flux.

[0010] According to an implementation, a trip signal is received and integration of the voltage of the transformer is prevented when the parameter associated with the voltage of the transformer is below the second threshold value.

[0011] According to an implementation, a trip signal is received and the integration of the voltage of the transformer is started on receipt of the trip signal. Further, the computed residual flux is discarded when the parameter associated with the voltage of the transformer is greater than the first threshold value at a time instant corresponding to an end of the integration.

[0012] According to an implementation, the residual flux is computed from the integration of the voltage of the transformer by averaging at least one power cycle of integration values.

[0013] According to an implementation, the integration is performed till a predefined criterion is satisfied.

[0014] According to an implementation, a signal indicating that the computed residual flux is invalid is generated when a magnitude of the residual flux is greater that an amplitude of an integrated transformer voltage in steady-state condition.

[0015] According to another implementation, a signal indicating that the computed residual flux is unreliable is generated when the parameter associated with the voltage of the transformer is greater than a second threshold value at a time instant corresponding to an end of the integration.

[0016] According to an implementation, the parameter associated with the voltage of the transformer is monitored in a plurality of phases and the comparison of the parameter associated with the voltage of the transformer and the determination of the validity of the residual flux are performed in each of the plurality of phases.

[0017] According to an implementation, the parameter associated with the voltage of the transformer is any one of a root mean square (RMS), an amplitude, a rate of change, an integral over a pre-determined time, or a quantity derived from at least one of these.

BRIEF DESCRIPTION OF DRAWINGS

[0018] The features, aspects, and advantages of the present subject matter will be better understood with regard to the following description and accompanying figures. The use of the same reference number in different figures indicates similar or identical features and components.

[0019] Fig. 1 illustrates signals and waveforms for various electrical parameters obtained during transformer de-energization for determining the residual flux based on a method known in the art.

[0020] Fig. 2 illustrates a block diagram of a two-source equivalent electrical network including a device for residual flux determination, in accordance with an embodiment of the present subject matter.

[0021] Fig. 3(a) illustrates a block diagram of a first example configuration of the two-source equivalent electrical network including the device for residual flux determination, in accordance with an embodiment of the present subject matter.

[0022] Fig. 3(b) illustrates signals and waveforms for various electrical parameters obtained during transformer de-energization in the first example configuration, in accordance with an embodiment of the present subject matter.

[0023] Fig. 4(a) illustrates a block diagram of a second example configuration of the two-source equivalent electrical network including the device for residual flux determination, in accordance with an embodiment of the present subject matter.

[0024] Fig. 4(b) illustrates signals and waveforms for various electrical parameters obtained during monitoring of a transformer in the second example configuration, in accordance with an embodiment of the present subject matter. [0025] Fig. 5 illustrates a third example of signals and waveforms for various electrical parameters obtained during transformer de-energization, in accordance with an embodiment of the present subject matter.

[0026] Fig 6. illustrates a fourth example of signals and waveforms for various electrical parameters obtained during transformer de-energization, in accordance with an embodiment of the present subject matter.

[0027] Fig. 7 illustrates a method to compute residual flux, in accordance with an embodiment of the present subject matter.

DETAILED DESCRIPTION

[0028] The present subject matter relates to determining residual flux in a power transformer. The following describes computation of residual flux and determining the validity of the residual flux for power transformers.

[0029] Conventionally, a controlled switching device receives a circuit breaker opening command or a trip command when a circuit breaker is opened to de-energize an associated transformer. On receiving the trip command, the controlled switching device starts integrating the load voltage that is acquired from a voltage measuring device associated with the transformer. The integration is performed for a predetermined period and the final integrated values in each phase are adopted as residual fluxes. The computed residual flux may be used for performing subsequent controlled switching actions, thereby avoiding degradation in power quality in the power system. [0030] Fig. 1 illustrates signals and waveforms for various electrical parameters obtained during transformer de-energization for determining the residual flux based on a method known in the art. Waveforms 102 depict transformer voltages monitored in three phases. Signal 104 depicts a trip command signal issued by a protection or control unit at time tl. On receipt of the trip signal at time tl, the transformer voltage is integrated in each of the three phases till a time t2 to obtain the residual flux in the three phases. The time period tl to t2, over which the integration is performed, is referred to as integration time window and typically corresponds to a predetermined amount of time. Waveforms 106 depict integrated transformer voltages (computed flux), where the flux values denoted by arrows 108, 110, and 112 indicate the computed residual flux obtained in the three phases by integration of the transformer voltage over the integration time window tl to t2. [0031] However, such techniques of residual flux computation may not provide accurate residual flux values in various scenarios. For example, in case where an upstream circuit breaker connected to the system trips, the transformer may be deenergized even before the circuit breaker associated with the controlled switching device is issued a trip command for the controlled switching device to start integrating transformer voltages, and hence the residual flux may not be accurately computed if the circuit breaker subsequently is issued a trip command. In another scenario, when the secondary side of the transformer is connected to the grid, the transformer may not start de-energising even if the circuit breaker associated with the controlled switching device may have tripped, resulting in erroneous computation of the residual flux. In other scenarios, the computed residual flux may be erroneous, for example, due to incorrect measurement of load voltages, oscillation of limb voltages, etc.

[0032] The present subject matter provides for accurate determination of residual fluxes of a transformer. In one example, a parameter associated with a voltage of a transformer is monitored and compared against a preset threshold value. The preset threshold value may be at least one of a first threshold value corresponding to an ON- voltage threshold and a second threshold value corresponding to an OFF-voltage threshold. The integration of the voltage of the transformer is started based on the comparison and the residual flux is computed from the integration. In another example, while the integration may be started based on receipt of a trip signal, it may be stopped or prevented if the monitored parameter is less than the OFF- voltage threshold value. In one example, once the integration is started, it may be performed till a predefined criterion is met. Hence, the integration window may change based on the predefined criteria. Further, the validity of the computed residual flux may be determined, for example, based on a magnitude of the computed residual flux or the value of the monitored parameter at the end of the integration window. Accordingly, a signal indicating whether the computed residual flux is valid may also be generated, based on which the residual flux value may be used or discarded.

[0033] As the monitored parameter is compared with at least one of an ON- voltage threshold and OFF-voltage threshold to initiate integration or prevent integration, it may be ensured that the residual flux computation is performed when the transformer is actually being de-energized. Further, the signal indicating the validity of the residual flux generated helps users or computing processes decide whether the computed residual flux may be reliably used subsequently. The present subject matter thus ensures high accuracy in computing the residual flux values in various scenarios and also provides for generation of validity signals to increase the reliability of the residual flux values.

[0034] The above and other features, aspects, and advantages of the subject matter will be better explained with regard to the following description and accompanying figures. Wherever possible, the same reference numbers are used in the drawings and the following description to refer to the same or similar parts. While several examples are described, modifications, adaptations, and other implementations are possible.

[0035] Fig. 2 illustrates a block diagram of a two-source equivalent electrical network including a device for residual flux determination, in accordance with an embodiment of the present subject matter. A two-source equivalent electrical network 200 comprises two terminals Bus M 201 and Bus N 202. The two electrical sources 203 and 204 supply power to Bus M 201 and Bus N 202 respectively. In one example, the sources 203 and 204 may be power generators, such as synchronous power generators or inverter-based resources. The electrical network 200 may transmit electric power at high voltages, such as in the range of kilovolts, and for long distances, such as for tens or hundreds of kilometres.

[0036] It will be understood that that the network 200 may include a plurality of additional components or devices for monitoring, sensing, and controlling various parameters that may be associated with the transmission lines but are not shown for brevity. For example, components such as circuit breakers, sensors, current transformers, voltage transformers, loads connected to the transmission lines, shunt reactors, intelligent electronic devices lEDs, protective relays, and the like may be connected to the transmission line. In one example, a power transformer 250, alternatively referred to as transformer 250, for which the residual flux may be computed may be connected to the network 200.

[0037] A device 208 may receive voltage measurements associated with the transformer 250. In another example, a protection and control unit 207 may be configured to provide a trip signal to a circuit breaker 252. In one example, the trip signal may be directly applied to start the integration of the transformer or can be derived by any other means to derive the status of the circuit breaker. In one example, the protection and control unit 207 may provide the trip signal to the device 208 to open the circuit breaker 252. In one example, the device 208 may be an intelligent electronic device (TED). In other examples, the device 208 may be any computing device, such as a server, a desktop device, a laptop, etc., which may receive the measurements from an ZED.

[0038] In an example, the present subject matter may be implemented by one or more modules. The modules may be implemented as instructions executable by one or more processors. For instance, in the example where the device 208 performs the method, the modules are executed by the processors of the device 208. In case the method is implemented in part by the device 208 and in part by a server, the modules (depending on the step) will be distributed accordingly in the device 208 and the server. [0039] In one example, the device 208 may be configured to receive input measurement signals from various measurement equipment connected to the network 200, such as current transformers, potential transformers, Rogowski coils, circuit breakers or other measurement sensors. In one example, the device 208 may receive voltage measurements of the power transformer 250 from the voltage measuring device 254. The device 208 may process the measurements obtained with the help of a processor 220. The processor 220 may be implemented as a dedicated processor, a shared processor, or a plurality of individual processors, some of which may be shared. The device 208 may comprise a memory 226, that may be communicatively connected to the processor 220. Among other capabilities, the processor 220 may fetch and execute computer-readable instructions, stored in the memory 226. In one example, the memory 226 may store a residual flux determination module 222. In other examples, the residual flux determination module 222 may be external to the memory 226. The memory 226 may include any non-transitory computer-readable medium including, for example, volatile memory, such as RAM, or non-volatile memory, such as EPROM, flash memory, and the like.

[0040] In one example, a method to compute the residual flux value of the power transformer 250, such as a three-phase power transformer, may be performed by the processor 220 by execution of the residual flux determination module 222. The residual flux determination module 222 may monitor a parameter associated with a voltage of the transformer 250. In one example, the parameter associated with the voltage of the transformer is any one of a root mean square (RMS), an amplitude, a rate of change, an integral over a pre-determined time, or a quantity derived from at least one of these. In one example, the voltage of the transformer may be the limb voltages of the transformer 250. [0041] Based on a comparison of the parameter associated with the voltage of the transformer 250 against a preset threshold value, the integration of the voltage of the transformer 250 may be started. The voltage may be integrated by the residual flux determination module 222 to compute the residual flux of the transformer 250. In one example, the preset threshold value may be at least one of a first threshold value corresponding to an ON-voltage threshold and a second threshold value corresponding to an OFF -voltage threshold. For example, if the parameter associated with the voltage of the transformer 250 is less than the ON-voltage threshold, it may indicate that the transformer 250 is being de-energized and hence the integration may be started irrespective of whether a circuit breaker is issued a trip signal or not. In another example, if the parameter associated with the voltage of the transformer 250 is less than the OFF-voltage threshold, it may indicate that the transformer 250 is already deenergized and hence the integration may not be started, or may be prevented, even if a circuit breaker receives a trip signal. Thus, the present subject matter facilitates the accurate computation of residual flux. Various example scenarios where the teachings of the present subject matter may be applied are explained later with reference to Figs. 3-6.

[0042] To compute the residual flux, the voltage of the transformer 250 may be integrated till a predefined criterion is satisfied. In one example, the voltage of the transformer 250 may be integrated till a predefined criterion is satisfied, for example, till a pre-determined amount of time has elapsed. In one example, the voltage of the transformer may be integrated for a predetermined time according to the predefined criteria. In another example, the voltage of the transformer may be integrated for a time period less than the predetermined time based on a comparison of a parameter of the transformer voltage or of the integrated transformer voltage against some other preset threshold value dependent on the parameter associated with voltage.

[0043] In one example, the residual flux may be computed from the integration of the voltage of the transformer by averaging at least one power cycle of integration values at the end of the pre-determined time for increased accuracy.

[0044] The residual flux determination module 222 may also be configured to determine the validity of the residual flux value that is computed. The validity of the residual flux may indicate if the residual flux value is valid and reliable and may be used for further computations or decision making, or if the residual flux values are not reliable and hence may be discarded. [0045] Further, the device 208 may comprise an output interface 224 to communicate the results obtained from the residual flux determination module 222, for example, to a server. The output interface 224 may include a variety of computer- readable instructions-based interfaces and hardware interfaces that allow interaction with other communication, storage, and computing devices, such as network entities, web servers, databases, and external repositories, and peripheral devices. In one example, the residual flux values, the validity signal, voltage and current measurements, and the like may be viewed on a display connected to the output interface 224 or integrated with the device 208.

[0046] The present subject matter hence provides for accurate computation of residual flux and also provides for determining the validity of the residual flux value, ensuring high reliability.

[0047] Fig. 3(a) illustrates a block diagram of a first example configuration of the two-source equivalent electrical network including the device for residual flux determination, in accordance with an embodiment of the present subject matter. The first example configuration 300 may be a radial power system where an upstream circuit breaker 302 is connected to the network 300. Operation of the upstream circuit breaker 302 may cause the transformer 250 to de-energize, even when the circuit breaker 252 associated with the device 208 is not tripped.

[0048] In one example, the device 208 of the present subject matter may be configured to monitor a parameter associated with the transformer voltage and compare it against a first threshold value that corresponds to an ON-voltage threshold. In one example, the first threshold value may be 80% of the nominal value of the parameter. For example, if the parameter is RMS voltage at each phase, the first threshold value may be 80% of the nominal RMS voltage of the transformer. In one example, the device 208 may be configured to start integrating the voltage of the transformer 250 on detecting a drop in the parameter associated with the voltage of the transformer 250 to a value below the first threshold value as shown in Fig. 3(b).

[0049] Fig. 3(b) illustrates signals and waveforms for various electrical parameters obtained during transformer de-energization in the first example configuration, in accordance with an embodiment of the present subject matter. Waveforms 306 depict transformer voltages of three phases of the transformer 250. Signal 308 depicts a signal issued by a protection and control unit 207 for tripping a circuit breaker 252 associated with the device 208. As shown in Fig. 3(b), the transformer de-energization may happen even without a trip signal to the circuit breaker 252, for example, due to the tripping of the upstream circuit breaker 302. Waveforms 310 depict the RMS voltages of the three phases of the transformer 250. Waveforms 312 depict the integrated transformer voltages of the transformer 250.

[0050] In operation, the device 208 monitors the RMS voltages 310 at the three phases of the transformer 250 and compares them against the first threshold value 314. When the RMS voltage 310 of the three phases of the transformer drops below the first threshold value, indicated by a point 316 on the waveform, the residual flux determination module 222 may start integrating voltage of the transformer at time tl. In another example, the residual flux determination module 222 may start integrating the voltage of the transformer at the point where at least one of the three RMS voltages 310 of the transformer drops below the first threshold value. The transformer voltage is integrated in each of the three phases till time t2, when a predefined criterion is satisfied. In one example, the voltage of the transformer 250 may be integrated till a predefined criterion is satisfied, for example, till a pre-determined amount of time has elapsed. In one example, the voltage of the transformer may be integrated for a predetermined time according to the predefined criteria. In another example, the voltage of the transformer may be integrated for a time period less than the predetermined time based on a comparison of a parameter of the transformer voltage or of the integrated transformer voltage against some other preset threshold value dependent on the parameter associated with voltage.

[0051] In one example, in order to ensure correct estimation of the residual flux values 320, 322, and 324, at least one cycle of transformer voltage prior to the voltage drop detection indicated by point 316 may be included in the integration. In one example, voltage samples from a time tO to tl may be collected. The time period from tO to tl, over which voltage samples are collected, is referred to as a pre-trigger buffer size. This may be achieved by continuously buffering a window (tO to tl) of the most recent voltage samples. In one example, the voltage of the transformer 250 may be integrated till a predefined criterion is satisfied, for example, till a pre-determined amount of time has elapsed. The time period tO to t2, over which the integration is performed, is referred to as integration time window. Residual flux values 320, 322 and 324 are thus obtained for each phase of the transformer 250. Thus, the residual flux may be accurately computed irrespective of whether the circuit breaker 252 receives a trip signal. [0052] Fig. 4(a) illustrates a block diagram of a second example configuration of the two-source equivalent electrical network including the device for residual flux determination, in accordance with an embodiment of the present subject matter. The second example configuration 400 may be a meshed power system where the transformer 250 is connected to the grid 402 on the secondary side. Hence, the transformer 250 may not be de-energized even when the circuit breaker 252 associated with the device 208 is tripped. In one example, the device 208 may be configured to start integrating the voltage of the transformer 250 when the circuit breaker 252 receives a trip command signal from the protection and control unit 207. Additionally, the device 208 may be configured to monitor a parameter associated with the transformer voltage and to compare it against a first threshold value that corresponds to the ON-voltage threshold. In one example, if the voltage does not drop below the first threshold value in all three phases after the circuit breaker trips, the device 208 may be configured to terminate or prevent the integration or to discard the result of the integration.

[0053] Fig. 4(b) illustrates signals and waveforms for various electrical parameters obtained during monitoring of the transformer in the second example configuration, in accordance with an embodiment of the present subject matter. Waveforms 406 depict transformer voltages of the three phases of the transformer 250. Signal 408 depicts a trip command to the circuit breaker 252 associated with the device 208, issued at time tl. The trip command signal may be issued by the protection and control unit 207. Waveforms 410 depict the RMS voltages of the three phases of the transformer 250. In one example, the device 208 monitors and compares the RMS voltages 410 in the three phases of the transformer 250 against the first threshold value 414 for a predetermined amount of time from time tl to time t2. The time period of tl to t2 is referred to as an integration time window corresponding to when the predefined criteria is satisfied. In one example, the predefined criteria may be satisfied when the predetermined amount of time has elapsed. In another example, the voltage of the transformer may be integrated for a time period less than the predetermined time based on a comparison of a parameter of the transformer voltage or of the integrated transformer voltage against some other preset threshold value dependent on the parameter associated with voltage. When the RMS voltage 410 of at least one of the three phases of the transformer does not drop below the first threshold value 414 at the end of the time t2, the residual flux determination module 222 stops integrating voltage of the transformer and may provide an output indicating that the transformer 250 was not de-energized. Thus, inaccurate computation of the residual flux may be prevented even if the circuit breaker 252 is tripped.

[0054] In some scenarios (not show in the figures), the circuit breaker might have tripped but the transformer may have already been de-energized. In such conditions, integrating the transformer voltage would result in wrong residual flux values. The present subject matter overcomes this problem by monitoring a parameter associated with the voltage of the transformer based on which integrating the transformer voltage may be started. The residual flux determination module 222 module, on receiving a trip command from the protection and control unit 207 to trip the circuit breaker associated with the device 208, may check whether the parameter associated with the transformer voltage is already below the second threshold value or the OFF-voltage threshold. If the transformer voltage is already below the second threshold value, the residual flux determination module 222 does not start integrating the transformer voltage and the residual flux values obtained during the preceding actual de-energization of the transformer are retained.

[0055] Fig. 5 illustrates a third example of signals and waveforms for various electrical parameters obtained during transformer de-energization, in accordance with an embodiment of the present subject matter. In one example, device 208 discussed with reference to Fig. 2 may be configured to determine the validity of the computed residual flux values. In one example, the computed residual flux values may be discarded when the parameter associated with the voltage of the transformer is greater than a first threshold value at a time instant corresponding to an end of the integration. Waveforms 506 depict transformer voltages of the three phases of the transformer 250. Signal 508 depicts a trip command from the protection and control unit 207 to trip the circuit breaker associated with the device 208 at time tl. Waveforms 510 depict the RMS voltages at the three phases of the transformer 250. Waveforms 511, 512, and 513 depict the integrated transformer voltages in the three phases of the transformer 250. As it can be observed from the Fig. 5, the circuit breaker 252 associated with the device 208 receives a trip command signal 508 at time tl. On receipt of the trip signal at time tl, the transformer voltage is integrated in each of the three phases till a time t2, where a predefined criterion is satisfied. In one example, the voltage of the transformer 250 may be integrated till a predefined criterion is satisfied, for example, till a predetermined amount of time has elapsed. In one example, the voltage of the transformer may be integrated for a predetermined time according to the predefined criteria. In another example, the voltage of the transformer may be integrated for a time period less than the predetermined time based on a comparison of the parameter of the transformer voltage or of the integrated transformer voltage against some other preset threshold value dependent on the parameter associated with voltage.

[0056] In one example, the device 208 monitors and compares the RMS voltages 510 at the three phases of the transformer 250 against the first threshold value 514 for a predetermined amount of time, indicated by a time period of tl to t2. When the RMS voltage 510 of at least one of the three phases of the transformer drops below the first threshold value 514, the residual flux determination module 222 may start integrating voltage of the transformer. The time period of tl to t2 is referred to as an integration time window. At the end of integration t2, the residual flux determination module 222 may check whether the absolute residual flux values in any phase is higher than the peak dynamic flux 518 or 519. The peak dynamic flux 518 or 519 refers to an amplitude of the integrated transformer voltage in steady-state condition calculated before deenergization. In one example, when a magnitude of the residual flux 515, 516, and 517 is greater that an amplitude of an integrated transformer voltage in steady-state condition, a signal indicating that the residual flux is invalid may be generated to raise a warning to the user or to a controlled switching unit. However, the user may decide to either still use the calculated residual flux values or to discard them and revert to a fall-back controlled switching strategy. Waveforms 511, 512, and 513 depict this scenario, where two out of the calculated residual fluxes 515, 516 and 517 are higher than the peak dynamic flux 518 and 519.

[0057] Fig 6. illustrates a fourth example of signals and waveforms for various electrical parameters obtained during transformer de-energization, in accordance with an embodiment of the present subject matter. In one example, device 208 discussed with reference to Fig. 2 may be configured to determine the validity of the residual flux values determined. Waveforms 606 depict transformer voltages of the three phases of the transformer 250. Signal 608 depicts a trip command from the protection and control unit 207 to trip the circuit breaker associated with the device 208 issued at time tl. Waveforms 610 depict the RMS voltages at the three phases of the transformer 250. Waveforms 611, 612, and 613 depict the integrated transformer voltages of the transformer 250.

[0058] As it can be observed from Fig. 6, the circuit breaker associated with the device 208 receives a trip command signal 608 from the protection and control unit 207 at tl. In one example, the device 208 monitors and compares the RMS voltages 610 at the three phases of the transformer 250 against the first threshold value 614 for a predetermined amount of time tl to a time t2 till a predefined criterion is met. In one example, the voltage of the transformer 250 may be integrated till a predefined criterion is satisfied, for example, till a pre-determined amount of time has elapsed. In one example, the predefined criteria may be a predefined time. In another example, the voltage of the transformer may be integrated for a time period less than the predetermined time based on a comparison of a parameter of the transformer voltage or of the integrated transformer voltage against some other preset threshold value dependent on the parameter associated with voltage. The time period from tl to t2 is referred to as integration time window. When the RMS voltage 610 of at least one of the three phases of the transformer drops below the first threshold value 614, the residual flux determination module 222 may start integrating voltage of the transformer till the time t2. In one example, the voltage of the transformer may be integrated for a pre-determined amount of time (tl to t2).

[0059] While integrating the transformer voltage following a trip command received by the circuit breaker or transformer de-energization, the residual flux determination module 222 may continuously monitor and compare the RMS voltage of the transformer 250 against the second threshold value 616. The second threshold value 616 may correspond to an OFF -voltage threshold. In one example, the OFF -voltage threshold may be 10% of the nominal value of the parameter. For example, if the parameter is RMS voltage at each phase, the second threshold value may be 10% of the nominal RMS voltage of the transformer. If, at the end of the pre-determined integration window, a time period of tl to t2 as indicated in Fig. 6, the transformer voltage RMS is still above that threshold in any phase, it may raise a warning to the user or to the device 208 that the residual flux values are unreliable. When the transformer voltage is still above the second threshold value 616, the residual flux values may oscillate and may not converge to a steady state value. In one example, a signal to indicate that the computed residual flux is unreliable when the parameter associated with the voltage of the transformer is greater than a second threshold value at a time instant corresponding to an end of the integration.

[0060] Optionally, the user may decide to either still use the calculated residual flux values or to discard them and revert to a fall back controlled closing strategy. In one example, the calculated residual flux values may be an average of integration values over a certain time window, such as one power cycle. In another example, the device 208 may be configured to take the average of the last cycle or an integer number of cycles such as 2 or 3 cycles of calculated residual flux values as residual flux value in that phase for higher accuracy. In one example, the residual flux values may be averaged for n power cycles, where the n power cycles may correspond to a time period t3 to t2.

[0061] The present subject matter thus provides devices and methods to provide reliable residual fluxes for optimal energization of a power transformer.

[0062] Fig. 7 illustrates a method to compute residual flux values, in accordance with an embodiment of the present subject matter. The order in which method 700 is described is not intended to be construed as a limitation, and some of the described method blocks may be performed in a different order to implement the method 700 or an alternative method. Furthermore, the method 700 may be implemented in any suitable hardware, computer readable instructions, firmware, or combination thereof. For discussion, the method 700 is described with reference to the implementations illustrated in Fig. 2.

[0063] In the method 700, at block 702 a parameter associated with a voltage of a transformer is monitored. In one example, the parameter associated with the voltage of the transformer is any one of a root mean square (RMS), an amplitude, a rate of change, an integral over a pre-determined time, or a quantity derived from at least one of these.

[0064] At block 704, the parameter associated with the voltage of the transformer is compared against a preset threshold value for starting integration of the voltage of the transformer. In one example, the preset threshold value is at least one of a first threshold value corresponding to an ON-voltage threshold and a second threshold value corresponding to an OFF-voltage threshold. In one example, on detecting a drop in the parameter associated with the voltage of the transformer to a value below the first threshold value, integration of the voltage of the transformer is started. Further, samples of the voltage are buffered during monitoring and at least one cycle of the voltage prior to the detection of the drop in the parameter associated with the voltage of the transformer in the integration to compute the residual flux are included. In one example, the method comprises receiving a trip signal and starting the integration of the voltage of the transformer on receipt of the trip signal. [0065] At block 706, the voltage of the transformer is integrated till a predefined criterion is met, to compute a residual flux value. In one example, the voltage of the transformer may be integrated for a predetermined time according to the predefined criteria. In one example, the voltage of the transformer 250 may be integrated till a predefined criterion is satisfied, for example, till a pre-determined amount of time has elapsed In another example, the voltage of the transformer may be integrated for a time period less than the predetermined time based on a comparison of a parameter of the transformer voltage or of the integrated transformer voltage against some other preset threshold value dependent on the parameter associated with voltage.

[0066] In one example, the residual flux is computed from the integration of the voltage of the transformer by averaging at least one power cycle of integration values at the end of the pre-determined time. In one example, on receiving a trip signal the integration of the voltage of the transformer may be prevented when the parameter associated with the voltage of the transformer is below the second threshold value.

[0067] Further, a validity of the residual flux may be determined. The method may provide a value of the residual flux and a signal indicating the validity of the residual flux. In one example, when a magnitude of the residual flux is greater than an amplitude of an integrated transformer voltage in steady-state condition a signal indicating that the residual flux is invalid may be generated. In another example, when the parameter associated with the voltage of the transformer at the end of the predetermined time is greater than a first threshold value, the computed residual flux values may be discarded.

[0068] In another example, when the parameter associated with the voltage of the transformer at the end of the pre-determined time is greater than a second threshold value at a time instant corresponding to an end of the integration, a signal indicating that the residual flux is unreliable may be generated.

[0069] The parameter associated with the voltage of the transformer may be monitored in a plurality of phases and the comparison of the parameter associated with the voltage of the transformer and the determination of the validity of the residual flux may be performed in each of the plurality of phases.

[0070] The present subject matter thus provides a method to compute accurate residual flux values and determine the validity of residual flux values to ensure a correct and high reliability of the residual flux values. [0071] Although the present subject matter has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the subject matter, will become apparent to persons skilled in the art upon reference to the description of the subject matter.

Claims

1/ We Claim:

1. A method comprising: monitoring a parameter associated with a voltage of a transformer; starting integration of the voltage of the transformer based on a comparison of the parameter associated with the voltage of the transformer against a preset threshold value; and integrating the voltage of the transformer to compute a residual flux.

2. The method as claimed in claim 1, wherein the preset threshold value is at least one of a first threshold value corresponding to an ON-voltage threshold and a second threshold value corresponding to an OFF-voltage threshold.

3. The method as claimed in claim 2 comprising starting the integration of the voltage of the transformer on detecting a drop in the parameter associated with the voltage of the transformer to a value below the first threshold value.

4. The method as claimed in claim 3 comprising buffering samples of the voltage during monitoring, and including at least one cycle of the voltage prior to the detection of the drop in the parameter associated with the voltage of the transformer in the integration to compute the residual flux.

5. The method as claimed in claim 2 comprising receiving a trip signal and preventing integration of the voltage of the transformer when the parameter associated with the voltage of the transformer is below the second threshold value.

6. The method as claimed in claim 1 comprising receiving a trip signal and starting the integration of the voltage of the transformer on receipt of the trip signal.

7. The method as claimed in claim 6 comprising discarding the computed residual flux when the parameter associated with the voltage of the transformer is greater than a first threshold value at a time instant corresponding to an end of the integration.

8. The method as claimed in claim 1, wherein the residual flux is computed from the integration of the voltage of the transformer by averaging at least one power cycle of integration values.

9. The method as claimed in claim 1 wherein the integration is performed till a predefined criterion is satisfied.

10. The method as claimed in claim 1 comprising generating a signal to indicate that the computed residual flux is invalid when a magnitude of the residual flux is greater than an amplitude of an integrated transformer voltage in steady-state condition.

11. The method as claimed in claim 1 comprising generating a signal to indicate that the computed residual flux is unreliable when the parameter associated with the voltage of the transformer is greater than a second threshold value at a time instant corresponding to an end of the integration.

12. The method as claimed in any one of the preceding claims, wherein the parameter associated with the voltage of the transformer is monitored in a plurality of phases and the comparison of the parameter associated with the voltage of the transformer and a determination of a validity of the residual flux are performed in each of the plurality of phases.

13. The method as claimed in any one of the preceding claims, wherein the parameter associated with the voltage of the transformer is any one of a root mean square (RMS), an amplitude, a rate of change, an integral over a pre-determined time, or a quantity derived from at least one of these.

14. A device comprising a processor to to execute the method of any one of claims 1 to 13.

15. A non-transitory computer readable medium comprising instructions that, when executed by a processor, cause the processor to execute the method of any one of the claims 1 to 13.