METHOD AND DEVICE FOR CONTROLLING AT LEAST ONE CIRCUIT BREAKER OF A POWER SYSTEM

Abstract

A power system comprises a power source, a transmission line coupled to the power source through a circuit breaker, a shunt reactor coupled to the transmission line, and a current transformer connected in series with the shunt reactor. A method for controlling the circuit breaker of the power system comprises processing an output signal of the current transformer to obtain the voltage on the transmission line by determining a time derivative of a current sensed by the current transformer. The method further comprises performing, by at least one control or protection device, a control or protection operation (e.g., auto-reclosing) based on the determined time derivative of the current sensed by the current transformer.

Claims

1. A method of controlling at least one circuit breaker of a power system, the power system further comprising a power source, a transmission line coupled to the power source through the circuit breaker, a shunt reactor coupled to the transmission line, and a current transformer connected in series with the shunt reactor, the method comprising: processing an output signal of the current transformer, comprising determining a time derivative of a current sensed by the current transformer; and performing a control and/or protection operation based on the determined time derivative of the current sensed by the current transformer.

2. The method of claim 1, wherein processing the output signal of the current transformer comprises determining a line voltage on the transmission line based on the time derivative of the current sensed by the current transformer.

3. The method of claim 2, wherein the control and/or protection operation comprises a controlled reclosing of the circuit breaker after tripping of the circuit breaker.

4. The method of claim 3, wherein the power system comprises a voltage transformer connected to the power source and adapted to sense a source voltage of the power source, wherein the control and/or protection operation comprises a controlled reclosing of the circuit breaker based on an output signal of the voltage transformer and on the time derivative of the current sensed by the current transformer.

5. The method of claim 4, wherein the controlled reclosing of the circuit breaker comprises controlling a target reclosing time at which the circuit breaker is reclosed based on the output signal of the voltage transformer and on the time derivative of the current sensed by the current transformer.

6. The method of claim 5, wherein the reclosing of the circuit breaker is controlled by a control or protection device comprising a point-on-wave controller.

7. The method of claim 6, wherein processing the output signal of the current transformer comprises determining a line voltage on the transmission line by multiplying the time derivative of the current sensed by the current transformer by an inductance of the shunt reactor or by another suitable scaling factor.

8. The method of claim 7 further comprising determining the inductance of the shunt reactor in a calibration that uses the output signal of the voltage transformer measured while the circuit breaker is closed and the time derivative of the current sensed by the current transformer while the circuit breaker is closed.

9. The method of claim 8, wherein the inductance of the shunt reactor is determined based on at least one peak value of the output signal of the voltage transformer while the circuit breaker is closed and at least one peak value of the time derivative of the current sensed by the current transformer while the circuit breaker is closed, or wherein the inductance of the shunt reactor is determined based on at least one calculated root mean square, RMS, value of the output signal of the voltage transformer while the circuit breaker is closed and at least one calculated RMS value of the time derivative of the current sensed by the current transformer while the circuit breaker is closed.

10. The method of claim 9, further comprising using the time derivative of the current sensed by the current transformer to perform at least one of: detecting an instant of line de-energization; detecting an extinction of temporary faults or secondary arcing; at least one protection function; line synchronization.

11. A control or protection device for controlling at least one circuit breaker of a power system, the power system further comprising a power source, a transmission line coupled to the power source through the circuit breaker, a shunt reactor coupled to the transmission line, and a current transformer connected in series with the shunt reactor, wherein the control or protection device comprises: an input to receive an input signal representing a current sensed by the current transformer or a time derivative of the current sensed by the current transformer; and a control circuit adapted to perform a control and/or protection operation based on the time derivative of the current sensed by the current transformer.

12. The control or protection device of claim 11, wherein the control or protection device comprises a point-on-wave controller adapted to perform controlled reclosing of a circuit breaker after tripping based on an output signal of a voltage transformer connected to the power source and on a line voltage signal derived from the time derivative of the current sensed by the current transformer.

13. The control or protection device of claim 11, wherein the control or protection device is adapted to perform controlled reclosing of a circuit breaker after tripping based on an output signal of a voltage transformer connected to the power source and on a line voltage signal derived from the time derivative of the current sensed by the current transformer.

14. The control or protection device of claim 13, wherein the control or protection device is adapted to perform the method of claim 10.

15. A power system, comprising: a power source, a transmission line coupled to the power source through a circuit breaker, a shunt reactor coupled to the transmission line, a current transformer connected in series with the shunt reactor, and the control or protection device of claim 14.

16. The power system of claim 15, further comprising a computing device adapted to calculate the time derivative of the current sensed by the current transformer.

17. The power system of claim 16, wherein the computing device is adapted to reconstruct a line voltage signal on the transmission line by multiplying the time derivative of the current sensed by the current transformer by an inductance of the shunt reactor or by another suitable scaling factor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The subject-matter of the invention will be explained in more detail with reference to preferred exemplary embodiments which are illustrated in the attached drawings, in which:

[0070] FIG. 1 is a schematic representation of a power system comprising a control or protection device according to an embodiment.

[0071] FIG. 2 is a schematic representation of a power system comprising a control or protection device according to an embodiment, wherein a computing device separate from the control or protection device is provided.

[0072] FIG. 3 illustrates signals for different phases of a power system.

[0073] FIG. 4 is a flow chart of a method according to an embodiment.

[0074] FIG. 5 is a flow chart of a method that comprises a calibration according to an embodiment.

[0075] FIG. 6 is a schematic representation of a power system employing a conventional control structure for controlled reclosing.

DETAILED DESCRIPTION OF EMBODIMENTS

[0076] Exemplary embodiments of the invention will be described with reference to the drawings in which identical or similar reference signs designate identical or similar elements. While some embodiments will be described in the context of controlled reclosing of a circuit breaker (CB) after tripping, the embodiments are not limited thereto. The features of embodiments may be combined with each other, unless specifically noted otherwise.

[0077] While the power system and its components are described and shown in single-phase representation, it is generally to be understood that a power system and its components usually comprise three phases. However, the invention with its embodiments is not limited to a specific number of system phases.

[0078] FIG. 1 is a schematic representation of a power system according to an embodiment. The power system may be a high voltage power system or a medium voltage power system.

[0079] The power system may comprise a power source 2 and a CB 1. The CB 1 is arranged to connect a transmission line 3 to the power source 2 when the CB 1 is closed and to disconnect the transmission line 3 from the power source 2 when the CB 1 is open. The transmission line 3 may be equipped with one or more shunt reactors 4, 9. The shunt reactor(s) 4, 9 is/are operative to compensate a capacitance of the transmission line 3. The shunt reactor 4 is arranged at the local end of the transmission line, close to the CB 1. Another shunt reactor 9 may be arranged on the far end of the transmission line 3 or in another location along the transmission line 3, but is not required for the techniques of this disclosure.

[0080] A control or protection device 10 may perform controlled reclosing of the CB 1 after tripping. The control or protection device 10 may be or may comprise a point-on-wave controller. The control or protection device 10 may be adapted to predict differential voltage signals across the CB 1 when the CB 1 is open to determine a target time for reclosing.

[0081] Contrary to conventional control strategies that use a voltage transformer 5 for directly measuring a line voltage U.sub.2 for identifying future beat minima (as illustrated in FIG. 6 and as explained above), the control or protection device 10 performs controlled reclosing using a source voltage U.sub.1 and calculated (rather than measured) line voltage L.Math.dI.sub.L/dt derived from a time derivative of a shunt reactor current I.sub.L. The control or protection device 10 may predict future beat minima at which a voltage difference across the open CB 1 is minimum, based on the measured source voltage U.sub.1 and the calculated line voltage L.Math.dI.sub.L/dt derived from the time derivative of the shunt reactor current I.sub.L to determine the target reclosing time of the CB 1. The target reclosing time may be determined individually for separate phases or poles of the CB 1. The control or protection device 10 may be adapted to forward individual reclosing commands to each circuit breaker pole, in accordance with the predicted future minima of the beat of the voltage difference across the open CB 1.

[0082] In order to provide the shunt reactor current, the power system 10 may comprise a current transformer 6 connected in series with the shunt reactor 4. A series connection of the current transformer 6 and the shunt reactor 4 may be connected between an end of the transmission line 3 and ground.

[0083] The control or protection device 10 may comprise circuitry to calculate the time derivative of the shunt reactor current I.sub.L sensed by the current transformer 6. The circuitry may comprise an integrated semiconductor circuit such as a processor, controller, or application specific integrated circuit that is programmed to calculate the time derivative. In another embodiment, as illustrated in FIG. 2, a separate computing device 8 may be connected between the current transformer 6 and the control or protection device 10. The computing device 8 may compute the time derivative of the shunt reactor current I.sub.L sensed by the current transformer 6. The computing device 8 may provide the time derivative of the shunt reactor current I.sub.L sensed by the current transformer 6, or the reconstructed line voltage signal on the transmission line 3 obtained by multiplying the time derivative of the shunt reactor current I.sub.L, sensed by the current transformer 6 by an inductance of the shunt reactor 4 or by another suitable scaling factor, to the control or protection device 10.

[0084] The control or protection device 10 or computing device 8 may use the output signal of the current transformer 6 arranged in series with the local shunt reactor 4 to reconstruct the voltage signal U.sub.2 on the local end of the transmission line 3. According to the basic electrical equation of a reactor,

[00001]$\begin{array}{cc}{U}_L=L.Math.\frac{{\mathrm{dI}}_L}{dt},& \left(1\right)\end{array}$

the time derivative of the shunt current signal I.sub.L is calculated and multiplied by the inductance L of the shunt reactor to yield the voltage U.sub.L across the shunt reactor, which is equal to the line voltage signal U.sub.2=U.sub.L. Example waveforms are shown in FIG. 3, which reflect that U.sub.2=U.sub.L.

[0085] The differential voltage U.sub.CB across the CB 1 after tripping may be determined using the calculated voltage U.sub.L,

[00002]$\begin{array}{cc}{U}_{\mathrm{CB}}=U_1-L.Math.\frac{{\mathrm{dI}}_L}{dt}.& \left(2\right)\end{array}$

[0086] The beat pattern in the differential voltage U.sub.CB across the CB 1 may be used by the control or protection device 10 to predict future minima of the beat, e.g., based on the recurring pattern of previous beats in U.sub.CB.

[0087] The calculation of the derived line voltage U.sub.L can be performed through an analog circuit or numerically, from sampled current values (obtained by the point-on-wave controller itself or via a digital communication system). The calculation can be done by the control or protection device 10, as illustrated in FIG. 1, or by a separate computing device 8 interposed between CT 6 and control or protection device 10, as illustrated in FIG. 2. Instead of the inductance value L in equation (1) a different scaling factor may be applied, for example to match the input ratings of the control or protection device 10.

[0088] The current transformer 6 may comprise a Rogowski coil, which by definition outputs a time derivative of the measured reactor current. In such an embodiment, the step of calculating the derivative of the shunt reactor current signal IL is implicitly included in the measurement of the shunt reactor current signal, and the derived line voltage signal U.sub.2 =U.sub.L is obtained simply by appropriate scaling of the output signal of the Rogowski coil, for example to satisfy equation 1.

[0089] While operation of the control or protection device 10 has been explained with reference to one phase, the power system 10 typically has several phases. A transmission line 3, shunt reactor 4, current transformer 6 and voltage transformer 7 may respectively be provided for each one of the various phases. The control or protection device 10 may initiate controlled reclosing of the circuit breaker 1 or of several circuit breaker poles for any of the plural phases that have been tripped, using the techniques described herein.

[0090] FIG. 3 shows waveforms 11, 12, 13 of the source voltage U.sub.1 for three different phases, the waveforms 21, 22, 23 of the line voltage U.sub.2 for the three different phases, the waveforms 31, 32, 33 of the reactor current I.sub.L for the three different phases, and the waveforms 41, 42, 43 of the calculated line voltage U.sub.L determined in accordance with equation (1) from the time derivative of the shunt reactor current. As seen in FIG. 3, the calculated line voltage U.sub.L determined in accordance with equation (1) from the time derivative of the shunt reactor current matches the line voltage U.sub.2 that would be obtained from a voltage transformer having good frequency response characteristics. The differential voltage U.sub.CB across the CB 1 when the CB 1 has tripped, calculated in accordance with equation (2), allows the beat minima of the differential voltage to be predicted with good accuracy.

[0091] Current transformers 6 usually have a better frequency response than capacitive voltage transformers 5. Hence, the accuracy of the derived line voltage signal U.sub.L determined in accordance with equation (1) is suitable for calculating a correct reclosing target, even at line frequencies significantly different from nominal power frequency.

[0092] FIG. 4 is a flow chart of a method 60 according to an embodiment. The method 60 may be performed to implement controlled reclosing of the CB 1 after tripping.

[0093] At step 61, the CB 1 of a power system may be tripped.

[0094] At step 62, the time derivative of the shunt current signal I.sub.L measured while the CB 1 is open may be calculated and multiplied by the inductance L of the shunt reactor, in accordance with equation (1), to yield a voltage that is equal to the line voltage signal U.sub.2=U.sub.L. Instead of the inductance value L, another suitable scaling factor may be applied.

[0095] At step 63, the differential voltage U.sub.CB across the CB 1 after tripping may be determined in accordance with equation (2). A beat pattern in the differential voltage U.sub.CB across the CB 1 after tripping may be used to predict future minima in U.sub.CB.

[0096] At step 64, controlled reclosing of the CB 1 may be performed using the beat pattern in the differential voltage U.sub.CB across the CB 1 after tripping determined in step 63. Target reclosing times may be set so that the various poles of the CB 1 are reclosed at beat minima of the differential voltage U.sub.CB for the respective phases.

[0097] The inductance L of the shunt reactor 4 may be read from a physical rating plate of the shunt reactor 4, from a data sheet of the shunt reactor 4, or from a computerized record representing technical data of the shunt reactor 4. The inductance L may be input into the control or protection device 10, for example, via a user interface or via a data interface.

[0098] Alternatively or additionally, a calibration routine may be executed to determine the inductance L of the shunt reactor 4.

[0099] FIG. 5 is a flow chart of a method 70 according to an embodiment. The method 70 may be performed to determine the inductance L of the shunt reactor 4 in a calibration.

[0100] At step 71, the calibration is started.

[0101] At step 72, the time derivative dI.sub.L/dt of the shunt reactor current signal I.sub.L measured while the CB 1 is closed may be received or calculated from the shunt reactor current signal I.sub.L.

[0102] At step 73, the source voltage U.sub.1 measured by the VT 7 while the CB 1 is closed may be received.

[0103] At step 74, the inductance L of the shunt reactor 4 may be calculated based on the source voltage U.sub.1 measured by the VT 7 while the CB 1 is closed and based on the time derivative dI.sub.L/dt of the shunt reactor current signal I.sub.L measured while the CB 1 is closed. Instead of the source voltage U.sub.1 measured by the VT 7, the load voltage U.sub.2 measured by the VT 5 may be used while the CB 1 is closed.

[0104] At step 75, the calculated inductance L of the shunt reactor 4 may be stored in the control or protection device 10 or in the computing device 8, for use in combination with the time derivative dI.sub.L/dt of the shunt reactor current signal I.sub.L measured while the CB 1 is open to calculate the line voltage U.sub.L in accordance with equation (1) for controlled circuit breaker reclosing or for other power system related functions.

[0105] The calibration routine may be performed while the CB 1 is closed. The calibration routine may use a source voltage U.sub.1 measured by the VT 7 while the CB 1 is closed and a time derivative of the shunt current signal I.sub.L measured by the current transformer 6 while the CB 1 is closed. Instead of the source voltage U.sub.1 measured by the VT 7, a load voltage U, measured by load VT 5 may be used while the CB 1 is closed.

[0106] In the calibration routine, the inductance L may be determined as

[00003]$\begin{array}{cc}L=\frac{{\hat{U}}_1}{\frac{dt}{}},& \left(3\right)\end{array}$

where .Math..sub.1 designates a peak value of the source voltage U.sub.1 measured by the VT 7 while the CB 1 is closed and

[00004]$\frac{dt}{}.$

designates a peak value of the time derivative dI.sub.L/dt of the shunt reactor current signal I.sub.L measured by the current transformer 6 while the CB 1 is closed. In equation (3) peaks of the same polarity are used for both .Math..sub.1 and

[00005]$\frac{dt}{}.$

[0107] Instead of using a single peak value, equation (3) may be evaluated by using an average of several peak values of the source voltage U.sub.1 measured by the VT 7, or of the load voltage U.sub.2 measured by the VT 5, while the CB 1 is closed as U.sub.1 and an average of several peak values of the time derivative of the shunt reactor current signal I.sub.L measured by the current transformer 6 while the CB 1 is closed as

[00006]$\frac{dt}{}.$

In a similar manner, equation (3) may be evaluated by using the calculated RMS value, or the average of several calculated RMS values, of the source voltage U.sub.1 measured by the VT 7, or of the load voltage U.sub.2 measured by the VT 5, while the CB 1 is closed as .Math..sub.1 and the calculated RMS value, or the average of several calculated RMS values, of the shunt reactor current signal I.sub.L measured by the current transformer 6 while the CB 1 is closed as

[00007]$\frac{dt}{}.$

Averages other than RMS may be used instead of the RMS values to calculate the inductance L during calibration.

[0108] While a calibration has been explained with reference to one phase, calibration may be performed for each one of plural different phases of a power system using the techniques described herein to determine the inductances of the shunt reactors of the plural different phases of the transmission line. Alternatively, the inductance value calculated in one phase may be applied to all phases, or an inductance value obtained by calculating the average of the inductance values calculated in all phases may be applied to all phases.

[0109] While embodiments of the invention have been described in association with controlled reclosing of a circuit breaker after tripping, the methods, devices, and systems may be used for other purposes. For illustration, the output signal of the current transformer 6 may be used to calculate the line voltage, obviating the need for using a voltage transformer 7 as in the conventional system illustrated in FIG. 6. The time derivative of the shunt reactor current may be used for detecting an instant of line de-energization, for detecting an extinction of temporary faults or secondary arcing, for performing one or several protection functions, and/or for line synchronization (e.g., synchro-check).

[0110] While the device 10 is referred to as a “control or protection device” herein, it will be appreciated that the device 10 can perform both control and protection functions. The word “or” as used herein is to be understood as being non-exclusive.

[0111] While peak values or RMS values of various signals have been described herein in the context of techniques for determining the inductance value of the shunt reactor, other values that are characteristic for a voltage amplitude and an amplitude of a time derivative may be used in the disclosed procedures.

[0112] While the invention has been described in detail in the drawings and foregoing description, such description is to be considered illustrative or exemplary and not restrictive. Variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain elements or steps are recited in distinct claims does not indicate that a combination of these elements or steps cannot be used to advantage; specifically, in addition to the actual claim dependency, any further meaningful claim combination shall be considered disclosed.