HIGH-VOLTAGE TRIGGERED PULSECLOSER WITH ADAPTIVE CIRCUIT TESTING

Abstract

A system and method for maintaining electrical stability of a high-voltage transmission power system in response to a fault. The method includes detecting the fault, opening a switch to clear the fault, performing a first pulse test for a predetermined duration of time to determine if the fault is still present, preventing a reclosing operation from occurring if the pulse test indicates that the fault is still present, and allowing the reclosing operation to occur if the first pulse test indicates that the fault is not present. One or more subsequent pulse tests can be performed if the first pulse test is inclusive about the persistence of the fault, where the reclosing operation is prevented from occurring if the pulse tests indicate that the fault is still present and the reclosing operation is allowed if the pulse tests indicate that the fault is no longer present.

Claims

1. A method for maintaining electrical stability of a power system in response to a fault, the method comprising: detecting the fault; opening a switch to clear the fault; performing a first pulse test for a predetermined duration of time to determine if the fault is still present; preventing a reclosing operation by the switch from occurring if the first pulse test indicates that the fault is still present; and allowing the reclosing operation to occur if the first pulse test indicates that the fault is no longer present.

2. The method according to claim 1 wherein the first pulse test is performed by a triggered vacuum gap (TVG) device.

3. The method according to claim 1 wherein the predetermined duration of time is ≤0.5 cycles.

4. The method according to claim 1 further comprising performing subsequent pulse tests if the first pulse test is inclusive about the persistence of the fault, wherein preventing a reclosing operation from occurring includes preventing the reclosing operation from occurring if any of the pulse tests indicates that the fault is still present and allowing the reclosing operation to occur includes allowing the reclosing operation to occur if any of the pulse tests indicates that the fault is no longer present.

5. The method according to claim 4 wherein the first and subsequent pulse tests are performed by a triggered vacuum gap (TVG) device.

6. The method according to claim 1 wherein the first pulse test is performed at a predetermined point-on-wave time.

7. The method according to claim 1 wherein the power system is a high-voltage transmission power system.

8. The method according to claim 1 wherein the reclosing operation is performed by the switch.

9. A method for maintaining electrical stability of a high-voltage transmission power system in response to a fault, the method comprising: detecting the fault; opening a switch to clear the fault; performing a first pulse test for a predetermined duration of time using a triggered vacuum gap (TVG) device to determine if the fault is still present; performing subsequent pulse tests by the TVG device for the predetermined duration of time if the first pulse test is inclusive about the persistence of the fault; preventing a reclosing operation by the switch from occurring if the pulse tests indicate that the fault is still present; and allowing the reclosing operation to occur by the switch if the pulse tests indicate that the fault is no longer present.

10. The method according to claim 9 wherein the predetermined duration of time is ≤0.5 cycles.

11. The method according to claim 9 wherein the first pulse test is performed at a predetermined point-on-wave time.

12. A system for maintaining electrical stability of a power network in response to a fault, the system comprising: sensors coupled to the power network and to a controller couple to the sensors, the sensors and controller operable to detect the fault; an actuator coupled to a switch, the actuator being operable in the presence of a fault to open the switch to clear the fault; a first pulse tester coupled to the power network, the first pulse tester being operable to perform a pulse test of the power network for a predetermined duration of time to determine if the fault is still present; the actuator being configured to prevent a reclosing of the switch where the first pulse test indicates that the fault is still present; and the actuator being further configured to provide the reclosing operation where the first pulse test indicates that the fault is not present.

13. The system according to claim 12 wherein the pulse tester comprises a triggered vacuum gap (TVG) device.

14. The system according to claim 12 wherein the predetermined duration of time is ≤0.5 cycles.

15. The system according to claim 12 the pulse tester being configured to perform subsequent pulse tests if the first pulse test indicates the fault being present, wherein the actuator is further configured to prevent a reclosing of the switch where the subsequent pulse test indicates that the fault is still present; and the actuator being further configured to provide the reclosing operation where the subsequent pulse test indicates that the fault is not present.

16. The system according to claim 15 wherein the pulse tester comprises a triggered vacuum gap (TVG) device.

17. The system according to claim 15 wherein the predetermined duration of time is ≤0.5 cycles.

18. The system according to claim 12 wherein the pulse tester is configured to perform the first pulse test at a predetermined point-on-wave time.

19. The system according to claim 12 wherein the actuator operates the switch to perform the reclosing operation.

20. The system according to claim 12 wherein the power network is a high-voltage transmission power network.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a block diagram of a pulse-closing system for a high voltage transmission power network;

[0015] FIG. 2 is a cross-sectional type view of a TVG device that can be used in the pulse-closing device shown in FIG. 1; and

[0016] FIG. 3 is a graph with time on the horizontal axis and voltage on the vertical axis showing sensitivity to pulse-testing and hard reclosing after one-cycle of fault clearing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The following discussion of the embodiments of the disclosure directed to a system and method for maintaining electrical stability of a high-voltage transmission power system in response to a fault using pulse testing is merely exemplary in nature and is in no way intended to limit the disclosure or its applications or uses.

[0018] It is critical that power transmission be stable by balancing between generation and consumption of power on an instantaneous basis, where this is typically provided by making sure power generation is kept on-line. When a fault event occurs in a power transmission network, a reclose operation in response thereto may cause destabilization of the network, which would subsequently cause more harm than good, i.e., the attempt to restore power causes a broader loss of power. The present disclosure determines how and when to perform a reclose operation after a fault has been cleared to ensure that the network does not become unstable. Various parameters, such as voltage angles in the network, are calculated and/or observed that determine whether the network is returning to stability or becoming unstable after a fault is cleared to determine if a reclosing operation can be performed. If the network is tending toward stability, then pulse-tests are performed to determine if the fault is still present before the reclosing operation is performed.

[0019] As will be discussed, the present disclosure proposes a system and method for rapid and repeated tests for the persistence of a fault without performing hard reclosing, where the system employs a switch having a TVG device. As soon as the system has detected and cleared the fault, the switch recloses using point-on-wave timing and other factors to optimize speed and network stability. The following discussion assumes that the “switch” consists of three separate poles (one for each phase in a three-phase system) whose operation is coordinated as a single three-pole switch. However, other embodiments allow independent single-pole operation, or separate timing for point-on-wave opening and closing of each pole in a three-phase implementation.

[0020] Once a fault is detected using one or more of the fast-fault-detection schemes, the controls issue an initial permissive to clear the suspected fault by actuating the switch that has been optimized for opening-speed. While the initial permissive-to-clear is being processed in the controls, double-checks are performed against parallel computations, such as DFT, spectral analysis and V-versus-I (impedance) measurements to ensure that a suspected fault was not erroneously detected. In addition, the type of fault, i.e., line-to-ground (LG), line-to-line (LL), line-to-line-to-ground (LLG), line-to-line-to-line (LLL) or line-to-line-to-line-to-ground (LLLG), is identified where possible to help identify the optimal scheme for subsequent triggered pulse testing to improve both accuracy and speed of determining when a fault of any type has cleared.

[0021] In one embodiment, the triggering electrode of the TVG device is located only in the stationary contact of the switch, here presumed to be a vacuum interrupter. This permits triggered pulse testing only once in a line-frequency cycle, and specifically in the positive voltage half-cycle, near the negative-going voltage zero-crossing. In another embodiment, triggering electrodes are located both in the stationary contact and in the moving-contact end of the vacuum bottle. This permits triggered pulse testing twice during a line-frequency cycle. If the parallel computations indicate that the fault detection may be a false-positive leading to a nuisance trip, then the initial permissive may be withdrawn within the timeframe that switch contacts can be actuated, and transition into the triggered pulse-closing mode is inhibited.

[0022] If the slower double-check computations are incomplete, inconclusive, or even contradictory to the initial fault detection, then the permissive is either reinforced or withdrawn according to a pre-configured selection to avoid false positives or false negatives, depending on which is considered worse for the application. Timing may also be configured for a specific application. Such timing is a system-level double-check of the initial permissive to actuate the switch to clear the fault.

[0023] If a true fault is detected, and switches have opened to clear the fault, then the device controls transition into a triggered pulse-closing mode to determine when the fault has cleared and consequently the earliest time the switches may be reclosed. To do this, the triggering electrodes in the switches are periodically energized at a precise point-on-wave to ignite a plasma arc across the open contact gaps. The resulting current, which is of a much lower magnitude than the available fault current, and of only a few milliseconds duration, is measured and analyzed to determine if the fault is still present or not on any of the phases. If the fault is determined to have cleared, then an initial permissive is issued to reclose the contacts. If the fault has not cleared, then no permissive is issued and the device waits for the next point-on-wave opportunity to trigger the pulse test.

[0024] Interfaces with external current and voltage monitors, as well as with external control and status signaling all of which may be available in substations, are accounted for. However, a communications infrastructure with sufficient bandwidth and data rates required to interface the local, external measurement and control systems to the triggered pulse testing may not exist at all such substations. Consequently, for the purposes of this discussion it is assumed that voltage and current sensing, and the control/status signals, are performed internally to the triggered pulse testing, although such sensing and signaling may be provided by external devices.

[0025] In other embodiments, the controls may wait a predetermined amount of time before triggered pulse-closing at a fixed periodic interval; the controls may wait a configured amount of time before triggered pulse-closing at a variable periodic interval; the controls may cease triggered pulse-closing if the fault is determined not to have cleared within a specified amount of time, such as within the critical reclosing interval; the controls may cease triggered pulse-closing if the fault is determined not to have cleared after a specified number of triggered pulse-closings; the controls may receive instructions from an external control system to start or stop triggered pulse-closing; the controls may be configured according to conventional TCC curves or other application-specific timing considerations; the controls may be configured not to perform triggered pulse-closing or to reclose at all; the controls may adjust the pulse-test interval based on the magnitude of the measured fault current, and/or the controls of one triggered pulse-closer may coordinate with other triggered pulse-closers to interleave their respective pulse-closing activities to gain additional situational awareness, such as fault-locating or integration with distance and differential relaying schemes.

[0026] FIG. 1 is a block diagram of a pulse-closing system 10 illustrating the components that can be used for maintaining electrical stability of a high-voltage transmission or medium voltage distribution power system in response to a fault as discussed above. The system 10 includes three high-voltage transmission lines 12, 14 and 16 one for each phase A, B and C that receive high voltage power from a power generator 18, such as a turbine. A pulse-closing device 20 is coupled to the lines 12, 14 and 16 and includes a switch assembly 22 having a reclosing switch 24, such as a vacuum interrupter, and a pulse testing TVG device 26 in the line 12, a switch assembly 28 having a reclosing switch 30 and a pulse testing TVG device 32 in the line 14, and a switch assembly 34 having a reclosing switch 36 and a pulse testing TVG device 38 in the line 16. The pulse-closing device 20 also includes an actuator control 40 that opens and closes the switches 24, 30 and 36 during the fault clearing and reclosing operation and a trigger control 42 that generates the plasma arc in the TVG devices 26, 32 and 38.

[0027] A voltage sensor 48 is coupled to the lines 12 and 14 at the line-side of the device 20 and a voltage sensor 50 is coupled to the lines 14 and 16 at the line-side of the device 20 to provide voltage measurements on the lines 12, 14 and 16. A voltage monitor 52 receives voltage measurements from the sensors 48 and 50. A current sensor 54 provides current measurements on the line 12, a current sensor 56 provides current measurements on the line 14 and a current sensor 58 provides current measurements on the line 16. A current monitor 60 receives the current measurements from the sensors 54, 56 and 58. This configuration of voltage monitoring uses line-to-line voltage measurements from the sensors 48 and 50. In an alternate embodiment, the voltage measurements may be line-to-ground measurements requiring three voltage sensors. A signal processor 62 receives voltage and current signals from the monitors 52 and 60, processes the signals and provides the processed signals to a fault detection and response logic controller 64 that commands the actuator control 40 and the trigger control 42 to control the switches 24, 30 and 36 and the TVG devices 26, 32 and 38 consistent with the discussion herein. The signal processor 62 is in communications with a communications device 66 to receive voltage and current signals, status signals, etc. from other components in the network.

[0028] The TVG devices 26, 32 and 38 can be any TVG device suitable for the purposes discussed herein. FIG. 2 is a cross-sectional type view of an exemplary embodiment of an electrically-triggered TVG device 70 to show one representative example. The TVG device 70 includes a vacuum enclosure 72 having a cylindrical insulator 74 and conductive end plates 76 and 78. In exemplary embodiments, the vacuum enclosure 72 is sealed at vacuum pressure of at least 10.sup.−6 mbar and less than 10.sup.−3 mbar. The TVG device 70 also includes a pair of opposing conductive electrodes 82 and 84 defining a trigger gap 86 therebetween. The electrode 82 is connected to a stem 88 that extends through a sealed hole in the plate 76 and the electrode 84 is connected to a stem 90 that extends through a sealed hole in the plate 78, where the stems 88 and 90 provide connection for the device 70 to other switching elements. The internal surface of the insulator 74 is protected from conductive deposits by a cylindrical metallic vapor shield 92.

[0029] A pulse-triggering circuit 94 produces a sufficiently high-voltage/low-current pulse across the trigger gap 86 to initiate the plasma arc, which is then sustained for several hundred microseconds thereafter by a lower-voltage/higher-current pulse. In exemplary embodiments, the duration of the initial higher-voltage/lower-current pulse is a few microseconds and the duration of the lower-voltage/higher-current pulse is a few hundred microseconds. The geometry of the arrangement between the triggering electrode and its target surface is such that the initial pulse may be focused on a very small area on the electrode 82 so that the power density of the trigger pulse on the electrode surface is magnified and the electrical trigger energy transferred to the electrode leads to almost instantaneous vaporization of electrode material and transition of vapor into a dense plasma cloud 96 that expands towards the electrode 74 as a plasma plume and leads to electrical breakdown of the gap 106 and creation of a vacuum arc between the electrodes 82 and 84. Gap breakdown occurs based on the magnitude of the voltage differential between the electrodes 82 and 84 after the plasma cloud 96 is created. The electrode material may be chosen based on its triggering ability, i.e., its ablation ability under laser pulses, in conjunction with its vacuum arc interruption ability and dielectric strength in vacuum.

[0030] FIG. 3 is a graph with time on the horizontal axis and voltage on the vertical axis showing sensitivity to pulse-testing and hard reclosing after one-cycle of fault clearing for a permanent fault. Graph line 110 represents the voltage on one of the lines 12, 14 or 16. A fault is detected by the current and voltage measurements and the appropriate recloser switches 24, 30 or 36 is opened to clear the fault at time location 112. The voltage on the line begins to stabilize thereafter and a reclosing operation is scheduled to be performed at time location 114 to determine if the fault is still present, which it will be since the fault is permanent. Graph line 116 represents and 0.5-cycle pulse-test, graph line 118 represents and 1.5-cycle pulse-test, graph line 120 represents and 2.5-cycle hard reclose and re-open, and graph line 122 represents and 3.5-cycle hard reclose and re-open that could occur at the time location 114. Graph lines 120 and 122 show that for the traditional hard reclose, the fault is introduced back into the network causing the generator 18 to trip, which could lead to system stability. Fast pulse testing, either 0.5-cycle or 1.5-cycle shown by the graph lines 116 or 118, during the presence of a permanent fault maintains generation online, and therefore system stability indirectly, compared to hard-reclosing, either 2.5-cycle or 3.5-cycle conventional hard-reclosing followed by a re-open. In this case, the generator voltage collapses, indicating a trip-offline, for the hard-reclosing compared to the pulse testing. The graph shows that the fault is still present, and the reclose operation is not performed.

[0031] The present disclosure performs a first point-on-wave pulse test, for example, a ≤0.5 cycle pulse test, using the proper TVG device 26, 32 or 38 at the time location 114 so as to detect for the presence of the fault, but without placing significant fault current on the line. If the system is satisfied that the pulse test showed the fault is gone, then reclosing is allowed. Since pulse testing is generally benign to the network compared to hard reclosing, repetitive pulse tests may be applied, or limited by configuration, until either the fault is declared permanent and the device locks out, or the fault is determined to be no longer applied to the system whereupon the reclosing device receives a permissive to reclose. Therefore, multiple pulse tests of the same or longer duration can be performed if the first pulse test did not provide an adequate determination that the fault was or was not still present.

[0032] Since the pulse testing is low-energy, i.e., the amount of current allowed by the device to flow is substantially less than the available fault current if the fault is persistent, both the obvious and latent damage caused by let-through current in transformers and circuit breakers is avoided. Pulse testing should occur within the critical reclosing interval in order to create as small a disturbance on the network as possible. However, simulation results suggest that it may be possible to de-stabilize a transmission network by pulse testing too soon in the presence of a persistent fault.

[0033] The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.