WO2000046896A1

WO2000046896A1 - Protection relay for a circuit breaker

Info

Publication number: WO2000046896A1
Application number: PCT/AU2000/000055
Authority: WO
Inventors: David Russell Murray
Original assignee: Nu-Lec Industries Pty Ltd
Priority date: 1999-02-02
Filing date: 2000-02-02
Publication date: 2000-08-10
Also published as: AUPP842799A0

Abstract

A protection relay (20) for a circuit breaker (10) which opens an electrical circuit (12) in response to line current in the electrical circuit exceeding a predetermined setting (48). The protection relay includes a timer (23) for accumulating time elapsed after changes in line current measurements; a current adjustment means (44) which automatically increases the current setting (48) in response to time elapsed after the line current goes to substantially zero, and automatically decreases the current setting after the line current has been restored from substantially zero for a further elapsed time; whereby the circuit breaker (20) is allowed to carry an increased load current, such as may result from loss of load diversity, during the further elapsed time period.

Description

TITLE OF THE INVENTION PROTECTION RELAY FOR A CIRCUIT BREAKER

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to a protection relay for an electrical circuit breaker. The invention relates particularly, but not exclusively, to protection relays for circuit breakers which may be employed in medium to high voltage electrical distribution circuits. Such protection relays are sometimes manufactured as stand alone products and sometimes incorporated into switchgear controllers, as in the case of automatically re-closing circuit breakers or "auto re-closers".

Discussion of the Background Art Existing protection relays for circuit breakers provide a wide variety of protection and automatic re-closing functions, including those implementing well-known inverse time protection, definite time protection and instantaneous protection schemes. An example of a prior art protection relay or tripping control device is set out in US Patent No. 4644438 assigned to Merlin Gerin.

The time-current protection characteristics of such schemes are usually the subject of published standards, such as the IEC 255 standard and the IEEE C37.112 standard. Digital electronic trip units including a microprocessor which conveniently facilitates user adjustable protection characteristics are also known, such as set out for example in US Patent No. 4351012 assigned to Westinghouse Electric Corporation.

However, many protection relays suffer from problems associated with unwanted or nuisance tripping of circuit breakers when the load supplied by the protected circuit has been disconnected for an extended period of time, typically in the order of hours. During the period of disconnection, temperature dependent loads tend to increase with elapsed time as the thermostats controlling heating and cooling devices activate. This phenomenon can be considered as a loss of load diversity. In normal circumstances thermostats or other control circuits for thermal loads would be operating in a random or diverse fashion, which tends to reduce the instantaneous load. Conventionally, manual intervention is required in order to temporarily modify the tripping characteristics of the relay in order to accommodate the increased load caused by loss of diversity. In a complex distribution system, the requirement for such intervention is inefficient and may lead to unexpected consequences for a protection scheme.

BRIEF SUMMARY OF THE INVENTION Object of the Invention

It is an object of the present invention is to provide a protection relay for a circuit breaker which ameliorates or overcomes at least some of the problems associated with the prior art.

Another object of the present invention is to provide a protection relay for a circuit breaker which automatically allows for loss of load diversity which occurs when the load supplied by a distribution circuit protected by the circuit breaker has been without supply for an extended period of time.

Further objects will be evident from the following description.

Disclosure of the Invention In one form, the invention resides in a protection relay for a circuit breaker which opens an electrical circuit in response to line current in the electrical circuit exceeding a predetermined setting, the protection relay including:

(a) a timer for accumulating time elapsed after changes in line current measurements;

(b) a current adjustment means which -

(i) automatically increases the current setting in response to time elapsed after the line current goes to substantially zero, and

(ii) automatically decreases the current setting after the line current has been restored from substantially zero for a further elapsed time;

(c) whereby the circuit breaker is allowed to carry an increased load current during the further elapsed time period.

The increased line current may result from loss of diversity of load in the electrical circuit. In preference a current adjustment means automatically increases the current setting in response to both time elapsed after the line current goes to substantially zero and a current multiplier

Preferably the current setting is increased and/or decreased as a monotonic function of time. Suitably the current setting is increased to a setting equal to the product of the current multiplier and the predetermined setting.

Preferably the current setting is decreased to a setting equal to the predetermined threshold setting.

Suitably the current setting is a threshold current setting.

In another form the invention resides in a method for adjusting a current setting in protection relay for a circuit breaker which opens an electrical circuit in response to line current exceeding a predetermined setting, said method including the steps of: (a) monitoring line current carried by the circuit breaker;

(b) automatically increasing the current setting in response to elapsed time after the line current goes to substantially zero; and

(c) automatically decreasing the current setting after the line current has been restored from substantially zero for a further elapsed time;

(d) whereby the circuit breaker is allowed to carry an increased line current during the further elapsed time period. The increased line current may result from loss of load diversity.

In preference, the current setting is automatically increased in response to both the time elapsed after the line current goes to substantially zero and a current multiplier.

Preferably the step of automatically increasing and/or the step of automatically decreasing the current setting is effected as a monotonic function of time.

Suitably the current setting is increased to a setting equal to the product of the current multiplier and the predetermined setting.

Preferably the current setting is decreased to a setting equal to the predetermined setting.

Suitably the current setting is a threshold current setting.

In further forms the invention resides in a circuit breaker including a protection relay as set out in the preceding paragraphs, along with a circuit breaker operated in accordance with the method as set out in the preceding paragraphs.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

To assist in understanding the invention preferred embodiments will now be described with reference to the following figures in which :

FIG. 1 is a block diagram of a protection arrangement for a medium voltage circuit including a circuit breaker controlled by a protection relay of a first embodiment of the present invention;

FIG. 1A is a conceptual diagram showing the relationship of the cold load function with the monitoring and protection functions of the relay; FIG. 2 is a graph showing the effect of a time multiplier on an inverse time protection curve;

FIG. 3 is a graph showing the effect of an additional time parameter on the inverse time protection curve;

FIG. 4 is a graph showing the effect of an instantaneous element on the inverse time protection curve;

FIG. 5 is a graph showing the effect of a minimum time to trip element on the inverse time protection curve;

FIG. 6 is a graph showing the effect of a maximum time element on the inverse time protection curve;

FIG. 7 is a graph showing the effect of a threshold multiplier on an inverse time protection curve; FIGS. 8A and 8B are graphs showing the interaction between instantaneous, threshold, minimum and maximum time on an inverse time protection curve;

FIG. 9A is an overview flowchart showing the cold load function;

FIG. 9B is a graph showing the operation of a cold load multiplier; and

FIGS. 10A, 10B and 10C are graphs showing the effect of the cold load multiplier on an inverse time protection curve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 , there is depicted a circuit breaker 10 which includes a vacuum interrupter 11 for opening and closing a medium voltage circuit, that is in the range of 12kV to 75kV. The circuit breaker is of a type suitable for operation with three phase circuit, thus having three lines or phases, however only a single line 12 is shown for purposes of clarity. A current transformer (CT) 13 and a capacitive voltage transformer (CVT) 14 are provided in operative relation to the high voltage line 12 in the circuit breaker 10. The vacuum interrupter is operated by an actuator 15 to open ("trip") or close the circuit. The circuit breaker 10 also houses auxiliary switches 16 and a memory circuit 17. The memory circuit holds data specific to the circuit breaker 10. The control cabling 18 associated with the circuit breaker 10 is connected to the CAPM 21 via a control cubicle entry module 19. A control cubicle 20 houses electronic systems for controlling the circuit breaker 10, which electronic systems include a control and protection module ("CAPM") 21 and external communications interfaces 22. The CAPM is centred around a micro-controller 23, which includes a microprocessor suitably a Motorola 68332 model device along with program ROM and scratch-pad RAM type memories. The microcontroller 23 is connected to the auxiliary switches 16 and a further external memory circuit 17 via a digital interface circuit 24. The microprocessor is coupled to an analogue-to-digital conversion circuit 25 which samples the respective outputs of the current transformer 13 and voltage transformer 14 via an analogue front-end circuit 26 which suitably conditions the raw analogue signals prior to conversion.

The external memory circuit 17 includes 2 MBytes of nonvolatile memory, typically in the form of a flash EEPROM, for holding configuration parameters and historical data relating to the circuit breaker.

The control and protection module 21 carries out the following functions, which are coordinated by the microprocessor under program control. The monitoring of the current and voltage transformers, calculation of RMS phase current and ground spill current in the circuit 12, protection relay functions and auto-reclose relay functions. The circuit breaker, electronics and power supplies are also monitored for correct operation.

The micro-controller 23 is also coupled to an actuator control circuit 27, which circuit controls charging of the "close" capacitor 28 and the "trip" capacitor 29 from the un-interruptible power supply ("UPS") 30. The control circuit 27 also controls the discharging of the close and trip capacitors into the circuit breaker actuator 15 for operation of the vacuum interrupter 11. The UPS 30 sources power from an internal battery 31 or an auxiliary supply 32 as required.

The CAPM 21 further includes a serial interface 33 and an on-board modem 34, typically of the V.23 type. The on-board modem 34 is coupled to a radio communications interface 35. The serial interface includes two RS232 ports, the first can be coupled to a personal computer ("PC") 36 and the second port can be coupled to a further modem 37 for communications via an alternate channel. The serial interface further includes a serial peripheral interface ("SPI") bus allowing the microprocessor to drive an operator sub-system panel 38 and a remote input/output card 39.

Upon power up, or when a circuit breaker 10 is connected via entry module 19 to the CAPM 21 , initialisation data is read from the non-volatile portion of the memory 17 in the circuit breaker. The initialisation data includes error checking codes which enable the CAPM to validate the correct operational status of the circuit breaker and displays status information included on the operator sub-system panel 38. In order to provide over-current protection, the CAPM 21 continually samples the current flowing in the secondaries of the current transformers 13 for each phase. The analogue phase currents from the CTs are converted and then summed digitally to determine the ground current for the purpose of ground fault protection. The currents are also digitally filtered to minimise problems with transient overreach. The phase currents are also summed as analogue signals to determine the spill current, which is then converted and digitally sampled for the purposes of sensitive ground fault protection ("SGF"). The spill current signal is conditioned by a low pass filter to reduce sensitivity to harmonics above 60Hz, which in turn reduces SGF sensitivity to transformer in-rush and other harmonic interference.

If any of the derived signals, phase current (for each respective phase), ground current or SGF current exceed their relevant "setting" currents, which may be entered and/or adjusted by an operator via the sub-system panel 38, the CAPM 21 logs the event which is referred to as a "pick-up". In the case of a definite time protection scheme, wherein the circuit breaker is caused to open a fixed time after pick-up, a timer begins recording elapsed time immediately. Whereas in the case of inverse time and instantaneous protection schemes, timing is initiated when the signal of interest exceeds a threshold current value. If a particular current signal should fall below 90% of its respective setting current for longer than a selected fault reset time, the relevant timer is reset and the protection sequence cancelled and re-initialised by the CAPM.

Whilst a three-phase circuit breaker is described in the preferred embodiment above, it will be appreciated that the invention also finds application in single-phase circuit breakers of voltage ratings from low to high voltage.

In the present embodiment the setting currents are set once for all circuit breaker trips in a sequence. Other protection parameters, for example inverse time curve, instantaneous or threshold multipliers and re-close times, are set separately for each trip in a re-close sequence. This allows, for example, instantaneous protection on the first trip in a sequence and inverse time protection on subsequent trips in a sequence. A variety of inverse time curves are provided which cause the circuit breaker to trip faster as the current rises. Inverse time curves may be specified on phase and ground current with separate setting currents.

Ground protection may be optionally disabled by the operator as required.

The inverse time curves, such as that shown in FIG. 2, for the protection relay functions may be implemented in software as follows.

The current signals are monitored by the microprocessor 23 after pick-up and when a current signal rises above the threshold current setting, a "time-to-trip" value is calculated to begin timing. The time-to-trip calculation is repeated periodically, typically every few milliseconds, whilst continuing to monitor the current signal. When the time to trip value reaches zero, the microprocessor 23 issues a trip request to the actuator control circuit 27 to discharge the trip capacitor 29, which in turn causes the circuit breaker actuator 15 to open the vacuum interrupter 11. If the current signal falls below the threshold current the timing stops but the protection relay does not reset. Accordingly, the relay will not cause a trip at currents below the threshold current under the inverse time protection scheme.

A series of adjustable parameters, in addition to the selection of a time-current curve and setting currents, are provided by the protection relay of the embodiment, as follows:

"time multiplier": multiplies the time-to-trip value and may be set independently for each trip, see FIG. 2;

"additional time": sets an additional time which is added to the time-to-trip value, which may also be set independently for each trip, see FIG. 3;

"instantaneous trip": an element which may be applied which will trip the circuit breaker if the current rises above a multiple of the phase or ground setting currents, see FIG. 4; • "minimum time": sets the minimum time to trip, which may be set independently for each trip, see FIG. 5;

"maximum time": sets the maximum time to trip, see FIG. 6; and "threshold multiplier": sets the multiplier applied to the setting current to obtain the threshold current (for phase current or ground current), see FIG. 7.

If the instantaneous multiplier is set below the threshold multiplier then an instantaneous trip will only occur after the line current exceeds the threshold current.

FIG. 8A illustrates the order in which the operator selected parameters may be applied to a selected inverse time curve by the processor. Preferably the order is (i) time multiplier, (ii) additional time, (iii) instantaneous element, (iv) maximum time, (v) threshold current and (vi) minimum time. FIG. 8B shows the modification of the curve of FIG. 8A using the following settings- maximum time: 2 seconds; instantaneous multiplier: x 10 and threshold current multiplier: x 2. In this example, the timing sequence will begin at twice the setting current and an instantaneous trip will occur at ten times the setting current. When a typical load supplied, for example from a substation of a power distribution utility company, has been without supply for an extended period of time, typically in the vicinity of one or more hours, the load loses its diversity. In general the longer the period without supply, the greater the loss of diversity and the higher the load current when supply is restored. The protection relay of the embodiment includes a "cold load" feature which allows for this loss of diversity automatically, allowing the circuit breaker to hold an increased load without tripping. This feature operates by timing the loss of supply to the load and increasing the threshold current accordingly.

FIG. 1A shows how the cold load function relates to other protection relay functions. Digitised line currents 40, provided by the digital to analogue conversion process 41 , are stored in a real-time database 42 by the microprocessor 23. The line currents 40 may then be accessed by the protection functions 43 and the cold load function 44 as required. When activated, the cold load function 44 supplies a threshold current multiplier 45 to the protection functions. Protection settings 46 and cold load settings 47 may be entered via the operator sub-system panel 38 for storage in a settings database 48 for access by the protection functions 43 and the cold load function 44, respectively.

The operator may set a multiplier and a time period to configure the cold load function. The phase and ground current threshold settings provided by the settings database are subject to a cold load multiplier derived from the formula below: M_CL = 1 + (TIMER L) (MCLS - 1 )) (1 )

where: M_CL is the cold load multiplier;

TIMER., is a timer which is increased while current is off and decreased while current is on, limited by 0 and t_CL; t_CL is the operator selected cold load time period; and

M_CLS is the operator selected cold load setting. By way of example, if the cold load time period t_CL is 2 hours and the cold load setting M_CLS is x2, when the current has been off for 1 hour the phase and ground setting currents are multiplied by the cold load multiplier M_CL of 1.5 to obtain revised current thresholds. Accordingly, the cold load function will increase the phase and ground current thresholds at a rate specified by the operator when the load is off - up to the limit specified. In another embodiment, the multiplier M_CL may be applied to the setting currents rather than the threshold currents. When supply is restored TIMER_! is reset and the load the current thresholds are decreased back to the levels established by the static multipliers. The protection relay suitably calculates a cold load multiplier every minute, as depicted in the flowchart in FIG. 9. It should be noted that it does not matter how the supply is lost, whether by operation of the circuit breaker 10 locally or otherwise from a failure at a substation supplying the line 12. Upon initialisation of the CAPM 21 at power up, the cold load function is placed in an idle mode on the assumption that the load is likely to be diverse.

FIG. 9A illustrates an overview of the steps in the above embodiment of the cold load function, when activated. The time delay at step 51 indicates that the function steps are executed every minute. The line current values are compared at step 52 with a minimum value (such as 2 Amps, which is considered substantially zero in this application). If line current is less than the minimum value (indicating that supply is not present) TIMER_T is incremented at step 53, to a value no greater than T_CL. Otherwise TIMER_T is decremented at step 54, to a value no lower than zero. The cold load multiplier M_CL is then recalculated 55 according to formula (1 ) above, for use in modifying threshold currents in the other protection functions. A further example of the cold load function is illustrated in

FIGS. 9B and 10, wherein the threshold current multiplier is set to x1 J , the instantaneous multiplier is set to x1.75, the cold load setting is x2 and the cold load time limit is set to 2 hours. Figure 9B shows how the current multiplier will vary according to the length of time supply is off, ie. from TO, and then with the passage of time after supply is restored at T1.

FIG. 10A shows the protection curve, as it would be at times TO and T3. FIG. 10B shows the protection curve in use when the supply is first restored and the current multiplier corresponds to 2 times the setting current. In this case an instantaneous trip will not occur until the line current exceeds twice the setting current. FIG. 10C shows the protection curve in use when the line current has been restored for 1 hour. This corresponds to a current multiplier of 1.5 times the setting current. Note that the instantaneous trip will now occur at the set value of 1.75 times the setting current. After supply has been restored for 1.8 hours, the cold load multiplier will revert to the static threshold multiplier settings and the protection curve will be as shown in FIG. 10A. Accordingly, it is apparent that the protection relay of the invention allows a circuit breaker controlled by the relay to carry an increased line current for a limited period after the restoration of supply after an extended outage. The automatic adjustment of current settings for the limited period accommodates loss of load diversity which conventionally causes nuisance tripping of circuit breakers after supply restoration.

Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features.

Claims

1. A protection relay for a circuit breaker which opens an electrical circuit in response to line current in the electrical circuit exceeding a predetermined setting, the protection relay including:

(b) a current adjustment means which -

(ii) automatically decreases the current setting after the line current has been restored from substantially zero for a further elapsed time; (c) whereby the circuit breaker is allowed to carry an increased line current during the further elapsed time period.

2. The protection relay of claim 1 wherein the increased line current results from a loss of diversity of load in the electrical circuit.

3. The protection relay of either claim 1 or claim 2 wherein the current adjustment means automatically increases the current setting in response to both the time elapsed after the line current goes to substantially zero and a current multiplier.

4. The protection relay of any one of claims 1 to 3 wherein the current setting is increased as a monotonic function of time.

5. The protection relay of either claim 3 or claim 4 wherein the current setting is increased to a setting equal to the product of the current multiplier and the predetermined setting.

6. The protection relay of any one of claims 1 to 5 wherein the current setting is decreased as a monotonic function of time.

7. The protection relay of claim 6 wherein the current setting is decreased to a setting equal to the predetermined setting.

8. The protection relay of any one of the preceding claims wherein the current setting is a threshold current setting.

9. A method for adjusting a current setting in protection relay for a circuit breaker which opens an electrical circuit in response to line current exceeding a predetermined setting, said method including the steps of:

(a) monitoring line current carried by the circuit breaker;

(d) whereby the circuit breaker is allowed to carry an increased line current during the further elapsed time period.

10. The method of claim 9 wherein the increased line current results from a loss of diversity of load in the electrical circuit.

11. The method of either claim 9 or claim 10 wherein the current setting is automatically increased in response to both the time elapsed after the line current goes to substantially zero and a current multiplier.

12. The method of any one of claims 9 to 11 wherein the step of automatically increasing the current setting is effected as a monotonic function of time.

13. The method of either claim 11 or claim 12 wherein the current setting is increased to a setting equal to the product of the current multiplier and the predetermined setting.

14. The method of any one of claims 9 to 13 wherein the step of automatically decreasing the current setting is effected as a monotonic function of time.

15. The method of claim 14 wherein the current setting is decreased to a setting equal to the predetermined setting.

16. The method of any one of claims 9 to 15 wherein the current setting is a threshold current setting.

17. A circuit breaker including a protection relay as claimed in any one of claims 1 to 8.

18. A circuit breaker with a protection relay operated in accordance with the method of any one of claims 9 to 16.