AU660302B2 - Control scheme for differential protection of electric railway feeder circuit breakers - Google Patents

Description

Our Ref: 425327 66030 P/00/011 2 Regulation 3:2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT see* a 0 Applicant(s): CC C S S

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CC C

S.C.

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Anthony Joseph Griffin 13 Bungalow Road PEAKHURST NSW 2210

AUSTRALIA

DAVIES COLLISON~ CAVE Patent Trade Mark Attorneys Level 10, 10 Barrack Street SYDNEY NSW 2000 Address for Serviaps: Invention Title: Control scheme for differential protection of electric railway feeder circuit breakers The following statement is a full description of this invention, including the best method of performing it known to me:- 5020 AMD/0526a CONTROL SCHEME FOR DIFFERENTIAL PROTECTION OF FEEDER CIRCUIT BREAKERS ON AN ELECTRIC RAILWAY The present invention relates to a device and method for controlling the protective circuitry of a feeder network, and in particular, relates to a device and method for controlling the operation of such protected circuitry by the differential comparison of electric signals at spaced apart locations on a feeder network. It will be appreciated from a reading of this specification that the invention is particularly applicable to electric railway applications.

In a small proportion of the feeders' on a normal power system, differential protection devices may be installed in addition to the normal distance, overcurrent, and inverse-definite-minimum-time relays. Commercially o available differential systems operate on the principle that, in an unfaulted feeder, what goes in at one end, comes out unchanged at the other. Any difference in magnitude or phase angle of the measured quantity between the two ends, is considered to be a fault condition. When a fault condition is detected, each relay operates to command the associated circuit breaker to open.

In electric railway traction system feeder applications, it is not possible to consider this difforence in load current between the two ends as a fault. A train must be considered as a moving load on the feeder. Both the magnitude of the load current and the electrical characteristics of the signal continuously~change as the train changes position along the feeder between the circuit 'o breakers, and depends on the size of the train and how it is driven.

The present invention seeks to overcome to disadvantages of the prior art as hereinbefore described.

The present invention in particular seeks to permit the trip settings on circuit breakers feeding iato the electrical section of a feeder network, to be adaptively altered, depending on the instantaneous load on the feeder network, whilst maintaining an adequate and a safe margin between minimum fault and maximum load conditions. That is, -2whilst conventional differential protection systems measure any spill between the physical quantities and interpret these to be a fault, the present invention seeks to provide a differential protection system, particularly applicable for electric railways, which permits a predefined variability of magnitude and/or phase angle of current and/or voltage to occur along the feeder network, prior to either altering the trip settings of the circuit breakers of the feeder network, or, commanding the opening of the circuit breakers.

In one broad form, the present invention provides a control system, comprising: at least two sensor means, to sense the magnitude and/or phase angle of the current and/or voltage of an electrical signal at spaced apart locations on an electrical feeder network; at least one circuit breaker in said feeder network; and a central processing means, including means to receive information data representative of each electrical signal sensed by each sensor means via a comnmunications link, means to correlate said information data to provide a resultant data representative of the combination and/or difference of the parameters of said electrical signals, means to compare said resultant data with a preset data precalculated as representing the parameters of a fault condition, and, means to provide at least one control signal to each circuit breaker or said feeder network via said communications link, to control the operation of said circuit breaker(s) and/or said feeder network depending upon the result .of said comparing means.

Preferably, said control signal opens said circuit breaker(s) when said resultant data falls within the range of preset data precalculated as representing the parameters of a fault condition.

Alternatively, or in addition, said control signal may contrui the trip setting of said circuit breaker(s).

Alternatively, or in addition, said control signal ac.tivates a visual and/or audible Salarm when said resultant data falls within the range of preset data precalculated as f representing the parameters of a fault condition, p:%wpdocs\amd\425327\ajc -3- Preferably, instead of tripping said circuit breaker said control signal causes said feeder network to be operated under predetermined restrictive conditions, such as altering the trip settings of said circuit breaker.

In a preferred embodiment of the invention, said correlating means receives and adds said information data from each sensor to provide said resultant data.

The communications link is preferably embodied as a conventional telephone line, an RF link or a microwave link.

The signals may be transmitted and/or processed in analog and/oi digital form.

;The present invention will become more fully understood from the following detailed description of a preferred but non-limiting embodiment thereof in connection with the accompanying drawings, wherein: Fig. 1 illustrates a schematic diagram of a feeder network having the protective circuitry of the present invention connected thereto; and, 20 Fig. 2 illustrates how a number of decision logic blocks may control the operation of each circuit breaker.

o S: As shown in Fig. 1, a feeder 1 is provided with at least one circuit breaker 2. The operation of each circuit breaker is controlled by a control signal 3, the control signal 3 being outputted from a central processing means 4. At least two sensors 5 are provided at spaced apart locations along the feeder network 1, to sense the characteristics of the electrical signals along the feeder network. For instance, the magnitude and/or phase angles of the currents and/or voltages along the feeder aetwork may be sensed. This information is transmitted by transmitters 6 and received at the central processing units 4 by receivers 7, forming the communication circuits 8. Any known means of communication of signals between the sensors and the control units may be embodied.

p:\wpdocs\amd\425327\ajc AMD/0526a 4 In Fig. 2 is illustrated, for example, how information from each sensor 5 is compared with information from a further sensor 5a, and compared by the decision logic circuitry 9 of the central processing unit 4 to provide one or more control signals 3 supplied to the circuit breaker 2.

The central processing unit 4 therefore utilises the information provided by the sensors 5 and 5a to electrically command a change in the trip setting of each of the circuit breakers, while the circuit breakers 2 are in service and carrying fault current. This control is best arranged as a fail-safe command where, in the absence of the command signal, the trip setting reverts to the lower or lowest of the possible settings. A deliberate command signal 3 is e therefore required to raise the trip setting of the circuit o breaker 2. It is anticipated that for most applications there will only be two settinga a "low" and a "high" *000 setting.

0.00 In some applications where there can be no valid intermediate load, such as an underground-type screened cable, or aerial feeder not being the railway overhead wiring, the control processing unit will be required to open the current breaker(s) which feed into the section of feeder.

The communications link provides the following

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functions. Firstly, it detects the state (open or closed) or the local circuit breaker and transmits that information to the remote circuit breaker(s) and/or to a central

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processor. It measures the physical properties of the voltage and current (numeric values) at the circuit breaker location and at selected location along the feeder and transmits that information to the remote circuit broaker(s) and/or to a central processor. The communications link may transmit the data over a conventional (public) telephone-type circuit, which may or may not be a dedicated pair.

At the receiving end of the central processor, the communications link converts the data into command signals which either select the trip setting of the circuit breaker or command one or more of the circuit breakers to open. If AMD/0526a 5 the c-nversion is done at a central processor, it may be necessary to retransmit some or all of the command signals to the circuit breakers. The communications link also provides a receiver at each circuit breaker, which, in the absence of a valid signal from the transmitters (or the central processor) give no command signal to increase the trip setting of the circuit breaker and therefore cause the circuit breaker to trip at the lowest of its pre-set settings, or, when a specific and valid command signal is received, opens the circuit breaker.

The communications link could preferably comprise discrete voice-frequency transmitter-receiver pairs', or serial-parallel transmitter-receiver pairs, modems, and similar devices which may or may not be designed to work over the public telephone network.

In use, a typical operation of the system of the present invention might be as follows.

0"4" Firstly, the local feeder current magnitude and direction is sensed by a sensor and the signal is typically

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transmitted in digital form to one or more other locations for processing at those other locations.

Additionally remote feeder current(s) (and other control information) is also received in digital form from one or more other locations.

Communications-type technology as hereinbefore described is then used to convey the feeder currents (and other control information) between locations.

To process the feeder currents (and other control information) in digital form received from the local sensor and from one or more other locations, the following steps typically occur. Firstly, the magnitude and direction of the signals are compared to determine the differential current and its direction. The magnitude and direction of the differential current is then compared with a specific pre-set permissible magnitude and direction of differential ,_cuTrent.(The pre-set permissible magnjtude and directionof the differential current will in most cases be site-specific), and when the magnitude and direction of the AMD/0526a differential current exceeds the specified pre-set permissible magnitude and direction of differential current for a period of ti.ne which is also specified and pre-set, a command is issued to the controlling circuit breakers which may, depending on the specific circumstances, trigger an alarm, change the trip setting of the controlling circuit breakers, or open the circuit breakers.

In most practical installations, the protection equipment will be duplicated at each end of a double-ended feeder. The pre-set permissible magnitude and direction of the differential current and the pre-set time delay will not necessarily be identical at each end of the feeder.

Data may be obtained from more than one location along the feeder. A typical example of this is a location where the feeder changes from cable to aerial construction, or where a cable connects to the overhead wiring. The protection rules each side of the discontinuity are different.

To achieve the desirable fail-safe characteristic, it is considered necessary to transmit more than one signal for p 0 each changeable device which is monitored, such that a change of state of the monitored device causes more than one signal to change state, and a command signal also requires more than one signal to issue commands. By way of example, the opening of a circuit breaker may be detected by a set of changeover contacts one signal will change from a binary one to a zero, while the other changes from a binary zero to a one. A valid change of state is therefore detected by change in both of these related signals, and any other 04 0.

cecmbination is to be recognised by the control scheme as an invalid combination which causes the low setting of the fault detecting device(s) to be selected on one circuit breaker, selected circuit breakers, or an all circuit breakers depending on the specifics of the installation.

In practice, in d.c. electric railway applications, not all circuit breakers can be adapted for changeable trip settings in a fail-safe manner. Most-of.the circuit breakers used on d.c. electric railways are polarised and magnetically held. The tripping of the circuit breaker is AMD/0526a 7 achieved, by the main circuit current setting up a magnetic flux which opposes the holding coil flux. By various methods, the interaction of the main circuit breaker and the holding coil flux, which causes the circuit breaker to trip, can be made to occur with less current in the main circuit.

For a.c. electric railways, the most practical method of changing the trip setting is considered to be the installation of two or more protection relays, each relay being set to a predefined pickup (and if necessary, time).

To change the trip setting of the circuit breaker, the trip setting command signal can be arranged to short circuit the current coil(s) of the lowest-set relays. When there is no command signal, the lowest-set relay picks up first to trip the circuit breaker. Whilst a valid command signal is being received, the lowest-set relay, which is not *6 P shovt-circuited, causes the circuit breaker to trip.

Because the command to open a feeder circuit breaker will cause a loss of supply and hence reduce the reliability of the electric railway, the command to open requires more validation than the command to change the trip setting.

Standard techniques exist for validating signals through communications circuits.

It will therefore be appreciated that the control V' system hereinbefore described, simultaneously monitors the feeder signals and sends a command to the circuit breakers and/or the feeder supply to control the operation of the 8:4 circuit breakers or the feeder supply. It will be 4.6t appreciated that the protection system hereinbefore described may be applied independently of any other form of protection, or may be combined with other forms of protection in a feeder network.

Numerous variations and modifications to this invention will become obvious to persons skilled in the art, and such alterations or modifications should be considered to fall within the scope of the invention as hereinbefore described and as hereinafter claimed.

Claims (9)

1. A control system, comprising: at least two sensor means, to sense the magnitude and/or phase angle of the current and/or voltage of an electrical signal at spaced apart locations on an electrical feeder network; at least one circuit breaker in said feeder network; and a central processing means, including means to receive information data representative of each electrical signal sensed by each sensor means via a communications link, means to correlate said information data to provide a resultant data representative of the combination and/or difference of the parameters of said electrical signals, means to compare said resultant data with a preset data precalculated as representing the parameters of a fault condition, and, means to provide at least one control signal to each circuit breaker or said feeder network via said communications link, to control the operation of said circuit breaker(s) and/or said feeder network depending upon the result ••oo 15 of said comparing means,

2. A control system as claimed in claim 1, wherein said control signal opens said circuit breaker(s) when said resultant data falls within the range of present data precalculated as representing the parameters of a fault condition,

3. A control system as claimed in claims 1 or 2, wherein said control signal controls the trip setting of said circuit breaker(s).

4. A control system as claimed in any one of claims 1 to 3, wherein said control signal activates a visual and/or audible alarm when said resultant data falls within the range of preset data precalculated as represtnting the parameters of a fault condition.

A control system as claimed in any one of claims 1 to 4, wherein, instead of tripping said circuit breaker said control signal causes said feeder network to be operated under predetermined restrictive conditions, such as altering the trip settings of said circuit breaker. 7/ 't

6. A control system as claimedl in any one of claims 1 to 5, wherein, said corfnlating p;%wpdos~Iatd\4252'Zbjc -9- means receives and adds said information data from each sensor to provide said resultant data.

7. A control system as claimed in any one of claims 1 to 6, wherein said communications link is embodied as a conventional telephone line, an RF link or a microwave link.

8. A control system as claimed in any one of claims I to 7, wherein said signals are transmitted and/or processed in analog and/or digital form,

9. A control system, substantially as herein described with reference to the accompanying drawings. DATED this 18th day of April, 1995 ANTHONY JOSEPH GRIFFIN *i By His Patent Attorneys DAVIES COLLISON CAVE p:%wpdocs\nmd425327\njc AMD/0526a ABSTRACT A control system having at least two sensor means, provided at spaced apart locations on an electrical feeder network, a circuit breaker in the feeder network and a central processing means. The central processing means receives information pertaining to the electrical signals sensed by each sensor means and provides at least one control signal to said circuit breaker. The processing is achieved by correlating the information data to provide a resvutant data representative of the combination and/or difference of the parameters of said electrical signals, and Scomparing the resultant data with a preset data, precalculated as represented in the parameters of the fault condition. 0*b* 0* 0**