Fault protection scheme

Details Fault protection scheme Fault protection scheme

1999-09-16

2002-04-09

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific quantity comparison means

Reexamination Certificate

active

06369996

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to improvements in apparatus for the protection of electrical power lines.
Several fault protection techniques are known for power lines (including electrical cables). They each have several common features and yet can be divided into at least two distinct groups.
In a general fault protection scheme, a fault protection circuit which includes a circuit breaker is provided in the electrical line. The breaker has a normally closed contact which connects two portions of the line when no fault is present. When a fault is detected, the contact is opened to isolate one part of the line from the other. A trip relay is provided which detects the current or voltage flowing in the line to provide an output signal. If the output signal is indicative of a fault on the line, the circuit breaker is opened. The exact construction of this fundamental building block of fault protection systems and its variants is well known and will not be described further herein.
The known fault protection schemes differ in the way in which the output signal is processed to control the opening of the circuit breaker. Two broad categories of scheme are known as non-unit protection and unit protection. Each has its own limitations.
An example of a non-unit fault protection scheme is the distance protection scheme. In this scheme a single circuit breaker protects a length of line section, for example over the ends ‘S’ and ‘R’ as shown in FIG.
1
(
a
).
A single measurement device (the “relay”) provided at a measurement point “M
1
” adjacent the circuit breaker provides an output signal indicative of the signal on the line. From the single measurement, the protected portion of line covered by the circuit breaker is divided into a number of zones, typically two zones. The first zone, zone
1
, may cover up to 80% of the line section from the circuit breaker. This is referred to as the reach point and is shown in FIG.
1
(
a
). Zone
2
is normally arranged to cover 50% of the shortest adjacent line beyond the protected line. Again this is shown in FIG.
1
(
a
). Since the impendance of a line is proportional to its length, the zone
1
can be defined using a relay capsule of measuring the impedance of the line up to a certain point. This is known in the art as a distance relay. It is designed to operate its circuit breaker only for faults occurring between the relay location and its reach point.
For a fault occurring within zone
1
, say at a point ‘F
1
’ the fault can be detected by monitoring the output signal from the relay. The circuit breaker is adapted to be opened instantly upon detecting the presence of a fault in zone
1
.
For a fault occurring within zone
2
and outside zone
1
, say at a point ‘F
2
’, the overall response time of the circuit breaker is chosen to be significantly longer. This provides enough time for an adjacent (remote) circuit breaker on the line nearer to the zone
2
fault to open, hopefully clearing the fault without the need to open the local circuit breaker. In the example, if the second circuit breaker employs a two zone protection scheme, F
2
will be within its zone
1
area causing an instant trip.
A problem with the non-unit protection technique described above is that the boundaries of the zones are not well defined, especially zone
2
.
Unit protection techniques, on the other hand, have a clear defined protection zone as shown in FIG.
1
(
b
). The boundaries are user defined by using information from the local relay and also from a remote relay at the other end of the protected section of line. The system effectively makes two point measurements at points “M
1
” and M
2
”, i.e. from each end of the protected section.
A communication link is used to transmit information about the system condition from one end of the protected section to the other. With this connection, the relays at both ends can be arranged to trip instantly to isolate the protected portion of the line when a fault occurs on the protected portion, say at “F
1
”. For a fault outside of the protected portion, the circuit breakers may both be prevented from opening, for instance for a fault at the point “F
2
”.
Whilst the unit protection scheme provides more defined zones than the non-unit protection scheme by allowing accurate location of the fault position, it is expensive to implement and the communication links are undesirable.
In a modification of the two schemes, it is known to combine the distance protection with the communication links in what is known as a “permissive undereach scheme”. In this scheme, shown schematically in FIG.
1
(
c
), a protection relay is provided at each end of a protected portion of a line. Each relay is adapted to make an instant fault trip of its associated circuit breaker for a fault occurring within a zone
1
distance along the line. This zone
1
typically defines 80% of the line section length. For example, for a fault at position “F
1
” in the Figure, both circuit breakers will be opened instantly. However, for a fault occurring outside of zone
1
at one relay, only one circuit breaker will be instantly tripped. A signal passed along a communical link between the two relays can be used to send information from the relay that has instantly tripped to the other relay. Only if an appropriate signal is received will the other circuit breaker be opened as the fault lies outside its protected zone
1
section. Again, this scheme suffers from the need for a communication link between the relays.
SUMMARY OF THE INVENTION
An object of the invention is to provide a fault protection scheme which at least partially alleviates the problems present in prior art systems incorporating communication links.
In accordance with a first aspect of our invention a section of a line is protected by providing a local fault protection assembly at one end of the section distal from at least one remote fault protection assembly at the other side of the section, each assembly comprising a circuit breaker, a line signal measurement means adapted to produce an output signal indicative of the voltage or current on the line, and a signal processing means adapted to process the output signal to produce a fault signal in the event that the output signal indicates the presence of a fault in the line, the method comprising the steps of monitoring the output signal from the signal measurement means at the local circuit breaker to detect the presence of a fault within a first zone extending over at least a part of the section from the local breaker or outside of this first zone, and in the event that the output signal is indicative of a fault within the first zone substantially instantly opening the local circuit breaker, and in the event that the fault is outside the first zone detecting the operation of the remote circuit breaker by monitoring the output signal from the local signal measurement means, monitoring the output signal to determine the condition of the section of line after the remote circuit breaker has been operated, and operating the local circuit breaker in the event that the fault condition meets a predetermined criteria.
By a part of the protected section, we mean 50% or 60% of the length of the section. It may be more than this, say 90%, or less, say 10%.
The invention therefore provides a method in which unit protection is implemented without the need for a remote communication link between the local and remote circuit breaker. This is made possible by suitable processing of the output signal.
By operation of the remote circuit breaker we may mean opening or closing of a trip contact of the remote circuit breaker provided in the line. By operating the local circuit breaker, we mean opening or closing the circuit breaker.
The method may comprise the additional step of substantially instantly opening the local circuit breaker regardless of whether a fault is detected which is within the first zone or outside the first zone. If the output signal from the signal measurement means indicates that the fault has not been cleared after the

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