Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-09-16
2002-04-16
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S066000, C361S069000
Reexamination Certificate
active
06373670
ABSTRACT:
This invention relates to improvements in fault protection apparatus for protecting electrical power lines, and in particular to an improved time graded fault protection apparatus.
One of the most important fault protection techniques for power lines is known as Overcurrent Protection. In this technique, a circuit breaker is provided which in normal operation is closed to connect two portions of a line. A current measurement means, typically a current transformer, monitors current flowing in the line through the circuit breaker. This is often referred to as a relay. The output of the current measurement means is monitored by a fault detection means, and in the event that the output corresponds to a current indicative of a fault condition the circuit breaker is opened to isolate one portion of the line from the other.
On extended lengths of power line, more than one circuit breaker (and associated current measuring means and fault detection means) are provided. These are spaced apart along the electrical line. In this case, to ensure that only the minimum number of circuit breakers are opened to clear a fault, a time grading strategy is employed whereby each circuit breaker is adapted to be opened after a predetermined time has elapsed following detection of a fault by its associated fault detection means. The current detection means are made directional, i.e. to monitor the current flowing in each direction, and the time delays for each circuit breaker are graded according to their position relative to other circuit breakers on the line, and the direction along the line from the circuit breaker in which the fault has been detected.
A typical prior art apparatus involving five circuit breakers is shown in FIG.
1
. The circuit breakers are denoted CB
1
, CB
2
, CB
3
, CB
4
and CB
5
and are provided in series between a first end ‘R’ of a protected line and a second end ‘R’. Each circuit breaker is provided with two current measurement relays, one for each direction. The time delays for each circuit breaker, starting with that nearest the first end are T
1
, T
2
, T
3
, T
4
and T
5
were T
5
>T
4
>T
3
>T
2
>T
1
.
These time delays relate to the time which elapses after a fault has been detected at a respective circuit breaker until the breaker is opened for a fault in the direction of the first end of the line, i.e. to the left of the circuit breaker.
For a fault in the direction of the second end of the line, the delays are (starting with the circuit breaker nearest the second end of the line) T
1
′, T
2
′, T
3
′, T
4
′ and T
5
′ where T
5
′>T
4
′>T
3
′>T
2
′>T
1
′. Each circuit breaker therefore has two time delays associated with the breaker. With the exception of the central breaker CB
3
, these will be different valued.
As can be seen from
FIG. 1
, the circuit breakers nearest an end of the line open after a shorter delay for current flowing from the direction of the near end.
A problem with the prior art time grading technique arises where line lengths are long and a high number of circuit breakers are provided. In such a case, the some of the time delay may be unacceptably high. Also, the time delays are fixed in value once the apparatus is installed.
In accordance with a first aspect of our invention in a fault protection apparatus for use in a time graded fault protection scheme comprising a circuit breaker means having a normally closed trip contact provided in a line, current measurement means adapted to produce an output signal indicative of the current in the line at the circuit breaker means, and fault signal processing means adapted to process the output signal from the current measurement means to detect an initial fault on the line at a first instance, the fault signal processing means is further adapted to process the output of the current measurement means after the initial fault has been detected and to produce an accelerated opening signal in the event that the output signal meets a predetermined criteria, and control means are adapted to open the circuit breaker trip contact in response to said accelerated opening signal.
The output signal may be adapted to meet the predetermined criteria when an event on the line indicative of the operation of a remote circuit breaker means on the line occurs and the output signal after the operation remains indicative of a fault on the line. Under these conditions on accelerated opening signal may be produced and the circuit breaker may then be opened instantly. If the output signal indicates that a remote circuit breaker has opened and the fault has been cleared from the protected portion of line, no accelerated trip signal will be generated and the circuit breaker may be kept closed.
The current measurement means may comprise a forward and reverse current measurement means adapted to produce an output signal indicative of the current flowing in the line on either side of the fault protection apparatus. This allows the apparatus to decide where the fault is on the line. Two fault signal processing means may then be provided, one for each direction. A directional response to faults can then be achieved.
The fault signal processing means may be adapted to produce an accelerated opening signal if a fault condition meeting the predetermined criteria is detected by the forward current measurement means but not by the reverse opening means. If a fault is initially detected by the reverse current measurement means, a time delayed opening signal may be generated which is used by the control means to open the circuit breaker after a predetermined time delay, say 0.5 seconds.
In accordance with a second aspect of our invention in a directional time graded fault protection apparatus for an electrical line comprising at least two fault protection apparatus respectively provided at a first and second end of a protected portion of the line, each fault protection apparatus comprising at least one circuit breaker means, a forward and reverse current measurement means associated with each circuit breaker means, the forward current measurement means being adapted to measure current flowing in the line from a first direction and, the reverse current measurement means being adapted to measure current flowing in the line from the second opposite direction, control means associated with each circuit breaker, each control means being adapted to open its respective circuit breaker after a first predetermined period of time in response to an initial fault detected by processing the output of the forward current measurement means associated with the breaker and fault signal processing means associated with at least one of said circuit breakers adapted to monitor the output of the reverse current measurement means so that in the event that the output from the reverse current measurement means meets a predetermined criteria after an initial fault has been detected, the control means is adapted to open the respective circuit breaker at an accelerated time.
If the output signal meets the predetermined criteria, the fault signal processing means may be adapted to produce an accelerated opening signal which is used by the control means to open the circuit breaker.
If the output signal does not meet the predetermined criteria, the control means may cause the circuit breaker to open after a predetermined time period has elapsed since an initial fault is detected by the reverse current measurement means.
Thus, the invention provides a time graded fault protection apparatus for a line in which at least one fault protection apparatus is adapted to make an accelerated opening decision for certain fault conditions which increases speed of response. In prior art systems, the slower reacting fault protection apparatus (those at a far end from a fault) can only open the circuit breaker after their predetermined time delay has elapsed following initial fault detection.
The current measurement means may, in an alternative, be replaced by a voltage signal measurement means adapt
Alstom UK Limited
Jackson Stephen W.
Kirschstein et al.
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