Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2000-06-13
2001-12-11
Hoynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S094000, C700S292000, C702S079000
Reexamination Certificate
active
06330141
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to protective relays for electric power distribution systems, and in particular, to an electronic protective relay having an overcurrent trip characteristic which allows its operation to be coordinated with the trip characteristics of prior art electromechanical protective relays found in existing electric power distribution systems.
2. Background Information
Typically, the trip functions of the protective relays controlling the response of a hierarchy of circuit breakers in an electric power distribution system are coordinated so that the breaker closest to the cause of the overcurrent is tripped first to isolate the fault and limit disruption in the system. Coordination is implemented by incorporating a time delay in the trip functions of the upstream protective relays in order to provide an interval for the downstream relays to respond first. If the downstream relay does not respond or the fault is above the downstream protective relay, a trip of the upstream circuit breaker is initiated when the delay times out.
Traditionally, electromechanical relays have been used in such applications. The electromechanical relays provide a delayed trip which is an inverse function of time. A common inverse function is the I
2
t function which is a measure of heat generated by an overcurrent condition, although other inverse functions of the general form I
p
where the power P can be less than, equal to, or more than 1, are used. In response to an overcurrent above a pickup value, the electromechanical protective relays integrate the selected inverse current function over time and generate a trip signal when the integrated value reaches a selected limit. The physical characteristics of the electromechanical protective relays have the effect of introducing an additional, fixed time delay into the response to an overcurrent. They also have a delay in resetting the integrated value when the current falls below the pickup value.
Electronic protective relays are replacing electromechanical relays for these applications. Typically, the electronic protective relays utilize a microprocessor to implement the selected, or selectable inverse current function. As many systems still incorporate electromechanical protective relays, it is required that the electronic protective relays emulate the electromagnetic relays to assure effective coordination between the two types of relays. Thus, the American National Standard Institute (ANSI) and the International Electrotechnical Commission (IEC) have both established the following response curve to be implemented by the electronic relays:
T
=
D
*
[
A
(
I
I
pu
)
P
-
1
+
B
]
Equation
1
Where: T=Trip Time in seconds; D=Time Multiplier Setting; I
pu
=Pickup Current Setting; A=Variable Time Delay Setting; B=Fixed Time Delay
As can be seen from Equation 1, precise application of this relationship requires a division operation which is computationally burdensome. As a microprocessor is typically performing additional functions such as metering, it is desirable to avoid the need to implement the division function. An approximation of the above function can be generated by just summing the values of I
p
for each digital sample properly scaled and adding in the fixed time B. This will work reasonably well were the overcurrent magnitude remains fairly constant which is reasonable in some industrial applications. However, in situations where the fault current changes appreciable in magnitude, significant error results from using this technique. Such changes in fault current can occur, for instance, where the impedance of a fault varies such as when a tree limb blows against a conductor, or where there is an inappropriate response by another protective relay in the system.
There is a need, therefore, for an improved protective relay for an electric power distribution system.
More particularly, there is a need for an improved electronic protective relay which can better emulate an electromechanical relay, but without an excessive computational burden or error.
SUMMARY OF THE INVENTION
These needs and others are satisfied by the invention which is directed to an electronic protective relay for an electric power distribution system which emulates an electromechanical protective relay according to a model which includes a variable inverse current time delay and a fixed time delay. The electronic protective relay in accordance with the invention comprises current sensing means measuring current in the electric power distribution system to generate a measured current value. A processor digitally integrates the measured current using an inverse current function to generate a variable delay tally. The processor also determines a corresponding proportionate amount of the fixed time delay to generate a fixed delay tally. The processor then generates a trip output in response to a combined attainment by the variable delay tally and the fixed delay tally of a selected trip tally. In a general sense, the processor forces the variable delay tally and the fixed delay tally to track one another. In a preferred embodiment of the invention, this is implemented by means within the processor which determines at repetitive intervals which of the variable delay tally and the fixed delay tally is smaller. If the variable delay tally is smaller, variable delay means increases the variable delay tally by using the selected inverse current function and the measured current. If the fixed delay tally is smaller, fixed delay means increases the fixed delay tally by a proportionate amount. In the most preferred embodiment of the invention, the interval is one cycle of the current, so that the measured current is the current for that cycle. The processor also includes means which generate a trip signal when both the variable delay tally and the fixed delay tally are above trip values. As the fixed delay tally is made proportional to the variable delay tally, these trip values can be the same.
Also in accordance with the preferred embodiment of the invention, the tallies are only increased when the measured current is above a pickup value. A reset feature becomes active when the measured current falls below the pickup value. The preferred reset feature comprises means which reduces the variable delay tally using a selected inverse current function, and then reduces the fixed delay tally by the same amount. As an alternative, the reset means can clear both of the tallies when the measured current falls below pickup.
The invention also embraces the method of electronically emulating overcurrent protection provided for an electric power distribution system by an electromechanical protective relay which includes repetitively, at selected intervals, measuring the current to generate a measured current signal, comparing the measured current signal to a stored pickup signal, when the measured current exceeds pickup current, electronically generating from the measured current signal a variable delay tally signal using an inverse current function. The method further includes electronically generating a fixed delay tally which proportionally tracks the variable delay tally and generating a trip signal when the variable delay tally signal and the fixed delay tally signal combined reach a trip signal level. The variable delay tally is increased by the inverse current function when the variable delay tally is less than the fixed delay tally. On the other hand, the fixed delay tally is increased by a corresponding proportion of the fixed delay tally when the fixed delay tally is smaller than the variable delay tally. Preferably, the interval is one cycle of the current.
The method also includes resetting the variable delay tally signal and the fixed delay tally signal when the measured current signal falls below the pickup signal. The resetting is accomplished by reducing the variable delay time by a selected inverse function of time and reducing the fixed delay tally to track reducti
Eaton Corporation
Hoynh Kim
Moran Martin J.
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