Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2003-08-01
2004-12-21
Nguyen, Vincent Q. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S508000, C361S076000, C361S079000, C361S080000, C361S081000
Reexamination Certificate
active
06833711
ABSTRACT:
DESCRIPTION
TECHNICAL FIELD
This invention relates generally to out-of-step (power swing) conditions on a power line, and more specifically concerns the detection of an out-of-step condition following clearance of an external fault on the line.
BACKGROUND OF THE INVENTION
In many power systems, particularly those in less developed countries, a condition known as power swing can occur, caused by various system conditions. A power swing can result in severe system disturbances, and is generally known in the art as an out-of-step condition. Basically, for a typical power system, during normal operation, the output of electric power from an electric power generator will produce an electric torque which balances the mechanical torque applied to the rotor shaft of the generator. Ideally, the electric power generator rotor runs at a constant speed, because of the balance of electric and mechanical torques. When a fault on the power system occurs, reducing the amount of power transmission from the generator, the electric torque, which normally balances the mechanical torque, will also decrease. If the mechanical power is not reduced during the time of the fault, the generator rotor will accelerate.
Referring now to
FIG. 1
, after a fault occurs, with the power output being reduced to P
F
from P
0
, the generator rotor swill start to accelerate, and the angle &dgr; between the two source generators on the line (power P is transferred between the two generators in operation of the power system) will start to increase. At the time that the fault is cleared, when the angle difference reaches &dgr;
c
, there is a decelerating torque acting on the rotor because the electric power P
C
at the angle &dgr;
C
is larger than the mechanical power input P
0
. However, because of the inertia of the rotor system of the motor, the angle &dgr; will not start to go back to &dgr;
0
immediately, but rather continue to increase to &dgr;
F
, when the energy lost during deceleration in area
10
of the power angle curve of
FIG. 1
is equal to the energy gained during the acceleration in area
12
. If &dgr;
P
is smaller than &dgr;
L
, then the system is transiently stable.
With sufficient damping, the angle difference &dgr; of the two sources eventually goes back to the original balance point of &dgr;
0
. However, in the situation where area
10
is smaller than the area
12
at the time that the angle reaches &dgr;
L
, then a further increase in the angle will result in an electric power output that is smaller than a mechanical power output, such that the generator rotor will accelerate again and &dgr; will increase beyond the point of typical operational recovery, resulting in a transiently unstable situation, which is shown in the curve of FIG.
2
. When such an unstable situation of the power system occurs, one equivalent generator rotates at a speed that is different than the other equivalent generator, which is a classic out-of-step (OOS) condition.
An out-of-step power system condition, besides providing inherent stability problems for the system, also may be evaluated by certain distance and phase overcurrent elements in the protective relay as a fault as opposed to an out-of-step condition. The protective relay will then operate to trip circuit breakers associated with the relay in response to the out-of-step condition, adding to the instability of the system. Such a response of the relay is thus undesirable.
Traditionally, such as described in U.S. Pat. No. 5,731,943, the rate of change of the positive sequence impedance (Z1) is monitored to detect an out-of-step condition; the operation of the distance protection elements are blocked if the impedance rate of change indicates an out-of-step condition rather than a fault. The positive sequence impedance measurement is used because the change of that impedance into a protection region defined by the protection elements of the protective relay is a slow process during an out-of-step condition, while the impedance moves rapidly from a load region into a protection region in the impedance plane when an actual fault occurs.
FIGS. 3A and 3B
show illustrative double binder impedance characteristics in the impedance plane used to detect an out-of-step condition and provide a blocking signal in the distance elements. In these examples, (
FIG. 3A
is exemplary), the outer protection boundary impedance element &dgr; is located inside the load region
22
. While an inner protection boundary impedance element is placed outside of the over-reaching zone
2
boundary.
Typically, in order to prevent distant elements in the protective relay from operating in response to an out-of-step condition, it is conventional to block the instantaneous zone
1
distance element and the forward direction overreaching zone
2
element used in a communication/assisted tripping scheme. The inner protection zone boundary impedance element must thus be located outside of the overreaching zone
2
region. Under certain conditions at the time of fault clearance, the positive sequence Z1 impedance measured by a distance relay may already be in a protection region. If after fault clearance occurs, the impedance does not stay between the inner and outer impedance measurement elements in the impedance plane, the conventional out-of-step logic will fail to operate and will not block the distance element of operation. This will aggravate the disturbance of a power system in an out-of-step condition.
Hence, while traditional double blinder out-of-step blocking (OSB) arrangements operate correctly under most circumstances to block the operation of distance elements during an out-of-step condition, an external multi-phase/three-phase fault which is slowly cleared may result in a failure to pick up the out-of-step blocking condition because the Z1 impedance is already within the inner impedance region of out-of-step detection logic and the impedance may not stay between the inner and outer impedance measurement elements. Hence, there is a need to be able to detect an out-of-step condition for the system condition following clearance of external faults.
SUMMARY OF THE INVENTION
Accordingly the present invention is an apparatus for use in a protective relay for detecting an out-of-step condition following clearing of an external multi-phase fault, on a power line, comprising: a circuit for detecting the presence of a multi-phase fault on the power line; a timing circuit for readying, i.e. “arming”, an out-of-step logic circuit if the multi-phase external fault remains for a preselected period of time; means for determining the positive sequence impedance on the power line; and a circuit for declaring an out-of-step condition and for blocking selected distance elements of the protective relay if the positive sequence impedance remains inside a selected impedance plane boundary of protection for a qualified period of time following clearing of the multi-phase external fault.
REFERENCES:
patent: 4825323 (1989-04-01), Wilkinson
patent: 5515227 (1996-05-01), Roberts et al.
patent: 5731943 (1998-03-01), Roberts et al.
patent: 5790418 (1998-08-01), Roberts et al.
patent: 5808845 (1998-09-01), Roberts
patent: 5883578 (1999-03-01), Roberts et al.
patent: 6369996 (2002-04-01), Bo
patent: 6518767 (2003-02-01), Roberts et al.
Hou Daqing
Mooney Joseph E.
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Nguyen Vincent Q.
Schweitzer Engineering Laboratories Inc.
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