Digital distance relay

Details Digital distance relay Digital distance relay

2001-06-27

2002-12-24

Riley, Shawn (Department: 2838)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific quantity comparison means

C324S525000

Reexamination Certificate

active

06498709

ABSTRACT:

FIELD OF THE INVENTION
This invention is related to a digital distance relay for measuring impedance up to the point of the fault with a fault point resistance in an AC power supply system with load current.
BACKGROUND OF THE INVENTION
Impedance of the distance relay in the prior art is set by estimating impedance measurement errors arising from fault with resistance of fault point in a power supply system with load current. At a sending terminal, tendency of so called “over-reach” is seen where reactance component of the impedance is seen smaller than actual value. At a receiving terminal, tendency of so called “under-reach” is seen where reactance component of the impedance is seen larger than actual value.
However, terminals where a distance relay is set are not fixed to the sending side or the receiving side, and impedance is usually set considering maximum of the error that can be generated under estimated system conditions.
FIG. 1
shows a digital distance relay
10
set in a power supply system. As shown in
FIG. 1
, the digital distance relay
10
receives total current “I+IL” of load current IL and fault current “I” from a transmission line
1
via a current transformer
2
, and voltage V transformed to an appropriate level via a voltage transformer
3
. The voltage V consists of a voltage drop component “R+jX” due to the impedance of the transmission line and fault point voltage VF due to fault current I/C (wherein C is a current division ratio of the fault current) through the fault point resistance RA at the fault point F, as shown in Equation (1) below:
V=R
·(
I+IL
)+
jX·
(
I+IL
)+
VF
  (1)
where VF=RA·I/C, and “j” is an imaginary unit of complex numbers. In Equation (1), the fault point voltage VF is a cause of error in impedance measurement of distance relays.
Equation (1) can be transformed to Equation (2) shown below:
V
/
(
I
+
IL
)
=
Zry
=
R
+
jX
+
(
RA
/
C
)
/
(
1
+
IL
/
I
)
_
(
2
)
because:
ZF
=
(
RA
/
C
)
/
(
1
+
IL
/
I
)
_
=
rf
+
jxf
The impedance of the underlined part of Equation (2) results in impedance measurement error as shown in FIG.
2
.
In
FIG. 2
, FB corresponds to RA/C in Equation (2), and FA corresponds to the fault impedance seen from the relay corresponding to the underlined part of Equation (2).
The ratio of the magnitudes of FA and AB is |IL/I|, and the phase difference &dgr; between FA and FB shows advanced phase of the load current relative to the fault current I. When the magnitude |IL/I| is changed with a constant phase difference &dgr;, the trace of the impedance Zry seen from the relay becomes a circle as shown in FIG.
2
. This circle is a trace of the point A where the circumference angle ∠FAB viewing a chord FB (=RA/C) is a constant of ∠FAB=&pgr;−&dgr;.
The resistance component rf and the reactance component xf of the measurement error of the impedance seen from the relay are calculated as follows:
Equation (1) is multiplied by (I+IL)*, which is a conjugate complex number of the current (I+IL) which is applied when the impedance is measured with the relay, and then the real and imaginary parts of both side are respectively equalized as shown in Equations (3) and (4) as follows:
Re[V
·(
I+IL
)*]=
R·|I+IL|
2
+Re[VF
·(
I+IL
)*]
Re[Zry]=R+Re[VF
·(
I+IL
)*]/|
I+IL|
2
  (3)
Im[V
·(
I+IL
)*]=
X·|I+IL|
2 +Im[VF
·(
I+IL
)*]
Im[Zry]=X+Im[VF
·(
I+IL
)*]/|
I+IL|
2
  (4)
The underlined parts of Equations (3) and (4) are the impedance measurement error “rf+jxf”.
Mathematically in general, Vectors A and B have the following relations:
[
A·B*]=|A|·|B
|·exp(
j
(&thgr;))
Im[A·B*]=|A|·|B
|·sin(&thgr;)
Re[A·B*]=|A|·|B
|·cos(&thgr;)
wherein “*” denotes conjugate complex number, &thgr;(=&thgr;A−&thgr;B) is an advanced phase of Vector A relative to Vector B. Considering the mathematics described above, Equations (3) and (4) show that, when the fault point voltage VF is delayed relative to the current “I+IL” which is used for impedance measurement of the relay, xf becomes negative, and the reactance Xry measured by the relay becomes smaller than the actual reactance X up to the fault point, which is an “over-reach” state. On the other hand, when the fault point voltage VF is advanced relative to the current “I+IL”, xf becomes positive, and the reactance measured by the relay becomes larger than the actual line reactance, which is an “under-reach” state.
From the explanation above, it is understood that the measurement error of the digital distance relay for measuring the impedance up to the fault point changes much depending the direction and magnitude of the load current, in case of a fault with a load current and with a fault resistance. However, in the prior art, the digital distance relay is set considering maximum measurement error calculated based on the condition of the power supply system and the estimated magnitude of the fault point resistance.
SUMMARY OF THE INVENTION
Accordingly, it is an objective of the present invention to provide a digital distance relay preventing over-reach and under-reach by detecting phase difference between the current for measuring the impedance of the digital distance relay and the current which is in a substantially same phase as the fault point current. The performance of the distance relay is adjusted on real-time basis with the change of phase difference.
According to a first aspect of the present invention, there is provided a digital distance relay for deciding whether a fault point is within a stipulated operation region by obtaining an impedance up to the fault point through a line equation of a transmission line including terms of resistances and reactances using data of voltage and current of AC power supply system periodically sampled, the relay comprising: a first means for calculating positive-phase resistance Rcal and positive-phase reactance Xcal: a second means for storing load current a stipulated time period prior to the fault was detected: a third means for detecting current in same phase as fault current flowing through the fault point: a fourth means for detecting relative phase of the current detected by the third means relative to current for directly calculating the positive-phase resistance Rcal and the positive-phase reactance Xcal: a fifth means for deciding whether the impedance is within a specified region by applying a value proportional to the relative phase detected by the fourth means: and a sixth means for deciding the fault point is within the stipulated operation region if: (a) when the load current stored by the second means indicates a sending current direction, the positive-phase resistance Rcal and positive-phase reactance Xcal are within a stipulated region based on a stipulated positive-phase resistance and a stipulated positive-phase reactance, and the impedance is decided within the specified region by the fifth means; or (b) when the load current stored by the second means indicates a receiving current direction, the positive-phase resistance Rcal and positive-phase reactance Xcal are within a stipulated region based on a stipulated positive-phase resistance and a stipulated positive-phase reactance, or the impedance is decided within the specified region by the fifth means.
By the first aspect of the present invention, when an impedance of the transmission line from the relay point up to the fault point is measured and it is decided whether the fault point is within the stipulated operation range, even if the load current is flowing and the fault point has a fault point resistance and there is an impedance measuring error, the relay oper

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