A Comparative Assessment of Power Swing Detection Techniques P.K. Nayak, J. Ganeswara Rao, P. Kundu,
FFT Abstract--Distance impedance,
may
relays,
which
mal-operate
due
respond to
to
power
changes swings.
in For
maintaining security, power swing blocking (PSB) function is utilized in modern distance relays to avoid unintended operation during power swing. However, if fault occurs during power swing, distance relays must operate. This paper reviews the
A.
K. Pradhan and P. Bajpai
analysis
asymmetrical
to
detect
faults)
a
fault
quickly
and
(both
symmetrical
reliably
during
and
power
swing. In [5], a new symmetrical fault detector for distance relay is proposed based on the presence of decaying DC in the current waveform using Prony method. This paper presents an overview of the behavior of the different conventional
shortcomings in the existing power swing detection techniques
power swing detection techniques, which have been used as
of distance relays. The concentric impedance characteristics
protective relays.
scheme,
the
Vcos'P,
the
decreased
resistance
and
the
superimposed current methods are the conventional techniques for power swing detection for which a comparative assessment is carried out.
II. SYSTEM DESCRIPTION In order to verify how zone-l setting of distance relay is affected by the power swing, an existing double circuit line
Index Terms-- Distance protection, power swing, symmetrical fault, single-phase fault.
OWER
P
model has been used by [6] to study power swings. The system is simulated using PSCAD/EMTDC software. In
I. INTRODUCTION system
system shown in Fig. 1 is selected for simulation. This system
faults,
line
switching,
normal conditions, power is transferred from M to N through generator
disconnection, and switching on/off large load result in
oscillations in rotor angles between different groups of
two parallel lines. The length of Line-l and Line-2 is 280 km each. Power angle 0 is the difference between the voltage angles at M and N.
generators and can result in severe power swings [l]. With ° the power angle difference close to 180 , the apparent impedance seen by a distance relay can be within the operating zone of the relay. This can be misinterpreted as a fault and cause the relays to trip unnecessarily. To ensure the stability of distance relay, PSB function is integrated in most of the modern distance relays to block the operation during pure power swing. However, if fault occurs during a power swing, this function must OmblockO and let the relay trip. When
a
power
swing
occurs,
at a relay location,
the
measurable electrical quantities such as voltage, current, apparent impedance, active power, and reactive power vary as a function of machine rotor angles. When a fault occurs, however, these quantities change abruptly. Such behaviors are used in conventional PSB devices as criteria to block or unblock the tripping. The concentric impedance characteristic schemes [2], the Vcoscp, the decreased resistance and the superimposed currents methods are conventional algorithms for power swing detection [3]. Ref. [4] provides an algorithm based on DC component of each phase current obtained from
The authors are thankful to DST, New Delhi for sponsoring the project100lIFD/63112009-2010 through which the research was conducted. The authors are with the Department of Electrical Engineering, IIT Kharagpur-721302.
[email protected]
Fig. I. Single line diagram of the study system
The distance relay at breaker Bl is considered for the study. Zone-l of the relay covers 80%. A three phase fault is created on Line-2 at point F at 0.6 s which is cleared after 0.1 s by the opening of breakers B3 and B4. Such disturbances cause a power swing or even, in severe cases, a pole slip. The ° 25 are shown in Fig.
current and voltage waveforms for 0
=
2(a) and 2(b), respectively. Fig. 2(c) shows the positive sequence apparent impedance (Z) seen by the relay at B1. The waveform in Fig. 2 indicates that as the system goes through a power swing, Z enters Zone-l at times.
the impedance locus travels in a very short time across the
5l
8
different zones. It crosses ZonefB to Zone-2 buffer in a very
-1 L-______�����__�__�__�� 2.5 2 1.5 0.5
(a) > � r--..�--�----� 6
200 0 S, '" -200
�
-�L-�____�______�______�____�
!'�[U � v y V � 1.5
0.5
2
2.5
(b)
1.5
1
0.5
2.5
2
Time (Sec) (c)
Fig. 2. (a) Current,(b) voltage,and (c) apparent impedance (Z) for a power swing with pre fault power angle () 25°. =
short time ( :::: 12 ms). The crossing time of Zone-2 to Zone-1 buffer is even faster «
10 ms). From the above observation, it
is obvious that, this scheme is not able to detect very fast power swings, because the time spent by the impedance locus in the buffers of areas between outer concentric circle and Zone-3 or areas between Zone-3 and Zone-2 is very much smaller than the delay time used in many blocking schemes which is usually between 30 ms and 2 s [7). Fig. 4 shows the impedance loci of phase-b and phase-c at relay location for a power swing during single pole open condition. Phase-a to ground fault is created at Is at the middle of the Line-l and the fault is isolated by removing phase-a at LIs, which results power swing on phase-b and
III. PERFORMANCE EVALUATION OF DIFFERENT POWER SWING DETECTION TECHNIQUES A.
phase-c. From Fig. 4, it is observed that detection of power swing during single pole open condition by this scheme is not possible, because the movement of the swing locus of phase-b
Concentric Impedance Characteristic Schemes In this scheme, PSB functions use the difference between
the rates of change of impedance to distinguish a fault from a power swing. To accomplish this differentiation, typically two
from outer concentric circle to Zone-1 is 30 ms which is faster than the delay time set by the PSB element in the concentric scheme. 300
concentric impedance characteristics are set and a timer records the duration of the impedance locus as it traverses between
the
two.
If
the
swing
crosses
the
concentric
characteristics before the set time, the relay declares the event as a fault. Otherwise, if the locus crosses the circles, the relay classifies the event as a power swing.
100
§ x
o
In order to demonstrate the effectiveness of concentric
-100
characteristics schemes to block the distance relay from
"-+--:Zb
operation during power swing and unblock if fault occurs
-200
during power swing, three different cases have been studied. Case1:
Slow and fast power swing condition, Case2: Power
swing during single pole open condition and Case3: Faults (both symmetrical and asymmetrical) during power swing. The swing loci of the apparent impedance Z seen by the relay at the location R of the study system (Fig. slow swing (fs
for both 5 Hz) is
shown in Fig.3.
-400
-300
-200
-100
0 100 R(Ohm)
300
400
500
Fig. 4. Portion of Impedance loci of phase-b and phase-c seen by the relay at location R, for fast swing. Time interval between successive points in the loci is 40ms.
A three phase fault is created at the middle of Line-lat 2 s during
power
swing
which persists for 0.3 s and the
corresponding Z and dZ/dt are shown in Fig. 5(a) and 5(b) respectively. From the figure, it is observed that after the occurrence of a three-phase fault, the apparent impedance remains nearly constant and is very close to zero. Hence, dZ/dt
]. It is claimed that, the method is able to detect slow and fast power swings
2.5
and can even detect three-phase fault during power swing [3].
Time (Sec) (e)
Fig. 5 Apparent Impedance (a) Z for three phase fault (b) dZ/dt for three phase fault (c) Z for high resistance (Rg 1000) single line to ground fault. =
B.
Magnitude of the swing-center voltage (SCV), defined as the voltage at the location of a two-source equivalent system where the voltage value is zero and the angles between the ° two sources are 180 apart, is used to distinguish faults from power swing [8]. An approximation of the SCV is Vcoscp, where V is the magnitude of locally measured voltage, and cp is the angle difference between V and the local current. The discrimination of faults from power swing is based on the fact that during power swing SCV changes continuously and during fault it remains almost constant. Therefore, the distance relay can be unblocked according to the criterion of
