15-06-2013, 04:55 PM
Phasor measurement unit based transmission line protection scheme design
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ABSTRACT
This paper presents an adaptive transmission line protection scheme based on synchronized phasor
measurement units. This scheme uses the positive-sequence voltage and current phasors at both ends
of a transmission line to determine the parameter of the transmission line and the location of a fault
on the transmission line. This scheme can be used for the protection of both single- and double-circuit
transmission lines. This scheme is also robust against power swing conditions. A novel adaptive single
pole auto re-closer is introduced based on the proposed scheme due to its capability of differentiating
transient and permanent faults. System simulation studies show that the proposed scheme is able to
operate fast and accurately for transmission line protection.
INTRODUCTION
Transmission line protection is the most elaborate and challenging
function in power system protection. About two-thirds of faults
in power systems occur on the transmission line network. Consequently,
it has received extensive attention from researchers and
designers in the area of power system protection [1,2].
Distance relaying technique has attracted considerable attention
for the protection of transmission lines. The principle of this
technique is to measure the impedance at a fundamental frequency
between the relay location and the fault point, and to determine
whether a fault is internal or external to a protection zone based
on the measured impedance. Voltage and current data at the relay
location are used for this purpose. The measured impedance is
affected by power swing, load current, and many other factors. For
a double-circuit transmission line, the measured impedance is also
affected by the mutual coupling effect caused by the zero-sequence
current of the adjacent parallel circuit. The erroneous trip decision
mayhave a serious impact on the dynamic stability of power system
[2].
Line parameter estimator
The impedance parameters of a transmission line can be calculated
using synchronized voltage and current phasors at both ends
of the transmission line based on PMUmeasurement. The accurate
measurement of the terminal voltages and currents will allow the
accurate computation of line parameters Z, Y, Zc and . It should
be noted that measurements should be made for various load conditions
and ambient temperatures to account for the variability of
impedance parameters, especially the correlation of the resistance
to temperature [15]. This helps to calculate Z and Y adaptively and
more accurately if used on a real-time basis. Z, Y, Zc and can be
calculated as follows based on the measured currents and voltages
at the sending and receiving ends.
Fault detector and classifier
Measured currents at the ends of a transmission line are subject
to changes when a fault occurs on a transmission line. Fault
detection/classification principle may be based upon the detection
of these changes. The principle of detecting the variation of fault
before and after the fault incidence is used in this paper and a fast
and reliable fault detector/selector module is designed to detect the
fault and classify the fault type.
Fault selector module outputs are made based on the combination
of current phasors using (5)–(7). In these equations Ta, Tb,
and Tc corresponds to phases A, B and C fault classifier outputs,
respectively. If the fault classifier output signal exceeds a pick-up
threshold for three consecutive samples, a fault on the relevant
phase is detected.
Evaluation of the proposed transmission line protection
scheme
Power system model
Using an electromagnetic transient program EMTDC a twomachine
three-phase 400 kV power system has been simulated for
the analysis of the proposed transmission line protection scheme.
The one-line diagram of the studied system is shown in Fig. 16
and its parameters are shown in Table 1. The transmission line is a
double-circuit line and simulated using the frequency-dependent
model. The physical structure of the modeled transmission line is
shown in Fig. 17.
The performance of the proposed relay has been tested for different
faults in a validation set with different fault types, fault
locations, fault inception times, source impedances, and pre-fault
power flow directions. Some test results are presented in the following
subsections.