L60 Signal Processing - GE L60 Instruction Manual

OVERVIEW CHAPTER 9: THEORY OF OPERATION
Figure 9-20: Three-terminal transmission line single phase model for compensation
In some applications, the effect of shunt reactors must be taken into account. Shunt reactors can be installed in very long
lines to provide some of the charging current required by the line. This reduces the amount of charging current flowing into
the line. In this application, the setting for the line capacitance is the residual capacitance remaining after subtracting the
shunt inductive reactance from the total capacitive reactance at the power system frequency.
With respect to shunt reactors, keep in mind that the inductance (of the reactor) and capacitance (of the line) cancel each
other for the fundamental frequency only. When considering transients, an inductor is not a 'negative capacitor.' Therefore,
it is prudent to exclude the reactors from the measuring zone by subtracting the reactor current from the line CT current
and configuring the charging current compensation for the entire amount of the line capacitive current (not for the net
between the line and installed reactors). This approach is not only technically correct, but also simplifies the application by
not requiring monitoring of the status (on/off) of the reactors.
Charging current is calculated and subtracted from the line current individually per phase. Depending on the number of
terminals on the line (two or three as configured by the 87PC function), half or one-third (in case of three-terminal line) of
the net line charging current is subtracted at each terminal. For breaker-and-a-half configurations, if the
87PC SIGNAL
setting value is "Two Sources Current," the charging current is subtracted per each breaker current individually
SOURCE
and proportionally to the current flowing through each breaker.

9.1.10 L60 signal processing

As a protection method, phase-comparison is a time-domain principle. It can be logically analyzed if implemented as a set
of operations on instantaneous signals, starting at the local AC currents and received DC voltages encoding the phase
information for the remote currents, and culminating at the trip integrators to measure the coincidence time between the
operating currents. Early (and still prevailing) implementations of microprocessor-based relays are generally based on
frequency domain processing. This means that instantaneous currents and voltages are first filtered and represented by
phasors, (that is, magnitudes and angles), then trip/no-trip decisions are based upon phasors or similar aggregated values.
Successful implementations of the L60 phase comparison principle are based on instantaneous values, not phasors. There
are several reasons for using instantaneous value, the main one being the analog nature of the remote information. The
transmitted/received analog signal is an on/off binary signal that encodes the information not on signal magnitude, but
rather on timing with respect to actual continuous time. In addition, this signal is subject to impairments that cannot be
9
alleviated by means of filtering, but by manipulations on its shape. Therefore, it is logical to process the communication
signals in the phase comparison relay in the time domain, and adjust the reminder of the algorithms to follow the
instantaneous approach, not vice versa. The time domain approach follows the methods of the last generation of analog
phase comparison relays, giving a chance for equally good performance.
The L60 samples currents and voltage inputs at a rate of 64 samples per cycle. Current samples are pre-filtered using
band-pass Finite Response Filters (FIR), with a weighted average of signal samples in a selected data window, to remove
the decaying DC component and low-frequency distortions. The L60 implementation uses a data window of 1/3rd of a
cycle, resulting in an extra signal (phase) delay of approximately 1 ms.
9-28 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL