Series Compensated Lines; Memory Polarized Directional Comparators - GE D90 Plus Instruction Manual

CHAPTER 15: THEORY OF OPERATION

Series compensated lines

Memory polarized directional comparators

PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Faults on or in close vicinity to series compensated lines can create problems for distance
protection.
Voltage or current inversion can lead to false direction discrimination by directional
elements. This can potentially include both a failure to operate on a forward in-zone fault
as well as misoperation on a reverse fault. Both distance and overcurrent directional
elements can be affected.
Series-capacitors and their overvoltage protection equipment (air gaps and/or metal-
oxide varistors) have a steady-state overreaching effect on the apparent impedance seen
by the relay—a forward fault can appear much closer to the relay as compared with the
actual fault location. The apparent impedance can be shifted towards the relay by as
much as the total reactance of the series capacitors placed between the potential source
of the relay and the fault point. This extreme steady-state overreach happens during low-
current faults when the air-gaps do not flashover or the MOVs do not conduct any
significant current.
In addition to this steady-state overreach effect, sub-synchronous oscillations in both
currents and voltages can cause significant transient overreach.
Distance protection elements of the D90
using 100% memory polarized directional comparators. As the memory duration is set
longer that the slowest fault clearing time for reverse faults, it is guaranteed that the
distance element does not pick-up on reverse faults if the voltage inversion happens.
At the same time, it is guaranteed that the distance element picks up for all forward faults
regardless of any voltage inversion as long as the memory voltage is used. Before the
memory expires, the relay responds to any fault on the protected line. Stepped distance
backup zones operate after the memory voltage expires. But the backup protection
responds to distant faults that do not cause any inversion of the positive-sequence
voltage. As a result, the time-delayed stepped-distance zones are guaranteed to operate.
Distance protection elements of the D90
using a multi-input comparator approach. If the current inversion happens, then the
distance elements are secure on reverse faults because multiple conditions involving fault-
loop, negative-sequence, and zero-sequence currents and the memory voltage are
checked prior to declaring a forward fault.
On close-in forward faults beyond the series capacitors as seen from the relaying point,
the current inversion phenomenon can take place for a short period of time. The condition
cannot sustain for a long time as very high fault currents occur, causing large voltage
drops across the series capacitors and prompting the overvoltage protection of the
capacitors to operate quickly. This effectively removes the series compensation and
eliminates the current inversion. However, when the currents used by distance comparator
(fault-loop current for ground and phase distance protection, and the negative-sequence
and zero-sequence currents for ground elements) stay shifted by more than 90° from their
natural fault position determined by the user as the element characteristic angle, the
distance elements can fail to pick-up on such a forward fault for the brief period of current
inversion. This is an inherent attribute of the 100% memory polarized mho element, and
not a weakness particular to the D90
Therefore, for dependability, it is recommended to use high-set phase overcurrent
protection for direct tripping on close-in faults potentially causing current inversion, and
overreaching ground fault directional overcurrent functions (such as negative-sequence,
ground or neutral) for communication-aided schemes.
Plus
deal with the problem of voltage inversion by
Plus
deal with the problem of current inversion by
Plus
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SERIES COMPENSATED LINES
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