GE D30 Instruction Manual page 430

9.4 SERIES COMPENSATED LINES
9.4SERIES COMPENSATED LINES
Faults on or in a close vicinity of series compensated lines may create problems for distance protection:
• Voltage and/or current inversion may lead to false direction discrimination by directional elements. This may 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 may appear much closer to the
relay as compared with the actual fault location. The apparent impedance may 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 the above steady-state overreach effect; sub-synchronous oscillations in both currents and voltages may
cause significant transient overreach.
Distance protection elements of the D30 deal with the problem of voltage inversion by using 100% memory polarized direc-
tional comparators. As the memory duration is set longer that the slowest fault clearing time for reverse faults, it is guaran-
teed that the distance element would not pick-up on reverse faults should the voltage inversion happen.
At the same time, it is guaranteed that the distance elements would pick-up for all forward faults regardless of any voltage
inversion as long as the memory voltage is used. Before the memory expires the relay would respond 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 D30 deal with the problem of current inversion by using a multi-input-comparator
approach as described in the Distance Characteristics subsection. Should the current inversion happen, the distance ele-
ments 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
may take place for a short period of time. The condition cannot sustain for a long time as very high fault currents would
occur causing large voltage drops across the series capacitors and prompting the overvoltage protection of the capacitors
to operate quickly. This would effectively remove the series compensation and eliminate the current inversion. However,
when the currents used by distance comparator (fault-loop current for ground and phase distance protection, and the nega-
tive- and zero-sequence currents for ground elements) stay shifted by more than 90 degrees from their natural fault position
determined by the user as the element characteristic angle, the distance elements may 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 D30 relay.
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 nega-
tive-sequence, ground or neutral) for communication-aided schemes.
The problem of steady-state overreaching due to the negative reactance of the series capacitors may be addressed in the
D30 in a traditional way by shortening the reach of an underreaching distance elements to the net inductive reactance of
the line between the potential source and the far end busbar(s). This generic approach has two major drawbacks. First, it
leaves large portion of the line uncovered by the directly tripping distance protection. Second, it does not solve the transient
overreaching problem caused by sub-synchronous oscillations.
Therefore, the D30 offers a unique option for dynamic reach control that is effectively based on the magnitude of the current
flowing through the series capacitor bank(s). The underreaching distance functions can be set as for plain uncompensated
9
line, i.e. using the impedance of the line alone, and the relay would control an effective reach accordingly using the current
magnitude as illustrated in the figure below.
The reach is reduced sufficiently to cope with both steady-state and transient overreach phenomena. For large degrees of
compensation and small-current faults, the transient overreach may be as high as 100%. This means that fast distance pro-
tection is not achievable. The adaptive D30's mechanism would guarantee security on external faults. Overreaching ground
fault directional overcurrent functions (such as negative-sequence, ground or neutral) shall be used for dependability.
9-18 D30 Line Distance Protection System
9 THEORY OF OPERATION
9.4.1 DESCRIPTION
GE Multilin