9-18 D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
SERIES COMPENSATED LINES CHAPTER 9: THEORY OF OPERATION
9
9.3.2 Example
Consider relay input signals as in the Distance Elements Analysis section and assume an offset impedance of 4 Ω and ECA
and limit angles of 88° and 90°, respectively. The relay calculates the following negative-sequence quantities:
V_2 = 6.39 V ∠–159.6°; I_2 = 1.37 A ∠–68.1°; I_1 = 2.94 A ∠–144.2°
and the following signals for the directional unit of the negative-sequence directional overcurrent element:
• Forward-looking element:
S_pol = 11.87 V∠20.2°
S_op = 1.37 V∠20.2°
• Reverse-looking element:
S_pol = 11.87 V∠20.2°
S_op = 1.37 V∠−160.0°
After comparing the angles, a solid forward indication is given.
Assume further the pickup setting of 0.25 A for both forward and reverse directions, and the “Negative-sequence” mode
setting entered for the overcurrent unit of the element. The relay calculates the operating signal using the positive-
sequence restraint:
I_op = |I_2| – |I_1| / 8 = 1.003 A > 0.25 A
The overcurrent unit then picks up and the element operates in the forward direction.
9.4 Series compensated lines
9.4.1 Description
Faults on or in close vicinity of series-compensated lines can create the following problems for distance protection:
• Voltage and/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 the e steady-state overreach effect, sub-synchronous oscillations in both currents and voltages can
cause significant transient overreach
Distance protection elements of the D30 deal with the problem of voltage inversion by using 100% memory polarized
directional comparators. As the memory duration is set longer than 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 elements 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.
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