Power Systems Lab · 18 modules · all computed in your browser
Series compensation
Series capacitor + MOV (Goldsworthy), voltage and current inversion, Z1 overreach, bus- vs line-side VT.
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Power Systems Lab · 18 modules · all computed in your browser
Series capacitor + MOV (Goldsworthy), voltage and current inversion, Z1 overreach, bus- vs line-side VT.
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500 kV series-compensated line (3φ faults)
What the distance relay sees
Apparent Z
29.9 Ω ∠ -88°
True m·XL
10.0 Ω
Relay current
26.5 kA ∠ -66° vs Es
Z1 decision
no Z1
Capacitor V / MOV
1058 kV · ipu 0.00
Equivalent Zc
0.0 − j39.8 Ω
Inversion
VOLTAGE inversion
Sub-sync. natural f
29.1 Hz
Learn it
A series capacitor cancels part of a long line's inductive reactance, which raises transfer capability and stability margins. For protection it is a nuisance: the impedance between the relay and the fault is no longer proportional to distance. If the capacitor sits between the VT and the fault, the relay sees X − Xc, so Z1 overreaches unless its reach is cut back. For a close-in fault the net reactance seen from the bus VT can go negative (voltage inversion), and if Xc exceeds the source plus line reactance to the fault, the current itself reverses (current inversion) and directional elements are fooled. A metal-oxide varistor (MOV) across the capacitor clips its voltage on large fault currents, which makes the effective compensation depend on the fault current itself.
Quiz · 0/3
1. The VT is on the bus side of a 50 % series capacitor. Z1 is set to 80 % of XL. What happens for a fault at 95 %?
2. What defines current inversion?
3. Why does a conducting MOV reduce the inversion risk?