Protection Against Transient Overvoltages Major causes of transient overvoltages • Capacitor switching • Lightning • Ferroresonance of transformers Fundamental Principles of Overvoltage Protection • • • • • Limit the voltage across sensitive insulation. Divert the surge current away from the load. Block the surge current from entering the load. Bond grounds together at the equipment. Reduce, or prevent, surge current from flowing between grounds. • Create a low-pass filter using limiting and blocking principles. Mitigation of capacitor switching transients • Modify switching times (clock control) so capacitors are switched slightly before load increase expected • Use capacitor switches with – preinsertion resistors, or – synchronous closing (each pole switches at the predicted voltage zero) • Move the capacitor to a new location Distribution transformers • Ferroresonance problem is likely if -Single pole switching (fuses) – Fed from UG cables (large capacitance to ground) – Low loss transformers – Wye (ungrounded)-delta connection – Can happen on wye-wye grounded, but less likely and maybe less damaging Mitigation of ferroresonance • • • • • Prevent the open phase condition Limit the overvoltage Limit the cable length Add load if possible Change switching strategy – switch at transformer terminals instead of at cable source side • Use YY grounded transformers supplied from cables – long cable runs may need three single-phase transformer in YY grounded bank im(t) v(t) lm(t) If a capacitor is switched with a large transformer, one of the harmonic currents in the transformer inrush current may coincide with a parallel resonance (Thevenin equivalent L in parallel with C). Parallel resonant circuit is a high Z, so harmonic voltages can be large on inrush. Mitigating nuisance trips of motor drives due to transient overvoltages • Capacitor switching transient may cause motor drives to trip (overvoltage condition) • One solution is to isolate drives with ac line chokes and isolation transformers Mitigation of Lightning 2 ft 8 in • Overhead Lines 2 ft 8 in 1 ft 6 in – Line shielding – Line surge arresters 3 ft 8 in 3 ft 8 in 3 ft 1 in 2 ft 5 in 4 ft 11 in Transformer transient inrush currents • Suddenly energizing a large transformer (which is unloaded) im(t) Ll1 v(t) lm(t) l(t) Lm switch closed: v(t)=dl/dt l(t) = Ll1 im(t) + lm(t) v(t) = Vp sin(wt) l(t) = Vp sin(wt) dt = - (Vp/w) cos(wt) + K assuming the switch closes at t = 0. If l(t) = 0, then K = Vp/w l(t) = (Vp/w)[1 - cos(wt)] However, in sinusoidal steady state l(t) = -(Vp/w) cos(wt) l(t) 2Vp/w l(t) lm(t) Vp/w imp peak inrush im(t) im(t) … t Transformer transient inrush current has dc component plus even-order and odd-order harmonics that are sustained for a long time compared to most electrical transients. Initial peak current can be large.