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
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•
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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.