1. Protection Against Transient Overvoltages

2. Major causes of transient overvoltages
Capacitor switching
Lightning
Ferroresonance of transformers

3. 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.

4. 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

5. 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

6. 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

7. 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.

8. 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

9. Mitigation of Lightning
Overhead Lines
Line shielding
Line surge arresters

10. 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)

11. 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)

12. l(t)
2Vp/w
l(t)
lm(t)
Vp/w
peak
inrush
im(t)
imp

13. 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.