1. Transient Overvoltages

2. Sources of transient overvoltages
Capacitor switching
switching a single capacitor
switching back-to-back capacitors
transient “magnification”
Other methods to improve power factor
- Synchronous condenser
- power electronics device (VSC/STATCOM)

3. Sources of transient overvoltages
Other switching transients
line energizing
faults
Lightning
Ferroresonance
May be sustained, not transient

4. Capacitor switching
Substation
Feeder impedance V
HV system
Thevenin
equivalent A
Current and voltage
measurements
Switched
capacitor
One-line diagram showing location of switched capacitor and measurement location on distribution feeder circuit

5. Capacitor switching ia Feeder Z va Switched capacitor HV system
Thevenin
equivalent
Sinusoidal forced response
(AC steady state) plus natural
(transient) response

6. Capacitor switching

7. Magnification of capacitor switching transient by customer capacitor
customer’s capacitor bank on LV side of service transformer will see the voltage transient produced by utility capacitor switching (on MV or HV system).
if the resonant frequency is close to the frequency produced by the utility capacitor, the customer’s capacitor bank will appear to magnify the oscillation locally.

8. Substation
Service transf
Load
HV system
Thevenin
equivalent L1 L2
capacitor
One-line diagram showing location of switched capacitor and customer with LV capacitor which may magnify switching transient

9. This may require customer to install high-energy MOV surge arresters
Customer may install a small inductor (called a reactor) in series with the capacitor bank
added advantage of improving harmonic performance (next chapter)
if a few drives are all that are affected, then the inductor can be added there

10. Back to back capacitor switching
Substation
Service transf
HV Thevenin
equivalent
capacitors
Charging one capacitor from the other through a very small inductance.
Rise time on inrush current waveform is very high, with high-frequency oscillation.

11. Lightning Lightning surge may enter from
strike to HV or MV (primary) line supplying customer
nearby strike to ground
strike to LV (secondary) line on or near customer premise
strike to customer building or other structure

12. Transformer model at high frequencies
Capacitively coupled impulse due to Interwinding capacitance of transformers A a … G G N N
vAG(t)
vaG(t) t t

13. Transformer model at high frequencies
vaG(t) N t G
Longer pulses may be conducted around transformer

14. Ferroresonance
Nonlinear resonance between unloaded transformer magnetizing branch and a series capacitor
May occur when distribution transformers fed from cables are switched one phase at a time
Especially a problem on ungrounded wye-delta transformers
May occur any time an unloaded iron-core coil is in series with a capacitor

15. Long-duration overvoltage caused by ferroresonance of transformer
2. Terms and definitions

16. Ferroresonant circuit
VL = jwL I
= E - VC VC VL E
VC = [1/(jwC)] I
=-j [1/(wC)] I
XC = 1/(wC)
XL jI
VL = XL jI = E + XC jI VL
E + XC jI jI

17. Ferroresonant circuit
VL = XL jI = E + XC jI
slope = XC
slope=XL VL jI

18. Ferroresonant circuitsB C A B C

19. Possible effects of ferroresonance
Overvoltage
Audible noise
Overheating
Sustained overvoltage => arrester failure
Flicker due to erratic voltage
Subharmonic voltages (e.g., at 1/3 or 1/2 of normal frequency)
Chaotic response