Transient Overvoltages

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)

Sources of transient overvoltages

• Other switching transients– line energizing– faults

• Lightning• Ferroresonance

– May be sustained, not transient

Capacitor switching

A

V

Switched capacitor

Feeder impedance

Current and voltagemeasurements

Substation

HV systemThevenin equivalent

One-line diagram showing location of switched capacitor and measurement location on distribution feeder circuit

Capacitor switching

Switched capacitor

HV systemThevenin equivalent

Feeder Z

va

ia

Sinusoidal forced response(AC steady state) plus natural(transient) response

Capacitor switching

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.

capacitor

Substation

HV systemThevenin equivalent

One-line diagram showing location of switched capacitor and customer with LV capacitor which may magnify switching transient

Service transf

Load

L1 L2

• 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

Back to back capacitor switching

capacitors

Substation

HV Thevenin equivalent

Service transf

Charging one capacitor from the other through a very small inductance.Rise time on inrush current waveform is very high, with high-frequency oscillation.

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

Transformer model at high frequencies

N

A

G

N…

a

G

t

vAG(t)

t

vaG(t)

Capacitively coupled impulse due to

Interwinding capacitance of transformers

Transformer model at high frequencies

A

G

N

a

Longer pulses may be conducted around transformer

t

vaG(t)

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

2. Terms and definitions 15

Long-duration overvoltage caused by ferroresonance of transformer

Ferroresonant circuit

VL

I

E VC

VL = jL I = E - VC

VC = [1/(jC)] I =-j [1/(C)] IXC = 1/(C)

VL = XL jI = E + XC jI VL

jI

XL jI E + XC jI

Ferroresonant circuitVL = XL jI = E + XC jI

VL

jI

slope=XL

slope = XC

Ferroresonant circuitsA

C

B

C

B

A

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