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
Substation
Feeder impedance
V
HV system
A
Thevenin
Current
and
voltage
equivalent
measurements
Switched
capacitor
One-line diagram showing location of
switched capacitor and measurement
location on distribution feeder circuit
Capacitor switching
ia Feeder Z
va
HV system
Thevenin
equivalent
Switched
capacitor
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.
Substation
Service transf
Load
HV system
L1
Thevenin
equivalent
L2
capacitor
One-line diagram showing location of
switched capacitor and customer with LV
capacitor which may magnify switching
transient
• This may require customer to install highenergy 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
Substation
HV Thevenin
equivalent
Service transf
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.
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
a
…
A
G
vAG(t)
Capacitively coupled
impulse due to
Interwinding capacitance
of transformers
G
N
vaG(t)
t
t
Transformer model at high
frequencies
A
a
vaG(t)
N
t
G
Longer pulses may be conducted
around transformer
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
Long-duration overvoltage caused
by ferroresonance of transformer
2. Terms and definitions
15
Ferroresonant circuit
I
E
VC
VL
XC = 1/(wC)
VL = XL jI = E + XC jI
VL = jwL I
= E - VC
VC = [1/(jwC)] I
=-j [1/(wC)] I
VL
XL jI
E + XC jI
jI
Ferroresonant circuit
VL = XL jI = E + XC jI
VL
slope
=XL
slope = XC
jI
Ferroresonant circuits
A
B
C
A
B
C
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