transient overvoltages. sources of transient overvoltages capacitor switching –switching a single...
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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