Transient Overvoltages

Sources of transient overvoltagesCapacitor switchingswitching a single capacitorswitching back-to-back capacitorstransient magnification

Other methods to improve power factor - Synchronous condenser - power electronics device (VSC/STATCOM)

Sources of transient overvoltagesOther switching transientsline energizingfaultsLightningFerroresonanceMay be sustained, not transient

Capacitor switchingAVSwitched capacitorFeeder impedanceCurrent and voltagemeasurementsSubstationHV systemThevenin equivalentOne-line diagram showing location of switched capacitor and measurement location on distribution feeder circuit

Capacitor switchingSwitched capacitorHV systemThevenin equivalentFeeder ZvaiaSinusoidal forced response(AC steady state) plus natural(transient) response

Capacitor switching

Magnification of capacitor switching transient by customer capacitorcustomers 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 customers capacitor bank will appear to magnify the oscillation locally.

capacitorSubstationHV systemThevenin equivalentOne-line diagram showing location of switched capacitor and customer with LV capacitor which may magnify switching transientService transfLoadL1L2

This may require customer to install high-energy MOV surge arrestersCustomer may install a small inductor (called a reactor) in series with the capacitor bankadded 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 switchingcapacitorsSubstationHV Thevenin equivalentService transfCharging one capacitor from the other through a very small inductance.Rise time on inrush current waveform is very high, with high-frequency oscillation.

LightningLightning surge may enter fromstrike to HV or MV (primary) line supplying customernearby strike to groundstrike to LV (secondary) line on or near customer premisestrike to customer building or other structure

Transformer model at high frequenciesNAGNaGtvAG(t)tvaG(t)Capacitively coupled impulse due to Interwinding capacitance of transformers

Transformer model at high frequenciesAGNaLonger pulses may be conducted around transformertvaG(t)

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

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

Ferroresonant circuitVLIEVCVL = jwL I = E - VCVC = [1/(jwC)] I =-j [1/(wC)] IXC = 1/(wC)VL = XL jI = E + XC jIVLjIXL jI E + XC jI

Ferroresonant circuitVL = XL jI = E + XC jIVLjIslope=XLslope = XC

Ferroresonant circuitsACBCBA

Possible effects of ferroresonanceOvervoltageAudible noiseOverheatingSustained overvoltage => arrester failureFlicker due to erratic voltageSubharmonic voltages (e.g., at 1/3 or 1/2 of normal frequency)Chaotic response