Tab 2 Voltage Stresses Switching Transients

Transient Overvoltages Decay with time, usually within one or two cycles Often called surges The two most common types of transient overvoltages: - Switching Surges - Lightning Surges Industry, Inc., Power Technologies International 2-2 2017 Industry, Inc., PTI

Time Functions & Transients Transient analysis considers many types of time varying functions: - Sine & Cosine waves Power frequency Harmonic frequency Resonant frequncy - Unit Step function - Exponential functions - Surge functions Industry, Inc., Power Technologies International 2-3 2017 Industry, Inc., PTI

Switching Surges Generally contain frequencies from power frequency to tens of khz Caused by the closing or opening of switching equipment (circuit breakers, disconnectors, etc.): - Energizing of lines cables transformers reactors buses - Line re-energization or high speed reclosing - Circuit breaker transient recovery voltages (TRV) - Faults Industry, Inc., Power Technologies International 2-4 2017 Industry, Inc., PTI

The Ideal Circuit Breaker or Switch Model When closed Z = 0 Y = 0 When open Z = Y = 0 Industry, Inc., Power Technologies International 2-5 2017 Industry, Inc., PTI

Closing 120 V equipment will not conduct until metal-to-metal contact is made HV equipment will pre-strike when the dielectric strength of the gap between the contacts is less than the instantaneous applied voltage Industry, Inc., Power Technologies International 2-6 2017 Industry, Inc., PTI

Energize Capacitor Bank at Various Angles: Voltages 200 [V] 150 100 50 0-50 -100-150 0 4 8 12 16 [ms] 20 (file capclose0.pl4; x-var t) v:cap 90- v:cap 45- v:cap 00- v:sine Industry, Inc., Power Technologies International 2-7 + V 2017 Industry, Inc., PTI

Energize Capacitor Bank at Various Angles: Inrush Currents 0.700 [A] 0.525 0.350 0.175 0.000-0.175-0.350-0.525-0.700 0 4 8 12 16 [ms] 20 (file capclose0.pl4; x-var t) c:cap 90- c:cap 45- c:cap 00- Industry, Inc., Power Technologies International 2-8 + V 2017 Industry, Inc., PTI

Three-phase voltages and currents captured by event recorder 2017 Industry, Inc., PTI

Switching Surges 200 [kv] 150 100 50 0-50 -100-150 -200 - Peak of the surge 0 10 20 30 40 50 60 70 80 [ms] - Have only one (or just a few) significant peaks - Have damped high frequency (100 Hz to 10 khz) components superimposed on the power frequency voltage - Waveform from an EMTP simulation of a line energization Steady state 60 Hz 2017 Industry, Inc., PTI

probability of exceeding V Switching Surge Results Are Statistical 99.5% 99.0% 98.0% 95.0% 90.0% 80.0% 70.0% 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 5.0% 2.0% 1.0% 0.5% - A switching surge magnitude and waveshape depends upon the instant in time that the circuit breaker closes. - Computer simulations can be repeated with various closing times to obtain a statistical distribution of the overvoltages. 0 11111 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 V (pu) 2017 Industry, Inc., PTI 2-11

Closing Resistors or Reactors Some equipment uses closing resistors or reactors to reduce transients Applications - capacitor banks - EHV lines Closes 1st Closes 2nd Pre-insertion impedance Industry, Inc., Power Technologies International 2-12 2017 Industry, Inc., PTI

Analysis of a Voltage Waveform Three phase to ground and neutral voltages at the substation bus from simulation of an SLG fault on a network feeder. 35.00 [kv] 26.25 21.21 kv = 1 pu 17.50 8.75 0.00-8.75-17.50-26.25 Steady state voltages Peak of C phase switching surge from fault is 32.8 kv (1.547 pu) Fault starts at peak of A phase TOV during fault is highest on B Phase Switching surges from fault clearing Almost back to steady state voltages -35.00 0.00 0.04 0.08 0.12 0.16 [s] 0.20 (file OA1.pl4; x-var t) v:nqbusa v:nqbusb v:nqbusc v:nqbusn 2017 Industry, Inc., PTI 2-13

Saturation Curve for Transformer Flux [Webers] 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.47 Wb, 1 ka 0 10 20 30 40 50 60 70 80 90 100 Peak Current on LV Winding [ka] air core inductance 2017 Industry, Inc., PTI 2-14

Simulation of a large autotransformer energizing in a weak system V (pu) i (ka) i (ka) 2.00 1.50 1.00 0.50 0.00-0.50-1.00-1.50-2.00 0.60-0.40 0.250 0.200 0.150 0.100 0.050 0.000-0.050-0.100 V at CB CB a CB b CB c I SA Surge arrester energy >1.4 MJ 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 2017 Industry, Inc., PTI 2-15

V (pu) Simulation of a large autotransformer energizing in a weak system 2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00-0.20-0.40-0.60-0.80-1.00-1.20-1.40-1.60-1.80-2.00 V at CB 0.190 0.200 0.210 0.220 0.230 0.240 0.250 0.260 0.270 2017 Industry, Inc., PTI 2-16

Three-phase set of waveforms of inrush currents from transformer energizing simulation 5000 [A] 3750 2500 1250 0-1250 -2500-3750 -5000 0.00 0.04 0.08 0.12 0.16 [s] 0.20 (file shot0040.pl4; x-var t) c:nqbusa-1q070a c:nqbusb-1q070b c:nqbusc-1q070c 2017 Industry, Inc., PTI 2-17

The Physics of the Interrupting Process Process starts when the metallic contacts part The last contact point melts & evaporates An arc is formed between the contacts Arcs contain a high temperature ionized gas (plasma) Arcing continues until it is extinguished Industry, Inc., Power Technologies International 2-18 2017 Industry, Inc., PTI

Arc Extinguishing Arc cooling exceeds arc heating Generally occurs at a current zero A 150 100 50 0-50 -100-150 Industry, Inc., Power Technologies International 2-19 time current zero 2017 Industry, Inc., PTI

Current Chopping A 12 10 8 6 4 2 0 Industry, Inc., Power Technologies International 2-20 time current chop - normal current zero - no chop 2017 Industry, Inc., PTI

Recovery Voltage Current interruption is immediately followed by a voltage across the contacts In a very short time, the conducting gas must change into an insulating medium Industry, Inc., Power Technologies International 2-21 2017 Industry, Inc., PTI

Resistive Load Switching Relatively easy duty 150 100 50 0-50 -100-150 current system voltage / / recovery voltage time interruption at current zero / Industry, Inc., Power Technologies International 2-22 2017 Industry, Inc., PTI

Capacitive Current Switching 1.5 1 0.5 0-0.5-1 -1.5 current / capacitor voltage \ -system voltage time Current zero interruption leaves a 1 pu trapped charge on the capacitor Recovery voltage reaches 2 pu on 1 phase circuits 2.0 pu recovery voltage Industry, Inc., Power Technologies International 2-23 2017 Industry, Inc., PTI

Inductive Current Switching Relatively small currents High frequency transient recovery voltage current chopping can create very high overvoltages 1.5 1 0.5 0-0.5-1 -1.5 current / time system voltage \ reactor voltage - 2.0 pu recovery voltage Industry, Inc., Power Technologies International 2-24 2017 Industry, Inc., PTI

Fault Current Interruption CBs must safely handle very high magnitude currents CBs must open quickly to minimize the impact of the fault on the system Fault Industry, Inc., Power Technologies International 2-25 2017 Industry, Inc., PTI

Transient Recovery Voltages (TRV) 2.5 2 1.5 1 0.5 0-0.5-1 40 30 20 10 0 system voltage \ initial rate of rise \ current average rate of rise switch recovery voltage - time time crest Industry, Inc., Power Technologies International 2-26 2017 Industry, Inc., PTI