Switching Over Voltages (SOV)
Temporary Over Voltage (TOV)
Presented by:
Dharshana Muthumuni
Lalin Kothalawala
Outline
The study approach to SOV investigation, using the PSCAD/EMTDC simulation tool, is discussed in
this webinar. The following topics are addressed:
• Switching over voltages and Temporary over voltages
• Power system modeling for switching studies
o System model
o Component models (transformers, breakers, shunt devices)
o Surge arresters
• Simulation of switching events
o Point-on-wave impact
o Trapped charge on lines/cables
o Line reactor impacts
• Transformer energizing transients
• Coupled line resonance examples
• PSCAD examples
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Switching Studies
Objectives of a switching study:
• Determine the over voltage levels due to switching events
• Verify equipment insulation levels will not be violated
• Verify surge arrester requirements and surge arrester ratings
• Identify potential network resonance issues
Types of studies:
• Switching frequency over voltage studies (SOV)
• Temporary over voltage studies (TOV)
• Transformer energizing
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SOV and TOV Frequency Spectrum
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Switching Over Voltages (SOV)
• Switching over voltages (SOV) result from
the operation of breakers and switches or
due to faults in a power system.
• Switching actions lead to travelling waves
on transmission lines, in addition to
initiating oscillations in local L-C elements.
• Such travelling waves and local oscillations
can appear as high frequency voltage
transients in the network. The switching
transient frequencies can reach up to a
few kHz (say 500 Hz – 2 kHz)
• Typically SOVs are well damped (due to
system losses and loads) – short duration
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Switching Over Voltages (SOV)
Travelling waves on Transmission Lines
Breaker Surge arrester
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Electromagnetic Transients in Power Systems - Characteristics
Travelling waves on Transmission Lines
1e6
Double circuit
RL RRL A
60 km G1 G2
H
V
12
TL_01_60KM TL_04_40KM TL_115KV
120 12.5
Station A Station B
C1 C2 C3
E1
TL_05_210KM
.45 12
Single circuit
290 km
E2 Tower: 3L1
TL_03_180KM
32 Conductors: chukar
Ground_Wires: 1/2_HighStrengthSteel
1e6
RRL A
Double circuit
RRL RL
0
RL
V 110 km
H
TL_02_110KM
Line travel time (approx.). 1 ms
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Electromagnetic Transients in Power Systems - Characteristics
Local lumped L-C Oscillations
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Temporary Over Voltages (TOV)
• Ferranti effect (Open end line
voltage)
• Single line to ground faults
• Load rejection
• Transformer energizing
• Parallel line resonance
TOVs => power frequency/low order
harmonics
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PSCAD Modeling Considerations
PSCAD Network Model
380 kV System example: 30743 31243
Equivalent
Voltage
Source
System model captures details up to 31260 31300 31290 31240
230 kV
around 2-3 buses from the switching Side
31226
location. 31262 31280 31223 31229
AL FADHILI 31227
Line under Study
SUDAIR 19011
11911 19010 19025 19009 19019
132 kV
Side 19008 19015
19030 19013
19006 21900 19001 19023
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PSCAD Model Development
Modelling Considerations
• System represented at least up to two buses away from point of interest
o The impact of the fast transients are limited to a local area around the station
o The transient itself is mainly influenced by the circuit elements (R-L-C) in close vicinity to where the
disturbance (e.g. breaker action/fault) occurred
• Frequency dependent transmission line models – Travelling waves and damping due to line
resistance
• Detailed transformer model including saturation data
• Shunt devices – Can influence network resonances
• Surge arrester non- linear characteristics- Main protective device limiting SOV
• Equivalent voltage source models to represent network boundaries/ generators/motors –
fast transients die out relatively fast compared to mechanical dynamics of generators can
influence SOV (in most cases)
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PSCAD Network Model
Network boundary equivalence
Model data:
• Bus voltage & angle
• Positive sequence
RRL
impedance
• Zero sequence impedance
RL
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Transmission Lines
G1 G2
9.96
Tower / Line Details C3
12.3
C5
• Geometrical arrangement of conductors
.457
14.32
• Ground clearance
Tower: TA_1
• Line sag 51.74 Conductors: ACAR
Ground Wire 1: OPGW
C1 Ground Wire 2: C4
OPGW
0
.457
14.72
Conductor Data Tower: TA_1
Conductor data
• Conductor type 40.74 Conductors: ACAR
Parameter Value C2 C6
• Radius
1 Conductor type xxx
• DC resistance 2 Outer radius (effective) 0.7025 [in]
.457
0 15.14
• Bundle data 3 DC resistance 0.0948 [ohm/mi] Tower: TA_1
• Ground wire data 4 Conductor sag 20 [ft.] 29.74 Conductors: ACAR
5 Bundle sub-conductors 2 [nos.]
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Transmission Lines
Line Transposition
1 1
I_aa11
Sect_1
a
Sect_1 Sect_1
I_aa22
1
1
Sect_2
Sect_2
Sect_2
1 1
I_bb11
Sect_3
b
Sect_3 Sect_3
I_bb22
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Cables
Cable model
• Bergeron model
o R,X,B (or Surge impedance and travel time)
• Frequency dependent model
o Based on Cable design data
Bergeron Model Options
Travel Time Interpolation: On
Reflectionless Line (ie Infinite Length): No
0 1 2
Cable # 1 Cable # 2 Cable # 3
1 1 1
Manual Entry of Y,Z
+ve Sequence R: 0.000045e-3
Conductor Conductor Conductor
Insulator 1 Insulator 1 Insulator 1 +ve Sequence XL: 0.001278e-3
Sheath Sheath Sheath
Insulator 2 Insulator 2 Insulator 2 +ve Sequence B: 1.003e-3
0 Sequence R: -estimated-
0.033 0.033 0.033
0.062 0.062 0.062 0 Sequence XL: -estimated-
0.06247 0.06247 0.06247
0.071 0.071 0.071 0 Sequence B: -estimated-
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Transformer Model
Model data
• General data
o Ratings, impedance
• Saturation data
Transformer Saturation Characteristics
1.5
1
Flux linkage (pu)
0.5
0
-100 -50 0 50 100
-0.5
-1
-1.5
Current (%)
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Shunt Devices
• Shunt reactor – with equivalent inductance (or as an non-linear inductor)
o Single phase units
o Three limbed core or five limbed core units
• Shunt capacitor – with equivalent capacitance
• Series compensation – with equivalent capacitance
𝑘𝑉 2
𝑥=
2.20 [uF]
3.19 H
120 MVAR 120 MVAR 𝑀𝑉𝐴𝑟
Reactor Cap Bank
𝑥 = ω𝐿 𝑜𝑟 1/𝜔𝐶
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Generators and Motors
Model data
• Bus voltage & angle
• Positive sequence impedance (Xd”)
• Zero sequence impedance (if available)
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Surge Arrester – A Non Linear Resistor
Model data
• Arrester rating – 360 kV
• V-I characteristic
• Energy absorption capability – 13 kJ/kV
2.6
2.2
Voltage (pu)
1.8
1.4
1.0
0 5 10 15 20 25 30 35 40 45
Current (kA)
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Arrester Non linear VI characteristics
Insulation withstand level/Voltage • Discharge voltage (protection level) is a
function of the rise time of the current
surge
• Faster surges result in a higher
discharge voltage (ex. lightning)
• The discharge voltage for a switching
System nominal voltage (peak)
surge could typically be 2% - 4% lower
than that for a comparable (current
peak) lighting surge.
• MCOV is typically 75% - 85% of the duty
cycle ‘voltage rating’.
Protective Ratio (PR) = (Insulation withstand level/Voltage at protected equipment)
Example: PR = BIL/Lightning Protective Level (LPL)
Protective Margin (PM) = (PR – 1).100
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Arrester Coordination current
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Arrester TOV Capability
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Arrester Energy Handling Capability
Provided by manufacturers as a data sheet item;
- kJ/kV (of arrester MCOV)
- kJ/kV (of arrester rating)
Arrester Energy = (V*I)* (duration of transient)
• How fast the transient gets damped out will
determine (mainly) the energy dissipation of
arrester
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Model Validation
Active Power Flow
Bus number PSCAD (MW) PSSE (MW)
Comparison with Field Data
19001-19008 625 686
19001-19009 399 379
19001-19012 324 325
19001-19024 103 121
19001-19062 757 837
19001- 434 436
18073&18003
19012-18088 647 650
19024-19008 405 409
19024-19061 210 203
19024-11924 237 238
Fault Level
Load Bus PSCAD (kA) PSSE (kA)
19001 47.7 46.9
19012 33.4 33.2
19024 48.0 46.9
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SOV Studies
Study Considerations - SOV
Simulation setup
• Point on wave impact - 100 points over a cycle
• Trapped charge on lines
• Network topology (credible scenarios)
Circuit A
Station X Station Y
Circuit B
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Study Considerations - SOV
Point on Wave impact
• Switching at different points over a 60 Hz cycle
100 points over a cycle 100 simulations
Breaker Pole Pre-Strike
1.5
0.5
Voltage (pu)
0
0 t1 t2 tx 0.01 0.02
-0.5
-1
-1.5
Time (seconds)
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Study Considerations - SOV
Point on Wave impact
• Switching at 100 different points over a 60 Hz cycle
o Multiple Run component
o Multiple Run additional recording
Meas-Enab
. Meas-Enab
Ch. 1 . V1
.
.
Ch. 2
Multiple
Ch. 3 Multiple Run
Run Addtional
Recording
Ch. 4
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Study Considerations - SOV
Trapped Charge
Simulation of trapped charge on
transmission line
• Line reactor out of service
Simulation of trapped charge on
transmission line
• Line reactor in service
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Study Considerations - SOV
Credible Scenarios
• 10 -20 different scenarios for each line
- 100 point on wave simulations for each scenario
Ex. 1) Reactors in service
2) Reactors out of service
3) Circuit B in service
4) Circuit B out of service
Circuit A
Station X Station Y
Circuit B
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SOV Results
Voltage
Surge Arrester
Energy
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SOV Results - Statistical Summary
Line switching results
• E.g. Double circuit line
o Circuit A energized from one end
o Monitor voltages at two ends and at points along the line
Voltage in kV
Closing Time E_19011 E_31227 E_A1 E_A2 E_B1 E_B2
Minimum: 0.4 441.6869422 328.437343 441.687283 648.5288508 41.0001304 53.75910884
Maximum: 0.4166 486.3028315 333.013673 486.303169 667.127313 78.50546719 84.10483459
Mean: 0.4083 469.2023583 330.033559 469.202736 656.3111978 64.56812464 73.44458982
Std Dev: 4.86E-03 12.5280621 1.01570766 12.5280421 5.347866178 12.1054595 8.943241286
2% Level: 0.398310918 443.472864 327.94755 443.473283 645.3280233 39.70655001 55.0774175
98% Level: 0.418289082 494.9318526 332.119567 494.932189 667.2943723 89.42969926 91.81176214
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TOV - Harmonic Resonance following Transformer
Energizing
Study Considerations - TOV
• Ferranti effect (Open end line voltage)
• Single line to ground faults
• Load rejection
• Transformer energizing
• Parallel line resonance
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TOV – Transformer Energizing
Transformer energizing
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TOV – Transformer Energizing
• Transformer inrush/magnetizing current - contains low order harmonics
• Network frequency scan – Parallel resonant points of network
1e6
Double circuit
60 km
RL RRL A
H
V
TL_01_60KM TL_04_40KM TL_115KV
120
Station A Station B
E1
TL_05_210KM
Single circuit
290 km
E2
TL_03_180KM
1e6
Double circuit RL
RL RRL A RRL
V 110 km
H
TL_02_110KM
Network Impedance
4000
3615 ohm
Impedance (ohm)
Z+ impedance
3000
2000
1000
0
0 120 240 360 480 600 720 840 960 1080 1200
Frequency (Hz)
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Parallel Line Resonance
TOV – Induced Voltage (on a de-energized line)
Induced voltage on an open transmission line
• Induced voltage on a de-energized line due to coupling between an energized
parallel line on the same right of way
• De-energized line may be connected/ not connected to line reactors
Circuit A
Station X Station Y
Circuit B
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TOV – Coupled Line Resonance
Induced voltage in transmission line
• Induced voltage on a de-energized line due to coupling between an energized
parallel line on the same right of way
• De-energized line is connected to line reactors
• Induced voltage due to coupled resonance can be above 1 pu
Circuit A
Station X Station Y
Circuit B
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TOV – Open Line Resonance
Open line resonance
• Transmission line with line reactors
• Resonance results when line is tripped from
both ends with reactors connected
• Reactor/current transformer over fluxing
issues
• Avoid this condition through proper
operational practices.
• Low frequency oscillations
Circuit A
Station X Station Y
Circuit B
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Breaker Stuck Pole Conditions
Without NGR With properly sized NGR
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PSCAD Examples
• Transformer energizing
o Voltage dips
o Sympathetic inrush conditions
o Harmonic resonance conditions
• Transmission line energizing
o Impact of POW
o Trapped charge
o Line reactors
• Coupled line resonance conditions (over fluxing concerns)
o Induced voltage
o Open line resonance
• Cable energizing
o Current zero miss condition
• Capacitor switching
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