The document discusses insulation coordination studies for selecting insulation strength consistent with expected overvoltages. It defines insulation coordination and describes understanding insulation stresses and strength. Methods for controlling stresses include surge arresters, pre-insertion resistors, and synchronous closing control. Insulation coordination studies evaluate overvoltages from very fast transients, lightning, switching, and temporary conditions to verify protection of electrical equipment. An example application performs a lightning surge analysis of a 550kV gas-insulated substation to evaluate protective margins of equipment.

1. Insulation Coordination Studies
“The Selection of Insulation Strength”
March 25, 2014
Adam Sparacino
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2. Definition of Insulation Coordination1
• Insulation Coordination (IEEE)
– The selection of insulation strength consistent with expected
overvoltages to obtain an acceptable risk of failure.
– The procedure for insulation coordination consists of (a)
determination of the voltage stresses and (b) selection of the
insulation strength to achieve the desired probability of failure.
– The voltage stresses can be reduced by the application of surge‐
protective devices, switching device insertion resistors and controlled
closing, shield wires, improved grounding, etc.
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(1) IEEE Std 1313.1‐1996 “IEEE Standard for Insulation Coordination ‐ Definitions, Principles, and Rules.

3. Four Basic Considerations
• Understanding Insulation Stresses
• Understanding Insulation Strength
• Designing Methods for Controlling Stresses
• Designing Insulation Systems
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4. Four Basic Considerations
• Understanding Insulation Stresses
• Understanding Insulation Strength
• Designing Methods for Controlling Stresses
• Designing Insulation Systems
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5. Definition of Overvoltages
• Overvoltage
– Abnormal voltage between two points of a system that is greater than
the highest value appearing between the same two points under
normal service conditions.2
• Overvoltages are the primary “metric” for “measuring” and
“quantifying” power system transients and thus insulation
stress.
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(2) IEEE Std C62.22‐1991 ‐ IEEE Guide for the Application of Metal‐Oxide Surge Arresters for Alternating‐Current
Systems, 1991.

6. Vocabulary of Voltage
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Peak line‐ground Voltage
RMS Voltage line‐ground = (Vpeak/√2)
Peak Voltage line‐ground = VL‐L_rms√2/√3

7. Illustration of Overvoltages
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8. Four Basic Considerations
• Understanding Insulation Stresses
• Understanding Insulation Strength
• Designing Methods for Controlling Stresses
• Designing Insulation Systems
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9. Electrical Insulation
• Insulation can be expressed as a dielectric with a function to
preserve the electrical integrity of the system.
– The insulation can be “internal” (solid, liquid, or gaseous), which is
protected from the effects of atmospheric conditions (e.g.,
transformer windings, cables, gas‐insulated substations, oil circuit
breakers, etc.).
– The insulation can be “external” (in air), which is exposed to
atmospheric conditions (e.g., bushings, bus support insulators,
disconnect switches, line insulators, air itself [tower windows, phase
spacing], etc.).
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10. Insulation Strength
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Source: IEEE Std 62.22-1997, IEEE Guide for the Application of Metal-Oxide Surge Arresters for AC Systems
Typical Volt Time Curve for Insulation Withstand
Strength for Liquid Filled Transformers

11. Insulation Strength
• Example for Transformers Windings
– Normal system operating voltage
• 345 kVL‐L_RMS (1.00 p.u.)
– Maximum continuous operating voltage (MCOV)
• 362 kVL‐L_RMS (1.05 p.u.)
– Basic switching impulse insulation level (BSL)
• 745/870/975 kVL‐N_Peak
– Basic lightning impulse insulation level (BSL)
• 900/1050/1175 kVL‐N_Peak
– Chopped wave withstand (CWW)
• 1035/1205/1350 kVL‐N_Peak
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12. Frequency of Different Events
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Transients
& Surges
Power System Control
& Dynamics
milliseconds microseconds
seconds
10-20 minutes
Power
Frequency

13. Four Basic Considerations
• Understanding Insulation Stresses
• Duty and Magnitude of applied voltage
• Understanding Insulation Strength
• Ability to withstand applied stress
• Designing Methods for Controlling Stresses
• Designing Insulation Systems
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14. Potential Overvoltage Mitigation
1. Surge Arresters
– Need to be sized and located properly to “clip” overvoltages.
2. Pre‐Insertion Resistors/Inductors
– Need to be sized according to equipment being switched (only help
during breaker operation) to prevent excessive overvoltages from
being initiated.
3. Synchronous‐Close/Open Control
– Need to use independent pole operated (IPO) breakers and program
controller based on equipment being switched (only help during
breaker operation) to prevent excessive overvoltages from being
initiated.
4. Surge Capacitors
– Need to be sized and located to “slow” the front of incoming surges
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15. Four Basic Considerations
• Understanding Insulation Stresses
• Duty and Magnitude of applied voltage
• Understanding Insulation Strength
• Ability to withstand applied stress
• Designing Methods for Controlling Stresses
• Designing Insulation Systems
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16. Insulation Coordination Process
1. Specify the equipment insulation strength, the BIL and BSL of
all equipment.
2. Specify the phase‐ground and phase‐phase clearances that
should be considered.
3. Specify the need for, location, rating, and number of surge
arresters.
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17. Insulation Coordination Studies
1. Very Fast Transients (VFT) Analysis (nanoseconds time frame)
– GIS disconnected switching.
– Quantify the overvoltages throughout the substation.
– Primary intent of determining location and number of surge arresters
within the substation.
2. Lightning Surge Analysis (microseconds time frame)
– Quantify the overvoltages throughout the substation.
– Primary intent of determining location and number of surge arresters
within the substation.
3. Switching Overvoltage Analysis (milliseconds time frame)
– Quantify the overvoltages and surge arrester energy duties associated
with switching events and fault/clear operations.
– Primary intent is to verify that transient overvoltage mitigating devices
(e.g., surge arresters, pre‐insertion resistors, synchronous close control)
are adequate to protect electrical equipment.
– Capacitor, Shunt Reactor, Transformer, and Line Switching Studies.
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18. Insulation Coordination Studies (cont.)
4. Temporary Overvoltage Analysis (seconds time frame)
– Quantify the overvoltages and surge arrester energy duties as produced
by faults, resonance conditions, etc.
– Primary intent is to verify conditions that cause problems within the
system and develop the necessary mitigation.
– Fault/Clear, load rejection, ferroresonance studies.
5. Steady State Analysis (minutes to hours time frame)
– Quantify voltage during various system configurations.
– Power flow/stability studies.
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EXAMPLE APPLICATION
STUDY FOR INSULATION COORDINATION
LIGHTNING SURGE ANALYSIS

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EAST 500 kV BUS
WEST 500 kV BUS
CB CB CB CB CB CB CB CB CB
DUMMY BUS (POSITION FOR
FUTURE BREAKER)
GML00
G762W
G762E
GEB06
G752E
G752W
G3A00
B3A01
B3A00
G952E
G952W
GWB06
G962W
G962E
G972W
G972E
GLU00
G872W
BLU01
BLU00
G872E
G4A00
G772W
G772E
B4A01
B4A00
la = 30.70
lb = 25.66
lc = 21.76
la = 21.19
lb = 20.74
lc = 23.64
la = 70.62
lb = 76.69
lc = 82.77
la = 70.15
lb = 76.25
lc = 82.30
la = 26.42
lb = 25.51
lc = 24.59
la = 23.47
lb = 22.56
lc = 21.64
la = 23.47
lb = 22.56
lc = 20.64
la = 26.42
lb = 25.51
lc = 24.59
la,b,c = 8.323
la,b,c = 19.59
la = 12.47
lb = 11.55
lc = 10.64
la,b,c = 19.59
la,b,c = 8.323
la = 9.518
lb = 8.603
lc = 7.689
la,b,c = 8.323
la,b,c = 5.634
la,b,c = 5.634
la,b,c = 8.323
BML00
BML01
500 kV LINE 500 kV LINE
Refer to Figure 2 for
details of line
terminations.
Refer to Figure 2 for
details of line
terminations.
XFMR Refer to Figure 3 for
details of XFMR
terminations.
Refer to Figure 3 for
details of XFMR
terminations.
XFMR
All lengths shown in meters.

21. Example for Line/XFMR Termination
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Notes
(1) Line traps only on phase A and C for 500 kV lines. In
EMTP model, phase B has a 2.53 m section of
conductor modeled in place of line trap.
550 kV GIS
To GIS
Bay #6
Line Trap1
CCVT
Gas-to-
Air
Bushing
Surge
Arrester
500 kV Line
350 MCM
Ground Lead
(38’)
550 kV GIS
To GIS
Bay
Gas-to-Air
Bushing
Surge
Arrester
To Transformer
350 MCM
Ground
Lead (38’)

22. Approach for Evaluation the Insulation Coordination of
the 550 kV Gas‐Insulated Substation
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Step 1: A severe voltage surge was injected into the substation for various
operating configurations to screen for maximum potential overvoltages.
Step 2: The resulting overvoltages were compared to the Basic Lightning Impulse
Insulation Level (BIL) of the equipment and the protective margin1 for the
equipment was calculated.
Step 3: If overvoltages resulted in less than a 20% protective margin in the initial
screening analysis for cases with the full system in or N‐1 contingencies, a more
detailed analysis was performed to identify the protective margins resulting from a
reasonable upper bounds lightning surge based on the configuration of the
substation and connected transmission lines.
– For the detailed analysis, specific details of the transmission lines such as conductor
characteristics, shielding design, ground resistivity, keraunic level, etc. are considered to
determine a reasonable upper bounds to place on the lightning surge impinging on the
substation.
(1) Protective Margin = [ BIL / Vmaximum_peak – 1] x 100%
Screening AnalysisDetailed Analysis

23. Lightning Surge Incoming From 500 kV Line
Phase‐to‐Ground Voltage of Incoming Lightning Surge
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0
1000
2000
3000
4000
0 5 10 15 20
MLFULL_halfSRC>MLSRCA(Type 1)
Voltage(kV)
Time (us)
Peak = 3264 kV (1.2 x 2720 kV CFO)
Time-to-peak = 0.5 microseconds.
Lightning surge impinges
substation from 500 kV Line.
Lightning surge initiated at
1.0 microseconds.

24. Lightning Surge Incoming From 500 kV Line
Highest Phase‐to‐Ground Voltage Observed in GIS
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0
500
1000
1500
2000
0 5 10 15 20
MLFULLB>G752WB(Type 1)
Voltage(kV)
Time (us)
Peak overvoltage =
1109 kV.
GIS Basic Impulse Insulation Level (BIL) = 1550 kV
Protective Margin = 40%
([1550/1109 – 1] x 100%)

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EXAMPLE APPLICATION
STUDY FOR INSULATION COORDINATION
TRANSMISSION LINE SWITCHING ANALYSIS

26. Transmission Line Switching Analysis
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• Excessive Transient Overvoltages and
the Possibility of a Flashover During
Energizing or Re‐Closing
• Overvoltages Exceeding Guidelines
Used to Develop Line Clearances
Potential Equipment Concerns
Transmission line is energized
(normal energizing or re-closing).
• Synchronous‐Close Control
• Pre‐Insertion Resistors/Inductors
• Surge Arresters
• Shunt Reactors
Potential Mitigation Techniques
• Basic Switching Impulse Level (BSL)
• Probability of Flashovers
Applicable Criteria

27. Statistical Switching Methodology
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Tclose
Three poles closing
centered around closing
time (Tclose)
3 = ¼ cycle ÷ 2 = 2.08 ms
Sliding ¼ cycle window for pole
closing shifted over a half cycle
timeframe using a uniform
distribution
Each pole can close at anytime
within the ¼ cycle window centered
around the closing time (Tclose) for
each energization. Random closing
times based on a normal (Gaussian)
distribution
¼ cycle window
Source-Side Voltage
Case simulated with
200-400 energizations

28. Electro‐Geometric Line Model
Example 345 kV Transmission Line
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14.5’ 14.5’
27’
B C A
27’
54’
(24’ at midpoint)
78’
(63’ at midpoint)
Center
Line
Line Length (total) = 85 mi
Untransposed
Ground resistivity = 37 Ohm‐m
Phase Conductor:
ACSR Lapwing
2/c Bundle 18” spacing
Outside diameter = 1.504”
RDC = 0.059 Ohm/mi
Thick/Diam = 0.375
Shield Wire:
Alumoweld 7#8
Outside diameter = 0.385”
RDC = 2.40 Ohm/mi

29. Statistical Switching Overvoltage Strength Characteristics
and SOV densities of the line
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30. Statistical Distr. Of Overvoltages Along 500 kV Line with
NO Surge Arresters
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0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
1.00 1.50 2.00 2.50 3.00 3.50 4.00
Probability to Exceed Overvoltage (%)
Peak Overvoltage (Per Unit on a 500 kV Base)
Statistical Distribution of Overvoltages Along Line
Sending End
1/4 Point
1/2 Point
3/4 Point
Remote End
Example CFO
Estimated insulation
withstand for the
transmission line: CFO = 3.53
p.u., f/CFO =5%.
E2 is the value in which the
overvoltages exceed 2% of the
switching operations.
Highest overvoltage at the
remote end of the line = 2.75
p.u. (1123 kV).
98% of the overvoltages along
the line are ≤ 2.62 p.u. (1070
kV).
Statistical
distribution based on
the case‐peak
method from IEEE
Std 1313.2‐1999.

31. Statistical Distr. Of Overvoltages Along 500 kV Line with
Line End Surge Arresters
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0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
1.00 1.50 2.00 2.50 3.00 3.50 4.00
Probability to Exceed Overvoltage (%)
Peak Overvoltage (Per Unit on a 500 kV Base)
Statistical Distribution of Overvoltages Along Line
Sending End
1/4 Point
1/2 Point
3/4 Point
Remote End
Example CFO
Estimated insulation
withstand for the
transmission line: CFO = 3.53
p.u., f/CFO =5%.
E2 is the value in which the
overvoltages exceed 2% of the
switching operations.
Highest overvoltage along the
line = 2.21 p.u. (902 kV).
98% of the overvoltages along
the line are ≤ 2.16 p.u. (882
kV).
Statistical
distribution based on
the case‐peak
method from IEEE
Std 1313.2‐1999.

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EXAMPLE APPLICATION
STUDY FOR INSULATION COORDINATION
SHUNT CAPACITOR SWITCHING ANALYSIS

33. Shunt Capacitor Switching Analysis
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• Contact Wear from Excessive Inrush
Current Duty
• Excessive Transient Overvoltages
• Induced Voltages and Currents in
Control Circuits
• Step and Touch Potentials During
Switching
Potential Equipment Concerns
Capacitor bank is energized and
transient inrush currents flow
through capacitor bank breaker
and voltage surges propagate
into the system.
• Current‐Limiting Reactors
• Synchronous‐Close Control
• Pre‐Insertion Resistors/Inductors
• Surge Arresters
Potential Mitigation Techniques
• ANSI/IEEE Inrush Current Limits
• Basic Switching Impulse Level (BSL)
• Breaker Capability Beyond Standards
• IEEE Std 80 for grounding
Applicable Criteria

34. Capacitor Bank Re‐Strike
During De‐Energization
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Current Through Switching Device Voltage on Each Side of Switching Device
Current is
interrupted
First restrike
occurs and
current is re‐
established
High frequency
current is
interrupted
Second restrike occurs and
current is re‐established
Voltage on capacitor
bank side of
switching device (DC
trapped charge)
Voltage on system
side of switching
device
Peak overvoltage
from 1st restrike
Peak overvoltage
from 2nd restrike

35. Voltage Magnification
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• When a shunt capacitor bank is energized with a nearby
capacitor at a lower voltage, the potential for voltage
magnification may exist when the following condition is true:
1 1 2 2
• Furthermore, when C1>>C2, and L1<<L2 the condition can be
exaggerated

36. Voltage Magnification (Cont.)
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Example 1.95 p.u. overvoltage at HV
bus when capacitor bank is switched.
Example 4.39 p.u. overvoltage at LV
bus when capacitor bank is switched.

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EXAMPLE APPLICATION
STUDY FOR INSULATION COORDINATION
SHUNT REACTOR SWITCHING ANALYSIS

38. Shunt Reactor Switching Analysis
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• Excessive Inrush Currents from
Energizing
• Transient and Temporary Overvoltages
from Resonance Conditions
• Generation of Harmonics
• Resonance from Parallel Lines
Potential Equipment Concerns
Shunt reactor is energized and
inrush current flows through the
system and circuit breaker.
• Synchronous‐Close Control
• Surge Arresters
• Appropriate Relay Settings
• Operational Limitations
Potential Mitigation Techniques
• Equipment Insulation Levels
• Voltage Sag/Dip Criteria
• Harmonic Distortion
Applicable Criteria

39. Resonance Overvoltages
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345 kV Substation
Voltage Measured on Energized Line
Line in service
(breakers closed
at both ends)
Line out of service
(breakers open at
both ends)
345 kV Substation
345 kV Substation 345 kV Substation

40. Resonance Overvoltages
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Peak overvoltage
= 2.94 p.u.
It is anticipated that the line equipment
would be capable of withstanding at
least 1.5 p.u. for 100 ms.
Line breakers open to
trip the line at 200 ms.
The shunt reactors should be tripped
within 550 ms of the line breakers
tripping to avoid excessive
overvoltages for this case.
Anticipated temporary overvoltage
(TOV) capability (1.5 p.u. for 100 ms).

41. Summary
• Insulation Coordination is the selection of insulation strength.
• Determine maximum insulation stress.
• Determine the minimum insulation strength with margin taking into
account stress reducers (surge arresters, pre‐insertion resistors,
synchronous close control, etc.) that can withstand the maximum
stress.
• Studies help in quantifying the maximum anticipated stress
and determining the rating/location of overvoltage mitigating
devices.
• A key component of insulation coordination is pairing the
correct strength to the correct stress.
• As a rule of thumb, the shorter the time the overvoltage is applied to
the insulation the greater the magnitude of overvoltage the insulation
can withstand before failure.
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THANK YOU FOR YOU ATTENTION