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Insulation Coordination Studies
“The Selection of Insulation Strength”
March 25, 2014
Adam Sparacino
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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.
(1) IEEE Std 1313.1‐1996 “IEEE Standard for Insulation Coordination ‐ Definitions, Principles, and Rules.
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Four Basic Considerations
•
•
•
•
Understanding Insulation Stresses
Understanding Insulation Strength
Designing Methods for Controlling Stresses
Designing Insulation Systems
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Four Basic Considerations
•
•
•
•
Understanding Insulation Stresses
Understanding Insulation Strength
Designing Methods for Controlling Stresses
Designing Insulation Systems
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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.
(2) IEEE Std C62.22‐1991 ‐ IEEE Guide for the Application of Metal‐Oxide Surge Arresters for Alternating‐Current
Systems, 1991.
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Vocabulary of Voltage
Peak line‐ground Voltage
RMS Voltage line‐ground = (Vpeak/√2)
Peak Voltage line‐ground = VL‐L_rms√2/√3
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Illustration of Overvoltages
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Four Basic Considerations
•
•
•
•
Understanding Insulation Stresses
Understanding Insulation Strength
Designing Methods for Controlling Stresses
Designing Insulation Systems
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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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Insulation Strength
Typical Volt Time Curve for Insulation Withstand Strength for Liquid Filled Transformers
Source: IEEE Std 62.22-1997, IEEE Guide for the Application of Metal-Oxide Surge Arresters for AC Systems
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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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Frequency of Different Events
10-20 minutes
seconds
Power System Control
& Dynamics
Power
Frequency
milliseconds
microseconds
Transients
& Surges
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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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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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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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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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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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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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500 kV LINE
Refer to Figure 2 for
details of line
terminations.
BML00
Refer to Figure 2 for
details of line
terminations.
BLU00
BML01
All lengths shown in meters.
BLU01
la = 30.70
lb = 25.66
lc = 21.76
la = 23.47
lb = 22.56
lc = 21.64
500 kV LINE
la = 21.19
lb = 20.74
lc = 23.64
la = 23.47
lb = 22.56
lc = 20.64
WEST 500 kV BUS
GWB06
la,b,c = 8.323
la,b,c = 19.59
G752W
G952W
CB
CB
G752E
la = 26.42
lb = 25.51
lc = 24.59
la = 9.518
lb = 8.603
lc = 7.689
G762W
G962W
CB
CB
G952E
CB
G762E
G772W
la = 12.47
lb = 11.55
lc = 10.64
EAST 500 kV BUS
CB
CB
G772E
G872E
GML00
la,b,c = 8.323
GEB06
la = 70.62
lb = 76.69
lc = 82.77
G972W
G872W
G4A00
DUMMY BUS (POSITION FOR FUTURE BREAKER)
XFMR
la,b,c = 5.634
CB
CB
G962E
la,b,c = 19.59
G3A00
la,b,c = 8.323
la = 26.42
lb = 25.51
lc = 24.59
G972E
GLU00
la,b,c = 8.323
la,b,c = 5.634
la = 70.15
lb = 76.25
lc = 82.30
B3A01
B4A01
B3A00
B4A00
Refer to Figure 3 for
details of XFMR
terminations.
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XFMR
Refer to Figure 3 for
details of XFMR
terminations.
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Example for Line/XFMR Termination
500 kV Line
Line Trap1
To Transformer
Surge
Arrester
Gas-toAir
Bushing
CCVT
To GIS
Bay #6
Gas-to-Air
Bushing
550 kV GIS
350 MCM
Ground Lead
(38’)
Surge
Arrester
Notes
To GIS
Bay
550 kV GIS
(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.
350 MCM
Ground
Lead (38’)
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Detailed Analysis
Screening Analysis
Approach for Evaluation the Insulation Coordination of the 550 kV Gas‐Insulated Substation
Step 1:
A severe voltage surge was injected into the substation for
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.
–
various
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%
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Lightning Surge Incoming From 500 kV Line
Phase‐to‐Ground Voltage of Incoming Lightning Surge
MLFULL_halfSRC>MLSRCA(Type 1)
4000
Peak = 3264 kV (1.2 x 2720 kV CFO)
Time-to-peak = 0.5 microseconds.
Voltage (kV)
3000
2000
Lightning surge impinges
substation from 500 kV Line.
1000
Lightning surge initiated at
1.0 microseconds.
0
0
5
10
Time (us)
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20
23
Lightning Surge Incoming From 500 kV Line
Highest Phase‐to‐Ground Voltage Observed in GIS
MLFULLB>G752WB(Type 1)
2000
GIS Basic Impulse Insulation Level (BIL) = 1550 kV
Voltage (kV)
1500
Protective Margin = 40%
([1550/1109 – 1] x 100%)
Peak overvoltage =
1109 kV.
1000
500
0
0
5
10
Time (us)
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EXAMPLE APPLICATION
STUDY FOR INSULATION COORDINATION
TRANSMISSION LINE SWITCHING ANALYSIS
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Transmission Line Switching Analysis
Potential Equipment Concerns
• Excessive Transient Overvoltages and the Possibility of a Flashover During Energizing or Re‐Closing
• Overvoltages Exceeding Guidelines Used to Develop Line Clearances
Transmission line is energized
(normal energizing or re-closing).
Applicable Criteria
•
•
Basic Switching Impulse Level (BSL)
Probability of Flashovers
Potential Mitigation Techniques
•
•
•
•
Synchronous‐Close Control
Pre‐Insertion Resistors/Inductors
Surge Arresters
Shunt Reactors
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Statistical Switching Methodology
Source-Side Voltage
¼ cycle window
Case simulated with
200-400 energizations
3 = ¼ cycle ÷ 2 = 2.08 ms
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
Tclose
Three poles closing
centered around closing
time (Tclose)
Sliding ¼ cycle window for pole
closing shifted over a half cycle
timeframe using a uniform
distribution
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Electro‐Geometric Line Model
Example 345 kV Transmission Line
14.5’
Shield Wire:
Alumoweld 7#8
Outside diameter = 0.385”
RDC = 2.40 Ohm/mi
14.5’
78’ (63’ at midpoint)
B
C
A
27’
27’
54’
(24’ at midpoint)
Center
Line
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Phase Conductor:
ACSR Lapwing
2/c Bundle 18” spacing
Outside diameter = 1.504”
RDC = 0.059 Ohm/mi
Thick/Diam = 0.375
Line Length (total) = 85 mi Untransposed
Ground resistivity = 37 Ohm‐m
28
Statistical Switching Overvoltage Strength Characteristics and SOV densities of the line
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Statistical Distr. Of Overvoltages Along 500 kV Line with NO Surge Arresters
Statistical Distribution of Overvoltages Along Line
110%
Statistical distribution based on the case‐peak method from IEEE Std 1313.2‐1999.
Probability to Exceed Overvoltage (%)
100%
Estimated insulation withstand for the transmission line: CFO = 3.53 p.u., f/CFO =5%.
90%
80%
70%
Sending End
60%
1/4 Point
98% of the overvoltages along the line are ≤ 2.62 p.u. (1070 kV).
50%
40%
1/2 Point
3/4 Point
Highest overvoltage at the remote end of the line = 2.75 p.u. (1123 kV).
30%
20%
Remote End
Example CFO
E2 is the value in which the overvoltages exceed 2% of the switching operations.
10%
0%
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Peak Overvoltage (Per Unit on a 500 kV Base)
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Statistical Distr. Of Overvoltages Along 500 kV Line with
Line End Surge Arresters
Statistical Distribution of Overvoltages Along Line
110%
Probability to Exceed Overvoltage (%)
100%
Estimated insulation withstand for the transmission line: CFO = 3.53 p.u., f/CFO =5%.
90%
80%
Statistical distribution based on the case‐peak method from IEEE Std 1313.2‐1999.
70%
Sending End
60%
1/4 Point
98% of the overvoltages along the line are ≤ 2.16 p.u. (882 kV).
50%
1/2 Point
3/4 Point
40%
Remote End
30%
Example CFO
20%
Highest overvoltage along the line = 2.21 p.u. (902 kV).
E2 is the value in which the overvoltages exceed 2% of the switching operations.
10%
0%
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Peak Overvoltage (Per Unit on a 500 kV Base)
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EXAMPLE APPLICATION
STUDY FOR INSULATION COORDINATION
SHUNT CAPACITOR SWITCHING ANALYSIS
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Shunt Capacitor Switching Analysis
Capacitor bank is energized and
transient inrush currents flow
through capacitor bank breaker
and voltage surges propagate
into the system.
Potential Equipment Concerns
• Contact Wear from Excessive Inrush Current Duty
• Excessive Transient Overvoltages
• Induced Voltages and Currents in Control Circuits
• Step and Touch Potentials During Switching
Applicable Criteria
•
ANSI/IEEE Inrush Current Limits
•
Basic Switching Impulse Level (BSL)
•
Breaker Capability Beyond Standards
•
IEEE Std 80 for grounding
Potential Mitigation Techniques
•
Current‐Limiting Reactors
•
Synchronous‐Close Control
•
Pre‐Insertion Resistors/Inductors
•
Surge Arresters
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Capacitor Bank Re‐Strike
During De‐Energization
Current Through Switching Device
High frequency current is interrupted
First restrike occurs and current is re‐
established
Second restrike occurs and current is re‐established
Voltage on Each Side of Switching Device
Peak overvoltage from 1st restrike
Voltage on system side of switching device Current is interrupted
Voltage on capacitor bank side of switching device (DC trapped charge)
Peak overvoltage from 2nd restrike
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Voltage Magnification
• 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
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Voltage Magnification (Cont.)
Example 4.39 p.u. overvoltage at LV
bus when capacitor bank is switched.
Example 1.95 p.u. overvoltage at HV
bus when capacitor bank is switched.
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EXAMPLE APPLICATION
STUDY FOR INSULATION COORDINATION
SHUNT REACTOR SWITCHING ANALYSIS
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Shunt Reactor Switching Analysis
Potential Equipment Concerns
•
•
Shunt reactor is energized and
inrush current flows through the
system and circuit breaker.
•
•
Excessive Inrush Currents from Energizing
Transient and Temporary Overvoltages from Resonance Conditions
Generation of Harmonics
Resonance from Parallel Lines
Applicable Criteria
•
•
•
Equipment Insulation Levels
Voltage Sag/Dip Criteria
Harmonic Distortion
Potential Mitigation Techniques
•
•
•
•
Synchronous‐Close Control
Surge Arresters
Appropriate Relay Settings
Operational Limitations
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Resonance Overvoltages
345 kV Substation
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
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Resonance Overvoltages
Line breakers open to
trip the line at 200 ms.
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.
Anticipated temporary overvoltage
(TOV) capability (1.5 p.u. for 100 ms).
The shunt reactors should be tripped
within 550 ms of the line breakers
tripping to avoid excessive
overvoltages for this case.
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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
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