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Tab 4 – Insulation Coordination Concepts Distribution System Engineering Course – Unit 10
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Insulation Coordination Definition
IEEE - The selection of the insulation strength of equipment in relation to the voltages, which
can appear on the system for which equipment is intended and taking into account the service environment and the characteristics of the available protective devices.
IEC - Selection of the dielectric strength of equipment in relation to the operating voltages
and overvoltages which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the available preventing and protective devices.
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Overvoltages can be reduced by
Proper shielding and grounding Other methods
(tripping and closing resistors, synchronous switching etc)
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4-4
Insulation Coordination Methods
Probabilistic Method Risk of Failure concept for self-restoring insulation (air)
Deterministic Method
Maximum Voltage Stress < Minimum Insulation Strength for non self-restoring insulation (internal to equipment)
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4-5
Probabilistic Method
Insulation strength and surges are statistical variables
Insulation withstand (strength) of air is usually represented by a Normal Distribution.
The magnitude and waveshape of each lightning or switching surge depends upon many conditions.
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Probability Concepts Refresher
Mean(μ) The arithmetic sum of a sample data divided by the number of units (n) in the sample
Variance (VAR)
The sum of the squares of the deviations of the data points from the Mean (μ) value of the sample n, divided by (n-1)
Standard Deviation (σ)
The positive square root of the Variance (VAR) of a sample of data
Coefficient of Variation (COV): The Standard Deviation(σ) of a sample divided by its Mean(μ)
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0
1
2
3
4
5
6
7
8
9
10
0.8 0.9 1 1.1 1.2
0.04
0.06
Normal Distribution with a Mean = 1 for 2 values of /mean
Density function Cumulative function
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.8 0.9 1 1.1 1.2
0.04
0.06
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4-8
Switching surge distributions can be approximated by Normal Functions
0.5%
1.0%
2.0%
5.0%
10.0%
20.0%
30.0%
50.0%
60.0%
70.0%
80.0%
90.0%
95.0%
98.0%
99.0%
99.5%
40.0%
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)
pro
ba
bil
ity
of
ex
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ing
V
0 11111
Normal functions are constant slopes on normal graphs.
Slope is proportional to
Surge distribution from simulation
mean Siemen
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0.1 0.3 0.5 0.7 0.9 1.1
V (pu CFO)
Pro
ba
bil
ity
Pw
Ps
Ps x Pw x 10
Insulation Coordination: Probabilistic Method
Probability density of surges (Ps) overlaps cumulative probability of insulation withstand (Pw) Risk of flashover is a function of the area under Ps x Pw Used mainly for EHV line design using computer programs
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4-10
Gaps in Parallel
p equals the probability of flashover of one gap
The probability of not flashing over of that gap is (1- p)
The probability of n gaps not flashing over is: (1-p)n
The probability of flashing over 1 of n gaps [1-(1-p)n] Siem
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Gaps in Parallel Impact Cumulative Probability
1 vs. 100 gaps
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.8 0.9 1 1.1 1.2
Fla
sh
ove
r P
rob
ab
ilit
y
V (pu CFO)
1
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4-12
Deterministic Method
BIL
overvoltages insulation strength
V
protective margin
1
0
p r o b a b i l i t y
Maximum Voltage Stress < Minimum Insulation Strength
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4-13
Minimum Protective Margins
Surge type Oil & Paper Air
Steep front 20% 20%
Lightning 20% 15%
Switching 15% 15%
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4-14
Protective Margin
100% x1Level Protective Arrester
WithstandInsulation
Margin Protective
Example:
BIL = 750 kV Arrester P.L. = 600 kV
25%100 x1600
750 Margin
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4-15
Insulation Coordination Process
Compare
Cost of: insulation arresters
to
Benefit of: lower equipment failures higher reliability Siem
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4-16
Insulation Coordination Process
Insulation Coordination Process
Insulation Levels
Arrester Locations/Ratings
Risk of Failure
Overvoltage Events
Operating Conditions Circuit Topology
Arrester Characteristics Insulation Characteristics
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4-17
Insulation Coordination Goals
Select: standard insulation levels for equipment
• BIL • BSL • May be predetermined based on voltage level and standard equipment ratings
air clearances creepage (leakage) distances
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4-18
Transmission System BIL Ratings
Nominal SystemVoltage
(kV)
ApparatusInsulator BIL
(kV)
46 250
69 350
115 550
138 650
161 750
230 900, 1050, 1175
345 1050, 1300
500 1550, 1800
765 2050Siemen
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4-19
Typical BIL Ratings for Power Cables
Nominal Voltage (kV rms L-L) 15 25 35 69
115 138 230 345 525 765
BIL (kV peak) 95 125 150 350 550 650 1050 1300 1550 2050
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4-20
Distribution (MV) System BIL Ratings
Basic Lightning Impulse Insulation Level (BIL)
System kV Class
Distribution Class Power Class
5 kV 60 kV 75 kV 15 kV 95 kV 110 kV 25 kV 125 kV 150 kV 35 kV 150 kV 200 kV Siem
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4-21
Purpose of Surge Arresters
To limit the overvoltages to reasonable levels
To reduce the probability of equipment flashover or insulation failure from overvoltages
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Insulation Strength vs. Duration of Voltage Stress
4-22
Note that lightning overvoltages are not related to the system voltage. They are related to lightning stroke current, grounding system impedance and shielding system effectiveness.
Evaluate Margins of Protection !
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4-23
Select Surge Arresters
Determine appropriate locations Select an arrester class Choose the minimum MCOV rating Evaluate TOV capability
select a higher MCOV if necessary Calculate protective margins for substation equipment
lightning surges switching surges
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4-24
To increase protective margins
Do one or more of the following: Use an arrester with lower protective characteristics
• Lower MCOV
• Another class
Place the arrester closer to the equipment
• when separation distance causes a problem
Add arresters at additional locations
Increase insulation level (i.e. next higher BIL)
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