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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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4-7

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

ce

ed

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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4-9

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

4-11

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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