CHAPTER 4 INSULATION CO ORDINATION · 8/7/2016  · BASIC SWITCHING IMPULSE INSULATION LEVEL (BSL) BSL is the electrical strength of insulation expressed in terms of the crest value

4/15/2013

1

CHAPTER 4 : CHAPTER 4 : INSULATION COINSULATION CO‐‐ORDINATIONORDINATION

When any over voltage appears in the electrical system , then there may be a chance of failure of its insulation system.

Probability of failure of insulation, is high at the weakest insulation point nearest to the source of over voltage.

Insulators in some points are easily replaceable and repairable compared to other.

In some points are not so easily replaceable and repairable and the replacement and repairing may be highly expensive and require long interruption of power.

Therefore failure of insulator at these points may causes bigger part of electrical network to be out of service.

So it is desirable that in situation of insulator failure, only the easily replaceable and repairable insulator fails.

4/15/2013

2

To arrange the electrical insulation levels of different components in the electrical system in such a manner, that the failure of insulator, if

occurs, confides to the place where it would result in the least damage of the system, easy to repair and replace, and results least disturbance

to the power supply.

Insulation level - An insulation strength expressed in terms of a withstand voltage

To ensure that the probability of insulation breakdown is limited to an acceptable value and that any breakdown is

restricted to self-restoring insulation

terms of a withstand voltage

4/15/2013

3

SelfSelf--RestoringRestoring

– Once the arc clears completely, insulation back at full strength– Insulator string in air (or other path in air)

NonNon----SelfSelf——RestoringRestoring

– Insulation failure results in damage to insulation– Will need to be isolated and repaired- Examples: • Underground cable p )• Transformers• Rotating Machines

• Voltage Stress: • Dielectric Strength of i l ti Voltage Stress:

– Magnitude of Surge– Duration of surge– Distribution of stress

• Current Stress:

insulation

• Surge protective devices– Device characteristics– Device placement– Magnitude of surge

- Duration of surge– Length of surge

– Device placement

• Cost

4/15/2013

4

1. Selection of the reliability criteria

2. Determine the electrical stress placed on the equipment or the air clearance

3. This stress is then compared to the insulation strength characteristic, from which a strength is selected

4. If the insulation strength or the clearance is considered to be excessive, then the stress can be reduced by use of ameliorating (improved) measures such as surge arresters, protective gaps, shield wires and closing resistors in the circuit breakers.

Nominal System Voltage – phase to phase voltage of the system for which the system is normally designed

Maximum System Voltage – the maximum allowable power frequency voltage which can occurs may be for long time during no load or low load condition of the power system.

Factor of Earth – ratio of highest Vrms phase-earth (during fault) to Vrms phase-phase (no fault)

Insulation Level (insulation strength)– Lightning Impulse and short duration power frequency withstand voltage (<300 kV), Switching Impulse (>300 kV)

4/15/2013

5

Statistical Impulse Withstand Voltage - The peak value of a switching or lightning impulse test voltage at which insulation exhibits, under the specified conditions, a 90% probability of withstand.

4/15/2013

6

4/15/2013

7

CONVENTIONAL OR

DETERMINISTICSTATISTICAL

METHODDETERMINISTICMETHOD

The conventional insulation coordination approach seeks thecoordination approach seeks the impulse voltage level at which the

equipment insulation will not show any disruptive discharge

For unknown probability of failure, i.e.For unknown probability of failure, i.e. non-self restoring

4/15/2013

8

The maximum amplitude of transient over voltages reach thecomponents, can be limited by using protecting device like lightningarrestor in the system.

To avoid insulation failure, insulation level of different types ofequipment connected to the system has to be higher than magnitude oftransient over voltages appeared on the system.

Usually insulation level is above protective level (safe margin 15 – 20% higher)

4/15/2013

9

Based on knowledge of overvoltage occurrence and flashover probability statistics – not the highest overvoltage possible.

Design based on acceptable risk of flashover

Risk of failure diminishes as the insulation is strengthened (larger gap or longer insulation string)

T tif th i k f f il f i l ti th h i l l iTo quantify the risk of failure of insulation through numerical analysis of the statistical nature of the overvoltage magnitudes and of electrical withstand strength of insulation.

P

1.0 r(V) = P(V) * f(V)(A d )(Area under curve)

P(V) = Probability of failure at level V

r(V) = risk of failure

V

f(V) = Frequency of surge at level V

4/15/2013

10

CONVENTIONAL

STATISTICAL

4/15/2013

11

Choice of methods is depending on the characteristics of the insulationinsulation

Refers to the ability of electric equipment such as transformers to withstand lightning surges. Transformers with a rating of 600 volts and below are designed to have a BIL of

10 kV10 kV.

Expresses the insulation withstand under a standard impulse of 1.2/50 µs

The BIL is tied to a specific waveshape in addition bring tied to standard atmospheric

conditions.

The BIL may be either a statistical BIL or a conventional BIL.

4/15/2013

12

Th t l f t d d li ht i i l f hi hThe crest value of standard lightning impulse for which the insulation exhibits 90% probability of withstand, a

10% probability of failure.

Th t l f t d d li ht i i l fThe crest value of a standard lightning impulse for which the insulation does not

exhibit distruptive

Note : Crest value ‐ The maximum value attained by the peak of a wave, surge or impulse

4/15/2013

13

BASIC SWITCHING IMPULSE INSULATION BASIC SWITCHING IMPULSE INSULATION LEVEL (BSL)LEVEL (BSL)

BSL is the electrical strength of insulation expressed in terms of the crest value of the standard switching impulse.terms of the crest value of the standard switching impulse.

Expresses the insulation withstand under a standard switching impulse of 250/2500 µs

The BSL may be either a statistical BSL or a conventional BSL.

Statistical BSL is applicable only to self-restoring insulations. Conventional BSL is applicable to non-selfrestoring insulations.

CONVENTIONAL BSLCONVENTIONAL BSL

The crest value of a standard switching impulse withstand for which the insulation does not exhibit disruptive discharge when subjected to a specific number of applications of this impulse.

4/15/2013

14

Statistical vs. Conventional Statistical vs. Conventional BIL & BSLBIL & BSL

The statistical BIL or BSL is defined statistically or probabilisticallyprobabilistically

Probability of F.O. (failure) for application of an standard impulse ( with crest of BIL or BSL) is 10%

Insulation strength characteristic may be represented by a cumulative Gaussian Distribution

4/15/2013

15

CFO A lt l l t th diti f th

CFO CFO –– Critical flashover voltageCritical flashover voltage

CFO = A voltage level at the condition of the insulation results in a 50% probability of flashover (half the impulse flashover)

Locating the BIL or BSL at the 10% point results in the definition that the BIL or BSL is 1.28 standard deviationsdeviations,

CFO CFO –– Critical flashover voltageCritical flashover voltage

BIL= CFO(1 – 1.28 σf /CFO)

BSL= CFO(1 – 1.28 σf /CFO)

The standard deviation (σ) is the voltagedifference between the 16% and 50% points or

between the 50% and 84% points

4/15/2013

16

CFO CFO –– Critical flashover voltageCritical flashover voltage

Coefficient of variation, σpv = σf/CFO

Statistical withstand voltage =(1 - 3 σpv )V50

Statistical flashover voltage =(1 + 3 σ )VStatistical flashover voltage =(1 + 3 σpv )V50

Sigma in per unit of the CFO is properly called the coefficient of variation

CFO CFO –– Critical flashover voltageCritical flashover voltage

coefficient of variation.

Thus a sigma of 5% is interpreted as a standard deviation of 5% of the CFO.

The standard deviation or sigma is 2%to 3% for lightningg g

For switching impulse, sigma ranges from about 5% for tower insulation to about 7% for station type insulations.

4/15/2013

17

EXAMPLEFINAL EXAM 2011/2012 – I

Q4(C)

In a laboratory, switching impulses are applied to a post insulator in order to determine the basic switching impulse insulation level (BSL) of the post insulator. The results of the test are shown in Table 1. These test results are then plotted on a linear graph paper as in Figure 1.

i.    Determine the critical flashover (CFO)

ii.   Calculate the BSL using statistical equation

iii.  Show in the graph of Figure 1the value of BSL and what is the percentage of flashover at this condition?

4/15/2013

18

Example 

Results of BIL test is shown inResults of BIL test is shown in Table 5c . Determine the BIL of the string insulator

Conventional BIL or BSLConventional BIL or BSL

Simply defined but has less meaning as regards Simply defined but has less meaning as regards insulation strength. One or more impulses having the insulation strength. One or more impulses having the

standard wave shape and having a crest value equal to standard wave shape and having a crest value equal to the BIL or BSL are to applied to the insulations. If no the BIL or BSL are to applied to the insulations. If no

flashover occur, the insulation is stated to possess a BIL flashover occur, the insulation is stated to possess a BIL or BSL.or BSL.

4/15/2013

19

Risk Failure calculationRisk Failure calculation