Lecture Slides Chapter 4 Insulation Coordination

To ensure that the probability of insulation breakdown is limited to an acceptable value and that any breakdown is restricted to self-restoring insulation Insulation level - An insulati

SEE 4463 HIGH VOLTAGE TECHNOLOGY

Dr Nouruddeen Bashir Umar

Email: nour@fke.utm.my

Tel: 0177696962

Course materials (notes, lecture slides and tutorials)

can be downloaded from:

http://nour.fke.utm.my/see-4463

Chapter 4 : Insulation Coordination

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

• Probability of failure of insulation, is high at the weakest insulation pointnearest to the source of over voltage

• Insulators in some points are easily replaceable and repairable compared toothers

• However, at other points, the insulators are not so easily replaceable andrepairable and the replacement and repairing may be highly expensive, andrequire long interruption of power

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

• So it is desirable that in situation of insulator failure, only the easily replaceableand repairable insulator fails

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.

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

restricted to self-restoring insulation

Insulation level - An insulation strength expressed in

terms of a withstand voltage

External insulation

• Is the distances in open air or across the surfaces of solid insulation in contact with open air that are subjected to dielectric stress and to the effects of the atmosphere

• Example transmission line insulators

Internal Insulation

• Internal insulation is the internal solid, liquid, or gaseous parts of the insulation of equipment that are protected by the equipment enclosures from the effects of the atmosphere

• Example transformer and bushing insulation

Self-restoring insulation

• Insulation that completely recovers insulating properties after a disruptive discharge (flashover) caused by the application of a voltage is called self-restoring insulation.

• This type of insulation is generally external insulation

Non Self-restoring insulation

• This is the opposite of self-restoring insulators, insulation that loses insulating properties or does not recover completely after a disruptive discharge caused by the application of a voltage.

• This type of insulation is generally internal insulation

• Surge protective devices

– Device characteristics – Device placement

• Cost

1 Selection of the reliability criteria

2 Determination of the electrical stress placed on the equipment or the air

clearance

3 Comparison of 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) measuressuch 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 forlong time during no load or low load condition of the power system

Insulation Level (insulation strength)

• Lightning Impulse and short duration power frequency withstand voltage (<300 kV),Switching Impulse (>300 kV)

Switching Impulse Waveshape

• Front time: The time from the actual zero to actual crest of impulse

• Tail Time: Time from actual zero to time to half value of tail

• The standard switching impulse waveshape is 250/2500 µs

Basic Lightning Insulation Level (B.I.L)

• Also known as lightning impulse withstand voltage

• It is defined as the electrical strength of insulation expressed in terms of the crest value of the

"standard lightning impulse.“

• In essence, the BIL is tied to a specific waveshape in addition being tied to standard atmospheric conditions

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

• The statistical BIL is applicable only to self-restoring insulations , whereas the conventional BIL is applicable to non-self-restoring insulations BILs are universally for dry conditions

• The statistical BIL is the crest value of standard lightning impulse for which the insulation exhibits a 90% probability of withstand, a 10% probability of failure.

• The conventional BIL is the crest value of a standard lightning impulse for which the insulation does not exhibit disruptive discharge when subjected to a specific number of applications of this impulse.

Fig Statistical BIL or BSL (or statistical Impulse Withstand Voltage

Basic Switching Insulation Level (B.S.L)

• Also known as Switching impulse withstand voltage

• It is defined as the electrical strength of insulation expressed in terms of the crest value of the

"standard switching impulse.“

• In essence, the BSL is tied to a specific waveshape in addition being tied to standard atmospheric conditions

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

• The statistical BSL is applicable only to self-restoring insulations , whereas the conventional BIL is applicable to non-self-restoring insulations BSLs are universally for wet conditions

• The statistical BSL is the crest value of standard lightning impulse for which the insulation exhibits a 90% probability of withstand, a 10% probability of failure.

• The conventional BSL is the crest value of a standard lightning impulse for which the insulation does not exhibit disruptive discharge when subjected to a specific number of applications of this impulse.

• For line insulation coordination, this includes transmission and

distribution lines

• The task is to specify all dimensions or characteristics of the transmission

or distribution line tower that affect the reliability of the line:

 The tower strike distances or clearances between the phase conductor and thegrounded tower sides and upper truss

 The insulator string length

 The number and type of insulators

 The need for and type of supplemental tower grounding

 The location and number of overhead ground or shield wires

 The phase-to-ground mid-span clearance

 The phase-phase strike distance or clearance

 The need for, rating, and location of line surge arresters

For station insulation coordination, similar to Line insulation coordination the task is similar in nature.

It includes generation, transmission and distribution It is to specify:

 The equipment insulation strength, that is, the BIL and BSL of all equipment

 The phase-ground and phase-phase clearances or strike distances

 The need for, the location, the rating, and the number of surge arresters

 The need for, the location, the configuration, and the spacing of protective gaps

 The need for, the location, and the type (masts or shield wires) of substationshielding

 The need for, the amount, and the method of achieving an improvement in lightningperformance of the line immediately adjacent to the station

CONVENTIONAL

OR DETERMINISTIC

METHOD

STATISTICAL METHOD OR PROBABILISTIC

METHOD

• The conventional insulation coordination approach seeks the impulse

voltage level at which the equipment insulation will not show any disruptive discharge

• This approach to insulation coordination requires the evaluation of the highest overvoltages to which an equipment may be subjected during operation and selection of standardized value of withstand impulse voltage with suitable safety margin

• Used primarily for unknown probability of failure, i.e non-self

restoring insulations

• The maximum amplitude of transient over voltages reaching the

components, can be limited by using protecting devices in the system like lightning arrestor.

• To avoid insulation failure, insulation levels of different types of equipment connected to the system have to be higher than the magnitude of transient over voltages that will appear on the system.

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

• In a power system various equipment like transformers, circuit breakers, bus supports etc have different breakdown voltages and hence the volt-time characteristics.

• Thus for proper protection of such equipment, it is therefore required that their insulations be properly coordinated with the insulation of the various protective devices.

• Conventional Method of Insulation Coordination involves the correlation

of the insulation of the various equipment in a power system to the

insulation of the protective devices used for the protection of those equipment against overvoltages.

• The basic concept of insulation coordination is illustrated

in Figure (i) showing the desired positions of the volt-time

curves of the protecting device and the equipment to be

protected.

• Curve A is the volt-time curve of the protective device and

B the volt-time curve of the equipment to be protected.

• Thus, any insulation having a withstand voltage strength

in excess of the insulation strength of curve B is protected

by the protective device of curve A

• The breakdown voltage for a particular insulation of

flashover voltage for a gap is a function of both the

magnitude of voltage and the time of application of the

voltage

• The volt-time curve is a graph showing the relation

between the crest flashover voltages and the time to

flashover for a series of impulse applications of a given

wave shape

Figure (i)

• Overvoltages are a random phenomenon and it is uneconomical to design plantwith such a high degree of safety that they sustain the infrequent ones.

• It is also known that insulation designed on conventional method basis doesnot give 100% protection and insulation failure may occur even in well designedplants and, therefore,

• It is desired to limit the frequency of insulation failures to the most economicalvalue taking into account equipment cost and service continuity

• Insulation coordination, therefore, should be based on evaluation and limitation

of the risk of failure than on the prior choice of a safety margin

• Therefore the modern practice is to make use of probabilistic concepts andstatistical procedures especially for very high voltage equipment

• This method is based on knowledge of overvoltage occurrence (the statistical distribution

of overvoltages) and flashover probability statistics and not the highest overvoltage possible.

• Relies on statistical approach which relates directly the electrical stress and the electrical strength

• This method requires a knowledge of the distribution of both the anticipated stresses and the electrical strengths of the insulation.

• Designed based on acceptable risk of flashover.

• Risk of failure diminishes as the insulation is strengthened

• This method is laborious however very useful

• Main aim of this method:

To 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.

To coordinate the electrical stresses with

electrical strengths the overvoltage

distribution is represented in the form of

probability density function (Gaussian

distribution curve and the insulation

breakdown probability by the cumulative

distribution function

The knowledge of these distributions enables us to determine the ‘risk

of failure’

f(V) = Frequency of surge at level V

r(V) = risk of failure

• Finding a suitable insulation such that the withstand distribution does not overlap with the overvoltage distribution is not practical ,

• Thus in the statistical method of analysis, the insulation is selected such that the 2% overvoltage probability coincides with the 90% withstand probability

STATISTICAL

1 Characteristics/type of the insulation

• The insulation strength of air is usually described by a normal cumulative distribution, so this strength distribution may be convolved with the stress distribution to determine the probability of flashover

• The insulation strength of transformer insulation is specified by a single value for BIL and BSL hence no statistical distribution of the strength is available and thus, the conventional method is applied

2 System voltage level

• Cost of insulation for system of the voltage more than 380 kV is proportional to square of the voltage.

• Recall the statistical BIL or BSL is defined

statistically or probabilistically

• For every application of an impulse having the

standard waveshape and whose crest is equal to

the BIL or BSL, the probability of a flashover or

failure is 10%

• In general, the insulation strength characteristic

may be represented by a cumulative Gaussian

distribution

• The mean of this distribution or characteristic is

defined as the critical flashover voltage or CFO

• CFO is the voltage level at the condition of the

insulation results in a 50% probability of flashover

(half the impulse flashover)

Critical flashover voltage (CFO)