UNIT-4SWITCHING COORDINATION & Insulation Coordination

Contents • Introduction• Definitions• Insulation Co-ordination curves• Basic Insulation Level (BIL)• Insulation levels of substation equipments• Lightning Arrestor selection & location• Selection of Arrestor voltage rating• Arrestor discharge voltage & current • Protective margin

Causes of Over voltage in Power System

• Increase in voltage for the very short time in power system is called as the over voltage. it is also known as the voltage surge or voltage transients. The voltage stresscaused by over voltage can damage the lines and equipment’s connected to the system,

• There are two types of causes of over voltage in power system

• 1. Over voltage due to external causes• 2. Over voltage due to internal causes

• Transient over voltages can be generated at high frequency (load switching and lightning), medium frequency (capacitor energizing), or low frequency.

• Over voltage due to external causes:This cause of over voltage in power system is the lightning strokes in the cloud.Now, how lightning strokes are produced. So when electric charges get accumulated in clouds due to thunder Strom caused due to some bad atmosphere process.This type of over voltages originates from atmospheric disturbances, mainly due to lightning. This takes the form of a surge and has no direct relationship with the operating voltage of the line.

• This takes the form of a surge and has no direct relationship with the operating voltage of the line. It may be due to any of the following causes:

• a) Direct lightning stroke• b) Electromagnetically induced over voltages

due to lightning discharge taking place near the line, called 'side stroke'.

• c) Voltages induced due to atmospheric changes along the length of the line.

• Over voltages are caused on power systems due to external and internal influencing factors. The voltage stress caused by over v o l t a g e c a n d a m a g e t h e l i n e s a n d equipment’s connected to the system.

• Over voltages arising on a system can be generally classified into two main categories as below:External Over voltagesThis type of over voltages originates from atmospheric disturbances, mainly due to lightning.

INSULATION CO-ORDINATION

It is selection of suitable insulation levels of various components in any electrical system and their rational arrangement. It is required to ensure:

1) Insulation shall withstand all normal stresses and majority of abnormal ones

2) Efficient discharge of over voltages due to internal /external causes

3) Breakdown shall be only due to external causes4) Breakdown shall be at such places where least

damage is caused5) Safety of operating personnel and public.

Determination of Insulation coordination

S t e p s i n t h e d e t e r m i n a t i o n o f I n s u l a t i o n coordination:

• Determination of line Insulation

• Selection of BIL and Insulation levels of other equipment

• Selection of Lightning Arrestors.

Definition:- Flash over voltages

• Dry flash over voltage (Dry FOV) :-Power frequency voltage which will will cause flashover of the Insulation.

• Wet flash over voltage( Wet FOV):- Power frequency voltage which will cause flash- over when sprayed with water of a resistance 9000-11000 ohm-cms drawn from a source of supply at a temp within 10°C of the ambient temperature in the neighbour- hood of insulation under testing and directed at an angle of 45° ,the volume of water being equivalent to precipitation of 0.305 cm /min.

Definition:- Flash over voltages

Impulse flash over voltage (Impulse FOV):-

• The voltage which will cause flash over of an Insulation when subjected to a 1.2x50µs (Indian Standard) impulse

• (British standards 1x50µ sec)• (American standards 1.5 x 40µsec)

Definition:- Flash over voltages

Basic Insulation level :-The crest voltage of standard wave that will not cause flashover of the insulation is referred to as “Basic insulation level”(Basic impulse insulation voltages are levels expressed in impulse crest voltage with a standard wave not longer than 1.2x50 µs (Indian standards)

Equipment insulation as tested shall be equal or above the BIL.

Impulse spark over volt- time characteristic

Time in microseconds

Cre

st

Impulse spark over volt- time characteristic

This characteristic is obtained by plotting : --Time which elapses between the moment the

voltage wave is applied and the moment of spark over -- on abscissa

-Voltage at the movement of spark over (i) Occurring on the wave front

(ii) Occurring on the wave peaks (iii) Crest of the voltage for spark over occurring

on the wave tail.

Impulse spark over volt- time characteristic

• This characteristic is established by means of a 1/50 impulse wave

• A line drawn meeting the breakdown values is the characteristic

• Proper insulation co-ordination will ensure that the voltage time Curve of any equipment will lie above the volt -time curve of the protective equipment, say, Lightning arrestor.

Insulation Co-ordination curves

DETERMINATION OF LINE INSULATION

Extra high voltage line can be made lightning proof by:

• Efficient shielding • Low tower footing impedance equal to or less

than 10 ohmswhen shielding angle is Transmission lines up to 220kV 30° 400 kV at and above 20°

T h e i n s u l a t i o n o f l i n e b a s e d u p o n t h e consideration of lightning,switching surges and power frequency over voltages.

Line insulation • Line insulation shall be sufficient to prevent a

flashover from the power-frequency over voltages and Switching Surges.

• I t sha l l take in to cons iderat ion the loca l unfavourable circumstances which decrease the flash over voltage (rain, dirt, Insulation pollution etc.,)

OVER VOLTAGE FACTORS

Line Voltage

s

Switching Surge flash

over

Power frequency flash over (Dry & Wet)

220kV 6.5 V pn 0.3400kV 5.0 V pn 3.3

Vpn = Phase to Neutral Voltage (rms)

OVER VOLTAGE FACTORS It is good to make an allowance to add one or

two more Insulators for each string to take care of the possibility of an insulator unit in a string becoming defective.

• Facilitate hot line maintenance • Up to 220 kV Line – 1 disc for each string• 400 kV Line – 2 discs for each string

FLASH OVER VOLATAGE(FOV) OF DISCS 254 X 145 mm

NO OF DISC

S

DRY FOV WET FOV ( kV rms)

Impulse FOV

(Standard full wave)

9 540 375 86010 590 415 94514 785 565 126515 830 600 134525 1280 900 2145

RECOMMENDED INSULATION LEVEL OF LINE

Normal syste

m Voltag

e

Vpn

In kV(Vph/√3)

Switching over volt. (Wet) kV

No of discs required

132kV 76 76 x6.5=495 5220kV 127 127x6.5=825 9400kV 231 231x5=1755 13

RECOMMENDED INSULATION LEVEL OF LINE

Normalsystem Voltage

VpnIn kV

Power freq. over volt

(wet) (kVrms)

No. of discs

req.

No. of disc

s recom.

As per practic

e

132kV 76 76x3=228 6 7 9/10

220kV 127 127x3=381 10 11 13/14

400kV 231 231x3=762 20 22 23/24

Co-ordination of line Insulation and Sub-Station Insulation

• Line Insulation is not directly related to the Insulation of equipment within the Sub-Station.

• Impulse flash over voltage of line Insulation determine the highest surge voltage that can travel into the sub-station.

• Current through l ight ing arrestor can be calculated from:

1 )Surge impudence of line2 )Surge voltage arriving over the line

Co-ordination of line Insulation and Sub-Station Insulation

• Discharge voltage of the LA on that current is the basic protective level of the substation equipment.

• Discharge voltage across LA varies with surge current.

INSULATION LEVELS OF SUBSTATION EQUIPMENT

• Transformers, Isolators, Instrument Transformers are manufactured for the standard Insulation level.

• Some times transformers, are manufactured for one step lower insulation level for the sake of economy. (LAs will be designed for a still lower level)

• Where LAs are provided right on the top of the transformer, some of the equipment may lie well out side the protective zone of the LA.

INSULATION LEVELS OF SUBSTATION EQUIPMENT

Protective zone is determined based on A With stand level of equipment B Discharge volt of LA C Distance between LA and equipment.

• Such equipment shall be designed for one step higher BIL.

• Generally BIL of substation equipment other than transformer are designed for10% higher BIL than that of Transformer .

Lightning Arrester A lightning arrester is a device used on electrical

power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal.

Types of Arresters Horn Gap Arresters: Horn gap arresters are named for their two horn-

shaped metal rods. These rods are arranged around a small air gap, and the distance between the two rods increases as they rise from the gap. The rods are placed on porcelain insulators. The horn is connected to two different wires: One attaches to the electrical line. Between it and the horn is a resistance and a choke coi l . The resistance regulates the level of current allowed into the arrester at once, and the choke coil increases the reactivity of the arrester when transient frequency occurs. The other wire is connected to a ground, which siphons the excess electricity to the ground.

Types of Arresters Multi-Gap Arresters: Multi-gap arresters are made from a series

of metal cylinders. These cylinders are all insulated from one another as well as separated by air gaps. The first cylinder gets connected to the electrical line, while all of the other cylinders are attached to the ground through a series resistance, which gradually wears down the power of the current. Some of the gaps between the later cylinders have a shunt resistance that catches a surge when there is an excess of v o l t a g e .

Types of Arresters Valve-Type Arresters: Valve-type arresters are commonly used in

more high-powered electrical systems. They consists of two main parts: a series of spark gaps and a series of non-linear resistor discs. Valve-type arresters work when excessive voltage causes the spark gaps to touch, and the non-linear resisters carry the voltage into the ground. Once the surge of excess power ends, the resisters push the spark gaps apart.

Working Principle of LA The earthing screen and ground wires can well

protect the electrical system against direct l ightn ing strokes but they fa i l to prov ide protection against traveling waves, which may reach the terminal apparatus. The lightning arresters or surge diverts provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground.

Working Principle of LA

Working Principle of LA It consists of a spark gap in series with a non-

l inear res istor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-l inear resistance is that its resistance increases as the voltage (or current) increases and vice-versa.

Lightning arrester selection & Location

• To determine the magnitude of the power frequency phase to ground voltage expected at the proposed arrester location during phase to ground fault, or other abnormal conditions which cause higher voltages to ground than normal.

• To make a tentative selection of the power frequency voltage rating of the arrester. This selection may have to be reconsidered after step (6) is completed.

• To select the impulse current l ike ly to be discharged through the arrester.

Lightning arrester selection • To determine the maximum arrester discharge

voltage for the impulse current and type of arrester selected.

• To establish the full-wave impulse voltage wi thstand leve l o f the equipment to be protected.

• To make certain that the maximum arrester discharge voltage is below the ful l wave impulse, withstand level of the equipment insulation to be protected, by adequate margin.

• To establish the separation limit between the arrester and the equipment to be protected.

Location of Arresters• Lightning arresters are typically located at the

top of a distribution pole wherever a pole-top transformer, capacitor bank or other critical piece of equipment is located.

• The arrester should be connected to ground to a low resistance for effective discharge of the surge current.

• The arrester should be mounted close to the equipment to be protected & connected with shortest possible lead on both the line & ground side to reduce the inductive effects of the leads while discharging large surge current.

Selection of Arrestor Voltage Rating

For purpose of selection of voltage rating of a LA three types of earthing are considered:

• ( I ) E f fect ive earthed system: A system is effectively earthed if under any fault condition the line to earth voltages of healthy phases do not exceed 80 % of the system line to line voltage.

• I f in a system a l l t rans formers have s tar connected winding with neutrally solidly earthed then the system is effectively earthed.

• However if only few transformers are earthed like that, it is not effectively earthed system.

Types of Earthing (II) Non effectively earthed system: a) If the line to earth voltage in healthy phases in

case of a fault exceed 80% of the line to line voltage but does not exceed 100% of it, the system is called non effectively earthed system.

b) System with few solidly earthed neutralsc) Systems with neutral Earthed through resistors or

reactors of low ohmic value or arc suppression coil.

(III) Isolated or un earthed neutral systems :- System neutrals are not earthed. Line to earth

voltage of healthy phases exceed 100% of the line to line voltage.

Selection of Lightning Arrestors Tentative selection of arrestor Voltage:

Arrestor Voltage rating shall not be less than product of system highest voltage x co-efficient of earthing.

Co-efficient of earthing :

– Effectively earthed system --- 80%

– Non effectively earthed system --- 100 %

and isolated earth system

Selection of lightning arrestors• In a 220 kV effectively earthed system

– Highest system voltage = 245 kV– Co-efficient of earthing = 80%– Arrestor voltage rating >= 245x0.8 = 196 kV– As per IS 3070 (part –I) 1965 the rating is 198

kV

• By going for a higher voltage rating for a surge arrestor, the degree of protection for equipment gets reduced.

Selection of arrestor discharge current

This can be calculated from:

(a)Spark over voltage of transmission l ine insulation(b) Surge impedance of the line(c)Residual discharge voltage of LA

Ia = 2E- Ea /Zwhere,

Ia = Arrestor discharge currentE = Magnitude of incoming surge voltageEa = Residual discharge voltage of an arrestorZ = Surge impedance of the line

Selection of Arrestor Discharge Current

– In a 220 kV sys tem us ing 11 insu la to rs Transmission line will not permit a traveling wave of a value more than 1025 kV crest

– As per IS 3070 (Part 1) -1965 the residual voltages of LA at a discharge current of 10kA is 649 kV.

– Considering the surge impedance as 450 ohms– Maximum value of discharge current of LA =

2(1025,000)-649000 /450 = 3100 Amps

– The LAs normally in 200 kV and 400 kV system have a discharge current rating of 10 kA.

Selection of Arrestor Discharge Voltage– This is the most important characteristic of LA

that determine the protection level of the substations.

– The arrestor discharge voltage shall be less than BIL of equipment for effective protection.

– Discharge voltage depends on(I) discharge current(II) rate of rise of current applied(III) Wave shape of current applied

– Discharge voltage of LA increases with discharge current. But increase is much restricted due to non –linear resistance properties of modern valve type LA.

– Increase in d ischarge from 5 kA to 20 kA produces only 25% rise in discharge voltage.

– Increase in rate of current from 1000 to 5000 Amps per micro second increases discharge voltage by only 35%.

Protective Margin of LA– Protective margin of LA = BIL of the equipment -

maximum discharge voltage of LA

– While determining protection level offered by a LA 10% allowances towards drop in lead length and manufacturing tolerance shall be allowed.

– Protective margin shall be 20% of the BIL of the equipment when closely located.

– In a 220 kV systemDischarge voltage of LA = 649 kVAllowing 10 % margin protection level = 713 kVBIL of equipment = 900 kVProtection margin = 900-713 = 187 kV There is more than 20 % of the BIL of the

equipment.

Protective margin of LA– In American System, the margin with average

values of the discharge voltage of station type arrester at 5000 amp is expressed as:

Average discharge voltage x 1.25 +40 kV = BIL protected

When adequate margin is not available LAs with lower rating shall be chosen taking risk.

Insulation Co-ordination Scheme– For 220 KV system.– L.A. Voltage rating=system highest voltage x

co-efficient of earthing =245x.8=196Kv.– Selecting standard rating from Table ,L.A.

voltage rating=198 Kv– Discharge current rating= 10KA (assumed)– Residual voltage table =649Kv (peak) – Protection level of the L.A. =649x1.1=714Kv– For a margin of 20% between the B.I.L. and the

protection level of L.A., the B.I.L. should be =714x1.2=857Kv.

– Choose standard B.I.L. Table =900 Kv,– The corresponding power freq. minute test

voltage =395kv– Switching surge flashover voltage =220/ √ 3

x6.5=825kv – Check it is less than B.I.L. of 900kv.– Power frequency over vol tage=220/ √ 3

x3=228kv rms

This is less than 395kv.– B . I . L . o f CBs , i n s t rument t r ans fo rmer ,

disconnect switches etc,.=900x1.1=990kv.– Choose standard B.I.L.=1050 kv.

The L.A. voltage rating

Rated system voltage

KV

Highest system voltage

KV

Arrester rating in

KV

132 145 120/132220 245 198/216400 420 336