L9-Impulse Voltages and Testing

8/12/2019 L9-Impulse Voltages and Testing

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Insulation Coordination

the selection of insulationstrength


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Kiski Valley

Substation - 1938


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Luxor substation 1933.before lightning arresters


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Insulation

Self restoring insulation recovers after aflashover external such as air switches,insulators, external surfaces of bushings(defined at standard atmosphericconditions)Non-self restoring insulation does not

recover after a flashover enclosed oil, paper, gas, air systems such as transformersand circuit breakers


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Standardshttp://www.techstreet.com/info/astm.

Voltages: ANSI C84.1- 1995 System VoltagesVoltages: IEEE 1312 1987 AC Electrical Systems and EquipmentOperating Voltages above 230kvBILs: ANSI 92 defines BILs for the equipment manufacturers to useIEEE std 4 1995 Standard Techniques for High Voltage TestingCircuit Breakers: ANSI C-37.06 1997Transformers: ANSI C57.12 1993 Liquid Immersed Distribution,Power and Regulating Power TransformersSwitches: IEEE C37.34 Standard Test Code for High VoltageSwitchesInsulators: ANSI C29.8 & C29.9 Cap and Pin Type & Post TypeInsulatorsArresters: IEEE C62.11 Metal Oxide surge arresters for AC PowerCircuits Arresters: IEEE C62.22 Guide for application of Metal Oxide surgearresters for Alternating Current Systems


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Impulse Voltages and Equipment

Requirements

Over-voltages

Transient WaveshapesEquipment testing


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Transient Waveforms

Lightning - 1.2 x 50 u-second

Front of WaveChopped WavesSwitching - 250 x 2500 u-second

60 hertz waves


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Peak or RMS

60hz voltages are expressed in rms ph-phDuty cycle and MCOV are in rms phase-groundLightning, chopped wave and switching surges are

expressed in peak volts phase to ground:

Peak = 2 rms ph-ph 3

Peak = 2 145kv = 118.4kv3


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Lightning ImpulsesVery fast rise can cause line insulation to fail

backflashes - rise in the 500-2000KV per u-secondrange and 10-20 u-sec tail (cfo of line - positive polaritywaves)

Direct strokes to phase wires (cfo due to negative

polarity waves) slightly slower rise and longer tail than backflashBIL: Basic Impulse Level is the electrical strength ofinsulation to the crest value of the standard lightningimpulse (for dry conditions)

BIL = CFO 1.28 x o f (10% probability of flashover)

Test wave doesnt reflect actual lightning but its easilyreproduced in test labLig htning can be positive or negative compared to the

earth most strokes are negative. The insulation withstandcan be different for each type


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Lightning Impulse1.2 x 50 u-seconds

BIL = 1.0

.5

t2

Virtual tail = 50 micro-seconds

IEEE std 4-1995 Standard Techniques for High Voltage Testing

Virtual front = 1.2 micro-seconds

.9

.3 A

B

t1

Virtual Zero

v

t

Tolerances:Front: +/- 30%Tail: +/- 20%


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Impulse Generators:

Impulse generators are stacks of capacitors and resistorsInitially the capacitors are connected in parallel and charged upThen the capacitors are re-connected in seriesThen they are discharged to provide the high voltage impulse waveResistors are used to shape the front and tail of the wave

+

-

+-

Charging circuit

Discharging circuit


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Chopped Wave Impulse1.2 x 3 u-seconds

BIL = 1.0

IEEE std 4-1995 Standard Techniques for High Voltage Testing

v

t

1.1 - 1.29 x BIL

t


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Switching Impulses

BSL: Basic Switching Level is the electricalstrength of insulation to the crest value of thestandard switching impulse (wet conditions)

BSL = CFO 1.28 x o f (10% probability offlashover)

Switching impulses are longer duration both onthe front and the tail (hundreds of u-seconds)Low line surge impedance (> 138kv overheadlines, also cable circuits)Re-closing into trapped charge on line


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SwitchingSurges:

Z 0= L/C

Voltage doubles when closing in on anopen line = 2 P.U. at open line terminal

Assume that High Speed Re-closing traps a negative 1 P.U. charge on the line.Then when the breaker re-closes the maximum voltage at the open end can

approach a maximum of 3.5 - 4.0 P.U. for multiple reflections depending ondamping (R):

Trapped charge = -1.0 P.U.

3.5 P.U.


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50% CriticalFlashover VoltageCFO

Strike Distance

SwitchingSurges: Insulation strength ofair gaps:

At operating voltages above 230kv switching transients becomethe constraint that drives system design

Breakers & switches are only tested for switching surge at345kv and above. Transformers 115kv & above.


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Switching Impulse250 x 2500 u-seconds

1.0

.5

t2 = 2500 u-secIEEE std 4-1995 Standard Techniques for High Voltage Testing

v

tt1 = 250 u-sec

Tolerances:Front: +/- 20%Tail: +/- 60%


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Power Frequency (60hz)

Maximum 60Hz OperatingVoltage

Nominal + 5%500kv system is nominal + 10%


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System Voltages ANSI C84.1 - 1995


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System Voltages ANSI C84.1 - 1995


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Power Frequency (60hz)Overvoltages

Rise on unfaulted phases for a line-ground fault(X/R and co-efficient of grounding)Loss of ground (backfeed)

Ferro-resonance line or cable capacitance withtransformer inductanceLoad rejection

Ferranti effect long unloaded transmission linesTransformer inrush


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Coefficient of Grounding


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Potential Rise on the Line

Voltage

Breaker open

..


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Equipment Insulation

Bus InsulatorsAir SwitchesCircuit BreakersTransformersBushings

Equipment External Clearances


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29IEEE 1313.1 1996

Equipment BILs (Class one equipment) :


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Equipment BILs/BSLs for EHV (Class two) :


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Equipment Insulation60 hz testing:

60 hz tests wet tests are done on insulators, airswitches and cijrcuit breakers to understandresponse during rain (IEEE std 4 1995)

and dew conditions (ANSI C-29.1 1988 section4.6)


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Bus Insulators

Air SwitchesCircuit BreakersTransformers

Bushings


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Bus Supports & AirSwitches ANSI C37.32

Withstand (10%)


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Standard AtmosphericConditions

Ambient temperature = 20 deg CAir Pressure = 760mm mercury (1 atmosphere)Absolute humidity = 11 gms water/Mtr 3 air

The test voltages are adjusted as needed whenconditions vary from the standard


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Bus Insulators


Bushings


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Bus Supports &Air SwitchesANSI C37.32


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EHV Switches:

Air SwitchesANSI C37.32


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Air Switch Coordination

- Gap flashover- Gap withstand

- Insulator flashover- Insulator withstand

Make the gap withstand 10%higher than the insulator flashover!


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Bus Supports &Air SwitchesANSI C37.32


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Passing the 3+9 test:

Air Switches

APs switch specification MS 1100 -10 requires transmission switches towithstand 15 impulses without flashover for the gap withstand and alsoflashover for 15 impulses to establish the insulator flashover value.


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Bus Insulators


Bushings


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ABB 550 PM


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Circuit Breaker CoordinationBreaker closed

Weakest point is typicallythe throat shield to ground


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ABB 550 PM


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ABB 550 PM Failure Doubs DL-55, July 8, 2000 Tube Flashover


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Circuit Breaker CoordinationBreaker openSwitching surge values are higher for the

gap than for phase to ground


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Ci i k


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Circuit Breaker500kv PM Design Tests

One minute dry withstand 60hz 860kvTen second wet withstand 60hz 775kvFull wave impulse (BIL) 1800kvChop wave impulse 2 u-sec to chop 2320kvChop wave impulse 3 u-sec to chop 2070kvSwitching impulse terminal to gnd 1175kv(breaker open)Switching impulse terminal to gnd 1175kv(breaker closed)Switching impulse terminal to terminal 1300kv(breaker open)


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Breaker Applications

Closing resistors are used to reduce theswitching surge created at the far end ofa transmission line when it is energized

Once breaker is closed the resistor is removedfrom the circuit.

Not designed to handle continuous load currentor fault current.

AP uses closing resistors on EHV breakers Typical value: 400-450 ohms.


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Closing resistors :

Z 0= L/C Rc

Rc

Z 0 V 0 Z 0

(Rc + Z 0 ) V 0 = V S

bypass

V S


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Transformer TestLevels


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Transformers:138-12kv delta-wye

lowvolage

wintding

Gas Space - N 2

highvoltagewinding

HV & LV BILs determinethe amount of line end

insulation ofthe windings

Turn-Turn failure at line end


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Turn Turn failure at line end

138kv disk winding 30/40/50mva


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Transformer

lowvolagewintding

Gas Space - N 2

highvoltagewinding

HV BIL determinesthe distance betweenthe high-low windingsand H-L insulation


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Transformer

lowvolagewintding

Gas Space - N 2

highvoltagewinding

Induced test determinesthe high-ground

insulation


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Transformer

lowvolagewintding

Gas Space - N 2

highvoltagewinding

LV BIL and thru-faultmechanical forcesstress low-groundinsulation


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Bus Insulators


Bushings

Bushings


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Bushings

Condenser type

bushings are built to

grade the voltage

from line to ground

evenly.

creep andcontamination

SF6 weathersheds

500 kV Bushing Type O


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g yp

500 kV Bushing Type O


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g yp


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External Equipment Clearances:

Need to compare external equipment withstands with bussupport and air switch clearancesCompare phase-to-ground and phase-to-phase equipmentdimensions with the bus design

Equipment manufacturers will set phase-phase and phase-ground dimensions to pass equipment test standards butthese are not coordinated with bus dimensionsTypically the clearances between live parts on theequipment are too small! Flashovers will occur at theequipment before the bus.Need to specify equipment clearances that willcoordinate with bus clearances


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Transformer Clearances: 138-12kvxfmr

PhaseTo phase

PhaseTo ground

138kv 49 in. 49 in.

12kv 6.5 in. 6.5 in.


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AP TransformerDesign Clearances:


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Transformer Terminated Lines

Initial Switching surge

..Breaker openWhen you close this breaker

a switching transient is createdthat doubles at the other end of

the line because the transformeris open on the low side

Reflected wave

Close linebreaker first

Then close thexfmr air switch

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L9-Impulse Voltages and Testing