1 insulation coordination “the selection of insulation strength”

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Insulation CoordinationInsulation Coordination

“the selection of insulation “the selection of insulation strength”strength”

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Kiski Valley Substation - 1938

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Luxor substation – 1933…………….before lightning arresters

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InsulationInsulation

Self restoringSelf restoring insulation recovers after a insulation recovers after a flashover – external such as air switches, flashover – external such as air switches, insulators, external surfaces of bushings insulators, external surfaces of bushings (defined at standard atmospheric (defined at standard atmospheric conditions)conditions)

Non-self restoringNon-self restoring insulation does not insulation does not recover after a flashover – enclosed oil, recover after a flashover – enclosed oil, paper, gas, air systems such as transformers paper, gas, air systems such as transformers and circuit breakersand circuit breakers

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Standards Standards http://www.techstreet.com/info/astm.http://www.techstreet.com/info/astm.

Voltages: ANSI C84.1- 1995 System VoltagesVoltages: ANSI C84.1- 1995 System Voltages Voltages: IEEE 1312 – 1987 AC Electrical Systems and Equipment Operating Voltages above 230kvVoltages: IEEE 1312 – 1987 AC Electrical Systems and Equipment Operating Voltages above 230kv BILs: ANSI 92 defines BILs for the equipment manufacturers to useBILs: ANSI 92 defines BILs for the equipment manufacturers to use IEEE std 4 – 1995 Standard Techniques for High Voltage TestingIEEE std 4 – 1995 Standard Techniques for High Voltage Testing Circuit Breakers: ANSI C-37.06 1997Circuit Breakers: ANSI C-37.06 1997 Transformers: ANSI C57.12 – 1993 Liquid Immersed Distribution, Power and Regulating Power Transformers: ANSI C57.12 – 1993 Liquid Immersed Distribution, Power and Regulating Power

Transformers Transformers Switches: IEEE C37.34 Standard Test Code for High Voltage SwitchesSwitches: IEEE C37.34 Standard Test Code for High Voltage Switches Insulators: ANSI C29.8 & C29.9 Cap and Pin Type & Post Type InsulatorsInsulators: ANSI C29.8 & C29.9 Cap and Pin Type & Post Type Insulators Arresters: IEEE C62.11 Metal Oxide surge arresters for AC Power CircuitsArresters: IEEE C62.11 Metal Oxide surge arresters for AC Power Circuits Arresters: IEEE C62.22 Guide for application of Metal Oxide surge arresters for Alternating Current Arresters: IEEE C62.22 Guide for application of Metal Oxide surge arresters for Alternating Current

SystemsSystems

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Impulse Voltages and Equipment Impulse Voltages and Equipment RequirementsRequirements

Over-voltagesOver-voltages Transient WaveshapesTransient Waveshapes Equipment testingEquipment testing

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Over-voltagesOver-voltageswhat causes over-voltages on the system?what causes over-voltages on the system?

LightningLightning Switching – breakers clearing faultsSwitching – breakers clearing faults Increases in power frequency voltageIncreases in power frequency voltage

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Transient WaveformsTransient Waveforms

Lightning - 1.2 x 50 u-secondLightning - 1.2 x 50 u-second Front of WaveFront of Wave Chopped WavesChopped Waves Switching - 250 x 2500 u-secondSwitching - 250 x 2500 u-second 60 hertz waves60 hertz waves

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Peak or RMSPeak or RMS

60hz voltages are expressed in rms60hz voltages are expressed in rmsph-ph ph-ph

Duty cycle and MCOV are in rms phase-groundDuty cycle and MCOV are in rms phase-ground Lightning, chopped wave and switching surges are Lightning, chopped wave and switching surges are

expressed in expressed in peakpeak volts phase to ground: volts phase to ground:

Peak = 2 rmsrmsph-phph-ph

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Peak = 2 145kv = 118.4kv 3

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Lightning ImpulsesLightning Impulses Very fast rise can cause line insulation to failVery fast rise can cause line insulation to fail

– ““backflashes” - rise in the 500-2000KV per u-second range and backflashes” - rise in the 500-2000KV per u-second range and 10-20 u-sec tail (cfo of line - positive polarity waves) 10-20 u-sec tail (cfo of line - positive polarity waves)

– Direct strokes to phase wires (cfo due to negative polarity Direct strokes to phase wires (cfo due to negative polarity waves) slightly slower rise and longer tail than backflashwaves) slightly slower rise and longer tail than backflash

BIL: BIL: BBasic asic IImpulse mpulse LLevel is the electrical strength of insulation to evel is the electrical strength of insulation to the crest value of the standard lightning impulse (for dry the crest value of the standard lightning impulse (for dry conditions)conditions)– BIL = CFO – 1.28 x BIL = CFO – 1.28 x oof f (10% probability of flashover)(10% probability of flashover)

Test wave doesn’t reflect actual lightning but its easily reproduced Test wave doesn’t reflect actual lightning but its easily reproduced in test labin test lab

Lightning can be positive or negative compared to the earth – most Lightning can be positive or negative compared to the earth – most strokes are negative. The insulation withstand can be different for strokes are negative. The insulation withstand can be different for each type each type

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Lightning Impulse 1.2 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:

•Impulse generators are stacks of capacitors and resistors•Initially the capacitors are connected in parallel and charged up•Then the capacitors are re-connected in series•Then they are discharged to provide the high voltage impulse wave•Resistors are used to shape the front and tail of the wave

+

-

+-

Charging circuit

Discharging circuit

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Chopped Wave ImpulsesChopped Wave Impulses

Allows a faster front wave to be applied without Allows a faster front wave to be applied without flashover occurring on the tail of the waveflashover occurring on the tail of the wave

Chopped wave impulses are standard lightning Chopped wave impulses are standard lightning waves that are shunted or “chopped” to zero on the waves that are shunted or “chopped” to zero on the tail of the wave within 2 or 3 u-secondstail of the wave within 2 or 3 u-seconds

Chopped wave tests are only used in ANSI (not Chopped wave tests are only used in ANSI (not IEC) standards and are meant to model surges IEC) standards and are meant to model surges created by the flashover of a post insulator or an created by the flashover of a post insulator or an arrester operationarrester operation

Stresses transformer turn to turn insulationStresses transformer turn to turn insulation Model the need for arresters at the “open breaker”Model the need for arresters at the “open breaker”

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Chopped Wave Impulse 1.2 x 3 u-seconds

BIL = 1.0


v

t

1.1 - 1.29 x BIL

t

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Switching ImpulsesSwitching Impulses

BSL: BSL: BBasic asic SSwitching witching LLevel is the electrical evel is the electrical strength of insulation to the crest value of the strength of insulation to the crest value of the standard switching impulse (wet conditions)standard switching impulse (wet conditions)

– BSL = CFO – 1.28 x BSL = CFO – 1.28 x oof f (10% probability of (10% probability of flashover)flashover)

Switching impulses are longer duration both on Switching impulses are longer duration both on the front and the tail (hundreds of u-seconds)the front and the tail (hundreds of u-seconds)

Low line surge impedance (> 138kv overhead Low line surge impedance (> 138kv overhead lines, also cable circuits)lines, also cable circuits)

Re-closing into trapped charge on lineRe-closing into trapped charge on line

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Switching Switching Surges:Surges:

Z0= L/C

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

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

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

Trapped charge = -1.0 P.U.Trapped charge = -1.0 P.U.

3.5 P.U.

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50% Critical Flashover Voltage CFO

Strike Distance

Switching Switching Surges:Surges:

1.2x5

0 lig

htnin

g

Positive switching

surge

Insulation strength of air gaps:

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

Breakers & switches are only tested for switching surge at 345kv and above. Transformers 115kv & above.

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Switching Impulse 250 x 2500 u-seconds

1.0

.5

t2 = 2500 u-sec


v

tt1 = 250 u-sec

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

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Power Frequency (60hz) Power Frequency (60hz) Maximum 60Hz Operating Maximum 60Hz Operating

VoltageVoltage

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) Power Frequency (60hz) OvervoltagesOvervoltages

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

Loss of ground (backfeed)Loss of ground (backfeed) Ferro-resonance – line or cable capacitance with Ferro-resonance – line or cable capacitance with

transformer inductancetransformer inductance Load rejectionLoad rejection Ferranti effect – long unloaded transmission linesFerranti effect – long unloaded transmission lines Transformer inrushTransformer inrush

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Coefficient of GroundingCoefficient of Grounding

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Potential Rise on the LinePotential Rise on the Line

Voltage

Breaker open

..

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Equipment InsulationEquipment Insulation

Bus InsulatorsBus Insulators Air SwitchesAir Switches Circuit BreakersCircuit Breakers TransformersTransformers BushingsBushings Equipment External ClearancesEquipment External Clearances

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BIL/BSL Testing

1.0

.5

CFO

BIL or BSL = 1.28 ooff

Pro

babi

lity

of F

lash

over

KV

.1

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 InsulationEquipment Insulation60 hz testing:60 hz testing:

60 hz tests – wet tests are done on insulators, air 60 hz tests – wet tests are done on insulators, air switches and cijrcuit breakers to understand switches and cijrcuit breakers to understand response during rain (IEEE std 4 – 1995)response during rain (IEEE std 4 – 1995)

and dew conditions (ANSI C-29.1 – 1988 section and dew conditions (ANSI C-29.1 – 1988 section 4.6)4.6)

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Bus InsulatorsBus Insulators Air SwitchesAir Switches Circuit BreakersCircuit Breakers TransformersTransformers BushingsBushings

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Bus Supports & AirSwitches ANSI C37.32

Withstand (10%)

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Standard Atmospheric Standard Atmospheric ConditionsConditions

Ambient temperature = 20 deg CAmbient temperature = 20 deg C Air Pressure = 760mm mercury (1 atmosphere)Air Pressure = 760mm mercury (1 atmosphere) Absolute humidity = 11 gms water/MtrAbsolute humidity = 11 gms water/Mtr3 3 airair The test voltages are adjusted as needed when The test voltages are adjusted as needed when

conditions vary from the standardconditions vary from the standard

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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

AP’s switch specification MS 1100-10 requires transmission switches to withstand 15 impulses without flashover for the gap withstand and also flashover for 15 impulses to establish the insulator flashover value.

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Circuit Switcher Coordination

In the model Mark V circuit switcherthe interrupter closes when theswitch blade reaches the full openposition so the open blade must coordinate with the post insulator.In the series 2000 the interrupter staysOpen when the blade opens!

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ABB 550 PMABB 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 ABB 550 PM

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ABB 550 PM Failure ABB 550 PM Failure Doubs DL-55, July 8, 2000 – Tube Flashover 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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Circuit Breaker Design TestsCircuit Breaker Design Tests

Breaker switching surgewithstand is higher across

the interrupter than to ground!

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Circuit BreakerCircuit Breaker500kv PM Design Tests500kv PM Design Tests

One minute dry withstandOne minute dry withstand 60hz60hz 860kv860kv Ten second wet withstandTen second wet withstand 60hz60hz 775kv775kv Full wave impulse (BIL)Full wave impulse (BIL) 1800kv1800kv Chop wave impulse – 2 u-sec to chopChop wave impulse – 2 u-sec to chop 2320kv2320kv Chop wave impulse – 3 u-sec to chopChop wave impulse – 3 u-sec to chop 2070kv2070kv Switching impulse – terminal to gnd 1175kv Switching impulse – terminal to gnd 1175kv

(breaker open)(breaker open) Switching impulse – terminal to gnd Switching impulse – terminal to gnd 1175kv 1175kv

(breaker closed) (breaker closed) Switching impulse – terminal to terminal 1300kv Switching impulse – terminal to terminal 1300kv

(breaker open)(breaker open)

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Breaker ApplicationsBreaker Applications

Closing resistorsClosing resistors are used to reduce the are used to reduce the switching surge created at the far end of switching surge created at the far end of a transmission line when it is energizeda transmission line when it is energized– Once breaker is closed the resistor is removed Once breaker is closed the resistor is removed

from the circuit.from the circuit.– Not designed to handle continuous load current Not designed to handle continuous load current

or fault current.or fault current.– AP uses closing resistors on EHV breakersAP uses closing resistors on EHV breakers– Typical value: 400-450 ohms.Typical value: 400-450 ohms.

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Closing resistorsClosing resistors::

Z0= L/CRc

Rc

Z0 V0

Z0

(Rc + Z0)V0 = VS

bypass

VS

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Transformer BIL Levels

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Transformer Test Levels

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Transformers:Transformers:138-12kv delta-wye138-12kv delta-wye

low

volage wintding

Gas Space - N2

highvoltagewinding

HV & LV BILs determine the amount of line end

insulation of the windings

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Turn-Turn failure at line end

138kv disk winding – 30/40/50mva

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TransformerTransformer

low

volage wintding

Gas Space - N2

highvoltagewinding

HV BIL determinesthe distance between the high-low windingsand H-L insulation

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low

volage wintding

Gas Space - N2

highvoltagewinding

Induced test determines the high-groundinsulation

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low

volage wintding

Gas Space - N2

highvoltagewinding

LV BIL and thru-fault mechanical forces stress low-groundinsulation

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BushingsBushings

Condenser type Condenser type

bushings are built to bushings are built to

grade the voltage from grade the voltage from

line to ground evenly.line to ground evenly.

““creep” and creep” and

contaminationcontamination

SF6 weathershedsSF6 weathersheds

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500 kV Bushing Type O500 kV Bushing Type O

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External Equipment Clearances:External Equipment Clearances:

Need to compare external equipment withstands with bus Need to compare external equipment withstands with bus support and air switch clearancessupport and air switch clearances

Compare phase-to-ground and phase-to-phase equipment Compare phase-to-ground and phase-to-phase equipment dimensions with the bus designdimensions with the bus design

Equipment manufacturers will set phase-phase and phase-Equipment manufacturers will set phase-phase and phase-ground dimensions to pass equipment test standards but ground dimensions to pass equipment test standards but these are not coordinated with bus dimensionsthese are not coordinated with bus dimensions

Typically the clearances between live parts on the Typically the clearances between live parts on the equipment are too small! Flashovers will occur at the equipment are too small! Flashovers will occur at the equipment before the bus. equipment before the bus.

Need to specify equipment clearances that will Need to specify equipment clearances that will coordinate with bus clearancescoordinate with bus clearances

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Transformer Clearances: 138-12kv138-12kv

xfmrxfmr

PhasePhase

To phaseTo phase

PhasePhase

To groundTo ground

138kv138kv 49 in.49 in. 49 in.49 in.

12kv12kv 6.5 in.6.5 in. 6.5 in.6.5 in.

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AP TransformerDesign Clearances:

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Proper Insulation Coordination

If lightning strikes the phase conductor and it generates enough voltage to flash the insulation I want it flashing over the cheapest, self restoring insulation first and I want it to go to ground

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Transformer Terminated LinesTransformer 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 transformer is open on the low side

Reflected wave

Close line breaker first

Then close thexfmr air switch

1 insulation coordination “the selection of insulation strength”

Documents

standard lightning impulse

standard lightning waves

type lightning impulse

line insulation

standard techniques

insulation recovers

impulse generators

ieee c62