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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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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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Equipment Insulation
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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Equipment Insulation
Bus Insulators
Air SwitchesCircuit BreakersTransformers
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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Equipment Insulation
Bus Insulators
Air SwitchesCircuit BreakersTransformers
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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Equipment Insulation
Bus Insulators
Air SwitchesCircuit BreakersTransformers
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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