Fachgebiet Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 7- 1 -...

FachgebietHochspannungstechnik

Overvoltage Protection and Insulation Coordination Chapter 7 - 1 -

Insulation Strength Characteristics

Topics to be covered in the following

bull Insulators under polluted conditionsbull Probability of flashover (Normal and Weibull distributions)bull Behavior of parallel insulationbull Coordination procedure deterministic and statistical approachbull Correction with altitude of installationbull Clearances in air gap factors



Probability of Disruptive Discharge of Insulation

Non-self-restoring insulationbull No method at present available for the determination of the probability of disruptive dischargebull Therefore it is assumed that the withstand probability changes from 0 to 100 at the value

defining the withstand voltage bull Withstand voltage usually verified by application of a limited number of test voltages at standard

withstand level with no disruptive breakdown allowed Procedure A of IEC 60006-1

[IEC 60060-1]

The breakdown process is statistical in nature to be taken into account especially for impulse voltage stress





Self-restoring insulationbull Withstand capability can be evaluated by tests and be described in statistical termsbull Therefore self-restoring insulation is typically described by the statistical withstand voltage

corresponding to a withstand probability of 90 bull Withstand voltage verified by application of a limited number of test voltages at standard insulation

level allowing a certain number of discharges Procedure B of IEC 60060-1 152-test usually applied procedure in the IEC world Procedure C of IEC 60060-1 3+9-test

See next three slides hellip




[IEC 60060-1]




[IEC 60071-2]

Comparison of Procedures B and C

Only here both procedures are equivalent

Examplebull equipment at the borderline rated and tested at its U10 has a 82 probability of passing the test in Procedure

Bbull a better equipment rated and tested at its U55 has a 95 probability of passing the test in Procedure Bbull a worse equipment rated and tested at its U36 has only a 5 probability of passing the test in Procedure B

with Procedure C its probability of passing would be higher the 5 probability of passing would be given for equipment rated and tested at its U63 (see also next slide)





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-3 -25 -2 -15 -1 -05 0 05 1 15 2 25

(U-U50)Z

Pb(U)

152

3+9

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Test voltage referred to conventional deviation

Probability of passing the test approx 82 at probability of breakdown of 10

P(U)

(U-U50)Z



Insulation Strength in Air

Factors influencing the dielectric strength of the insulation

bull magnitude shape duration and polarity of the applied voltagebull electric field distribution in the insulationbull homogeneous or non-homogeneous electric fieldbull electrodes adjacent to the considered gap and their potentialbull type of insulation

bull gaseousbull liquidbull solidbull combination of two or all of thembull impurity content and the presence of local inhomogeneities

bull physical state of the insulationbull temperaturebull pressurebull other ambient conditionsbull mechanical stressbull history of the insulation (aging damage)

bull chemical effectsbull conductor surface effects

Covered by equations U50RP = f(d)where U50RP hellip 50 probability breakdown voltage of a rod-plane-configuration d hellip gap spacing

Covered by equations U50 = f(U50RP K)where K hellip gap factor

K is a factor indicating how much higher the electrical strength of a particular electrode configuration is in comparison with the rod-plane-configuration (which gives least dielectric strength) factors K were experimentally found for standard switching impulse voltage stress



Gap factorsGap factors (Table G1 of IEC 60071-2)
















Gap factorsGap factors [Electra No 29 (1973)pp 29-44)]

Electrode configuration K

Rod-plane

Rod-structure (under)

Conductor-plane

Conductor-window

Conductor-structure (under)

Rod-rod (h = 6 m under)

Conductor-structure(over and laterally)

Conductor-rope(under and laterally)

Conductor-crossarm (end)

Conductor-rod (h = 6 m under)


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Insulation response to power-frequency voltages (IEC60071-2 Annex G)

1250RP 750 2 ln(1 055 )U d 1250RP 750 2 ln(1 055 )U d

withU50RP hellip crest value in kVd in m d le 3 m

50 50RPU U50 50RPU U

250 50RP 135 035U U K K 250 50RP 135 035U U K K for d gt 2 m

exact for d lt 1 m conservative for 1 m le d le 2 m

0 5009U U 0 5009U U


bull influence of rain in an air gap negligible but for insulators to be consideredbull pollution for insulators to be consideredbull altitude correction required

asymp 300 kVm (rms value)


(assuming U0 = U50 - 4Z and Z = 003 U50)



Insulation response to slow-front overvoltages (IEC60071-2 Annex G)

50RP 1080 ln(046 1)U d 50RP 1080 ln(046 1)U d with

U50RP hellip in kV for positive polarity at most critical front-time (see Ch 3) d in m d le 25 m

0650RP 500U d 0650RP 500U d


withU50RP hellip in kV for positive polarity standard switching impulse voltage (see Ch 3) d in m d le 25 m

50 50RPU KU50 50RPU KU Note for K ge 145 U50neg may become lower than U50posNote for K ge 145 U50neg may become lower than U50pos





0 50075U U 0 50075U U (assuming U0 = U50 - 4Z and Z = 006 U50)

bull influence of rain in an air gap negligible but for insulators to be consideredbull altitude correction required





For phase-to-phase insulation similar gap factors as for phase-to-earth insulation can be applied

But the influence of negative and positive components has to be taken into account by a factor α

peak negative component=

sum of peak negative and positive components peak negative component

=sum of peak negative and positive components

[IEC 60071-2]



Insulation response to fast-front overvoltages (IEC60071-2 Annex G)

50RP 530U d 50RP 530U d with

U50RP hellip in kV for positive polarity d in m d le 10 m


Note for negative LI voltages dielectric strength is higher and increases non-linearly with gap spacing


ie linear increase with gap spacing

The gap factors K (found for SI voltages) cannot be directly appliedFrom experimental investigations

ff 074 026K K ff 074 026K K with

K+ff hellip fast-front overvoltage gap factor

for positive polarity K hellip gap factor for SI voltage

according to tables50 ff 50RPU K U 50 ff 50RPU K U




50 neg line insulator 700U d 50 neg line insulator 700U d


bull influence of insulators to be considered particularly for range IIbull less influence from long insulators without metallic parts (long rod composite station

post) than for cap-and-pin insulatorsbull altitude correction requiredbull virtually no influence of rain neither for air gaps nor for insulators

Estimation of negative line insulator flashover voltage (in order to determine lightning overvoltages impinging on a substation

Conventional deviationZ asymp 003U50 for air gaps and positive polarityZ asymp 005U50 for air gaps and negative polarityZ asymp (005 hellip 009)U50 across insulators





Independent from the theoretical and empirical background given so far IEC 60071-2 offers tables on minimum clearances in air (Annex A) Not all values of these tables can be derived from above equations as they additionally take into account

bull withstand values instead of U50-valuesbull feasibilitybull economybull experiencebull average influence of environmental conditions (pollution rain insects hellip)




[IEC 60071-2]



[IEC 60071-1]

Procedure for Insulation Coordination in Four Steps

Flow chart of IEC 60071-1(Figure 1)

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Performance criterion IEC 60071-2 Cl 32Performance criterion IEC 60071-2 Cl 32

According to definition 322 of IEC 71-1 the performance criterion to be required from theinsulation in service is the acceptable failure rate (Ra)

The performance of the insulation in a system is judged on the basis of the number ofinsulation failures during service Faults in different parts of the network can have differentconsequences For example in a meshed system a permanent line fault or an unsuccessfulreclosure due to slow-front surges is not as severe as a busbar fault or corresponding faults in a radial network Therefore acceptable failure rates in a network can vary from point to point depending on the consequences of a failure at each of these points

Examples for acceptable failure rates can be drawn from fault statistics covering the existingsystems and from design projects where statistics have been taken into account Forapparatus acceptable failure rates Ra due to overvoltages are in the range of 0001year up to 0004year depending on the repair times For overhead lines acceptable failure rates due to lightning vary in the range of 01100 kmyear up to 20100 kmyear (the greatest number being for distribution lines) Corresponding figures for acceptable failure rates due to switching overvoltages lie in the range 001 to 0001 per operation Values for acceptable failure rates should be in these orders of magnitude




Altitude correctionAltitude correction

In general withstand or breakdown voltages must be corrected for air density (pressure temperature) and absolute humidity

Temperature and absolute humidity tend to cancel out each other Thus correction is mainly required for pressure which has its strongest influence in the altitude of installation

Therefore in the procedure of insulation coordination an altitude correction must be performed in the step from the coordination withstand voltage Ucw to the required withstand voltage Urw

Air density vs altitude 8150eH

(regression of experimental data)

where H hellip altitude above sea level in m

Voltage correction depends on voltage shape (the kind of pre-discharges) thus a voltage-dependant factor m is introduced

8150eH

mk





[IEC 60071-2]

m = 1 for LI voltagem = acc to Figure 9 for SI voltagem = 1 for short-time alternating voltagem = 05 for long-time alternating voltage and tests under pollution

Final altitude correction factor

8150a

1e

Hm

Kk

8150a

1e

Hm

Kk

approx 13 per 100 m (for m = 1) approx 13 per 100 m (for m = 1)

Note this has been proven only up to 2000 m For higher altitudes investigations still to be done EPRI China Tibet test station in 4300 m altitude




[IEC 60071-1]



we are here




From Ucw Urw

rw a s cwU K K U rw a s cwU K K U where Ka hellip altitude correction factorKs hellip safety factor taking into account

bull differences in equipment assemblybull dispersion in product qualitybull quality of installationbull aging effectsbull other unknown influences

Internal insulationbull no altitude correction (Ka = 1)bull Ks = 115 )


External insulationbull Ka = f(mH) = exp(mH8150)bull Ks = 105


) for on site tests on complete GIS a factor Ks = 125 is sometimes recommended in order to take volume effects into account




From Ucw Urw

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V

Assumption Ucw = 1000 kV

Internal insulation Urw = 1151000 kV = 1150 kVExternal insulation U rw

= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here



Insulation Coordination

For calculation examples see IEC 60071-2 Annex H







Non-self-restoring insulationbull No method at present available for the determination of the probability of disruptive dischargebull Therefore it is assumed that the withstand probability changes from 0 to 100 at the value

defining the withstand voltage bull Withstand voltage usually verified by application of a limited number of test voltages at standard

withstand level with no disruptive breakdown allowed Procedure A of IEC 60006-1

[IEC 60060-1]













[IEC 60060-1]




[IEC 60071-2]










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P(U)

(U-U50)Z
































Rod-plane


Conductor-plane

Conductor-window








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1250RP 750 2 ln(1 055 )U d 1250RP 750 2 ln(1 055 )U d





0 5009U U 0 5009U U











0650RP 500U d 0650RP 500U d



















[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here



















[IEC 60060-1]




[IEC 60071-2]










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P(U)

(U-U50)Z
































Rod-plane


Conductor-plane

Conductor-window








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1250RP 750 2 ln(1 055 )U d 1250RP 750 2 ln(1 055 )U d





0 5009U U 0 5009U U











0650RP 500U d 0650RP 500U d



















[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

2000

0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here











[IEC 60071-2]










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Pb(U)

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P(U)

(U-U50)Z
































Rod-plane


Conductor-plane

Conductor-window








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1250RP 750 2 ln(1 055 )U d 1250RP 750 2 ln(1 055 )U d





0 5009U U 0 5009U U











0650RP 500U d 0650RP 500U d



















[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

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= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here












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P(U)

(U-U50)Z
































Rod-plane


Conductor-plane

Conductor-window








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1250RP 750 2 ln(1 055 )U d 1250RP 750 2 ln(1 055 )U d





0 5009U U 0 5009U U











0650RP 500U d 0650RP 500U d



















[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here







































Rod-plane


Conductor-plane

Conductor-window








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1250RP 750 2 ln(1 055 )U d 1250RP 750 2 ln(1 055 )U d





0 5009U U 0 5009U U











0650RP 500U d 0650RP 500U d



















[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here











1250RP 750 2 ln(1 055 )U d 1250RP 750 2 ln(1 055 )U d





0 5009U U 0 5009U U











0650RP 500U d 0650RP 500U d



















[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here













0650RP 500U d 0650RP 500U d



















[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here























[IEC 60071-2]











ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here


















ff 074 026K K ff 074 026K K with






















[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here


























[IEC 60071-2]



[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here










[IEC 60071-1]



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8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here


























8150eH

mk





[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here












[IEC 60071-2]



8150a

1e

Hm

Kk

8150a

1e

Hm

Kk






[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

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0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here










[IEC 60071-1]



we are here




From Ucw Urw











From Ucw Urw

1000

1500

2000

0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here











From Ucw Urw











From Ucw Urw

1000

1500

2000

0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here











From Ucw Urw

1000

1500

2000

0 1000 2000 3000 4000Hm

Uk

V



= 105exp(H8150)1000 kV

Example (for m = 1)

asymp



[IEC 60071-1]



we arrived here










[IEC 60071-1]



we arrived here








Fachgebiet Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 7- 1 -...

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Transcript of Fachgebiet Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 7- 1 -...