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Hochspannungstechnik Overvoltage Protection and Insulation Coordination / Chapter 6

Numerical Example 1 - 1 -

Insulation Co-ordination: Numerical Example 1

Case 1: Without Capacitor Switching in Sub.2

Case 2: With Capacitor Switching in Sub.2

New Substation with Un=230kVExisting SubstationUs=Um=245kVPollution level: HeavyAltitude from sea level: 1000m

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Range I: Um

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Step 1:determination of the representative overvoltages (Urp)

-Power Frequency Voltage

Us=Um=245kVminimum Creepage Distance for insulators =25mm/kV (Pollution: Heavy)

-Temporary Overvoltages

earth fault factor

load rejection factor

earth fault and load rejection factor

-Slow-front Overvoltages

surge arrester effect

line entrance equipments

other equipment

-Fast-front Overvoltages

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earth fault factor

3 . .. 2k

1.4k

1.4 1.8k

3 ...1.85k

HV side of power transformerin Sub.1 is solidly earthed.

System study: k=1.5 !(global network simulation)

P-E Overvoltage:

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System study: k=1.4(overspeed genarator in Sub. 1)

moderately extended systems: < 1.2 p.u. for up to several minutes

widely extended systems: 1.5 p.u. for some seconds

close to turbo generator: 1.3 p.u.

close to salient pole (German: "Schenkelpol") generator: 1.5 p.u.

P-E Overvoltage:

P-P Overvoltage:

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Will be opened after load rejection

Earth Fault factorextremely decreases.(D/Y grounded of Tr.)

k load rejection* k earth fault

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Us=Um=245kVminimum Creepage Distance for insulators =25mm/kV (Pollution: Heavy)


earth fault factor k=1.5; Urp=212kV

load rejection factor k=1.4; Urp=198kV(P-E);

Urp=343kV(P-P)

earth fault and load rejection factor not applicable

Urp=212kV(P-E)

Urp=343kV(P-P)

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energization and re-energization

Range of 2% slow-front phase-to-earthovervoltages at thereceiving end due to line energization (Approximately Value)

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Range of 2% slow-front phase-to-earthovervoltages at thereceiving end due to line energization (Exact Value)

System study :

Overvoltages originating from Sub.1

Ue2=1.9 p.uUp2=2.9 p.u


Ue2=3.0 p.uUp2=4.5 p.u

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Probability distribution of the representative amplitude of theprospective overvoltage phase-to-earth

Using phase-peak method:


Ue2=1.9 p.uUet=1.25*1.9-0.25=2.125 p.u


Ue2=3.0 p.uUet=1.25*3.0-0.25=3.5 p.u

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Probability distribution of the representative amplitude of theprospective overvoltage phase-to-phase

Using phase-peak method:


Up2=2.9 p.uUpt=1.25*2.9-0.43=3.195 p.u


Up2=4.5 p.uUpt=1.25*4.5-0.43=5.195 p.u

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Surge Arrester effect

O.V from Sub. 1 O.V from Sub. 2

phase-earth Uet=425kV Uet=700kV

phase-phase Upt=639kV Upt=1039kV

energization and re-energization without S.A:

Surge Arrester Characteristics:

Using Surge Arrester:

Phase-to-earth: Ure= Ups = 410kV < 425 & 700

Phase-to-phase: the lower value of

Urp= 2 Ups= 820kV < 1039 but >639 Urp= Upt = 639kV < 820

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Us=Um=245kVminimum Creepage Distance for insulators =25kV/mm (Pollution: Heavy)


earth fault factor




surge arrester effect

line entrance equipments

other equipment

-Fast-front Overvoltages: Simplified Statistical Approach (see step2)

Urp=212kV(P-E)

Urp=343kV(P-P)

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Step 2:determination of the co-ordination withstand voltages (Ucw)


Ucw=Urp (Kc=1)

Urp=212kV(P-E)

Urp=343kV(P-P)

Ucw=212kV(P-E)

Ucw=343kV(P-P)

-Slow Front Overvoltages

S.A Characteristics:

System study :


Ue2=1.9 p.uUp2=2.9 p.u


Ue2=3.0 p.uUp2=4.5 p.u

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Line entrance equipments:

Ups / Ue2 = 410/600 = 0.68 Kcd = 1.10

Ucw= 1.10 * Urp

2Ups/ Up2 = 820/900 = 0.91 Kcd = 1.00

Ucw= 1.00 * Urp

All other equipments:

Ups / Ue2 = 410/380 = 1.08 Kcd = 1.03

Ucw= 1.03 * Urp

2Ups/ Up2 = 820/580 = 1.41 Kcd = 1.00

Ucw= 1.00 * Urp

S.A Characteristics: Ups= 410kV

Line entrance equipments:

Ue2=3.0 p.u=600kVUp2=4.5 p.u=900kV

All other equipments:

Ue2=1.9 p.u=380kVUp2=2.9 p.u=580kV

(a)

(b)

(a)

(b)

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Step. 1:

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-Fast Front Overvoltages

Simplified Statistical Approach

S.A Characteristics:

n=2

n: minimum Nos. of lines connected to the Sub.

Upl = 500A = 4500n = 2

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L = 30m for internal insulationL = 60m for external insulation

Lsp= 300m Span Length

Ra= acceptable failure rate = 1 in 400 yearsRkm= outage per 100km per year = 1

La= Ra/ Rkm= 1/400 = 0.25 km = 250 m

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Step 3: determination of the required withstand voltages (Urw)

-Ks: Safety Factor

for any type of Overvoltages (P-E & P-P)

internal insulation : Ks=1.15

external insulation : Ks=1.05

-Ka: Altitude Factor

Only for external insulation !

H=Altitude above sea level = 1000 m

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For Switchingwithstand voltages:

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Step. 2: For Switchingwithstand voltages:

Ucw=451 , curve (a): m=0.94

Ucw=820 , curve (c): m=1.00

For Lightningwithstand voltages:

For Power Frequencywithstand voltages:

m = 1.00

m = 0.50

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For Switchingwithstand voltages:

Ka= e 0.94.(1000/8150)= 1.122 (P-E)Ka= e

1.00 . (1000/8150)= 1.130 (P-P)

For Lightningwithstand voltages:

For Power Frequencywithstand voltages:

Ka= e1.00 . (1000/8150)= 1.130 (P-E & P-P)

Ka= e0.50 . (1000/8150)= 1.063 (P-E & P-P)

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Urw= Ucw . Ks . Ka

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Step 4: Conversion to withstand voltages normalized for range I

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Step 5: Selecting of standard withstand voltage values

Summary:

Urw(s): minimum required withstand voltage obtained directlyUrw(c): minimum required withstand voltage obtained by conversion

l l l

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Short duration power frequency withstand voltages are selected according to direct values Urw(s)

Lightning withstand voltages are selected as a highest value of Lightning Urw(s) orconverted valueof switching withstand voltage into lightning withstand voltage Urw(c)

S 5 S l i f d d i h d l l

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Results:minimum required withstand voltage

PFWV(kVrms) / LIWV (kVpeak) : 395/950

only P-P minimum required withstand voltage for line enterance equipement is 1127kVwhichis more than 950 kV standardized value and next standard value of 1175kVshall be selected.

Since there is no three phase equipment in line entrance, minimum P-P clearance can bespecified instead of testing. (IEC 71-2: page:101; )

St 5 S l ti f t d d ith t d lt l

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LIWV (kVpeak) P-P & P-E for other equipments: 950

LIWV (kVpeak) P-P for line equipments: 1175

2.35m P-P clearane for line equipments and 1.9m

P-P and P-E clerance for other equipments.

St 5 S l ti f t d d ith t d lt l

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Results (cont.):

Refeinment in system study leads to lower value of PFWV for external insulations. So 360kVfor PFWVand 850kVfor LIWVmay be possible to select as an insulation levels.

for internal insulation, it is economic to select 750and 850kVlightning withstand voltageforPhase to Earthand Phase to Phase, respectively. But PFWVshall be at least 395kV.

I l i C di i N i l E l 1

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Case 2: With Capacitor Switching in Sub.2

With Capacitor Switching

C s 2: C p cit r S itchin in Sub 2 (St p 1: U )

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Case 2: Capacitor Switching in Sub. 2 (Step.1: Urp)

Having a capacitor switching in Sub.2 affects only in Slow-front Overvoltages.

System study shows the following maximum amplitude of O.V in new Sub. Due to capacitorswitching in Sub. 2:

Ups< Uet and 2 Ups< Upt

Surge Arrester Characteristics:

Urp= 410 kV (P-E)Urp= 820 kV (P-P)

Case 2: Capacitor Switching in Sub 2 (Step 2: U )

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Case 2: Capacitor Switching in Sub. 2 (Step.2: Ucw)

Ups= 410 kV (P-E)2 . Ups= 820 kV (P-P)

IEC 60071-2 page: 215, Mistake!!!Kcd= 1.075 , Ucw= 441 kV;

Case 2: Capacitor Switching in Sub 2 (Step 3: U )

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Case 2: Capacitor Switching in Sub. 2 (Step.3: Urw)

Ucw= 441 kV (P-E)Ucw= 820 kV (P-P)

Ucw=441 , curve (a): m=0.94Ucw=820 , curve (c): m=1.00

Ka= e 0.94(1000/8150)= 1.122 (P-E)Ka= e

1.0(1000/8150)= 1.130 (P-P)

Safety Factor :internal insulation : Ks=1.15external insulation : Ks=1.05

-External insulation:(P-E): Urw= 441 * 1.05 * 1.122 = 519 kV(P-P): Urw= 820 * 1.05 * 1.13 = 973 kV

-Internal insulation:(P-E): Urw= 441 * 1.15 = 507kV(P-P): Urw= 820 * 1.15 = 943 kV

Case 2: Capacitor Switching in Sub 2 (Step 4: Conversion)

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Case 2: Capacitor Switching in Sub. 2 (Step.4: Conversion)

-Conversion to short-duration power-frequency withstand voltage (SDW):-External Insulation:(P-E): SDW = 519 * (0.6 + 519 / 8500) = 343 kV;(P-P): SDW = 973 * (0.6 + 973 / 12700) = 658 kV;-Internal Insulation:(P-E): SDW = 507 * 0.5 = 254 kV;(P-P): SDW = 943 * 0.5 = 472 kV.

-Conversion to lightning impulse withstand voltage (LIW):-External Insulation:(P-E): SDW = 519 * 1.3 = 675 kV;(P-P): SDW = 973 * (1.05 + 973 / 9000) = 1127 kV;-Internal Insulation:(P-E): SDW = 507 * 1.1 = 558 kV;

(P-P): SDW = 943 * 1.1 = 1037 kV.

Case 2: Capacitor Switching in Sub 2 (Step 5)

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Summary:

Urw(s): minimum required withstand voltage obtained directlyUrw(c): minimum required withstand voltage obtained by conversion

Urw

- kV rms for PFWV

-kV peak for SIWV or LIWV

External Insulation Internal

Insulation

Urw(s)

Urw(c)

Urw(s)

Urw(c)

PFWV

P-E 237 343 243 254

P-P 383 658 395 472

SIWV

P-E 519 - 507 -

P-P 973 - 943 -

LIWV

P-E 884 675 715 558

P-P 884 1127 715 1037

Case 2: Capacitor Switching in Sub. 2 (Step.5)

Case 2: Capacitor Switching in Sub 2 (Step 5)

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Case 2: Capacitor Switching in Sub. 2 (Step.5)

Results:

minimum required withstand voltagePFWV(kVrms) / LIWV (kVpeak) : 395/950

only P-P minimum required withstand voltage for all equipments(not onlyline enteranceequipement) is 1127kVwhich is more than 950 kV standardized value and next standard valueof 1175kVshall be selected.

2.35m P-P clearane for all external equipments needed if no P-P test would like to beprovided.

for internal insulation, 460/1050 kV (due to capacitor switching in Sub.2)

Case 2: Capacitor Switching in Sub 2 (Comparision)

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Case 2: Capacitor Switching in Sub. 2 (Comparision)

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