73638665 auto transformer insulation coordination study

Surge Transfer Study for Power Transformer Using

EMTDC/PSCAD

Veerabrahmam Bathini, Chandra Shekhar Reddy Atla, Dr. K. Balaraman and K. Parthasarathy

1Abstract- The lightning and switching surges can be transferred

from one voltage level to another through transformer couplings.

A distribution system, which may not be directly exposed to the

overvoltages of atmospheric origin, but connected to a utility

system through a transformer of high turns ratio will be exposed

to overvoltages on the secondary side due to overvoltages on the

primary windings. The resulting stresses on the distribution

system may exceed the BIL levels. This paper presents modeling

of high frequency autotransformer and frequency dependent

surge arrester models and results of simulations for lightning and

switching surges transferred through 502 MVA, 380/132/13.8 kV

autotransformer using EMTDC/PSCAD. Surge arresters are

usually provided on the high voltage side and low voltage side of

the autotransformers. The purpose of the present paper is to

analyze the surges transferred towards tertiary of

autotransformer. If these surges are to be controlled to safe levels

it may be necessary to provide the surge arresters at tertiary side

also. This aspect has been highlighted in present paper.

I. INTRODUCTION

The most common primary distribution voltage in industrial

systems is 13.8/11 kV. However, for large power demands, the

utility system voltage may be as high as 380/400 kV. The

surge transfer through the transformers depends upon the

voltage turn ratio, as well as electrostatic and electromagnetic

couplings of the windings. The lightning and steep fronted

waves are partially transferred through the electromagnetic

coupling, which is the mechanism that governs the

transformer operation at power frequencies and depends upon

the turns ratio. The magnitude of these surges transferred

through electromagnetic coupling is far less than the

magnitude of surges transferred through electrostatic coupling

hence electrostatic effects dominate the coupling of transients

from the primary to the secondary windings. For slower

switching surges, the electromagnetic coupling effect

predominates [1]. The overvoltages caused by transfer of

lightning and steep fronted waves or switching surges are

compared with BIL of the equipments on low voltage side. In

case the magnitude of transferred overvoltages exceed the

BIL levels, mitigation techniques like provision of properly

rated surge arresters (SA), surge capacitors etc., have to be

1

Veerabrahmam Bathini, Sr. Power system Engineer, is with M/s

Power research and development consultants Pvt. Ltd, Bangalore,

India. (e-mail:[email protected])

Chandra Shekhar Reddy Atla, Power system Engineer, is with M/s

Power research and development consultants Pvt. Ltd, Bangalore,

India. (e-mail:[email protected])

Dr. K Balaraman, CGM, Power System Group, is with M/s PRDC Pvt. Ltd., Bangalore. (e-mail: [email protected])

Prof. K Parthasarathy, Retired Professor from IISc, Bangalore.

employed to control these overvoltages. This paper

concentrates on mitigation technique provided by surge

arrester.

The selection of an appropriate surge arrester is an important

consideration. System overvoltages under normal and faulted

conditions, system grounding and ground fault clearance times

should be considered in selecting a surge arrester. The

selection procedure is as follows [2] [7].

Arrester rated voltage (Vn): selected based on maximum temporary overvoltages (TOV) appearing

in the power network, considering earth fault factor.

Maximum continuous operating voltage (MCOV): selected based on the maximum system steady state

operating voltage.

Energy Capability: selected based on switching and lightning overvoltage studies.

This paper presents the modeling of high frequency

autotransformer and frequency dependent surge arrester to

conduct surge transfer studies for 502 MVA, 380/132/13.8 kV

autotransformer using EMTDC/PSCAD. Considering a worst

case scenario for simulation, the lightning impulse or

switching impulse injected currents at high voltage (HV) and

low voltage (LV) terminals of the autotransformer are selected

based on the V-I characteristics of corresponding surge

arresters. The modeling methodologies, data considered for

case study and simulation results are presented in following

sections.

II. MODELING

This section presents the modeling details of 502 MVA,

380/132/13.8 kV autotransformer and surge arresters.

A. Autotransformer Model

The parameter specifications of 502 MVA, 380/132/13.8 kV

autotransformer provided by manufacturer are presented in

Table 1. TABLE 1

AUTOTRANSFORMER PARAMETERS

S.No. Parameter Value

1 Rated capacity 502 MVA

2 Rated voltages (High/medium/low) 380/132/13.8 kV

3 Lightning BIL (High/medium/low) 1300/650/95 kV

4 Switching BIL (High/medium/low) 1050/650/95 kV

5 frequency 60 Hz

6

Type of system grounding

HV Solidly

LV Solidly

TV Effectively

7 Common neutral (autotransformers) Solidly

8 Short circuit Impedances : %Z (on 500 ZHL=19.3%

16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010 548

Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.

S.No. Parameter Value

MVA base) ZHT=268.75%

ZLT=243.38%

9 Terminal to ground capacitances and terminal to terminal capacitances

CHG

CLG

CTG

CHL

CHT

CLT = 5033.8pF

In PSCAD/EMTDC, the built in model for

autotransformer with tertiary is not available.

modeling implementation is described in this section.

single phase three winding transformers are used to

three phase autotransformer with tertiary as shown in Fig

The leakage impedances for three single phase three winding

transformers can be determined from the leakage impedances

of the autotransformer by using the procedure

Appendix [3]. The input data required for this model can be

extracted from the data provided in Table 1. To represent high

frequency model for the autotransformer

capacitances mentioned in Table 1 are connected as shown in

Fig. 2.

Fig. 1: Autotransformer model in PSCAD/EMTDC.

B. Frequency Dependent Surge Arrester Modeling

Surge arrester dynamic characteristics are significant for

studies involving lightning and other fast transient surges. The

time to crest for surges used in lightning studies can range

from 0.5 s to several s. For a given current magnitude in an

arrester, the voltage developed across the arrester can increase

by approximately 6% as time to crest of current is decreased

from 8 s to 1.3 s. One approach for an arrester model for

lightning studies would be to use a simple non

characteristics based on 0.5 s discharge voltage. This would

give conservative results (higher voltages) for surges with

slower time to crest. The frequency dependent model will give

good results for current surges with times to crest from 0.5

to 40 s [4]. The surge arrester model proposed by Pinceti [

derived from IEEE model [4] is used in the present paper for

#2

#3

#1

#2

#3

#1

#2

#3

#1

HVa

HVb

HVc

Value

=268.75%

=243.38%

= 5854.1pF

= 8057.6 pF

= 14424.4 pF

= 3023.8 pF

= 1889 pF

= 5033.8pF

model for three phase

is not available. Hence the

modeling implementation is described in this section. Three

used to represent

as shown in Fig.1.

The leakage impedances for three single phase three winding

transformers can be determined from the leakage impedances

procedure described in

The input data required for this model can be

the data provided in Table 1. To represent high

autotransformer, terminal

connected as shown in

model in PSCAD/EMTDC.

Modeling

Surge arrester dynamic characteristics are significant for

studies involving lightning and other fast transient surges. The

time to crest for surges used in lightning studies can range

to several s. For a given current magnitude in an

arrester, the voltage developed across the arrester can increase

by approximately 6% as time to crest of current is decreased

. One approach for an arrester model for

would be to use a simple non-linear V-I

discharge voltage. This would

give conservative results (higher voltages) for surges with

slower time to crest. The frequency dependent model will give

o crest from 0.5 s

model proposed by Pinceti [5]

[4] is used in the present paper for

performing surge transfer study. The surge arrester model is

presented in Fig. 3.

Fig. 2: High frequency Autotransformer model

Fig. 3: Frequency dependent surge arrester model proposed by Pinceti

This model is composed by two sections of non

resistance usually designated by

separated by inductance L1 and L0. T

M) is added to avoid the numerical problems.

and L0 are computed based on the procedure described in

The computation procedure is described in flow chart shown

in Fig. 4. Vn is arrester rated voltage (

voltage (kV) for the discharge current of

impulse, Vr1/T2 is the residual voltage

current 10 kA, 1/T2 s steep front

can vary between 2 and 20 s. The nonlinear resistors

A1 can be modeled as a piecewise linear

characteristic of A1 arrester is selected from manufacturer data

sheet and V-I characteristic of A0 is selected based on curves

proposed by IEEE W.G.3.4.11 [4] which are shown in

Fig. 4: Flowchart to calculate elements

LVa

LVb

LVc

TVa

TVb

TVc

e1t

e1l

HV

5854.1e-6 [uF]

8057.6e-6 [uF]3023.8e-6 [uF]

1889e-6 [uF]

K=Vr1/T2/Vr8/20

K

The V-I characteristic of A0 and value of L1 in the model have

to be properly adjusted to match the manufacturers data with

respect to switching and lighting characteristics.

Adjustment of V-I characteristics of A0 to match switching

surge Voltages:

The V-I characteristics of A0 are adjusted in surge arrester

model to get a good match between model and manufacturers

switching surge voltages and currents.

Fig. 5: Characteristics of nonlinear elements A0 and A1

proposed by IEEE W.G. 3.4.11 [4].

Adjustment of L1 to match V8/20 voltages:

The value of L1 in model is adjusted with V-I characteristic

of A1 and modified V-I characteristics of A0 to obtain a good

match between the manufacturer data and model discharge

voltages for an 8/20 s current.

The frequency dependent surge arrester models used for the

case studies are presented Tables 2, 3 and 4 and L0 and L1

values are presented in Table 5.

TABLE 2

360 kV FREQUENCY DEPENDENT SURGE ARRESTER PARAMETERS Rated arrester voltage (kVrms) 360

MCOV- Maximum Continuous Operating

Voltage (kVrms)

289

Leakage current at MCOV (mA) 5

TOV 1 sec rating (kVrms) 410


Maximum residual

voltage (kV crest) at

discharge of (kAp, kVp)

[provided by

manufacturer, A1]

30/60s switching

surge current

(0.5 , 674) , (1, 692), (2, 712),

(3, 725)

8/20s lightning surge current

(5, 761), (10, 792), (20, 856), (30, 899)

0.5 s steep front

current

(10, 856), (20, 927)

V-I characteristics of A0 (Adjusted )

(kAp, kVp)

(0.5, 721), (1, 739), (2, 764),

(3, 783), (5, 798), 10, 856),

(20, 927)

Line discharge class [Energy Absorption] 4 [ 4320 KJ]

TABLE 3


MCOV (kVrms) 97




Maximum residual

voltage (kV crest) at discharge of

(kAp, kVp)

[provided by manufacturer, A1]

30/60s switching

surge current

(0.5 , 233) , (1, 240 ), (2, 255),

(3, 258)

8/20s lightning

surge current

(5, 264), (10, 273), (20, 291)

0.5 s steep front current

(10, 335), (20, 372 )

V-I characteristics of A0 (Adjusted)

(kAp, kVp)

(0.5, 289), (1, 296), (2, 306),

(3, 314), (5, 320), 10, 335),

(20, 372)

Line discharge class [Energy Absorption] 4 [ 1440 kJ]

TABLE 4


MCOV (kVrms) 10.2


TOV 1 sec rating (kVrms) 14.0

TOV 10 sec rating (kVrms) 13.2

Maximum residual

voltage (kV crest) at discharge of

(kAp, kVp)

[provided by manufacturer, A1]

30/60s switching

surge current

(0.5, 25.7) , (1, 26.7)

8/20s lightning

surge current

(1.5, 27.6 ), (3, 29.1), (5, 30.2),

(10, 32.4), (20, 35.9), (40, 40.2)

0.5 s steep front current

(10, 40), (20, 44)

V-I characteristics of A0 (Adjusted )

(kAp, kVp)

(0.5, 34.3), (1, 35.2), (2, 36.3),

(3, 37.2), (5, 38), 10, 40),

(20, 44)

Line discharge class [Energy Absorption] 4 [ 90 kJ]

TABLE 5

L0 and L1 VALUES FOR 360 kV, 120 kV and 12 kV SURGE ARRESTERS Surge Arrester rating L0

[H]

L1(Adjusted)

[H]

360 kV 2.42 35.0

120 kV 1.2 3.6

12 kV 0.12 0.36

III. CASE STUDIES

Case studies have been performed for 502 MVA,

380/132/13.8 kV autotransformer in order to find the surges

transferred to tertiary voltage (TV) side with lightning or

switching or steep front impulse applied at HV or LV

terminals. The Basic Insulation Levels (BIL) for the auto-

transformer is presented in Table 1.

Considering a worst case scenario for simulation, the

lightning impulse or switching impulse injected currents at HV

or LV terminals of the autotransformer are selected based on

the V-I characteristics of corresponding surge arresters. The

generated impulse currents namely 3 kA, 30/60 s switching

impulse, 20 kA, 8/20 s lightning current impulse and 20 kA,

0.5/20 s steep front current impulse, presented in Fig. 6, 7

and 8 respectively, are used in the simulation.

Case study 1: Switching current impulse of 3kA, 30/60 s

The study results with switching current impulse of 3 kA,

30/60 s as shown in Fig. 7 at autotransformer terminals are



presented in Table 6 and the corresponding voltage waveforms

at the tertiary are shown in Figures 9 and 10.

Case study 2: Lightning current impulse of 20kA, 8/20 s

The study results with lightning current impulse of 20 kA,

8/20 s as shown in Fig. 8 at autotransformer terminals are


at the tertiary are shown in Figures 11 to 13.

Case study 3: Steep front current impulse of 20kA, 0.5/20 s

The study results with lightning current impulse of 20 kA,

0.5/20 s as shown in Fig. 9 at autotransformer terminals are


at the tertiary are shown in Figures 14 to 16.

Fig. 6: switching impulse current, 3kA, 30/60 s

Fig. 7: Lightning impulse current, 20kA, 8/20 s

Fig. 8: Steep front impulse current, 20kA, 0.5/20 s

TABLE 6

SWITCHING SURGES TRANSFERRED THROUGH TRANSFORMER Case Switching

surge

applied at transformer

terminal

[SA location]

Energy absorbed

by SA at [KJ]

Voltage at Transformer

terminal [kVp] and

Corresponding %BIL

Refer

Figures

HV side

LV side

TV side

HV LV %BIL TV %BIL

1 HV side

[HV, LV]

114.6 22.0 - - 220 34 87 93 Fig. 9

2 LV side [HV, LV]

9.0 33.3 - 612 - 58.3 63 66.3 -

3 HV side

[HV, LV,TV]

119 17.5 1.5 - 220 34 23.2 25.2 Fig. 10

4 LV side

[HV, LV,TV]

12.1 32.4 0.1 612 - 58.3 23 24.2 -

TABLE 7

LIGHTNING SURGES TRANSFERRED THROUGH TRANSFORMER Case Switching

surge

applied at transformer

terminal

[SA location]

Energy absorbed by SA at [KJ]

Voltage at Transformer terminal [kVp] and

Corresponding %BIL

Refer Figures

HV side

LV side

TV side

HV LV %BIL TV %BIL

1 HV side

[HV, LV]

339 6.8 - - 220 34 121 128 Fig. 11

2 LV side

[HV, LV]

11 116 - 610 - 47 86 91 Fig. 12

3 HV side

[HV, LV,TV]

335 7.1 5.0 - 220 34 29 30.5 -

4 LV side

[HV, LV,TV]

7.4 114 0.15 610 - 47 30 31.6 Fig. 13

TABLE 8

STEEP FRONT SURGES TRANSFERRED THROUGH TRANSFORMER Case Switching

surge

applied at

transformer terminal

[SA

location]

Energy absorbed by SA at [KJ]

Voltage at Transformer terminal [kVp] and

Corresponding %BIL

Refer Figures

HV

side

LV

side

TV

side

HV LV %BIL TV %BIL

1 HV side

[HV, LV]

433 17.7 - - 249 38.5 155 163 Fig. 14

2 LV side

[HV, LV]

22.5 142.6 - 615 - 47.3 128 135 Fig. 15

3 HV side

[HV,

LV,TV]

434 17.2 5.3 - 222 34 43 45.3 Fig. 16

4 LV side

[HV,

LV,TV]

16.6 143 0.2 615 - 47 45 47.4 -



Fig. 9: Voltage at TV side of transformer for case 1, Table 6.










According to IEC Standard 60099-5[6], considering a safety

factor of 1.15, the acceptable overvoltages must be within

87% of BIL values. From the case studies, it is seen that with

surge arresters at autotransformer HV and LV terminals the

overvoltages at tertiary side of the transformer are high and

beyond the recommended BIL levels. Hence in addition to

provision of surge arresters at HV and LV sides of

transformer, 12 kV surge arrester at tertiary side of transformer

is required to limit the overvoltages to safe levels. It is also

seen that the energy absorbed by the three surge arresters are

well within the allowable ratings as presented in Tables 2, 3

and 4.

IV. CONCLUSIONS

The paper presents the modeling details of high frequency

autotransformer and frequency dependent surge arrester for

surge transfer studies. The results of simulations for lightning

and switching surges transferred through 502 MVA,

380/132/13.8 kV autotransformer using EMTDC/PSCAD are

presented. Three case studies have been performed to

determine need for surge arrester at tertiary of

autotransformers. Based on these studies it is observed that in

addition to surge arresters at HV and LV side of

autotransformer, surge arrester is required at tertiary side of

the transformer to limit the overvoltages to safe levels.

REFERENCES [1] J.C. Das, Surges transferred through transformers, IEEE Conference

on pulp and Industry technical conference, 2002, pp. 139-147.

[2] IEC 60071-2, Insulation co-ordination: part 2: Application guide, third edition, 1996-12.

[3] V. Brandwajn, H.W. Dommel, I.I. Dommel, Matrix Representation of

three-phase N-winding transformers for the steady state and transient studies, IEEE Transactions on Power Apparatus and Systems, Vol.

PAS-101, No.6, June 1982, pp.1369-1378.

[4] IEEE working Group 3.4.11, Application of surge protective devices subcommittee, Surge protective Devices Committee, Modeling of

Metal Oxide Surge Arresters, IEEE Transactions on Power Delivery,

Vol. 7, no.1, January 1992, pp. 302-309. [5] Micaela Caserza, Marco Giannettoni, Paolo Pinceti, Validation of ZnO

Surge Arresters Model for Overvoltage Studies, IEEE Transactions on

Power Delivery, vol. 19, no.4, Oct. 2004, pp-1692-1695. [6] IEC 60099-5, Surge Arresters- Part 5 Selection and Application

Recommendations, edition 1.1, March 2000. [7] Andrew R. Hileman, Insulation Coordination for Power Systems,

Taylor & Francis Publications, 1999.

APPENDIX: AUTOTRANSFORMER

Fig. 17: Autotransformer with Tertiary Winding

For a accurate representation of autotransformer the high and

low voltage terminals should be represented with the actual

common winding II and series winding I, as shown in Fig. 17

[3] for autotransformer with a tertiary winding III.

This requires a re-definition of the short circuit data in terms

of windings I, II, III, with their voltage ratings

VI = VH - VL

VII = VL (1)

VIII = VT

The test between H and L is already the correct test between

I and II, since II is shorted and the voltage is applied across I

with b and c being at the same potential through the short

circuit connection. Therefore, simply change ZHL to the new

voltage base VI,

2

,. .H

I II HL

H L

VZ Z in p u

V V

=

(2)

No modifications are required for the test between II and III,

ZII,III = ZLT in p.u. (3)

For the test between H and T, the modification can best be

explained in terms of the equivalent star-circuit of Fig. 17,

with the impedances being ZI, ZII, ZIII, based on VI, VII, VIII, in

this case. With III short circuited, 1 p.u. current (based on VIII

= VT) will flow through ZIII. This current will also flow

through I and II as 1 p.u. based on VH, or converted to bases

VI, VII, II = (VH VL) / VH and III = VL / VH. With these

currents, p.u. voltages become

, . .H LI I III

H

V VV Z Z in p u

V

= + (4)

, . .LII II III

H

VV Z Z in p u

V= + (5)

Converting VI and VII to physical units by multiplying eq.(4)

with (VH VL) and eq. (5) with VL, adding them up, and

converting the sum back to a p.u. value 2 2

.H L LHT I II III

H H

V V VZ Z Z Z in p u

V V

= + +

(6)

Eqs. (2), (3) and (6) can be solved for ZI, ZII, ZIII since ZI,II =

ZI+ZII and ZII,III = ZII+ZIII,

, 2. .

( )

H V H L

I III HL HL LT

H L H LH L

V V V VZ Z Z Z in p u

V V V VV V= +

(7)

The autotransformer of Fig. 17 can therefore be treated as a

transformer with 3 windings I, II, III by simply re-defining the

short circuit input impedances with eqs. (2), (3) and (7).



73638665 auto transformer insulation coordination study

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