16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
548
Surge Transfer Study for Power Transformer Using
EMTDC/PSCAD
Veerabrahmam Bathini, Chandra Shekhar Reddy Atla, Dr. K. Balaraman and K. Parthasarathy
Abstract- 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.
1
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 turn’s 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
Veerabrahmam Bathini, Sr. Power system Engineer, is with M/s
Power research and development consultants Pvt. Ltd, Bangalore,
India. (e-mail:veerabrahmam@prdcinfotech.com)
Chandra Shekhar Reddy Atla, Power system Engineer, is with M/s
Power research and development consultants Pvt. Ltd, Bangalore,
India. (e-mail:sekhar.atla@gmail.com)
Dr. K Balaraman, CGM, Power System Group, is with M/s PRDC
Pvt. Ltd., Bangalore. (e-mail: balaraman@prdcinfotech.com)
Prof. K Parthasarathy, Retired Professor from IISc, Bangalore.
1
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
Type of system grounding
60 Hz
HV Solidly
LV Solidly
TV Effectively
6
7
Common neutral (autotransformers)
8
Short circuit Impedances : %Z (on 500 ZHL=19.3%
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
Solidly
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
Value
HV
LV
e1t
CLT = 5033.8pF
5033.8e-6 [uF]
Terminal to ground capacitances and
terminal to terminal capacitances
3023.8e-6 [uF]
9
ZHT=268.75%
ZLT=243.38%
CHG = 5854.1pF
CLG = 8057.6 pF
CTG = 14424.4 pF
CHL = 3023.8 pF
CHT = 1889 pF
8057.6e-6 [uF]
MVA base)
performing surge transfer study. The surge arrester model is
presented in Fig. 3.
e1l
LVa
5854.1e-6 [uF]
In PSCAD/EMTDC, the built in model for three phase
autotransformer with tertiary is not available. Hence the
modeling implementation is described in this section. Three
single phase three winding transformers are used to represent
three phase autotransformer with tertiary as shown in Fig.1.
The leakage impedances for three single phase three winding
transformers can be determined from the leakage impedances
of the autotransformer by using the procedure described in
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, terminal
capacitances mentioned in Table 1 are connected as shown in
Fig. 2.
TV
e1r
14424.4e-6 [uF]
S.No. Parameter
549
1889e-6 [uF]
Fig. 2: High frequency Autotransformer model
Fig. 3: Frequency dependent surge arrester model proposed by Pinceti [5].
HVa
#2
TVa
#1
#3
LVb
HVb
#2
#1
TVb
#3
HVc
LVc
#2
#1
TVc
#3
This model is composed by two sections of non-linear
resistance usually designated by A0 and A1 which are
separated by inductance L1 and L0. The resistance R (about 1
MΩ) is added to avoid the numerical problems. The values L1
and L0 are computed based on the procedure described in [5].
The computation procedure is described in flow chart shown
in Fig. 4. Vn is arrester rated voltage (kV), Vr8/20 is the residual
voltage (kV) for the discharge current of 10 kA, 8/20 µs
impulse, Vr1/T2 is the residual voltage (kV) for the discharge
current 10 kA, 1/T2 µs steep front impulse. The fall time T2
can vary between 2 and 20 µs. The nonlinear resistors A0 and
A1 can be modeled as a piecewise linear V-I curves. V-I
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. 5.
Fig. 1: Autotransformer model in PSCAD/EMTDC.
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 µs to several µs. 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 µs to 1.3 µs. One approach for an arrester model for
lightning studies would be to use a simple non-linear V-I
characteristics based on 0.5 µs 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 µs
to 40 µs [4]. The surge arrester model proposed by Pinceti [5]
derived from IEEE model [4] is used in the present paper for
Data Sheet
B.
Vr1/T2
K=Vr1/T2/Vr8/20
K<1.18
−V
1 V
L1 = . r1/ T 2 r 8/ 20 .Vn
4
Vr 8/ 20
L0 =
L1=0.03 Vn
L0=0.01 Vn
1 Vr1/ T 2 − Vr 8/ 20
.
.Vn
12
Vr 8/ 20
Fig. 4: Flowchart to calculate elements L0 and L1 [5].
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16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
The V-I characteristic of A0 and value of L1 in the model have
to be properly adjusted to match the manufacturer’s 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 manufacturer’s
switching surge voltages and currents.
of nonlinear elements A0 and A1
proposed by IEEE W.G. 3.4.11 [4].
Fig. 5: Characteristics
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 µs 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.
550
TABLE 3
120 kV FREQUENCY DEPENDENT SURGE ARRESTER PARAMETERS
Rated arrester voltage (kVrms)
120
MCOV (kVrms)
97
Leakage current at MCOV (mA)
5
TOV 1 sec rating (kVrms)
139
TOV 10 sec rating (kVrms)
132
Maximum residual
30/60µs switching (0.5 , 233) , (1, 240 ), (2, 255),
voltage (kV crest) at
surge current
(3, 258)
discharge of
8/20µs lightning
(5, 264), (10, 273), (20, 291)
(kAp, kVp)
surge current
[provided by
0.5 µs steep front
(10, 335), (20, 372 )
manufacturer, A1]
current
V-I characteristics of A0 (Adjusted)
(0.5, 289), (1, 296), (2, 306),
(kAp, kVp)
(3, 314), (5, 320), 10, 335),
(20, 372)
Line discharge class [Energy Absorption]
4 [ 1440 kJ]
TABLE 4
12 kV FREQUENCY DEPENDENT SURGE ARRESTER PARAMETERS
Rated arrester voltage (kVrms)
12
MCOV (kVrms)
10.2
Leakage current at MCOV (mA)
5
TOV 1 sec rating (kVrms)
14.0
TOV 10 sec rating (kVrms)
13.2
Maximum residual
30/60µs switching (0.5, 25.7) , (1, 26.7)
voltage (kV crest) at
surge current
discharge of
8/20µs lightning
(1.5, 27.6 ), (3, 29.1), (5, 30.2),
(kAp, kVp)
surge current
(10, 32.4), (20, 35.9), (40, 40.2)
[provided by
0.5 µs steep front
(10, 40), (20, 44)
manufacturer, A1]
current
V-I characteristics of A0 (Adjusted )
(0.5, 34.3), (1, 35.2), (2, 36.3),
(kAp, kVp)
(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
L1(Adjusted)
Surge Arrester rating L0
[µH] [µH]
360 kV
2.42
35.0
120 kV
1.2
3.6
12 kV
0.12
0.36
III. CASE STUDIES
TABLE 2
360 kV FREQUENCY DEPENDENT SURGE ARRESTER PARAMETERS
Rated arrester voltage (kVrms)
360
MCOV- Maximum Continuous Operating
289
Voltage (kVrms)
Leakage current at MCOV (mA)
5
TOV 1 sec rating (kVrms)
410
TOV 10 sec rating (kVrms)
388
Maximum residual
30/60µs switching (0.5 , 674) , (1, 692), (2, 712),
voltage (kV crest) at
surge current
(3, 725)
discharge of
8/20µs lightning
(5, 761), (10, 792), (20, 856),
(kAp, kVp)
surge current
(30, 899)
[provided by
0.5 µs steep front
(10, 856), (20, 927)
manufacturer, A1]
current
V-I characteristics of A0 (Adjusted )
(0.5, 721), (1, 739), (2, 764),
(kAp, kVp)
(3, 783), (5, 798), 10, 856),
(20, 927)
Line discharge class [Energy Absorption]
4 [ 4320 KJ]
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 autotransformer 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 µs switching
impulse, 20 kA, 8/20 µs lightning current impulse and 20 kA,
0.5/20 µs 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 µs
The study results with switching current impulse of 3 kA,
30/60 µs as shown in Fig. 7 at autotransformer terminals are
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
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 µs
The study results with lightning current impulse of 20 kA,
8/20 µs as shown in Fig. 8 at autotransformer terminals are
presented in Table 7 and the corresponding voltage waveforms
at the tertiary are shown in Figures 11 to 13.
Case study 3: Steep front current impulse of 20kA, 0.5/20 µs
The study results with lightning current impulse of 20 kA,
0.5/20 µs as shown in Fig. 9 at autotransformer terminals are
presented in Table 8 and the corresponding voltage waveforms
at the tertiary are shown in Figures 14 to 16.
Fig. 6: switching impulse current, 3kA, 30/60 µs
Fig. 7: Lightning impulse current, 20kA, 8/20 µs
551
TABLE 6
SWITCHING SURGES TRANSFERRED THROUGH TRANSFORMER
Refer
Case Switching Energy absorbed Voltage at Transformer
surge
by SA at [KJ]
terminal [kVp] and
Figures
applied at
Corresponding %BIL
transformer
HV LV TV HV LV %BIL TV %BIL
terminal
side side side
[SA
location]
1
HV side 114.6 22.0
- 220 34
87
93
Fig. 9
[HV, LV]
2
LV side
[HV, LV]
9.0
33.3
-
612
-
58.3
3
HV side
[HV,
LV,TV]
119
17.5
1.5
-
220
34
4
LV side
[HV,
LV,TV]
12.1
32.4
0.1 612
-
58.3
63
66.3
23.2 25.2
23
24.2
Fig. 10
-
TABLE 7
LIGHTNING SURGES TRANSFERRED THROUGH TRANSFORMER
Case Switching Energy absorbed Voltage at Transformer
Refer
surge
by SA at [KJ]
terminal [kVp] and
Figures
applied at
Corresponding %BIL
transformer
HV LV TV HV LV %BIL TV %BIL
terminal
side side side
[SA
location]
1
HV side
339 6.8
- 220 34
121 128
Fig. 11
[HV, LV]
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 Energy absorbed Voltage at Transformer
Refer
surge
by SA at [KJ]
terminal [kVp] and
Figures
applied at
Corresponding %BIL
transformer
HV LV TV HV LV %BIL TV %BIL
terminal
side side side
[SA
location]
1
HV side
433 17.7
- 249 38.5 155 163
Fig. 14
[HV, LV]
2
LV side
[HV, LV]
22.5 142.6
3
HV side
[HV,
LV,TV]
434
4
LV side
[HV,
LV,TV]
16.6
-
615
17.2
5.3
-
143
0.2 615
Fig. 8: Steep front impulse current, 20kA, 0.5/20 µs
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
- 47.3
128 135
Fig. 15
222 34
43
45.3
Fig. 16
- 47
45
47.4
-
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
552
Fig. 9: Voltage at TV side of transformer for case 1, Table 6.
Fig. 13: Voltage at TV side of transformer for case 4, Table 7.
Fig. 10: Voltage at TV side of transformer for case 3, Table 6.
Fig. 14: Voltage at TV side of transformer for case 1, Table 8.
Fig. 11: Voltage at TV side of transformer for case 1, Table 7.
Fig. 15: Voltage at TV side of transformer for case 2, Table 8.
Fig. 12: Voltage at TV side of transformer for case 2, Table 7.
Fig. 16: Voltage at TV side of transformer for case 3, Table 8.
Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, A.P, INDIA.
16th NATIONAL POWER SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010
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
553
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
V I = 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
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]
[2]
[3]
[4]
[5]
[6]
[7]
J.C. Das, “Surges transferred through transformers”, IEEE Conference
on pulp and Industry technical conference, 2002, pp. 139-147.
IEC 60071-2, “Insulation co-ordination: part 2: Application guide”, third
edition, 1996-12.
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.
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.
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.
IEC 60099-5, “Surge Arresters- Part 5 – Selection and Application
Recommendations”, edition 1.1, March 2000.
Andrew R. Hileman, “Insulation Coordination for Power Systems”,
Taylor & Francis Publications, 1999.
APPENDIX: AUTOTRANSFORMER
 VH 
Z I , II = Z HL 
(2)
 in p.u.
 VH − VL 
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
V − VL
VI = Z I H
+ Z III , in p.u.
(4)
VH
VL
(5)
+ Z III , in p.u.
VH
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
VII = Z II
2
2
 V − VL 
 VL 
Z HT = Z I  H
(6)
 + Z II 
 + Z III in p.u
 VH 
 VH 
Eqs. (2), (3) and (6) can be solved for ZI, ZII, ZIII since ZI,II =
ZI+ZII and ZII,III = ZII+ZIII,
VH VV
VH
VL
Z I , III = Z HL
+ Z HL
− Z LT
in p.u. (7)
VH − VL
VH − VL
(VH − VL ) 2
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).
Fig. 17: Autotransformer with Tertiary Winding
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