International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Reduction of Inrush Current for Transformer Using
Sequential Switching Method
Ketan Gohil
Department of electrical engineering
G.H.Patel college of engineering
Anand, India
gohilketan369@gmail.com
Jatinkumar Patel
Department of electrical engineering
G.H.Patel college of engineering
Anand, India
jatinpatel@gcet.ac.in
Chirag Parekh
Plant coordinator
Atlanta electricals Pvt. Ltd.
Anand, India
chirag.parekh@atlantaelectricals.com
Abstract- Nowadays, solar power becomes one of the most
suitable renewable energy source for power generation. In practice,
inrush current is produced in grid connected transformer(s) during
routine switching process of solar power plant. This inrush current
effects to winding of transformer as well as produces harmonics
and power quality issues. In this paper the calculation of inrush
current and its effect on power system network is elaborated.
Possible cost effective solution for reduction of inrush current by
suitable techniques is discussed in this paper. In sequential
switching technique, better suppression of inrush current and
harmonics is obtained.
Index terms—Modelling, Matlab, Transformer, Inrush current,
solar power plant, switching method.
I. INTRODUCTION
A demand of electric power is increasing due to industrial and
mankind growth rapidly. Therefore to fulfill the requirement of
Electric power, apart from the conventional source of energy, the
renewable source of energy is being more popular compared to
other renewable sources of energy. Growth of solar power
installation has been linearly increased and became key player
for produce of electricity. Total installed capacity of solar power
in India 5130 MW and Gujarat state is 1024.15 MW up to the till
date [1].
Solar power plants are switched off at night due to some
problems such as reverse power flow and reactive power
compensation problem. Next day, at early morning, when solar
plant is switched on in case of grid tide, due to switching effect,
high value of inrush current is drawn by transformer due to
instantaneous switching effects. Also transformer draws high
amount of current when it converts its state from de-energized to
energized state which measured as inrush current which should
be mitigate effectively to overcome the power quality issues.
Amount of inrush current is 8 to 10 times higher than rated
current [2].
During starting, amount of high inrush current for several
cycles has been fed to grid. But it slowly decreases and finally
reaches at normal rated current which is shown in Fig. 1.
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
Fig.1. pattern of inrush current
Electrical power is unable to generate during night time. As a
result, solar power plant auxiliaries draw the certain amount of
current and solar plant behaves like a load. Therefore, in
uninstructed manner, plant(s) have been disconnected from main
grid by power plant operators. Next day when plant is connected
to grid through transformer high amount of magnetizing current
(Im) drawn by the transformer due to residual flux of transformer.
As a result, grid tied transformer of solar plant experiences the
heavy amount of inrush current which becomes daily practice for
solar power generation. To prevent from such an undesirable
phenomenon, certain techniques has been developed, however
rigorous research is required in this area. To reduce inrush
current, different techniques such as series compensation
techniques, virtual air gap techniques, asymmetric winding, pre
fluxing, optimal losing, point on wave switching, ultra-low
power frequency power source, bridge type inrush current
limiter, grounding resistance at neutral point and sequential
switching method are discussed in this paper. From above
discussed methods, Sequential switching method is adopted to
reduce the inrush current.
A. Effects of inrush current.
1. Voltage distortion: when transformer impedance, smaller
than the source impedance during energization of it, consider
voltage drop will occurs due to inrush current event. Voltage
distortion (0.1 pu to 0.9 pu) will affect the some sensitive load,
industrial costumer, medical equipment etc. [3] [4].
2. Harmonics: Due to inrush current, harmonics are generated
with different orders and its magnitude indicated in Table I.
Second order harmonics are dominant because its quarter wave
symmetric in nature. Inrush current star in either positive
direction or negative direction. [i.e. For supply of 50Hz
frequency, freq. of second harmonics will be 50 x 2 = 100Hz.]
[5].
TABLE I. HARMONICS CONTENT OF INRUSH CURRENT
Harmonic order
Fundamental
DC
2nd
3rd
Magnitude (%)
100%
40 to 60%
30 to 70%
30 to 50%
3. Stress on transformer winding: During inrush current in
transformer, two main forces acts on winding.
 Radial force.
 Axial force.
Radial force occurs during inrush current, which cause the
increase the diameter of winding. It also forces during short
circuit occurrence which is more damaging compared to inrush
current. The axial force drives the winding towards ground
which changes the dimensions and also able to damage the
transformer winding [6] [7].
4. Malfunctioning of protective relay: In actually to compute
the highest value of inrush current should be very affected
parameter in designing and to determine the setting of over
current relay used in power transformer maloperation of relay is
occurred. Due to malfunctioning of relays, circuit breakers
operates for tripping which is undesirable [8].
B. Factors for inrush current
1. Switching angle: If applied voltage is zero value at that
instant inrush current is maximum. Because if voltage is zero
value at that time flux is maximum and hence, inrush current is
maximum. Flux in transformer is given by following equation
[9].
φ = (φr + φm) cos(α) −φm cos(ωt + α)
(1)
The flux value at different instant of switching is calculated
using (1) and presented in Table II. Table. II seen that inrush
current depends on two factors; residual flux and switching
angle of voltage. If α=90; ϕ=2ϕm, for α= 0; ϕ=3ϕm. Therefore,
voltage at zero crossing inrush current is maximum.
TABLE II. VALUE OF THE FLUX AT DIFFERENT SWITCHING INSTANT
Switching Angle
Value of flux
α = 0°
∅ = 3∅m
α = 45°
∅ = 2.12∅m
α = 90°
∅ = ∅m
2. Residual flux density: Transformer is made from
ferromagnetic material. Hence, always residual flux will be
present in it due to hysteresis effect which is the main cause of
inrush current in windings. Which depends upon core material
characteristics. Its extreme value considered for cold rolled
and hot rolled material are 80% and 60% respectively of the core
saturation value [10]. Value of residual flux in transformer is
measured by de-energization instant of transformer. Residual
flux measurement is used for calculate of suitable instant for
energization of transformer [11]. Table III. shows residual flux
in different rating of transformer.
TABLE III. RESIDUAL FLUX OF DIFFERENT RATING OF TRANSFORMER
Transformer capacity
Residual flux
300 kVA
0.33 Wb
800 kVA
0.38 Wb
50 MVA
13 Wb
300 MVA
83 Wb
3. Series resistance: Line resistance between the sources and
transformer is another factor of inrush current generation. Due to
damping effect, series resistance between transformer and
sources, not only decreases maximum preliminary inrush current
but also increase its decay rate. Transformer near to generator
usually have high inrush current because of the line resistance
between generator side transformers and generator is minimum
[10].
4. Inrush under load: If transformer is energize with
connected load, some value of load power factor affected by
inrush current in transformer. If under heavy load and at unit
power factor transformer is switched on, the maximum value of
inrush current is low. Inrush current peak value is high for
reduction of power factor [9].
5. Source impedance: Inrush current is also defined by
impedance of source of power system. If transformer primary
winding and source impedance is same or impedance of source
is more than impedance of transformer primary winding inrush
current in that situation is maximum. The transformer inrush
current causes momentary voltage sag due to the impedance
between the energized transformer and the source [12].
6. Size of transformer: The size of the transformer reproduces
the inner transformer impedance of winding. The small
transformer (< 1000 kVA) generates high inrush current for
small duration (100 ms.). The high rated transformer (>1000
kVA) generates low inrush current for long duration in range of
seconds [13].
Above factors are responsible for causes the inrush current.
Main factors for reducing the inrush current are switching angle
(α) and residual flux (ϕres) after designing of transformer. Inrush
current value is high as compare to normal rated current,
therefore harmonics at that instant is also high. Harmonics
contents in inrush current waveform shown in Table I.
II.
METHODS FOR MITIGATE THE INRUSH CURRENT
Several methods has been proposed by various authors to
reduce inrush current in transformer in articles [14-29]. The
methods elaborated here are developed in time frame
chronologically which has shown in Fig.2. Most of the methods
are able to mitigate inrush current effectively however sequential
switching method is able to provide most effective solution
compared to others which is simulated for power transformer
used for distributed generator at large.
Virtual air gap method
Optimal closing method
Series compensation method
transformer, power-on angle of circuit breaker and calculation of
residual flux. Inverter based series compensator using a currentmode control for reducing the inrush current during energization
of transformer [17]. By using asymmetric winding the inrush
current was minimizes based on increasing the inrush equivalent
inductance by changing the value of internal film and external
film in coil winding. [18]. In this method all three phases of
transformer is energized at proper instant. If each phase of
transformer is energized in sequential manner at suitable instant
inrush current is reduce. In this method not required the residual
flux calculation. Inrush current reduce by this method by using
of zero crossing detector and knowledge about closing time of
circuit breaker [19]. By using this method reduce the residual
flux in core of transformer by applying voltage across tertiary
winding before energization of transformer [20] [21]. In this
method use the diode bridge technique for reduce the inrush
current in primary grounded transformers. The advantages of
this method are its simple circuit, easily implement to
transformer [22]. By using sequential phase energization method
each phase is energized in sequential manner with uses of
neutral grounding resistance [24] [25]. In prefluxing inrush
current reduction scheme reduce the remaining residual flux
which remaining in transformer core at startup instant of
transformer. Prefluxing equipment consist of capacitor, diode,
and fuse. This method does not need measuring of the residual
flux [26-29].
Asymmetrical winding
III. SIMULATION FOR INRUSH CURRENT INDICES
Point on wave switching method
Ultra low frequency power source
Transformer inrush currents are generates high magnitude
amount harmonic rich currents at the instant of transformer
energization. Inrush currents are categorized in three types [30].
Table IV shows the rating of transformer specially used for solar
power plant.
TABLE IV. PARAMETERS OF TRANSFORMER
Bridge type Inrush current limiter
Transformer
Grounding resistance at neutral point
Fig. 2. Methods of inrush current reduction
A concept of virtual air gap method is use of virtual air gap
which equal thickness differs in function of controllable limits
changed to the alignment of magnetic circuit and the related
control system. The AGW (air gap windings) current is either set
to a definite value using a current sensor or external source, in
the main magnetization winding of the magnetic circuit [14]. By
using controlled switching or optimal closing method required
the calculation of residual flux in core of transformer at deenergization instant. In this method to minimize the inrush
current not required the independent pole circuit breaker but it
required the calculation of residual flux of transformer [15] [16].
Series compensator technique is an inverter-based series
compensator which is comprised of a single-phase inverter and
series transformer. This technique is easy to implement because
it does not requires information about the parameters of
Secondary
Rating of
winding
winding
transformer
66 kV
20 MVA
Freq.
voltage
Star-Star
Prefluxing method
Primary
(Step up)
11 kV
50
Hz
1. Energization inrush current: During Energization of
transformer, energized inrush current is result of re-energized
transformer due to residual flux is can be zero or depending on
de-energization time of transformer.
2. Recovery inrush current: Recovery inrush current is flow
through transformer when supply voltage is restored after having
been reduced by system disturbances.
3. Sympathetic inrush current: Sympathetic inrush current
occurs in transformer when multiple transformers are connected
in system and one of them is energized.
Power transformer of 11/66 kV, 20MVA has been simulated for
the given switching instant, normal inrush, recovery inrush and
sympathetic inrush current difference has been achieved by
suitable modelling in Matlab/ Simulink environment which are
shown in Fig. 3(a), 3(b) and 3(c).
Simulated results which reflects the difference between
normal inrush current, recovery inrush current and sympathetic
inrush currents of transformer are as shown in Fig. 4(a), 4(b) and
4(c).
By analyzing the simulated result of energization inrush
current in Fig. 4(a) show the inrush current peak is more than
1500A. Harmonics content and THD generated due to
energization inrush current shown in Table V.
TABLE. V. HARMONICS CONTENT AND THD OF ENERGIZATION INRUSH CURRENT
Phase
Phase R
Phase Y
Phase B
THD
90.92%
111.69%
110.75%
2nd harmonics
63.93%
72.98%
73.49%
3rd harmonics
31.32%
52.85%
53.53%
Fig. 3 (a). Matlab/Simulink based model of energization inrush current
Fig. 4(a). Energization inrush current
Recovery inrush current for 20MVA transformer are
elaborated in 4(b). During starting (0 to 0.6 sec) is
characterization of energization inrush current. For single phase
fault (0.9 to 1.2 sec) has been achieved for rated power
transformer. After recover the fault recovery inrush current is
drawn by transformer after 1.2 sec as shown.
Fig. 3(b). Matlab/Simulink based model of recovery inrush current
Fig. 4(b). Recovery inrush current
Fig.3(c). Matlab/Simulink based model of sympathetic inrush current
Figure 4(c) indicates the waveform of transformer T1 current
having energization inrush current and sympathetic inrush
current (1 to 2 sec.) due to transformer T2 is energized. When
transformer T2 is energized after 1 sec. it is observed that a
value of sympathetic inrush is low as compare to energization
inrush current in terms of magnitude.
TABLE. VI. PEAK VOLTAGE INSTANT OF PHASE AT DIFFERENT FREQUENCY
Frequency
(Hz)
48.5
49.0
49.5
50.0
50.5
51.0
51.5
Peak voltage
instant of Phase
R (msec.)
5.150
5.100
5.050
5.000
4.950
4.900
4.854
Peak voltage
instant of Phase
Y (msec.)
12.016
11.900
11.783
11.666
11.550
11.433
11.326
Peak voltage
instant of Phase
B (msec.)
18.883
18.700
18.516
18.333
18.150
18.700
17.798
In Fig. 5 show that the Matlab/Simulink model of sequential
switching method and in Fig. 6 show the waveform of current in
transformer using sequential switching method. Simulation is
carried out for 50Hz frequency and its phase R, Y, and B phase
switching instant is 5 msec. 11.666 msec. and 18.333 msec.
respectively.
Fig. 4(c). Sympathetic inrush current
IV. INRUSH CURRENT REDUCE BY SEQUENTIAL SWITCHING
METHOD
In this paper to reduce of transformer inrush current by using
sequential switching method is described. In this method
transformer each phase is energized at peak value of supply
voltage of each phase. An instant of Peak value of supply
voltage for each phase is different. For 50 Hz supply frequency
switching instant of phase R, Y, B is calculated as follows,
Therefore, 10 msec. time is required for half cycle (180°
degree). 5 msec. is for 90° degree. For circuit breaker of phase
RYB closing instant is, firstly assume that we supply phase R
voltage at 90° (maximum) in 5 msec. Next phase Y is connected
at 120° apart from phase R that means 90° + 120° = 210°. Time
required for 0° to 210° is 11.666666 msec. After energization of
phase Y next phase B is energized at 120° apart from phase Y.
time required for 210°+120°=330°. Time duration for 0° to 330°
is 18.3333333 msec. [19]. But in actual supply frequency is not
remaining 50 Hz constant. For different frequency peak instant
of voltage is different. Some different value of frequency and
their switching instants are shown in Table VI.
Table VI show that the peak voltage instant of three phase for
different frequency. In proposed method, transformer must be
energized at peak value of phase voltage. In actual it is possible
by zero crossing detector such as IC TCA 785 and other zero
crossing detector. But the limitation of zero crossing detector
only use for particular value of frequency. Because, for different
value of frequency peak instant of voltage is different. For
example from Table V for 50 Hz frequency peak voltage instant
of phase R, Y and B phase is 5 msec. , 11.666 msec. and 18.333
msec. respectively.
Fig.5. Matlab simulation mode of sequential switching method
Figure 6 show that inrush current is reduce by effectively by
using sequential switching method for 11/66 kV, 20 MVA
transformer.
Fig. 6: Inrush current reduce by sequential switching method
From Table VII show that the harmonics content after
implementation of sequential switching method.
TABLE VII. HARMONICS CONTENT AND THD AFTER IMPLEMENT OF SEQUENTIAL
[11]
Goran Petrović, Tomislav Kilić, Stanko Milun “Remanent flux
measurement and optimal energization instant determination of power
transformer” XVII IMEKO World Congress Metrology in the 3rd
Millennium, Dubrovnik, Croatia, June 22 −27, 2003.
[12]
Lin, C.E., Cheng, C.L., Huang, C.L., and Yeh, J.C. “Investigation of
magnetizing inrush current in transformers- Part I: Numerical
simulation” IEEE Transactions on Power Delivery, Vol. 8, No. 1, pp.
246–254, January 1993.
[13]
Al-Khalifah A. K. and Al-Saadany E. F. "Investigation of
magnetizing inrush current in a single-phase transformer" IEEE
Conference on Power Engineering, 2006 Large Engineering Systems,
pp. 165-171, July 26-28, 2006.
V. Molcrette , J. L. Kotony, J. P. Swan and J. F. Brudny, "Reduction
of inrush current insingle-phase transformer using virtual air gap
technique" IEEE Trans.Magn. , vol. 34, pp.1192 -1194, 1998.
J. Brunke, K. Frohlich, “Elimination of Transformer Inrush Currents
by Controlled Switching-Part I: Theoretical considerations” IEEE
Transactions on Power Delivery vol 16 no.2 April 2001.
SWITCHING METHOD
Phase
R
THD (%)
14.21
2nd order
harmonics
(%)
3rd order
harmonics
(%)
Reduction
of
harmonics
(%)
3.34
3.35
70.76
Y
24.92
6.35
2.11
86.77
B
37.52
2.99
2.92
73.23
V. CONCLUSION
During switching of solar plant, produces inrush current in
transformer effects the transformer winding, generates the
harmonics and power quality issues. Different techniques to
reduce the inrush current are discussed in this paper. Sequential
switching method is used because it does not required
calculation of residual flux and easy to implement in system.
From simulation and analysis of inrush current it conclude that
harmonics reduce in R, Y, and B phase are 71%, 87%, and 73%
respectively
References
[14]
[15]
[16]
J. Brunke, and K. Frohlich, “Elimination of Transformer Inrush
Currents by Controlled Switching Part II: Application and
performance considerations” IEEE Transactions on Power Delivery
vol 16 no.2 April 2001.
[17]
Juei - Lung Shyu ," A Novel Control Strategy to Reduce Transformer
Inrush Currents by Series Compensator ", IEEE PEDS 2005.
[18]
J. Chhen, T. Liang, C. Cheng, S. Chen, R. Lin and W. Yang,
“Asymmetrical winding configuration to reduce inrush current with
appropriate short-circuit current in transformer” IEE Proc.-Electr.
Power Appl., Vol.152, No.3, May 2005.
[19]
F. Ali Asghar and K. P. Basu, "Reduction of three phase transformer
magnetizing inrush current by use of point on wave switching ", IEEE
Student Conference on Research and Development, Malaysia, pp368-370, Nov. 16-18 2009.
B. Kovan, F. de Leon, D. Czarkowski, Z. Zabar, and L. Birenbaum,
“Mitigation of inrush currents in network transformers by reducing
the residual flux with an ultra-low-frequency power source,” IEEE
Trans. Power Del., vol. 26, no. 3, pp. 1563–1570, Jul. 2011.
[1]
http://pib.nic.in/newsite/PrintRelease.aspx?relid=134497.
[2]
Hemchandra Madhusudan Shertukde “distributed photovoltaic grid
transformer” CRC publication book, 2014.
[3]
Seo, HC & Kim, CH 2008, “The analysis of power quality effects
from the transformer inrush current: A case study of the Jeju power
station, Koria”, Power and energy society general meeting-conversion
and delivery of electrical energy in the 21st century 2008, IEEE, PA
Pittsburgh, pp. 1-6, 20-24 July 2008.
[4]
M. Nagpal, T. G. Martinich, A. Moshref, K. Morison, and P.
Kundur,"Assessing and limiting impact of transformer inrush current
on power quality," IEEE Transactions on Power Delivery, vol. 21, pp.
890-896, 2006.
[5]
Al-Khalifah A. K. and Al-Saadany E. F. "Investigation of
magnetizing inrush current in a single-phase transformer" IEEE
Conference on Power Engineering, 2006 Large Engineering Systems,
pp. 165-171, July 26-28, 2006.
[21]
D. P. Balachandran, K. R. Sreerama and V. P. Shimnamol,
"Transformer Inrush Current Redution by Power Frequency Low
Voltage Signal Injection to the Tertiary Winding" IEEE conf
Powertech “07, pp.1953 -1958, 2007.
[6]
Neves, W, Fernandes, D, Baltar, F, Rosentino, A, Saraiva, E, Delaiba,
A, Guimaraes, R, Lynce, M & de Oliveira, J 2011, “A comparative
investigation of electromechanical stress on transformers caused by
inrush and short-circuited currents”, International conference on
electrical power quality and utilisation (EPQU), 2011, Libson, 17-19
October 2011.
[22]
Seyed Majid Madania, Mehrdad Rostamia, G.B. Gharehpetianb, Reza
Hagh Maram “Improved bridge type inrush current limiter for
primary grounded transformers” Electric Power Systems Research ,
vol.95, pp. 1– 8, feb.2013.
[23]
Faiz, J., Ebrahimi, B.M., Abu-Elhaija, W. “Computation of static and
dynamic axial and radial forces on power transformer windings due to
inrush and short circuit currents” Applied Electrical Engineering and
Computing Technologies (AEECT), 2011 IEEE Conference, Jordan,
2011.
Yu Cui, Sami G. Abdulsalam, Shiuming Chen, Wilsun Xu, “A
Sequential Phase Energization Technique for Transformer Inrush
Current Reduction— Part I: Simulation and Experimental Results”
IEEE transactions on power delivery, vol. 20, no. 2, April 2005.
[24]
Fahrudin Mekic, Ramsis Girgis, Zoran Gajic, Ed teNyenhuis “Power
Transformer Characteristics and Their Effect on Protective Relays”
33rd Western Protective Relay Conference, October 17-19, 2006.
Yu Cui, Sami G. Abdulsalam, Shiuming Chen, Wilsun Xu, “A
Sequential Phase Energization Technique for Transformer Inrush
Current Reduction—Part II: Theoretical Analysis and Design Guide”
IEEE transactions on power delivery, vol. 20, no. 2, April 2005.
[25]
D.I.Taylor, N.Fischer, J. D. Law, and B. K. Johnson, “Using LabVIEW to measure transformer residual flux for inrush current
reduction,” presented at the North Amer. Power Symp., Starkville,
MS, Oct. 4–6, 2009.
[26]
Douglas I. Taylor, Joseph D. Law, Brian K. Johnson, and Normann
Fischer, “Single phase transformer inrush current reduction using
prefluxing” IEEE Transactions On Power Delivery, Vol. 27, No. 1,
January 2012.
[7]
[8]
[9]
Y.G. paithankar and S.R. bhide “fundamental of power system
protection” PHI Pvt. Ltd, 2003.
[10]
S.V.kulkarni and S.A.kharpade “Transformer engineering design and
practice” MARCEL DEKKER, INC. 2004.
[20]
[27]
Douglos I. taylor “system apparatus, and method for reducing inrush
current in a three phase transformer” united states patent, patent no.
US 8,878,391 B2, date: nov, 4, 2014.
[28]
V. Oiring de Castro Cezar, L-L. Rouve, J-L. Coulomb, F-X.
Zgainski, O. Chadebec, and B. Caillault “Elimination of inrush
current using a new prefluxing method. Application to a single-phase
transformer”IEEE, 2014.
[29]
Vaddeboina, V, Taylor, G & Proudfoot, C 2012, “Switching large
transformers on weak transmission netroks – A real time case study”,
Universities power engineering conference 2012, 47th international
conference, , IEEE, London, pp. 1-6, 4-7 September 2012.