Effect of DC Bias on Magnetization Current Waveforms ... - IEEE Xplore

2012 IEEE International Power Engineering and Optimization Conference (PEOCO2012), Melaka, Malaysia: 6-7 June 2012

Effect of DC Bias on Magnetization Current Waveforms of Single Phase Power Transformer Syafruddin Hasan, Soib Taib, Risnidar Chan, Siti R.M

Abstract-- An investigation of magnetizing current waveforms of single phase power transformer due to DC bias injection through simulation and experimental study is present in this paper. A method base on MATLAB simulation that is capable of predicting of magnetizing current without and with DC bias is used. The experimental study is done by the DC bias current injected simultaneously with AC source to primary winding of transformer and the secondary side is open circuit. The results show that the waveforms distorted when the DC bias exist. The pulsated waveforms are pushed to half cycle in which the bias current is in the same direction as magnetizing current (unsymmetrical pulsated) and even harmonics are significant. The simulated waveforms have good agreement with the measured one. Index Terms—DC bias, even harmonics, magnetizing current, single phase transformer, waveform.

I. INTRODUCTION

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ow days, worldwide globalization includes intercontinental distribution of electric power which comprises the rising introduction of high voltage direct current (HV-DC). One of advantage of HV-DC is the lowered energy losses. However, the combination of AC and DC power equipments yields the problem that AC machines such as power transformers may be affected by DC bias. Geomagnetically Induced Current (GIC) flow on the earth surface due to Solar Magnetic Disturbances (SMD) which may also called Geomagnetic Disturbance (GMD), HVDC monopole transmission system, quasi-direct or direct current

The authors wish to thank to the Centre of Excellent for Renewable Energy and School of Electrical System Engineering, Universiti Malaysia Perlis (UniMAP) for the financial and technical support as well. Syafruddin Hasan is with PPK-SE UniMAP and FT-USU Medan (e-mail: [email protected]). Soib Taib is with School of Electrical & Electronic Engineering, University Sains Malaysia (e-mail: [email protected]). Risnidar Chan is with PPK-SE and FT-USU Medan UniMAP (e-mail: [email protected]). Siti Rahayu Mohamed is with PPK-SE UniMAP (e-mail: [email protected]

978-1-4673-0662-1/12/$31.00 ©2012 IEEE

will flow into the nearby transformers whose neutral point is grounded. The DC current also could be due to power electronic operating under normal conditions or under abnormal conditions such as imperfect devices switching [1]. Depending on the level and duration of DC injection, possible adverse effects that may cause the saturation of transformer core during each half cycle. The core is saturated during the half cycle in which the bias current is in the same direction as the magnetizing current [2]. Consequently, as a result, transformer magnetizing current will greatly increase and will be in rich in harmonics, which in turn could cause overall increase in transformer reactive power consumption and additional core losses, as well as eddy current losses due to the higher leakage flux [3]-[4]. Then some other problems can be generated, such as local overheating, the vibration and noise of the transformer, corrosion of grounding equipment, metering errors and malfunction of protective equipment which are directly related with the harmonic in the current of transformer [5]-[6]. DC bias can originate from a number of sources, some of which are Geomatically Induced Currents (GIC), Photovoltaic Systems and AC and DC Drives [7]-[8] This paper focuses on the prediction of voltage and magnetizing current distortion resulting from DC offset currents on the primary part of single phase transformer. Therefore, the DC bias appears explicitly on the primary side of the transformer model and formed part of the magnetizing current during laboratory testing. II. TRANSFORMER WITH DC BIAS When a transformer is connected to a pure symmetrical load, the flux that is produced in the secondary will be opposed by the flux created from the additional component of primary current. Hence the mutual flux remains relatively unchanged. However, when the load current through that transformer is not perfectly symmetrical, which means the transformer is supplying a secondary current containing a DC component; a unipolar flux is established in the core. This unipolar flux is not opposed by the flux created from the additional component of primary current. It will add up with the mutual flux in the core and push the net flux in the core towards saturation. Consequently, the core is saturated

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during the half cycle in which the bias current is in the same direction as the magnetizing current [2]. This additional flux "bias" or "offset" will push the alternating flux waveform closer to saturation in the positive cycle then the negative cycle. Therefore, the core material is said to experience half-cycle saturation. The core material will enter half-cycle saturation earlier if the magnitude of DC component is greater. Fig.1 displays the excitation characteristics, exciting current and flux of a power transformer iron core.

Fig. 1. Excitation characteristics, excitation current and flux of a power transformer with and without dc bias

Explanation of Fig.1, ac = ac flux produced by ac current dc = dc flux produced by dc current = ac + dc = total flux in the iron core s = saturation flux = the minimum angle in one cycle at which = s M = knee point N = saturation point Is = saturation current Idc = direct current due to dc bias k1 and k2 are gradients of piecewise magnetizing characteristic

III. SIMULATION In this investigation, the magnetizing (exciting) current prediction program is developed by using MATLAB software. The concept of the program is to return a graphical approximation of the B-H relationship when a transformer’s core magnetizing characteristic is known and a certain level of DC bias is assumed. To realize this concept, the magnetizing of a transformer core under no-load operating must be known. Once the plot is obtained, a piecewise linear approximation for the mid point locus is made. Next, program will prompt to enter the level of DC bias. The value entered will come in the form of a core flux bias. With the level of DC bias determined, the resultant biased flux in the first quarter of the B-H curve can be calculated from equation 1) = ac + dc = max sin ωt + dc Flux density, B is given by formula 2) where A is cross sectional area of core and constant. Therefore B~ Since the B-H curve is assumed to be sections of straight line, with the flux density and slope, the corresponding value of field intensity can be calculated by applying simple concept of linear function. Magnetizing current is calculated by dividing the field intensity values yield from previous calculation by the number of primary winding. Fig.2, Fig.3 and Fig.4 show plot of magnetizing current without and with DC bias injection by using MATLAB.

It can be seen that the exciting current is symmetrical to time axis without dc bias, and it becomes asymmetrical with dc bias occurring. Under such condition, the dc flux generated by the dc current offsets the ac flux in the transformer iron core. This is the so called half-cycle saturation of transformer under dc bias [9]. Since a typical power transformer needs little magnetizing current that set to the knee point at rated condition, a small amount of direct current will sufficient apt to cause significant half-cycle saturation. From Fig.1, we can found that the exciting current waveform with dc bias is sharper than without dc bias. Refer to flux or magnetizing current waveforms that exhibit asymmetrical characteristics, i.e., i(t) ≠ - i(-t), the even harmonics will produced significantly.

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Fig. 2. Magnetizing current versus time without DC bias


approach that we use to investigate the effect of DC bias on a single phase power transformer. The experiment is carried out at Centre of Excellent for Renewable Energy (CERE) Universiti Malaysia Perlis.

Fig. 3. Magnetizing current versus time with 20% of DC Bias

Fig. 5. Experimental Set up of DC bias of power transformer

V. RESULTS AND DISCUSSION For comparative need, the no-load test with input voltage, V = 225 Volt is made first, Fig.6 shows the laboratory experiment result of input voltage and current waveforms without DC bias and its individual harmonic distribution is shown in Fig.7. The waveform is recorded for two cycles (720 degree) and its individual harmonics is recorded from fundamental (order 1) to order 15.

Fig. 4. Magnetizing current versus time with 50% of DC Bias

IV. EXPERIMENTAL The experiments were conducted on single-phase power transformer 12/240 V. The arrangement of equipment set-up (connection diagram) of DC bias experiment which used in this research work is as shown in Fig.5. The power analyzer PM 300, conjunction with personal computer (PC) and software using VPAS Lite is used to monitor the voltagecurrent waveforms and harmonics. The AC input voltage to primary transformer winding is connected to AC variable voltage power supply (0 – 250 V, max 5 A) and the DC current injected is come from DC regulated power supply (0 – 50 V, max 1 A), by ED-345 equipment produced by ED laboratory The experiment was conducted with various values of DC current and the transformer test (TUT) is in open circuit condition. This a new

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Fig. 6. Voltage and Current waveforms without DC bias for V = 225 V


Fig. 7. Individual harmonic distribution of input current without DC bias for V = 225 Volt

Fig. 10. Individual harmonic distribution of input current with 0.1A of DC bias

For DC bias effect on input voltage and current waveforms, Fig.8 and Fig.9 show its sample waveforms with 0.1A and 0.3A of DC current injection. Meanwhile, the individual harmonic distributions are displayed in Fig.10 and Fig.11.

Fig. 11. Individual harmonic distribution of input current with 0.3A of DC bias

Fig. 8. Voltage and current waveforms with 0.1 A of DC bias

Referring to Fig.6, Fig.8 Fig.9, Table 1 displays the peak amplitudes difference of positive and negative cycles of magnetization current without and with DC bias. TABLE I PEAK VALUES OF MAGNETIZATION CURRENT WITHOUT AND WITH DC BIAS.

Input Voltage (V) 225

Fig. 9. Voltage and current waveforms with 0.3 A of DC bias

Without DC Bias +0.340 and – 0.330

Ipeak (A) 0.1 A Idc +6.100 and – 0.215

0.3 A Idc + 6.300 and – 0.220

Fig.6 shows the input voltage and current waveforms without DC bias under 225 V AC voltage feeding. The voltage waveforms are symmetrical. It is also seen that the input (magnetization) current waveforms is still symmetrical about the coordinate axis, although there are a little difference in its peak values (refer to Table 1). Fig.8 and Fig.9 displays the waveforms of magnetization current and voltage of test transformer when 0.1 A and 0.3 A of DC current are injected into the primary winding of the transformer. The Figures show that the magnetizing currents

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are distorted severely. There is a big differeent between peak value of magnetizing current in one direction (positive cycle) and the other cycle (negative cycle). For exxample, 0.1 A of DC current injection, the peak value of maggnetization current is 6.1 A in positive direction and just -0.2555 A in the other direction. If the transformer is operating cclose to the knee point on the saturation curve, it could now w be pushed into saturation by the additional DC flux. The presence of DC biased flux had shifted the negative cycle away from its negative saturation point towards positive sidde. Without DC bias, in line with the magnnetization current waveforms in Fig.6, that the waveform is syymmetrical about the coordinate axis, the odd harmonics of inddividual harmonic distribution are significant rather than even haarmonic. Meanwhile, Fig. 10 and Fig.11 show w the individual harmonic distribution of magnetization current with DC bias occur. Because of the current waveformss are not further symmetrical (Fig.8 and Fig.9), the even ordeer harmonics (2nd, 4th and 6th) is shown a clear pattern. It can be also seen from current indivvidual harmonics distribution with respect to various DC curreent injections that the amplitude of the harmonics is increased with the DC bias current.

[7]

Spooner E.D., “A New Australian Stan ndard for Small Grid-Connected Renewable Generation System Conn nected via Inverters”, Australian CRC for Renewable Energy (ACRE), School of Electrical Engineering & Telecommunications, The University y of New South Wales [8] Swammy M. M., and Rossiter S.L., “Harmonic Interaction Between 1500 kVA Supply Transformer and VFD V Load at an Industrial Plant”, IEEE transactions on Industry Applica ation, vol.34, No.5, pp. 897-903 1998. [9] Jinxia Yao, Min Liu, Changyun Li,, Qingmin Li, “Harmonic and Reactive Power of Power Transform mers with DC Bias”, Power and Energy Engineering Conference (APPE EEC), 2010 Asia-Pacific [10] John A.Orr and Alexander E.Manuael,, “On the Need for Strict Second Harmonic Limits”, IEEE Transaction on o Power Delivery, Vol.15, No.3, pp. 967-971July 2000

V. CONCLUSION The results of this investigation are usefull in understanding voltage-magnetizing current waveforms phenomena of transformer without and with DC bias injecteed. The computation of the magnetizing curreent waveforms in single phase power transformer under DC bias with MATLAB required the B-H curve data of tthe core material and a piecewise linear approximation for thee mid point locus is made. The investigation results show that the transformer magnetization current waveforms are unssymmetrical and contain a lot of harmonics. The peak valuue of magnetizing current increases with the increasing of thhe injected direct current and the even harmonic content is verry significant with the existing of DC component in magnetizingg current. VI. REFERENCES [1] [2]

[3] [4] [5]

[6]

A.Ahfock and A.J.Hewitt, “DC Magnetizatioon of Transformers”, IEEProceeding Electric Power Applications, 20055 O.Biro, S.AuBerhofer, G.Buchgraber, K.Preiis and W.Seitlinger, “Prediction of magnetising current waveform in a single-phase power transformer under DC bias”, IET Science Measureement Technology, pp. 2-5, 2007, I. (1), Y.You, E.F.Fuchs, and P.R.Barnes, “Reactivee Power Demand of Transformer with DC Bias”, Industry Applicatioon Society Magazine, Vol. 2, No. 4, pp.45-52,July/August 1996. Philip R.Price, “Geomagnetically Induced Current Effects on Transformer”, IEEE Transactions on Power Deelivery, Vol.17, No.4, pp. 1002-1008, October 2002. Yinghui Chen, Tiebing Lu, and Zhibin Zhhao, “Study on the Electromagnetic Influence of DC Bias on the Power Transformer”, Electric Power Science and Engineering, .pp 10-113, 2009, 25(3). Shu Lu, Yilu Liu, and Jaime De La Ree, “Harmoonic Generated from a DC Biased Transformer”, IEEE Transaction on P Power Delivery, Vol.8, No.2, pp. 725-731, April 1993

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VII. BIOGRAPPHIES Syafruddin Hasa an, was born in July 1, 1959 in Pidie, Aceeh-Indonesia. He received his BSc from Uniiversitas Sumatera Utara (USU) Medan in i 1985 and MSc from Institut Teknolo ogi Bandung (ITB) Bandung-Indonesia , 1993 all in Electrical Engineering. He is i a lecture in UniMAP and USU. He is currently c PhD student in Universiti Malaysiia Perlis (UniMAP). His field of interesst includes Electrical Machines, Power Electronic Drives and Power Quality.

w born on December 28, Risnidar Chan was 1949 in Medan, Indonesia. She received her B Sc from m the Sumatera Utara University (USU U) in 1977 and Master Degree from ITB B Bandung in 1995. She is currently a Ph..D student in UniMAP Malaysia and teeaches the Power System Analysis & Conttrol. Her interests include Power System An nalysis and Power Quality especially harmo onics analysis in Power System.

Siti R. M. was bo orn in Kelantan, Malaysia, on January 11, 1984. She graduated in Electrical Systeem Engineering at the University of Malaysia Perlis (UniMAP). She received her B.Sc (Hons) in Electrical System Engineeriing at UniMAP on 2010. Her research in nterest is in transformer design and her research interest is in ne design. electrical machin