[IEEE 2014 IEEE/PES Transmission & Distribution Conference & Exposition (T&D) - Chicago, IL, USA (2014.4.14-2014.4.17)] 2014 IEEE PES T&D Conference and Exposition - Field studies on the Inrush Current for 33/11kV transformer and the effect on voltage interruption during switching operation of 10MW wind farm

Field Studies on the Inrush current for 33/11kV Transformer and the Effect on Voltage Interruption During Switching Operation of 10MW Wind Farm Abstract—The Inrush Current for transformer switching operation study is an electromagnetic transient analysis (EMT) and are performed in order to investigate the network response after the energized the two 33/11 kV, 7.75MVA transformers in the Layafa S/S. The Inrush current associated with transformer energizing can cause some reactions that may adversely affect other loads in the power networks system, for instance voltage sags or over-voltage phenomena which could cause nuisance tripping for protective devices or loads. Protection devices may mis- interpret these events as fault currents, if the protection devices are not properly coordinated well. In this paper, the inrush current analysis was performed for the generators-transformers of the wind farm switching operation. In the study analysis, the 33/11 kV transformers will not be energized at the same time. Each transformer will be energized at no load condition, with the tap changer set to the maximum step of the HV side (higher voltage) in order to minimize the inrush current values. The simulation analysis was performed which the aid of DigSilent and EMT computer simulation to establish this study while considering the minimum short circuit power in the grid and the Wind Farm disconnection from the system. This paper is based on the field results test of the newly installed wind farm generator- transformer in relation to network interruption on the 33 kV and 11 kV systems during the switching on operation especially transient voltage phenomena. Index Terms—Inrush Current, Layafa Substation, Switching operation, DigSILENT, Transformer energization. This work was supported in part by the Major State Basic Research Development Program 2009CB219700 , in part by National High Technology Research and Development Program of China(2011AA05A109), and in part by Ph. D Programs Foundation of Ministry of Education of China (20110142110055). I. INTRODUCTION During the switching on operation, energizing of transformer is most times results in the transformer drawing large inrush current, which eventually decays down to a small magnetizing current. The time it takes inrush current to decay depends on the resistance and reactance of the circuit, including the transformer’s magnetizing reactance. Since the magnetizing inductance of transformer is high, the inrush current can take a long time to reach its steady state value [1] . This inrush current may temporary cause voltage drop or over-current in the power system. Transformers are very important equipment in power system, which facilitate the transmission of electric power at high voltages over long distances. The energizing can lead to excessive transient inrush current, especially when the transformer core has remnant flux that adds to the flux build-up after switching. Inrush current sags the system voltage, thereby affecting the power quality of the network in proximity of the transformer. The extent to which power quality is affected depends on short circuit MVA at the source bus, and the magnitude and decay time constant of the transient current. The scope of study is inrush transformer switching operation during the energizing of the two 7.5 MVA, 33/11 kV transformer in Layafa S/S. The study involves two 33/11 kV transformers with rated power equal to 7.5 MVA each. Both transformers can be energized from the HV side but energizing the second transformer has to be performed when the other machine is in steady state conditions. Transformers generally before energizing them, have a residual magnetizing flux due to the previous operation status. The amount of residual flux before the energizing the transformer will determine the inrush current phenomena and should be considered in the switching calculation of 80% of residual magnetizing flux [2] . According to the Nigeria grid code in continuous duty, the minimum admissible rated voltage at the 33 kV systems is equal to 0.94 p.u. This value will be considered as the reference limit for Layafa S/S in Dji- Dkoio Izuchukwu Wuhan University of Technology Wuhan 430074, China Owolabi Sunday Adio, Xiangning Lin, IEEE Senior member, Jinwen Sun, Pengyu Yang, Muhammad S. Khalid State Key Laboratory of Advanced Electromagnetic Engineering and Technology Huazhong University of Science and Technology(HUST) Wuhan 430074, China 978-1-4799-3656-4/14/$31.00 ©2014 IEEE steady state conditions. Energizing a transformer also has harmonics inrush current which can excite system resonances and cause dynamic over-voltages. For short term voltage deviations, a minimum reference value equal to 0.85 p.u. is considered in the present study. Voltage values at Layafa S/S lower than 0.85 p.u. during transient phenomena are considered not acceptable. II. DESCRIPTION OF TRANSFORMER EQUATION DURING INRUSH CURRENT. The central part of the transformer model includes winding resistances, leakage inductances, iron core model and zero sequence inductance. Present the field application of these elements, [2] [3]. it is necessary to use the expression that can accurately describe the inrush current waveform. This section describes the transformer model and equations that shows the behavours of transformer inrush current during the first phase energizing which can be expressed by the following electric circuit equivalent and equations, [3] [4] . Figure 1. Transformer electrical equivalent circuit (per-phase) model. From figure 1, pr and pL is primary resistance and leakage reactance, mL is the nonlinear inductance of the iron core as function of the magnetizing current. Secondary side resistance spr and leakage reactance spL as referred to primary side are also shown. While pv and sv represent the primary and secondary phase to ground terminal voltage respectively. From the circuit above the following equation can be deduced: 0 1sin( ) /p m p Lv v t i r N d dt (1) Where 0 is the phase of primary voltage at t =0, i is magnetize current, L is core flux and 1N is the number of turn in primary side, then equation (1) becomes 0 1 1 1sin( ) / /m L p Lv t N r L N dt (2) And linking to primary inductance 1L and calculate equation (2) for L The transformer core flux equation is given here in equation (3) below 10( cos ) pr t L t m r me 1 cos cosp o ot m r m r t tLe (3) where m is the max of L and r is residual flux At θ0 =π/2 in Eqn. 3 we have Eqn 4 : 1 sin p mt t r tLe (4) The equation (3) and (4) are necessary to reduce the transient flux of the transformer thereby reduce transformer inrush current. 11 1 2 p p sp tr rt t IL li i e e (5) Where I is the nominal current, and lsp< 1L , then transient current produced here with load current is damped very fast. Also to find magnetizing current, for this case, transient flux exists with r magnitude and time constant equal τ= 1L / pr , the max of magnetizing current can be derived from equation (6). 0 112 5 im r m t A i A (6) Where Ai area of the core, At is area of the core with the winding and μ0 is the permittivity in air. The estimation for the maximum inrush current also can be deduced from flux- current characteristics of transformer as follows: ( ) ( ) 0 2 ( ) m s t s s s i t t t t tL t tL (7) 1 1 0 max 0 1 1 1 0 cos 1 sin( ) cos cos / ( cos ) p p ms m s p m L p Lo ot m r m r t L t m r m rv v t t t N r L NL e vvi L e (8) This equation find out how the residual flux, switching angle in form of (cosθ0) and saturation inductance affects the inrush current of a transformer. III. CALCULATION AND SIMULATION ANALYSIS. The simulation model is built using DigSILENT software. The system consists of 37 wind farm generator-transformer connected to a single bus. The bus is connected to an external grid that represents the rest of the network. All the transformers and the generators are to 33/11 kV with Delta/ Wye connection. The transformers specifications are provided in Appendix I. The typical model for this is shown in (Figure 1) and (Figure 2) that represent first and second energizing of the transformer. The model consists of breaker used to control the de-energizing and energizing transients in the simulation. The magnitude of the inrush current in a transformer being energized through windings with Load Tap Changer (LTC) can be significantly reduced when the tap is conveniently positioned to allow a much higher number of turns to be excited. Fig 2. Layafa 33/11 kV transformersEnergizing of the first transformer Fig 3. Layafa 33/11 kV transformer Energizing of the second transformer A. Energizing of the transformer From figure 4 to Figure 10, the EMT computer simulation results for the energizing of the first 7.5 MVA 33/11 kV transformers in Layafa S/S are showing below: Fig 4. Phase current during transformer energizing The maximum in rush current is 370 Apeak without taking into consideration residual flux while is 560 Apeak considering the residual flux. The value is lower than rated value of DigSilent tool while in the normal operation flow in series unit having a rated value of 390 Arms during the switching on operation, all the in rush flows in the excitation winding that has rated current of 125 Arms.. Fig.5. Magnetizing flux during transformer energizing Fig.6. Magnetizing flux (phase A) and phase voltage at Layafa S/S during transformer energizing. In the above figures, this shows that in rush current is 185A and 1.82 pu. The magnitude of residual flux in the transformer is the main parameters to change magnitude of inrush current, when circuit breakers are opened transformer is isolated from network, the residual flux remains in transformer and when transformer is energized inrush current will be increased to clear this effect, the tap changer is set to the maximum step of the HV side (higher voltage) at no load condition. Fig.7. Fourier analysis of phase voltage during transformer energizing Fig. 8. Hysteresis curve during transformer energizing Fig 9. Magnetizing current during transformer energizing Fig. 10. Magnetizing flux (phase A) and phase voltage at Layafa S/S during transformer energizing. IV. SIMULATION RESULTS. A. Simulation Results Analysis. From the figures 3 to figure 10 above, are energizing simulation analysis for the impact of the 33/11 kV transformers on the MV system at Layafa Substation. The main results of the computer calculation are presented here in table 1. Here in the analysis, the time it takes inrush current to reduce and the transient voltage to decay is control by tap changer set to the maximum step of the HV (High Voltage). As the tap changer is changing to higher step of HV, the transient voltage is equally improved from .08 p.u to 0.93 p.u (this represent 14.9% reduction) and the system frequency lasted from half cycle to 15 secs. TABLE I. THE RESULTS CALCULATION. According to the calculation results, no overvoltage phenomena are highlighted in the 33 kV and 11 kV systems during the transformer energizing. As indicated in Table I, the voltage values at the 33 kV systems at Layafa and Katsina S/S in the first instants after the transformer energizing (sub-transient phase) were higher than the minimum reference Parameter Calculated minimum value [p.-u.] Reference voltage limit for transient phenomena [p.u.] Minimum/ permissible value (continuous duty) [p.-u.] Restoration time within minimum continuous permissible value (0.94 p.u.) [ms] Total simulation time [s] Energizing of the first transformer Phase A voltage at 33 kV Layafa S/S 0.926 0.92 0.94 105 3 sec Phase A voltage at 33 kV Katsina S/S 0.926 0.91 0.94 85 3 sec Overvoltage Within permissible limits Magnetizing flux (d-axis) 2.043 n.a. n.a. 3 sec Magnetizing flux (phase A) 1.87 3 sec Magnetizing current 5.108 n.a. n.a. 3 sec Energizing of the second transformer Phase A voltage at 33 kV Layafa S/S 0.926 0.92 0.94 483 3 sec Phase A voltage at 33 kV Katsina S/S 0.926 0.91 0.94 135 3 sec Overvoltage Within permissible limits Magnetizing flux (d-axis) 2.045 n.a. n.a. 3 sec Magnetizing flux (phase A) 1.812 3 sec Magnetizing current 5.489 n.a. n.a. 3 sec limits for transient phenomena. However, the instantaneous values of the voltage at both the Katsina and Layafa 33 kV S/S should be within the permissible voltage range for continuous duty (0.94 -1.04 p.u.) after 103ms for the energizing of the first transformer and 463ms for the energizing of the second transformer. For the above considerations, the energizing of the two 33/11 kV transformers were verified within an acceptable values. In addition, the energizing of the second transformer has to be performed only when the transient phenomena relevant to the energizing of the first transformer are extinguished. This was performed with the tap changer of the primary winding set in the maximum position (higher voltage) this increased the number of turns to be excited. B. Contribution and Significant of the Study. Most of the papers written before about inrush current were focused more on the effect of the inrush current on the protective devices and how to mitigate it. However, not much research works have been carried out on the impact of the inrush current on the wind generator –transformer in-term of voltage stability and the voltage quality at the grid end, weather it would be within the acceptable limit of the power system network as the results shown in table I.. As it was discussed in A, Transient voltage of 14.9% above permissible value caused grid interruption for duration of 2 cycles, when allow to into the grid without a precaution. The significant of the paper is to determine the limit of inrush current which can cause the transient voltage (Under-voltage or over- voltage) that can lead to the grid instability. This will help on when to mitigate the inrush current that can cause the voltage sag or over- voltage, thereby improve the voltage quality of the wind farm and making the network stability for future reference. V. CONCLUSION The simulation has been performed for energizing the two 33/11 kV, 7.5MVA transformers considering the minimum short circuit power of the grid and the Wind Farm disconnection from the system. During the energizing of the transformers, the relevant circuit breakers were closed at the time when the voltage of phase A is crossing the zero point. This condition presents the higher value of inrush current. The extent to which power quality is degraded depends on short circuit at the source bus, the magnitude and decay time constant of the transient current. In the study case presented, the permissible voltage transient limit was 0.94 p.u. But when wind farm generator was energized during switch on- operation, the transient voltage increases to1.08 p.u which is above the system limit as a result of inrush current in the wind generator transformer. The paper also shows the transient of 14.9% different from the system nominal voltage with duration of 2 cycles will affect the network stability, thereby cascading into power interruptions due to unsteady bus system. However, changing the tap changer position was able to reduce the value to 0.93 p.u this values is within acceptable limit. References cvv[1] Mukesh Nagpal, Terrence G. Martinich, Member, IEEE, Ali Moshref, Kip Morison, and P. Kundur, Fellow, IEEE. “Assessing and Limiting Impact of Transformer Inrush Current on Power Quality” “ IEEE Transaction on power delivery, Vol. 21, NO. 2, April 2006”. [2] Y. Cui, S.G. Abdulsalam, S. Chen, and W. Xu, A Sequential Phase Energization Method for transformer inrush current reduction, Part I: Simulation and Experimental Results, IEEE Trans. on Power Delivery, Vol. 20, pp. 943-949, April 2005. [3] W. Xu, S.G. Abdulsalam, S.Chen, and X. Liu, A Sequential Phase Energization Method for transformer inrush current reduction, Part II: Theoretical Analysis and Design Guide, IEEE Trans. on Power Delivery, Vol. 20, pp. 950-957, April 2005. [4] Federal Ministry of power, Network study on the kastina wind farm, 2011. [5] R.Rahnavard, M.Valizadeh, A.A.B.Sharifian, S.H.Hosseini, Analytical Analysis of Transformer inrush current and some new Techniques for its reduction. [6] Chen S. D., Lin R. L., Cheng C. K. Magnetizing inrush model of transformers based on structure parameters // IEEE Trans. Power Delivery, 2005. – Vol. 20. – No. 3. – P. 1947-1954 [7] Xu W., Abdulsalam S. G., Cui Y., Liu X. A sequential phase energization technique for transformer inrush current reduction— Part II: Theoretical analysis and design guide // IEEE Trans. Power Delivery, 2005. – Vol. 20, No. 2. – P.950-957. [8] Robert J. Meredith (2008, June 23). Atp modeling of core-form transformers by magnetic circuit analysis, including finite sectioning. 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[10] Mark McGranaghan, EPRI Solutions, and Fred Kinder, Dranetz-BMI, “Electrical Construction and Maintenance” Evaluating Motor and Transformer Inrush Currents, “ 2006. /ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 200 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages false /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 400 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 600 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /CreateJDFFile false /Description > >> setdistillerparams > setpagedevice