Modeling of UHV Power Transformer and Analysis of Electromagnetic Transient

Linjun Zeng, Xiangning Lin, IEEE Senior Member, Jingguang Huang, Zhiqian Bo, IEEE Senior Member

College of Electrical Engineering and Information Technology

Three Gorges University

443002, Yichang, P. R China

Abstract1 To correctly apply transformer differential protec-tion in the environment of UHV (Ultra High Voltage), it is neces-sary to model the UHV power transformer reasonably and carry out corresponding electromagnetic transient simulations. There-fore, according to the equivalent circuit of three winding auto-transformer, we firstly set up the three winding autotransformer model by means of UMEC (Unified Magnetic Equivalent Circuit) transformer model provided by EMTDC software. Then, the pa-rameters of UHV transformer are converted to those of the UMEC model. In this way, the UHV transformer model is built. Based on this model, the internal faults model of UHV transfor-mer is set up. Under the UHV environment, the energization and internal fault of UHV power transformer are simulated, and the simulated data are utilized to investigate the operation reliability of the well-applied differential protection combined with the crite-rion of second harmonic blocking. It is shown with the simulation results that the second harmonic ratios of three phase inrush of the UHV power transformer are all below 10% in the case of some energization scenarios. In this case, the mal-operation of the diffe-rential protection with 15%~20% second harmonic restraint ratio cannot be avoided, even though it adopts such a blocking strategy that the protection will be blocked as the second harmonic ratio of any one phase exceeds the threshold. Besides, in some light fault conditions, the second harmonic ratio of the fault current is rela-tively high in the beginning of fault inception, leading to the short time delay of operation of protection.

Index TermsUHV power transformer; autotransformer; UMEC; EMTDC; magnetic inrush; internal fault current; harmonic

I. INTRODUCTION

here has been a problem of distinguishing between magnet-ic inrush and internal fault current for the differential pro-

tection of the power transformer, and the scenario of the 1000kV UHV (Ultra High Voltage) transformer protection is possibly more serious. Compared with EHV (Extra High Vol-tage) power systems, the electromagnetic environment of UHV system is more complex. Meanwhile, the configuration and parameters of UHV transformer differ from EHV transformer

1 This work is supported by National Natural Science Foundation of China (50777024) and by Program for New Century Excellent Talents in Universi-ty(NCET-07-0325)..

L. Zeng, X. Lin, J. Huang are with College of Electrical Engineering and In-formation Technology, Three Gorges University, Yichang 443002, Hubei Province, China. X. Lin is also with the Department of Electrical Engineering, Huazhong University of Science and Technology (HUST), Wuhan430074, China (e-mail: linxiangning@hotmail.com). Z. Q. Bo is with the AREVA T&DAutomation & Information Systems, Stafford ST17 4LX, U.K.

obviously. In this case, modeling the UHV power transformer reasonably and carrying out the corresponding electromagnetic transient simulations are the preconditions of applying trans-former differential protection correctly.

Autotransformer is the main type of UHV transformer, but the model of autotransformer provided by most simulation software is absent. An ordinary way is to replace autotransfor-mer by the common transformer in electromagnetic transient simulations. In this way, only the effect of magnetic coupling is included, but the electric relation between primary side and secondary side is ignored. The model proposed in reference [1] adopts flux linkage as state variable and includes the nonlinear-ity of transformer core. It is clear in terms of concept but too complex to perform in many cases. In reference [2], a new transient simulation model of the three-phase autotransformer is described, in which the controlled voltage and current sources are developed with the modified damping trapezoidal method which is engaged to form its synthetic simulation mod-el. In this case, both efficiency and precision of simulations are improved. However, this type of investigation can be more reasonable if it considers the nonlinearity of magnetizing im-pedance. Furthermore, the electromagnetic transient simula-tions in the UHV electromagnetic environment is a new chal-lenge, especially including the UHV transmission line with distributed parameters.

PSCAD/EMTDC is an appropriate simulation software ap-plied in various fields of power systems. In particular, it is suitable for electromagnetic transient simulations. In this paper, according to the equivalent circuit of three winding autotrans-former, we set up the UHV autotransformer model and its in-ternal faults model by means of UMEC (Unified Magnetic Equivalent Circuit) transformer model provided by EMTDC software. This new model considers both particularity of UHV transformer and nonlinearity of transformer core. Based on this model, we carried out simulations in several cases included energization, inter-turn short-circuit fault, phase to ground short-circuit fault and phase-to-phase short-circuit fault. Finally, we analyzed the current waveforms and evaluated the issues of the transformer differential protection with 2nd harmonic blocking scheme applied in UHV transformer protection.

II. MODELING OF UHV POWER TRANSFORMER

A. Basic configuration of UHV Power Transformer Autotransformer is applied widely in 220kV and higher sys-

tems due to many merits such as low cost, high efficiency, low exciting power and so on. The tremendous capability and the high requirement of insulation lead to the huge bulk and prodi-

T

978-1-4244-4241-6/09/$25.00 2009 IEEE

gious weight of UHV transformers as the single-phase capabil-ity of UHV transformer is up to 1000 MVA. In view of the convenience of transportation, adopting single-phase configu-ration becomes necessary. The UHV transformer produced in China is exactly single-phase autotransformer [3]. The three-phase configuration is implemented with the single-phase transformer bank.

Autotransformer has the tertiary winding, namely low-voltage winding. The tertiary winding is unloaded, instead, its functionality is to circulate third harmonic. Three phases of the tertiary winding are connected by delta-type and earthed through a low-voltage reactor.

To satisfy the demand of isolation, the off-line vol-tage-regulating from the neutral terminal is adopted and the voltage regulator and compensation transformer separately are set by the UHV transformer. The principle can be illustrated by Fig.1.

Fig.1 the principle diagram of UHV transformer voltage regulation

SVCVLVTVEVLELT respectively represents series

winding, common winding, low-voltage winding, voltage reg-ulation winding, magnetizing winding, low-voltage magnetiz-ing winding, low-voltage compensation winding. Due to this special type of coupling of windings, short-circuit impedance of UHV transformer is much bigger than that of ordinary transformer.

Since the currents of each sides of UHV transformer are the main concerns, the main transformer and the corresponding voltage-regulating compensation transformer are equivalent to be a three winding autotransformer.

B. The equivalent circuit of three winding autotransformer

No mater how the windings are arranged, we can study the three-winding autotransformer by means of a equivalent Y-type equivalent circuit[4]. In the following, the equivalent circuit of UHV transformer based on the series, common and tertiary winding are modeled.

'cI&

'BU&

QU&

QI&

'cU&

'BI&

Fig.2 Three winding auto transformer theory Diagram

As seen in Fig.2, after converting electrical quantities to

common winding side, 'cU& and 'cI& are the voltage and the

current respectively of the series winding. The voltage and the

current of the common winding are denoted by QU& and QI& .

Besides, 'BU& and 'BI& represent the voltage and the current of

the tertiary winding.

Similar to equation derivation of ordinary three winding transformer, the following equation can be deduced when the exciting current is ignored, as given by:

' ' '

' ' ' ' ' '

C Q C C Q Q

C B C C B B

U U I Z I Z

U U I Z I Z

= +

= +

& & & &

& & & &

(1)

Where, 'CZ is the leakage impedance converted from the

series winding; QZ is the leakage impedance of the common

winding; 'BZ is the leakage impedance converted from the low-voltage winding.

According to (1), its Y-type equivalent circuit can be de-duced, as seen in Fig.3:

'CZ

QZ'

BZ

C

B

Q Fig.3 Three ports Y-type equivalent circuit

The parameters of the equivalent circuit can be obtained by

three-winding ordinary transformer test. By this token, the three-winding autotransformer can be simulated based on three winding ordinary transformer.

C. Models of UHV transformer for simulation

EMTDC is a widely-used simulation software in power sys-tem analysis. The typical application is to calculate the varia-tion of electric parameters momentarily when power system is disturbed or the system parameters are changed. In this paper, modeling of UHV transformer and simulation of electromag-netic transient are both carried out by EMTDC. However, EMTDC does not provide the three-winding autotransformer models directly. According to the above analysis, and in view of the "electric" relation between series winding and common winding of autotransformer, we connect two windings of the UMEC three-winding transformer model to form the high-voltage winding and the medium-voltage winding. In this way, UHV transformer model can be achieved.

Fig.4 Model of UHV transformer

As seen in Fig.4, 1# winding, 2# winding and 3# winding

form the low-voltage winding, the series winding and the common winding respectively. The effectiveness of the equi-valence is to guarantee the leakage impedances of correspond-ing windings being equal between the equivalent model and the original model. Significantly, the parameters of the UHV transformer should be converted to the tertiary winding, name-ly low-voltage winding.

The UMEC transformer model is built primarily based on core geometry. Unlike the classical transformer model, the magnetic coupling between windings of different phases, in addition to coupling between windings of the same phase, are

taken into account in the UMEC model. The piecewise linear technique is used to control the model equivalent branch con-ductance. The non-linearity of the core is input directly into the model as a piece-wise linear U-I curve, which makes full use of the interpolation algorithm for the calculation of exact instants in changing of state range.

Internal faults of transformer include inter-turn short-circuit fault, inter-turn to ground fault, and phase to phase short-circuit fault and phase to ground fault. At present, diversified phase to phase fault or phase to ground fault are available by means of FAULTS module provided by EMTDC. Therefore, the model of internal winding fault is the main concern of this paper.

When an inter-turn fault occurs on double-winding transfor-mer, the fault turns of the faulty winding can be seen as a ter-tiary winding. It can be regarded as the fault of the tertiary winding of a three-winding transformer[5].Based on this con-cept, faulty turns of three winding transformer can be simulated by a fourth winding, see Fig.5.

Fig.5 internal faults model of transformer

In Fig.5, 2# winding denotes the fault turns, and the fault types can be controlled by the breakers. Leakage reactance X2 of 2# winding and leakage reactance X3 of 3# winding can be calculated, as given by:

2 3

22 3 2 3/ ( / )

cX X X

X X N N

+ =

=

(2)

In (2), Xc is known as the leakage reactance of series winding. N2, N3 are respectively the turn quantities of 2# winding and 3# winding. Practically, N2/N3 nearly is equal to the ratio of 2# winding's rated voltage to 3# winding's.

III. SIMULATION AND ANALYSIS

Due to the nonlinearity of transformer core, the magnetizing inrush possibly occurs when a transformer is energized, which easily leads to the mal-operation of the differential protection if no blocking strategy is included. Therefore, identifying inrush current accurately is the premise of correct operation of diffe-rential protection. The above two models were used to carry out the simulations of energization and internal faults of UHV transformer in the following. In this way, the operation beha-vior of the protection can be investigated.

A. System Model and correlative parameters

The system model comes from Jindongnan Nanyang Jingmen 1000kV AC test and demonstration project of CHINA in process, and all the parameters in the model system are from the real UHV project.

The transmission lines parameters: JindongnanNanyang: length =363km; Positive sequence resistance in per km R1=0.00758/km, positive sequence reactance in per km X1 =0.26365/km, positive sequence capacitance in per km C1= 0.01397F/km. Zero sequence resistance R0=0.15421/km, zero sequence reactance X0=0.7821/km, zero sequence capa-

citance C0=0.008955F/km. NanyangJingmen: length= 291km. Positive sequence resistance in per km R1 =0.00801/kmpositive sequence reactance in per km X1= 0.2631/km positive sequence capacitance in per km C1=0.013830F/km. Zero sequence resistance R0=0.1563 /kmzero sequence reactance X0=0.8306/kmzero sequence capacitance C0 = 0.009296F/km .

The UHV autotransformer parameters are as follows: Rated capabilities of the high-voltage side, the medium-voltage side and low-voltage side are respectively 1000 MVA, 1000MVA and 334MVA. Voltage ratings (RMS)of the high-voltage side, the medium-voltage side and the low-voltage side are respec-tively 1050 kV, 525kV and 110kV; short-circuit impedances (based on rated capabilities of high voltage side): 18% in High-medium side, 62% in High-low side, and 40% in me-dium-low side. No-load loss is 0.07%; magnetizing loss is 155kW.

Rated capability of high voltage reactors: 960MVA in Jin-dongnan side of JindongnanNanyang transmission line, while 720MVA in Nanyang side; 720MVA in Nanyang side of Na-nyangJingmen transmission line, and 600MVA in Jingmen side.

In view of the influences which result from the energization transient of the transmission lines and high voltage reactors, the energization position is at the high voltage side of UHV trans-former at Jingmen side.

The system model is shown in Fig.6.

Fig6. Model system As seen in Fig.6, the UHV source is connected to the high-

voltage side of the UHV transformer via UHV transmission lines. The medium-voltage side is linked with a equivalent load, while low-voltage winding is connected in delta-type and is grounded through a reactor and a capacitor for compensation.

Actually, the source of the UHV project is provided by the medium-voltage side of the UHV transformer at Jindongnan. It is no harm to replace Jindongnan by equivalent source since the emphasis lies in the energization at Jingmen. Besides, the reactors are modeled by parallel inductances, and capacitors are modeled by capacitances. The remnant flux is modeled by DC source which is put on the high-voltage side of transformer.

B. Simulation and analysis of energization

Energization simulations were carried out in terms of diverse initial angles and remnant flux densities. A scenario of typical inrush waveforms in three phases is shown in Fig.7. As seen, the harmonics of the inrush is more abundant than the trans-formers in EHV and lower voltage level systems, leading to the more abnormal waveforms.

Fig.7 Magnetic inrush currents in the condition of typical energization, Initial

angle of phase A is 0o; remnant flux densities of the three phase are all 0 The UHV transformer adopts Y-d-11 type. Therefore, our

concern is the differential current with regard to the transformer differential protection. Differential current is the summation of three-side influx currents, which should be equipped with phase and magnitude compensation. Namely, if the influx currents of the high, medium, low voltage sides of phase A are ahI& amI&

alI& , and the influx currents of the high, medium, low voltage

side of phase B are bhI& , bmI& , blI& , in view of the phase com-pensation and magnitude compensation, the differential current of phase A is

( ) ( )525

11031050 1050

3 3ah bh am bm alI I I I I + +& & & & & .

Because the transformer is energized at the high-voltage side, there are no currents in other two sides, so the differential cur-

rent of phase A is ( )ah bhI I& & exactly. Tab.I provides the har-monic ratios of the three phase differential currents in the con-dition of various energizations.

Tab.I Harmonic analysis of inrush currents

Remnant flux density

Initial angle of phase A

( o)

2nd harmonic ratio (%)

PhaseA

PhaseB

PhaseC

Phase A:0Bm Phase B:0Bm Phase C:0Bm

0 30.4 40.4 15.1

30 31.8 22.6 14.8

60 37.0 23.7 34.3

Phase A:0.7Bm Phase B:-0.5Bm Phase C:-0.5Bm

0 16.0 18.9 10.1

30 17.0 15.4 1.9

60 30.3 15.0 3.7

Phase A:0.9Bm Phase B:0Bm

Phase C:-0.9Bm

0 12.8 17.7 4.0

30 9.8 6.9 6.1

60 17.0 17.0 7.8

According to Tab.I, when the initial angle of phase A is 30o, the harmonic ratio of one phase will be under 15%, even if there is no remnant flux. When the remnant flux is taken into account, the harmonic ratio of phase C will fall below 1.9%, as shown in fifths row of Tab.I. This indicates that it is unrealistic to adjust harmonic restraint ratio only to avoid the mal-operation of differential protection. Only by virtue of adopting such a blocking strategy that the protection will be blocked as the second harmonic ratio of any one phase exceeds the threshold, and regulating harmonic restraint ratio to the value lower than 15%, the scenario of above mal-operation can be avioded. Furthermore, when the remnant flux densities of three phase are 0.9Bm, 0 and -0.9Bm and the initial angle of phase A is 30o, the 2nd harmonic ratios of three phase differential currents are all under 10%. In this case, even the above strict countermeasure cannot allow the protection to survive.

It is impossible to simulate all the conditions involving the diverse initial angles, remnant flux densities and different op-eration states of systems. However, the simulation results pre-sented in this paper at least suggest that the 2nd harmonic cha-racteristic of the inrush of UHV transformer is weaker than that in EHV and lower voltage level systems. This scenario should be paid attention to when the differential protection of UHV transformer is put into service.

Besides, the high order harmonic, especially odd harmonic of the inrushes are more abundant than ordinary transformer. It possibly has some impact on the methods identifying inrush by means of waveform characteristic.

C. Simulation and analysis of internal faults

Inter-turn short-circuit fault, turn-to-ground fault simulations of different faulty turns ratios have been carried out and shown in Fig.6. All the faults occurred in phase A exemplarily.

Moreover, several phase to ground faults were simulated by means of FAULTS module provided by EMTDC, including phase A to ground, phase A to phase B short-circuit fault and phase A-phase B short-circuit-to-ground fault.

Several current waveforms of phase A in different fault conditions are shown in Fig.8.

aInter-turn short-circuit fault

bturn to ground fault

cphase A to Phase B short-circuit fault

Fig.8 Currents for internal faults

According to waveforms, no mater inter-turn short-circuit or turn to ground fault occurs, the smaller the short-circuited turns, the lower the primary current. When a terminal fault occurs, the fault current is big and abnormal.

Due to the distributed capacitive effect of UHV transmission line and the particularity of UHV transformer itself, there is abundant harmonics within the fault currents. However, for most inter-turn faults, the ratio of the 2nd harmonic of differen-tial currents of diversified faults is under 15%. Therefore, it would not influence the rapidity of the protection operation in a mass of fault conditions.

However, the exception still exists. For instance, when a 2% inter-turn fault occurs, the 2nd harmonic of the faulty phase is up to 22.6%, which exceeds the conventional 2nd harmonic re-straint ratio setting, leading to the time delay of the protection operation.

According to the simulation result, 2nd harmonic blocking scheme can distinguish between inrush and fault current on the whole. The differential protection with 2nd harmonic blocking can do well when applied in UHV transformer protection if the restraint ratio is chosen reasonably. However, the settings and the restraint mode should be chosen carefully.

Besides, a great deal of beneficial works on the aspect of identifying inrush current were presented[8-12].we can assess the adaptability of these methods to UHV transformer based on the energization and internal fault model of UHV transformer proposed in this paper. Accordingly, the operation performance of the differential protection of UHV transformer can be im-proved further.

IV. CONCLUSION

Based on the elementary transformer model provided by EMTDC, the UHV transformer and its internal fault model possessing the characteristic of autotransformer are built in this paper. We carry out the corresponding electromagnetic tran-sient simulations in UHV environment, and offer the reasona-ble precondition for investigating the protection operation of the UHV transformer, especially for proving its applicability to the UHV test and demonstration project in CHINA. The em-phasis is to evaluate the operation reliability of the differential protection combined with 2nd harmonic blocking. It is shown with the simulation results that the 2nd harmonic characteristic of the inrush in UHV transformer is weaker than EHV and lower voltage level systems. The 2nd harmonic ratios of three phase differential currents may be all under 10% in certain conditions. In the fault conditions, all the 2nd harmonic ratios of differential currents are under 10% except the light inter-turn fault. Making a comprehensive view of simulations of inrushes and fault currents, the differential protection with 2nd harmonic blocking scheme still has redundancy when applied in UHV transformer protection. REFERENCES [1]Z. Zhao and Z. Feng, "Digital real time simulation model and digital integral of autotransformer" , Journal of TongJi Universityvol. 29 ,no.4,pp.416- 420, 2001. (In Chinese) [2]L. Zhao and C. Chen, "Study of model of three phase autotransformer in electric system transient simulation", Proceedings of the EPSA, vol. 16, no.1, pp. 83, 2004. (In Chinese) [3]S. Sun and M. Fang, et al, "Development and design of 1000kV autotrans-former", Electrical Equipment, vol. 8, no. 4, pp. 6-10, 2007. (In Chinese)

[4]T. Yang, C. Shi, X. Tan, Autotransformer and its applications (In Chinese). Beijing, China: China Electric Power Press, pp.28-30. [5]W. Wang and B. Hou, Theoretical Basis of the Protection Principle of the Utility-Type Unit (In Chinese). Beijing, China: China Electric Power Press, pp.63-64. [6]X. Wang and Z. Wang, "Study of simulation of transformer with internal faults", Power System Technology, vol. 28, no. 12, pp.51, 2004. (In Chinese) [7]Y. Huang and Q. LI, et al, "Simulation for magnetic inrush and fault current of three-phase transformer based on EMTDC", RELAY, vol. 35, no. 1, pp. 26-29, 2007. (In Chinese) [8]B. He and X. Xu, "Protection based on wave comparison", Proceedings of the CSEE, vol. 18, no.6, pp. 395-398, 1998. (In Chinese) [9]S. Jiao and W. Liu, "A novel scheme to discriminate inrush current and fault current based on integrating the waveform", Proceedings of the CSEE, vol. 19, no.8, pp. 35-38, 1999. (In Chinese) [10]X. Lin, P. Liu, C. Yang, "Studys for identification of the inrush based on improved correlation algorithm ", Proceedings of the CSEEvol. 21, no. 5, pp. 56-60, 2001. (In Chinese) [11]D. Chen , X. Yin, et al, "Virtual third harmonic restrained transformer differential protection principle and practice", Proceedings of the CSEE, vol. 21, no. 8, pp. 19-23, 2001.(In Chinese) [12]J. He and J. Li, B. Yao, et al, "A new approach of transformer inrush de-tected based on the sine degree principle of current waveforms", Proceedings of the CSEE, vol. 24, no. 4, pp. 84-59, 2007.(In Chinese) BIOGRAPHIES Linjun Zeng received Bachelor degree from China Three Gorges University (CTGU) in 2006. He is presently a master candidate at the department of Elec-trical Engineering in CTGU. His research interests are power system protection and control.

Xiangning Lin received a Master and a Ph.D degree from the Huazhong Uni-versity of Science & Technology (HUST) respectively in the Electrical Engi-neering. He is currently a professor titled of "Chutian Scholar" in Three Gorges University, Yichang, Hubei Province, China. His research interests are modern signal processing and its applications in the power systems, power system protective relaying and control.

Jingguang Huang is presently an associate professor at the department of Electrical Engineering in CTGU. His research interest is modern signal processing and its applications in the power systems. Zhiqian Q. Bo received the B.Sc. degree from Northeastern University, She-nyang, China, in 1982 and the Ph.D. degree from The Queens University of Belfast, Belfast, U.K., in 1988. From 1989 to 1997, he was with the Power Systems Group, University of Bath, Bath, U.K. Currently, he is with AREVA T&DAutomation and Information Systems, Stafford, U.K., responsible for new technology developments. His main research interests are power system protection and control.

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