Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection

IJSTE - International Journal of Science Technology & Engineering | Volume 2 | Issue 07 | January 2016 ISSN (online): 2349-784X

Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection Mansi Bhanot M. Tech Student Jannayak Ch. Devi Lal Vidyapeeth, Sirsa,

Gurpreet Singh Assistant Professor Guru Jambeshwar University, Hisar

Abstract When a transformer is energized, magnetizing inrush current arises due to short-term overfluxing in the transformer core. The quantity of inrush current may be as high as ten times of transformer rated current which causes maloperation of differential protection system. This inflow current comprises decaying DC and superior second harmonic components which may cause unwanted effects like poor power quality and minimized mean lifespan of transformer. At the moment of transformer energizing if fault transpires, the protection system crash to distinguish between inrush and fault current. This paper provides a method for ejection of DC component and discrimination between inrush and fault condition to intend power transformer protection. For elimination of DC component, a compensating signal is generated with help of algorithm based on Taylor series expansion of inrush current. As second harmonic component is supreme in inrush current, discrimination is carried out utilising harmonic restraint. Based on ratio of second harmonic component to fundamental component algorithm is evolved to discriminate inrush from fault current, which is validated on a case study developed in Matlab/Simulink.

Keywords: DC Components, Inrush Current, Taylor Series, SHR ________________________________________________________________________________________________________

I. INTRODUCTION Transformer is one of the extortionate equipments in the power system network which needs to be isolated fast in the event of a fault. Network operators have authority towards the consumers, in providing authentic and continuous power without causing immense blackouts in the network. Hence, it is of paramount interest to shield the power transformer from being subjected to inrush and faults during inceptive energization and short circuit. Differential protection is one of the most favorible schemes used for the safeguard of power transformers. However, the main problem encountered while using differential protection is its limited ability to discrimination between inrush and fault current. While it is observed that magnetizing current of transformer may be only 1-2 percent of rated current at steady state operation, it could reach 10-20 percent of rated current during energization of transformer. This genesis not only spurious triggering of differential protective relay but also leads to maloperation of other protective system. Researchers and engineers have evolved few methods in the last few years to distinguish between inrush and fault current including the second harmonic restraint, dead-angle restraint, voltage restraint and the flux-based inrush restraint.

II. SIMULINK MODELS The inrush current phenomenon and its discrimination from fault current is investigated on a three phase transformer of 22kV/220kV, 250 MVA by simulink using MATLAB as shown in Fig 2. As explained in Section II, when an unloaded transformer is energized at zero degree, the inrush current arrive as shown in Fig 3. It is seen that, the peak value of inrush current is very high as compared to normal magnetizing current and it is unsymmetrical because of non-liner magnetizing characteristic of transformer.

In addition to this, the magnetizing inrush current is 10-15 times of the steady state current value, which flows only in one side of the transformer and leans to operate the differential relay unless some form of constraint is not provided. Because of the nonsymmetrical wave shape, the transformer inrush current contains all harmonics and DC components. The harmonic content of the phase ‘A’ inrush current is calculated using a discrete fourier transform (DFT).

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Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection (IJSTE/ Volume 2 / Issue 07 / 033)

Fig. 1 Model for Harmonic Analysis of Inrush and Fault Current

A digital filter whose variables are determined using (7) and (8) is used to filter out the DC components present in the magnetizing inrush current.

Fig. 2: Digital Filter Used to Compensate DC Component

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Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection (IJSTE/ Volume 2 / Issue 07 / 033)

The digital filter whose output is shown in Fig 3, designed so that its output is exactly opposite to that of DC component. From Table I it can be seen that SHR is more for inrush current as compared to fault current. Thus, SHR can be used to discriminate inrush from fault current such that, if SHR is less than threshold value then that condition can be considered as fault otherwise it is inrush condition. Table I shows the values of SHR for various switching conditions of CB which is shown in Fig 1 in two different scenarios of fault and without fault operating conditions.

III. RESULTS AND WAVEFORMS The performance of the proposed structure is computed by a computer simulation that uses MATLAB Software. The study case is dedicated to test the dynamic performance of the proposed structure.

Fig. 3: Magnitude of Inrush Current at Zero Degree Energization

Fig. 4: Harmonic contents present in phase ‘A’ inrush current

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Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection (IJSTE/ Volume 2 / Issue 07 / 033)

Fig. 5: Inrush and Fault Current

Fig. 6: DC Component in Inrush Current

Fig. 7: Signal for Compensation of DC Component

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Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection (IJSTE/ Volume 2 / Issue 07 / 033)

IV. FUTURE WORK In future workout we can extend this project by using an inrush current limiter, a series-compensator based circuit for limiting inrush current of transformer will be used. This compensator based circuit composed of a diode-bridge type DC reactor connected in series with each phase of the transformer. The ICL consists of a diode bridge and a DC reactor as shown in Fig.6

Fig. 8: The Single-Phase Power Circuit Topology

Fig. 9: Inrush Current and DC Reactor Current.

The termination shows that the inrush current duration dimnished. The DC component varies according to the inrush current as illustrated in figure.

Fig. 10: Magnitude of Inrush Current at Zero Degree Energization with ICL

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Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection (IJSTE/ Volume 2 / Issue 07 / 033)

Fig. 11: DC Component in Inrush Current with ICL

Fig. 12: Compensation of DC Component with ICL

From Table I it can be seen that SHR is more for inrush current as compared to fault current. Thus, SHR can be used to discriminate inrush from fault current such that, if SHR is less than threshold value then that condition can be examined as fault otherwise it is inrush condition. Table I shows the values of SHR for various switching conditions of CB which is shown in Fig 1 in two different scenarios of fault and without fault operating conditions.

Table - 1 Rms Value of Inrush Currents for Various Switching Instants (with and without Fault Conditions)

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Mitigation of DC Component and Suppression of Inrush Current for Power Transformer Protection (IJSTE/ Volume 2 / Issue 07 / 033)

V. CONCLUSION The proposed method presents a discrimination strategy between inrush and fault current for the protection of power transformer based on harmonic analysis and elimination of DC decaying component using digital filters. Detailed analysis of harmonic contents of the magnetizing inrush current has been estab lis hed to be important for the design of the harmonic restraint and has been convoluted in this paper. The domination of second harmonic component in the inrush current is an important factor for discrimination In this future workout, this is extended by using power electronic converter to suppress the transformer inrush current are studied and corroborated. Later on exercise we can elevate this venture by using a DC reactor and the outcomes reveal that the inrush current reduces. Main advantages of this method is simple power circuit, reliable operation, the cost of this current limiter is less, no information required about residual flux, switching instant, etc.

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S. R. Wagh, Shantanu Kumar and Victor Sreeram, "Extraction of DC component and Harmonic Analysis for Protection of Power Transformer",2013 IEEE 8th Conference on Industrial Electronics and Applications (ICIEA) [2] Paul C., Y. Ling and Amitava Basak, “Investigation of Magnetizing Inrush Current in a Single-phase Transformer,” IEEE Transactions on magnetics. Vol 24, No. 6, Nov. 1988.

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[5] J. S. Thorp and A. G. Phadke, “A microprocessor based three phase transformer differential relay,” IEEE Trans. Power Appl. Syst., vol. PAS-101, pp. 426– 432, Feb. 1982.

[6] Mrs. Manisha Wani, Prof. Kalyani Kurundkar, Prof. M. P. Bhawalkar, "Use of Power Electronic Converters to Suppress Transformer Inrush Current", 2012 IEEE International Conference on Power Electronics, Drives and Energy Systems, December16-19, 2012 [7] S.S.Rao “Switchgear and Protection” Khanna Publishers 1992.

[8] Y.G Paithankar, S.R Bhide “Fundamentals of Power System Protection” PHI Learning Pvt.Ltd.2004 [9] Taylor D. I., Law J. D., Johnson B. K. and Fischer N. “Single-phase transformer inrush current reduction using prefluxing “. IEEE Transactions on Power Delivery, Vol. 27, No.1, January 2012, pp. 245-252.

[10] M. Tarafdar Hagh and M. Abapour “DC Reactor Type Transformer Inrush Current Limiter” IEEE Electric Power Applications, IET, Vol.1 No. 5, August 2007 , pp. 808-814.

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