Discrimination of Internal Faults and Inrush Currents For Large Modern Power Transformer

Wael Hamdy Yousef , IEEE Student Member Doaa khalil Ibrahim, IEEE Member, and Essam Abo El-Zahab Power Generation Engineering Electrical Power and Machines Department and Services Company PGESCO Faculty of Engineering, Cairo University

whyousef@pgesco.com doaakhalil73@hotmail.com, zahab0@yahoo.com

Abstract - This paper presents a differential protection approach for large high quality power transformers that have low inrush currents of about 3.14 times rated currents. Such transformers have low second harmonic level, which results in conventional differential protection malfunction. A scheme for internal faults and inrush currents discrimination based on both discrete Fourier transform DFT and discrete Wavelet Transform DWT is proposed. The proposed method has been designed based on the percentage of the sum of wavelet transform coefficients D1 and fundamental differential current (based on a 1 kHz sampling rate). It has been tested by extensive EMTP/ ATP simulations for different faults and switching conditions on El Tebbin Power Plant 220/21 kV generator/power transformer. It is proved that it has a high capability for discrimination even in the case of inrush with low second harmonic content and internal fault currents with high second harmonic component. All tests proved that the proposed scheme is reliable, accurate, and fast. Index Terms – Differential protection, inrush current, power transformer, Wavelet Transform.

I. INTRODUCTION

ARGE power transformers are very expensive and vital components in electric power systems. Therefore, it is very important to minimize the frequency and

duration of unwanted outages, that results in a high demand imposed on power transformer protective relays; this includes the requirements of dependability associated with no mal-operations, security associated with no false tripping, and operating speed associated with short fault clearing time to avoid extensive damage and/or to preserve power system stability and power quality. Discrimination between internal faults and magnetizing inrush currents has long been recognized as a challenging power transformer protection problem. However, magnetizing inrush currents generally contain large second harmonic component in comparison to internal faults, conventional transformer protection systems are designed to achieve required discrimination by sensing that large second harmonic content [1]. Protection of large power transformers is a very challenging problem in power system relaying since the level of the second harmonic in inrush is significantly decreased due to the improvement of core steel in modern power transformers. Moreover, a large second harmonic

component can also be found during transformer internal fault currents if a shunt capacitor or a distributive capacitor is connected to a transformer in a long extra high voltage (EHV) transmission line. Consequently, the methods based on the measurement of the second harmonic are not sufficiently effective for differential protective relays [2].

Recently, several new protective schemes have been proposed to deal with the foregoing problem in power transformer protection. Most of them have mainly focused on transform voltage and current waveforms. A method has been proposed to distinguish inrush versus internal fault currents by tracking the variation of the equivalent instantaneous inductance, which alters with respect to the changes of the transformer core state [3] that avoids the use of second harmonic as the restraint principle. The main disadvantage of these methods includes the need to use voltage transformer that increases protective algorithm cost. A blocking scheme called waveform correlation has been proposed in [4]. It compares the symmetry between the first and second half cycles of current waveform in a whole cycle, thus internal fault or inrush currents can be distinguished if the correlation coefficients between the first and second half-cycles exceed given threshold. However, it is still difficult to identify symmetric inrush currents by using such scheme. On the other hand, a significant number of relaying formulations has been developed, based on finite elements, artificial neural networks (ANN) [5], fuzzy logic and dynamical principal components analysis [6]. These formulations however, are applied on specific systems and have a lot of generalization difficulties. Moreover, several protective schemes have also been proposed based on wavelet analysis [7-9]. These approaches are still liable to cause malfunction of relays in the case that inrush currents have low second harmonic content while internal fault currents have high values. Furthermore, most of these techniques use high sampling frequencies which considered the main drawback to add additional cost by changing existing physical relays. In addition, performance of some of these techniques that incorporate high frequency signals information in their operation had been validated using inadequate low frequency transients transformer modeling that increases doubts about their actual performance when be applied [10].

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Proceedings of the 14th International Middle East Power Systems Conference (MEPCON’10), Cairo University, Egypt, December 19-21, 2010, Paper ID 178.

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In this paper, a powerful protection scheme for three phase large modern power transformers using Discrete Wavelet Transform DWT is proposed. The proposed technique uses wavelets decompositions and a logic decision algorithm that identifies and discriminates correctly inrush currents and internal transformer faults. The proposed technique uses same microprocessor hardware requirement as existing conventional relays (1 kHz sampling rate: 20 samples per power frequency cycle based on 50 Hz). Therefore, it doesn’t need to add further component or additional cost which considered one of its main advantages. In order to analyze the proposed formulations efficiency, it was built in MATLAB platform and tested with simulated fault cases under ATP/EMTP. Comparative test results with differential protection formulations with harmonic restraint shows that the logic decision algorithm proposed provides an efficient and reliable operation.

II. DIFFERENTIAL PROTECTION

Differential protection is widely applied on power transformers protection, buses protection, large motors, generators protection and transmission lines protection. Considering power transformers above 10 MVA, the percentage differential relay with harmonic restraint is the most used protective scheme. A percentage differential function is applied to the fundamental component of the currents to decide whether an internal fault has occurred or not. It converts the primary and secondary currents to a common base and compares the operating current with a restraining current. The difference between the operating and restraining currents is small for normal operating conditions and external faults, while it becomes significant during internal faults. The operating current of percentage current differential protection can be obtained by: | I-I=|I sPop (1) Where: IP and IS are the primary and secondary currents respectively. The restraining current, Irt in most common modes can be obtained by: | II|k=I sPrt + (2)

Where: k is a compensation factor, usually taken as 1 or 0.5. The differential relay generates a tripping signal if the operating current, Iop, is greater than a percentage of the restraining current, Irt as follows: rtop I.SI LP> (3)

Where: SLP is the straight line defining the relay minimum pickup current. The relay operating region is located above the SLP characteristic (Equation 3), and the restraining region is below the SLP characteristic. Digital differential protection relay uses Discrete Fourier Transformation (DFT) filtration to extract Iop and Irt fundamental differential current every sample using the above equation.

III. WAVELET TRANSFORM

Power system disturbances associated with fast electromagnetic transients are typically non-periodic signals which contain both high-frequency oscillations and localized impulses superimposed on the power frequency and its harmonics. These characteristics present problems for traditional discrete Fourier transform (DFT) as it is best reserved for periodic signals. To reduce the effect of non-periodic signals on the DFT, the short-time Fourier transform (STFT) is used. It assumes local periodicity within a continuously translated time window [11]. DWT analysis was developed as an alternative method to STFT to overcome problems related to its frequency and time resolution properties. More specifically, unlike the STFT which provides uniform time resolution for all frequencies, the DWT provides high time resolution and low frequency resolution for high frequencies and high frequency resolution and low time resolution for low frequencies. DWT provides a compact representation of a signal in time and frequency domains and therefore has been extensively used in power system analysis in recent years. DWT uses a time function with finite energy and fast decay that called mother wavelet, several mother wavelets are known. In this paper, it is necessary to detect singularities (abnormal frequency changes) in the signals with the highest possible precision. In order to achieve this requirement, the mother wavelet used should consider the number of its vanishing moments. With more vanishing moments, higher precision can be achieved in the singularities detection. However, with more vanishing moments, the mother wavelet has also need more samples, limiting the number of details in which a specific signal could be analyzed, since the mother wavelet suffers dilation as the details increase. Therefore, the mother wavelet used in this paper is the Daubechies4, since it has the better relation between the number of its coefficients and its vanishing moments [12]. If the original signal is being sampled at Fs Hz, the band of frequencies between Fs/2 and Fs/4 would be captured first detail; similarly, the band of frequencies between Fs/4 and Fs/8 would be captured in second detail, and so on.

IV. PROPOSED SCHEME

It is proposed in this paper to develop a protection algorithm that can be used as a subroutine in an existing conventional differential relay. The identification and discrimination between internal faults and inrush currents are achieved from the analysis of the three-phase differential current signals, obtained through current transformers (CTs). Therefore, extracting first wavelet detail d1 using a data window of two cycles: one pre-fault cycle and one post-fault cycle of the differential signals is illustrated in Fig. 1.

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Fig. 1. Decomposition of inrush current using Wavelet Transform.

To verify whether the system is subjected to an internal

fault or not, the discrimination index IB is calculated. This index quantifies the characteristic of the first detail of the differential signals. The IB value is defined as the minimum of the three phases' sum of the absolute ratio of the first details coefficients values and differential fundamental currents extracted using DFT. Therefore, IB is calculated as follows:

CBA

n

n df

r

Id

,,1

B1

1MinI

= ∑

=

(4)

Where: IB = Discrimination index value. d1 = First detail coefficient (256–512 Hz). nr = number of samples in the window (40 samples). Idf1 = Fundamental frequency differential current. Min = Minimum value of the three phases A, B, C.

IB is compared against a predetermined threshold value (σ = 0.6). In this aspect, extensive series of fault cases with different conditions, cases of contaminated normal switching with faults and different inrush cases have been carried out and thus, the optimal setting for σ is determined. This setting value of this threshold is dependent on system parameters. Detailed proposed flowchart is illustrated in Fig. 2.

V. SIMULATION TESTS AND RESULTS

The system under consideration is the simplified machine model of a 21/220 kV transformer, star connected with grounded neutrals at high side and delta at low side as shown in Fig. 3. The system is simulated using BCTRAN subroutine

under ATP/EMTP. System parameters are given in the Appendix.

The simulations provide samples of currents in each phase when the transformer is energized, when a fault occurs on the system or when both occur simultaneously. Simulations have been divided into three main categories: simultaneous fault and inrush conditions, magnetizing inrush conditions and faulty conditions (LG, LL, 2LG and 3LG,), when feed of network side (back energization).

Inrush currents are simulated at different energization angles of: 0º, 30º, 60º, 90º, 120º and 150º with respect to phase-A voltage zero-crossing and different remnant flux by closing switch S1 in the high voltage side. Simulated fault resistances were 0.2, 10, 20 and 40 Ω.

Fig. 2. Proposed Scheme Flowchart.

Fig. 3. Single line diagram of the studied system.

Extracting Fundamental Operating and Restraining

currents using DFT: Idf1, Irf1

Idf1> SLP.Irf1

n>nr

Extracting first detail of Id using DWT

Calculating Discrimination index IB

IB < σ

Trip action

No

No

Yes

Yes

Shifting Next Sample

An internal fault detected

An inrush case detected

1 kHz sampled Primary and Secondary

3ph currents

∆ Y

G Relay

480MVA 220/21kV 1/15000

1/1600

Net

wor

k

S1 S2

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Simulation results of 48 inrush cases are given in Table I a-b for 15% and 20% second harmonic restrain setting, while the simulation results of 252 fault cases (contaminated switching with fault cases) are given in Table II a, b. Where: G denotes group number, N denotes number of cases studied related to same group tests, AVR denotes block operation of differential relay when the average of the three phases exceeds setting value. 3/3, 2/3 and 1/3 denotes block operation of differential relay when 3 phases, 2 phases and only 1 phase exceed setting value respectively. WD denotes the proposed scheme. Different performances are expressed with the number of mal-operations. As illustrated in Table I a-b, the performance of the proposed scheme had been tested compared to the performance of the possible second harmonic restrain setting types in existing relays ( 1 of 3, 2 of 3, 3 of 3 or AVR) at different energization cases (48 cases). Obviously, the proposed algorithm ensured efficient block during inrush currents. Moreover, AVR and 1 of 3 algorithms ensure same accurate operation in all cases tested irrespective of setting value either 15 or 20%.

However, the results of contaminated switching with fault cases tested ensured that the performance of proposed scheme is fully accurate in detecting all simulated faults irrespective of fault conditions. 3 of 3 algorithm also ensures same accurate operation in all cases tested irrespective of setting value either 15 or 20%, while none of other existing second harmonic restraint setting and types ensures accurate trip in all fault cases tested. Therefore, the proposed scheme is the only scheme that ensures accurate action in both normal inrush currents and contaminated inrush with fault cases.

TABLE I.a SIMULATION RESULTS OF INRUSH CASES WITH 15% SECOND HARMONIC

SETTING

G N Remnant fluxφ/φmax No. of Mal-operations A B C AVR 3/3 2/3 1/3 WD

1 12 0 0 0 0 0 0 0 0 2 6 -0.6 0.9 0.2 0 1 0 0 0 3 6 -1.5 0.9 1.3 0 3 0 0 0 4 6 -1.4 1.3 1.1 0 1 0 0 0 5 6 -0.9 0.3 1.4 0 3 0 0 0 6 6 -0.8 0.3 0.9 0 1 0 0 0 7 6 0.5 -0.6 0.1 0 3 0 0 0 Total no. of Mal-operations 0 12 0 0 0

TABLE I.b

SIMULATION RESULTS OF INRUSH CASES WITH 20% SECOND HARMONIC SETTING

TABLE II.a SIMULATION RESULTS OF CONTAMINATED SWITCHING WITH FAULT CASES WITH

15% SECOND HARMONIC SETTING

TABLE II.b

SIMULATION RESULTS OF CONTAMINATED SWITCHING WITH FAULT CASES WITH 20% SECOND HARMONIC SETTING

The three most typical challenges of power transformer differential relays are: avoiding faulty trip due to inrush currents even low values, avoiding faulty block for internal fault conditions and ensuring fast tripping for internal faults. The performance of proposed relaying scheme had been tested for tens times to ensure its dependability associated with no mal-operations, security associated with no false tripping, and operating speed associated with short fault clearing time to avoid extensive damage and/or to preserve power system stability and power quality. Due to the limitation of the length of this paper, only some cases of these tested are illustrated as examples in details in the following subsequent sections. A. Avoiding faulty trip due to low inrush currents

A magnetizing low inrush current is simulated on the high voltage side of the power transformer at time 0.085 sec and energization angle 90o with residual flux of -0.65, 0.96 and 0.18 of maximum flux value as shown in Fig. 4. As illustrated, the second harmonic content was so low and a mal-operation tripping is activated by conventional differential relay. However, using wavelet based discrimination index, the proposed scheme has successfully identify such case and accurately block relay operation.

B. Avoiding faulty blocking for internal fault conditions

A double line to ground fault is simulated on the high voltage side of the power transformer at time 0.0883 sec as shown in Fig. 5. As illustrated, the second harmonic content exceeds the setting value and a mal-operation blocking is activated by conventional differential relay. However, using wavelet based discrimination index that doesn't exceed the

G N Remnant fluxφ/φmax No. of Mal-operations A B C AVR 3/3 2/3 1/3 WD

1 12 0 0 0 0 0 0 0 0 2 6 -0.6 0.9 0.2 0 4 0 0 0 3 6 -1.5 0.9 1.3 0 3 1 0 0 4 6 -1.4 1.3 1.1 0 4 0 0 0 5 6 -0.9 0.3 1.4 0 3 1 0 0 6 6 -0.8 0.3 0.9 0 4 0 0 0 7 6 0.5 -0.6 0.1 0 3 1 0 0 Total no. of Mal-operations 0 21 3 0 0

G N Fault Type No. of Mal-operations AVR 3/3 2/3 1/3 WD

1 36 3LG at LV side 0 0 0 0 0 2 36 2LG at LV side 1 0 0 23 0 3 36 LL at LV side 1 0 0 23 0 4 36 3LG at HV side 0 0 0 0 0 5 36 LG at HV side 36 0 36 36 0 6 36 2LG at HV side 35 0 0 36 0 7 36 LL at HV side 33 0 0 36 0 Total no. of Mal-operations 106 0 36 154 0

G N Fault Type No. of Mal-operations AVR 3/3 2/3 1/3 WD

1 36 3LG at LV side 0 0 0 0 0 2 36 2LG at LV side 0 0 0 8 0 3 36 LL at LV side 0 0 0 8 0 4 36 3LG at HV side 0 0 0 0 0 5 36 LG at HV side 31 0 30 36 0 6 36 2LG at HV side 22 0 0 36 0 7 36 LL at HV side 17 0 0 36 0 Total no. of Mal-operations 70 0 30 124 0

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predetermined setting, the proposed scheme has successfully identify such fault case and accurately activate trip action at 0.111 sec within only 0.0227 sec.

Fig.4. Magnetizing inrush current simulation a) 3 phase differential current in pu b) Percentage of second harmonic c) Tripping signal for conventional relay d) Discrimination index of the proposed scheme e) Tripping signal for proposed scheme.

C. Ensuring fast tripping for fault cases

Fig. 6 illustrates the performance of the proposed scheme during double line to ground fault at time 0.0883 sec against the performance of conventional differential relay. As shown, the conventional one activates trip action at 0.123 while the proposed scheme activates trip at 0.111 sec (a delay of 0.013 sec is avoided).

Fig. 5. LLG fault current simulation a) 3 phase differential current in pu b) Percentage of second harmonic c) Tripping signal for conventional relay d) Discrimination index of the proposed scheme e) Tripping signal for proposed scheme.

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Fig. 6. LLG fault current simulation a) 3 phase differential current in pu b) Percentage of second harmonic c) Tripping signal for conventional relay d) Discrimination index of the proposed scheme e) Tripping signal for proposed scheme.

CONCLUSIONS

A scheme for differential protection of large high quality three-phase power transformers, based on currents transient analysis, is proposed using both Discrete Fourier and Wavelet Transforms. Extensive tests have been carried out including the variation of energization conditions and fault conditions to evaluate proposed scheme performance.

The proposed scheme presented herein has a number of distinct advantages: § It avoids faulty trip operation due to inrush currents even

for low values. § It avoids faulty block for internal fault conditions even for

high second harmonic content or simultaneous fault and inrush conditions.

§ It ensures fast tripping for internal fault. § It could be used for updating, improving, and

refurbishing of the existing transformer differential protection systems, since it uses a low sampling frequency as existing digital relay microprocessor.

APPENDIX

A. Network Parameters: Z = 1.115 + j 29.88 Ω. B. Transformer Parameters: Three phase Y/∆ 480 MVA, 50

Hz, 220/21 kV, 15% impedance. Maximum inrush current is 3.14 times at high voltage side.

C. Generator Parameters 437 MVA with power factor ranges: 0.85 lagging to 0.89 leading. The generator voltage is 21 ± 5% kV, X/R=26.8.

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