Faults Detection in Single-Core Symmetrical Phase Shifting Transformers Based on Wavelets

Transcription

1 Faults Detection in Single-Core Symmetrical Phase Shifting Transformers Based on Wavelets 1 Meenakshi Sahu, 2 Rahul Rahangdale 1,2 Department Of Electronics And Communication Engineering School of Engineering and I.T.,Mats University, Aarang Kharora Highway, Aarang, Raipur, (C.G.), India Abstract: This paper presents application of wavelet transform in fault detection of single-core symmetrical phase shifting transformer. The first part of this paper describes the modeling of a single core symmetrical phase shifting transformer. The second part of the paper discusses the problem associated with the conventional protection scheme by evaluating the simulation results, and the application of wavelet transform for detecting the inter-turn fault on it. A phase shifting transformer (PST) is usually used for varying the voltage phase angle between the two systems, which provides controlling of active power transfer. Some types of PSTs allow controlling phase shift in a certain defined range, which means that phase shift between the two systems, can be varied on-line. Therefore, this type of phase shifting transformers must be protected by relays, which are able to on-line compensate for additional phase shift introduced by PST. It has been observed that for certain situations standard differential relays may requires tap changing information and sometimes mal-operate. In this paper application of Wavelet Transform is used for faults detection in singlecore symmetrical phase shifting transformers. In this technique, it is not necessary to provide tap changing position of the transformer.matlab Simulink, is used for the simulation of delta-hexagonal Phase Shifting Transformer (PST) using On Load Tap Changers (OLTC). Relaying algorithm has also developed using MATLAB Simulink. Keyword: Phase shifting transformer, Relaying algorithm, Transformer Protection, Wavelet Transform, Computational Technique. I. INTRODUCTION Increased energy demand, deregulation, and privatization of the power supply industry often cause utilities to operate and stress transmission systems to, and occasionally beyond, their original design capabilities. Maintaining reliable, secure, and economical operation of interconnected networks under these conditions requires that transmission operator s better control and manages network power flows. Specially designed power system equipment can control the flow of active or reactive power in interconnected power systems by affecting one or more parameters. Traditionally, the only device available to power system operators that controlled both the magnitude and direction of power flow was the PST. Today other devices are also available to control network power flows, which are categorized under different power electronics based FACTS devices, Further based on the controlling parameter and working principle they are classified as Series and Shunt FACTs devices. Such as Fixed Series Capacitor, Thyristor Controlled Series Capacitor, Unified Power Flow Controllers, Thyristor- Controlled Phase-Angle regulators, and Inter-phase Power FlowControllers [1]. This paper focuses on the PST. A phase shifting transformer (PST) is one of several devices that can be classified to series elements of Flexible AC Transmission Systems (FACTS) technology. When the PST is series-connected to line reactance between two systems then the equation for calculating the active power flow through transmission line can be rearranged and is as follows [2], [3], [4]: 22 -(1) It is clearly visible form (1) that due to use of a PST it is possible to provide controlling of active power transfer capability by adding (or subtracting) an additional phase shift φ to the existing phase angle δ between the line sending Vs and receiving V R end voltages. In the other words, higher or lower amount of active power can be transferred through a transmission line for smaller value of δ. Another advantage is a possibility of improving power system stability and providing flexible power flow control. Generally, PSTs are usually installed between two systems (or cross border inter connections) being connected with one or more parallel transmission lines (paths) examples of using of PST are presented in details in [5], [6], [7], [8]. A differential relay is always used as a unit protection. The operating principle of the differential relay is to monitor the current entering and leaving the zone of protection. During the normal or external fault conditions, the current measured into the zone is always equal to the current leaving the zone, and therefore the vector sum of both currents is always zero (ideally). However, in the event of internal fault conditions it

2 measures a large differential current. Differential protection is commonly used for the protection of transformers, motors, generators and short-transmission lines. The three main objectives of transformer protection are: 1. Detection of internal faults with high sensitivity 2. High speed isolation of the transformer in the event of a fault 3. Security/stability against the external fault, or nofaulted, system conditions for which tripping of the transformer are not required. Proper protection minimizes the cost of repair, production loss, adverse effect on the balance of the system, damage to the adjacent equipment and the period of unavailability of the damaged equipment [9]. The conventional differential protection principles for the two commonly used PST types, two-core symmetrical and delta-hexagonal PST, are proposed by [10]. However, due to PST unique design and construction, it brings new challenges in addition to the aforementioned traditional challenges associated with the standard transformer differential protection. Commonly known challenges are non-standard phase shift between two ends, saturation of the winding exposed to high voltages, turn-turn fault protection. Various new solutions, also based on the differential philosophy, have been proposed by [11]-[12] to cope with these challenges. However, no new technique has reached the practical level yet.however, there is still much room for developing efficient protective relaying for such devices. The approach presented in this paper is one of the attempts for accomplishing that. Based on fault transients, several algorithms have been reported for fault detection and classification. For all of the proposed algorithms, how to extract the transients features from the original fault signal is the most important issue. Wavelet transform (WT), which is the perfect time-frequency localization ability, has been chosen as an effective tool for analyzing the fault transients [13] [15]. A generalized algorithm based on wavelet MRA using, Discrete wavelet transform (DWT) has been verified for the detection and classification of the faults in uncompensated transmission line. Wavelet MRA, is found to be most suitable for extracting the information from transient fault signals. Second and third order harmonics are dominant in the fault signals and are hence chosen for the analysis (d6 coefficients) and Db4 as mother wavelet. Using wavelet MRA technique, the summation of detail coefficients for sixth level are extracted from the current signal. From the magnitude of detail coefficient summations, the presence of fault in a particular phase is detected in [16]. Reference [15] proposed an effective feature extraction method using WT, WTs are well suited for the analysis of the non-stationary signals measured by the protection devices, and the WT has the ability to perform local analysis of relaying signals without losing the timefrequency information. WT is used to capture the highfrequency traveling waves for fault detection, classification, and phase selection of faults. Reference [17] used the discrete wavelet transform (DWT) to design the fault classification tool for the boundary protection of series-compensated transmission lines, and [18] described the DWT-based technique in detail and pointed out that DWT is an excellent online tool for relaying applications. Wavelet MRA is the computing algorithm used by DWT with the automatically adjusted window to extract sub band information from fault transients, and it has been proved as an effective tool in analyzing fault transients. Differential current measuring principles proposed by various approaches are reviewed in previous section followed by the application of WT in relaying has been discussed, the detail comparative analysis and discussions about PST in next section, further the WT is discuss than simulation study and results will show us application of WT in fault identification of PST. Followed by Conclusion in next section. II. PHASE SHIFTING TRANSFORMERS Phase shifting transformers are widely used for the control of power flow over parallel transmission lines. Power flow control becomes necessary in today s deregulated power system market, when parallel transmission paths are owned or operated by different operators. PST offers a complete, reliable and more economical solution for the control of power flow as compared to FACTS devices. PSTs are available in unique designs and constructions when compared to the standard power transformers. Moreover, they are among the most expensive transformer kinds in their family. The advantage of the symmetrical design over asymmetrical is that the phase shift angle is the only parameter that influences the power flow.a Phase- Shifting Transformer is a device for controlling the power flow through specific lines in a complex power transmission network. The basic function of a Phase- Shifting Transformer is to change the effective phase displacement between the input voltage and the output voltage of a transmission line, thus controlling the amount of active power that can flow in the line as shown in equation (1) for active power control. Phaseshifting transformer using on-load tap changers (OLTC) for introducing a phase shift between three-phase voltages at two buses in a transmission system. Controlling phase-shift on a transmission system will affect primarily flow of active power. Although the phase-shifting transformer does not provide as much flexibility and speed as power-electronics based FACTS, it can be considered as a basic power flow controller. The dynamic performance of the phase-shifting transformer can be enhanced by using a thyristor-based tap changer instead of a mechanical tap changer. The delta hexagonal connection consists of three pairs of windings interconnected in a hexagonal configuration.simulated model in MATLAB Simulink [19] has been utilized here to test differential protection algorithm, the operation of delta-hexagonal Phase 23

3 Shifting Transformer (PST) using On Load Tap Changers (OLTC) is also tested with differential protection algorithm. One 120 kv 1000 MVA networks are interconnected through a phase shifting transformer (PST). The phase shift can be varied on load by means of On Load Tap Changers (OLTC).In order to observe impact of phase shift on power transfer, the phase shift is increased from zero to 32.2 degrees lagging (tap +5), then phase shift is reduced to zero and increased again up to 32.2 degrees leading. This is performed by sending 5 pulses to the "Up" input, and then, 10 pulses to the "Down" input ". As the tap selection is a relatively slow mechanical process (3 sec per tap as specified in the "Tap selection time" parameter of the block menus), the simulation Stop time is set to 50 s As the tap of the transformer changes for requirement of active or reactive power demand, protection algorithm needs to be reconfigured. The OLTC is used to change tap online but relay algorithm needs to reconfigure offline. Therefore a reliable and sensitive relaying algorithm required for this purpose.the simulated model is shown in Fig. 1. Where g( n) ( 1) n h(1 n) A sequence of hn ( ) defines a mother wavelet. In this study, preferred mother wavelet is Daubechies (Db) to analyze the sample signal. Many researchers [20, 21] have discussed the properties and application of this mother wavelet. Mathematical expression for wavelet coefficients can be determined by the following expression. -(3) CA1[ k] x( n) l[2 k n] n -(4) CD1[ k] x( n) h[2 k n] n CA1[ k] and CD1[ k ] are the coefficients of wavelet decomposition which essentially quantity the contribution strength of analyze signal at level 1. The coefficients also yield with the initialize of Mother Wavelet. To obtain another level of decomposition cascading of procedure are applied. The output signal of low frequency band will be the initial input of the next layer as shows in Fig. 2. The g(n) and h(n) present the low and high pass filter, respectively which this 2 parameter significant associate with the desire mother wavelet. The transmission line faults in power system are usually classified as Symmetrical faults and Unsymmetrical fault whereas the three-phase fault is termed as a symmetrical type of fault. Fig. 1 Simulated model in MATLAB III. WAVELET TRANSFORM Wavelet transform (WT) is one of the most efficient methods to analyzed voltage and current of transient behaviour. The powerful feature is non-uniform division of frequency domain. Wavelet can be extracted data from a shot window at a high frequency range. In contract, during a long period of window the low frequency components can be addressed. According to this powerful feature wavelet transform is suitable to analyzed both time and frequency domain. The fundamental functions called decomposition which form as a high and low pass filter. The signal can be decomposed into various frequency bands, which are extracting by the mother wavelet. Mother wavelet selection also shows a different output. WT may consist of 2 main functions namely, scaling function () t and Fig. 2.Wavelet transform Multi- Resolution decomposition wavelet function () t are defined by the following IV. SIMULATION STUDY AND RESULTS equations The differential algorithm by using WT is tested using ( t) 2 h( n) (2 t n) -(1) simulated fault events in MATLAB/SIMULINK software. The sampling frequency of three phase current ( t) 2 g( n) (2 t n) -(2) signals is considered as 1.2 khz. Both primary and secondary windings faults are considered for validation of differential algorithm.three phase current signal from both primary and secondary is obtained from bus 1 and 24

4 bus 2 respectively through instrument transformers. Here instrument transformer is assumed ideal. The Wavelet toolbox in MATLAB software has been used for data acquisitionprocess, detailed coefficient obtained through DWT are used for relaying algorithm. The behaviour of current signals during different position of Tap and there second harmonics behaviour on normal operating condition are shown in Fig. 3 and Fig. 4. Fig. 3Currentsignal (fundamental) of PST secondary side (in per unit) for all the three phase (no fault) Fig. 4Second harmonic component of current signal of PST during tap change In this paper the three-phase current signals are fed through a discrete wavelet decomposition filter to decompose transients into a series of wavelet components, each of which corresponds to a time domain signal that covers a specific octave frequency band containing more detailed information. Such wavelet components appear to be useful for detecting, localizing, and classifying the sources of transients. Wavelet transform is largely due to this technique, which can be efficiently implemented by using only two filters, one high pass (HP) and one low pass (LP) at level (k). The results are down-sampled by a factor two and the same two filters are applied to the output of the low pass filter from the previous stage. The high pass filter is derived from the wavelet function (mother wavelet) and measures the details in a certain input. The low pass filter on the other hand delivers a smoothed version of the input signal and is derived from a scaling function, associated to the mother wavelet. The choice of mother wavelet is very important in detecting and localizing different types of fault transients. The daubechies (db) is the commonly used mother wavelet wavelet is used which decomposes the signal effectively. The filter output consists of high frequency details, which can be down sampled by two to get level- 1 high frequency detail coefficients HFDR, HFDY, and HFDB in the range of 600 to1200 Hz. A fault detector must detect the fault inception and to issue an output signal indicating this condition. During normal operating conditions, the currents and voltages of the transformer are sinusoidal signals. Load variation with time may produce slow amplitude changes in current signals and, in a lesser extent, in voltage signals. The inception of the fault introduces abrupt changes of amplitude and phase in current and voltage signals. Fault signals can be contaminated with different transient components such as exponentially-decaying dc-offset (mainly in current signals) and high-frequency damped oscillations (mainly in voltage signals), among other components. These changes of amplitude and phase, and the appearance of transient components, can be used to detect the inception of a fault. Figure.5. presents the block diagramof the decomposition algorithm. Phase currents Ia, Ib, Ic are obtained when a disturbance is detected. Then these currents are subjected to decomposition using discrete wavelet decomposition filter to extract high frequency details from the current signals.further Standard Deviation (SD) of extracted details coefficientfor comparison with the threshold value. If the SD of the first difference of the high frequencydetail coefficients of the corresponding line currents is greater than a threshold value then fault is detected on that particular line. After the fault has been detected in that line the output value is logic1 indicatesthe presenceof fault or logic 0 indicates absence of fault. Figure.5. Block diagram of faultdetection algorithm Fault type Fault Detection (0 or 1) R Y B RG YG BG suitable for protection applications. In this paper db4 RYG

5 YBG RBG RYB RYBG Table 1 performance of fault detection logic Fault detection algorithm is implemented in MATLAB. The algorithm output is 1 if there is any inter-turn or turn to ground fault present in the phase shifting transformer otherwise output will be 0 as shown in Table 1. V. CONCLUSION In this paper, the effects of the PST on the differential relay algorithm were investigated first and found that it can mal operate during several situations of tap changing. It was noted that the PST affects the current entering and leaving to the relaying point, which is calculated by the differential relay. It is also observed that differential relay mal-operated during external fault conditions which is not acceptable in high voltage application. Further to overcome this difficulty DWT and computational technique is used to obtained accurate protection algorithm for identifying the turns fault present in the of single-core symmetrical phase shifting transformer. It is noticeable here that no need of any information required for tap changing position in this method. Obtained simulation results declare that the accuracy and reliability of the proposed protection algorithm. REFERENCES [1] N. G. Hingorani and L. Gyugyi, Understanding FACTS Concept and Technology of Flexible AC Transmission System, IEEE Press, [2] D. A. Tziouvaras, R. Jimenez, "138 kv phase shifting transformer protection: EMTP modeling and model power system testing", in Proc. Eighth IEE International Conference on Developments in Power System Protection, 5-8 April 2004, Vol. 1, pp [3] L. Sevov, C. Wester, "Phase angle regulating transformer protection using digital relays", in Proc. Eighth IEE International Conference on Developments in Power System Protection, 5-8 April 2004, Vol. 1, pp [4] J. Verboomen, D. Van Hertem, P.H. Schavemaker, W.L. Kling, R.Belmans, "Phase shifting transformers: principles and applications", in Proc. International Conference on Future Power Systems, 18 Nov. 2005, DOI: /FPS [5] P. Bresesti, M. Sforna, V. Allegranza, D. Canever, R. Vailati1,"Application of Phase Shifting Transformers for a secure and efficient operation of the interconnection corridors" in Proc. IEEE Power Engineering Society General Meeting, 2004, pp [6] B. K. Patel, H. S. Smith, T. S. Hewes, W. J. Marsh, "Application of phase shifting transformers for Daniel-Mcknight 500kV interconnection", IEEE Transactions on Power Delivery, Vol. 1, No. 3, July 1986, pp [7] A. S. Siddiqui, S. Khan, S. Ahsan, M.I. Khan, Annamalai, "Application of Phase Shifting Transformer in Indian Network", in Proc. International Conference on Green Technologies (ICGT), 2012, pp [8] IEEE PSRC Working Group K1, "Protection of Phase Angle Regulating Transformers", IEEE Special Publication, Oct [9] IEEEStd C37.91, IEEE Guide for Protecting Power Transformers, [10] Protection of Phase Angle Regulating Transformers IEEE Power System Relaying Committee prepared by Working Group K1, 1999 [11] TammamHayder, Ulrich Schaerli, Kurt Feser, Fellow, IEEE, Ludwig Schiel, Universal Adaptive Differential Protection for Regulating Transformers IEEE Transactions on Power Delivery, vol. 23, no. 2, pp , April [12] B. Kasztenny, E. Rosolowski, Modeling and Protection of Hexagonal Phase-Shifting Transformers Part II: Protection IEEE Transaction on Power Delivery, vol. 23, no. 3, pp , July [13] Allipilli, Y.; Rao, G.N., "Detection and classification of faults in transmission lines based on wavelets," in Electrical, Electronics, Signals, Communication and Optimization (EESCO), 2015 International Conference on, vol., no., pp.1-6, Jan [14] A. I. Megahed, A. M. Moussa, and A. E. Bayoumy, Usage of wavelet transform in the protection of series-compensated transmission lines, IEEE Trans. Power Del., vol. 21, no. 3, pp , Jul [15] O. A. S. Youssef, Online applications of wavelet transforms to power system relaying, IEEE Trans. Power Del., vol. 18, no. 4, pp , Oct [16] Allipilli, Y.; Rao, G.N., "Detection and classification of faults in transmission lines based on wavelets," in Electrical, Electronics, Signals, Communication and Optimization (EESCO), 2015 International Conference on, vol., no., pp.1-6, Jan [17] A. I. Megahed, A. M. Moussa, and A. E. Bayoumy, Usage of wavelet transform in the 26

6 protection of series-compensated transmission lines, IEEE Trans. Power Del., vol. 21, no. 3, pp , Jul [18] O. A. S. Youssef, Online applications of wavelet transforms to power system relaying, IEEE Trans. Power Del., vol. 18, no. 4, pp , Oct [19] SimPowerSystems. User Guide, TheMathWorks, Inc. Natick, MA. [20] Chul Hwan Kim and Raj Agganrval, "Wavelet transforms in power systems Part I General introduction to the wavelet transforms," Power Engineering Journal, April [21] Chul Hwan Kim and Raj Agganrval, "Wavelet transforms in power systems Part 2 Examples of application to actual power system transients," Power Engineering Journal, April