Single-Core Symmetrical Phase Shifting Transformer Protection Using Multi-Resolution Analysis

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1 IJEEE, Volume 3, Spl. Issue (1) Single-Core Symmetrical Phase Shifting Transformer Protection Using Multi-Resolution Analysis Meenakshi Sahu 1, Mr. Rahul Rahangdale 1, Department of ECE, School of Engineering And I.T., Mats University, Raipur, (C.G.), India 1, Abstract: This paper presents performance evaluation of conventional current differential protection 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. 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 online. 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. The main attention is paid to testing of differential protections for transformer energization and external faults for different phase shift values. It has been observed that for such situations, standard differential relays may sometimes mal-operate. MATLAB Simulink, is used for the simulation of delta-hexagonal Phase Shifting Transformer (PST) u sing On Load Tap Changers (OLTC). Relaying algorithm has also developed using MATLAB Simulink. Key words: Current differential protection, Phase shifting transformers, FACTS devices, Fault identification and classification, Relaying 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 inter connected 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 Flow Controllers [1]. This paper focuses especially 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 [], [3], []: (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 interconnections) being connected with one or more parallel transmission lines (paths) examples of using of PST are presented in details in [], [], [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 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: Detection of internal faults with high sensitivity RES -1 1

2 IJEEE, Volume 3, Spl. Issue (1) High speed isolation of the transformer in the event of a fault Security/stability against the external fault, or no-faulted, 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 [1]. 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]- [1] to cope with these challenges. However, no new technique has reached the practical level yet. The provision of various new and old PST differential based approaches is to detect and discriminate the internal/external faults. However, they are different in differential current measuring principle. The standard transformer design is solely based on the concept of magnetic-coupling therefore, differential current measuring principle reflects the ampere-turn relation of the magnetically coupled transformer windings. However, design and construction of the PST is such that it represents both magnetic-coupling and electrically connected circuits. Therefore, PST differential current measuring principle can be the representation of magnetically coupled or electrically connected windings or simply the vector sum of the compensated currents entering and leaving the two ends of PST without considering the PST design and construction. Differential current measuring principles proposed by various approaches are reviewed in previous section followed by the detail comparative analysis and discussin about PST in next section, further the differential protection scheme is discussed then simulation study and results will show us the problems associated with conventional PST differential protection. 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. Phase-shifting transformer using onload 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 phaseshifting 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 [13] has been utilized here to test differential protection algorithm, the operation of delta-hexagonal Phase Shifting Transformer (PST) using On Load Tap Changers (OLTC) is also tested with differential protection algorithm. One 1 kv 1 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 3. degrees lagging (tap +), then phase shift is reduced to zero and increased again up to 3. degrees leading. This is performed by sending pulses to the "Up" input, and then, 1 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 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 off-line. Therefore a reliable and sensitive relaying algorithm required for this purpose. The simulated model is shown in Fig. 1. III. CURRENT DIFFERENTIAL PROTECTION OF PHASE SHIFTING TRANSFORMERS From many years, current differential protection has been widely used for the protection of bus-bars, generators, transformers. Speed and selectivity are the two main advantages of differential protection, and it therefore only responds to internal faults. The differential protection technique is based on the comparison of the differential current with the restraining current [1]. Differential current is measured using Kirchoff s law by taking the vector sum of the current entering and leaving the zone of protection. Fig. 1 Simulated model in MATLAB RES -1

3 IJEEE, Volume 3, Spl. Issue (1) The zone of differential protection is normally defined by the positioning of the current transformers (CTs). However, the zone of protection can be the representation of either a magnetically coupled circuit (e.g. transformer) or electrically connected circuit (e.g. bus -bar or lines). Any change in the defined zone leads to an imbalance between the current entering and leaving the zone and, therefore, results in the differential current. Therefore, current differential protection is sensitive to most of the internal faults. However, the sensitivity of differential protection is dependent on the faultcurrent level. There are various old and new differential protection-based approaches [1] [17] could be distinguished based on the differential current measuring principle that reflects the type of circuit it is representing. IV. SIMULATION STUDY AND RESULTS The current differential algorithm is tested using simulated fault events in MATLAB/SIMULINK software. The sampling frequency of three phase current signals is considered as 1. 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 3 and bus respectively through instrument transformers. Here instrument transformer is assumed ideal. The Digital signal processing toolbox in MATLAB software has been used for data acquisition process, fundamental component obtained through DFT 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. and Fig. 3. Fig. Differential current signal of PST during single turn fault (AG) Fig. Differential current signal of PST during single turn fault (BG) x x x 1 x 1 x 1 x 1 x x x x Fig. Current signal of PST secondary side (in per unit) for all the three phase (no fault) Fig. 3Second harmonic component of current signal of PST during tap change x 1 x 1 x 1 x Fig. Differential current signal of PST during single turn fault (CG) x Fig. 7 Differential current signal of PST during inter turn fault (ABG) x 1 x 1 x 1 RES -1 3

4 IJEEE, Volume 3, Spl. Issue (1) Fig. 8 Differential current signal of PST during inter turn fault (BCG) Fig. 9 Differential current signal of PST during inter turn fault (CAG) x Fig. 1 Differential current signal of PST during inter turn fault (ABCG) Fig. to Fig. shows that differential current algorithm correctly identified any single turn short circuit fault occurred in the PST. For simulating different types of single turn fault on secondary side of the PST three phase switch is used in MATLAB. During single turn to ground fault conditions it is clear that from Fig. to Fig. before no fault condition differential current is near about zero value but when fault occurred in PST it will larger than the threshold value set. Here threshold value is set based on the different fault analysis done including all internal and external fault condition. Whenever differential quantity is greater than threshold value it indicate internal fault presence in PST. In similar manner for inter turn fault condition has evaluated and shown in Fig. 7 to Fig. 1. Previously mentioned result x 1 x 1 x 1 x 1 x 1 x 1 x 1 x 1 for differential algorithm is for secondary side inter-turn fault condition. This differential algorithm is also successfully identifying primary side fault of PST. It is here by noted that this algorithm is working on some of the common case only. This algorithm fails where phase shift of the PST is larger than 9 degree. It has also observed from the above case study that threshold value is set based on different fault scenario, which should be set manually during time to time according to tap position of the PST. During external fault condition near by the transformer connection this algorithm mal-operate and gives false tripping signal to circuit breaker. V. CONCLUSION In this paper, the effects of the PST on the differential relay algorithm were investigated. Research results revealed that the during PST presence in the system, differential algorithm mal-operates. 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. Therefore, there is a need for an improved algorithm for protecting phase shifting transformers with better stabilization for external disturbances and more sensitivity to internal fault. In addition, further simulation testing should also be done for the newly developed protection criteria. In the scope of interest are the solutions based on intelligent techniques and adaptation concepts. REFERENCES [1] N. G. Hingorani and L. Gyugyi, Understanding FACTS Concept and Technology of Flexible AC Transmission System, IEEE Press,. [] 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, -8 April, 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, -8 April, Vol. 1, pp [] 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., DOI: 1.119/FPS..3. [] 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,, pp [] B. K. Patel, H. S. Smith, T. S. Hewes, W. J. Marsh, "Application of phase shifting transformers for Daniel-Mcknight kv interconnection", IEEE Transactions on Power Delivery, Vol. 1, No. 3, July 198, 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), 1, pp [8] IEEE PSRC Working Group K1, "Protection of ngle Regulating Transformers", IEEE Special Publication, Oct [9] IEEE Std C37.91, IEEE Guide for Protecting Power Transformers, 8. RES -1

5 IJEEE, Volume 3, Spl. Issue (1) [1]Protection of ngle Regulating Transformers IEEE Power System Relaying Committee prepared by Working Group K1, [11]TammamHayder, Ulrich Schaerli, Kurt Feser, Fellow, IEEE, Ludwig Schiel, Universal Adaptive Differential Protection for Regulating Transformers IEEE Transactions on Power Delivery, vol. 3, no., pp 8-7, April 8. [1]B. Kasztenny, E. Rosolowski, Modeling and Protection of Hexagonal Phase-Shifting Transformers-Part II: Protection IEEE Transaction on Power Delivery, vol. 3, no. 3, pp , July 8. [13]SimPowerSystems. User Guide, TheMathWorks, Inc. Natick, MA. [1]IEEE Std C37.91, IEEE Guide for Protecting Power Transformers, 8. [1] Protection of ngle Regulating Transformers, IEEE Power System Relaying Committee prepared by Working Group K1, [1]Michael J. Thompson, Hank Miller, John Burger, AEP Experience With Protection of Three Delta/Hex ngle Regulating Transformers, in Proc.Advanced Metering, Protection, Control, Communication, and DistributedResources, 7, pp [17]Z. Gajic, Use of Standard 87T Differential Protection for Special Three-Phase Power Transformers-Part 1: Theory, IEEE Transaction on Power Delivery, vol. 7, no. 3, pp 13-1, July 1. [18]Solak, K.; Rebizant, W.; Schiel, L., "Differential protection of singlecore symmetrical phase shifting transformers," in Electric Power Engineering (EPE), 1 1th International Scientific Conference on, vol., no., pp.1-, - May 1. RES -1