An Improved Algorithm for Variable Slope Differential Protection of Distribution Transformer using Harmonic Restraint

Transcription

1 An Improved Algorithm for Variable Slope Differential Protection of Distribution Transformer using Harmonic Restraint B S Shruthi National Institute of Technology Karnataka, Surathkal, India shruthibs123@gmail.com K Panduranga Vittal National Institute of Technology Karnataka, Surathkal, India vittal.nitk@gmail.com Abstract In the context of reliable protection of distribution transformer, differential protection is considered to be the best. The infiltration of Distributed Energy Resources into the grid has necessitated the security of transformer to be increased many fold. In this paper, an improved variable slope differential method is proposed for a 1 MVA, 11/0.433 kv /Y connected distribution transformer considering provision for accounting challenges of DES penetration. By using second and fifth harmonic restraint, along with negative sequence component, it is possible to effectively differentiate between various conditions like inrush, over excitation and internal fault including inter turn fault with few shorted turns. Apart from these, cases for remanence and external fault are also taken up to test the reliability of the scheme. Effective simulation studies have been performed to demonstrate the efficiency of the proposed scheme using PSCAD/EMTDC. Index Terms differential protection, distributed generation, Distribution transformer, harmonic restraint. I. INTRODUCTION Distribution transformers are generally considered to be those which provide transformation from 11kV or lower voltages down to the level of final distribution. The main intention of transformer protection is to provide the ability to detect internal faults with high sensitivity, along with a high degree of immunity to operation on system faults for which tripping of transformer terminal breaker is not required. It is based on the fact that the differential current will be high only in the case of faults internal to the zone. Security of transformer differential protection schemes is dependent on detecting the magnetizing inrush currents of the protected transformer and associated blocking of differential operation due to inrush related, non-fault and other unbalance currents. Variable slope differential protection is one of the schemes that provide restraint against inrush and under overexcitation. Manuscript received March 1, 2013; revised June 10, With the connection of DG, the system loses radial configuration. Since the DG sources also contribute to the fault, the system coordination could be lost. M. Mañana et. al. [1] concluded that magnetizing inrush currents of power transformers have to be limited, especially when they share a common bus with DG. With the proliferation of DG installed at the customer load site, the delta-wye distribution transformers become sources of fault current that can be difficult to detect and isolate. Under worstcase scenarios, without proper protection, the customer generation may keep the faulted utility circuit energized even after the utility source breakers have opened. Several schemes to detect and isolate the transformer under such condition are discussed in [2]. Current differential relaying is the most commonly used type of protection for transformers of approximately 10 MVA three-phase and above. Although utility practice for protection of distribution transformers as of now is only through remote back up or fuses local to transformer, it is envisaged in future with DG penetration that differential protection becomes inevitable. General protection issues with DG proliferation are gaining importance [3]-[5] but its impact on distribution transformer protection is not sought much. Detailed analysis and publications are not available for reliable transformer protection and hence the main objective of this work is to address these concerns. Improvement of the existing protection scheme is taken up as the first phase and is the focus of this paper. In the context of protection, transformer inrush is of large importance and appropriate models for these transient behaviors needs to be chosen. Unified magnetic equivalent circuit (UMEC) model in PSCAD/EMTDC has the ability to accurately reproduce the unbalance operations. A 1 MVA, 11/0.433 kv /Y connected transformer is opted for the purpose of this study. The following section discusses the general differential protection for a transformer along with the issues related to the same followed by variable slope differential protection. Section 3 briefs about the proposed relay logic. 61

2 Few simulation results to support the developed scheme are presented in the last section. II. TRANSFORMER DIFFERENTIAL PROTECTION Transformer is one of the most important equipment in power systems and it is important that adequate measures are taken for protection. Differential protection is a fast, selective method of protection against short circuits in transformers. In a basic differential protection scheme, currents on both sides of the transformer are compared. Under normal conditions, primary and secondary currents are equal and hence no difference current flows. There has been renewed interest in devising a reliable scheme for the protection of transformer using various methods. B. Kasztenny et. al. [6] reviewed the principles of protection against internal short circuits in transformers of various constructions. A new approach for transformer differential protection using current-only inputs is described in [7]. Various protection schemes with neural network algorithms and wave-shape recognition techniques have also been proposed [8 11]. Sequence components have also been proved useful. H. Khorashadi-Zadeh [12] describes the complete design of a prototype digital protective relay for three-phase transformers using positive sequence current whereas usage of negative-sequence currents in order to both detect and determine the position of the fault with respect to the protected zone is dealt with in [13]. The nature of power transformers creates several complications for the application of such differential relays. Magnetizing inrush currents created by transformer transients Phase shift between primary and secondary for delta-wye connections Variable ratio of the power transformer caused by a tap changer High exciting currents caused by transformer overexcitation Mismatch between the CT ratios and CT Saturation Differential relays are prone to maloperation due to one or more of the above mentioned reasons. Harmonic restraint or blocking methods ensure relay security for a very high percentage of inrush and overexcitation cases. The selection of CTs along with choice of percentage slope for differential relaying and the effects of magnetizing inrush are discussed in detail in [14]. A variable-percentage or dual-slope characteristic, further increases relay security for heavy CT saturation. Several common methods for defining restraint and slope characteristics are examined and guidance on selection of the correct slope setting is provided in [15]. Modern day transformers make use of IEDs designed for protection, control, measurement and. Parameter setting and DFR records can be accessed using the IEC protocol. Oscillographic files are available to any Ethernet-based application in the standard COMTRADE format. Flexible user settings afford programming for different CT ratios and transformer phase shifts [16]. Digital relaying technique thus provides value addition to the well-established percentage differential relays. III. DEVELOPMENT OF RELAYING SCHEME In transformer differential applications, CTs are selected to accommodate a maximum fault current along with preserving low current sensitivity. As a minimum goal, CT saturation should be avoided for the maximum symmetrical external fault current. The CT ratio and burden capability should also permit operation of the differential instantaneous element for the maximum internal fault. Detailed procedure for selection of CT is given in [17]. Inrush or overexcitation conditions of a power transformer produce distorted currents because they are related to transformer core saturation. The distorted waveforms provide information that helps to discriminate these conditions from internal faults. The restrained differential is designed to sensitively clear internal faults while remaining secure under other unbalance conditions. Digital relays use unrestrained differential that provides a very fast clearance of severe internal faults with a high differential current regardless of their harmonics. The second harmonic restraint, together with the waveform based algorithms, ensures that the restrained differential does not trip due to the transformer inrush currents. The fifth harmonic restraint ensures that the restrained differential does not trip on apparent differential current caused by a harmless transformer over-excitation [16]. Among the several research work that describe similar relaying schemes, fault discriminants for differential protection of transformer have been defined in [18] using the information available from harmonic analysis. The second and fifth harmonics along with transformer primary voltage amplitude provides accurate distinction between over excitation and internal fault condition. A parabolic characteristic digital differential relay has been developed based on the above discriminants. But the traditional transformer differential percentage protection is not sensitive enough to detect minor internal winding faults. A short circuit of a few turns of the winding will give rise to a heavy fault current in the short-circuited turns, but changes in the transformer terminal currents will be very small, because of the high ratio of transformation between the whole winding and the short-circuited turns. Addition of DG sources might cause significant effects to the existing scheme. Issues like sympathetic inrush could add to problems such as increase in fault current due to the additional source and directionality of flow. This calls for certain improvements in the conventional relay logic, including the capability to detect internal winding faults. Instantaneous differential and bias currents are calculated from the CT outputs and harmonic analysis is carried out. The fundamental, second and fifth harmonic of the differential current is extracted along with its 62

3 negative sequence component and the fundamentals of restraining current and the primary voltage. The threshold values for each of these are chosen based on the simulation results. For severe internal faults, unrestrained operation is desired. If the fundamental harmonic of differential current is higher than a set value, and second harmonic component is below a threshold, immediate trip decision is given. Parabolic characteristics are used in other cases and trip signal is issued when where I d1 and I r1 are the fundamental components of differential and restraining currents. The values of a and b in this case are 0.3 and 0.08 respectively. Fault discriminant K is used to give a stable discrimination between inrush and internal fault condition. Harmonic analysis of the differential current leads us to the result shown below. Table I shows the maximum value of various harmonic components as a percentage of fundamental under inrush and internal fault condition. It can be seen that second harmonic is highly predominant in the case of inrush. Armando Guzmán et. al. [19] summarize the existing methods for discriminating internal faults from inrush and overexcitation conditions along with comparative studies between harmonic restraint and blocking methods. Following are the various cases that were analyzed to check the dependability of the above mentioned protection scheme. TABLE I. Harmonic DIFFERENTIAL CURRENT AS A PERCENTAGE OF FUNDAMENTAL Differential Current Component (%) Inrush Internal Fault Fundamental If K and I d2 are greater than their threshold values, inrush indicator flag is set. To check for overexcitation condition, verify whether the voltage level is greater than times normal voltage (say =1.1), and also whether fifth harmonic component, I d5 has increased. Finally if the voltage at primary terminals has collapsed below times normal voltage (say = 0.9), and the negative sequence component is higher than a set threshold, increment the trip counter for inter-turn fault. In any case the trip counter is incremented up to 3 before giving the trip decision. This approach ensures security under external faults, inrush and overexcitation, and dependability for internal faults along with maintaining sensitivity. There can be several ways to use the harmonic restraint scheme. The approach generally in practice is to consider the second and the fifth harmonic as a percentage of fundamental. The restraint can be applied by considering each phase separately or all the three phases together. In case of choosing a common restraint, considering the sum of harmonics of each of the three phases gives an additional advantage of setting a higher value of restraint compared to averaging over the three phases. Adaption of the appropriate technique depends on the application. Second Third Fourth Fifth Sixth Seventh A. Magnetizing Inrush During transformer energization, the inrush current can be as high as 8 30 times the rated current for few cycles, depending upon the transformer core and system resistance. Other factors that affect the inrush current are the point on voltage waveform during switching and residual flux in core. Following figure 2shows the harmonic components of Phase A primary current under typical energization. IV. SIMULATION RESULTS The performance is evaluated through extensive simulations using the PSCAD/EMTDC software. Few typical results obtained are given in this section. A 1 MVA, 11/0.433 kv common core, three limb transformer was taken up for study, the primary being connected in delta and secondary in star. The system simulated is as shown in the figure 1 below. Figure 1. PSCAD model used for simulation. Figure 2. Harmonics of Phase A primary current after energization. Sympathetic inrush phenomenon occurs when a transformer is switched on in a power system network containing other transformers which are already energized. Results show that the inrush current in transformers being connected to system where there are other transformers already in operation, decays slower. 63

4 B. Remanent Flux In Core When a transformer is de-energized, the magnetizing voltage is taken away, the magnetizing current goes to zero while the flux follows the hysteresis loop of the core. This results in a residual flux in the core. Remanence may be as high as 80-90% of the rated flux, resulting in large peak values and heavy distortions of magnetizing current. Simulations were carried out with various amounts of remanent flux in core. The Fundamental and Second harmonic extracted from FFT analysis with a remanence of -60% in phase A, 50% and -20% in phase B and C respectively is as shown in the figure 3 below. Figure 4 shows the inrush flag set after energization. should be given before the flow of this high current causes damage to the winding or insulation. The trip decision is given at sec, which is considerably good from the transformer safety point of concern. Figure 6. Inrush and trip signal under ACG fault. Figure 3. Fundamental and 2nd harmonic under remanence condition. C. Internal Fault Figure 4. Inrush flag with remanent flux. The internal electrical faults that can occur are winding short circuits, winding to ground or turn to turn faults. Various types of temporary and permanent faults were simulated on both the windings of the transformer. It was observed that in every case, the trip signal was issued at around 20ms from the time of fault inception. As an example, a temporary phase A to C to ground fault on the primary winding of the transformer is considered. Fault is applied from 0.2 sec to 0.3 sec. Turn to ground and turn to turn faults were simulated by creating new components with the help of FORTRAN programming. The change in inductance matrix under fault was derived using the basic equations based on consistency, leakage and proportionality. Satisfactory results were obtained for various levels of shorted turns which justify the necessity of using negative sequence component as an additional discriminant. D. External Fault From the point of view of differential protection, it is also important to study the effects of out of zone faults. In some cases, there might be unusual maloperation of the differential protection after removal of external fault. The reason is that saturation states of CTs on either side are different after an external fault occurs [20]. An example of phase A to C double line fault on the secondary side considered. The main difference is that restraining current becomes higher than the differential current which is seen in figure 7. Inrush and trip indicator is as seen in figure 8. Figure 7. Differential and restraining current under AC fault. Figure 5. Differential and restraining current under ACG fault. Figure 5 shows fundamental of differential current rising to an extremely high value and quick trip decision Figure 8. Inrush flag under AC external fault. 64

5 E. Over Excitation The magnetic flux inside the transformer core is directly proportional to the applied voltage and inversely proportional to the system frequency. Overvoltage and/or under frequency conditions can produce flux levels that saturate the transformer core. Over excitation condition was simulated by increasing the voltage in steps. It was observed that for every step increase in voltage, a new level of inrush current is reached and it also causes a rise in the fifth harmonic. Figure 9. 5th harmonic during overexcitation. Figure 10 shows the inrush and over excitation flag. Though the transformer core saturated, the relay was found to be stable even up to 150% of rated voltage. Figure 10. Inrush and overexcitation flag. V. CONCLUSION Internal electrical faults in transformers are very serious and cause immediate damage. Short circuits and ground faults in windings and terminals are normally detected by variable slope differential protection. Second and fifth harmonic restraint provides discrimination against inrush and overexcitation condition. Under inter turn fault, lesser the number of turns shorted, lesser will be the current and hence detection through harmonic analysis is not always feasible. Under such situations negative sequence restraint provides better sensitivity. A relay logic combining all these along with voltage restraint is proposed in this paper. Simulation results have proved that the relay gives acceptable decisions under various operating and fault conditions of the transformer. Addition of DER to a distribution or sub-transmission system has the potential to impact relay systems well beyond the point of common coupling. Distribution systems are basically designed as radial networks and revision of the existing protection schemes become necessary with DG integration. Different DG sources show different characteristics towards their ability to provide short circuit current needed to trip the relay. Distribution transformers, in particular face problems like inrush, ferroresonance, etc. along with CT saturation. Sympathetic inrush phenomenon also becomes more evident and sensitivity of the relays becomes important. Hence the work carried out here will aid in taking adequate measures for distribution transformer protection in networks where there is flow of negative sequence current and intermittency. REFERENCES [1] M. Mañana, L.I. Eguíluz, A. Ortiz, G. Díez, C.J. Renedo, and S. Pérez, Effects of Magnetizing Inrush Current on Power Quality and Distributed Generation, IX Spanish Portugese Congress on Electrical Engineering, Marbella, July [2] K. Behrendt, Protection for Unexpected Delta Sources, Schweitzer Engineering. Laboratories, Inc., Oct [3] M.Paz Comech, Miguel Garcia-Gracia, Samuel Borroy, and M.Teresa Villen, Protection in distribution generation, CIRCE, Feb 2010, pp [4] J.A. Martinez and J. 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Tripathy, Power Transformer Differential Protection Based on Neural Network Principal Component Analysis, Harmonic Restraint and Park s Plots, Research article published in Advances in Artificial Intelligence, Volume 2012, Jan [9] A. Guzmán, S. Zocholl, G. Benmouyal and Héctor J. Altuve, A Current-Based Solution for Transformer Differential Protection Part II: Relay Description and Evaluation, IEEE Transactions on Power Delivery, Vol. 17, No. 4, Oct 2002, pp [10] H. Khorashadi-Zadeh, Power Transformer Differential Protection Scheme Based on Symmetrical Component and Artificial Neural Network, 7th Seminar on Neural Network Applications in Electrical Engineering, IEEE Sep 23-25, [11] H. Khorashadi-Zadeh, M. Sanaye-Pasand, Power transformer differential protection scheme based on wavelet transform and artificial neural network algorithms, Proc. of the 39nd International Universities Power Engineering Conference, 2004, pp [12] H. 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6 Part I: Problem Statement, IEEE Transactions on Power Delivery, Vol. 16, No. 4, Oct 2001, pp [20] X. Lin, H. Weng, P. Liu, B. Wang, and Z. Bo, Analysis of a Sort of Unusual Mal-Operation of Transformer Differential Protection Due to Removal of External Fault, IEEE Transactions on Power Delivery, Vol. 23, No. 3, July 2008, pp B S Shruthi was born in Bangalore, in She received her Bachelor of Engineering degree in Electrical and Electronics Engineering from The National Institute of Engineering, Mysore in She is currently perusing M.Tech by Research in the field of Power and Energy Systems at The National Institute of Technology Karnataka (NITK). Area of Research is Development of Integrated Protection Schemes for Protection of Transformers Supporting Deregulated Distribution Systems. She is a Student Member of IEEE along with IEEE Power Engineering Society and IEEE Women in Engineering. Her areas of interest include Power System Protection, Power Transformers, Distributed Generation and Embedded Systems. Dr. Panduranga Vittal K was born in Bellary, in He received his B.E. (E & E) degree from Mysore University in the year 1985, M.E. (Applied Electronics) degree in 1989 from PSG College of technology, Coimbatore. Then he earned Ph D. degree from NITK during the year Presently, Dr.Vittal is serving as Professor and Head of Dept. of Electrical & Electronics Engineering, National Inst. of Technology, Karnataka Surathkal, Mangalore. He is a senior member of IEEE, Member of IEEE-Power Engineering Society, Life member of ISTE and IE (India). He has published 50 technical research papers in various National and International conferences and 15 papers in International Journals. He has chaired several international conferences in India and abroad. Recently he has served as chairman for IEEE, ICIIS-2010 an international conference and NSC-2010 a national conference. He has been principal investigator in various funded projects from Indian government and University programs of industries. He has guided and also presently guiding several research scholars for the award Ph.D. degree. His research areas of interest include power system protection, power quality and design of embedded systems. 66