Negative-Sequence Based Scheme For Fault Protection in Twin Power Transformer

Transcription

1 Negative-Sequence Based Scheme For Fault Protection in Twin Power Transformer Ms. Kanchan S.Patil PG, Student Prof.Ajit P. Chaudhari Associate Professor ABSTRACT A power transformer is a vital component in any power system network and any of its failure may cause disturbance in the proper operation of electrical power system network. Negative sequence current flows in transformers primary and secondary which can be computed by using digital relay along with their phase differences. Magnitude and phase information of negative sequence currents can be used to detect minor transformer windings fault. Negative sequence current and voltage based algorithms are very useful in the determination of faults in electrical power systems. Negative sequence algorithm becomes insensitive when current is not flowing through the secondary windings of transformer. A relay system for the protection of transformer using negative sequence current and voltage is introduced in this paper. The proposed protection scheme was evaluated with different cases, which included different numbers of shorted turns of the transformer, inrush currents. The experimental results presented in this paper indicate that the algorithm proposed in this paper is faster and more sensitive and is capable of detecting turn-to-turn faults occurring during transformer energization. Keywords Negative Sequence Algorithm, Power Transformer. 1. INTRODUCTION The faults occur in a transformer are classified in two types: external and internal faults. External faults are those that occur outside of the transformer: over voltage, over-fluxing, under frequency, and external System short circuits. Internal faults are those that occur Inside of the transformer: winding turn-to-turn, turn-to-ground, over-fluxing. From the last few decades a continuous growth has observed in the power system and the progress will continue in the upcoming years. A transformer, being an integrated part of the power system, is an important link between a generating power station and a point of power utilization.[1] Due to various kinds of intricate loads and their control systems, transformer is prone to faults. Internal winding faults in transformers can cause huge damages in a very short time, and in some cases the damages are repairable, and also about 70%-80% of transformer failures are caused by internal faults. Among these faults, Prof.Girish K. Mahajan Associate Professor girishmhajan_16@rediffmail.com Prof.Gaurav P. Tembhurnikar Asst.Professor tgaurav81@gmail.com Winding turn to turn fault is challenging to monitor and detect, especially at lower magnitude of the fault current. Internal faults involve a magnitude of fault current which is low relative to the power transformer base current. This indicates a need for high speed and high sensitivity to ensure good protection. According to the IEEE Standard documents, there is no one standard way to protect all power transformers against minor internal faults and at the same time to satisfy basic protection requirements: sensitivity, selectivity, and speed. The most difficult internal turn-to-turn fault is the fault which initially involves only a few turns [2]. Turn to turn faults can be calculated by numerous techniques. Fuzzy logic based techniques are also available to detect turn to turn faults which involves 16% of transformer windings [3]. To distinguish between healthy and faulted transformer, negative sequence algorithm having some assumptions such as the faults are unlikely to be three phase faults. Zero sequence current based schemes are also proposed to detect turn to turn faults [4]. The algorithms used in zero sequence current protection schemes are very sensitive to faults and is intended to be used in conjunction with current differential protection scheme during energization. During normal operations 1% turn to turn faults can be detected by using negative sequence current based algorithms [5]. Magnitude and phase difference of primary and secondary negative sequence current can be used by the algorithms to detect turn to turn faults. During energization, the transformer's secondary breaker is open. Inrush current flows on the primary side of the transformer while no current flows on the secondary side of the transformer. Therefore, the phase information of the negative-sequence current on the secondary side of the transformer is not useful during energization. [1]Voltage differential algorithms are used to protect the transformer from third harmonics voltage which is achieved by negative sequence algorithm. The transformer protection scheme is 166

2 based on the changes in flux linkages which occur due to turn to turn faults which is applied to both Y-Y winding arrangement and Y- winding arrangements. The algorithm described in [6] was found to operate during transformer energization and over excitation conditions. The algorithm's sensitivity was able to detect turn-to-turn faults comprised of 10% of the windings. Voltages as well as Currents also get affected by the turn to turn faults. The algorithms introduced in this paper compares primary and secondary voltage magnitude to detect the turn to turn faults.[5] Algorithm was developed in order to protect the transformer during energization and normal operation. Proposed algorithm was tested using real time simulator. Relay system is proposed in order to protect the transformer using real time simulator. The simulated transformer model was capable of simulating transformer energization current along with turn-to-turn faults[7]. To recognise value of inrush currents in constructed model was calculated using iterative process given in [8] and according to the example given in [9]. The calculations for inrush currents can be provided for the transformer under study. A brief discussion on the methods used to detect turn to turn faults in power transformer is given in section II. Proposed negative sequence based schemes are discussed in section III. Section IV describes the relay system and how it was tested. Section V provides the prototype test results. 2. TURN TO TURN FAULTS DETECTION For transformer protection, negative sequence current protection based relays are in use [10]. This relay makes use of a negative-sequence percentage differential current element which is calculated by the vector addition of all currents entering the protection zone. Sensitivity of algorithm is high which can detect faults involving 2% of the transformer's windings. The negative-sequence current differential element blocks the relay during inrush conditions. External fault region 180 o 90 o 270 o Internal fault 0 o region Fig:1 Detection of fault using negative sequence currents The primary and secondary negative-sequence currents along with their phase differences are used in the relay description [11]. Figure:1 describes the algorithm visually. Both primary and secondary negative-sequence current magnitude must be larger than in order for a phase comparison to occur. If either or both negative-sequence currents are less than, the phase is set to 120. This angle is outside the Relay Operating Angle (ROA) and ensures no trip signal is issued if the negativesequence current is too small to obtain an accurate phase angle. If the negative-sequence currents magnitudes are larger than, the phase difference between the primary and secondary negative-sequence currents is examined. This phase difference must fall within the region described by the ROA. 3. SCHEME BASED NEGATIVE SEQUENCE 3.1 Scheme based Negative sequence current A negative-sequence current-based algorithm (NSCA) for turn-to-turn faults sensing is proposed in [5]. First of all, the negative sequence current is calculated for both the primary side and secondary side of the transformer. Form this calculations we get, two negative-sequence current phasors. Let I p and I s denote the negative-sequence current phasors calculated for the primary and secondary side of the transformer. The next step of this algorithm is to check the magnitudes of I p and I s to ensure that they are both above a minimum threshold as shown in equation (1) and (2). This is important not only to prevent false tripping due to minor imbalances but also to ensure that the phase angle of the negative-sequence currents are reliable. I 2 p > Base current (Primary side) (1) I 2 s > Base current (Secondary side) (2) If the magnitude satisfy above equations then the phases of I p and I s are compared. I 2 p - I 2 s < 60 o (3) If equation (3) is satisfied, then there is trip. The current transformers (CT) are arranged such that negative-sequence current caused by external faults result in phase differences of 180. Ideally, an internal fault would results in a zero phase difference. Current Transformer saturation is the main cause of excursions from the ideal phase difference [5], making it necessary to allow for a range of angles from 0 to 60 o 3.2 Proposed Scheme By using advanced numerical technology, it is now possible to protect power transformers with new differential protection principle, which has much higher sensitivity than traditional transformer differential protection for low-level internal faults. The basic requirement of the algorithm is to satisfy phase comparison of equation (1) & (2). Negative sequence 167

3 current magnitudes of both primary and secondary currents must be greater than the threshold values given in equation (1) & (2). When current is flowing on the primary and secondary sides of the transformer inrush current can occur during the removal of a fault or the energization of transformer. When transformers primary is switch on by keeping secondary open, current will not flow in secondary windings while inrush current starts to flow in primary windings causing the negative-sequence current-based turn-to-turn fault detection method to block for any severity of fault. Currents as well as voltages will be affected by the turn to turn faults occurring in the transformer. Also the transformer s turn ratio would be affected by the turn to turn faults. Due to this voltages between all the phases gets disturbed causing imbalance between negative sequence current or voltages in three phase system. When transformer is loaded the current method of negative sequence fault detection is effective. But as there is no load current flowing during energization, this method is blind to turn-to-turn faults. manner similar to differential current protection. Equation (4), (5), (6), and (7) are valid for Y-Y connected transformers. = = Vp (5) V p (6) [ ] = [ ] [ ] (7) Similarly the secondary side turn-to-turn fault will change the effective turns ratio, from N s to N s. A fault on the primary side causes a decrease in secondary phase voltage while a fault on the secondary side causes an increase in phase voltage. Voltage exists on the load side of the transformer whether current is flowing or not. Inrush current affects the terminal voltages which allow negative sequence voltage algorithm to react faster than the current differential algorithm. the negative-sequence voltages for the primary and secondary side of the transformer can be calculated if the primary phase voltages and secondary phase voltages are available. The algorithm for comparing these two negative-sequence voltages is similar to the differential current algorithm. The pick-up negative-sequence voltage is set to 1% of the rated phase voltage. In order to illustrate how a voltage imbalance is detected, a single phase transformer with a turn-to-turn fault will be discussed in detail. It represents one phase of a 3-phase transformer experiencing a turn-to turn fault. Two scenarios will be discussed: a turn-to-turn fault on the primary side or a turn-to-turn fault on the secondary side. A small portion of the primary windings are shorted causing a small amount of additional current I 2 p to be drawn. This does not create a significant change in e p since the source resistance is assumed to be low. Therefore the negative- sequence voltage contributed by the primary side, given by V 2 p in (4), will be negligible [ ] = [ ] [ ] (4) The current travelling through the short circuit changes the mmf contribution of the faulted winding causing a change in the effective turns ratio from N p to N p. Therefore the secondary side contributes a large amount of negativesequence voltage, as given by in (6). The two negativesequence voltage magnitudes and are compared in a Fig:2 Algorithm for negative sequence current 4. NEGATIVE SEQUENCE ALGORITHMS First of all primary and secondary side negative sequence currents are considered. Figure 2 shows the algorithm for negative sequence currents. After that obtain primary and secondary side negative sequence current magnitude and also phase. If negative-sequence current exists only on the primary side of the transformer but not on secondary side, the primary side of the transformer is being energized. For transformer discrete Fourier transform and negative sequence transform on the primary and secondary side three phase currents was obtained. Here only the magnitude of the primary and secondary negative-sequence current is considered. Both the primary negative-sequence current and the secondary negative-sequence current, transformed to the primary side of the transformer, must be larger than 1% of the rated primary 168

4 current. This prevents erroneous tripping due to small imbalances. An energization on the secondary side of the transformer also results in the selection of the negative-sequence voltage algorithm. Figure 3 shows the algorithm for negative sequence voltages. the magnitude of the primary and secondary negative-sequence voltage is of interest in this case. Notice that the secondary negative-sequence voltage is transformed to the primary side of the transformer. Both the primary negative-sequence voltage and the secondary negative-sequence voltage, transformed to the primary side of the transformer, must be larger than 1% of the rated primary voltage which prevents erroneous tripping due to small imbalances in transformer. The restraining and differential voltages are calculated using (8) and (9). If the differential voltage exceeds the restraining voltage equation, a trip is warranted. V r = (Average of primary and secondary voltage magnitude) x (nominal turns ratio) (8) 5. SIMULATION RESULTS Fig:4 Voltage and current waveform of transformer on primary side when 2% windings are shorted V d = (Difference of of primary and secondary voltage magnitude) x (nominal turns ratio) (9) IEEE standard [13] requires that the transformer winding voltages, at no load, be within 0.5% of the nameplate voltage. If a 0.5% imbalance is introduced to the otherwise healthy system, it produces a negative-sequence voltage well below the threshold. Fig:5 Voltage and current waveform on secondary side of transformer Fig: 3 Algorithm for negative sequence voltages Fig:6 Pattern for inrush currents in three phases of 169

5 transformer when 2% windings are shorted on primary side Fig:7 Voltage and current waveform of transformer on primary side when 20% windings are shorted Fig.8 Voltage and current waveform on secondary side of transformer Fig:9 Pattern for inrush currents in three phases of transformer when 20% windings are shorted on primary side Figure 4 shows voltage and current waveforms of transformer when primary side of transformer is shorted by 2%. Voltage waveform are shown by upper part and lower part shows current waveform. Normal operation of transformer is seen till 1second, at the instant of 1 second fault occurs which causes disturbance in the operation of power transformer. But when only 2% turns are shorted during turn to turn faults, faults could not be detected properly and hence there is no trip at instant of 1second, it is clearly seen in figure 4, 5, 6. Figure 5 shows voltage and current waveform of transformer on secondary side. Pattern for inrush current is shown in figure 6 when 2% winding are shorted. Inrush currents for all three phases of transformer on primary side is shown in figure. Figure 7 shows Voltage and current waveform of transformer on primary side when 20% windings are shorted. Primary side of transformer is taken into consideration. Occurrence and clearance of fault during instant of 1 sec is clearly shown in figure 7. Fault is cleared within 0.02 seconds in power transformer when 20% windings are shorted on primary side. Voltage waveform are shown by upper part and lower part shows current waveform. The changes in magnitude of current during, after and before occurrence of fault is to be noted. Figure 8 shows voltage and current waveforms on the secondary side of power transformer. The voltage and current waveforms are shown during energization of power transformer. The waveforms can be seen before, after and during energization of the primary side of transformer. Pattern for inrush currents in all the three phases of transformer is shown in figure 9 when 20% windings are shorted on primary side. At the instant of one second fault occurs and cleared within a very short period of 0.02 seconds and after words pattern becomes uniform again. The working of transformer can be observed before occurrence of fault, during fault and after clearing the fault. 6. CONCLUSION In this paper, an efficient protection scheme based on negative sequence currents for detecting minor internal turn to-turn faults in power transformers was described. The proposed scheme is simple to implement The proposed protection scheme was evaluated with different cases, which included different numbers of shorted turns of the transformer, different values of the system parameters, different connections of the power transformer, and the inrush current. By using the proposed algorithm turn-to-turn faults involving 5% of the transformer's windings were detected. The negative-sequence-based algorithm was seen to be more sensitive and faster than the current differential algorithms. The reliability of protection system using negative sequence algorithm is increased. 170

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