Modern Philosophies of Inrush Current Detection Algorithm and their Impact on Transformer Protection

Transcription

1 Modern Philosophies of Inrush Current Detection Algorithm and their Impact on Transformer Protection 1 Mohamed A. Ali, 2 Ahmed F. Bendary 1 Faculty of Engineering, Shoubra, Benha University, Egypt 2 Faculty of Engineering, Helwan University, Egypt 1 mohamed.mohamed02@feng.bu.edu.eg, 2 Fahmybendary10@gmail.com Abstract This paper presents a comparative analysis between the operating principal between the most commonly used protection relays for inrush current detection in Egypt. The paper presents the detailed philosophies of sample manufacturer's relays, also, testing the relays under several operation scenarios with several levels of second harmonic during energizing the circuits to show the adaptability and reliability of each algorithm during the energization with high/low inrush current levels. Matlab simulation is employed for testing the algorithms. Also, practical testing using secondary injection tester at Central Protection Sector of Egypt is performed. Index Terms Inrush Current Detection Algorithm, Matlab, Cold Load Pickup (CLP) I. INTRODUCTION Transformer inrush currents can be divided into four categories: Energization inrush, recovery inrush, sympathetic inrush, and cold-load pickup inrush. The first, energization inrush, results from the reapplication of system voltage to a transformer which has been previously de-energized. The second, recovery inrush, occurs when transformer voltage is restored after having been reduced by a nearby short circuit on the system. The third, sympathetic inrush can occur while an unloaded Transformer, which is being switched on, experiences an inrush, an adjacent Transformer, which is in service, may also experience a smaller degree of inrush. This sympathetic inrush may also occur when two or more transformers are operated in parallel. Offsets in inrush currents can circulate in transformers already energized, which in turn causes a mild inrush. Energization inrush is the most commonly investigated form of inrush, and can result in the large current magnitudes. The fourth, cold-load pickup inrush is an inrush current due to energizing a set of loads being de-energized for long period. The reason of that inrush current is due to load diversity is abnormal due to switching all loads at a time for the first energizing periods which extended to minutes. Magnetizing inrush current makes transformer protection a challenge to researchers. M. Jamali et al [1] investigated the effects of some parameters on the characteristics of inrush current in MATLAB Simulink. Their results showed that increasing switching angle at a positive remanent flux or source resistance will decrease the amplitude of inrush current. It has been shown that largest second harmonic content may not necessarily appear at the first cycle. Thus, the second harmonic restraining scheme is not reliable for all cases. Abdolmutaleb Abou-Safe et al [2] presented a mathematical model for an unloaded saturated transformer. The model uses non-linear core parameters (R and L), which vary according to the magnetic state of the non-linear core. A comparison between experimental and simulated results showed a good agreement and proved the validity of this model for studying in-rush current. The results of their research showed the risks of connecting an unloaded power transformer to the power system. It was recommended that this phenomenon is taken into account when protection devices on the Transformer are adjusted, to avoid mal-operations and consequent tripping of the transformer circuit breaker. The scholars have researched a lot, some conventional techniques to distinguish between inrush current and internal fault currents in transformers are 2109

2 reported here based on different principles. The second harmonic restraint method is the most common one used by various relay manufacturers and application engineers. There are a few variations of harmonic restrained differential protection. Jialong Wang [3], presented the analysis of factors affecting the second harmonic ratio in inrush current, and described various harmonic restraint methods and compared their performance. The above mentioned methods are successful to some extents but practically there are some conditions where predetermined threshold is not sufficient to make a decision, and mal-operation of relay occurs. Problems in identifying inrush using second harmonics component are: firstly, the magnitude of the second harmonic in fault current can be close to or greater than that present in the magnetizing inrush current. Next, the second harmonic components in the magnetizing inrush currents tend to be relatively small in modern large power transformers. Consequently, differential protection technique based on the second harmonic restraint may fail. In Fifth Harmonic blockade technique, Ouahdi Dris et al [4]has stated in their paper, based on the fact that the inrush current has a second harmonic component of the differential current which is much larger in the case of inrush than for a fault, and the over-excitation current has a larger fifth-harmonic component. And as the use of digital protection offers the advantage to implement complexes algorithms such as DFT to ensure better extraction of fundamental and other harmonic components, then the use of the second and the fifth harmonics for restraining and blocking, by the differential protection will give a possibility to discriminate between the faulty and the normal state of power transformer. Most of the conventional transformer protection relays employs the harmonic analysis approach to identify the type of current that flow in the protected transformer. There been different algorithms to carry out the harmonic analysis, among these algorithms, Sine Cosine correlations, Rectangular Transform, Discrete Fourier Transform DFT, Least Square Method, Walsh Function, Harr Function and Kalman filtering Techniques etc. The wavelet Packets (WPT) algorithm approach for determining different types of currents for power transformer is evaluated by the authors Iswadi HR et al [5], the use of WPT as feature extraction naturally emphasizes the difference between fault, non-fault and inrush currents as generated by MATLAB/SIMULINK, since their frequencies are very different. The results of simulation showed that the algorithm successfully distinguish between fault, non-fault and magnetizing inrush currents condition in less than 1/5 cycle. The classification scheme is powerful and need simple calculation. Saeed Jajebi et al in their paper [6], presented a combinatorial scheme based on hidden Markov models (HMM) and wavelet transform (WT) to discriminate between magnetizing inrush currents and internal faults in power transformers. HMMs are powerful tools for transient classification which compute the maximum likelihood probability between training and testing data signals for identification. The WT is employed to extract certain features which reduce the computation burden of HMMs and enhance detection accuracy. The newly extracted feature efficiently discriminated between faults by different trends. The k-means clustering technique is applied to reduce the training procedure time investment. Since the discrimination method is based on the probabilistic characteristics of the signals without application of any deterministic index, more reliable and accurate classification was achieved. Their method is independent of the selection thresholds. Based on the proposed algorithm a high-speed relay response (1/4 cycle) can be achieved. Some techniques to increase reliability, speed and robustness of existing digital relays are reported in recent literature. Those techniques are based on Fuzzy logic, ANN approach and adaptive neuro-fuzzy approach. These developments are discussed in details [7:16]. In This paper, further analysis to the most commonly used techniques for second harmonic detection in Egypt is presented to show the effectiveness and validity of an adequate one for overcurrent and differential relays. The analysis will be applied at two sample relays. Both of Matlab simulation and practical second injection testing are presented. II. ANALYSIS OF SAMPLE NO.01 The relay under analysis is chosen to be SAMPLE NO.01 protection relay. The SAMPLE NO.01 relay comprises two independent functions for inrush current detection. The first one is cold load pickup while, the other is built-in DFT filter for harmonic extraction [17]. The following section will step into details for describing the mentioned two functions. Cold load pickup in SAMPLE NO.01: The element avoids undesired operation of the overcurrent elements in case of high currents produced when energizing a line that has been open for a long period of time. The element starts to operate when the current values of the three phases are lower than 4% of the rated current (In) i.e. the circuit breaker is opened. At this moment, a timer (T IN) is started. If the currents return to values above 4% In before the timer has timed out, the unit returns to its original status. When the timer has timed out, the tap settings of the instantaneous phase elements (50P1 and 50P2) are multiplied by a constant K50P, and the phase TOC (51P) tap settings are multiplied by 2110

3 a constant K51P. A status signal turns on indicating that the Cold Load Pickup (CLP) is enabled, and the corresponding event is generated. For exiting the CLP status, when the current values of the three phases are above 8% In, a T OUT timer is started. If the current values fall below 8%In before the timer countdown has finished, the unit remains in CLP status. If the timeout finishes and current values are still above 8%In, the CLP status is disabled and the relay returns to its original settings. The CLP status signal is deactivated, and the corresponding event is generated. The T IN and T OUT timer values, as well as the multiplying constants K50P and K51P, and the function permission are settings associated to the CLP. Figure.1 depicts the CLP in SAMPLE NO.01 relay. III. ANALYSIS OF SAMPLE NO.02 Cold load pickup in sample No.02 relays: In this relay, a binary input wired from a manual close contact or by time control, it is possible to switch the overcurrent pickup settings to less sensitive settings for a programmable duration of time (dynamic setting change). After the set time has expired, the pickup settings automatically return to their original setting. This can compensate for initial inrush when energizing a circuit without compromising the sensitivity of the overcurrent elements during steady state conditions [2]. Inrush Restraint or Second harmonic trip inhibition in sample No.02 relays: The relay features second harmonic restraint [2]. If second harmonic content is detected during the energization of a transformer, the pickup of non-directional and directional elements are blocked. The element must have a pre-adjusted setting for second harmonic level from 10% up to 45% as shown in table.1. Table.1: Setting of harmonic restraint in SAMPLE No.02 Figure.1: Cold load pickup in SAMPLE NO.01 protection relay Harmonic detection in SAMPLE NO.01: The MIF II uses a complete cycle recursive DFT (Discrete Fourier Transformation) [17, 19-22] in order to obtain the resulting measure phasor. The Fourier transformation consists of decomposing a signal into a series of sinusoidal signals with frequencies that are multiples of the fundamental frequency. Once these signals have been obtained, all harmonics are extracted to get the phasor value corresponding to the fundamental frequency; therefore, it acts as a digital harmonic filter and all the relay protection and measurement elements work only with the fundamental component of each signal. So that, the SAMPLE NO.01 relay definitely will not trip for any inrush cases. Figure.2 depicts the main operating principal for harmonic detection and relay operation. Figure.2: Sample No.01 operating principal for harmonic detection and relay operation Figure.3: shows the second harmonic restraint algorithm in sample No.02 relay. Figure.3: Sample of sample No.02 relays operating principal for harmonic trip inhibition. In Other manufacturer's relays, the setting of second harmonic level may lead to mal operation of the device. In case of wrong setting the relay may give abnormal blocking or tripping. But, as per GE philosophy all harmonics are extracted for reliable tripping decision. The following section will highlight the actual performance of the two relays under several operation scenarios to show the impact of power system harmonics on the two relays. 2111

4 Figure.4: Test System IV. IMPACT OF POWER SYSTEM HARMONICS USING MATLAB SIMULATION In this section several energizing cases with different second harmonic levels are presented and tested on test system shown in figure 4. Two sample protection relays (sample No.01 and sample No.02) are employed to analyze the performance of each second harmonic detection algorithm and the impact on the associated transformer protection. Case No.1: The circuit under protection with SAMPLE NO.01 relay is energized with 21% second harmonic. As per SAMPLE NO.01 algorithm the DFT filter will extract all harmonics and only the fundamental component will be enter to the measurement and protection functions for tripping decision. Here it better to clarify that, the SAMPLE NO.01 relay did not produce blocking signal for tripping like Other manufacturers because in SAMPLE NO.01 relay, the harmonics don't enter the protection functions for more reliable operation and the tripping decision dependent only on the fundamental component, so that, the relay did not trip for such harmonic level or any other harmonic level or harmonic order as shown in figure.4. Figure.5: No trip due to 21% second harmonic at sample No.01relay. Case No.2: The circuit under protection with sample No.02 relay is energized with 21% second harmonic. According to its philosophy, the relay detects the 21% second harmonic and it is found that it was greater than the I 2f /I setting which equals 20% so that, the relay blocks and did not trip for such harmonic level according to the relay setting as shown in figure.5. Figure.6: No trip due to 21% second harmonic at sample No.02 relay. Case No.3: The circuit under protection with SAMPLE NO.01 relay is energized with 19% second harmonic. As per SAMPLE NO.01 algorithm the DFT filter will extract all harmonics and only the fundamental component will be enter to the measurement and protection functions for tripping decision so that, the relay did not trip for such harmonic level or any other harmonic level or harmonic 2112

5 order as shown in figure.6. V. IMPACT OF POWER SYSTEM HARMONICS USING PRACTICAL INJECTION TESTING In this section, the analogue and some digital signals of sample No.01 protection relay are depicted according to secondary injection testing using OMICRON 256 for different second order harmonic levels. All cases as shown in figure 9 through figure 13 depict the stability and reliability of this algorithm during any second harmonic level. Figure 9 through figure 13 is COMTRADE files- according to IEEE retrieved from the relay from the "waveform capture" option in the relay. All the following results from figure 9 through figure 13 are a practical testing by the author at Central Protection Sector of Egypt. Figure.7: No trip due to 19% second harmonic at sample No.01 relay. Case No.4: The circuit under protection with sample No.02 relay is energized with 19% second harmonic. According to its philosophy, the relay detects the 19% second harmonic and it is found that it was lower than the I 2f /I setting which equals 20% so that, the relay gives wrong trip command as depicted in figure.7. Figure.9: No trip due to 20% second harmonic at sample No.01 relay. Figure.10: No trip due to 40% second harmonic at sample No.01 relay. Figure.11: No trip due to 60% second harmonic at sample No.01 relay. Figure.8: Abnormal trip due to 19% second harmonic at sample No.02 relay. 2113

6 VIII. REFERENCES [1] M. Jamali, M. Mirzaie, S. Asghar Gholamian, Calculation and Analysis of Transformer Inrush Current Based on Parameters of Transformer and Operating Conditions, Electronics And Electrical Engineering ISSN , No. 3(109). Figure.12: No trip due to 80% second harmonic at sample No.01 relay. [2] Abdolmutaleb Abou-Safe and Gordon Kettleborough, Modelling and Calculating the In-Rush Currents in Power Transformers Damascus Univ. Journal Vol. (21)-No. (1)2005. [3] Jialong Wang, Analysis of transformer inrush current and comparison of harmonic restraint methods in transformer protection Protective Relay Engineers, st Annual Conference 1-3 April Figure.13: No trip due to 100% second harmonic at sample No.01 relay. VI. CONCLUSION According to the previous analysis, the two philosophies of the mentioned relays are different in dealing with harmonics. Sample No.02 relay is dealing only with second harmonic and make blocking if the second harmonic level is greater than a pre-adjusted values, so that, if wrong setting is entered to the relay, it is expected to result an abnormal tripping or blocking. Take into account that, the second harmonic due to inrush conditions is random depending on the loading level that varies with time according to the daily load curve. For that reason, this philosophy is not adaptive with all operation scenarios and user setting leads for abnormal behavior. But, sample No.01 relays is dealing with second harmonic and all harmonic levels. The relay extracts all harmonic level and only the fundamental component enters for tripping decision. Take into account that, the current magnitude increases with harmonic on the power system so, the relay may measure higher currents than actually exist in the fundamental and leads to wrong tripping as happened in sample No.01 relay, but, in sample No.02 relay all harmonics have no effect or influences on the operation of the relay, so that, the philosophy of sample No.01 is considered more reliable and adaptive with all operation scenarios. VII. ACKNOWLEDGMENT Thanks to the Consolidated Consultancy Group (CCG) to give the opportunity to use the sample relays for testing and analysis. [4] Ouahdi Dris, Farag. M. Elmareimi and Rekina Fouad, Transformer differential protection scheme with internal faults detection algorithm using second harmonics restrain and fifth harmonics blocking logic. [5] Iswadi HR, Redy Mardiana, Differential power transformer protection techniques using the wavelet packet transform approach Proceedings of the International Conference on Electrical Engineering and Informatics Institute Teknologi Bandung, Indonesia June 17-19, [6] Saeed Jazebi, Behrooz Vahidi and Seyed Hossenien A Novel Discriminative Approach Based on Hidden Markov Models and Wavelet Transform to Transformer Protection Journal simulation Vol 86 Issue 2 Feb [7] T.J. Ross, a book on Fuzzy logic with Engineering applications, University of New Mexico, USA, [8] Ahmed Abdulkader Aziz Prof. Dr. Abduladhem Dr. Abbas H. Abbas Abdulkareem Ali, Power Transformer Protection by Using Fuzzy Logic Iraq J. Electrical and Electronic Engineering Vol.5 No [9] Iman Sepehri Rad, Mostafa Alinezhad, Seyed Esmaeel Naghibi and Mehrdad Ahmadi Kamarposhti Detection of nternal Fault in Differential Transformer Protection Based on Fuzzy Method, American Journal of Scientific Research ISSN X Issue 32(2011), pp. 2114

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