2013 IEEE GCC Conference and exhibition, November 17-20, Doha, Qatar
Numerical Differential Protection Algorithm for Power Transformers Abdelrahman H. Hamouda, Fadel Q. Al-Anzi, Hussein K. Gad, Adel Gastli, Senior Member, IEEE, Department of Electrical Engineering, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar
[email protected] [email protected] [email protected] [email protected]
Abstract—The differential protective system establishes the main protection against short circuits occurring on transformer windings. This protective system should operate rapidly when internal faults occur. However, it should not operate under nonfault conditions such as a flowing inrush current. In digital differential relays, differential currents or other required signals are sampled and processed according to protective algorithms. Most of the existing fault detection algorithms in the digital differential protection of power transformers are based on processing the harmonic content of the differential currents. These methods utilize the fact that the ratio of the second harmonic to the fundamental component of differential currents under inrush current conditions is greater in comparison to that under fault conditions. In this paper, transformer terminal voltage and current signals are sensed and utilized to enhance the reliability of differential protection. The proposed algorithm is tested by simulation and compared to a conventional harmonic detection-based algorithm. Key words — Differential Protection, Differential Relay, Inrush Current, Microcontroller, Power Transformer, Winding Faults
I.
INTRODUCTION
T
RASFORMER failures can have external or internal causes. This research will concentrate mainly on the protection against internal faults. The internal faults can be classified into the followings: a)
Winding failures due to short circuits (turn-turn faults, phase-phase faults, phase-ground, open winding).
b) Core faults (core insulation failure, shorted laminations). c)
Terminal failures (open leads, loose connections, short circuits).
d) On-load tap changer failures (mechanical, electrical, short circuit, overheating). e)
Abnormal operating conditions (overfluxing, overloading, overvoltage).
One of the important protections against internal transformer faults, such as winding turns’ short-circuit, is the differential protection. The principle of the differential protection, which is illustrated in Fig. 1, is simple.
978-1-4799-0724-3/13/$31.00 ©2013 IEEE
Fig. 1: Schematic of transformer differential protection
One current transformer (CT) is used in the primary winding and a second one in the secondary winding. The outputs of the CTs are connected in such a way that the current of each CT opposes the other, thus, leading to normally a zero sum of the two currents. If the sum is not zero, this may mean that the ratio of one of the windings has been changed because of a short between the windings and consequently a trip signal can be sent by the protection relay to the circuit breaker in order to trip and disconnect the transformer from the power line and load. The major operating challenge to the transformer differential protection is maintaining security during CT saturation for external faults (e.g. short circuit in the secondary of the transformer) while maintaining sensitivity to detect low magnitude internal faults. CT saturation reduces the secondary output current from the CT, and causes a false differential current to appear to the relay [1]-[3]. Besides, when connecting a transformer to the power line, transformer energization takes place and produces an inrush current which develops a condition that resembles the condition of an internal fault. If no prevention mechanism is considered and provided, the differential relay will send a false trip signal to the circuit-breaker [1]. To enhance the reliability of differential protection, several published methods [1][4]-[10] have utilized current signals with sophisticated artificial intelligent algorithms for the detection of transformer internal faults and distinguishing from inrush current effect. For instance, Fuzzy Logic, Artificial Neural Networks, Wavelets have been used in [5]-
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2013 IEEE GCC Conference and exhibition, November 17-20, Doha, Qatar
[ 10]. Most of these methods recognize fault conditions by the distortion characteristic of the differential current waveform which leads to the appearing of some special order current harmonics. Conventional transformer protection schemes use the second harmonic component as the discriminating factor between an inrush and internal fault current. Most commonly used methods involve the detection of the 2nd harmonic content of the inrush currents which is used to block the differential function. Typically, when the ratio of the second harmonic current component with respect to the fundamental current component exceeds a preset level (e.g. 15%), the transformer differential protection in that phase is blocked [2]. There is still a room for more research and innovation in the differential protection especially with the development of more advanced and better performing microcontrollers and sensors at lower costs.
false tripping of the circuit breaker. Therefore, the RMS of the resulting differential current is calculated and subtracted from the RMS value of the differential current of the next cycle. The sign of the resulting value is checked. In case of an internal fault, the difference between a cycle and the next one must be greater or equal to zero. While in the case of an inrush current it is negative. This procedure follows the flow chart illustrated by Fig. 2.
This paper proposes a new design an efficient numerical differential protection algorithm for power transformers. The proposed method is reliable enough to distinguish between the transformer internal faults and external faults and inrush currents. The paper is organized as follows. Section II introduces the algorithms of the new proposed method and a conventional harmonic detection method which will be compared to. Section III describes the modeling and simulation of these algorithms using Matlab/Simulink. Section IV presents and discusses the results and finally section V concludes the paper. II.
ALGORITHMS DESCRIPTION
The proposed method depends upon two facts. The first one is that the initial voltage during the starting of the transformer excitation associated with the inrush current is zero. The second one is based on the fact that the peak of the differential current due to inrush current starts with a maximum peak at the first cycle then it decreases every next cycle after energizing the transformer until reaching almost zero while in the case of internal fault, the differential current will remain constant or increases depending on the development of the internal fault. The procedure of this method is as follows: the primary and secondary voltages and currents are first measured using the potential and current transformers. Then, the primary voltage is delayed for a short time and then compared to a threshold voltage to make sure that the first peak of the differential current is omitted. If the actual voltage is below the threshold value, then the transformer was not energized or has been just energized. This satisfies the first fact. Another process takes place during the previous analysis where it satisfies the second fact. Both primary and secondary currents are measured. The primary current is referred to the secondary side using the turn ratio. At the same time, the secondary current goes through a time delay which compensates for the phase shift between the primary and secondary. A small amplitude adjustment is also applied to compensate for the magnetizing current and core loss. After that, the resulting current value is subtracted from the calculated one (the primary current after referring it to the secondary). In normal operation, the resultant, which is the differential current, is zero. However, in the case of inrush current or or CT saturation this difference may not be zero and may lead to
Fig. 2: Flow chart of the proposed method
To compare the effectiveness of the proposed method to conventionally applied ones nowadays, the harmonic method was also considered. This method is based on measuring both primary and secondary currents and then referring the secondary current to the primary side. The difference between the two currents is calculated which forms the differential current. The differential current is compared to a pickup value. If the differential current is less than the minimum pick up, it means that the transformer works normally and there is no
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2013 IEEE GCC Conference and exhibition, November 17-20, Doha, Qatar
internal fault. Therefore, the protection method will repeat and go through the measurements again for the next cycle. However, if the differential current is higher than the minimum pickup value that means that an abnormal condition is detected and the differential protection scheme has to check whether this abnormal condition is an inrush, over-excitation, or fault current [12]. Therefore, the percentage of the second and the fifth harmonics are calculated to check whether they are in the normal range or not. This is dictated by the fact that the inrush current has high second harmonic which is above 20% [11] of the fundamental while over-excitation has high third harmonic. If the transformer is delta-connected, then the third harmonic will be trapped. Hence, it is more convenient to measure the fifth harmonic which is above 20% [11] of the fundamental. Although the magnitude of the fifth harmonic is small compared to the third harmonic, it is still higher than the fifth harmonic during the normal operating conditions. If the differential current is an inrush or an over-excitation current, no trip signal will be generated. On the other hand, if both percentages were out of the specified range, then the differential current is considered due to an internal fault. Therefore, a trip signal will be generated and sent to the circuit breaker. The flow chart of the conventional harmonic method is shown in Fig. 3.
Start
III.
MATLAB/SIMULINK MODELING AND SIMULATION
A. Proposed new method Fig. 4 shows the Matlab/Simulink block diagram of the model as it is built and simulated.
Fig. 4: Modeling of a 225 KVA, 2400/600 V three-phase core-type transformer
As it can be seen from Fig. 5, the simulation was done on 225 KVA, 2400/600 V three-phase core-type transformer. Three phase source is connected to RL circuit to implement a transmission line and then circuit breaker. After the circuit breaker, measurement tool is placed to measure the voltage and current on the primary side of the transformer. Another measurement tool is placed after the transformer to measure the secondary voltage and current. Then, a load is connected. The differential protection which is the “Trip Signal” block is shown in details in Fig. 5.
Fig. 5: The simulation block diagram of the tripping signal generation
This tripping mechanism follows the proposed algorithm that was discussed earlier in section 2. Fig. 3: Flow chart of the conventional harmonic method
It is important to highlight the choice of the threshold values which is the base of the tripping mechanism. The threshold value of ‘swithc2’ is the maximum allowed voltage deviation. For example, if the allowed voltage deviation is
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2013 IEEE GCC Conference and exhibition, November 17-20, Doha, Qatar
70%, the value has to be set to 0.7, while any value below that will be considered as noise. The threshold value of ‘swithc1’ is zero based on the fact that during a fault, current is increasing, hence; the difference between the two cycles will be positive and the relay should trip. The number of allowed short windings can be determined by ‘swithc3’. In other words, the sensitivity of the relay is controlled by this switch. ‘Swithc4’ has to be set to one since it is the dot product that combines the results of the current and the voltage branches of the simulation block. Thus, the change can be done only on threshold values 2 and 3 depending on the application, while other threshold values have to remain unchanged.
Notice that the block diagram of the harmonic restrained method differential protection starts by reading the primary current of the transformer. Then, it reads the secondary current and refers it to the primary side using the ratio of the transformer in order to perform the calculations. Then, the differential current is calculated by subtracting the referred secondary current from the primary current. After that, the absolute value of the differential current is taken and compared with a threshold value. Before going through the logic of the differential relay, the harmonic of the primary current is calculated by the red block (harmonic calculator) shown in Fig. 8 in details.
B. Conventional harmonic restrained method Fig. 6 shows the block diagram of harmonic restrained method simulation.
Fig. 6: General block diagram of Simulink implementation of the conventional harmonic restrained method.
It can be seen that the simulation was done on 132/33 KV, 54 MVA three-phase transformers connected to two circuit breakers and two three-phase measurement tools on each side. In addition, a three-phase source and series RL load is connected to the circuit. As illustrated in the figure, the circuit breakers take their commands from the differential protection block diagram shown in Fig. 7.
Fig. 8: Simulink block diagram of harmonics calculation
From Fig. , the fundamental, second and fifth harmonics of each phase are measured in order to calculate the percentage of the second and fifth harmonics (calculated based on the fundamental component) for each phase and compare them with the specified percentage of the second harmonic of the inrush current and the fifth harmonic of the over-excitation current. After determining the differential current and percentages of the second and fifth harmonic, if there differential current amplitude is above the threshold value while the percentages of the fifth or second harmonic is above the specified range then there will be no trip signal. But, if there is a high differential current while the percentages are within the normal range, then it will trip.
IV.
RESULTS & DISCUSSIONS
C. New Proposed Method 1) Case of inrush current without internal fault In this case, first the energization of the transformer is simulated without any internal fault.
Fig. 7: Simulink block diagram of the implementation of the relay control algorithm.
Fig. 9 shows phase differential currents rms values obtained in the case of inrush current cause by the transformer energization at 0.04ms. Notice how the rms differential current increases during the first cycle and then decays gradually.
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2013 IEEE GCC Conference and exhibition, November 17-20, Doha, Qatar
120
Mag (% of Fundamental)
0.18 0.16 0.14
0.1 d if
i , [A ]
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Fig. 9: rms of the differential current waveform of phase a during inrush current
Fig. 9 shows that there is no tripping signal generated because the differential current continued decaying gradually to zero steady-state value. So even though the differential current value was above the threshold, the controller was able to recognize that is due to an inrush current. 2) Case of an internal fault Now, an internal fault was introduced after energizing the transformer and the controller was able to detect it and trip the circuit breaker within 2 cycles. Note that the transformer was energized at 0.02s and percentage of short-circuited winding turns in phase a were 5%. 0.16
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Fig. 12: The spectrum of phase a primary current
Notice that the second and fifth harmonic components can be identified and evaluated compared to the fundamental component. In this case the second is 22% and the fifth is 5%. Conducting such test and measurement on a new or healthy power transformer can provide the minimum and maximum values of the percentages of second and fifth harmonics during transformer energizing and over excitation. The measured values can be used as thresholds during the practical implementation of the control algorithm. 2) Case of internal faults Now, an internal fault occurring 3s after energizing the transformer is simulated and the corresponding differential current for phase a is given in Fig. 13. Idiff phase A [A]
0.14 0.12
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Fig. 13: Phase a differential current before and after an internal fault occurring at 3s.
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Notice that the circuit breaker has tripped at 0.06s which is equivalent to 2 cycles. D. Conventional Harmonic Resytrained Method 1) Case of inrush current without internal fault Fig. 11 shows phase a differential current after transformer energizing. Notice that the differential current starts with high value and then decreases until it reaches its steady-state value which is close to zero.
Notice that the protection tripped immediately one cycle after the fault took place. . 3) Case of an external fault A three-phase short-circuit occurring after 8s and cleared at 12s was simulated. The three-phase current waveforms are shown in Fig. 14. 5000
Ia [A]
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Fig. 10: rms of phase a differential current waveform during inrush current and internal fault.
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After analyzing the primary current spectrum during the few first cycles using the Fourier Transformation it is possible to obtain the harmonic components of the differential current. Fig. 12 shows the spectrum for phase a differential current.
Ic [A]
Fig. 11: Phase a differential current during the normal condition
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Fig. 14: Three-phase primary currents before, during and after an external fault.
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2013 IEEE GCC Conference and exhibition, November 17-20, Doha, Qatar
Notice that the currents returned back to their initial values after the external fault is cleared which indicates that the differential protection has not detected the external fault as an internal fault. E. Discussions Both methods gave almost the same results where the inrush current and CT saturation as well as over-excitation took place and no tripping signals were sent to the circuit breaker. The conventional harmonic method involves a lot of calculation where the harmonics of the differential current are calculated during each cycle. Thus, the microcontroller required for the implementation of this method should be fast enough to perform the required task. Usually a DSP is required for this kind of calculations. The advantage of this method is that it requires only one cycle to determine whether the differential current is due to internal faults or not. The proposed new method, as described in the modeling and simulation section, is based on very simple calculations such as subtractions and if/then statements. Such calculations require very simple and cheap microcontrollers. However, as stated earlier, the proposed new method requires at least two cycles to identify the nature of the differential current and to decide if a trip signal is required or not, while the harmonic restrained method requires only one cycle. V.
CONCLUSION
The paper introduced a numerical differential protection based on a new algorithm which was modeled an simulated using Matlab/Simulink. The results obtained from this new method showed that the proposed algorithm works properly. A conventional harmonic retrained method was also presented and discussed in this paper and a comparison between the two methods was conducted. Both methods showed that they can detect internal faults and effectively differentiate between these faults and external ones and also inrush current occurring during transformer energization and over-excitation. However, the conventional method takes only one cycle to detect the fault while the proposed new method needs two cycles to distinguish between internal faults and inrush current situations.
REFERENCES [1]
Zoran Gajiü, “Differential Protection for Arbitrary Three-Phase Transformer,” Doctoral Dissertation, Department of Industrial Electrical Engineering and Automation, Lund University, 2008 (ISBN: 978-91-88934-47-5). [2] G. Zigler, Siemens, Numerical Differential Protection, March 2005, Book, ISBN 3-89578-234-3. [3] A. Mohamed, M. F. El-Naggar and E. H. Shehab El-Din, “A New Current-Based Technique Power Transformer Protection,” GCC Power 2012, Oman – Muscat, 12-14 November 2012. [4] J.P. Patra, “A Discussion on Power Transformer Magnetizing Inrush, Remedy, Fault Detection in Matlab–Simulink Environment”, International Journal of Electrical Engineering, Vol. 4, No. 1, 2011, pp.83-102. [5] M.M.Saha, E. Rosolowski and J. Izykowski: "Artificial Intelligent Application to Power System Protection". NPSC 2000 - Eleventh National Power Systems Conference, Bangalore, India, 19-22 Dec. 2000. Vol. 2, pp. 595-600. [6] P. Dey P. Das and A. K. Chakrabothy, “Implementation of Power Transformer Differential Protection Based on Clarke’s Transform and Fuzzy Systems”, International Journal of Engineering Research & Technology (IJERT)Vol. 1, No. 7, September, 2012, pp. 1-11. [7] Iswadi HR, Redy Mardiana, “Differential Power Transformer Protection Technique Using the Wavelet Packet Transform Approach”, Proceedings of the International Conference on Electrical Engineering and Informatics, Institut Teknologi Bandung, Indonesia June 17-19, 2007, pp. 818-821. [8] A. Zabardast , M.R. Feyzi, and A. Mehraein, "Distinction between Inrush Current of Power Transformer and Fault Currents in Differential Protection Using Wavelet Transformer", DRPT2008 6-9 April 2008 Nanjing China. [9] SRParaskar, M.A.Beg, G.M.Dhole , “Discrimination between Inrush and Fault in Transformer: ANN Approach,” International Journal of Advancements in Technology http://ijict.org/ ISSN 0976-4860, Vol. 2, No. 2 (April 2011), pp. 306-318. [10] M. Tripathy, "Power Transformer Differential Protection Based on Neural Network Principal Component Analysis, Harmonic Restraint and Park’s Plots", Hindawi Publishing Corporation, Advances in Artificial Intelligence, Vol. 2012, Article ID 930740, 9 pages. [11] Esmaeeli, B. V. (2010). MATLAB-SIMULINK Based Simulation for Digital Differential Relay Protection of Power Transformer For Educational Purpose . [12] Areva, Network Protection & Automation Guide, 3rd ed. Paris, France: Cayfosa, 1995.
On the other hand, the microcontroller needed for its implementation can be a very simple 8-bit microcontroller compared to a DSP that is required to implement the conventional harmonic method. Thus, the proposed method can be implemented practically with much lower cost compared to the conventional harmonic method. Finally, the use of the proposed new method as a numerical differential relay is very reliable and can be easily combined with solid state circuit breakers.
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