Discrimination of magnetic inrush current from fault current in transformer - A new approach

Transcription

1 Volume 114 No , ISSN: (printed version); ISSN: (on-line version) url: ijpam.eu Discrimination of magnetic inrush current from fault current in transformer - A new approach Yashasvi Tripathi 1, Kushagra Mathur 2, Dr S.V.N.L. Lalitha 3, Dr M. Ramamoorty 4 Department of EEE, KL University Green fields, Vaddeswaram, Guntur, AP, , INDIA yashasvi434@gmail.com 1,kushagramathur1234@gmail.com 2, lalitha@kluniversity.in 3, mrmoorty@gmail.com 4 Abstract It is a challenge for the power engineers all around the globe to find a fast and accurate method of discriminating magnetic inrush currents from fault currents in Power Transformers. Though the previously known inrush current detection techniques are able to do it but they are less reliable and slow to respond due to use of filter. A new approach of discriminating inrush current from fault current in a fast and precise manner is developed. Based on the asymmetry of inrush current waveform, a unique criteria for discrimination is established. MATLAB coding is developed to model a transformer for the analysis. Various switching instants on the supply voltage waveform have been considered at intervals of 90 0 from 0 0 to with different residual flux in magnetic core. Keywords: Inrush current, Power transformers, Discrimination, Asymmetry, Residual flux. 1. Introduction Transformer is a static device which transfers power from one electrical circuit to another at a constant frequency. It is a valuable electrical component in power system both at transmission as well as distribution end. The power is transmitted at a very high voltage by using step up transformers to reduce the transmission line losses in the power system which improves the overall efficiency of the transmission. At the distribution end step down transformers are used to get the voltage at distribution level, so transformer is having dual action. 615

2 Due to the above properties Transformers are one of the most important part of Power System. So it is anticipated from the power engineers to have a proper and fast protective scheme for its safety and to ensure its hassle free operation. Among many one of the most common reason for mal operation of differential relays used for transformer protection is high magnetic inrush current. The relay sometimes is unable to discriminate between an inrush current and fault current within the first cycle hence it issues a trip signal to the circuit breaker which causes an unwanted interruption in power. When a transformer is energized at no load or lightly loaded condition for the first few cycles it draws a very high amount of current known as magnetic inrush current. Magnetising current is the one which is responsible for the development of rated flux in the transformer. Inrush current peak is in the order of 5 to 10 times of the full load current. Due to this high current magnitude it becomes difficult for a relay to distinguish between inrush and fault current which causes its mal operation by giving tripping signal which causes discontinuity in power transmission. So an accurate identification of currents in a transformer is the key necessity for preventing mal-operation of the protection system under different high current conditions which include magnetising inrush current, external and internal fault current, etc. The requirement for transformer protection has become major issue due to the need for precise, quick, and reliable distinction between magnetic inrush current and internal fault current. The magnetic inrush current has a good percentage of second harmonics unlike this fault do not have large second harmonic. The percentage of second harmonic component of the current to its fundamental component is utilized (also known as SHR) for discriminations.in this method the relay should block the current when its ratio exceeds the pre-set value [1]. Most of the viable differential relays are provided by the SHR method to prevent tripping due to inrush current conditions. The ratio has been usually set in the range of 12 20%. But this method may fail to differentiate high inrush currents when transformers are energized with some significant residual flux. The next method utilises the duration of gap between the zero crossing instants of the current which is known as gap detection technique [2] is being used by some relays to identify the fault and inrush currents. But, this gap detection method is liable to mal-operate when saturation of CT takes place which in general is caused by high DC component present in the inrush current. A number of algorithms have also been developed to overcome this serious problem which consists of Wavelet Transform [3], ANN [4] and fuzzy logic [5]. Yet some of these methods require a huge data for online training purpose, computational liability on the relay is increased, and are difficult to predict counter to high frequency noise signal [6]. Due to the above mentioned drawbacks these methods have not reached a practical level yet. So the SHR and gap detection are widely used methods in practise in spite of their limitations of detecting high inrush currents till date. 2. Transformer Protection Transformer is a static device which can transfer power from one electrical circuit to another maintaining constant frequency. It is an important electrical component used in power system to reduce the transmission losses. We know from Faraday s 2 nd law emf equation, E = N dɸ...( 2.1) dt Where, N= number of turns, ɸ= flux linkage, E= Em sin ωt, Em = Peak value of voltage, ω = Angular frequency. 616

3 Flux linkage is, ɸ = Applying KVL in primary loop, So current through transformer, Em sin ωt N Em sin ωt i*r1 L1 di dt...(2.2) dt =0 (2.3) i = Em {sin (ωt + ) α} + c e( r1 t L1 ).(2.4) Z where α = ωl1, r1= Primary winding resistance, L1 = Primary winding reactance. r1 If the magnetic core is not saturated, then the inductance L1 is high and constant. The current i is magnetising current with small amplitude. If the core gets saturated then the inductance L1 is very low and current i will have very large amplitude. Fig. 1: Transformer under no load The above fig. represents a transformer under no load condition with sinusoidal supply. The equivalent circuit of transformer is shown in the figure below. where, N1 = Primary turns, N2 = Secondary turns Fig. 2: Transformer equivalent circuit A. Transformer protection using differential relay: In general, for transformer protection differential relays are popularly used. The differential relay works on the principle of difference current flowing through primary and secondary coils. If the differential current also known as spill current is zero then the relay does not operate or remains in the blocking region but if the spill current is non-zero then the relay operates. Differential relay only works for internal faults and remains un-operative for external (through) faults. Fig. 2.3 shows a typical differential relay scheme for transformer protection against internal fault. Fig. 3: A differential relay for transformer protection 617

4 In differential protection the difference of two currents is fed to the relay operating coil. So for the external faults the currents in the two C.Ts must be equal in magnitude and opposite in phase hence the spill current becomes zero. B. Cause for mal operation of differential relay: Differential Relays are widely used in the transformer protection but may maloperate due to system disturbances, such as: 1. Over excitation: Over excitation [7] of transformer implies the level of magnetic flux is higher than the designed level. This leads to saturation of the core drawing large current. This can leads to severe fault and mal-operation of the differential relay. 2. CT saturation: It is a physical phenomenon that happens when all magnetic domain on ferromagnetic material are already aligned and further flux increment does not takes place. The current transformer [8] implication on secondary current may be different. Saturated core does not imply constant flux increase or high current on secondary. Generally improper selection of CT ratio and high DC offset in line current under fault current result in saturation of CTs. 3. Large magnetizing inrush current: The large current drawn by transformer on no load when the transformer is energised and will last only for few cycles. The magnitude of inrush[9] is generally several times more than rated current.as its magnitude is near to fault current so there is a chance of tripping of over current relay. 3. Inrush currents and types A. Inrush current and its types: Magnetic inrush currents are the transient no load current which has high magnitude (5 to 10 times full load current) drawn by the primary winding of transformer. As discussed earlier primary current develops rated flux in the transformer core rate of change generates counter emf in the winding. But sometimes its magnitude becomes comparable to the fault current and hence the relay is unable to discriminate between the inrush and fault current. A typical inrush current waveform is shown in fig Fig. 4: An inrush current waveform 618

5 The inrush currents are mainly classified in three types: (i) Inrush current during energisation It occurs when transformer is brought from off state to energised state. (ii) Inrush current during recovery It occurs when the voltage is recovered after a small dip or disruption is restored. (iii) Inrush current during operation or Sympathetic inrush current It occurs when transformer is energised which is in parallel to an already excited transformer. The sudden drop in voltage caused due to energisation can cause inrush current in already excited transformer. Among the above three types of inrush currents [10] energisation is the most common which generates the largest current magnitude.. To understand inrush analytically we have to recognize the relationship between voltage applied to transformer and transformer s magnetic core flux.that relation is E= dλ(t) / dt Or λ (t) = e(t) dt + λ (0).(3.1) Where λ = Flux linkage and E =Induced emf λ (0) =Residual flux It is observed that the extreme possible value of λ after energisation is 2 λm + λ (0). The relation between inrush current and λ is given by the saturation characteristics of transformer core. The protection system must be able to discriminate between the inrush current and actual short circuit. B. Methods for Discrimination of inrush current from fault current: 1. Second harmonic Restraint Method Inrush current is dominated by second harmonic which is used in most of the differential relays for transformer, to discriminate inrush current and fault current. The harmonic sensing relays most commonly block operation if the harmonic(s) exceed a given percentage of the fundamental component. Some relays use the harmonic to increase the restraint current. Generally differential relays are aided by second harmonic restraint to block tripping due to magnetic inrush current. Pickup ratio is in range of nearly 12 to 20 %. An S.H.R. relay is shown in the figure below. Transformer Through bias Restraining coil Harmonic bias XC XL High Set Unit Operation coil Fig. 5: S.H.R. Relay circuit 2. Gap Detection Approach As shown in the figure below, the time difference in each cycle is called as dwell time [11] in this region differential current is almost zero. So to identify the inrush current check where current is becoming less then even 5% of the rated current. 619

6 This criterion is known as the gap detection approach. CT saturation due to both the short circuit fault and inrush currents adversely affects the detection criteria. When CT saturation happens after a small period subsequent to an internal fault, output currents of CT may be distorted so huge harmonic components are also present. Consequently, there is possibility that second harmonic criteria operates inaccurately and blocking of differential relay takes place. Fig. 6: Gap detection technique Following observations about the two methods discussed above, CT saturation due to both short circuit fault and inrush currents adversely affects inrush current detection. 3 A variety of algorithms recently presented include artificial neutral network (ANN), fuzzy logic and wavelet analysis ANN is a novel approach which is used in online detection to discriminate the inter turn fault and magnetizing inrush current, and also the fault location i.e., whether the turn to turn fault lies in secondary winding or primary winding through the use of discrete wavelet transform and artificial neural networks. Wavelet Transform [12] is used to analyse the signal for short duration for its spectral contents. Another method using operational matrices and Hartley transform [13] is proposed for evaluation of inrush current and its simulation. Following observations are made, These methods need a large data set for training. Impose a high computational burden on relay. Depend on the transformer parameters or initial conditions. Seem to be unpredictable against high frequency noise. C. Proposed strategy: A new approach of discriminating inrush current from fault current in fast and precise manner is developed. Based on the asymmetry of inrush current waveform a unique criteria for discrimination is established. It is observed that the instant at which the switching takes place with respect to voltage waveform has a significant role on the peak value of current. Several case studies have been conducted for various switching instants such as when the voltages waveform crosses zero, at its positive peak, at its negative peak, etc. Intermediate switching instants have also been considered at an interval of 90 0 from 0 0 to on the supply voltage waveform which are thoroughly discussed in results and conclusions. 620

7 4. Output Waveforms For the proposed approach single phase transformer is modelled for two different conditions which are Inrush current with open secondary, Short circuit at secondary with fault resistance and Internal fault for primary turns shorted. The tool used is MATLAB 2011 version to carry out all the simulations. A. Parameters: 1 ɸ Saturable Transformer having 2 winding with following parameters, Power Rating: 9.6 KVA Voltage Rating: 300 V/ 150 V Primary Winding: r1= 0.06 Ω x1 = 0.3 Ω Secondary winding: r2= 0.03 Ω x2 = 0.15 Ω Core loss resistance: Rc = 1200 Ω Unsaturated magnetizing reactance: Xm = 600 Ω Unsaturated inductance: Lu = 1.91 H Saturated inductance: Ls = H Peak value of rated magnetising current in steady state: im = A Frequency: 50 Hz 3 separate conditions are considered for various phase angles: Condition 1: Inrush current with open secondary. Condition 2: Secondary short circuited with fault resistance. Condition 3: Internal fault for some primary turns shorted. 300 V (rms) supply is given to the transformer under the conditions mentioned above and the developed model is run for 0.2 seconds (10 cycles) for various phase angles. Based on that the waveforms are obtained then FFT is performed to obtain fundamental, 2 nd harmonic, 5th harmonic and 7 th harmonic components. On the basis of the results obtained, logic for the relay is developed. For fault case the fault resistance is varied for 4 different values which are ωl, ωl ωl and.where, ω = 314 rad/s and L=0.191 H then for internal fault two cases were considered i.e., 5% and 10% primary turns were shorted. ωl, 10 B. Output waveforms: For Magnetising current For Residual flux = 0 621

8 For Residual flux = 2.7 mwb Fig. 7 : Magnetising current for zero residual flux For Internal fault: For 5% turns shorted: Fig. 8 : Magnetising current for 2.7 mwb residual flux Fig. 9: Fault current for 5% turns shorted 622

9 C. Observation Table : FFT analysis of the waveforms above is done and both +ve and ve peaks for the first cycle are noted and tabulated below for various switching instants : For Residual flux =0 For α = Current Inrush Internal DC comp (%) Fund (%) nd (%) 5 th (%) th (%) st peak nd peak -3.9e For α = Current Inrush Internal DC comp (%) Fund (%) nd (%) 5 th (%) th (%) st peak nd peak For α = Current Inrush Internal DC comp (%) Fund (%) nd (%) 5 th (%) th (%) st peak nd peak For α = Current Inrush Internal DC comp (%) Fund (%) nd (%) 5 th (%) th (%) st peak nd peak -2e CONCLUSION This paper highlights a simple, fast and cost effective way of distinguishing magnetising current from a fault current just by checking the positive and negative peaks of the current waveform. For inrush current case, its magnitude is always less than magnetising current peak value (i.e., A) in one of the half or in either of the halves for first few cycles. In case of through fault current, its magnitude will be always greater than magnetising current peak value in either of the halves for first few cycles. In case of an internal fault it takes 3 to 4 cycles for exceeding magnetising current in both cycles. So for internal fault logic will be similar to external fault if both halves 623

10 exceed A value otherwise both halves are of same polarity so it can be properly discriminated from inrush current. This method can help fast detection of magnetising current whereas previously known methods utilise filter which causes time delay and increases computational burden on the relay. So just by incorporating comparator the fault and inrush currents can be discriminated. REFERENCES [1] Pei L, Malik OP, Deshu C, Hope GS, Yong G. Improved operation of differential protection of power transformers for internal faults. Power Deliv, IEEE Trans 1992;7: [2] Mekic F, Girgis R, Gajic Z, tenyenhuis E. Power transformer characteristics and their effect on protective relays. Presented at the 33 rd Western protective relay conference, October 17 19, [3] Rasoulpoor M, Banejad M. A correlation basedmethod for discrimination between inrush and short circuit currents in differential protection of power transformer using discrete wavelet transform: theory, simulation and experimental validation. Int J Electr Power Energy Syst October 2013; 51: [4] Puneet Kumar Singh, D K Chaturvedi. Modeling and Simulation of Single-Phase Transformer Inrush Current using Neural Network. Control Theory and Informatics Vol.3, No.2, National Conference on Emerging Trends in Electrical, Instrumentation & Communication Engineering [5] Wiszniewski A, Kasztenny B. A multi-criteria differential transformer relay based on fuzzy logic. Power Deliv, IEEE Trans 1995;10: [6] Hooshyar A, Afsharnia S, Sanaye-Pasand M, Ebrahimi BM. A new algorithm to identify magnetizing inrush conditions based on instantaneous frequency of differential power signal. Power Deliv, IEEE Trans 2010;25: [7] %2F protection-against-overexcitation-of-atransformer&usg=afqjcnej7qjq. [8] UKEwjrmfTo57bQAhXBv48KHfZ_A_sQFgg2MAQ&url=http%3A%2F%2Fwww.gohz.com%2Fwhathappens-in-current-transformer-core-during-ct- saturation &usg=afqjcngi 5p3ReC9A0uR5bJEnh- GjnNi4cQ&sig2=cwniyL--lPm-gexXmTgeUQ [9] %2FInrush.pdf&sa=D&sntz=1&usg=AFQjCNHcWsfLj7hHRHmh9V9oOMjTk1Lx0wnSKvx0w3RlUV AdPtrK7EQ&sig2=6PZQp9z9_0cEhaJeG0RvTA. [10] LbQAhUErI8KHRvlATQQFggqMAM&url=http%3A%2F%2Fwww.electrical4u.com%2Fmagnetizingi nrush-current-in-power-transformer%2f&usg=afqjcnhvw6y-4t9pqp AjymyL4HkpK56fMA&sig2=LXnJaAfouFiXDjpec2QhzQ. [11] Dashti-Hamed, Mahdi Davarpanah, Majid Sanaye-Pasand, Hamid Lesani. Discriminating transformer large inrush currents from fault currents electrical power and energy systems 75 (2016) [12] Maya.P, S.Vidya shree, Roopasree.K, K.P.Soman. Discrimination of Internal Fault Current and Inrush Current in a Power Transformer using Empirical Wavelet Transform [13] M. A. Taghikhani, A. Sheikholeslami and Z. Taghikhani. Harmonic Modeling of Inrush Current in Core Type PowerTransformers using Hartley Transform. Iranian Journal of Electrical & Electronic Engineering, Vol. 11, No. 2, June

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