International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 12, December 2014) Transformer Inrush Current and Related Challenges Saurabh Shrivastava1, Ashfaque Khan2, Amita Mahor3 Factors contributing to the magnitude and duration of inrush current are Magnitude of residual flux in transformer core, Nonlinear magnetizing characteristic of transformer core, Magnitude of source voltage at the switching instant and Impedance and short circuit power of the source including VAR abortion capacity. The key adverse effects include- Abstract—Energizing of large power transformers is considered a critical event in the operation of an electric power system. When a transformer is energized by the grid or utility, it takes very high value current known as inrush current, the typical value of this current could be ten times. This inrush causes many problems like mechanical stress on transformer & harmonics injection to generator windings, and system protection malfunction. This transient phenomenon needs detail analysis for system protection. A. Mechanical and electrical stresses in windings The amplitude of inrush current can be equal to that of the short circuit current and may last longer depending on system configuration. This can seriously damage the windings through excessive mechanical stresses [5]. Keywords—Transformer inrush current, transformer transients, inrush current model of transformer. I. INTRODUCTION The energizing of large power transformers is considered a critical event in the operation of an electric power system. When a transformer is energized by the grid or utility, it takes very high value current known as inrush current, the typical value of this current could be ten times. This inrush causes many problems like mechanical stress on transformer & harmonics injection to generator windings, and system protection malfunction. With the development of smart grid, distributed generation from independent power producers is growing rapidly & to energize the system in a situation like black start has long been considered as challenging. The main reason for this criticality is the unpredictable system transients post energization. In the subsequent sections we will attempt to focus on phenomenon, reasons, effects and mitigation plans of this aspect and to ensure both steady state and transient stability aspects of system. B. Harmonic resonant over voltages Transformer inrush currents are rich in harmonics and in the event of resonance, a sustained Harmonic resonant over voltages may exist and if these over voltages last for a long period of time, they may eventually damage the equipment. C. Mall operation of protective relays D. Voltage dips: Due to high magnitude and asymmetrical nature of inrush current a voltage dip is observed by the system. The magnitude, duration and unbalance of voltages in the respective phases are function of system impedance, source, transformer capacities etc.[6] III. MITIGATION PLAN As the phenomenon of transients is dynamic in nature and largely depends on system configuration. Therefore, it is not possible to set any rule of thumb to access the precise behaviour of transients or suitability of system. The only reliable way is through system simulation (Electromagnetic modelling) in available electrical software. Still it is experienced that a unit is capable to charge a transformer of equal or moderately higher capacity without losing the system stability. Though there are ways to mitigate the transient but they are more system specific than a generalized solution. Some of the mitigation methods are – II. REASONS & EFFECT When the transformer is charged, a transient current known as magnetizing inrush current (of magnitude as high as 10times of rated current) flows in to the system. This is due to the nonlinear relationship of flux and magnetizing current as transformer core is in saturation mode. It is not only the high magnitude of inrush current but its composition (rich in DC component and harmonics) and duration are also the cause of concern which severely affects the stability of the system. 450 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 12, December 2014) A. Pre Insertion Resister One of the solutions to mitigate the risk related to transformer inrush currents is reducing the magnitude of these currents. Traditionally it is done by using preinsertion resistors in the circuit breakers. The voltage drop across the pre-insertion resistor produce by the inrush current will decrease the voltage on the transformer windings, which in turns decreases the magnetic over flux in the core. As a result, the magnitude of the transient magnetizing currents will be reduced as well.[5],[6] Capacitor banks in the high voltage network which are used to help control voltage under normal conditions generally cannot be used in black start conditions for two reasons: They are generally sized for normal short circuit levels. Under black start conditions the short circuit levels are significantly lower and thus switching the capacitor bank might cause excessive voltage change at the bus where it is switched. The excessive capacitance may result in generator self‐ excitation. Under self-excitation conditions, the high voltage bus could theoretically rise in range of 2 to 4 p.u. Even allowing for the action of arresters, this level of overvoltage would be sufficient to damage equipment. The amount of reactive margin that needs to be maintained is dependent on the composition of the load, including both steady state power factor and dynamic behavior. The minimum loading of a generator is unit dependent and is typically 30 to 60% of the generator full load rating. B. Low Impedance Charging If system configuration permits then charge the main transformer through auxiliary transformer. As small transformer has not only relatively low magnitude inrush current but duration is also small which will be considerably lower than charging the main transformer first Steps for fig – Charge the Ta and T1 (via Ta) on G1 Once inrush is decayed close the breaker of T1 towards G1 IV. INRUSH CURRENT EVALUATION THROUGH ATP Inrush current analysis performed for 15/6.6 kV, YY winding transformer, testing data of open circuit and short circuit used to model the transformer in ATP. External inductance used to model the saturation characteristics of the transformer. In model show blow the transformer is charged through grid and the value of inrush current is being analysed for different angle of supply voltage. Transformer discharging case is also analysed and effect of transient due to residual flux is being analysed. As the transformer charging current also depends on the pole discrepancy, circuit breaker is connected in the incomer of the transformer and effect of pole discrepancy on inrush current is included in the results. Figure 1: Typical Circuit diagram of power station [7] C. Soft Charging Another way to energize the generator step up autotransformer by connecting it prior to starting the generator. Then the transformer would be energized together with the generator under excitation control. This would avoid any inrush effects. Unfortunately this procedure cannot be followed on transformers which are not unit transformers. D. Load Composition It is recommended to load the generator to its minimum possible load prior to proceeding with energization of other system components. Load consists of active & reactive components and there are limitations on capacity of the connected generators to supply or absorb reactive power. Ideally the nature of load should not be highly capacitive like capacitor banks or long transmission line etc. Figure 2: ATP model of transformer 451 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 12, December 2014) V. CONCLUSION 1100 [A] It is accepted beyond doubts that transformer energization is a transient phenomenon which not only affects the system stability but to the system components as well. No rule of thumb can be assumed. Based on the details of the event it is a crucial call which largely relies on the experience of the engineer and his knowledge of the system. 900 700 500 300 REFERENCES 100 -100 0.00 0.05 0.10 0.15 0.20 [s] [1] IEC 60071-2, Insulation co-ordination – Part 2: Application Guide. (Third Edition, 1996) [2] H. K. Hoidalen, ATPDraw for Windows version 5.5, 2010. [3] J. A. Martinez and F. C. Aranda, "Tower Modeling for Lightning Analysis of Overhead Transmission Line," ( Proceedings of 2005 IEEE Power Engineering Society General Meeting, Vol. 2, June 1216, 2005, p.p.1212- 1217). [4] IEEE Standard for Insulation Coordination—Definitions, Principles, and Rules, ( IEEE Std. 1313.1-1996). [5] L.Prikler, G.Banfai, G.Ban and P.Becker, Reducing the Magnetizing Inrush current by means of Controlled Energization and deEnergization of Large Power Transformer, International Conference on Power System Transients, IPST 2003. [6] W. Xu, S.G. Abdulsalam, S.Chen, and X. Liu, A Sequential Phase Energization Method for transformer inrush current reduction, Part II: Theoretical Analysis and Design Guide, IEEE Trans. on Power Delivery, Vol. 20, pp. 950-957, April 2005. [7] Salman Kahrobaee, Marcelo C. Algrain, Sohrab Asgarpoor, ― Investigation and Mitigation of Transformer Inrush Current during Black Start of an Independent Power Producer Plant‖ Electric Power Division, Caterpillar, Inc., Peoria, USA 0.25 (f ile tr.inrush.pl4; x-v ar t) c:X0030A-TR_15A Figure 3: Transformer Charging Inrush 7.0 [A] 3.8 0.6 -2.6 -5.8 -9.0 0 5 10 15 20 25 30 [ms] 35 (f ile tr.inrush.pl4; x-v ar t) c:X0030A-TR_15A Figure 4: Transformer De-energizing Inrush Value of inrush current at various phase angle of supply voltage has been analysed and maximum value obtained in indicated in figure3 and 4, from above figures it can be concluded that for modelled 4MVA transformer the maximum value of inrush current is 1100Amps, which is seven times the nominal current value. 452