Mitigation of an Inrush Current of Power Transformer by using PWM-Inverter based Series Voltage Compensator

Transcription

1 Mitigation of an Inrush Current of Power Transformer by using PWM-Inverter based Series Voltage Compensator Apurva Kulkarni, Priyadarshani engg college,nagpur Vinesh Choudhari, Faculty of Electronics Engg Department Dr DYPatil Engineering college,puna Rupesh Juware, GHRCEMAmravati Yogesh Pawar KVNNAIK college of engg, Nashik Abstract Transformers are key component for electrical energy transfer in power system Stability and security of transformer protection are important to system operation Transformer inrush currents are high-magnitude, harmonic-rich currents generated when transformer cores are driven into saturation during energization These currents have undesirable effects, including potential damage or loss-of-life to the transformer, protective relay disoperation, and reduced power quality on the system Over the years, lot of techniques were invented to reduce the magnitude of inrush current In this paper, a PWM- Inverter based voltage series compensator is used to inject the voltages in respective phases at the instant of transformer energization The voltages injected by a compensator are controlled according to active and reactive power requirement of a system With this kind of voltage generation by a compensator, not only the inrush current is mitigated but it reduces the harmonic distortion Inrush current on power transformer with and without compensation is modelled in MATLAB and the simulation results are given Key Words: MATLAB, Simulink, PWM Inverter, Inrush Current, Series Compensator [I] INTRODUCTION Transformer is a essential device for power transmission system When a unloaded or lightly loaded transformer connect to a random supply, than it create large flux asymmetries and one or more core of the transformer can be saturated This saturation results in high magnitude currents that are rich in harmonics and have a high direct current component This current called inrush current or surge current The ratio of inrush current to full load current can be 5 to 10 times greater This current can create many problems in transformer like false operation of relays, potential damage in transformer etc Over the years, so many strategies have been implemented to mitigate the inrush current It includes the use of a resistor in the neutral of a transformer, POW, controlled switching in the case of 3-phase transformer, prefluxing the core etc The most popular method is controlled switching or POW (point-on-wave) But it all requires the knowledge of the residual fluxes in the core before the start The modern prefluxing method allow us to neglect residual flux but again, setting up the desired value of flux in the transformer and then switching it on after flux equalization is still a bit complicated strategy Voltage series compensators have been conventionally being used for reactive power compensation But this unique method can also be used to serve this purpose The logic in this model is very simple It is just to have a compensator in the circuit till the time the input current reaches it steady-state value The voltage of a compensator is controlled by PWM-inverter The magnitude of the compensator voltage reduces with time and becomes zero after the steady state is reached keeping the input current free from too much harmonic distortion In the later sections, we shall see the modelling of inrush current, simulation of inrush current without and with compensation, and FFT of both [II] Analytical expression After a transformer is connected to current pass a transient mode and then reaches the study state mode Voltage waveform, instant of closing the switch, amplitude and direction of the magnetic residual are the factors that can last the transient mode The primary current with secondary open circuit may rise to several

2 times the rated current of the transformer Ignoring the winding s resistance value, the relationship between the voltage and flux is describe following dϕ( t) E m sinω t = N1 dt Where N 1 = The number of turns in primary winding (t)= Flux in primary So flux can be obtained as Fig 1:- Flux and winding current at their negative peaks Where e = Voltage I= Coil current (t)= Magnetic flux Where constant and [III] System Under Study For t=0, Where r =The remanence in the iron core Now the inrush current represents as During the period of transient inrush current, since the transformer s core normally enters a state of saturation, the magnitude of inductance is reduce, the current increases quickly due to the decrease in inductance This phenomenon has some dangerous effects Instant in time when voltage is zero during continuous operation This is the point in time where both flux and winding current are at their negative peaks, experiencing zero rate of change As the voltage build to its positive peak, the flux and current waveforms build to their maximum positive rate of change, and on upward to their positive peaks as the voltage descends to a level of zero, as shown in fig, Initially, without any compensation, system consists of a 3-phase generator of 15kv feeding a transformer of 15kv/400kv, 250MVA with secondary open circuited It means that transformer is acting like a load and generator as a source Specifications: Generator: 250 MVA capacity 15 kv line to line voltage R g =01 pu, X g =07 pu Transformer: 250 MVA capacity, 15kv/400kv R p =001 pu, X p =01 pu, L m =500 pu Equivalent circuit: When circuit is switched on, the expression for current i(t) is given as!"# $$% Where & and &' $ ( ) *+*,*- *- Where s= Flux at which core saturates

3 r=residual flux in the core m= Maximum(peak) prospective flux In the system s= 17 pu m= 14 pu r= 08 pu, -08 pu, and 07 pu for the three phases A,B and C respectively As time t tends to infinity, current i(t) reaches it s steady state value = 155 A (RMS) So, reactive power consumed by transformer at steady state = 3*V phase *155 = 04 MVAr It can clearly be seen that system is drawing 02 MVAr extra inductive power So, to minimize inrush, compensator needs to be designed such that it provides 2MVAr capacitive power Current i(t) is simulated in MATLAB Simulink for all the three phases Reactive Power Reactive power Reactive power consumed at steady state required 06 MVAr 04 MVAr 02 MVAr [IV] Harmonic Distortion Inrush current is peaky in nature FFT analysis of an input current gives So, it can be easily concluded that the current is hardly sinusoidal in the beginning It contains a lot of harmonics which may result to undesirable resonances Input Peak(A) RMS(A) Current(without compensation) Phase A Phase B Phase C As the transformer is unloaded, the input current is mostly magnetising current So it lags the voltage by 90 In other words, power drawn by the transformer during excitation is reactive power Reactive power in any phase can be expressed as V phase *I phase Total reactive power drawn is calculated as Q= 06 MVAr [V] Series Compenation Series Compensator sends capacitive current into the system In other words, it provides the required reactive power to the system in the initial stage By series compensation, voltages are injected in the lines of a transformer Series compensation in one phase can be given as Where i p =magnetizing input current

4 I inj =compensating current I inj comes from a compensator, which is nothing but a PWM inverter It can be expressed as I inj = K R *n*i p Where K R is called as reduction factor Range of K R is from 0 to 1 For good compensation K R must be near to 1 Equivalent Circuit of Series Compensator Fig 71 Basic Control Scheme To decide the value of capacitor to dc side of inverter: The compensator is supplying reactive power from the stored energy in the capacitor Also, compensator is supplying power only for the time required by system to reach the steady state Firstly, this time needs to be calculated It depends upon the time constant of the overall system Time constant T=L/R=014 sec So, time taken by system to reach steady state= 5*T= 07 sec Current I inj can be expressed as I inj = V out /Z eq Where V out is the output voltage of an inverter and Z eq is the equivalent impedance of the system looking from the compensator side V out of inverter is controlled by controlling the modulation signal going to PWM generator Hence, energy supplied by compensator = power supplied * 07 Now, power supplied= 02MVAr, so energy supplied= 02*07*10^6 joules Energy stored in a capacitor = ½ CV dc 2 If dc voltage is fixed, the value of capacitor can be determined If V dc = 10kv, C= 1400F [VI]Application of Series Compensator The motive of the compensation is to bring down the reactive power of the system to it s steady state values Now at steady state, Q= 3*i p 2 *X = 04 MVAr P= 3*i p 2 *R= 005 MW These are the reference values for the compensator P * =005, Q * =04 [VIII] Results With C =1400F, results are simulated for V inj and I inj Injected Currents Fig 84 Injected current in Phase A Also we have 0 1 / " " and "5 8 0 So, the reference currents i d * from the references P * and Q * and i q * can be generated [VII]Controlling PWM Fig 85 Injected current in Phase B

5 Fig 86 Injected current in phase C RMS Phase A Phase B Phase C V inj 1902 V 1825V 1703V I inj 1746A 1871A 1817A Net reactive power injected by a compensator = V inja *I inja + V injb *I injb + V injc* I injc =019MVAr Reactive Power Required Reactive Power Supplied 02 MVAr 019MVAr After compensation, inrush can be simulated in all the three phases as Fig 88Modulating Signal to PWM [IX] Conclusion Inrush current is mitigated in all the three phases using a PWM-inverter based series compensation The magnitude in the beginning of the input current is very close to the steady state value of it FFT analysis is carried out on both models, with and without compensation, and it is observed that the harmonic distortion of the current improves if this strategy is used References: Fig 85Mitigated Current in Phase A Fig 86 Mitigated Current in Phase B Fig 87 Mitigated Current in Phase C Inrush currents with compensation Phase Peak Amp A 72 B 23 C 26 [1] Abdolmutaleb Abou-Safe, Gordon Kettleborough Modeling and calculation the inrush current in power transformer - Damascus Univ Journal Vol (21)-No (1)2005 Abou-Safe Kettle borough [2] Honkui Li, Yan Li, Xi Sun, Dongxu Li,Youteng Jing Analysis of three phase power transformer windings forces caused by magnetic inrush and short circuit current -IEEE conference on applied superconductivity and electromagnetic device Chengdu, China,Sep 25-27, 2009 [3]An Inrush Mitigation Technique of Load Transformers for the Series Voltage Sag Compensator Yu-Hsing Chen, Student Member, IEEE, Chang-Yi Lin, Jhao-Ming Chen, and Po-Tai Cheng, Senior Member, IEEE [4] An Inrush Current Mitigation Technique for the Line-Interactive Uninterruptible Power Supply Systems Yu-Hsing Chen Po-Tai Cheng CENTER FOR ADVANCED POWER TECHNOLOGIES (CAPT) Department of Electrical Engineering National Tsing Hua University Hsin-Chu, 30013, TAIWAN [5] Characteristics of Inrush Current of Present Designs of Power Transformers Ramsis S Girgis, Fellow, IEEE ABB Inc Ed G tenyenhuis, Member, IEEE,ABB Inc