DRIVE FRONT END HARMONIC COMPENSATOR BASED ON ACTIVE RECTIFIER WITH LCL FILTER

Transcription

1 DRIVE FRONT END HARMONIC COMPENSATOR BASED ON ACTIVE RECTIFIER WITH LCL FILTER P. SWEETY JOSE JOVITHA JEROME Dept. of Electrical and Electronics Engineering PSG College of Technology, Coimbatore, India. Dept. of Instrumentation and Control Engineering PSG College of Technology, Coimbatore, India. K. SUDHEERKUMARREDDY Department of Electrical and Electronics Engineering PSG College of Technology, Coimbatore, India. Abstract : Harmonic mitigation is a key issue in industrial as well as commercial drive applications. There are many types of filters available for this purpose. Shunt active filters provide better results when compared to passive filters. Presently IGBT based rectifiers [Active front end] are used as drive front end for AC to DC conversion, instead of using diode rectifier, to achieve less than 5% Total Current Harmonic Distortion (ITHD) at drive input as per IEEE standards. This paper proposes a dual purpose drive that can be used as a conventional drive and also to cancel out harmonics in line and also to act as rectifier to provide DC supply to the Inverter fed Induction motor drive. The proposed Harmonic compensator based on active rectifier is designed, modelled and its performance is simulated in MATLAB/Simulink. The proposed work demonstrates the effectiveness of the drive front end Harmonic compensator for power quality improvements at AC mains and it is found that the ITHD is 3.2% which is well within the IEEE 519 standards. Keywords : Active Front End (AFE), harmonic compensator, passive filter, Active rectifiers, power quality, harmonic distortion. T I.INTRODUCTION he wide use of nonlinear loads, such as front-end diode rectifiers connected to the power distribution systems for dc supply or inverter-based applications causes significant power quality degradation in power distribution networks in terms of current/voltage harmonics, power factor, and resonance problems. Restrictions on current and voltage harmonics maintained in many countries through IEEE in the USA and IEC /IEC in Europe standards, are associated with the popular idea of clean power [1][2]. Passive LC filters (together with capacitor banks for reactive power compensation) are simple, low-cost, and high-efficiency solutions. However, their performance strongly depends on the source impedance and can lead to unwanted resonance phenomena with the network. In addition, passive solutions are not effective for applications in which the nonlinear load exhibits fast transients. Multi-Pulse drives connected through phase shifting transformers can produce less magnitude of lower order harmonics compared to diode rectifiers. Active rectifiers with LCL filter can maintain 5% THD at the input of the drive[5]. In contrast, shunt Active Power Filters (APF) are recognized as a flexible solution for harmonic current compensation since they are capable of compensating harmonic currents generated by many types of nonlinear loads as well as providing fast responses to load variations.[4] Proposed active rectifier with LCL can be used such that front end part works as the active filter[5] so that it will cancel the all harmonics generated by other non linear loads. II. SYSTEM MODEL OF COMPENSATION PRINCIPLE The implementation of the active rectifier as a controlled current source first requires the generation of the reference signal and then the regulation of the generated reference signal. For this purpose, the control algorithm of the active rectifier is combination of current reference generator and 1

2 Magnitude (db) Journal of Electrical Engineering using quite small values of inductors and capacitors. Hence, the LCL filter[5] is used to limit switching frequency ripple in the application.[12] 3.1 Limitations on Filter Parameters: 1. The value of the capacitance is limited by the decrease of the power factor that has to be less than 5% at the rated power. Fig. 1 Schematic Circuit of Proposed Active Rectifier current controller. The current reference generator block involves two blocks; the harmonic current reference generator and the DC bus voltage regulator. The harmonic current reference generator extracts the harmonic components of the load current by the utilization of the measured load current (I L ) and terminal voltage (V F ). Moreover, this unit extracts the reactive power and the negative sequence current component of the load current, if the active rectifier is desired to compensate these currents. The DC bus voltage regulator with DC bus voltage (V dc ) feedback creates a current reference to hold DC bus voltage at its desired value. The output signals of the reference harmonic current generator and DC bus voltage regulator constitute the total current reference of the Active rectifier. This current reference is sent to current controller, which regulates the reference signal with the feedback signal of Power Active Filter current and creates switching signals to the Voltage Source Inverter (VSI) for the desired current at the APF output terminals.active rectifier works as a current source and it is controlled in such a way that to produce harmonic currents having opposite phase to those harmonic currents produced by the non-linear load. When a Shunt Active Filter is connected in parallel with a non-linear load its harmonic currents are compensated and the network is loaded with almost fundamental current only. III. LCL FILTER Power device switching frequencies can cause higherorder harmonics that can disturb other sensitive loads/equipment on the grid and can also produce losses. To reduce the current harmonics around the switching frequency a high value of input inductance should be used. However, for applications above several kilowatts, it becomes quite expensive to realize higher value filter reactors. The system dynamic response may become poorer. An alternative solution to this problem is to use a LCL filter. With this solution, optimum results can be obtained in the range of power levels up to hundreds of kilovolt-amperes 2. The total value of the filter inductance has to be less than 0.1 p.u. for low power filters. However, for high power levels, the main aim is to avoid the saturation of the inductors 3. The resonance frequency of the filter should be higher than 10 times the grid frequency and than half of the switching frequency.[3] 3.2 Selection of Damping Resistor: LCL filter transfer function is given by Bode plot is plotted by considering different values of resistances, and choose the minimum value of the resistor to make LCL filter transfer function stable Without damping resistor With 6 ohm Damping resistor Frequency (Hz) Fig. 2 Bode plot of LCL filter transfer function with and without damping resistor (1) 2

3 reference current can be extracted from the load currents using a simple LPF with feed-forward effect. The currents in the synchronous reference can be decomposed into two terms as (3) Only the alternating terms which are related to the harmonic contents will be seen at the output of the extraction system. [7] Moreover, using the extraction system just on the d axis, all the q axis component will be used as compensation reference. This way, the reactive power consumed by the load will be compensated in addition to the harmonics. The reference currents will be then (4) In order to find the active rectifier currents in three phase system, the inverse Park transform can be used as follows: (5) Fig. 3 Flow chart to find LCL values IV HARMONIC DETECTION METHOD In this method, called also the method of instantaneous currents i d, i q, the load currents are transformed from three phase frame reference abc into synchronous reference in order to separate the harmonic contents from the fundamentals [11]. It gives better performance even in the case where the three phase voltage is not ideal. Where, θ is the angular position of the synchronous reference. It is a linear function of the fundamental frequency. This reference is turning in a synchronous constant speed with the three phase voltage. The harmonic (2) Fig. 4 Block diagram of Harmonic detector V HYSTERESIS CURRENT CONTROLLER The current control strategy plays an important role in fast response current controlled inverters such as the active power filters and Active rectifiers. The hysteresis current control method is the most commonly proposed control method in time domain. This method provides instantaneous current corrective response, good accuracy and unconditioned stability to the system. Besides that, this technique is said to be the most suitable solution for current controlled inverters. [6] 3

4 Hysteresis current control is a method of controlling a voltage source inverter so that an output current is generated which follows a reference current waveform. The hysteresis control strategy aims to keep the controlled current inside a defined region around the desired reference current. The status of the switches is determined according to the error. [8-11] When the current is increasing and the error exceeds a certain positive value, the status of the switches changes and the current begins to decrease until the error reaches a certain negative value, then the switches status changes again. In the fixed hysteresis band control of the VSI, the switching frequency is a function of the derivative of the output current. This one depends on the value of the inductance of the decoupling filter and the voltage drop around it. It is important to notice that the coupling filter affects the switching frequency and the dynamic behavior of the active filter. VI RESULTS AND DISCUSSIONS Fig.6 shows the simulated waveforms. Load currents are non-sinusoidal because of diode rectifier with filter. Due to energy storage elements, it takes 0.14 seconds to attain the steady state condition. Load consumes a current of 75 A rms and 28.52% ITHD. It is to be noted that load current is composed only of 6n +/- 1 (n=1, 2, 3, 4 ) harmonics. Rectifier current (I f ) illustrates the waveform pattern of compensation currents generated by active rectifier. The magnitude of lower order harmonics are high compared to higher order harmonics. The harmonics generated by the Active rectifier is equal in magnitude and Phase shift to the harmonics generated by non linear load, so that all harmonics in the load current are cancelled out by rectifier. Source current becomes a sinusoidal, consists 3.2% ITHD. Speed control of Induction motor is done by connecting an inverter to the active rectifier dc link. To control the 2HP Induction motor drive, conventional switching table based Direct Torque Controller is used. Fig. 5 Simulink model of proposed system Proposed model is implemented in Matlab/Simulink. When the load is supplied through a three-phase diode rectifier, harmonics of order 6k +/- 1 (k = 1, 2,3...) of the fundamental frequency exist. Such load current waveform given as input to the harmonic detector.[10] Harmonic detector is implemented in the synchronous reference frame, so that 2 nd order Low Pass Filter is enough to separate harmonics from total load current waveform. Output of the harmonic detector is harmonic load current, which is exactly equal and degree out of phase with the load harmonics. In the proposed system, hysteresis controller is used because high controller bandwidth is easily achievable by using nonlinear regulators. By giving the reference current and Active rectifier current as input, controller generates the pulses for the rectifier. TABLE I Simulink Parameters SYMBOL PARAMETER VAULE V S Supply Voltage 415 V F Supply frequency 50 L L Load side Ac inductance 132uH U DC * Dc voltage reference 800 C DC Dc link capacitor 4700uF CDC-L Dc link capacitor load side 4700uF RL Load resistor 7 LDC Load side DC lnductor 1.4mH S Grid short circuit MVA 7 LM Main side inductor 735uH LC Converter side inductor 288uH CF Ac filter capacitor 4uF R Damping resistor 6 Ohm -- Rating of Induction motor 2HP VII CONCLUSION This paper has proposed a synchronous reference frame harmonic detector and Hysteresis current controller controlled active rectifier as drive front end. So that it cancels the harmonics generated by non linear loads which are connected in parallel with this rectifier. By using the rectifier s dc link, direct torque control based speed control 4

5 of induction motor is implemented so that drive can be used for two purposes. Results shows effective harmonic compensation at front end and the Current THD is 3.2% which is well below IEEE-519 Standard. Switching frequency of hysteresis controller is not constant which causes a difficulty in design of an output filter and results in unwanted resonance problems with the networks. Resonant controller based harmonic detector and SPWM controller can be used to operate at constant switching frequencies. Fig. 6 Simulation results of proposed Active Rectifier Fig. 7 Load Current Harmonic Spectrum 5

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