MULTILEVEL INVERTER BASED ACTIVE FILTER

Abstract

In recent years, use of power electronic converters, in industrial as well as domestic applications is consistently increasing for high level of automation and enhancing energy efficiency. Most of the times these converters generate harmonic current and reactive power in the supply system and thus cause the problem of power quality. Nonlinear equipments like transformers, induction machines, electric arc furnaces, welding equipment, fluorescent lamps (with magnetic ballasts), industrial loads such as AC and DC drives, battery chargers etc. are also responsible for generation of harmonics in electric power systems. Beside this, there are different types of disturbances ranging from sub cycle duration to long term steady state problems which lead to waveform distortion. A harmonic survey has been conducted at different commercial and high power industrial loads and a brief report is presented to know the existing level of harmonics present in the system. The harmonic distortion causes different undesirable effects in power system viz. reduced system efficiency, low power factor, deteriorated performance of induction motors, increase of losses in transformer and line, resonance phenomenon, interference with nearby communications networks, neutral burning, mal-operation of relays, blowing of fuses and so on. Harmonic regulations or guidelines such as IEEE 519-1992, /EC 61000, etc. are currently applied to limit the level of current and voltage harmonic. These guidelines are helpful in promoting better practices in design of both power systems and nonlinear equipment. To meet these requirements, passive and Active Power Filters (APFs) have been used in combination with the conventional converters. Classically, shunt passive filters consisting of tuned LC filters and/or high pass filters are used to suppress the current harmonics. However, these filters suffer from the problem of bulkiness, high cost, fixed compensation, as well as resonance with the system parameters. Moreover, to filter out multiple harmonics, multiple filters are required in parallel combination. On the other hand, modern APFs are superior in filtering performance, smaller in physical size, and more flexible in application, compared to traditional passive filters. Shunt APF acts as a harmonic current source which injects an anti phase but equal magnitude to the harmonic and reactive load current and thus improves quality of power from the source. Use of APFs is increasing due to reductions in cost of power semiconductor devices, their auxiliary parts, and integrated digital control circuits. In addition, APF also acts as power conditioning device which provides cluster of multiple functions such as harmonic filtering, damping, isolation, load balancing, reactive power control for power factor correction and voltage regulation. Recently many controlled processes in industry are being carried out at high voltage levels. The range of induction motor drives has reached up to 20 MW for supply voltages from 3.3 kV to 13.8 kV at 50 Hz or 60 Hz. The large size passive filters for different harmonic components are used in high voltage DC (HVDC) systems which result in high space and high cost. Alternatively, application of APF in high voltage system is gaining importance and interest of researchers. The conventional APF have limitations in medium and high voltage applications due to semiconductor's reverse voltage rating constraint, high losses caused by switching high voltage and high current, consequent high dv/dt, adverse affect on the communication system due to EMI as well as insulation degradation in electronic and electrical systems. APF can be used in high voltage system in two ways. In first method series connected semiconductor switches are used but, this configuration faces problems of unbalanced static and dynamic voltage sharing due to deviations of device characteristics and drive circuits. It further, requires large number of snubber components which cause additional losses and adversely affect reliability of the system. The second way is to use step up transformer with APF. In this method the Volt-Ampere (VA) rating of transformer is much higher than the rms VA rating of system due to harmonic reactive power. In literature three winding transformer is used wherein filter current is injected into third winding to compensate source side harmonics. Transformers used in such scheme, require special designing with K factor derating, which makes the system costly as well as bulky. The simulations for two-level APF with step up transformer in high voltage system are carried out and the results show that the compensation is affected by the rating of transformer. It was observed that even with the use of transformer of rating twice that of load kVA, for the same compensation, Total Harmonic Distortion (THD) at source side remains above the prohibitive level of IEEE standards. The use of transformer also causes difficulties in control due to DC magnetizing and surge overvoltage problems causing saturation of the transformers in transient states, hence making overall system more bulky and costly. MLIs are effective in high voltage applications which can eliminate the use of transformer. MLI includes an array of power semiconductors and capacitors. The commutation of the switches permits the summation of capacitor voltages, which reach high value at the output without use of transformer, while the power semiconductors must withstand only reduced voltages. MLI has many advantages in comparison with the hard-switched two-level pulse width modulation (PWM) inverter such as capability to operate at high voltage with lower dv/dt across switches, high efficiency and low electromagnetic interference (EMI) etc. Increase in number of DC levels, helps to achieve nearly sinusoidal voltage of high magnitude at the output of MLI. The MLIs are further classified as diode- clamped, flying capacitors and CMLI. Among these three, CMLI has a modular structure and requires least number of components as compared to other two topologies and as a result, it is receiving increasing attention for use in many different applications. The separate DC sources are replaced by DC capacitors for APF applications. In this work an exhaustive literature survey on topologies, control techniques and applications of APF has been done and a comprehensive report is presented. A complete design of power circuit parameters such as the selection of DC capacitance and coupling inductance is presented based on the amount of energy required to correct harmonic components and magnitude of DC link voltage. The conventional PI controller is designed based on power balance principle between AC and DC side of APF and used for initial verification of system performance. The quality and performance of APF as compensator depends mainly on, the topology of APF, method of estimation of compensating signal and the modulation techniques used to generate gating signal. A number of algorithms in time as well as frequency domain have been proposed to estimate the compensating current in the literature. The proposed controller combines compensation current extraction and capacitor voltage balancing. The instantaneous reactive p-q theory is used to find the real and reactive powers of the load power. The average capacitor voltage for each iii phase is compared with reference voltage and error is processed through PI controller to get the loss component of APF which is added to real power component of load power to derive reference source currents. The comparison of actual source current with the reference source currents is used to generate reference currents for APF. These reference currents are compared with phase shifted triangular carrier waves to generate switching signal for devices used in MLI. The advantage of p-q theory is that the real and reactive powers associated with fundamental component are DC quantities. These quantities can be extracted with low pass filter. Since the signal to be extracted is DC, a-p reference frame is insensitive to any phase shift errors introduced by low pass filter, improving compensation characteristics of the APF. The simulation model of five level CMLI based APF using p-q theory for current reference generation and carrier phase shifted pulse width modulation(CPS-PWM) for gating signal generation is developed in MATLAB/SIMULINK and the performance of APF for different loading in steady state as well as transient state is analyzed. The p-q theory gives satisfactory results, when supply voltages are sinusoidal, but in case of distorted mains voltage, prevalent in most industrial power system, the p-q theory generates errors in reference currents and limits the compensation of harmonics. In such cases average power method is used which uses supply voltages through Phase Locked Loop (PLL) and load currents to calculate the instantaneous power and average power over one sixth of a cycle. This in turn is used to obtain the peak current component of fundamental load current. The loss component of APF is obtained by comparing actual capacitor voltage and reference capacitor voltage. The peak component of reference source current is obtained by adding the peak value of load current component and the peak value of current providing the APF losses which is obtained from PI controller. The three phase reference source currents are obtained by multiplying the peak value of reference source current with unit current templates, Generally, conventional PI controller is used to regulate the DC link voltage for the estimation of reference currents. PI controller requires precise linear mathematical model of the system, which is difficult to obtain due to inevitable nonlinearities in the system. Therefore, an artificial neural networks (ANN) based control scheme is developed. ANN based controllers have self-adapting and high-rated calculation IV characteristics. A feed forward neural network is designed with three layers, the input layer with 7, the hidden layer with 21 and the output layer with 3 neurons respectively. The network is trained with large data of source current, reference DC voltage, power loss component and reference compensation current from conventional PI method using tan sigmoidal and pure linear activation functions in the hidden and output layers respectively. The ANN-based APF performance is evaluated through simulation results and it is found that the performance of APF improves with the use of ANN as compared to that with conventional PI based controller. Carrier phase shifted pulse width modulation (CPS-PWM) scheme is implemented to generate switching signals in which the reference currents of APF are compared with triangular carriers having same frequency and peak to peak amplitude but with a phase shift. In the CPS-PWM method, the equivalent switching frequency of the whole converter is (m-1,) times as the each power device switching frequency, for m-level MLI. It is observed that CPS-PWM can achieve a high equivalent switching frequency effect at very low real device switching frequency which is most useful in high power applications. Space vector pulse width modulation (SVPWM) scheme is implemented to improve the overall performance of APF. Various SVPWM algorithms are used for MLIs to get high quality output voltage for AC drives in the literature. The proposed algorithm is developed such that it doesn't need any lookup tables and required memory space is lesser than other available algorithms due to use of MATLAB sfunctions for generation of the switching states. The switching sequences are generated with respect to the triangle number, such that there will be one switching per state. The on-times are calculated based on on-time calculation for two level SVPWM, irrespective of triangle number, so that complex calculations and lookup tables are not required. The simulation and experimental results show that SVPWM provides excellent output performance, optimized efficiency, and high reliability as compared to CPS-PWM. Verification of the simulated results, has been done with that of prototype of three phase, five-level CMLI based APF that was developed during research work. Insulated gate bipolar transistor (IGBT) has been selected as switching device. Different hardware components required for experimentation such as delay circuit, pulse amplification and isolation circuit, voltage and current sensors circuit etc. have been designed and developed. The firing pulses have been generated using real-time simulation with the help of digital signal processor (DSP) of dSPACE 1103 board and output signals are available at CP1103 connector panel. The control desk of dSPACE facilitates to observe or store output signal and/or any variables in the model. The real time workshop (RTW) of MATLAB and real-time interface (RTI) feature of dSPACE result in the real-time simulation of the SIMULINK model of controller which results in the generation of control pulses. These generated control pulses are given to semiconductor devices of each H-bridge through delay and isolation and amplification circuit in real time. DC capacitor voltages have been sensed and are given to analog to digital converter (ADC) channels of dSPACE for comparison with reference values in order to generate error signals. The performance of MLI based APF is verified for different loading in steady state and transient conditions. The results of simulation and experiment are seen to be reasonably corroborative. To summarize, a CMLI based APF simulation model is designed using MATLAB/SIMULINK PowerSystem toolbox. Extensive simulation studies have been carried out to verify the performance for different type of loading under steady state and transient operating conditions. Afast acting ANN based controller is designed and trained off line. The performance of APF with ANN controller is investigated and compared to that of PI controller. The performance of APF for CPS-PWM and SVPWM is investigated and it shows that APF using SVPMW gives higher compensation and reduction in APF losses, resulting in improved performance as compared with other modulation techniques used for MLI based APF. A prototype model of a MLI based APF is designed and developed in the laboratory. Experimental results are obtained under different loading conditions to investigate the steady-state and transient performance of the APF. The proposed control schemes are validated by experimental results. Results show the good overall performance of CMLI based APF in high voltage system.