PERFORMANCE INVESTIGATION OF UNIFIED POWER QUALITY CONDITIONER

Abstract

In recent years, the use of power electronic controllers has increased manyfolds in domestic, commercial and industrial applications, ranging from few watts to MWs. These controllers suffer from the drawbacks of harmonic generation and reactive power flow from the source and offer highly non-linear characteristics. Non-linear equipments like transformers, induction machines, electric arc furnaces, welding equipments, fluorescent lamps (with magnetic ballasts), etc. are also responsible for generation of harmonics in electric power systems. Besides, there are different disturbances reported ranging from sub cycle duration to long term steady state condition which leads to waveform distortion. These are short interruptions, voltage sag, voltage swell, voltage and current transients, harmonic/waveform distortion, voltage fluctuations, voltage flicker, voltage unbalance, phase angle jump, over voltage, under voltage and power frequency variations. Among all these disturbances, waveform distortion, voltage flicker and voltage imbalance at the distribution system are considered to be more serious and regulation of voltage imbalance and compensation of harmonics become more of concern. Any significant deviation in the magnitude of the voltage, current and frequency, or their waveform purity may result in a potential power quality problem. In addition to this, most of these devices also cause the flow of reactive current component in the system. The generation of harmonics and reactive power flow in the power systems has given rise to the 'Electric Power Quality' problems. These power quality problems are reflected in the system in the form of reduced system efficiency, deteriorated performance of induction motors, interference with nearby communications networks, neutral burning, mal-operation of relays, blowing of fuses and so on. Thus there is a growing concern of power quality with the proliferation of AC/DC converters in adjustable-speed drives, power supplies, SMPS, DC motor drives, and so on. Harmonic regulations or guidelines such as IEEE 519-1992, IEC61000, etc. are currently applied to keep current and voltage harmonic levels in check. In addition, these guidelines promote better practices in both power systems and equipment design hence helping to minimize the operational cost. To meet the requirements of harmonic regulation, passive and active power filters are 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 harmonics. However, these filters suffer from the problem of bulkiness, high cost, fixed compensation, and they can also excite resonance phenomena with the system parameters. Moreover, to filter out all harmonics, one needed as many tuned filters are needed as the number of harmonics to be eliminated. On the other hand, modern active filters are superior in filtering performance, smaller in physical size, and more flexible in application, compared to traditional passive filters using capacitors, inductors and/or resistors. At present, active power filters are becoming affordable due to cost reductions in power semiconductor devices. (IGBTs), their auxiliary parts, and integrated digital control circuits. In addition, active power filter also acts as power conditioning device which provides cluster of multiple functions such as harmonic filtering, damping, isolation and termination, load balancing, reactive-power control for power-factor correction and voltage regulation, voltage-flicker reduction, and/or their combinations. Conventionally, shunt active power filters are used for conditioning the current flow from a load to the network, while series active power filters are used to improve the quality of the voltage supplied by the network to the load. However, when the distortions are of serious nature, because of the varying network parameters, it becomes difficult to get the exact compensatory current. This means the performance of shunt compensator unsatisfactory. Similarly, performance of series active power filter depends upon magnitude and nature of load current. Based on working principles of the above-mentioned conditioners, a new advanced custom power controller called Unified Power Quality Conditioner is in use. Resent research focuses on use of Universal Power Quality Conditioning device called Universal Power Quality Conditioner (UPQC) to compensate power quality problems such as voltage imbalance, reactive power, negative sequence and current harmonics by a single unit. In the present work an exhaustive literature survey on topologies, control techniques and applications of unified power quality conditioner for power quality improvement has been done and a comprehensive report is presented for. Most of these conditioners are built using pulse width-modulated (PWM) voltage-source converter with a dc capacitor as their power circuit, time domain technique for extraction of distorted signal and analog or low-cost microcontrollers for real time implementation. The voltage source topology of UPQC has drawbacks, mainly inside the series filter, such as a rather slow control of converters output voltage and has no short circuit protection. Also, when active shunt filter of UPQC is used as a power factor corrector, DC bus voltage oscillations appear which make the control of the series filter output voltage more difficult. The current source converter topology offers inbuilt short circuit protection, higher efficiency at low power loads, simple open loop current control and effective filtering of harmonics. The 11 drawback of CSI based active filter is with their energy storage element which is not only bulky but possesses high dc link losses. The requirement of series diode for limiting reverse current flow increases control complexity. With the use of superconducting magnetic energy storage (SMES) coils this problem can be over come. The availability of reverse blocking IGBT avoids the use of series diode in conventional bridge simplifying control complexity and also reduces components count as well. It is reported in literature that, the use of current source converter in active power conditioning is increasing due to its inbuilt short circuit protection capability and simplicity in operation. Inspite of this, very little work has been reported in literature about the use of CSI based UPQC. Evaluation of simulation results are presented on performance evaluation of a CSI-based UPQC using IRP theory. It uses synchronously rotating frame to derive reference signals, which has increased time delay in filtering dc quantities. However, the performance of UPQC mainly depends upon accurate determination of compensation signals. In present day use of CFL, computer, television, battery charger, electronic regulators etc are very common in domestic as well as commercial sectors. Similarly, use of SMPS UPS, microcontrollers, ASD and controlled or uncontrolled rectifiers are very much common. All these devices are non-linear in nature, hence, are responsible for emission of harmonics These devices aggregated in thousands causing serious concern for the utilities. A harmonic survey is conducted at domestic as well as industrial sector to know the level of harmonic injected by different non-linear. A brief report of analysis is presented. To analyze the performance of the UPQC in steady-state and transient conditions, a complete mathematical model is developed using single phase equivalent circuit diagram. A complete design of power circuit parameters such as the selection of filter capacitance and DC link inductance is presented based on the resonant frequency of the LC tank circuit and amount of energy required to correct harmonic components and magnitude of dc link current respectively. The conventional PI controller is designed and used for initial verification system performance. A simulation model using MATLAB / Sim Power System tools is developed based on design consideration to analyze the system. Initially the performance of series active power filter and shunt active power filter is analyzed individually. A PLL based unit vector template is used for extraction of distorted signal for control of series filter. The reference load voltages are obtained by multiplying unit vector templates with peak magnitude of supply voltage to control voltage harmonics. In order for the load voltage to be sinusoidal, the actual source voltage are compared with reference load voltage, the difference is compared in in hystersis controller to generate the gating signals for series filter. The load voltage is regulated through injection of series filter voltage to the system voltage. The performance of series filter is analyzed for distorted and non distorted load current. It is observed from the analysis that, the series active power filter performs satisfactorily only for non-distorted load current. Similarly, the performance of shunt active power filter is verified for regulation of current harmonics in a distorted environment. Fundamental real and reactive power is extracted from distorted source using Akagi's d-q theory. Actual reference currents are derived by adding corresponding real power drawn from shunt controller for controlling DC link current to the extracted real power. A PI controller is used for regulating DC link current. The shunt filter fails to perform satisfactorily at distorted environment. A simulation model of UPQC is obtained through integration of series active and shunt active power filter with designed DC link inductor and AC side filter capacitor. Performance of UPQC with distorted source feeding to different non linear loads is studied for both steady state and transient conditions. The nature of waveforms such as source voltage, load voltage, compensated voltage, source current, load current, compensated current, DC link current etc are studied. A prototype model of the current source unified power quality conditioner is developed in the laboratory by integrating the power circuit, the control hardware and the DSP, is tested experimentally to validate the simulation results. Ultra fast speed IGBT ( IRG4PH40U, 21 A 1200V, 40 kHz) is used in series with Ultra fast recovery diode (Intersil, RURP15120, 15A, 1200V) for power circuit of current source inverter. A suitably designed snubber circuit is connected across each device for its protection and mounted on suitable heat sink to ensure proper heat dissipation. A PACIFIC ASX -series programmable power source (320 ASX-UPC3, 3 phase, 2.0 kVA, 0-150 V iH, ) is used for simulating distorted source . A DSP DS1104 of d-SPACE is used for the real-time simulation and implementation of control algorithm. The control algorithm is first designed in the MATLAB /Sim Power System tools. The Real-Time Workshop of MATLAB automatically generates the optimized C-code for real-time implementation. The interface between MATLAB/Simulink and Digital Signal Processor (DSP, DS1104 of d-SPACE) allows the control algorithm to be run on the hardware, which is an MPC8240 processor. The supply voltages, line currents, compensation voltages and DC-link current are measured using multiple A/D converters. Gating signals generated from controller are connected to switches through proper isolation and protection. A proto type model of the line is also developed to investigate the performance of the UPQC. To investigate level of harmonic compensation THD of source IV voltage, load voltage, source current and load current are measured using Fluke power quality analyzer. Moreover, in order to check the correct implementation of proposed control algorithm in real time, d-SPACE Control Desk animation feature is used for capturing the waveforms like unit vector templates, reference current, reference voltage etc. Performance of prototype model is experimentally validated and compared with simulation results. The use of conventional PI controller resulted in unsatisfactory performance of UPQC under parameter variations nonlinearity load disturbance, etc. The performance of UPQC depends upon type of control strategy and technique adopted in current and / or voltage control. Rapid detection of disturbance signal with high accuracy, fast processing of the reference signal and high dynamic response of the controller are the prime requirements for desired compensation. Recent studies have shown that artificial neural networks (ANN) are reliable in improvement of power electronic system control; in fact, they have self-adapting and highrated calculation characteristics that allow them to handle high nonlinearities, uncertainties susceptible to occur in a controlled nonlinear system. A neural network based soft controller is designed and implemented for improved control action. A feed forward neural network is designed with three layers. The input, hidden and the output layer are designed with 2, 21 and 1 neuron respectively. The network is trained with large data of source current, reference load 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 UPQC performance is evaluated through simulation results and it is found that the ANN-based UPQC gives improved performance as that of conventional PI based controller. The performance of UPQC for steady state and transient conditions with designed ANN controller algorithm is investigated experimentally. A comparative analysis of UPQC performance for PI and ANN controller is presented. To summarize, a CSI based UPQC simulation model is designed using MATALAB /Sim Power System tools. Extensive simulation studies are carried out to verify the performance for different type of loading and also for transient as well as steady state operating conditions. A fast acting ANN based controller is designed and trained off line with huge data of conventional PI controller. The performance of UPQC with designed ANN controller is verified. A prototype model of a current source unified power quality conditioner is designed and developed in the laboratory. The designed power circuit and controller parameters are validated through the results obtained. A soft controller using ANN is designed for improved control action. Extensive simulation and experimental results are discussed under different loading conditions to investigate the steady-state, transient state and dynamic performance of the UPQC. The simulation results show the excellent performance of the proposed universal conditioner for mitigation of both voltage as well as current harmonics by a single unit. In spite of d-SPACE controller limitations for sampling time, the experimental results are found to be encouraging both for conventional as well designed ANN controller.