INVESTIGATIONS ON SHUNT ACTIVE POWER FILTER FOR POWER QUALITY IMPROVEMENT

Abstract

In recent years, many power electronic converters utilizing switching devices are being widely used in industrial as well as in the domestic applications, ranging from few watts to MWs. It is desired to draw purely sinusoidal current from the distribution network, but this is no longer the case with new generation of loads consisting of power electronic converters. In addition to the numerous advantages, these power electronic converters suffer from the problem of drawing harmonics and reactive components of current from the source, and offer highly nonlinear characteristics. Increase in such non-linearity causes different undesirable features like, low system efficiency, poor power factor, disturbance to other consumers, and interference in nearby communication networks etc. The current harmonics produced by these nonlinear loads further results in voltage distortion, and leads to various power quality problems, led to implementation of standards and guidelines such as IEEE 519-1992, for controlling harmonics on the power system. Classically, shunt passive filters, consists of tuned LC filters and/or high pass filters are used to suppress the harmonics. These passive filters have the problem of fixed compensation, large size and can also excite resonance conditions. Also, if a good level of correction is targeted, one needs as many filters as the number of harmonics to be eliminated. On the other hand, power capacitors are employed to improve the power factor by supplying reactive power demand. Traditional methods ofstatic var compensation based on fixed capacitors or Thyristor Switched Capacitors and Thyristor Controlled Reactors have the problem that, the var generated or absorbed is directly proportional to the energy storage capacity of the passive elements. The size of these elements will increase considerably with the increment in var to be compensated. Active Power Filters (APFs) are researched as a viable alternative over the classical methods to compensate harmonics as well as reactive power requirements of the non-linear loads. The objectives of the active filtering are to solve these problems by combining the advantages of regulated systems with a much-reduced rating of the necessary passive components. With the remarkable development and advances in switching speed and capacity of the power semiconductor devices, various topologies of the APFs are developed for the compensation of voltage and current harmonics, reactive power, neutral current, unbalance, voltage-flicker etc. Among them, Shunt Active Power Filters (SAPFs) are proved to be effective, to absorb current harmonics, compensation of reactive power and current imbalance at the customer end. Most of the APFs are developed using instantaneous reactive power theory, which is based on sensing harmonics and var requirement of the load and difficult to implement. Also, performance of this theory is significantly affected under distorted mains conditions. Although, several improved algorithms are proposed, most control circuits are complicated and not easy to implement. Another approach is proposed, which does not require sensing harmonics and var requirement of the nonlinear load. This method is simple and easy to implement. However, does not cover complete design aspects. Keeping this in mind, it is decided to design, develop and investigate a shunt active power filter, as a simple and cost effective solution, to improve the power quality, by compensating harmonics and reactive power requirement of the nonlinear loads. A field survey of voltage and current harmonics is conducted at the consumer as well as utility end, to know the existing level of harmonics present in the system and taken up as a first investigative object. Design and development of a three-phase Shunt Active Power Filter based on current controlled voltage source PWM converter, for the compensation of harmonics and reactive power, is taken up as the next investigative object. The control scheme is based on sensing line currents only and does not require to sense harmonics or var requirement of the load, unlike the instantaneous reactive power theory. The control is simple, thereby enhancing the system reliability, which is applicable for both three-phase as well as single-phase systems. Complete design aspects of power and control circuit parameters are described, and system performance is investigated to validate the design. Detailed simulation results are presented to investigate the steady state as well as transient performance. A dedicated micro-controller based experimental set-up is developed and tested in the laboratory for experimentation. PWM pattern generation is based on carrier less hysteresis based current control to obtain the switching signals. A simple hysteresis current controller is designed and developed with minimum hardware, incorporating a lockout delay between switchings of upper and lower devices. It is observed that the performance of the APF deteriorates for the nonlinear loads with sharp rising and falling edges. A hybrid series passive/shunt active filter is investigated for such loads with high distortion. Detailed simulation results are presented to investigate the performance with different load conditions and verified experimentally. Series passive filter (smoothing reactor) connected in front of the nonlinear load, reduces the bandwidth of the active filter, thereby reduces burden on it. In many commercial and industrial applications power is distributed through a three-phase, four-wire system. Load unbalance as well as single-phase nonlinear loads, leads to excessive neutral current, which can cause overloaded power feeders and transformers, voltage distortion, and common mode noise. Athree-phase, three-wire active power filter does not perform satisfactorily in such cases. Three single-phase units can be connected between each line and neutral, but it is cumbersome and expensive. The concept of instantaneous active and reactive power theory and theory of symmetrical components is proposed in literature for the case of three-phase, four-wire systems. But, the implementation is more complicated and costly solution. In place of three-phase converters, four-wire converters are proposed in the literature. The four wire converters can take either of two forms, a four-leg four switching-pole topology or a standard threephase converter with a mid point capacitor. Athree-phase, four-wire, shunt active filter based on four switching-leg topology is developed and investigated. Various simulation results are presented to validate the scheme and verified experimentally. Four-leg topology is preferred over the capacitor mid-point topology due to its better controllability. 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 some times due to nonlinear nature of the system. Therefore, a fuzzy logic based control scheme is developed, which is based on the simple linguistic variables and offer more freedom of design. Various simulation results are presented during transients and steady states, verified experimentally, and compared with the conventional PI controller. The fuzzy control is implemented through an inexpensive 8-bit micro-controller. Although, the generated voltage waveforms are always sinusoidal but there are many devices that distort the mains voltage, and these distortions are propagated all over the network. If such distorted supply is being used by a customer, which is true in general, the harmonics generated may be much more than the harmonics generated in case of harmonic free supply. Presently, APFs are designed to absorb all the harmonics generated and/or reactive power required by the load and make the source current sinusoidal, even if mains voltages are distorted. Performance of the instantaneous reactive power theory is affected under distorted mains conditions. Also, reactive power requirement is not completely compensated due to the waveform difference between the mains voltages and currents. A new control algorithm is therefore proposed for the compensation of customer generated harmonics and reactive power. It is capable to maintain similar waveform distortion in the compensated current, as present in the utility voltage. So that load iii behaves as a linear/resistive load and the resultant source current will have the same waveform as that of the supply voltage. As both the source voltages and currents have similar shape, the reactive power is compensated completely. Hence, unity power factor operation can be achieved, even under distorted mains. The proposed scheme provides an additional feature of compensation of only harmonics, in addition to the compensation of both harmonics and reactive power simultaneously. Also, unlike instantaneous reactive power theory, it is applicable for single-phase systems as well. The control algorithm is described in detail to extract the compensating current component, from the distorted voltage and current signals. A simulation model is developed and various simulation results are presented under both ideal and distorted mains conditions, for the compensation of only harmonics as well as both harmonics and reactive power simultaneously. Its viability is ascertained by implementing it on a laboratory prototype. It allows same distortion level in compensated source current as present in the utility voltage, thereby attributing the responsibility of the utility and the customer at the PCC, in deregulated power production system. So that utilities are also forced to maintain the voltage distortion within specified limits, and customer is not penalized for that. To summarize, first of all, a survey is carried out at the consumer as well as utility end, to know the existing level of harmonic pollution present in the system and future trends. A Shunt Active Power Filter is then designed, developed and investigated for power quality improvement, by compensating harmonics, reactive power requirement of the nonlinear loads, and input current balancing in three-phase, three-wire systems, and neutral current as well in three-phase, four-wire systems. Extensive simulation and experimental results are obtained under different loading conditions, to investigate the steady state and transient performance. A fuzzy logic controlled system is developed to improve the performance and compared with the conventional PI controller. Limitations of the available schemes are identified and an attempt is made to overcome them. A new control algorithm is proposed for the compensation of customer generated harmonics and reactive power. Its viability is ascertained by simulation results and verified experimentally.