Semester 3 Seminar Apf Kaviraj

GOVT. COLLEGE OF ENGG. AMRAVATI

1.INTRODUCTION Active power filter (APF) is a very useful tool for eliminating harmonic pollution in power systems. As compared with conventional passive filters APF has significant advantages such as good controllability, fast response and high control accuracy etc. APF can also compensate non-characteristic harmonics, which makes it extremely attractive in certain circumstances. With the development of power electronic technology, APF finds its wide use in the modern industry. Power harmonic pollution due to nonlinear loads used in industry products, thyristor controlled inductors used for the static var compensator, and ac/dc converters for ac traction system and high voltage dc transmission has been serious in the power quality of transmission or distribution system. The harmonic currents result in degrading the power quality in distribution system. Many circuit configurations of filters have been proposed to limit the line current distortion. Passive filters with low impedances at the dominant harmonic frequencies were used to reduce the harmonics for the consideration of hardware cost. However, these circuit configurations have several drawbacks. The passive filters with fixed compensation characteristics are ineffective to filter the current harmonics. The series or parallel resonance is happen between the system impedance and passive filters. The developments and applications of active filters have been researched because of the increasing concern the power quality at the consumer or distribution side. Active filters overcome the drawbacks of passive filters by using the switching mode power converter to perform the harmonic current elimination. Shunt active filters are developed to suppress the harmonic currents and compensate reactive power simultaneously. The shunt active filters are operated as a current source parallel with the nonlinear load. The power converter of active filter is controlled to generate a compensation current which is equal-but-opposite the harmonic and reactive currents generated from the nonlinear load. In this situation, the mains current is sinusoidal and in phase with mains voltage. However, the construction cost of active filters in a practical industry is too high. The power rating (utility voltage and harmonic currents) of power converter in active filters is very large. These limit the applications of active filters used in the power system. Hybrid active filter topologies have been developed to solve the problems of harmonic currents and reactive power effectively. Using low cost passive KAVIRAJ

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filters in the hybrid active filter, the power rating of active converter is reduced compared with that of pure active filters. Hybrid active filters retain the advantages of active filters and have not the drawbacks of passive and active filters. The hybrid active filters are cost-effective and become more practical in industry applications. Active power filters have been used to improve electric power quality through harmonic mitigation correction of voltage sags and balancing the unbalanced line currents .Almost all of the existing active power filters are realized by one single-phase or three-phase bridge converter. The power rating and the switching frequency of the converter are determined by the magnitude of the distortion current and the desired bandwidth of the filter, respectively. The combination of high power and high switching frequency results in excessive amounts of power losses. Furthermore, the reliability of the existing active filters is a major concern, as the failure of the converter results in no compensation at all. The inherent nonlinear characteristics of loads such as compact fluorescent lamps, color televisions, computers, light dimmers and adjustable speed drives for heat pumps and air-conditioning have made harmonic distortion a common practice in electrical distribution systems. Harmonic currents injected by such loads are, individually, too small to cause a pronounced distortion in distribution feeders. However, due to the proliferation of such loads, this situation will likely change. Yet, when it operates in large numbers, the cumulative effect of these loads has the capability of boosting the distribution system harmonic distortion levels.

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2. PRINCIPLE OF WORKING Active filters are the modern approach to the harmonic filtering .They act as an ideal current or voltage sources connected in shunt or in series with the ac supply to cancel the harmonics terminal voltages or to supply harmonic currents produced by both the internal harmonic voltages of the supply and non-linear load. The principle of an active filter is to inject in ac system a controlled current consisting of harmonic currents in phase opposition so that a cancellation of harmonic current occurs thus eliminating harmonics in the A.C. system. The active filter injects the harmonic current spectrum in opposite phase to the measured distorting harmonics currents. The original harmonics are their by cancelled. The harmonic current on the ac bus generated by active filter have opposite phase to the harmonic current present in the ac system. The control of an active filter in combination with the active generation of the compensating current allows for a concept that the active filter may not be overloaded and thus current exiding the capacity of active filter will remain on the network but the filter will operate and eliminate all harmonic currents up to its capacity. The active filters uses power electronic switching to generate harmonic currents that cancel the harmonic currents from a non-linear load (present on the ac bus). Active filters were developed to mitigate the problem of the passive filters. The optimal active filter is an application and utility interface. The active filter were developed to mitigate the problems of passive filter.There are many more configurations of an active filters. The active filters were developed in the later 80’s up to the time passive filter were in existence. Hence now days HYBRID ACTIVE FILTER are more common since they are the combination of an existing passive filter with a newly added active filter unit. The hybrid active filter provides a cost effective and practical harmonic compensation particularly for high power non-linear loads. Hybrid active filter improve the compensation characteristic of the passive filters. The active filter uses power electronic switching to generate harmonic currents that cancel the harmonic currents from a nonlinear load. The active filter configuration investigated based on a pulse-width modulated (PWM) voltage source inverter that interfaces to the system through a system interface filter as shown in Figure 1. In this

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configuration, the filter is connected in parallel with the load being compensated. Therefore, the configuration is often referred to as an active parallel filter. Figure 1 illustrates the concept of the harmonic current cancellation so that the current being supplied from the source is sinusoidal. The active filter uses power electronic switching to generate harmonic currents that cancel the harmonic currents from a nonlinear load. The active filter configuration is based on a pulse-width modulated (PWM) voltage source inverter that interfaces to the system through a system interface filter as shown in Figure 1. In this configuration, the filter is connected in parallel with the load being compensated. Therefore, the configuration is often referred to as an active parallel filter. Figure 1 illustrates the concept of the harmonic current cancellation so that the current being

supplied

from

the

source

is

sinusoidal.

Figure 1. Diagram illustrating components of the shunt connected active filter with waveforms showing cancellation of harmonics from non-linear load.

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The voltage source inverter used in the active filter makes the harmonic control possible. This inverter uses dc capacitors as the supply and can switch at a high frequency to generate a signal which will cancel the harmonics from the nonlinear load. One leg of the inverter is shown in Figure 2 to illustrate the configuration.

Figure 2. One line diagram for one leg of the active filter.

The active filter does not need to provide any real power to cancel harmonic currents from the load.The harmonic currents to be canceled show up as reactive power. Reduction in the harmonic voltage distortion occurs because the harmonic currents flowing through the source impedance are reduced. Therefore, the dc capacitors and the filter components must be rated based on the reactive power associated with the harmonics to be canceled and on the actual current waveform (rms and peak current magnitude) that must be generated to achieve the cancellation.The current waveform for canceling harmonics is achieved with the voltage source inverter and an interfacing filter. The filter consists of a relatively large isolation inductance to convert the voltage signal created by the inverter to a current signal for canceling harmonics. The rest of the filter provides smoothing and isolation for high frequency components. The desired current

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waveform is obtained by accurately controlling the switching of the insulated gate bipolar transistors (IGBTs) in the inverter. Control of the current wave shape is limited by the switching frequency of the inverter and by the available driving voltage across the interfacing inductance. The driving voltage across the interfacing inductance determines the maximum di/dt that can be achieved by the filter. This is important because relatively high values of di/dt may be needed to cancel higher order harmonic components. Therefore, there is a tradeoff involved in sizing the interface inductor. A larger inductor is better for isolation from the power system and protection from transient disturbances. However, the larger inductor limits the ability of the active filter to cancel higher order harmonics.

Fig:3 load current of thyristor controlled induction motor drive

Fig: 4 compensating current fed by active filter

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Fig:5 load current after compensation by active filter

Thus the implementation of active filter has high performance characteristics. We can use active filters for active filtering ,power factor compensation and load unbalance compensation. It estimates and impose sinewave current reference even in the case of highly distorted system voltages. In transient operation the unity power factor is obtain immediately. The active filters are superior than any other filter in the view that the active filters may not be overloaded as the control of active filter in combination with active generation of compensating currents follows for a concept that they may not be overloaded. The active filter has another advantage that unlike passive filters it requires less space and do not require huge structures like supporting beam, big insulators etc.

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3.Benefits of Active Harmonic Filters � Provide sustainable system benefits from: 1. Reduced Electrical Power Bills because of reduced consumption 2. Reduced Electrical Power Bills by limiting distortion 3. Improved Manufacturing Output by reducing intermittent batch losses 4.Improved Quality through increased integrity of systems,controls and data. 5.Improved Productivity through decreased troubleshooting and maintenance 6. Allows for local power-conditioning within user-defined compensation areas or facility-wide compensation to: a) Improve power factor ratings through harmonic recombination b) Isolate harmonic currents c) Minimize equipment problems through THD cancellation d) Reduce wasted energy f) Minimize current carried in plant cables.

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4. REFERENCE 1)

Mark McGranaghan: 'Active filter design and specification for control of Harmonics in Industrial and Commercial facilities'. Electrotek Concept Inc., Knoxville TN,USA.

2)

Lawu Zhou, Shanjun Qi, Lijun Huang, Zhiguang Zhou, Yinghao Zhu,: 'Design and Simulation of a Control System with a Comprehensive Power Quality in Power System.'

College of Electrical and Information Engineering, Hunan University, Changsha 410082, P.R.China 3)

Murat Kale, Engin Ozdemir,: 'An adaptive hysteresis band current controller for shunt active power filter.' Electrical Education Department, Technical Education Faculty,

Kocaeli University, 41100 Izmit, Turkey 4)

M.Karimi-Ghartemani, M.R.Iravani and F. Katiraei.: 'Extraction of signal for harmonic, reactive current and network un-balance compensation.' IEE Proc-Gener. Transm.

Distrib., Vol.152 No. 1, January 2005

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