Analysis and Simulation of Shunt Active Power Filter For Harmonic Cancellation of Non Linear Loads

Use of nonlinear loads, s uch a s power co nverters, pow er sources, uninterruptible power supply (UPS) units, and a rc devices like electric furnaces and fluorescent lamps and large adjusta ble speed motor drives, is expected to grow rapidly. All of these loads inject harmonic currents and reactive power into the power system. This paper presents a study a nd simulation of a three phase active power filter. A mult ilevel PWM inverter is used with current control technique. The proposed control system is very simple an d therefore practical impleme ntation of active power filters is available. The presented system is able to compensating current harmonics, reactive power a nd unbalance current of non linear loads. The total harmonic distortion is calculated for deferent load before and aft er filtering wit h deferent typ e of filter. A comparison is made between passive filter and active. The results have been obtained using softwa re cal led P SIM, which has demonstrated i ts re liability f or almost 10 years of simulations, which have been approved with real experimental results.


1-Introduction
Ideally, an electricity supply should always show a perfectly sinusoidal current wave form at every customer location.However, for many reasons, utilities, it is difficult to keep such conditions.The deviation of the current waveforms from sinusoidal is described in terms of the waveform distortion, often expressed as harmonic distortion.[1] The increasing in use of nonlinear loads in industry is keeping harmonic distortion in distribution networks to increase.The most used nonlinear device is perhaps the static power converter so widely used in industrial applications such as the steel, paper, and textile industries.[1,2] A harmonic component in an AC power system is defined as a sinusoidal component of a periodic waveform that has a frequency equal to an integer multiple of the fundamental frequency of the system.
Where (h) is an integer.
Conventionally, passive filters were the choice for the elimination of harmonics and to improve power factor.These passive filters have the disadvantages such as large size, resonance and fixed compensation.Active filters avoid the disadvantages of passive filters by using a switch mode power electronic converter to supply harmonic currents equal to those in the load currents.[2] 2-Non linear load Nonlinear loads are loads in which the current waveform does not take the shape of the applied voltage waveform due to a number of reasons, for example, the use of electronic switches that conduct load current only during a fraction of the power frequency period.Therefore, we can describe the nonlinear loads as those in which Ohm's law cannot describe the relation between V and I.There are many nonlinear loads drawing nonsinusoidal currents from electrical power systems as shown in figure (1).These non sinusoidal currents pass through different impedances in the power systems and produce voltage harmonics.These voltage harmonics propagate in power systems and affect all of the power system components.[1,3,4] Total harmonic distortion (THD) is an important index used to describe power quality in power system.It is define as the contribution of every individual harmonic component on the signal.THD is defining for current as follow: -[3] pulse width modulation (PWM) drives produce harmonics.[1, 4] Among the most common nonlinear loads in power systems are all types of rectifying devices like those found in power converters, power sources, uninterruptible power supply (UPS) units, and arc devices like electric furnaces and fluorescent lamps.Even linear loads like power transformers can act nonlinear under saturation conditions.Figures (2) & (3) represents the source current and its harmonics for different types of load (half wave rectifier and full wave rectifier) are taken as examples in this paper because these type of non linear loads are the most popular kind which are used in most electrical devices such as mobile cell charger, LCD, PC computer and monitors, and they are used in a very large range in life.[4] Except devices such as ovens and furnaces, which produce heat, most of the other electrical loads are sensitive to harmonics.In fact, harmonics may lead to improper operation.The effects of harmonics in power systems and electrical loads are described as the disturbance to Electric and Electronic Devices, and higher losses.[4]

4-Basic Configuration of Shunt Active Filter
Active filters are categorized into two main groups: single-phase and three-phase.Three-phase active filters may be with or without neutral connection.Single phase active filters are used to compensate power quality problems caused by single phase load, and three phase active filters are used for high-power nonlinear loads.Also active filter can be classified according to the type of the utilized inverter to current source inverter or voltage source inverter.Generally; Shunt active power filters compensate current harmonics by injecting a compensating current harmonics equal in magnitude but opposite in direction, i.e. injecting the harmonic components generated by the load but phase shifted by 180.As a result, components of harmonic currents contained in the load current are cancelled by the effect of the active filter, and the source current remains sinusoidal and in phase with the respective phase to neutral voltage.This principle is applicable to any type of load considered as a harmonic source.Moreover, the active power filter can also compensate the load power factor.In this way, the power distribution system sees the non-linear load and the active power filter as an ideal resistor.
The compensation characteristics of the shunt active power filter is shown in Figure .(4). [5] The operation of an active power filter depends basically on the characteristics of the current controller, the method implemented to generate the reference signal and the modulation technique.Most of the modulation techniques used in active power filters are based on PWM strategies with hysteresis band control method.The Hysteresis Band method switches the transistors (Q1 and Q4) when the current error exceeds a fixed magnitude to force the output of the inverter to follow the reference signal as can be seen in frequency is not determined, but it can be estimated because it is affected by the band of the hysteresis controller and the frequency is varying inversely with the bandwidth. .Under the limit condition of zero hysteresis bandwidth, the current error can be forced to zero, so that this condition implies an infinite frequency for the switch commutations, which is, of course, not practical.In real-life implementations, the hysteresis bandwidth is kept sufficiently small to minimize the tracking error without implying too high switching frequencies.[5] Figure (5) shows that the status of the power transistors (Q1 and Q4) of phase (A) are changed whenever the actual current (i) goes beyond a given reference current ( i * ) 2 i ∆ ± .
And identical controllers are used in phase (B) and (C). [6]

5-Control system of active power filter
The model of the proposed three phase multi-level PWM active filter is simulated by using PSIM simulation tool.Figure ( 6) shows a complete system diagram.The three phase source (3-Φ) of 220 volt (r.m.s), 50Hz is feeding a contaminating load, such as a power rectifier.The parameter of the load are R=5Ω, C=470µf, L=0.3mH, this value give a sufficient distorted load current.Figure (7a,7b) shows the load current wave form and harmonics spectrum respectively with THD equal to (46.68%).The instantaneous load current is measured using current sensor and filtered by a second order band-stop filter (notch filter) with center frequency equals to (50) Hz and stopping band equal to (20) Hz. Figure (8) shows the amplitude response of Band Stop Filter.If this center frequency were selected to be equal to 50Hz, the filter circuit shows infinite impedance to 50-Hz currents.Therefore, the band stop filter will block the fundamental frequency, while passing most of the other frequency components.The center frequency is the cut off frequency, and the different between the low cutoff frequency and high cutoff frequency is called the stopping band frequency.Generally the band stop filter must be tuned to eliminate the fundamental frequency of the load current and allow other components of the current to passing through it, i.e. the output of the band-stop filter is the harmonics of the current except the fundamental frequency as shown in Figure (9).The output of the band-stop filter is considered to be the reference current signal (control signal) of the hysteresis band current controller which is forced the multi-level inverter.Table (1) shows the transistor state for one cycle in all phases, figure (10) shows the wave form of all transistors state.[5] Now it is clearly that the output waveform of the inverter is similar to harmonics waveform and this is the advantages of the hysteresis current control as shown in Figure ( 11).[1,5] The output of the inverter must be filtered to eliminate the switching frequency; passive filter is more suitable for this stage by tuning the cutoff frequency of the passive filter equal to switching frequency and PDF created with pdfFactory Pro trial version www.pdffactory.com

6-Conclusions
In this paper a three phase active power filter is proposed using multilevel PWM inverter with hysteresis current control.The active power filter is very effective and flexible and also has lower cost comparing with passive filter, because the passive filter is tuned to eliminate a single frequency, but the active filter is suitable for all frequencies.Also the multilevel PWM technique gives the active filter a good performance comparing with passive filter.Table (2) shows the total harmonic distortion (THD) for different loads with different types of filters, in this paper we chose passive filter, two-level and multilevel PWM active power filter.

Table (1) 3-Φ Transistor state of one cycle
Phase Q1 Q2 Q3 Q4 Phase A Phase shift = 0 This equation can be described as the ratio between the (r.m.s) values of signals including harmonics and signals considering only the fundamental frequency.[3] The total harmonic distortion of two kinds of loads which are taken as examples are: -THD of load figure (a) = 0 THD of load figure (b) = 0.373-Harmonics Source and EffectsGenerally every device that uses power electronic components such as renewable electrical power generation systems, cycloconverters, electronic phase control loads, and PDF created with pdfFactory Pro trial version www.pdffactory.com Figure (5), this type of control needs a single comparator with hysteresis per phase.In this case the switching PDF created with pdfFactory Pro trial version www.pdffactory.com source line current to cancel the harmonics and only the fundamental current is drain from the source.Figure (12a and 12b)  shows the source current and harmonic spectrum respectively.[5] Figure (1-a) Linear Load Current

Table ( 2) THD Comparison between different loads
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