Review On Active Power Filter

the general categories of power quality problems. A swell is defined as an increase to between 1.1 and 1.8 p.u. rms voltage at the network fundamental frequency with duration from 0.5 cycles to one minute. The term momentary 

overvoltage is also used as a synonym for swell. Switching off a large inductive load or energizing a large capacitor bank is typical system maneuvers that cause swells.Although not as common as voltage sags, swells are also usually associated to system faults. The severity of a 

voltage swell during a fault condition is a function of the fault location, system impedance, and grounding. During a single phase-to ground fault on an impedance grounded system, i.e. with some zero sequence impedance, the non-faulted phase-to-ground voltages can increase up to three times the per-unit value (in the case of a non-grounded or high impedance grounded system). The difference in the zero- and positive-

sequence impedance causes a change in the non-faulted phases, not only in magnitude but also in phase. For voltage swells the start threshold is equal to 110% of the reference voltage. The end threshold is usually set 1 - 2% of the reference voltage below the start threshold. In other words, 

the duration of a voltage swell is measured from when one phase rises above 110% of the reference voltage until all three phases have again fallen below 108% - 109% of the reference voltage.

GENERAL CAUSES OF VOLTAGE SWELLS

Voltage swells are usually associated with system fault condition- just like voltage sags but are much less common. This is particularly true for ungrounded or floating delta system, the sudden change in ground reference result in a voltage swell in phase.Power system harmonics

Power system harmonics are integer multiples of the fundamental power system frequency. Power system harmonics are created by non-linear devices connected to the power system.Harmonics are voltage and current 

frequencies riding on top of the normal sinusoidal voltage and current waveforms. The presence of harmonics (both current and voltage) is viewed as `pollution' affecting the operation of power systems.The most common source of harmonic distortion is electronic equipment using switch-mode power supplies, such as computers, adjustable-speed drives, and high-efficiency electronic light ballasts. Harmonic waveforms are characterized by their amplitude and harmonic number. When a sinusoidal voltage is applied to a certain type of load, in which the load cause the current to vary disproportionately with the voltage during each cyclic period. These are classified as nonlinear loads, and the current taken by them will be a non sinusoidal waveform. When there is 

significant impedance in the path from the power source to a nonlinear load, these current distortions will also produce distortions in the voltage waveform at the load. Waveform distortion can be mathematically analyzed to show that it is equivalent to superimposing additional frequency components onto a pure sine wave (figure 2.1). These frequencies are 

harmonics (integer multiples) of the fundamental frequency, and can sometimes propagate outwards from nonlinear loads, causing problems elsewhere on the power system.The harmonics generated by the most common non-linear loads have the following properties:

• Lower order harmonics tend to dominate in amplitude.

• If the waveform has half-wave symmetry there are no even harmonics.

• Harmonic emissions from a large number of non-linear loads of the same type will be added.

The figure 2.1 shows the effect of harmonics on normal voltage or current waveform [5]. Odd harmonics 

waveforms contribute more to power system instability. In the figure the combined waveform shows the result of 

adding the harmonics on to the fundamental.

Harmonics in power systems can become the source of a variety of unwelcome effects. For example, 

harmonics can cause signal interference, over voltages, data loss, and circuit breaker failure, as well as equipment 

heating, malfunction, and damage. Any distribution circuit serving modern electronic devices will contain some degree 

of harmonic frequencies. The greater the power drawn by nonlinear loads, cause greater the level of voltage distortion. 

Potential problems (or symptoms of problems) attributed to harmonics include:

• Malfunction of sensitive equipment

• Random tripping of circuit breakers

• Flickering lights

• Very high neutral currents

•Overheated phase conductors, panels, and transformers 

•Premature failure of transformers and uninterruptible power supplies (UPSs)

•Reduced power factor

• Reduced system capacity (because harmonics create additional heat, transformers and other distribution equipment cannot carry full rated load) In addition, the harmonic currents produced by nonlinear loads can interact adversely with a wide range of power system equipment, most notably capacitors, transformers, and motors, causing additional losses, overheating, and 

overloading. These harmonic currents can also cause interferences with telecommunication lines and errors in metering devices. Because of the adverse effects that harmonics have on power quality, Standard has been developed to define a reasonable framework for harmonic control. Harmonic distortion in power distribution systems can be suppressed using different approaches. One among them is the use of active power filters.

Active power filters (APF)

Harmonic distortion in power distribution systems can be suppressed mainly by, passive and active filtering. The passive filtering is the simplest conventional solution to mitigate the harmonic distortion. The uses of passive 

elements do not always respond correctly to the dynamics of the power distribution systems. Passive filters are known to cause resonance, thus affecting the stability of the power distribution systems. Frequency variation of the power distribution system and tolerances in components values affect the passive filtering characteristics. As the regulatory requirements become more stringent, the passive filters might not be able to meet future revisions of a particular Standard. This may required a retrofit of new filters.Remarkable progress in power electronics had spurred interest in Active Power Filters (APF) for harmonic distortion mitigation. Active filtering is a relatively new technology, practically less than four decades old. The basic 

principle of APF is to utilize power electronics technologies to produce specific current components that cancel the harmonic current components caused by the nonlinear load.

APFs have a number of advantages over the passive filters. First of all, they can suppress not only the supply current harmonics, but also the reactive currents. Moreover, unlike passive filters, they do not cause harmful resonances with the power distribution systems. Consequently, the APFs performances are independent on the power distribution system properties. Active filtering is a relatively new technology, practically less than four decades old. There is still a need for further research and development to make this technology well established.

Working of APF

The below Figure  shows the components of a typical APF system and their connections. The compensation reference signal from the estimator drives the overall system controller. This in turn provides the control for the gating 

signal generator. The output of the gating signal generator controls the power circuit via a suitable interface.

Finally, the power circuit in the generalized block diagram can be connected in parallel, series or parallel/series 

configurations depending on the interfacing inductor/transformer used. An unfavorable but inseparable feature of APF 

is the necessity of fast switching of high currents in the power circuit of the APF.

An active power filter can be considered as a compensator for power system harmonics. The working of active 

power filter consists of mainly three stages [7]. They are:

1. Signal conditioning

2. Derivation of compensating signal.

3. Generation of gating signal.

Signal conditioning refers to the detection or sensing of harmonics in the power distribution line. As shown in Figure , the reference signal to be processed by the controller is the key component that ensures the correct operation of 

APF. The reference signal estimation is initiated through the detection of essential voltage/current signals to gather accurate system variables information. The voltage and current variables in power system is sensed by using potential 

transformers, current transformers, isolation amplifiers etc. The voltage variables to be sensed are AC source voltage, DC-bus voltage of the APF, and voltage across interfacing transformer. Typical current variables are load current, AC 

source current, compensation current and DC-link current of the APF. Based on these system variables feedbacks, reference signals estimation in terms of voltage/current levels are estimated in frequency-domain or time-domain.

The next stage is the derivation of compensating signal from the disrupted wave consists of both fundamental wave and the harmonic content. It can be done by two different methods-frequency domain approach and time domainapproach. Frequency domain approach use Fourier transformation method for this purpose. While Time domain approach uses different methods like Instantaneous Reactive-Power Theorem, Synchronous-Reference-Frame Theorem, Synchronous Detection Theorem, Sine Multiplication Theorem, notch filter method etc.

The third stage is the generation of gating signal for harmonic suppression. So many control techniques like space Vector PWM, repetitive control, hysteresis current control, one-cycle control, dead-beat control, sliding mode 

control, fuzzy control and the artificial neural network method have been introduced and applied to various configurations of active power filters. Gating signal generator in the general block diagram of APF is used for this 

purpose.