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APPLICATION OF A SHUNT ACTIVE POWER FILTER TO COMPENSATE MULTIPLE NON-LINEAR LOADS
ABSTRACT
In this project, the implementation of a shunt active power filter with a small series reactor for a three-phase system is presented. The system consists of multiple non-linear loads, which are a combination of harmonic current sources and harmonic voltage sources, with significant unbalanced components. The filter consists of a three-phase current-controlled voltage source inverter (CC-VSI) with a filter inductance at the ac output and a dc-bus capacitor. The CC-VSI is operated to directly control the ac grid current to be sinusoidal and in phase with the grid voltage. The switching is controlled using ramptime current control, which is based on the concept of zero average current error. The simulation results indicate that the filter along with the series reactor is able to handle predominantly the harmonic voltage sources, as well as the unbalance, so that the grid currents are sinusoidal, in phase with the grid voltages and symmetrical.
2. INTRODUCTION
Non-linear loads, especially power electronic loads, create harmonic currents and voltages in the power systems. For many years, various active power filters (APF) have been developed to suppress the harmonics, as well as compensate for reactive power, so that the utility grid will supply sinusoidal voltage and current with unity power factor.
Conventionally, the shunt type APF acts to eliminate the reactive power and harmonic currents produced by non-linear loads from the grid current by injecting compensating currents intended to result in sinusoidal grid current with unity power factor. This filter has been proven to be effective in compensating harmonic current sources, but it cannot properly compensate for harmonic voltage sources. Many electronic appliances, such as switched mode power supplies and electronic ballasts, are harmonic voltage sources. A voltage sourcing series active power filter is suitable for controlling harmonic voltage sources, but it cannot properly compensate for harmonic current sources.
In many cases, non-linear loads consist of combinations of harmonic voltage sources and harmonic current sources, and may contain significant load unbalance (ex. single phase loads on a three phase system). To compensate for these mixed non-linear loads, a combined system of a shunt APF and a series APF can be effective .
In this paper, a combination of a grid current forcing shunt APF with a series reactor installed at the Point of Common Coupling (PCC) is investigated to handle the harmonic and unbalance problems from mixed loads 3. SHUNT ACTIVE POWER FILTER OPERATION
The three-phase shunt active power filter is a three-phase current controlled “voltage source inverter” (CC-VSI) with a mid-point earthed, split capacitor in the dc bus and inductors in the ac output .
Conventionally, a shunt APF is controlled in such a way as to inject harmonic and reactive compensation currents based on calculated reference currents. The injected currents are meant to “cancel” the harmonic and reactive currents drawn by the non-linear loads. However, the reference or desired current to be injected must be determined by extensive calculations with inherent delays, errors and slow transient response.
3.1 Series Inductance
A key component of this system is the added series inductance XL (see Figure 2), which is comparable in size to the effective grid impedance, ZS. Without this inductance (or a series active filter), load harmonic voltage sources would produce harmonic currents
through the grid impedance, which could not be compensated by a shunt APF. Currents from the APF do not significantly change the harmonic voltage at the loads. Therefore,
there are still harmonic voltages across the grid impedance, which continue to produce harmonic currents..
3.2 Direct Control of the Grid Current
In this scheme (see Figure 1), the CC-VSI is operated to directly control the ac grid current rather than it’s own current. The grid current is sensed and directly controlled to follow symmetrical sinusoidal reference signals in phase with the grid voltage. Hence, by putting the current sensors on the grid side, the grid current is forced to behave as a sinusoidal current source and the grid appears as a high-impedance circuit for harmonics. By forcing the grid current to be sinusoidal, the APF automatically provides the harmonic, reactive, negative and zero sequence currents for the load, following the basic current summation rule:
igrid = iAPF + i load
The sinusoidal grid current reference signal is given by:
iref = k vgrid-1
where vgrid-1 is the fundamental component of the grid voltage, and k is obtained from
an outer control loop regulating the CC-VSI dc-bus voltage.