ACTIVE POWER FILTERS (Electric Motor)

10.3
With the increase of nonlinear loads such as ASDs drawing nonsinusoidal currents, power quality distortion has become a serious
Operating modes of the modified C-dump converter.
FIGURE 10.16 Operating modes of the modified C-dump converter.
Phase voltage and current and supply voltage and current waveforms.
FIGURE 10.17 Phase voltage and current and supply voltage and current waveforms.
problem in power systems. Active filters (AFs) are used for harmonic mitigation as well as reactive power compensation, load balancing, voltage regulation, and voltage flicker compensation. Based on topology, there are two kinds of active filters: current source and voltage source. Current-source active filters (CSAFs) employ an inductor as the DC energy storage device. In voltage-source active filters (VSAFs), a capacitor acts as the energy storage element. VSAFs are cheaper, lighter, and easier to control compared to CSAFs. There are also four types of active filters based on the system configuration.
Current-source active filters use a current-source inductor. This type of energy source is commonly used in shunt-type active filters. Different configurations for this kind of active filter have been developed in forms of low-power, single-phase or high-power, three-phase three-wire or four-wire systems. In the three-phase, four-wire system, load current unbalance can be compensated in addition to current harmonics and reactive current.
In CSAFs, the DC current of the energy storage inductor must be greater than the maximum harmonic of the load (maximum deviation of source current from reference value). If the current of the DC inductor is too small, the inverter cannot do proper compensation. This DC current should not be too much. If the current is too much, excessive loss results in the inductor and inverter; a passive filter cannot cancel switching frequency. There is no need for the DC power supply because an active filter only delivers reactive power and a small amount of fundamental current needed to compensate the AF losses.
A small capacitor is used to protect switches against over-voltages and also to make a low-pass LC filter with the inductor between the active filter and system to suppress switching frequency. For preventing resonance, the resonance frequency of the passive filter must be greater than the highest frequency of harmonics and considerably less than the switching frequency. The control strategy must be well designed to prevent this resonance.
The most dominant type of active filter is the voltage-source inverter (VSI) active filter. Their design has been improved and they have been used for many years; now they are at the commercial
stage. They are lighter, cheaper, and easier to control compared to the current-source inverter (CSI) type. Their losses are less than CSAFs, and they can be made in multilevels and multisteps.
Voltage-source active filters employ a capacitor as the DC energy storage. They are presented in single-phase or three-phase, three-wire or four-wire systems. This kind of active filter is convenient in uninterruptible power supply (UPS) systems. In UPS systems, DC energy storage is available and a DC/AC inverter is also ready. Only a control strategy is needed to convert the UPS to an AF when the source is in normal condition. Different kinds of control techniques are used to control VSAFs. The well-known control techniques are the instantaneous d-q theory, synchronous d-q reference frame method, and synchronous detection method.
In VASF, the DC voltage of the energy storage capacitor must be greater than the maximum line voltage. For proper operation of the active filter, at any instant the voltage of the DC capacitor should be 1.5 times of the line maximum voltage. A linking inductor establishes a link between the filter and system. The AF delivers its current to the system through the inductor. For controllability of AF, this inductor should not be large.
Active filters can also be classified as shunt, series, and hybrid. The most popular type of AF is the shunt type. Shunt AFs can be single-phase or three-phase, VSI or CSI. Shunt AFs are used to compensate the current and voltage harmonics of nonlinear loads, to perform reactive power compensation, and to balance unbalance currents. A shunt AF senses the load current and injects an appropriate current into the system based on its control function. Shunt AFs are currently commercially available.
A shunt AF acts as a current source. The sum of its current and load current is the total current, which flows through the source. Therefore, controlling the output current of an AF can control the source current. Ratings of series and shunt AFs have been compared in some papers. Based on those studies, the shunt AFs has approximately half the switch power rating of series AFs. The peak voltage over switches in series AFs is about one-third the peak voltage over switches in shunt AFs.
Series AFs can also be single-phase or three-phase and employ voltage-source or current-source inverters. Series AFs are mostly used to compensate voltage harmonics produced by nonlinear loads as well as voltage regulation and voltage unbalance compensation.
Series AFs are located in series between source and nonlinear loads. In the presence of source-side impedance, voltage harmonics of the nonlinear load appear at the point of common coupling. Series AFs sense the load-side voltage and produce the harmonic of load voltage in the negative direction and makes the voltage at the point of common coupling free of harmonics.
The main purpose of using a hybrid of active and passive filters is reducing the initial cost of the filter and improving the efficiency. Many configurations and combinations of active and passive filters have been studied and developed. Experimental results of combination series and shunt AFs with shunt passive filters are presented in many papers. Usually, the passive filter is tuned to specific frequency to suppress that frequency, decreasing the power rating of AF. Shunt passive filters should also be high pass to cancel the switching frequency of the AF and high-frequency harmonics. In this case, the switching frequency of the AF will decrease.
Another problem which AFs are faced with is high fundamental current through series AFs and high fundamental voltage across shunt AFs. Paralleling of series AFs with a passive filter can solve high current problems in series AFs. A proper control strategy should be adopted to avoid the possibility of resonance. High voltage across shunt AFs is reduced by putting the shunt AF in series with a passive filter.
Unified power quality conditioners (UPQCs), also known as universal AFs, are ideal devices to improve power quality. A combination of series and shunt AFs forms the UPQC. Series AFs suppress and isolate voltage harmonics, and shunt AFs cancels current harmonics. Usually, the energy storage device is shared between two AFs, either in CSI or VSI. There are two kinds of UPQC. In the first type, a shunt AF is placed near the source and a series AF is placed near the load. The series AF is used to compensate voltage harmonics of the load and the shunt AF is used to compensate residual current
harmonics and improve power factor or to balance the unbalanced load. In the second type, a shunt AF is placed near the load to compensate current harmonics of load, and a series AF is placed near the source to compensate voltage harmonics of the source or regulate the voltage.
In conclusion, IEEE and other international standards are imposing limits on harmonic voltages and currents. Many power electronic circuit designs have been proposed to deal with these standards. Effectiveness of active PFC is normally not a problem, but the cost involved in the additional power electronic circuit could be a major obstacle to acceptance. The simplest power factor correction method is to use passive LC filters to comply with IEC and IEEE standards. Although these passive PFC methods comply with the standards, the problems of EMI, EMC, and size of the passive elements involved must be addressed. Thus, the design of cost-effective power electronic equipment that complies with harmonic standards without introducing side effects or system interaction problems remains an open challenge to power electronic and motor drive engineers.


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