25-01-2013, 03:04 PM
Shunt Active Filter for Power Quality Improvement
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Abstract
This paper describes the development of a low cost shunt active power filter with digital control, which allows
dynamic power factor correction and both harmonics and zero-sequence current compensation. The active filter
controller is based on the instantaneous power theory (p-q theory) and was implemented using a standard 16 bits
microcontroller. The p-q theory is introduced followed by the presentation of some active power filters topologies. Then
a brief description of the implemented solution is made, including references to software tools used for simulation and
system development. Experimental results are also presented, showing the good performance of the developed active
filter.
Introduction
Due the intensive use of power converters and other non-linear loads in industry and by consumers in general, it
can be observed an increasing deterioration of the power systems voltage and current waveforms. The explanation is
simple. Accordingly to Fig. 1, which presents a single-phase system, the voltage across the load terminals is:
where Δv is the voltage drop in the power lines impedances. Even if the supply voltage vs is a pure sinusoid, the
non-linear load input current is not, and as a result, the supply current
includes harmonics which makes both the voltage drop Δv and the load voltage (vL) non-sinusoidal.
The p-q theory
In 1983, Akagi et al. [1, 2] have proposed the "The Generalized Theory of the Instantaneous Reactive Power in
Three-Phase Circuits", also known as p-q theory. It is based in instantaneous values in three-phase power systems with
or without neutral wire, and is valid for steady-state or transitory operations, as well as for generic voltage and current
waveforms. The p-q theory consists of an algebraic transformation (Clarke transformation) of the three-phase voltages
and currents in the a-b-c coordinates to the α-β-0 coordinates.
The p-q theory applied to active filters
From all the power components obtained through the p-q theory, only p and p0 are desirable, as they
correspond to the energy transferred from the supply to the load. The other quantities can be compensated using a shunt
active power filter (Fig. 3). Even p0 , which is related to a load unbalance (an undesirable operation condition), should
be compensated whenever possible. Watanabe et al. [3, 4] presented a way to compensate p0 , without the need of
using any power supply in the active filter. They showed that the value of p0 can be delivered from the power source to
the active filter through the α-β coordinates, and then the active filter can supply this power to the load through the 0
coordinate (see Fig. 3). This means that the energy previously transferred from the source to the load through the
zero-sequence components of voltage and current, is now delivered from the source phases through the active filter, in a
balanced way.
It is also possible to see in Fig. 3 that the active filter capacitor is only necessary to compensate p~ and 0 p ~ ,
since these quantities must be stored in this component at one moment to be later delivered to the load. The
instantaneous imaginary power (q), which includes the conventional reactive power, can be compensated without any
capacitor.
Active Filters
There are basically two types of active filters: the shunt type and the series type. It is possible to find active
filters combined with passive filters as well as active filters of both types acting together.
Fig. 4 presents the electrical scheme of a shunt active filter for a three-phase power system with neutral wire,
which is able to compensate for both current harmonics and power factor. Furthermore, it allows load balancing,
eliminating the current in the neutral wire. The power stage is, basically, a voltage-source inverter controlled in a way
that it acts like a current-source. From the measured values of the phase voltages (va, vb, vc) and load currents (ia, ib, ic),
the controller calculates the reference currents (ica*, icb*, icc*, icn*) used by the inverter to produce the compensation
currents (ica, icb, icc, icn). This solution requires 6 current sensors and 4 voltage sensors, and the inverter has 4 legs
(8 power semiconductor switches).
For balanced loads (three-phase motors, three-phase adjustable speed drives, three-phase controlled or
non-controlled rectifiers, etc) there is no need to compensate for the current in neutral wire. These allow the use of a
simpler inverter (with only three legs) and only 4 current sensors. It also eases the controller calculations.
Fig. 5 shows the scheme of a series active filter for a three-phase power system. It is the dual of the shunt active
filter, and is able to compensate for distortion in the power line voltages, making the voltages applied to the load
sinusoidal (compensating for voltage harmonics). The filter consists of a voltage-source inverter (behaving as a
controlled voltage source) and requires 3 single-phase transformers to interface with the power system.
The series active filter does not compensate for load current harmonics but it acts as high-impedance to the
current harmonics coming from the power source side. Therefore, it guarantees that passive filters eventually placed at
the load input will not drain harmonic currents from the rest of the power system. Another solution to solve the load
current harmonics is to use a shunt active filter together with the series active filter (Fig. 6), so that both load voltages
and the supplied currents are guaranteed to have sinusoidal waveforms.
Simulation tools
Simulation is a powerful way to reduce development time and ensure the proper fulfilment of critical steps.
During the development process of the shunt active filter, simulations were performed, which allowed the study of its
behaviour under different operation conditions, and permitted the tuning of some controller parameters together with
the optimisation of the active filter components values. There are not many simulation tools that allow working with
electrical systems, power electronics and control systems, in the same integrated environment. Matlab/Simulink and the
Power System Blockset were used as simulation tools in this case and are briefly described in the following paragraphs.
Matlab/Simulink
Matlab is a high-level language oriented toward engineering and scientific applications. It has evolved over a
ten-year history to become a popular, flexible, powerful, yet simple language. It has served as an effective platform for
more than twenty toolboxes supporting specialised engineering and scientific applications, covering areas from
symbolic computation to digital filter design, control theory, fuzzy logic, and neural nets. It is to be used interactively,
and supports also the ability to define functions and scripts, and dynamically links with C and Fortran programs. Recent
trends in the Matlab language have focused on an object-oriented graphics capability that permits a rich Graphical User
Interface (GUI) construction [5].
The implemented shunt active filter
The implemented shunt active filter consists, basically, of a digital controller and a current-controlled voltage
source inverter (Fig. 4).
The controller implementation is based in a standard 16-bit microcontroller (Intel 80296SA) with minimum
additional hardware to interface with the sensors and the inverter. Using the p-q theory, the controller calculates the
reference compensation currents to be synthesized by the inverter. These calculations were optimised (namely using
integer programming) to save CPU time and memory and improve the active filter dynamic and steady-state response.
The microcontroller software was developed using the Tasking EDE (Embedded Development Environment) for
196/296. This compiler, specific for the Intel MCS96 family of microcontrollers, allows the development of programs
in C or Assembler language.
The inverter, detailed in Fig. 7, uses a single capacitor in the DC side (the active filter does not need any power
supply). Current-control is implemented, basically, comparing the reference currents calculated by the controller with
the measured values of compensation currents, in order to produce command signals for the inverter semiconductor
switches. An inductive filter is placed at the inverter output to limit the ripple of the compensation currents. An RC
high-pass filter is set in the active filter output to filter the inverter commutation frequencies.
The capacitor voltage, Vdc , must be regulated, which is guaranteed by the same microcontroller that generates
the current references.
Conclusions
This paper presents a shunt active power filter as a reliable and cost-effective solution to power quality
problems.
The active filter controller is based on the p-q theory, which proved to be a powerful tool, but simple enough to
allow the digital implementation of the controller using a standard and inexpensive microcontroller with minimum
additional hardware.
The filter presents good dynamic and steady-state response and it can be a much better solution for power factor
and current harmonics compensation than the conventional approach (capacitors to correct the power factor and passive
filters to compensate for current harmonics). Besides, the shunt active filter can also compensate for load current
unbalances, eliminating the neutral wire current in the power lines. Therefore, this active filter allows the power source
to see an unbalanced reactive non-linear load, as a symmetrical resistive load.
The proposed low-cost solution allows the use of a large number low-power active filters in the same facility,
close to each problematic load (or group of loads), avoiding the circulation of current harmonics, reactive currents and
neutral currents through the facility power lines. This solution reduces the power lines losses and voltage drops, and
avoids voltage distortions at the loads terminals.