17-08-2013, 03:15 PM
A Simplified Control Algorithm for Three-Phase, Four-Wire Unified Power Quality Conditioner
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Abstract
In this paper, a simplified control algorithm for a three-phase, four-wire unified power quality conditioner (UPQC) is presented
to compensate for supply voltage distortions/unbalance, supply current harmonics, the supply neutral current, the reactive power
and the load unbalance as well as to maintain zero voltage regulation (ZVR) at the point of common coupling (PCC). The UPQC
is realized by the integration of series and shunt active filters (AFs) sharing a common dc bus capacitor. The shunt AF is realized
using a three-phase, four leg voltage source inverter (VSI) and the series AF is realized using a three-phase, three leg VSI. A
dynamic model of the UPQC is developed in the MATLAB/SIMULINK environment and the simulation results demonstrating
the power quality improvement in the system are presented for different supply and load conditions.
I NTRODUCTION
The main objective of electric utility companies is to sup-
ply their customers with uninterrupted sinusoidal voltage of
constant magnitude. However this is becoming increasingly
difficult to do, because the size and number of non-linear
and poor power factor loads such as adjustable speed drives,
computer power supplies, furnaces and traction drives are
increasing rapidly. Due to their nonlinear nature, these solid
state converters cause excessive neutral currents in three phase
four wire systems. Moreover, in the case of the distribution
system, the overall load on the system is seldom found to be
balanced. In the past, the solutions to mitigate these identified
power quality problems [1] were through using conventional
passive filters. But their limitations such as, fixed compen-
sation, resonance with source impedance and the difficulty
in tuning time dependence of filter parameters have ignited
the need for active and hybrid filters [2]–[4]. The rating of
active filters is reduced through augmenting them with passive
filters [5], [6] to form hybrid filters, which reduce overall
cost. Also they can provide better compensation than either
passive or active filters.
S YSTEM C ONFIGURATION
Fig. 1 shows the proposed 3-phase, 4-wire UPQC connected
to a power system feeding an unbalanced nonlinear load.
It consists of a three leg voltage controlled VSI used as
a series active filter and a four leg current controlled VSI
used as a shunt active filter. The dc link of both active
filters is connected to a common dc capacitor of 1500μF.
The series filter is connected between the supply and load
terminals using three single phase transformers with turns
ratios of 2:1. The primary windings of these transformers are
star connected and the secondary windings are connected in
series with the three-phase supply. In addition to injecting the
voltage, these transformers are used to filter the switching
ripple of the series active filter. A small capacity rated RC
filter [6] is connected across the secondary of each series
transformer to eliminate the high switching ripple content in
the series active filter injected voltage. The voltage source
inverters for both the active filters are implemented with
IGBTs (Insulated Gate Bipolar Transistors). The four leg shunt
active filter is connected ahead of a series filter through a small
capacity rated inductive filter. This four leg VSI based shunt
active filter is capable of suppressing the harmonic currents
both in the three phases,
C ONTROL S CHEME OF S ERIES F ILTER
A simple algorithm is developed to control the series
and shunt filters. The series filter is controlled such that it
injects voltages (vca , vcb , vcc ), which cancel out the distortions
and/or unbalance present in the supply voltages (vsa , vsb , vsc ),
thus making the voltages at the PCC (vla , vlb , vlc ) perfectly
balanced and sinusoidal with the desired amplitude. In other
words, the sum of the supply voltage and the injected series
filter voltage makes the desired voltage at the load terminals.
The control strategy for the series AF is shown in Fig. 2.
Since the supply voltage is unbalanced and or distorted, a
phase locked loop (PLL) is used to achieve synchronization
with the supply [21]. Three phase distorted/unbalanced supply
voltages are sensed and given to the PLL which generates
two quadrature unit vectors (sin θ, cos θ). The sensed supply
voltage is multiplied with a suitable value of gain before being
given as an input to the PLL.
R ESULTS AND D ISCUSSION
The developed model of a UPQC system in the MAT-
LAB/SIMULINK environment is shown in Fig. 4. A 5th
harmonic voltage source of 17.5% of the fundamental supply
voltage is connected in series with the supply voltage to
introduce distortion in the supply voltage. Moreover a 23%
unbalance in the supply voltage is considered to verify the
effectiveness of the UPQC. The combination of a 15kW non-
linear and a 6kW, 0.8pf linear load is considered as the
system load. A diode rectifier drawing constant dc current is
considered as the non-linear load. In order to consider the
unbalance/neutral current, a 5kW single phase diode rectifier
drawing constant dc current is connected between the phase
‘a’ and the neutral wire of the system. The series transformer,
ripple filters and active filters are developed using the Power
System Blockset toolbox.
C ONCLUSIONS
The effectiveness of the UPQC has been demonstrated
in maintaining three-phase balanced sinusoidal constant load
voltages, harmonic elimination, power factor correction, load
balancing and supply neutral current compensation. Supply
currents and load voltage harmonic levels are maintained
below IEEE-519 standards under all conditions. Moreover, the
supply neutral current is also eliminated, without sensing the
AF or the load neutral current. The computational delay and
the number of sensors are reduced by indirectly controlling
the three phase supply currents.