27-08-2014, 03:19 PM
A Novel Method of Load Compensation Under
Unbalanced and Distorted Voltages
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Abstract
—In this paper, load compensation techniques under
unbalanced and distorted voltages have been discussed. Two kinds
of compensation methods are considered. In the first category,
synchronous detection and modified equal current strategies are
used under the unbalanced but sinusoidal source voltages. In
the second method, load compensation based on instantaneous
symmetrical component theory with positive sequence extraction
is proposed and is shown to work under unbalanced and the
non-sinusoidal source voltages. To support the proposed method,
a 440 V (L-L) three-phase, four-wire distribution system is considered. The compensator is realized using a three-phase voltage
source inverter (VSI) operated in the current control mode.
I. INTRODUCTION
ANUMBER of methods are available for unbalanced and
nonlinear load compensation in the literature [1]–[5].
However these methods do not provide satisfactory compensation when the source voltages are unbalanced and distorted.
Under these conditions, the synchronous detection methods
provide a simple solution for the load compensation [6], [7].
These methods discuss load compensation for unbalanced and
nonlinear loads supplied by source voltages that are unbalanced
both in magnitude and phase.
In this paper, the synchronous detection methods are extended
for load compensation under the unbalance in source voltages,
with an additional freedom of setting a desired power factor of
the source currents with respect to source voltages in the respective phases. However, the extended synchronous detection
methods assume that the source voltages are sinusoidal. Another
drawback of these methods is that they need synchronization
for all three phases to generate reference currents. In general,
using these methods, a zero sequence current flows even after
compensation.
For unity power factor operation . The term in
(1) is the dc or mean value of the load power and is computed
using a moving average filter that has an averaging time of
half cycle or one cycle of supply voltage waveform depending
upon whether load current contains odd or both odd and even
harmonics. The term in (1) accounts for the losses in the
voltage source inverter while realizing the actual compensator.
With the ideal inverter given in Fig. 1, is equal to zero.
III. EXTENDED SYNCHRONOUS DETECTION METHODS
In [6] and [7], synchronous detection methods namely equal
power criteria, equal current criteria and equal resistance criteria
are discussed. In this, it is assumed that the source voltages have
magnitude and phase unbalance without distortion.
A. Equal Current Criterion
Under this criterion, it is assumed that the source currents are
equal in magnitude, i.e.,
where the subscript denotes the source and the subscript
denotes the maximum or peak value. If it is desired that the
source current should lag by radian in the respective phases,
the instantaneous source currents are expressed as
(4)
The average power supplied by the source, which is also average
load power denoted by , is given as
B. Equal Power Criterion
In equal power criteria, the real power consumed in each
phase after compensation is to be shared equally
C. Equal Impedance Criterion
In the equal impedance criterion, the source should see the
same impedance in each phase after compensation
D. Modified Equal Current Strategy
Let there be unbalance in magnitudes and in phase angles
of the source voltages as given by (2). Let the fictitious set of
three-phase voltages be balanced and are given by
E. Using Shunt Algorithm With Positive Sequence Extraction
of Source Voltages
When the source voltages are distorted, they can no longer be
directly fed to the shunt algorithm as illustrated in Section II.
To improve the performance of the algorithm,
IV. SIMULATION STUDIES
The compensator topology chosen for simulation as well as
experimental study is the neutral clamped inverter circuit as
shown in Fig. 4. It consists of two dc storage capacitors of the
same rating and a three-phase voltage source inverter. Each leg
of the inverter has two insulated-gate bipolar transistor (IGBT)
switches with antiparallel diodes across them. In this circuit,
the junction of the capacitors is connected to the neutral
point of the load. The clamped neutral allows a path for the
zero sequence current and therefore three currents can be in dependently controlled. The computed reference compensator
currents ( , and ) in the methods discussed above are
tracked by using a VSI in a hysteresis band current control.
V. EXPERIMENTAL RESULTS
The prototype compensator model given in Fig. 4 is devel oped in the laboratory. The voltage and current signals in the
power circuit are sensed using Hall effect voltage and current transducers. For synchronization a clock is derived from
phase- source voltage. An IBM compatible PC acquires thesesignals through analog-to-digital converter (ADC) channels of
a data-acquisition card PCI 9118DG. Based on these quantities,
the program written in C/C++ for the shunt algorithm as discussed in Section III-E is implemented. The program computes
instantaneous reference currents for the compensator. These
reference currents are converted to analog values through three
DACs.
The actual compensator quantities are obtained using Hall
effect voltage and current transducers. The two currents in each
phase are compared and the logic signal and its complementary
signal are generated. These logic signals are given to a blanking
circuit to create dead time in order to avoid short circuit during
the ON-OFF transition state of the two switches (i.e., and in
Fig. 4) on the same leg of the inverter. The above is also referred
as PWM controller. The output signals from the PWM controller
act as input signals to the driver circuit of the IGBT. The gate
drive circuit of the IGBTs is supplied by an isolated supply. The
dc voltages and (in Fig. 4) for the inverter are regulated
to their reference value using PI controller. The dc power supply
provides required dc voltage levels to the PWM control circuit.
VI. CONCLUSIONS
In this paper, it is shown that under unbalanced source voltages, the direct application of the shunt algorithms generally
available in literature will result in erroneous compensation. Various modified algorithms namely equal current, equal power and
equal impedance methods have been suggested. None of these
methods provide completely balanced source currents.posed modified equal current algorithm alleviates this problem.
This algorithm provides balanced and sinusoidal source currents
irrespectiveofthemagnitudeandphaseangleunbalanceinsource
voltages. However, this scheme does not work when the source
voltages are distorted. Under this condition, shunt algorithm with
positive sequence extraction of source voltages is used for the
generation of the reference compensator currents.