11-02-2013, 02:21 PM
New Control Scheme for a Unified Power- Quality Compensator-Q With Minimum Active Power Injection
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Abstract
Voltage sags are one of the most frequently occurring
power-quality problems challenging power systems today. The unified
power-quality conditioner (UPQCs) is one of the major custom
power solutions capable of mitigating the effect of supply voltage
sags at the point of common coupling (PCC) or load. A UPQC-Q
employs a control method in which the series compensator injects
a voltage that leads the supply current by 90 so that the series
compensator at steady state consumes no active power. However,
the UPQC-Q has some disadvantages. First, there are limitations in
rating when using UPQC-Q for series compensation. Second, there
is a phase difference between the input and output voltage in proportion
to the severity of voltage sags. As a result, it cannot offer effective
compensation for voltage drops. This paper proposes a new
control scheme for the UPQC-Q that offers minimum active power
injection. The proposed power injection method takes into consideration
the limitations of the rated voltage capacity of the series
compensator, and the phase difference during voltage sag events.
The validity of the proposed control scheme has been investigated
by simulation and experimental results for less than 50% voltage
sag conditions.
INTRODUCTION
DUE to the widespread automation of critical processes
throughout industry, the importance of precise voltage
control and voltage stability has increased dramatically. Sensitive
loads are greatly affected by power-quality (PQ) disturbances
in the system. Many nonlinear loads emerging recently
in the power networks are the main cause of harmonic currents
that decrease PQ.
The harmonic currents flowing through the source impedance
of the utility supply can cause voltage distortion at the point of
common coupling (PCC). This results in a malfunction of control,
protection, and metering equipment used in other loads and
system monitoring devices. Harmonic currents can also cause
unwanted system resonance, overloading of capacitors, a decrease
in efficiency due to increased losses,
UPQC-Q
Fig. 1 shows a simple configuration of the UPQC-Q. The series
compensator, which consists of PWM 1 inverter and filters
shown as and , compensates voltage sags by injecting
that leads the source current by 90 . The parallel compensator
by using the PWM 2 converter and low-pass filters and
performs power factor correction through reactive power compensation,
harmonics elimination, and dc-link voltage regulation.
Fig. 2 shows the phasor diagram of voltages and currents
during voltage sag compensation mode. In this operation,
voltage dips are regulated by injecting reactive powers by using
series compensator. Each abbreviation in this paper has the
following meaning.
Control of Negative Sequences
It was mentioned before that a negative-sequence source
voltage results in a 120-Hz dc-link ripple and an increase in
the rating of the inverter. Therefore, the negative-sequence
voltage should be removed from the reference voltage if the
imbalance levels in the source voltage are not serious. To do
this, the negative sequence should be separated from the source
voltage [18], [19]. In the case of unbalanced load conditions,
it is possible to use the same analysis applied to the case of
unbalanced source voltage [20], [21].
CONCLUSION
This paper discussed the control methods for a UPQC-Q
using a minimum active power injection algorithm. The conventional
UPQC-Q cannot compensate for the voltage sag
effectively with limitations on the rating of the series compensator
and a phase difference between the input and the output
voltage. When there are limitations on the rating and a phase
difference, the proposed control scheme can compensate for the
voltage sag effectively and economically by using minimum
active power. The control algorithm and mathematical models
were proposed, and then simulation and experiment results
were presented to verify the performance of the proposed
control strategy.