Artificial Intelligence Based Three-Phase Unified Power Quality Conditioner

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Abstract:
Power quality is an important measure of the performance of an electrical power system.
This paper discusses the topology, control strategies using artificial intelligent based controllers and
the performance of a unified power quality conditioner for power quality improvement. UPQC is an
integration of shunt and series compensation to limit the harmonic contamination within
5 %, the limit imposed by IEEE-519 standard. The novelty of this paper lies in the application of
neural network control algorithms such as model reference control and Nonlinear Autoregressive-
Moving Average (NARMA)–L2 control to generate switching signals for the series compensator of the
UPQC system. The entire system has been modeled using MATLAB 7.0 toolbox. Simulation results
demonstrate the applicability of MRC and NARMA-L2 controllers for the control of UPQC.
Key words: APF, MRC, NARMA-L2, VSI, DVR
INTRODUCTION
The better controllability, higher efficiency, higher
current carrying capability, and fast switching
characteristics of static power converters are promoting
major changes in controlling the power flow of
transmission and distribution systems. On the other
hand the nonlinear characteristics of these switching
devices introduce many undesirable features such as
low power factor, poor voltage regulation, zerosequence
currents, imbalances, and harmonics.
Traditionally passive filters, synchronous condensers,
capacitors, and phase advancers were used to improve
the power quality. The undesirable features such as
lower efficiency, bulkiness, fixed compensation,
resonance, and electromagnetic interference of
traditional compensators urged power electronics and
power system engineers to develop an adjustable and
dynamic solution for power quality problems. Active
power filters (APF) were introduced in order to
compensate reactive power, to cancel current
harmonics, to correct current imbalances and to control
zero-sequence currents[1]-[12], where as the disturbances
in terminal voltages shall be compensated using
dynamic voltage restorer (DVR)[13]-[17]. More recently
UPQC has been introduced as a one shot solution to
improve power quality. A multilevel converter using
diode-clamped inverters that can handle higher voltages
with extra degrees of freedom in the form of redundant
voltage states has been proposed in[18]. A new topology
and control circuit proposed in[19] has been successfully
tested for various operating conditions. In[20] authors
have identified and tested new functionalities such as
elimination of voltage sags resulted from short circuits.
A new methodology for the evaluation and control of
losses taking place in a UPQC has been proposed in[21].
A new control strategy aimed to compensate reactive
power, negative sequence current, current harmonics
and also to regulate any voltage imbalance has been
proposed in[22]. A neural network controlled UPQC
without injection transformer has been designed and
reported in[23]. Another control structure[24] using linear
quadratic regulator (LQR) along with hysteresis control
is successfully tested for various operating conditions.
UPQC implemented in[25] uses a control circuit without
reference calculation. Complicated control structures
of UPQC[17]-[25] have been replaced by a simple control
technique in[26]. Voltage interruption can also be
eliminated by the use of a unified power quality
conditioner with distributed generation[27]. Recent
research shows that AI based controllers are very
promising in the field power system and power
electronics[28]-[49]. In[29] and[34] the conventional PI
controller is replaced by a fuzzy logic controller [FLC]
for the determination of the reference current in a shunt
active power filter. Reference[32] shows the successful
application of FLC to generate the switching signals
required in an active filter realized using current
controlled PWM inverter. In[37], T-S fuzzy model is
used to predict future harmonic compensating current in
an APF system. Successful application of artificial
neural network (ANN) controllers for the
implementation of APF system has been reported[38]-[47]
and many disadvantages of conventional controllers
could be eliminated by the use of ANN controllers.
In this paper UPQC has been proposed for the
cancellation of harmonic currents, to compensate
reactive power, to eliminate voltage harmonics, to
improve voltage regulation, to correct voltage and
current imbalances and to avoid voltage interruption.
J. Computer Sci., 3 (7): 465-477, 2007
466
The novelty of this paper lies in the application of NNC
algorithms such as MRC, and NARMA– L2 control to
generate switching signals for the series compensator of
the UPQC system. The control strategies of UPQC are
detailed in second part of this paper. Simulation results
in the third part illustrate the successful implementation
of UPQC using NARMA-L2 and MRC.
UPQC WITH INTELLIGENT CONTROLLERS
Principle of UPQC: UPQC is one of the custom power
devices used at the electrical power distribution systems
to improve the power quality of distribution system
customers. UPQC could be used to cancel current
harmonics, to compensate reactive power, to eliminate
voltage harmonics, to improve voltage regulation, to
correct voltage and current imbalances, to correct
voltage sag or swell and to avoid voltage interruptions.
A UPQC consists of both shunt and series
compensators. A shunt compensator is used to cancel
the disturbances in current whereas series compensator
is used to cancel disturbances in voltage. Shunt
compensator could be connected to the left or right of
the series compensator. Ideally, shunt compensator
injects current to achieve purely balanced sinusoidal
source currents in phase with the supply voltages at
rated magnitude and frequency. On the other hand
series compensation is used to inject voltage to
maintain terminal voltage at rated magnitude and
frequency.
Control circuit of UPQC: A three-phase system has
been selected to study the performance of the UPQC
system. (Fig. 1) shows the schematic diagram of the
UPQC system. Voltage source inverters are used for
shunt and series compensation. One may note that both
voltage source inverters are supplied from a common dc
link capacitor. One of the voltage source inverters is
connected in parallel with the a.c. system while the
other one is connected in series with the a.c. system
through injection transformers. The inverter connected
in parallel, together with its control circuit forms the
shunt compensation circuit. On the other hand the
inverter connected in series with appropriate control
circuit forms the series compensation circuit. For the
successful operation of the UPQC, the dc capacitor
voltage should be at least 150 % of maximum line-line
supply voltage. To regulate the capacitor voltage
constant, either a PI controller or a fuzzy controller
could be used. Thus the control structure of UPQC has
been divided into shunt compensator and series
compensator control circuits.
Shunt compensation control circuit: A current
controlled VSI connected in parallel to the source
through booster inductors functions as the shunt
compensator. (Fig. 2) shows the block diagram of the
control circuit of the shunt compensator[11]. The control
circuit consists of a voltage control loop and two
current control loops. The ideal requirements of the
shunt compensator circuit are (i) to maintain the
capacitor voltage at a constant steady value and (ii) to
maintain the source current purely sinusoidal in nature
and in phase with the supply voltage.
a) Voltage control loop: The voltage control loop is
used to determine the amplitude of the reference source
current. Under steady state conditions the d.c. load
removes energy from capacitor at constant average rate
and the capacitor voltage can be maintained constant
only if the incoming power from the a.c. side is equal
to the output power demand. Thus the variation in
capacitor voltage is a measure of the amplitude of the
reference current. The voltage control loop senses the
voltage across the capacitor, increases the current
drawn from the supply if the capacitor voltage tends to
decrease from the reference value and decreases the
current drawn from the ac side when the capacitor
voltage tends to increase. Thus the voltage control loop
monitors the output voltage and determines the
amplitude of the source current. The amplitude of the
reference source current could be evaluated as follows: