08-05-2012, 09:59 AM
Simulation Analysis of DVR Performance for Voltage Sag Mitigation
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Abstract
Voltage sag is literally one of power quality problem
and it become severe to industrial customers. Voltage sag can
cause miss operation to several sensitive electronic equipments.
That problem can be mitigating with voltage injection method
using custom power device, Dynamic Voltage Restorer (DVR).
This paper presents modeling and analysis of a DVR with pulse
width modulation (PWM) based controller using
Matlab/Simulink. The performance of the DVR depends on the
efficiency of the control technique involved in switching the
inverter. This paper proposed two control techniques which is PI
Controller (PI) and Fuzzy Logic Controller (FL). Comprehensive
results are presented to assess the performance of each controller
as the best power quality solution. Other factors that also can
affect the performance and capability of DVR are presented as
well.
Keywords- Voltage sag; Dynamic Voltage Restore; Pulse Width
Modulation (PWM); PI Controller; Fuzzy Logic Controller
I. INTRODUCTION
Recently, power quality problems become a major concern
of industries due to massive loss in terms of time and money.
Hence, there are always demands for good power quality,
which positively resulting in reduction of power quality
problems like voltage sag, harmonic and flicker [1]. Voltage
sag is always considered as one of the major power quality
problems because the frequency of occasion is so high.
Moreover, according to the data recorded by Tenaga Nasional
Berhad (TNB), 80% of power quality complaints by
consumers in Malaysia were outlined to be associated with
voltage sag [2]. The common causes of voltage sag are faults
or short circuit in the system, starting of large loads and faulty
wiring [3]. This will lead to increase in both production and
financial loss for industries. Therefore, it is vital to mitigate
voltage sag.
Two main characteristics that explain the voltage sag are
depth/magnitude and duration of voltage sag itself. The
depth/magnitude and duration of voltage drop that said to be
voltage sag is between 0.1 to 0.9 pu with time interval, t about
0.5 cycles to 1 minute [4]. This classification is based on IEEE
standard 1159-1995.
There are various types of voltage sag mitigation
equipment that available nowadays such as Uninterrupted
Power Supply (UPS), flywheel, and the flexible ac technology
(FACTS) devices which have been widely used in the power
system due to the reliability to maintain power quality control
[5]. One of the most FACTS devices that have been created in
improvement the performance of power quality is Dynamic
Voltage Restorer (DVR) also known as custom power devices.
In this paper, DVR which consists of the injection transformer,
filter unit, PWM inverter, energy storage and control system is
used to mitigate the voltage sag in the power distribution
system.
Control unit is the heart of the DVR where it main function
is to detect the presence of voltage sags in the system,
calculating the required compensating voltage for the DVR
and generate the reference voltage for PWM generator to
trigger on the PWM inverter. The components of control
system unit are the dq0-transformation, Phase-lock-loop (PLL)
and the PI or FL Controller. PI Controller is a feedback
controller which drives the plant to be controlled with a
weighted sum of the error (difference between output and and
desired set-point) and the integral of that value.
II. METHODOLOGY
A. Mathematical Calculation
When voltage drop occurred at load, DVR will inject a
series voltage through transformer so that load voltage can be
maintained at nominal value as shown in Fig.1. Thus,
DVR L th L th V =V + Z I −V
(1)
*