23-06-2012, 11:19 AM
Voltage Sag Mitigation with D-STATCOM In Distribution Systems
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INTRODUCTION
Both electric utilities and end users of electrical power are becoming increasingly concerned about the
quality of electric power. The term power quality has become one of the most prolific buzzword in the power
industry since the late 1980s . The issue in electricity power sector delivery is not confined to only energy
efficiency and environment but more importantly on quality and continuity of supply or power quality and
supply quality. Electrical Power quality is the degree of any deviation from the nominal values of the voltage
magnitude and frequency. Power quality may also be defined as the degree to which both the utilization and
delivery of electric power affects the performance of electrical equipment .From a customer perspective, a
power quality problem is defined as any power problem manifested in voltage, current, or frequency deviations
that result in power failure or disoperation of customer of equipment. Power quality problems concerning
frequency deviation are the presence of harmonics and other departures from the intended frequency of the
alternating supply voltage. On the other hand, power quality problems concerning voltage magnitude deviations
can be in the form of voltage fluctuations, especially those causing flicker. Other voltage problems are the
voltage sags, short interruptions and transient over voltages. Some common reason for voltage sags are
lightning strikes in power lines, equipment failures, accidental contact power lines, and electrical machine starts.
Despite being a short duration between 10 milliseconds to 1 second event during which a reduction in the
RMS voltage magnitude takes place, a small reduction in the system voltage can cause serious consequences.
Distribution Static Compensator (DSTATCOM):
In its most basic function, the DSTATCOM configuration consist of a two level voltage source converter
(VSC), a dc energy storage device, a coupling transformer Connected in shunt with the ac system, and
associated control circuit as shown in Figure 1. The VSC converts the dc voltage across the storage device
into a set of three phase ac output voltages. These voltages are in phase and Coupled with the ac system
through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the
DSTATCOM output voltages allows effective control of active and reactive power exchanges between the
DSTATCOM and the ac system.
The VSC connected in shunt with the ac system provides a multifunctional topology which can be used
for up to three quite distinct purposes , Voltage regulation and compensation of reactive power, Correction of
power factor; and Elimination of current harmonics. The design approach of the control system determines the
priorities and functions developed in each case. In this case the control is based on sinusoidal PWM and only
requires the measurement of the rms voltage at the load point.
Sinusoidal PWM-Based Control Scheme.
Conclusion:
Nowadays, reliability and quality of electric power is one of the most discuss topics in power industry.
There are numerous types of power quality issues and power problems and each of them might have varying
and diverse causes. The types of power quality problems that a customer may encounter classified depending
on how the voltage waveform is being distorted. There are transients, short duration variations (sags, swells,
and interruption), long duration variations (sustained interruptions, under voltages, over voltages), voltage
imbalance, waveform distortion (dc offset, harmonics, inter harmonics, notching, and noise), voltage fluctuations
and power frequency variations. Among them, two power quality problems have been identified to be of major
concern to the customers are voltage sags and harmonics, but this project is focusing on voltage sags.
Voltage sags are huge problems for many industries, and it is probably the most pressing power quality
problem today. Voltage sags may cause tripping and large torque peaks in electrical machines. Generally,
voltage sags are short duration reductions in rms voltage caused by faults in the electric supply system and
the starting of large loads, such as motors. Voltage sags are also generally created on the electric system when
faults occur due to lightning, which are accidental shorting of the phases by trees, animals, birds, human error
such as digging underground lines or automobiles hitting electric poles, and failure of electrical equipment.
Sags also may be produced when large motor loads are started, or due to operation of certain types of
electrical equipment such as welders, arc furnaces, smelters, etc.