A Voltage Controlled Dstatcom for Power Quality Improvement

IOSR Journal of Electrical and Electronics Engineering (IOSR-JEEE)
e-ISSN: 2278-1676,p-ISSN: 2320-3331, Volume 10, Issue 6 Ver. I (Nov – Dec. 2015), PP 27-34
www.iosrjournals.org
DOI: 10.9790/1676-10612734 www.iosrjournals.org 27 | Page
A Voltage Controlled Dstatcom for Power Quality
Improvement
L.Pattathurani1
, Rajat Kumar Dwibedi2 ,
Dr.P.Sivachidambaranathan3
1
(Assistant Professor/EEE, Jeppiaar Institute of Technology, Sriperumbudur, Chennai,India)
2
(Assistant Professor/ECE, SRR Engineering College, India )
3
( Professor& Head /EEE, Sathyabama university, India )
Abstract: Due to increasing complexity in the power system, voltage sag is becoming one of the most significant
power quality problems. Voltage sag is a short reduction voltage from nominal voltage, occurs in a short time.
If the voltage sags exceed two to three cycles, then manufacturing systems making use of sensitive electronic
equipments are likely to be affected leading to major problems. It ultimately leads to wastage of resources (both
material and human) as well as financial losses. This is possible only by ensuring that uninterrupted flow of
power is maintained at proper voltage levels. This project tends look at the solving the sag problems by using
custom power devices such as Distribution Static compensator (D-STATCOM).Proposed scheme follows a new
algorithm to generate reference voltage for a distribution static compensator (DSTATCOM) operating in
voltage-control mode. The proposed scheme ensures that unity power factor (UPF) is achieved at the load
terminal during nominal operation, which is not possible in the traditional method. Also, the compensator
injects lower currents therefore, reduces losses in the feeder and voltage-source inverter. Further, a saving in
the rating of DSTATCOM is achieved which increases its capacity to mitigate voltage sag. Nearly UPF is
maintained, while regulating voltage at the load terminal, during load change. The state-space model of
DSTATCOM is incorporated with the deadbeat predictive controller for fast load voltage regulation during
voltage disturbances. With these features, this scheme allows DSTATCOM to tackle power-quality issues by
providing power factor correction, harmonic elimination, load balancing, and voltage regulation based on the
load requirement.
Keyword:-Distribution Static compensator; Pulse Width Modulation; Active Power Filters; Battery Energy
Storage Systems; Distribution STATIC Synchronous Compensators; Distribution Series Capacitors;
Distribution static compensator; Surge Arresters; Super-conducting Magnetic Energy Systems; Static
Electronic Tap Changers; Solid State Transfer Switches; Solid State Fault Current Limiter; Static Var
Compensator; Thyristor Switched Capacitors; Uninterruptible Power Supplies.
I. Introduction
Both electric utilities and end users of electrical power are becoming increasingly concerned about the
quality of electric power. The term power qualityhas become one of the most prolific buzzword in the power
industry. 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 equipment. Power quality is certainly a major concern in the present era, it becomes
especially important with the introduction of sophisticated devices, whose performance is very sensitive to the
quality of power supply. Modern industrial processes are based a large amount of electronic devices such as
programmable logic controllers and adjustable speed drives. The electronic devices are very sensitive to
disturbances and thus industrial loads become less tolerant to power quality problems such as voltage dips,
voltage swells, harmonics, flickers, interruptions and notches.
II. Power Quality Problems
2.1 Voltage SAG
A Voltage Sag is a momentary decrease in the root mean square (RMS) voltage between 0.1 to
0.9 per unit, with a duration ranging from half cycle up to 1 min. It is considered as the most serious problem of
power quality. It is caused by faults in the power system or by the starting of large induction motor.
2.2 Voltage Swell
Voltage swell is defined as an increase in the root mean square (RMS) voltage from 1.1 to 1.8 per

A Voltage Controlled Dstatcom For Power Quality Improvement
unit for duration from 0.5 cycles to 1 min. Voltage swells are not as important as voltage sags
because they are less common in distribution systems. The main causes for voltage swell are switching of
large capacitors or start/stop of heavy loads.
2.3 Harmonics
The fundamental frequency of the AC electric power distribution system is 50 Hz. A harmonic frequency is any
sinusoidal frequency, which is a multiple of the fundamental frequency. Harmonic frequencies can be even or
odd multiples of the sinusoidal fundamental frequency. The main causes for harmonic distortion are rectifiers
and all non-linear loads, such as power electronics equipment including VSDs.
2.4 Voltage Transients
They are temporary and undesirable voltages that appear on the power supply line. Transients are high
over-voltage disturbances (up to 20kV) that last for a very short time.
2.5 Flicker
Oscillation of voltage value, amplitude modulated by a signal with frequency of 0 to 30 Hz. The main
causes are frequent start/stop of electric motors (for instance elevators), oscillating loads.The following
Fig. 1 shows the sketch of a voltage waveform.
Fig.1 voltage waveform.
III. Solution Of Power Quality Problems
There are two approaches to tackle power quality problems. (a) Actions taken from the customer side
or (b) Actions taken from the utility side. The first approach is called load conditioning, which ensures that the
equipment is less sensitive to power disturbances, allowing the operation even under significant voltage
distortion. The second approach consists of line conditioning systems that suppress or counteracts the
power system disturbances. Currently, line conditioning systems are based on pulse width modulation (PWM)
converters connected to low voltage and medium voltage distribution system in shunt mode or in series.
However, with the restructuring of the power sector and with shifting trend towards distributed and dispersed
generation, the line-conditioning systems or utility side solutions will play a major role in improving the
inherent supply quality .Though there are many different methods to mitigate voltage sags and swells, the use of
a custom Power device is considered to be the most efficient method.
The term custom power refers to the use of power electronics controllers in a distribution
system, especially, to deal with various power quality problems. There are many types of Custom Power
devices. Some of these devices include: Active Power Filters (APF), Battery Energy Storage Systems
(BESS), Distribution STATIC Synchronous Compensators (DSTATCOM), Distribution Series Capacitors
(DSC), Distribution static compensator(DSTATCOM), Surge Arresters (SA), Super-conducting Magnetic
Energy Systems (SMES), Static Electronic Tap Changers (SETC), Solid -State Transfer Switches (SSTS),
Solid State Fault Current Limiter (SSFCL), Static Var Compensator (SVC), Thyristor Switched Capacitors
(TSC), and Uninterruptible Power Supplies (UPS). In this paper,an overview of the DSTATCOM, its
functions, configurations, components, operating modes, voltage injection methods and closed loop control of
the DSTATCOM limitations.

IV. Block Diagram of Proposed Method
A distribution system suffer from current as well as voltage related power quality problems, which
include poor power factor, distorted source current and voltage disturbances .A STATCOM connected to the
point of common coupling has been utilized to mitigate both types of PQ problems .When operating in CCM it
injects reactive and harmonics components of load current to make source current balanced ,sinusoidal and in
phase with the point of common coupling voltages. In VCM the STATCOM regulates PCC voltages at a
references value to protect critical loads from voltage disturbances such as sag, swell and unbalances. However
the advantages of CCM and VCM cannot be achieved simultaneously with one active filter device, since two
modes are independent of each other.
A load works satisfactorily for a permissible voltage range. Hence, it is not necessary to regulate the
PCC voltage at 1.0 p.u. While maintaining 1.0-p.u. voltage, DSTATCOM compensates for the voltage drop in
feeder. For this, the compensator has to supply additional reactive currents which increase the source currents.
This increases losses in the voltage-source inverter (VSI) and feeder. Another important aspect is the rating of
the VSI. Due to increased current injection, the VSI is de-rated in steady-state condition. Consequently, its
capability to mitigate deep voltage sag decreases. Also, UPF cannot be achieved when the PCC voltage.
STATCOM can be replaced by DSTATCOM. The proposed block diagram is shown in the fig.2.It is applicable
for both linear as well as non linear load.
Fig.2 proposed block diagram
V. Distribution Static Compensator (D-Dstatcom)
5.1 Introduction
A D-STATCOM (Distribution Static Compensator), which is schematically depicted in Figure, consists
of a two-level Voltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected
in shunt to the distribution network through a coupling transformer. 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
D-STATCOM output voltages allows effective control of active and reactive power exchanges between the D-
STATCOM and the ac system. Such configuration allows the device to absorb or generate controllable active
and reactive power. The VSC connected in shunt with the ac system provides a multifunctional topology which
can be used for up to three quite distinct purposes:
1. Voltage regulation and compensation of reactive power;
2. Correction of power factor; and
3. Elimination of current harmonics.
Here, such device is employed to provide continuous voltage regulation using an indirectly controlled converter.
The basic configuration of the D-STATCOM is shown in the fig.3.

5.2 Configuration Of Dstatcom
Fig.3 Configuration of a DSTATCOM.
5.3 Modelling Of Dstatcom
The singlephase equivalent circuit of STATCOM is shown in the fig.4.
Fig.4 single phase equivalent circuit of STATCOM
The state-space equation for the circuit is given by

5.3 Advantages of Dstatcom
 Power factor correction
 Harmonic elimination
 Load balancing, and voltage regulation
 Based on the load requirement.
VI. Simulation Results
6.1 Conventional Sag Condition And Sag Compensated Voltages
6.1.1 Sag Condition
6.1.2 Sag Compensated Voltage

6.1.2 Power Factor Correction
VII. Conclusion
The performance of the proposed scheme is compared with the traditional voltage controlled
DSTATCOM. The proposed method provides the following advantages- at nominal load, the compensator
injects reactive and harmonic components of load currents, resulting in UPF; nearly UPF is maintained for a
load change; fast voltage regulation has been achieved during voltage disturbances and losses in the VSI and
feeder are reduced considerably, and have higher sag supporting capability with the same VSI rating compared
to the traditional scheme. Different types of voltage sag conditions should applied compensated in simulink
environment. Additionally power factor correction and voltage regulation the harmonics are also checked, 20%
voltage sag eliminated under t=0.5 to 1sec, thus the simulation results show that the proposed scheme provides
DSTATCOM, a capability to improve several Power Quality problems (related to voltage and current).
References
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A Voltage Controlled Dstatcom for Power Quality Improvement

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A Voltage Controlled Dstatcom for Power Quality Improvement