International Journal of Recent Advances in Engineering & Technology (IJRAET)
ISSN (Online): 2347-2812, Volume-1, Issue -3, 2013 175
MITIGATION OF POWER QUALITY PROBLEMS BY USING D-STATCOM
1Shaik Khaja Gareeb Nawaz, 2Shaik Hameed
1(PG Scholor), Department of EEE, QCET
2Associate Professor, Department of EEE, QCET E-mail: 1[email protected], 2[email protected]
Abstract - This paper presents the systematic procedure of the modeling and simulation of a Distribution STATCOM (DSTATCOM) for power quality problems, voltage sag and swell based on Sinusoidal Pulse Width Modulation (SPWM) technique. Power quality is an occurrence manifested as a nonstandard voltage, current or frequency that results in a failure of end use equipments. The major problems dealt here is the voltage sag and swell. To solve this problem, custom power devices are used. One of those devices is the Distribution STATCOM (D-STATCOM), which is the most efficient and effective modern custom power device used in power distribution networks. D- STATCOM injects a current in to the system to correct the voltage sag and swell. The control of the Voltage Source Converter (VSC) is done with the help of SPWM. The proposed D-STATCOM is modeled and simulated using MATLAB/SIMULINK software.
Index Terms- Distribution STATCOM (D-STATCOM), MATLAB/ SIMULINK, Power quality problems, Sinusoidal Pulse Width Modulation (SPWM), Voltage sag and swell, Voltage Source Converter(VSC)
I. INTRODUCTION
Now a days, modern industrial devices are mostly based on the electronic devices such as programmable logic controllers and electronic drives. The electronic devices are very sensitive to disturbances and become less tolerant to power quality problems such as voltage sags, swells and harmonics. Voltage dips are considered to be one of the most severe disturbances to the industrial equipments . Voltage support at a load can be achieved by reactive power injection at the load point of common coupling. D-ST A TCOM injects a current into the system to correct the voltage sag and swell.
These power quality devices are power electronic converters connected in parallel or series with the lines and the operation is controlled by a digital controllers.
The modeling of these complex systems that contains both power circuits and control systems can be done
different bases. One of those power electronic solutions to the voltage regulation is the use of a Distribution ST A TCOM (DSTA TCOM). D-ST A TCOM is a class of custom power devices for providing reliable distribution power quality. They employ a shunt of voltage boost technology using solid state switches for compensating voltage sags and swells. The DST A TCOM applications are mainly for sensitive loads that may be drastically affected by fluctuations in the system voltage
II. POWER QUALITY PROBLEMS
The power disturbances occur on all electrical systems, the sensitivity of today's sophisticated electronic devices make them more susceptible to the quality of power supply. For some sensitive devices, a momentary disturbance can cause scrambled data, interrupted communications, a frozen mouse, system crashes and equipment failure etc [5]. A power voltage spike can damage valuable components. Power quality problems encompass a wide range of disturbances such as voltage sags, swells, flickers, harmonic distortion, impulse transients, and interruptions.
III. DISTRIBUTION STATIC COMPENSATOR (D-STATCOM)
A D-STATCOM (Distribution Static Compensator), which is schematically depicted in Fig. 5.1, 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. 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 D-STATCOM has been utilized mainly for
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regulation of voltage, correction of power factor and elimination of current harmonics. Such a device is employed to provide continuous voltage regulation using an indirectly controlled converter. In this paper, the D-STATCOM is used to regulate the voltage at the point of connection.
The control is based on sinusoidal PWM and only requires the measurement of the rms voltage at the load point.
Fig. Schematic diagram of a D-STATCOM
From the Fig. 5.1, the shunt injected current ISH corrects the voltage sag by adjusting the voltage drop across the system impedance ZTH. The value of ISH can be controlled by adjusting the output voltage of the converter.
The shunt injected current ISH can be written as,
ISH =IL ─ IS Where IS = VTH − V L
ZTH Therefore
ISH = IL ─ IS = IL ─ VTH − V L
ZTH Or
ISH∠η = IL∠−θ −VTH
ZTH∠ (δ−β) + VL
ZTH∠−β
The complex power injection of the D-STATCOM can be expressed as,
SSH = VL ISH
It may be mentioned that the effectiveness of the D- STATCOM in correcting voltage sag depends on the value of ZTH or fault level of the load bus. When the shunt injected current ISH is kept in quadrature with VL, the desired voltage correction can be achieved without injecting any active power into the system. On the other hand, when the value of ISH is minimized, the same voltage correction can be achieved with minimum apparent power injection into the system.
IV. METHODOLOGY
To enhance the performance of distribution system, D- STATCOM was connected to the distribution system.
D-STATCOM was designed using MATLAB simulink version R2009a. Fig. 5.6 below shows the flowchart for the methodology:
Fig. Flowchart for the Methodology
D-STATCOM Simulations and Results for Voltage Sags
Fig. 6.1 shows the test system used to carry out the various D-STATCOM simulations presented in this
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section. The test system composes a 230 kV, 50 Hz generation system, represented by a Thevenin equivalent, feeding into the primary side of a 3-winding transformer. A varying load is connected to the 11 kV, secondary side of the transformer. A two-level D- STATCOM is connected to the 11 kV tertiary winding to provide instantaneous voltage support at the load point.
D-STATCOM Simulations and Results for THD Total harmonic distortion, or THD, is the summation of all harmonic components of the Voltage or current waveform compared against the fundamental component of the voltage or current wave:
THD = Sum of square of amplitudes of all harmonics
Fundamental component Power factor(PF) is defind as the ratio between the average power and the product of the rms values of the input voltage and cureent; that is,
Power factor = average power apparent power = KW
KVA The relationship between PF and THD for non- linear loads can be determinied by,
power factor = 1
1+(THD )2
Fig. Simulink model for the test system for Voltage Sags
D-STATCOM without LCL Passive Filter
Table Results of THD and PF for different types of fault without LCL passive filter
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Table shows the percentage of THD shows that it is not within the IEEE STD 519-1992. The percentage of power factor is low in the range of 74.82 to 91.35 lagging.
Fig. Distortion output current without LCL Passive Filter
Fig. shows the waveform of distortion output current and spectrum of distortion output current.
D-STATCOM with LCL Passive Filter
Table Results of THD and PF for different types of fault with LCL passive filter.
SLG fault
LL fault
DLG fault
TP fault
TPG fault THD
(%)
1.15 0.66 1.11 1.11 1.10 Power
Factor (%)
99.99 99.99 99.99 99.99 99.99
Table shows that with LCL Passive filter, the percentage of THD has reduced. Now the THD is within the IEEE STD 519-1992. The power factor increases close to unity.
Fig. Output current with LCL Passive Filter
Fig. shows the waveforms of output current and spectrum of output current. It is sinusoidal with LCL Passive filter was connected to the D-STATCOM.
V. CONCLUSION
This paper has presented the power quality problems such as voltage sags, swells and total harmonic
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distortion in the distribution system and simulation techniques of a D-STATCOM. The simulation results show that the voltage sags and swells (such as LG, LL, DLG and TPG) can be mitigate by inserting D- STATCOM to the distribution system. By adding LCL Passive filter to D-STATCOM, the THD reduced within the IEEE STD 519-1992. The power factors also increase close to unity. Thus, it can be concluded that by adding D-STATCOM with LCL filter the power quality is improved.
VI. SCOPE OF FUTURE WORK
Further investigation of the D-STATCOM applications the newly development of the semiconductor devices and the rising demands inutility application provide a lot of opportunities of power flow control. With the promotion and development of smart grid and renewable energy application in the power system, the D- STATCOM applications will extend to different areas, from the high voltage transmission system to the residential distribution system. Further improvement of D-STATCOM power stage design Modular converter topology, ETO semiconductor device enable the low cost, high reliability and transformer less connection of D-STATCOM in transmission system application. With the new proposal of the D-STATCOM applications, the power stage design with different voltage/current rating in different application areas should be paid more attentions.
Further D-STATCOM control development the controller response to the disturbance and parameter variation also need to be considered in the future study of D-STATCOM applications. The D-STATCOM fault detection and protection although the fault tolerant design has been proposed, the D-STATCOM converter protection scheme is still one open research topic for real applications.
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