Digital simulation of dynamic voltage restorer (DVR) for voltage sag mitigation.
With the rapid technology advancement in industrial control processes, electric utilities are experiencing more demanding requirements on the power quality from the large industrial power consumers. Such power quality problems have been better appreciated when the price paid due to the economic losses caused by them is large. These concerns are reflected in the newer versions of power quality standards, such as IEEE 1159-1995  and IEC61000-4-30 . Trends of deregulation happening in Europe and America exert pressures on the utilities to accommodate such demanding requirements in a competitive electricity market environment.
Among the various power quality problems, the voltage sag, usually resulting from the faults on parallel transmission/distribution feeders, is attracting quite a large amount of attention of researchers from both industry and academia -. A definitive solution to this problem at large power levels has been commonly called dynamic voltage restorer (DVR), under the rubric of the custom power concept introduced by EPRI . The main function of DVR is to mitigate the voltage sag, although sometime additional functions such as harmonics compensation and reactive power compensation are also integrated to the device. It has also been shown in a previous study that the series compensation device such as the DVR as shown in Fig. 1 is preferred over shunt compensation strategy as shown in Fig. 2 for stiff systems , typical of large industrial load installations.
Much of the published literature on DVRs deals with a voltage source converter (VSC) realized using two-level converters, which are well suited for 480-V systems , . While in high power applications such as at distribution voltage levels, a multilevel converter is a more attractive solution, whose application in a DVR has not been well addressed. On the other hand, for the control of DVR, the open loop feedforward technique is found to be a common practice, which generally results in poor damping of the output harmonic filter . In this paper, the power of circuit design and controller design considerations of an ac stacked multilevel converter using cascaded H-bridges is studied for DVR application. The paper is organized as follows: the power architecture, principle and control are explained in section-II followed by digital simulation and results in section-III. In section IV summary presented as conclusion.
Dynamic Voltage Restorer
Dynamic voltage restorer (DVR) is designed to protect voltage sags on lines feeding sensitive critical equipment. Unlike UPS, the DVR is specifically designed for large loads (ranging from (2MVA to 10MVA) served at distribution voltage.
In this paper, a simple single phase DVR and a three-phase DVR model are developed using PSCAD/EMTDC program. The DVR is typically designed to inject the missing voltage on to the power line through a series transformer such as shown in figure 8. Figure 8 depicts the proposed single line DVR circuit.
Basically, the DVR comprises of three main parts, which are the inverter, DC energy storage unit and control system. The inverter configuration modeled in this circuit model is a PWM full bridge inverter. The switching inverter consisting IGBTs is employed in the model. The DC energy storage unit for the proposed model, which is interfaced to the inverter, is simply a capacitor. It is adopted to reduce complications during simulation. This capacitor draws the charging energy from the main power. In this model, a large capacitor is used to store the energy. Further more, the control system is employed to ensure only the missing voltage is injected to the series transformer. In addition, a Low pass filter is connected at the inverter side to attenuate high frequency voltage. The load type considered here is a linear load. This circuit uses the similar sag generator used in UPS model.
[FIGURE 1 OMITTED]
The Operational principle
The basic idea of the DVR is to inject the missing voltage into the system through series injection transformer when it is subjected to voltage sags. Therefore, the sag is unseen by the loads. During normal operation, the capacitor receives energy from the main source. When voltage drop is detected, the capacitor starts to recharge and deliver DC supply to the inverter. The inverter fully functions during voltage sags. During undisturbed power supply condition, the inverter only supplies a small injection voltage to the transformer.
For the proposed DVR model, only the missing voltage should be injected to the series transformer. Therefore, the PWM modulation scheme for the inverter is configured to produce the injection voltage at the output of the inverter. This configuration uses the same PWM scheme as UPS but with different control. The difference between the nominal voltage waveform and voltage sags waveform is set as a voltage reference for the PWM scheme that can be seen Figure 1, a control to detect the start and the end of voltage. Sags are developed so that the inverter will only supply the injection voltage at that duration.
A single-phase power system without compensation is shown in Figure 3. The voltage across load 1 and load 2 are shown in Fig 4. It can be seen that the voltage decreases when the second load is connected.
Power system model with basic DVR is shown in figure 5. The injected voltage is shown in figure 6.
Load voltage waveforms are shown in Figure 7. Load is connected at 0.2 sec the voltage sags as shown in Fig. The DVR comes in to action at 0.5 sec. It can be seen that the voltage reaches to normal value after 0.5 sec.
DVR circuit using inverter is shown in Figure 8. Inverter is represented as sub system. The subsystem alone is shown in Figure 9. The waveforms of inverter output and load voltage are shown in Figure10. The load voltage is distorted, since the inverter output is a square wave. This problem can be solved by using LC filter at the output of the inverter.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
The simulation results demonstrated the capability of the dynamic restorer (DVR) to compensate the voltage sag.
The circuit model for DVR is developed and it is used for simulation studies. DVR system is found to inject required voltage to compensate the sag. The simulation results indicate that DVR is preferred in the case of sensitive loads. The performance of induction motors is improved with the addition of DVRs
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Chintapudi. Sasikala *, Vyza. China Veera Reddy (1) and Vyza. Usha Reddy (2)
Research Scholar *, Professor (1), Assistant Professor (2) Department Electrical & Electronics Engineering Sri Venkateswara University, Tirupati, Andhra Pradesh, India E-mail: firstname.lastname@example.org, email@example.com firstname.lastname@example.org
* corresponding author
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|Author:||Sasikala, Chintapudi; Reddy, China Veera; Reddy, Usha|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Nov 1, 2009|
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