Optimal utilization of PV Based DVR to Reduce Voltage Sag and Harmonics

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International Journal of Recent Advances in Engineering & Technology (IJRAET)

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ISSN (Online): 2347 - 2812, Volume-3, Issue -10, 2015 28

Optimal utilization of PV Based DVR to Reduce Voltage Sag and Harmonics

1D. Kavitha, ²K.M.Kavitha, ³T.R Narasimhegowda, ⁴B.Kantharaj

1,2,3,4Department of Electrical and Electronics Engineering, Adichunchanagiri Institute of Technology, Chikkamagaluru.

Abstract- A dynamic voltage restorer(DVR) is a custom power electronic controller that can protect sensitive loads from disturbances in the supply system. The DVR can restore the load voltage within few milliseconds. This paper presents a photovoltaic(PV) array fed three phase three wire DVR for handling voltage regulation in a low voltage(LV) distribution system against imbalance or harmonics in the source side. Besides the voltage regulation, the proposed DVR reduces the energy consumption from three phase utility grid by utilizing the real power generation during daytime. The synchronous reference frame theory is used for the conversion of voltages from rotating vectors to the stationary frame. The compensation of the voltage sag reduction of harmonics are demonstrated. The results of the simulation studies performed in the MATLAB

software.

Index Terms-Dynamic Voltage Restorer(DVR), power quality, voltage sag, Harmonics, PV cells, SRF theory.

I. INTRODUCTION

The growing concern over the increasing energy consumption, environmental degradation resulting from combustion of fossil fuels and fluctuating oil prices of the nations has increased the use of renewable power generation in the power system application. PV based DVR has become favorable solutions for a home or a small industry, particularly in Tamilnadu, India rural areas have a substantial amount of insolation and have more than three hours of frequent power interruptions in a day. This may occur in the developing countries, where the generated electrical power is less than their demand.

The concept of utilizing PV solar system inverter as DVR, for the mitigation of voltage variations with power saver capability at the load side for single-phase is presented in [1]. The reference [2] has proposed the concept of utilizing the DVR for voltage sag and outage mitigation without PV system. The rating and design of series injection transformer of the DVR is presented in [3]. The design and control of DVR have been carried out in [4].

The custom power devices are developed and installed at consumer point to meet the power quality standards such as IEEE-519. When the fast variations in the source voltage cannot be ignored, these can affect the performance of critical loads such as semiconductor fabrication, plants paper mills, food processing plants and

automotive assembly plants. Voltage sags are the common reasons for interruption in production plants and for end-user equipment malfunctions in general. In particular, tripping of equipment in a production line can cause production interruption and significant costs due to loss of production. One solution to this problem is Dynamic Voltage Restorer (DVR). Its primary application is to compensate for voltage sags and swells. Also, a DVR is expected to respond fast (less than 1/4 cycle) and thus employs PWM converters and IGBT[5]. An energy-optimized control of DVR is discussed in [6]. The design of a capacitor-supported DVR that protects sag, swell, distortion, or unbalance in the supply voltages is discussed in[7]. The performance of a DVR with the high-frequency-link transformer is discussed in [8]. In this paper, the control and performance of a DVR are demonstrated with a reduced-rating voltage source converter (VSC).

The synchronous reference frame (SRF) theory is used for the control of the DVR

II. COMPONENTS OF DVR

Dynamic Voltage Restorer is one of custom power device specially used to inject a (fundamental frequency) voltage in each phase of required magnitude and phase to maintain the load voltage constant in the distribution system. Fundamental of DVR is shown in fig1. The power circuit of DVR shown in fig2.

Fig 1. Fundamental of DVR.

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Fig 2. The power circuit of DVR

The DVR consists of boost/series injection transformer, voltage source converter, passive filters, energy storage device and control circuit. Three single phase transformers are connected in series with the distribution feeder to couple the VSC (at the lower voltage level) to the higher distribution voltage level. In DVR voltage source converter is used to either completely replace the supply voltage or to inject the missing voltage. The missing voltage is the difference between the nominal voltage and the actual one. Switching device used is Insulated Gate Bipolar Transistors(IGBT). The passive filters placed at the high voltage side of the DVR to filter the harmonics. Energy storage is required to provide active power to the load during deep voltage sags. Lead- acid batteries, flywheel or SMES can be used for energy storage. Control circuit continuously monitors the supply voltage. The function of control system is to detect the disturbance in the supply voltage, compare it with the set reference value and then generate the switching pulses to the VSC to generate the DVR output voltages which will compensate the voltage sag.

III DVR COMPENSATION TECHNIQUES.

Compensation by DVR depends upon limiting factors such as power rating, load conditions, and voltage-sag type. Different methods of compensation are

Pre-sag/Dip compensation method(PDC): This method is recommended for the non linear loads which needs both voltage magnitude as well as phase angle to be compensated. Drawback of this method is that it requires higher capacity energy storage device as well as large voltage injection transformer.

Fig 3. Vector diagram of PDC method

In-phase compensation method: In this method only voltage magnitude is compensated. It is recommended for linear loads.

Fig 4. Vector diagram of In-Phase compensation method

Energy optimized compensation (or) In-phase advanced compensation(IPAC) In this method the real power spent by DVR is minimized by decreasing the power angle between the sag voltage and the load current.

Voltage tolerance method with minimum energy injection: Generally the voltage magnitude between 90%-110% of the nominal voltage and phase angle variations between 5-10% of the nominal state will not disturb the operation characteristics of loads. This method can maintain the load voltage in the tolerance area with small change of voltage magnitude.

Fig 5. Voltage tolerance method with minimum energy injection.

IV MODELING OF PHOTOVOLTAIC ARRAY

PV array is a system which uses two or more solar panels to convert sunlight into electricity. Photovoltaic array is a linked collection of solar cells. The use of new efficient photovoltaic solar cells has emerged as an alternative source of renewable green energy conversion. In the proposed DVR, PV array provides a DC source for the DVR. The electrical system powered by solar array requires DC/DC converter due to varying nature of the generated solar power resulting from sudden changes in weather conditions which change the solar irradiation level as well as cell operating temperature. Solar arrays are built up with combined parallel/series combination of solar cells.

The PV array is designed and modeled with a low step up boost converter to charge the batteries. The PV model is developed using basic equations of photovoltaic cells including the effects of temperature changes and solar irradiation [9]-[11]. The PV cell output voltage is a function of the photo current that mainly determined by load current depending on the

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solar irradiation level during the operation. The ideal photovoltaic equation is below.

where Ipv, cell is the current generated by the incident light (it is directly proportional to the Sun irradiation), Id is the Shockley diode equation, I0,cell [A] is the reverse saturation or leakage current of the diode [A], q is the electron charge [1.60217646 ・ 10−19C], k is the Boltzmann constant [1.3806503 ・ 10−23J/K], T [K] is the temperature of the p-n junction, and a is the diode ideality constant. Fig. 3 shows the equivalent circuit of the ideal photovoltaic cell.

Fig.6. Single-diode model of the theoretical photovoltaic cell and equivalent circuit of a practical photovoltaic

device including the series and parallel resistances.

V MODELING OF DVR WITH SOLAR PANEL FOR VOLTAGE SAG

Fig.7. shows a MATLAB model of the DVR with solar panel in which the SRF theory is used for reference signal estimation. The voltages at the PCC VS and at the load terminal VL are sensed for deriving the IGBTs’ gate signals. The reference load voltage V*L is extracted using the derived unit vector [5]. Load voltages (VLa, VLb, VLc) are converted to the rotating reference frame using abc−dqo conversion using Park’s transformation with unit vectors (sinθ, cosθ) derived using a phase-locked loop as

Similarly, reference load voltages (V*La, V*Lb , V*Lc) and voltages at the PCC vS are also converted to the rotating reference frame. Then, the DVR voltages are obtained in the rotating reference frame as

vDd = vSd – vLd (2) vDq = vSq – vLq (3)

The reference DVR voltages are obtained in the rotating reference frame as

v*Dd =v*Sd – vLd (4) v*Dq =v*Sq − vLq (5)

The error between the reference and actual DVR voltages in the rotating reference frame is regulated using proportional–integral-derivative (PID) controllers. Reference DVR voltages in the abc frame are obtained from a reverse Park’s transformation taking V*Dd from (4), V*Dq from (5), V*D0 as zero as

Reference DVR voltages (v∗dvra, v∗dvrb, v∗dvrc) and actual DVR voltages (vdvra, vdvrb, vdvrc) are used in a pulse width modulated (PWM) controller to generate gating pulses to a VSC of the DVR. The PWM controller is operated with a switching frequency of 10 kHz.

Fig.7. MATLAB based model of the Solar Panel supported DVR connected system.

VI SIMULATION RESULTS

A. System Analysis without DVR

The simulation results for system with-out DVR are given below. The voltage sag during overloading condition at the interval 0.2-0.4secs are shown in Fig 8.

Fig.8. Simulation of the system without DVR.

B. System Analysis with DVR

The simulation results for system with DVR are given below. The Load voltage is regulated to constant

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amplitude under sag during overloading condition at the interval 0.2-0.4secs are shown in Fig.9.

Fig.9. Simulation of the system with DVR

Fig.10. Load voltage and harmonic spectrum during the disturbance

Fig:11. Load voltage and harmonic spectrum with DVR during the voltage sag.

VII. CONCLUSION

Dynamic Voltage Restorer (DVR) with Solar Panels is an effective device for power quality enhancement due to its quick response and high reliability. The SRF theory has been used for estimating the reference DVR voltages. The proposed DVR with PV cells reduces the energy consumption from the utility grid. The conclusion is that it is an effective apparatus to reduce the Voltage Sag during disturbances. The load voltage is maintained sinusoidal by injecting the DVR voltage and the harmonics are reduced with DVR.

REFERENCES

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