A Photovoltaic System Used in Energy Harvesting for Low Power Application

Akihiro Torii(1), Atsushi Nakata(2), Kae Doki(1)

(1) Aichi Institute of Technology, 1247 Yachigusa Yakusa, Toyota, Aichi 470-0392, Japan(2) Shizuoka Institute of Science and Technology, 2200-2 Toyosawa, Fukuroi, Shizuoka 437-8555, Japan

Abstract:This paper describes a photovoltaic system using a tapped transformer for energy harvesting. The photovoltaic system converts energy from the sunlight into electric DC power by solar modules, and the DC power into AC power by a pulse width modulation inverter. The AC power is fed to the grid through the tapped transformer which changes the amplitude of the AC voltage. By changing the tap, the boosting ratio of the transformer is changed so that the generated power should flow into the grid. The proposed system is suitable not only for large-scale solar power systems but also for small-scale solar power systems. The proposed system scavenges small electric power which is not used for the power supply.

1. INTRODUCTION

There are various kinds of discussion against global warning, and environmentally friendly power generation is required. We have many types of power generation. The power station usually provides electric energy to electric power network which distributes the energy to consumers. In general, commercial power plants such as thermal, hydraulic and nuclear power stations are large, and their electricity generating capacities vary from hundreds kW to MW. These power plants furnish enormous electric energy to the world. Most of conventional power plants unfortunately have some negative points. Thermal power plants emit greenhouse gasses, nuclear power plants have an issue of radioactive materials, and hydraulic power plants require the protection from the environmental destruction. Some hydro, wind, and photovoltaic power stations began operations and generated electric power [1,2].

Energy harvesting is one of the biggest issues in the world. Many small-scale solar power units started operation. They provide electric energy for modern society without emitting greenhouse gas. A lot of research groups are investigating solar, wind, and geothermal power are renewable electricity [3]. We are focusing on renewable energy, especially sunlight.

A photovoltaic (PV) system is a system which supplies electric power from sunlight as the power source. Although the eco-friendly solar power unit is smaller than commercial solar power station called solar farm or solar station, the small-scale solar power unit is also connected to the electricity grid and it supplies the electric power to the grid. The solar cell generates a direct current (DC) electric power as a function of the intensity of sunlight, although the power system network transmits high voltage alternative current (AC) electric power. The solar cells need connecting in series and parallel to generate large energy. The solar cells need power conversion to AC energy in order to transmit the induced energy.

In a power conversion system used in a PV system, an inverter converts DC energy generated by the PV panel into AC energy. A conventional power conversion system in the PV system consists of a solar panel, an inverter and a transformer. A pulse width modulation (PWM) inverter converts DC energy into AC energy. The primary side of a transformer is connected to the utility, and the secondary side of the transformer is connected to the PWM inverter. The transformer is an electrical device for converting between two different voltages. The voltage at the secondary side, i.e. the PWM inverter’s voltage, is increased by the transformer. The system, however, cannot send electric power from the DC side to the AC side in case of the DC voltage drop caused by the weak sunlight.

In this paper, we discuss a PV system which supplies electric power from sunlight as the power source to the grid. Since the voltage generated by a solar cell is determined by the intensity of the sunlight and by an energy gap of an individual solar cell, the electric energygenerated by the weak sunlight cannot flow into the utility network. This paper describes a countermeasure against the weak sunlight. A tapped transformer which increases the voltage of the AC energy is introduced at the AC side of the inverter. We harvest the small energy which is so small that the electric power is not transmitted to the power utility network by using the conventional power conversion systems.

Figure 1: Conventional PV system.

Figure 2: Conventional PV system with a boost chopper.

2. PHOTOVOLTAIC SYSTEM

2.1 Conventional system

A conventional power conversion system in the PV system consists of a solar panel, an electrolytic capacitor Cdc, an inverter and a transformer as shown in Figure 1. The electrolytic capacitor stored energy generated by the panel. A PWM inverter converts DC energy into AC energy. The primary side of a delta-wye transformer is connected to the utility whose voltage is expressed as Vs. The secondary side of the transformer is connected to the PWM inverter. In Figure 1, L1 and L2 denote the leakage inductance of the primary and secondary side, and R1 and R2 denote winding resistance of the transformer. This is the simplest configuration of the PV system. The system, however, cannot send electric energy to the utility when the intensity of the sunlight is weak.

A boost converter, which is a DC-to-DC power converter with an output voltage greater than its input voltage, is sometimes used in the DC side of the PWM inverter as shown in Figure 2. The boost converter is a switching power supply containing semiconductor

switches (a diode and a transistor) and an energy storage element (a capacitor or an inductor). If the weak sunlight reduces the output voltage of the PV panel, the boost converter increases the voltage so that the PV system provides the electric energy to the utility. The disadvantage of the boost converter however is energy loss in the converter.

A transformer used in both Figures 1 and 2 is an electrical device for converting between two different voltages. There are two coils in the transformer. A turns’ ratio of the coils determines the ratio between the primary and secondary voltage. We assume that the line-to-line voltage of the primary and secondary side is 6.6 kV and 210 V, respectively. The voltage at the secondary, i.e. the PWM inverter’s voltage, is increased by the transformer. The system, however, cannot send electric power from the DC side to the AC side in case of the DC voltage drop caused by the weak sunlight, since the step-up ratio of the transformer is constant and the voltage of the primary side of the transformer should be higher than that of the utility voltage Vs.

2.2 Proposed System

Table 1: Specifications of PV module

Open circuit voltage 600 VShort circuit current 345.5 A

Voltage @ maximum power 480 VCurrent @ maximum power 312.5 A

Rated power 150 kW

In Figure 3, the proposed PV system is illustrated. A tapped transformer is introduced. We switch the taps from one to another as a function of the DC voltage Vdc of the capacitor Cdc. A tap in a transformer is a wire that is attached to a coil. There are a reduced number of turns between one end of coil and the tap. The voltage will change with the ratio of two numbers of turns. We used a transformer with three taps on the secondary side. The line-to-line voltages obtained by tap 1, tap 2, and tap 3 are 210 V, 105 V, and 52.5 V, respectively. In the proposed system, no boost chopper is used at the DC side of the PWM inverter. Therefore, we do not have additional loss in the proposed system.

3. SIMULATION

3.1 PV panel and I-V curveSpecifications of the solar panel are summarized in Table 1. Open circuit voltage, short circuit current, and voltage and current at the maximum power point are summarized.These values are obtained from a practical PV system.

The I-V (current- voltage) curve of a PV module describes its energy conversion capability at the existing conditions of sunlight level and temperature. Figure 4shows an I-V curve drawn by polynomial approximation. The approximated I-V curve is obtained by a practical PV module. As shown in Table 1 and Figure 4, the maximum

power is obtained when the output DC voltage Vdc is 480 V and the output DC current Idc is 312.5 A. Tap selection is also imposed in Figure 4. According to the output voltage Vdc, one tap is selected. For example, tap 1 is used when Vdc is between 320 V and 620 V. The tap selection is described precisely in the next section.

3.2 Tapped transformer

The leakage inductance L1 and L2, and the winding resistance R1 and R2, which are usually declared in the specification sheet of the transformer, are also indicated in Table 2. We assume that the leakage inductance and the winding resistance are proportional to the length of the windings. We select one of three taps (tap1, tap2, tap3) of the transformer according to the capacitor voltage Vdc which is shown in Figure 2. The transformer used in the conventional system has one tap which corresponds to tap1 in the proposed system. Assume that the weather condition is getting worse and the intensity of the sunlight becomes weak. If the capacitor voltage Vdc decreases below 320 V, the suitable AC voltage 210 Vac is not obtained any more. Theoretical minimum DC voltage Vdc, which is converted into 210 Vac, is 320 Vdc. When the AC voltage decreases, tap2 which changes boost ratio is selected. The number of winding of tap2 is a half of the number of windings of tap1. Similarly, the number of winding of tap3 is one-fourth of the number of windings of tap1. By using tap2 and tap3, the boost factor of AC voltage is two and four, respectively.

In order to prevent the frequent change of the tap, the tap selection has hysteresis. While the capacitor voltage Vdc decreases from 500 V to 300 V, we change tap from tap1 to tap2 at 320 V. On the contrary, the capacitor voltage Vdc increases from 300 V to 500 V, tap1 is used after Vdc is greater than 370 V. The overlap of the arrows in Figure 3 indicates the hysteresis of the tap selection.

Figure 3: Proposed PV system with a tapped transformer.

Figure 5: Control system for gate signals for PWM inverter.

Figure 4: Proposed PV system with tapped transformer.

Table 2: Leakage inductance and winding resistance of a transformer with tap.

Primary sideL1 R1

23.1 mH 2.18 Secondary side

Tap number L2 R21 23.4 H 44.1 m2 11.7 H 22.1 m3 5.85 H 11.0 m

The output voltage of the PWM inverter is increased by the transformer, and the increased AC power is supplied to the grid. In order to feed the electric power from the PV system to the grid, the voltage of the AC side of the PV system is higher than that of the voltage of the network.

3.3 PWM inverter

The PWM inverter, which converts from DC power to AC power, is a three arm inverter and composed with six insulated gate bipolar transistors (IGBTs). Since IGBTs have the advantage for high-speed and high-power switching, they are the preferred component for constructing PWM inverters.

Figure 5 shows control system used in the proposed system. The reference voltage Vdc* of the capacitor Cdc, the measured capacitor voltage Vcd, the utility voltage Vs, three phase current Irec are used to obtain gate signals of IGBTs. The gate signals which operate IGBTs are obtained by the control strategy shown in Figure 4.

The switching frequency of the PWM inverter is 6 kHz and the dead time of IGBTs is 6 s. The voltage controlling IGBTs is processed by comparator circuits. The comparator compares a desired signal to a reference signal and turns IGBTs on or off depending on the results of the comparison. The frequency of the desired signal is 60 Hz used in western part of Japan, and the frequency of the reference signal is 6 kHz triangle wave. In order to prevent short circuit of the arm of the PWM inverter, a dead time is needed. The dead time of the IGBT inverter

(a) Tap 1

(b) Tap 2

(c) Tap 3

Figure 6: Simulation results.

is 6 s which is the largest value of the normal IGBT control since IGBTs can be controlled by high frequency. The PWM inverter is operated so that large AC voltage is obtained. The third-order harmonics synchronized with the source voltage is superimposed. The amplitude of the harmonics is 15% of the fundamental frequency (60 Hz).Parameters (K1, K2, K3, K4, T1, T2, T3, T4) shown in Figure 5 are tuned numerically.

4. SIMULATION RESULTS

We simulate the operation of the proposed PV system. Figure 6 shows simulation results. U-phase current flowing from the PWM inverter into the grid through a delta-wye transformer is demonstrated. Six periods of the U-phase current which is 60 Hz is the evidence that the electric energy flows from the DC side to the grid. The output voltage Vdc of the PV panel which corresponds to the capacitor voltage is also shown.

Figure 6(a) shows a simulation result obtained by a sufficient sunlight. The output voltage Vdc of the PV panel is 480 V which is the optimal value of the PV panel. The panel generates its maximum electric power, and the root mean square (rms) value of the U-phase AC current is 412 Arms.

Even if the weather worsens and the DC voltage of the capacitor decreases, the current flows into the network. The DC capacitor voltage Vdc is 250 V in Figure 6(b) and 75 V in Figure 6(c). When we use a conventional PV system shown in Figure 1, these DC power is not converted to the sufficient AC voltage (210 V). The proposed system, however, converts the DC voltage to lower AC voltage, and the AC voltage is boosted to 210 V by using tap2 and tap3 of the transformer. In Figure 5(b), the U-phase current is 499 Arms. In Figure 6(c), it is 281 Arms. Although the U-phase current contains harmonics and switching noise of the PWM inverter, the electric power generated by the PV panel is transmitted to the grid. These simulation results imply that we could harvest small energy generated by the PV panel, even ifthe insufficient weather conditions. The insufficient potential of the AC electric energy is transmitted to the grid by the tapped transformer. An appropriate tap of the transformer provides the proper potential of the AC energy and the AC electric energy can be fed into the grid. Although the output electric power is small, the harvested energy can drive small electric load.

5. SUMMARY AND FUTURE WORK

This paper presents a PV system using a tapped transformer which boosts the AC voltage of the electric

energy. In a conventional PV system, the minimum DC voltage which supplies electric energy from the PV module to the grid is 320 V. In the proposed PV system, the minimum DC voltage is 75 V. This result implies that the small DC electric energy generated by a solar panel under insufficient

In the future, we will evaluate efficiency of the proposed PV system. We will determine an optimal tap operation of the tapped transformer.

ACKNOWLEDGMENT

This work was supported by MEXT Grant for the Strategic Research Foundation at Private Universities.

REFERENCES

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