Analysis of transient operating point characteristics of constant voltage tracking photovoltaic water pump system

Photovoltaic water pump system is a typical high-tech opto-mechanical integration, and is the preferred technology recommended by the United Nations International Development Agency (UNDP) to developing countries. It is reported that tens of thousands of photovoltaic water pump systems of different specifications have been put into operation around the world, and their application scale is expanding year by year, especially in developing countries such as Asia, Africa, and Latin America. It is reported that India has installed about 4,000 new photovoltaic water pump systems in the past five years, and plans to promote the installation of 50,000 more sets. It is expected that by 2010, there will be 500,000 photovoltaic water pump systems in operation in the world. After more than ten years of efforts, my country has successfully developed 2.5kW and 5kW photovoltaic water pump systems, and has been put into field operation in different regions. At present, these systems basically use constant voltage trackers (CVT) instead of maximum power point trackers (MPPT). For areas with large temperature differences between winter and summer throughout the year, since CVT cannot adapt to the changes in the volt-ampere characteristics of the solar cell array with light intensity and temperature, the system operating point deviates from the maximum power point of the solar cell array, resulting in system mismatch losses. This paper introduces the composition of a 2.5kW photovoltaic water pump system and typical field operation test data, and analyzes and discusses the typical daily transient operating point characteristics of the system.

2 Basic system structure

The photovoltaic water pump system is mainly composed of four parts: solar cell array, inverter controller, motor and water pump. The solar cell array consists of four groups in parallel, each group consists of 18 35W monocrystalline silicon solar cell modules in series. The working voltage of a single module is about 17V, and the working current is about 2A. The working voltage of each group is about 306V, and the working current is about 2A. The total output working voltage of the array is about DC306V, the total working current is about 8A, and the nominal total output power is 2500W.

The inverter controller converts the DC power output by the solar cell array into three-phase AC power, with an input voltage of DC300V, a rated output voltage of AC220V, and an initial operating frequency of 25Hz. It mainly consists of six parts: constant voltage tracker (CVT), Duck DC/DC converter, controllable voltage controlled oscillator (V/f), ring distributor, inverter drive and main circuit, and DC/DC converter for powering the control circuit.

The three-phase asynchronous motor and submersible pump form a submersible electric pump assembly, which operates with variable frequency. The motor has a nominal power of 1.5kW and a rated working voltage of three-phase 220V. The water pump is a 6-inch 5-stage submersible pump with a rated head of 45m.

3 Field operation data collection

The solar array is installed at 10° south-west, with an adjustable inclination of 30° to 55°. The inverter controller is placed in a cool, ventilated and dry place in the pump station. The submersible pump is placed 15m below the dynamic water level in the well, and the outlet pipe is connected with a flange. After the system is installed, the output voltage of the solar array is checked first, and then the four solar arrays are connected to the inverter controller in parallel one by one, and the inverter drive motor is started to run the experiment and collect data. Figure 1 is a real photo of the solar array on site.

Figure 1 Photo of the solar cell array of the photovoltaic water pump system

The instruments used for data collection are

1) Domestic DT9907C digital multimeter;

2) Nissan HCL-60 digital thermometer;

3) Class II standard solar cells (standard value Isc=159.11mA). Fix the standard solar cells on the solar cell array surface and measure the instantaneous solar radiation intensity incident on the array surface. At the same time, measure the array output working current, working voltage, component temperature and ambient temperature. Collect data every 1 hour. The typical day's field operation data is shown in Table 1.

Table 1 Typical daily field operation data of 2.5kW photovoltaic water pump system

4 Data Analysis and Discussion

It can be seen from the typical daily operation data that the instantaneous working voltage of the system basically tracks around 296V and drifts with the changes in solar radiation intensity and component temperature. Analysis of the data in Table 1 shows that when the solar radiation intensity is 730W/m2, the component temperature is 50℃, and the array working voltage is 296V, the array working current reaches 6.4A. When the solar radiation intensity is 830W/m2, the component temperature is 58℃, and the array working voltage is 298V, the array working current is 6.0A. The array working current decreases with the increase of solar radiation intensity, reflecting that the system transient working point deviates from the maximum power point. Figure 2 shows the instantaneous changes of solar radiation intensity, array working voltage and working current on a typical day. It can be further seen that when the array working voltage is basically constant, the array working current begins to increase linearly with the increase of solar radiation intensity, and when it reaches a certain value, it decreases with the increase of solar radiation intensity. When the solar radiation intensity decreases, the array working current begins to increase slightly and then decreases linearly. During periods of strong solar radiation, the array operating current shows abnormalities. This is because the array volt-ampere characteristics deteriorate as the component temperature rises, and the CVT cannot adapt to this transient change, causing the system to deviate from the maximum power point, resulting in power loss.

Figure 2 Instantaneous changes in solar radiation intensity, array operating voltage and operating current

Figure 3 shows the instantaneous change of the module temperature with the intensity of solar radiation. It can be seen that the ambient temperature is basically constant, and the module temperature changes approximately linearly with the change of solar radiation intensity. When the ambient temperature is 30℃ and the solar radiation intensity is 750W/m2, the module temperature reaches 60℃. The change of solar radiation intensity and module temperature causes the drift of the system working point. Figure 4 shows the change of the transient working point of the system on a typical day. Comprehensive analysis shows that the working voltage is set at 296V, which is too high during the period of strong solar radiation in summer, and the system cannot work effectively.

Figure 3 Instantaneous change of component temperature with solar radiation intensity

Figure 4 Typical day system transient operating point changes

5 Conclusion

The photovoltaic water pump system uses a constant voltage tracker (CVT), which is simple to manufacture and low in cost, but the system's transient operating point cannot adapt to the transient changes in the solar cell array's volt-ampere characteristics with temperature and light intensity. When the array's operating voltage is basically constant, the array's operating current changes abnormally with the change in solar radiation intensity. The system's operating point deviates from the array's maximum power point, resulting in system power loss. With the development of microelectronics and power electronics technology, in order to enable the photovoltaic water pump system to play its due role, the maximum power point tracking (MPPT) should be truly realized from a technical point of view to make the system more economical and reasonable.