Research on photovoltaic grid-connected power generation system based on 110kV Zhuishan substation

【Abstract】The structure of the photovoltaic grid-connected power generation system of the 110kV Zhuishan substation is introduced, the relevant technologies of the photovoltaic grid-connected power generation system are studied, and the benefits of the solar photovoltaic system are analyzed. From the perspective of solar power generation and energy-saving and emission reduction benefits, it is believed that the solar photovoltaic power generation system not only solves the problem of self-use electricity in the substation, but also supplements the urban electricity, while reducing energy consumption and pollution.

0 Introduction

Solar energy is the most abundant renewable energy with unique advantages and huge potential for development and utilization. Making full use of solar energy is conducive to maintaining the harmonious coexistence of man and nature and the coordinated development of energy and environment. Since the 21st century, the world's solar photovoltaic power generation industry has developed rapidly and the market application scale has expanded rapidly. In order to study the application technology of photovoltaic grid-connected power generation, Bengbu Power Supply Company applied for a scientific and technological project and established a photovoltaic grid-connected power generation system at Zhuishan Substation.

1 System Configuration

The total designed capacity of the photovoltaic system of Zhuishan Substation is 30kW, mainly including photovoltaic modules, grid-connected inverters, AC/DC lightning protection distribution cabinets and other parts. Photovoltaic modules convert solar energy into DC power under the photovoltaic effect. DC power flows into the grid-connected inverter through the AC/DC lightning protection distribution cabinet. The grid-connected inverter converts it into AC power that meets the power quality requirements of the power grid. It is connected to the AC380V/50Hz three-phase AC power grid through the AC/DC lightning protection distribution cabinet for grid-connected power generation. The system uses high-power monocrystalline silicon solar cell modules. The total effective area of ​​photovoltaic modules is 220m2, covering an area of ​​about 470m2.

Each solar cell string is designed to be connected in series with 8 battery modules, and the system can be divided into 21 solar cell strings. In order to reduce the connection lines between the battery modules and the inverter and facilitate future maintenance, 4 photovoltaic array lightning protection junction boxes are configured on the DC side. The DC output is transmitted to the secondary room through cables, and then merged into a DC bus through the DC unit of the AC/DC lightning protection distribution cabinet and input to the SG30K3 grid-connected inverter, and then connected to the AC380V/50Hz three-phase AC low-voltage power grid through the AC unit of the AC/DC lightning protection distribution cabinet.

The entire photovoltaic grid-connected power generation system is equipped with a monitoring device, which provides the system's working status, operating data and environmental parameters to professional technicians for real-time monitoring through RS485 or Ethernet communication interface.
The system schematic diagram is as follows:

2 System Technical Requirements

2.1 The photovoltaic grid is required to supply power to the station load during the day, and the excess power is fed into the grid. When the power generation is insufficient at night or on rainy days, the station load is supplied by the mains.
2.2 The system is required to have the maximum power point tracking control function to enable the system to obtain the maximum power output;
2.3 The system is required to have grid-connected protection functions such as over/under voltage protection, over/under frequency protection, short circuit protection, and anti-islanding effect;
2.4 The system is required to have remote communication function to realize remote monitoring of system operation.

3 Design of solar cell array

3.1 Solar cell power generation principle and classification

When the solar cell array is illuminated, the cells absorb light energy, and opposite charges accumulate at both ends of the cells, which generates "photovoltaic voltage", which is the "photovoltaic effect". Under the action of the photovoltaic effect, electromotive force is generated at both ends of the solar cell, converting light energy into electrical energy. Solar cells are energy conversion devices. When sunlight shines on the semiconductor PN junction, voltage is generated on both sides of the PN junction, causing the PN junction to short-circuit, and current will be generated. This current increases with the increase of light intensity. When the intensity of the received light is constant, the solar cell can be regarded as a constant current source. There are mainly the following types of solar cells: single crystal silicon cells, polycrystalline silicon cells, amorphous silicon cells, cadmium telluride cells, copper indium selenide cells, etc. At present, nano-titanium oxide sensitized cells, polycrystalline silicon thin films and organic solar cells are also being studied. However, the actual application is mainly single crystal silicon cells and polycrystalline silicon cells. Among them, single crystal silicon cells have the highest conversion efficiency, so the photovoltaic grid-connected power generation system of Zhuishan Substation uses single crystal silicon cells.

3.2 Design of PV array

A total of 168 180W monocrystalline silicon photovoltaic cell modules are used, divided into 21 units. Each unit array bracket can hold 8 solar cell modules. The modules are placed facing south, with a total of 5 rows of arrays, of which the 4 rows in the north contain 5 unit arrays each, and the single unit array in the south is a row. According to the latitude of Bengbu, the optimal design bracket is tilted at 27.4° to the horizontal ground.

The entire power generation system uses 8 modules connected in series as one unit, with a total of 21 branches connected in parallel, and input into 4 combiner boxes, 3 of which are connected to 5 inputs each, and the other combiner box is connected to 6 inputs. After the convergence, the cable passes through the cable trench and enters the AC/DC distribution cabinet in the main control room, and is connected to the SG30K3 grid-connected inverter through the DC unit of the AC/DC distribution cabinet, and finally from the grid-connected inverter to the 380V low-voltage grid through the AC unit of the AC/DC distribution cabinet.

4 Research on PV Grid-connected Inverters

4.1 Performance characteristics

The photovoltaic grid-connected inverter adopts the DSP control chip of TI Company in the United States. The main circuit is assembled with the most advanced intelligent power IPM module in Japan. It uses the current-controlled PWM active inverter technology and high-quality imported high-efficiency isolation transformer. It has high reliability, complete protection functions, and has the advantages of high power factor sine wave current on the grid side and power supply without harmonic pollution. Its structure is shown in Figure 2.

The grid-connected inverter converts the DC voltage of the photovoltaic array into a high-frequency three-phase AC voltage through a three-phase inverter, and then filters it into a sine wave voltage through a filter, and then isolates and boosts it through a three-phase transformer before being connected to the grid for power generation. In order to enable the photovoltaic array to generate power at maximum power, an advanced MPPT (Maximum Power Point Tracking) algorithm is used on the DC side.

4.2 Solar Cell Maximum Power Point Tracking Control Algorithm Based on Fuzzy Control

4.2.1 Solar Cell Characteristics

The intensity of sunlight greatly affects the output current of the solar array. Figure 3 shows the typical IV and PV characteristics under different sunlight intensities.

Figure 4 is the output power characteristic PV curve of the solar cell array. It can be seen from the figure that when the array operating voltage is less than the maximum power point voltage Vmax, the array output power increases with the increase of the solar cell terminal voltage Vpv; when the array operating voltage is greater than the maximum power point voltage Vmax, the array output power decreases with the increase of Vpv. The realization of MPPT is essentially a self-optimization process, that is, by controlling the array terminal voltage Vpv, the array can intelligently output the maximum power under various sunshine and temperature environments.

This project adopts the MPPT control algorithm based on fuzzy logic, achieving good dynamic response speed and accuracy.

4.2.2 MPPT based on fuzzy logic controller Based on fuzzy theory of fuzzy sets and fuzzy algorithms, a series of fuzzy control rules can be derived, which can be executed very simply by DSP. The design of fuzzy logic controller mainly includes the following contents:

1) Determine the input and output variables of the fuzzy controller;
2) Summarize and summarize the control rules of the fuzzy controller;
3) Determine the methods of fuzzification and defuzzification;
4) Select the domain and determine the relevant parameters. The composition of the fuzzy logic controller that implements MPPT is shown in Figure 5.

4.2.3 Fuzzy Inference Algorithm

In fuzzy theory, there are many inference methods for fuzzy control, but the most commonly used in fuzzy control are Mamdani inference, Larsen inference, Tsukamoto inference and Takagi-Sugeno inference. This paper uses Mamdani inference as an example to illustrate the control algorithm, and the inference rules are defined in the form of "if...then..." statements.

For example: "if (dP/dI is PB) and (△dP/dI is PB) and (△UDC (k-1)is P) then (△UDC (k) is PB)". As shown in Figure 6, "dP/dI is PB" means that the current working point of the solar array is located in the left half of the PI curve with a larger slope, so the FLC output △UDC (k) should be positive. Considering that "△UDC (k-1) is P" means that the last FLC output △UDC (k) was positive, that is, the grid-connected output power increased; then consider "△dP/dI is PB", which means that the slope change is positive, that is, the current slope is significantly larger than the previous slope. It can be seen from the PI curve that if the sunshine remains unchanged, in the left half of the curve, P increases, then the slope dP/dI is decreasing, while the current slope dP/dI is increasing, which means that the current sunshine has increased significantly. Therefore, the output △UDC (k) of FLC should also be a large positive value. So △UDC (k) is PB.

4.2.4 Defuzzification of exact output

The purpose of defuzzification is to find the exact value that can represent the effect of all fuzzy output quantities. That is, to take out the single value that best represents the fuzzy set from the fuzzy set obtained by inference. For this article, the output of FLC should be a specific △UDC (k) provided to the DC voltage outer loop control program of the subsequent stage.

Different methods can be used for anti-fuzzy judgment, and the results obtained by different methods are also different. Commonly used methods are: maximum membership method, centroid method, coefficient weighted average method and membership limit element average method. This paper adopts the centroid method. Its calculation expression is as follows:
In the above formula, μ(Di) is the membership of the i-th fuzzy output quantity, that is, the result of fuzzy reasoning; Di is the position of the i-th fuzzy output single point or the position of the central element; n is the number of fuzzy output quantities of the defined system, and n is 5 in this project.

5 Operational benefits

The total power of the photovoltaic system of the 110kV Zhuishan substation of Bengbu Power Supply Company is 30kW, the efficiency of the DC part of the system is 90%, the efficiency of the AC part is 90%, and the average peak sunshine time in Bengbu area is 3 hours per day. The theoretical annual power generation of the system is: 30×3×365×90%×90%=26608 kWh. The actual power generation in 2009 was 25818 kWh.

Traditional thermal power generation mostly uses coal as fuel and produces a lot of pollution, mainly dust, carbon dioxide, sulfur dioxide, thermal pollution and chemical pollution. A large amount of pollution treatment and environmental protection costs are required every year. Solar power generation does not require fuel consumption and does not pollute the air. It can save standard coal and reduce greenhouse gas emissions (including CO2, NOx, etc.), realize the clean use of energy, and make great contributions to environmental protection and energy conservation. Calculated based on 374g standard coal consumed per kilowatt-hour of electricity, 9.724 tons of standard coal are consumed for 26,000 kWh of electricity. Calculated based on 170gCO2 and 7.68gSO2 can be reduced for each kilowatt-hour of electricity, 26,000 kWh of photovoltaic power generation can reduce CO2 emissions by 4.42 tons and SO2 emissions by 199.68kg.

6 Conclusion

The photovoltaic grid-connected power generation system of the substation uses solar energy, a green, environmentally friendly and pollution-free clean energy. It is energy-saving and environmentally friendly, and it increases the reliability of the substation power system. When the power generation is insufficient at night or on rainy days, the power grid supplies power to the station power load, and generally achieves "zero consumption" of the city electricity for the station power load throughout the year. Although the cost of solar photovoltaic power generation systems is relatively high at present, and its economic benefits are not obvious compared to other forms of power generation, with the industrialization of photovoltaic power generation and the scarcity of conventional energy, its investment cost will inevitably decrease gradually and the economic benefits will become more and more obvious.