Applied to solar photovoltaic power station grid-connected inverter power supply system

1. System Function Description

The wind-solar photovoltaic power station inverter system developed using LPCS2000B has the main function of inverting the DC power generated by solar cells into three-phase AC power and sending it to the power grid. It also solves the problems of maximum conversion efficiency, harmonic interference, protection, etc. in grid-connected inverter.

2. System Function Block Diagram

3. Description of system control part

The function of the control part is to control the power part to generate a voltage in phase with the grid and to track the actual output power so that the output power of the power supply is equal to the maximum output power of the solar cell.

The control part is the LPCS2000B power control board, and the core device of the control board is the industrial DSPTMS320F2801 produced by Ti. Through the DSP multi-channel high-speed ADC sampling, the grid voltage, solar cell terminal voltage, solar cell output current, and DC voltage regulation output voltage are monitored respectively. At the same time, the DSP generates switch tube actions for voltage regulation and commutation, and generates a sine wave output.

The system can be controlled via a local control panel and also provides remote control based on RS232 interface.

4. System power description

a) Solar cell array

It is composed of 50Wp polycrystalline silicon solar cell modules, 24 modules are connected in series, and 9 modules are connected in parallel. The open circuit voltage is DC510V, the short circuit current is DC27A, the best working voltage is DC408V, and the best working current is DC25A.

b) DC voltage regulation circuit

The DC voltage generated by the solar cell is modulated into a 100Hz, 220V (RMS) steamed bun waveform.

c) Full-bridge commutation circuit

The 100Hz unipolar steamed bun waveform is commutated into a 50Hz bipolar AC waveform.

d) Three-phase transformer

It is coupled to the transmission network through a three-phase transformer.

5. MPPT to improve solar energy utilization efficiency

The maximum power point tracking control (MPPT) strategy monitors the output power of the photovoltaic array in real time, uses the interference observation control algorithm to predict the maximum power output of the array under the current working conditions, and changes the current impedance to meet the maximum power output requirements. When the temperature and light intensity are constant, the relationship between the terminal voltage of the solar cell and the output power.

6. System protection description

a) Grid connection protection

As a part of the power system, the photovoltaic grid-connected system needs to be connected to a protection device. On the one hand, it protects the photovoltaic power generation system to prevent islanding effects, etc. On the other hand, a relay protection device needs to be installed to prevent line accidents or power instability.

These protection functions include undervoltage protection, overvoltage protection, underfrequency protection, overfrequency protection, overcurrent protection and islanding protection.

b) Island problem

The inverter loses the grid system voltage that it relies on for reference, which is called the islanding effect. The generation of the islanding effect may complicate the reconnection of the grid and cause harm to the components in the grid.

This system uses both passive and active islanding detection technologies to ensure reliable detection of islanding phenomena and ensure equipment safety.

7. System electromagnetic compatibility interference processing

a) Filtering

By installing a differential mode filter at the output wires between the controller and the photovoltaic array, the influence of electromagnetic interference caused by the photovoltaic array and the wires between the photovoltaic array and the controller is eliminated.

By connecting capacitors of appropriate capacitance to the positive and negative poles of the photovoltaic array output, the interference caused by the voltage fluctuation of the photovoltaic array output can be eliminated.

In a photovoltaic power generation system with button or relay operation, a filter capacitor is added to the control circuit.

b) Lightning protection

On the one hand, lightning rods are installed, and on the other hand, lightning arresters are installed at the connection points of the photovoltaic array output wires.

c) Grounding

Connect the system casing and the grounding terminal of the lightning arrester to the ground to ensure the safety of the system and personnel.

The signal grounding adopts single-point grounding to prevent conducted interference between circuits and common ground conducted interference.

d) Electromagnetic shielding

Install conductive pads in the shell, apply conductive materials at the joints, adjust the spacing between fastening nails, install metal mesh at large openings, turn large holes into multiple small holes, and use single-point grounding for the entire shielding layer.