In view of the fact that once the isolated island phenomenon occurs in the photovoltaic grid-connected system and the photovoltaic system is not cut off in time, it may cause equipment damage and threaten personal safety, so the detection of isolated island needs to be fast and accurate, and the power quality should be taken into account. A design method of software and hardware for islanding detection controller is proposed, which can realize the rapid detection of photovoltaic grid-connected islanding.
New renewable energy micro-grid systems, thermostatic element such as wind power generation and photovoltaic power generation, systematically combine generators, new energy sources, loads, energy storage devices and control devices to form a single controllable unit, while supplying electricity to users and power grids. However, while microgrid provides supplement and support for the large power grid, there are also some hidden dangers for the large power grid. For example, the harmonic content and current distortion injected into the large power grid can not exceed the limit, and can not affect and damage other equipment in the large power grid; in addition, when the large power grid is blackout due to failure or maintenance, if the micro-grid continues to supply power to the large power grid, it will endanger the safety of equipment and maintenance personnel in the large power grid, resulting in. Incalculable loss. Based on this, the large power grid puts forward two requirements for access to the micro-grid: one is the quality of power injected into the large power grid, the other is the security of the large power grid. This requires that the microgrid system has fast and accurate islanding detection capability, while ensuring that the quality of power injected into the grid meets the requirements of the large grid.
Therefore, the islanding detection controller of microgrid has important significance for the development of microgrid. The island detection controller plays the role of connecting the large power grid and the micro-grid, which requires the ability of fast detection and fast disconnection from the large power grid. Therefore, the islanding detection controller of microgrid has the detection functions of DC voltage and current, AC voltage and current and DC bus voltage on the whole. According to the detected data, the maximum power output of new energy is controlled.
Then, according to the output power of new energy, the current control of grid connection is controlled.
At the same time, the voltage and current of AC side are also controlled. The detection result calculates the frequency and active power of the system, and introduces the fluctuation of active power into the active frequency shift islanding detection method with positive feedback, which improves the speed of islanding detection and reduces the distortion of grid-connected current. The whole controller mainly includes detection and operation (as shown in Figure 1). The control of switch Q5 achieves maximum power point tracking of photovoltaic system and voltage stabilization of DC bus, while switch Q1, switch Q2, switch Q3 and switch Q4 achieves reasonable control of four switches of inverters, and controls the realization of grid-connected current control and islanding detection method (as shown in Figure 2). The ultimate goal of the control is to get appropriate switching control signals through certain operation processing, so that the photovoltaic inverters can operate correctly, efficiently and reliably.
The controller must have strong operation ability and real-time response speed. It mainly needs to complete high-speed real-time functions such as duty cycle disturbance MPPT algorithm, joint PWM control of grid-connected current amplitude and waveform and islanding effect detection algorithm, while reserving communication ports to complete low-speed functions of data exchange with the host computer. The hardware structure of the islanding detection controller of the microgrid is shown in Fig. 3. The control process of photovoltaic grid-connected system can be summarized as: voltage signal acquisition, switch duty cycle calculation based on MPPT, grid-connected current control and island detection scheme, digital PWM modulation and integrated output of PWM signal with fault protection logic, etc.
Among them, the hardware design of the controller is closely related to analog acquisition, switch duty ratio calculation, digital PWM modulation, fault protection logic synthesis of output PWM signal and communication interface with PC. The active frequency shift island detection method based on active power variation is mainly based on the positive feedback active frequency shift method. In islanding detection, two problems need to be solved: one is the sampling and tracking of the current frequency of the power grid, which requires the speed and accuracy of detection; the other is the selection of frequency offset, which has a large offset value and a large distortion under normal conditions, which will greatly affect the power quality of the injected power grid, while the frequency offset value is small and the frequency offset frequency is high. The slow speed of rate limit will result in the decline of detection ability.
Therefore, in order to better select the appropriate offset, the active frequency shift islanding detection method with active power change is adopted. According to the actual situation that the islanding phenomenon often accompanies the active power change at the common end, the change of active power is introduced into the frequency offset to increase the offset speed of frequency shift and shorten the isolation. Time of island detection. At the same time, a smaller frequency offset is adopted when no islands occur, which makes the distortion of the injected power grid smaller and keeps the power quality clean. The flow chart of active frequency shift method based on active power is shown in Fig. 4. When the power grid is disconnected, the variation of active power increases the frequency deviation and accelerates the frequency offset.
When the output frequency reaches the frequency protection threshold and islanding phenomenon is detected, the photovoltaic system stops working.
Compared with the positive feedback active frequency shift islanding detection method, the detection time of islanding by this method is shorter, and the output current waveform distortion changes little compared with the normal operation. The design of photovoltaic grid-connected islanding detection controller includes hardware and software. This paper presents the overall design of the whole control system.
Around the realization of islanding detection function in grid-connected photovoltaic inverters, the hardware circuit of the whole back-stage inverters (such as driving circuit, sampling circuit, power grid frequency capture circuit, etc.) and the software of the whole system are designed, and the active frequency shift based on active power change is given. Flow chart of islanding detection program. The controller can realize fast and accurate detection of photovoltaic grid-connected islands.