Control of the Photovoltaic System for Flexible Power Generation

Abstract

This thesis presented a new maximum power point tracking (MPPT) technique to improve the efficiency of the photovoltaic (PV) system under partial shading conditions. This method uses a combination of particle swarm optimisation (PSO) and perturb and observe (P&O) methods to accelerate MPPT searching. The proposed MPPT method was validated and compared with other MPPT methods by simulation and experimental studies under different partial shading conditions. The results show the proposed accelerated PSO method can easily distinguish the global MPP maximum power point (MPP) from local MPPs in every test condition. Additionally, the accelerated PSO method can also successfully detect the occurrence of partial shading condition, and hence can easily capture the new global MPP. Moreover, compared with a standard PSO method, the accelerated PSO method provides faster convergence speed and better dynamic response. A novel MPPT method based on an oversampling technique was then proposed to enhance the performance of conventional MPPT methods, such as the P&O method under low irradiance conditions. The conversion resolution of 8-bit analog to digital converter (ADC) can be improved using this method by running the ADC at 16 times of the MPPT perturb rate and then using the average method. The performance of the novel MPPT method and P&O method was then evaluated under different low irradiance conditions by simulation and experimental tests. The results demonstrate the superiority of the novel MPPT method which exhibited better performance in all test conditions. A modified MPPT algorithm with integrated active power control was then developed for an off-grid PV-battery system. When the battery charging voltage or current is below the threshold, the proposed controller works in MPPT mode. However, when the battery voltage or current exceed the threshold, the proposed controller will automatically enable active power control to reduce the battery charging current and voltage. The proposed method was validated by simulation and experimental studies. Varied DC loads conditions were tested to evaluate the performance of the proposed off-grid PV system management strategy. The simulation and experimental results show that the proposed MPPT method with integrated active power prevents overcharging of the battery by curtailing PV output power. Therefore, the lifespan of the battery can be improved, and the operational costs of an off-grid PVbattery system reduced. A new control strategy was then proposed to prevent the negative impact caused by PV and load variation within the DC microgrid from transferring to the main grid. This was based on a double loop current control combined with a battery DC-DC converter which absorbed the PV and load variations. A detailed investigation was then conducted to develop inner and outer current controllers for the DCDC converter, whereby two transfer functions were derived. This described the dynamic response of the inductor current of the battery converter corresponding to the duty cycle variation and the dynamic response of grid interface current corresponding to the inductor current variation. The visibility of the proposed control strategy was evaluated through an experimental study. The results confirm that the proposed control strategy is capable of keeping the grid interface current between the DC microgrid and main grid constant regardless of the PV and load disturbances.