DC side control technology
1. Maximum power point tracking technology
The efficiency of the photovoltaic power generation system is the product of the photovoltaic conversion efficiency of the solar panel, the maximum power point tracking (MPPT) efficiency and the inverter efficiency. Improving the efficiency of MPPT is also of great significance to the efficiency improvement and cost reduction of photovoltaic power generation systems. Under different light intensity and battery temperature, the maximum output power points (I-V characteristics) of photovoltaic cells are different. In order to get the best energy utilization, measures must be taken to make the battery output automatically track changes in environmental conditions. MPPT technology is proposed to address this issue.
MPPT technology is a hot spot in photovoltaic power generation technology research, and a large number of research papers are published every year. Common MPPT algorithms at this stage include open circuit voltage method, short circuit current method, disturbance observation method, incremental conductance method, and intelligent tracking algorithms based on fuzzy and neural network theories. The open-circuit voltage method and the short-circuit current method generate the tracking reference of the photovoltaic cell in a fixed ratio, which is simple to implement, but the control accuracy is low, and it is always lower than the actual maximum power point. The disturbance observation method gradually adjusts the operating point of the system to about the maximum power point by regularly increasing and decreasing the output voltage of the photovoltaic cell. When the light intensity changes smoothly, the tracking effect is better, the implementation is simple, and the sensor requirements are not high. It is currently one of the most commonly used methods to achieve MPPT. However, when the light intensity or ambient temperature changes sharply, such as the occlusion of floating clouds, the disturbance observation method may fail to track. Incremental conductance method is to change the control signal to track to the maximum power point by comparing the increment of the output conductance of the photovoltaic cell and the instantaneous conductance value.
From the above analysis: it can be seen that there are deficiencies in a single MPPT algorithm. The composite MPPT method based on the combination of two or more MPPT methods is a beneficial attempt, which improves the MPPT performance to a certain extent; in view of the uncertainty of solar light intensity and array temperature, the use of modern control methods such as fuzzy logic control can achieve ideal results; the application of modern control technology has opened up a broad space for the research of high-performance MPPT technology.
2. Half-bridge inverter capacitor voltage balance control
In addition to the above-mentioned MPPT control technology, non-isolated grid-connected inverter topology will also cause additional DC side control requirements. In particular, the DC input end of the three-level neutral point clamped (Three-level Neutral Point Clamped, 3L-NPC) half-bridge topology is a capacitor series structure. Due to the non-ideal factors of the circuit components, the series capacitance in the half-bridge topology has the problem of voltage imbalance. Under the condition of closed-loop control, when the capacitor voltage is unbalanced, the AC output voltage will be distorted, including DC and even harmonics; in addition, the unbalanced capacitor voltage makes the switch tube withstand voltage inconsistent. If improperly controlled, it will even further aggravate the unevenness of the capacitor voltage and eventually cause the system to collapse. Therefore, a suitable method must be adopted to balance the capacitor voltage on the DC side of the half-bridge inverter. At present, capacitor voltage balance control is mainly divided into hardware control method and software control method.
The hardware control method mainly includes multiple direct current source method, parallel large resistance voltage divider method, and additional switch circuit method. In the multiple DC source method, multiple DC sources are used to replace the DC side capacitors. This method can simplify system control, but the hardware cost is high, and it is difficult to generate multiple isolated DC sources. In the method of connecting capacitors in parallel with large resistance, the resistance value is large so that the voltages of the two capacitors are approximately equal. The circuit of the additional switch circuit method is shown in Figure 1. When the voltages of the two capacitors are unbalanced, energy transfer is achieved through the voltage-balancing inductor LB, and usually LB works in discontinuous mode. Figure 2 shows the equivalent path of energy transfer when VC1>Udc/2>VC2, as shown by the dotted line, where VC1 and VC2 are the average voltages of capacitors C1 and C2, respectively. When VC1>Udc/2, the switching tube T1 is turned on, and the capacitor C1 transfers energy to the inductor LB; when VC1=Udc/2, the switching tube T1 is turned off, and the inductor LB transfers energy to the capacitor C2. The method of adding hardware circuits increases the volume, cost and loss of the converter, and complicates the system structure, which affects the reliability of the entire system.
Different from the hardware control method, the software control method mainly uses additional control algorithms to achieve capacitor voltage balance, including SVPWM modulation method, capacitor voltage difference control method, and modulation wave DC component feedback method. The SVPWM modulation method realizes the equalization of the capacitor voltage by adjusting the action time of the small vector. The capacitor voltage difference control law is to directly perform feedback control on the capacitor voltage difference to achieve voltage balance. In addition, considering that the corresponding DC component will appear in the modulating wave when the capacitor voltage is unbalanced, the method of bringing out the DC component in the modulating wave for feedback control is also a feasible way to eliminate the capacitor voltage difference.