The commonly used grid-connected control of photovoltaic systems mainly includes two closed-loop control links: the maximum power point control link and the output waveform control link.
1. Maximum power point control link
According to the output characteristics of photovoltaic cells, under certain sunshine intensity and temperature, the output power of photovoltaic cells can reach the maximum value only at a certain output voltage. The point at which this value is located is called the maximum power point (MPP). The actual output characteristics of solar cells are affected by various external factors such as sunshine intensity, ambient temperature and load conditions, and the output voltage and current will vary greatly, thereby affecting the output power and reducing the efficiency of the photovoltaic system. Therefore, the adjustment of the operating point of the photovoltaic array to make it work at the maximum power point is one of the key technologies of photovoltaic system control.
For ease of illustration, the output characteristics of the PV array are plotted as shown in Figure 1. Assume that curve 1 and curve 2 in the figure are the output characteristic curves of the photovoltaic array under two different sunshine intensities, and points A and B are the corresponding maximum power output points respectively; and it is assumed that the system runs at point A at a certain moment. When the sunlight intensity changes, that is, the output characteristic of the photovoltaic array rises from curve 1 to curve 2. At this time, if the load 1 is kept unchanged, the system will operate at point A’, which deviates from the maximum power point under the corresponding sunshine intensity. In order to continue to track the maximum power point, the load characteristics of the system should be changed from load 1 to load 2 to ensure that the system operates at the new maximum power point B. Similarly, if the change of the sunlight intensity reduces the output characteristic of the photovoltaic array from curve 2 to curve 1, the corresponding operating point changes from point B to point B’, and load 2 should be reduced to load 1 accordingly to ensure that the system still operates at maximum power point A when the sunlight intensity decreases.
The current photovoltaic system often adopts the method of maximum power point tracking (MPPT) to change the working state of the system in real time, so as to track the maximum power operating point of the photovoltaic array and realize the maximum power output. Common maximum power tracking control methods mainly include constant voltage tracking method, disturbance observation method, incremental conductance method, etc. The next article will describe these three methods in detail.
2. Output waveform control method
The output waveform control is mainly to enable the output current of the inverter to track the grid voltage in real time. At present, the commonly used output waveform PWM control methods include triangular wave comparison method, hysteresis comparison method, timing comparison method and so on.
(1) Triangular wave comparison method. The triangular wave comparison method compares the command current iref with the real-time value of the grid-connected current i, and the obtained current error is adjusted by the PI regulator and compared with the triangular wave to control the current error to the minimum. Figure 2 shows the control principle diagram of this method.
From the schematic diagram of the triangular wave comparison method and its working principle, it can be seen that this method still has disadvantages such as large following error, and the output waveform contains the same frequency harmonics as the triangular carrier wave.
(2) Hysteresis comparison method. The hysteresis comparison method compares the command current with the real-time value of the grid-connected current, and the obtained current error is used as the input of the hysteresis comparator, and the PWM signal that controls the switch in the main circuit is generated through the hysteresis comparator. The drive circuit controls the on-off of the power device, thereby controlling the change of the grid-connected current. Figure 3 shows the schematic diagram of the hysteresis comparison method.
The hysteresis comparison method has many advantages, such as simple and easy-to-control hardware circuit, real-time control, fast current response, and no need for carrier waves. However, the output current of this method contains more high-order harmonics, and because the width of the hysteresis loop is fixed, the error range of the current following is fixed, and the switching frequency of the switching device changes, which will result in a wider current spectrum and increase the difficulty of filter design.
(3) Timing comparison method. The timing comparison method uses a timing-controlled comparator to sample the command and the controlled signal with a fixed sampling period, and then controls the on-off of the inverter switching device according to the polarity of the deviation, so that the PWM signal changes at least once in one clock period. Figure 4 is a schematic diagram of the timing comparison method. The timing comparison method can prevent the switching frequency of power devices from being too high, but the disadvantage is that the current control error does not have a certain loop width, and the control accuracy is low.