A new PWM control method for photovoltaic controller

In remote areas far from the power grid, solar power generation uses photovoltaic controllers, battery groups, and photovoltaic panels to form independent photovoltaic power stations, among which the photovoltaic controller is the core of the entire power station. The topological structure of photovoltaic controllers is usually divided into two categories: DC/DC type and direct type [1]. The DC/DC type can be further divided into MPPT type [2] and resonant type. However, due to the presence of large inductive components, the volume, weight and heat of DC/DC controllers will increase sharply when large currents are used, limiting their practical application in the field of high power. Direct type controllers have advantages in the field of high power. Even if the photovoltaic current reaches several hundred amperes, its volume, weight and heat are relatively small. Therefore, direct type controllers have been widely used in high power fields such as mobile communication base stations and border checkpoints. However, direct type controllers still have some defects. The advantages and disadvantages are analyzed below.
1 Deficiencies of existing control methods
Existing direct type photovoltaic controllers usually use three types of charge and discharge control modes to control the charge and discharge of batteries. (1) Step-by-step input system [3], which divides the photovoltaic cell into N independent photovoltaic sub-arrays and defines N battery voltage control points Vi ₍ i=1, 2, ...N; Vi _< Vi ₊₁ ). When the battery voltage is greater than Vi, the i-th photovoltaic sub-array is turned off, otherwise it is turned on. In this way, as the battery voltage increases, the charging current decreases step by step; otherwise, it increases step by step. Advantages: This charging control method basically meets the charging needs of the battery, the control logic is simple and easy to implement, and the switching energy loss of the electronic power switch device is very small; Disadvantages: The control accuracy is not high, the voltage fluctuation range is large, and some advanced automatic control algorithms cannot be implemented. (2) On this basis, an improved control method with a time factor is added, and the battery voltage control point is set to 1 control point Vs. When the battery voltage is greater than Vs, the i-th photovoltaic sub-array is turned off. After a fixed delay, if the battery voltage is still greater than Vs, the i+1-th photovoltaic sub-array is turned off, and so on, until the N-th photovoltaic sub-array is turned off; otherwise, it is turned on, and the conduction process also has the above delay. Advantages: This charging control method reduces the variation range of the battery voltage and has the advantages of the previous charging control method; Disadvantages: It is easy to cause controller oscillation, especially the selection of delay time, which must be changed with the configuration of solar cells, battery capacity and load, otherwise it will lead to loss of control, and in severe cases, the battery will be overcharged or over-discharged and scrapped. (3) Pulse width modulation system (full control type PWM control method), that is, the photovoltaic cells are not divided into sub-arrays, and all photovoltaic sub-arrays are connected in parallel to form a total photovoltaic cell array, and then a high-power electronic switch is used for full-on and full-off PWM control. This method can accurately control the battery voltage at one voltage point. Advantages: high voltage control accuracy, various advanced automatic control algorithms can be used; Disadvantages: the switching power loss of power electronic switching devices is large. Under the same voltage level, the current level of power electronic switching devices is very high, and the requirements on devices are harsh. For high-power photovoltaic controllers, the heat sink volume is large.
2 New control method of fine and coarse adjustment combined PWM
Aiming at the shortcomings of the above three schemes, this paper proposes a new control method of fine and coarse adjustment combined PWM control. The photovoltaic cells are still divided into N independent photovoltaic sub-arrays with the same configuration (i=1, 2, ...N), but only the first photovoltaic sub-array (i=1) adopts PWM control, and the remaining photovoltaic sub-arrays (i=2, 3, ...N) still adopt ordinary switch control. The control method is: assuming that the total photovoltaic current when all N photovoltaic sub-arrays are turned on is I, then the photovoltaic current when each photovoltaic sub-array is turned on individually is I/N. If the PWM control duty cycle of the first photovoltaic sub-array varies from 0 to K, the PWM current of the first photovoltaic sub-array can be accurately controlled to (j/K)×(I/N), where j=0~K. The precise control of WM and the rough control of the switches of the remaining N-1 photovoltaic sub-arrays can be combined to obtain any precise current output within the current variation range of 0~I, and its value is: (j/K+m)×(I/N), where m is the number of the remaining N-1 photovoltaic sub-arrays that are turned on, m=0~N-1 (m=0 means that the remaining N-1 photovoltaic sub-arrays are all turned off); the controller only needs to select and calculate the values ​​of m(0~N-1) and j(0~K) to control the precise photovoltaic current output, and the current resolution accuracy is I/(KN), which is equivalent to the control effect of the PWM duty cycle variation range of 0~KN in the aforementioned third type of full-control PWM control method.
3 Implementation of fine and coarse combined PWM control
The microprocessor of this controller adopts the C8051F020 single-chip microcomputer [4], as shown in Figure 1. Through two external current sensors and voltage detection circuits, the parameters such as photovoltaic current, load current and battery voltage are obtained through AD conversion inside the microprocessor. The microprocessor sends out N switch control signals at the same time, of which the first signal is generated by the PWM control unit inside the microprocessor, and the second to N signals are generated by the ordinary digital I/O port (non-PWM) inside the microprocessor. When the i-th power electronic device is controlled to be turned on, the i-th photovoltaic sub-array charges the battery and supplies power to the load. The principle of battery charging control is to perform different constant voltage charging at different time periods. The charging process is divided into four processes: strong charging, equalized charging, absorption and floating charging. Except for strong charging, the three stages of equalized charging, absorption and floating charging are all constant voltage control. Various intelligent control algorithms can be used for the constant voltage control of the battery. This controller specifically adopts the PI (proportional integral) regulation algorithm, which is combined with the fine and coarse adjustment combined PWM control method for comprehensive implementation.

The transfer function structure of the control system is shown in Figure 2. VS is the set value of the battery voltage, VO is the actual output value of the battery voltage, and the difference △V between the two is input into the PI regulator to obtain the desired output current IO. The fine and coarse adjustment combined PWM is used to implement IO, and the implementation flow chart is shown in Figure 3. That is: divide IO by (I/N), take the remainder to get j, and take the integer to get m. Then let the PWM duty cycle of the first photovoltaic sub-array be j, let m of the remaining photovoltaic sub-arrays be turned on, and the remaining photovoltaic sub-arrays be disconnected, then the precise IO output is obtained: IO=(j/K+m)×(I/N). This current is provided to the battery and the load, and the battery output voltage VO is maintained at a constant voltage through the PI algorithm. In a control system composed of 6 photovoltaic sub-arrays, the PWM voltage, current and total photovoltaic current waveforms of the first photovoltaic sub-array are shown in Figure 4. The voltage here refers to the voltage across the power electronic switch, and in a relative time, the voltage and current of the second to sixth photovoltaic sub-arrays change very little (unless the coarse adjustment is in action), otherwise it is a straight line.

In this scheme, only one PV sub-array adopts PWM control, and the rest of the PV sub-arrays still adopt ordinary switch control. Compared with the overall PWM control after all PV arrays are connected in parallel, the PWM precise control realized by this fine and coarse adjustment combination reduces the PWM switch energy loss by (N-1)/N (N is the number of PV sub-arrays), and reduces the volume of the heat sink. Since multiple independent PV sub-arrays are still controlled separately, under the same voltage level, the current level requirement for the power switch device is very low. Low-cost power switch devices can be connected in parallel to realize one sub-array [5], which reduces the cost. At the same time, it also has high-precision current output for PWM control of all PV arrays. After testing, the system voltage regulation output meets the national standard [6]. Since the current involved in PWM chopping is small and the electromagnetic compatibility is good, it has passed the electromagnetic compatibility standard test and obtained CE certification. It has been actually applied to a series of PV controllers with a nominal voltage of -48 V and a current range of 30 A~400 A. Operation practice shows that this scheme has fully achieved the expected design effect.