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A Novel PV Controller PWM Control Method
- May 14, 2018 -

In remote areas far from the grid, solar power uses photovoltaic controllers, battery packs, and photovoltaic panels to form an independent photovoltaic power station. The photovoltaic controller is the core of the entire power plant. The topological structure of photovoltaic controller is usually DC/DC type and direct-through two types [1], DC/DC type can be further subdivided into MPPT type [2] and resonance mode, etc., but the DC/DC type controller Due to the presence of large inductive components, its volume, weight, and heat increase rapidly during high-current applications, which limits its practical application in high-power applications. The direct-through controller is relatively advantageous in high-power applications. Even if the photovoltaic current reaches several hundred amps, its volume, weight, and heat will not be relatively large. Therefore, the direct-through controller has been widely used in high-power fields such as mobile communication base stations and border posts. However, the through-type controller still has some defects. The following analysis of its advantages and disadvantages.




1 Insufficient existing control methods


    The current through-type photovoltaic controller generally controls the charge and discharge of the battery using the three types of charge and discharge control modes. (1) Step-by-step input system [3], that is, photovoltaic cells are divided into N independent photovoltaic sub-arrays, and N battery voltage control points Vi (i=1, 2, ..., Vi; Vi+1) are defined. 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, with the increase of the battery voltage, the charging current is gradually reduced step by step; otherwise it is increased step by step. Advantages: This charging control method basically meets the need of battery charging. The control logic is simple and easy to implement. The switching power loss of the electronic power switch device is very small. Disadvantage: The control accuracy is not high, the voltage fluctuation range is large, and some advanced automatic control. The algorithm cannot be implemented. (2) Based on this, an improved control method of time factor is added, and the battery voltage control point is set to one control point Vs. When the battery voltage is greater than Vs, the i-th photovoltaic sub-array is turned off. After a fixed time delay, if the battery voltage is still greater than Vs, the i+1th sub-array is turned off, and so on, until the Nth. The photovoltaic sub-array is turned off; otherwise, it is turned on. Advantages: This kind of charge control method reduces the range of battery voltage change, and has the advantages of the former type of charge control method; Disadvantages: It easily leads to controller oscillation, especially the choice of delay time, with the solar battery, battery capacity Changes in the configuration of the load may cause a loss of control. In severe cases, the battery may be overcharged or overdischarged and discarded. (3) Pulse width modulation system (full-control PWM control mode), ie, non-molecular array of photovoltaic cells, all photovoltaic sub-arrays are connected in parallel to form a total photovoltaic cell array, and then all high-power electronic switches are used to make all Full-off PWM control, this method can accurately control the battery voltage at a voltage point. Advantages: high voltage control accuracy, can use a variety of advanced automatic control algorithms; Disadvantages: power electronic switching devices, switching power loss is greater, at the same voltage level, the power electronic switching device current level requirements are very high, The device is demanding. For high power photovoltaic controllers, the heat sink is larger.



2 Coarse adjustment and combination of new PWM control methods


    Aiming at the shortcomings of the above three schemes, this paper proposes a new control method of fine-tuned combinational PWM control. The photovoltaic cell is still divided into N independent photovoltaic subarrays of the same configuration (i=1, 2,... N), but only the first photovoltaic subarray (i=1) is PWM controlled and the rest of the photovoltaic subarrays (i =2,3,...N) are still controlled by ordinary switches. The control method is as follows: Assuming that the total photovoltaic current when all N sub-arrays are turned on is I, then the photovoltaic current when each sub-array is turned on individually is I/N, if the PWM control duty cycle of the first PV subarray changes from 0 to K, the PWM current of the first PV array can be precisely controlled to (j/K)×(I/N). Where j=0~K varies; if the precise PWM control of the first photovoltaic subarray is matched with the coarse control of the switches of the remaining N-1 photovoltaic subarrays, the current variation range between 0 and I can be obtained The precise current output has the following value: (j/K+m)×(I/N), where m is the number of conductions of the remaining N−1 photovoltaic sub-arrays, and m=0~N−1 (m= 0, indicating that all remaining N-1 photovoltaic subarrays 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. Current resolution accuracy is I/(K N), which is equivalent to the control effect of the PWM duty cycle of the above-mentioned third-type full-control type PWM control method, which is 0 to KN.



3 Coarse adjustment and combination of PWM control


    The controller's microprocessor uses the C8051F020 microcontroller [4], as shown in Figure 1. Two external current sensors and a voltage detection circuit are used to obtain parameters such as a photovoltaic current, a load current, and a battery voltage through AD conversion in the microprocessor. The microprocessor sends N switch control signals at the same time. The first signal is generated by the microprocessor's internal PWM control unit. The 2nd to Nth signals are generated by the microprocessor's internal digital I/O ports (not PWM). . When the ith 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 charge control of the battery is to perform different constant voltage charging in different periods. The charging process is divided into four processes of strong charging, uniform charging, absorption and floating charging. Except for strong charging, the three stages of constant charging, absorption and floating charging are constant voltage control, and various kinds of intelligence can be used for the constant voltage control of the battery. The control algorithm, this controller uses the PI (proportional-integral) adjustment algorithm, and then combined with the coarse-combined combinational PWM control method.


Control system transfer function structure shown in Figure 2, VS is the battery voltage set value, VO is the actual output voltage of the battery voltage, the difference between the two △V input PI regulator, to get the desired output current IO, the IO adopts coarse adjustment Combined PWM implementation, implementation of the flow chart shown in Figure 3. That is: Divide IO by (I/N), take the remainder to get j, and take the integer to get m. Let the PWM duty cycle of the first PV subarray be j, so that there are m conductions in the remaining PV subarrays, and the remaining PV subarrays are disconnected, then the exact IO output is obtained: IO=(j/K+ m) x (I/N). The current is supplied to the battery and the load, and the battery output voltage VO is maintained at a constant voltage by the PI algorithm. In a control system consisting of six photovoltaic sub-arrays, the PWM voltage, current, and total photovoltaic current waveforms of the first 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 coarse motion occurs), otherwise it is a straight line.


Only one PV subarray of this scheme adopts PWM control. The rest of the PV subarrays are still controlled by common switches. Compared with the overall PWM control after all the PV arrays are connected in parallel, this coarse and fine adjustment combination achieves precise control of the PWM. The PWM switch energy loss is reduced by (N-1)/N (N is the number of PV arrays), and the heat sink volume is reduced; since multiple independent PV subarrays are still used for separate control, at the same voltage level, The current level requirement of the power switch device is very low. A low-cost power switch device can be used in parallel to realize a subarray [5], which reduces the cost, and at the same time, it also has a high-precision current output for PWM control of all photovoltaic arrays. The system regulated output meets the national standard [6]. Due to the small current involved in PWM chopping and good electromagnetic compatibility, it has passed the electromagnetic compatibility standard test and obtained CE certification. It has been applied to a series of photovoltaic controllers with a nominal voltage of -48 V and a current range of 30 A to 400 A. Operational practice shows that this scheme fully achieves the desired design effect.