CN107831818A - A kind of MPPT methods for solar power generation - Google Patents

Abstract

An MPPT method for solar power generation with strong universality, simple principle and superior performance, A. Control the output power P _L of the converter to 0; B. Increase the output power of the converter every cycle T ₁ , P _Ln = P _{Ln ‑1} +△P ₁ , P _Ln‑1 and P _Ln are the output power of the converter in the adjacent preceding and following periods respectively, and △P ₁ is the increase value of the output power _of the converter in period T1 _; C. Detection of each period T2 Photovoltaic panel output power P _PV and output voltage V _C , if ΔP _pv ·ΔV _C >0, then reduce the output power of the converter, P _Ln =P _Ln‑1 ‑△P ₂ , △P _pv and △V _c respectively are the output power increase value and the output voltage increase value of the photovoltaic panel in the adjacent preceding and subsequent periods, and △P ₂ is the decrease value of the output power of the converter in the period T ₂ . The invention is suitable for power control of solar photovoltaic battery panels.

Description

A kind of MPPT method for solar power generation

technical field

The invention belongs to the field of new energy power generation, and relates to maximum power point tracking (MPPT) control of solar power generation.

Background technique

With the continuous consumption of petrochemical energy and the increasingly severe environmental pollution problems, the human demand for clean and renewable new energy continues to grow. Solar energy has become a hot spot in new energy research because of its wide distribution, easy access, clean, efficient and sustainable characteristics.

Solar panels are the medium through which solar energy is converted into electricity. The output of electric solar panels is nonlinear, and its P-V curve presents a single-peak characteristic. In order to obtain the maximum amount of solar energy, it is necessary for the solar panel to work at the maximum power point (MPP). Most of the current mainstream MPPT methods use the perturbation and observation method (P&O) and the conductance incremental method (INC) and some improved methods (such as variable step size, zero oscillation, etc.). These two methods are simple in principle and easy to implement. However, these two methods have some common disadvantages: 1. Disturbance speed and accuracy cannot be balanced; 2. Energy will be lost due to oscillation at the steady-state operating point; 3. Only work at the maximum power point, and when there is a power balance requirement The occasion cannot achieve self-balancing with the load power; 4. When the light changes, there is a power impact on the load. These shortcomings are inherent in the algorithm itself and can only be improved but not eliminated. Therefore, there are some other methods such as Fuzzy Logic, neural network algorithm, etc., but these algorithms are very complicated, so not many are used.

Contents of the invention

The present invention aims to solve the above-mentioned problems existing in the MPPT method of solar power generation in the prior art, including that disturbance speed and precision cannot be balanced, energy will be lost due to oscillation during steady-state operation, output power cannot achieve self-balancing with load power, and illumination changes When there is a power impact on the load, etc. In order to solve the above problems, the present invention proposes a new MPPT method for solar power generation, which is generally applicable to various photovoltaic application systems, and has a simple principle and superior performance.

In order to solve the problems referred to above, the present invention adopts the following technical solutions and its special features are:

A. Control the output power P _L of the converter to be 0;

B. Increase the output power of the converter every cycle T ₁ , P _Ln = P _Ln-1 + △P ₁ , P _Ln-1 and P _Ln are the output power of the converter in the adjacent previous and subsequent cycles respectively, △P ₁ is the cycle The output power increase value of the converter in T ₁ ;

C. Detect the output power P _PV and output voltage V _C of the photovoltaic panel every cycle T ₂ , if ΔP _pv ·ΔV _C >0, then reduce the output power of the converter, P _Ln =P _Ln-1 -△P ₂ ,△ P _pv and △V _c are the output power increase value and output voltage increase value of the photovoltaic panel in the adjacent preceding and subsequent periods, respectively, and △P ₂ is the decreasing value of the converter output power in period T ₂ . In the present invention, T ₁ is preferably much greater than T ₂ , and ΔP ₁ is greater than or equal to ΔP ₂ .

Further, the present invention sets the photovoltaic cell panel output lower limit voltage threshold V _off , and when it is detected that the photovoltaic cell panel output voltage V _C drops to V _off , the output power of the converter is reduced to 0, and the system turns to restart.

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

Description of drawings

Figure 1 is a schematic block diagram of a common photovoltaic system structure;

Fig. 2 is a graph of the P-V characteristic curve of the photovoltaic cell panel;

Figure 3(a) is the working characteristic diagram when the photovoltaic system works on the left plane of the characteristic curve and the power of the converter increases;

Figure 3(b) is the working characteristic diagram when the photovoltaic system works on the left plane of the characteristic curve and the power of the converter is reduced;

Figure 3(c) is the working characteristic diagram when the photovoltaic system works on the right plane of the characteristic curve and the power of the converter increases;

Figure 3(d) is the operating characteristic diagram when the photovoltaic system works on the right plane of the characteristic curve and the power of the converter is reduced;

Fig. 4 is a logic diagram of the steps of the present invention.

Detailed ways

The traditional method is to realize MPPT by controlling the input voltage of the converter (that is, the output voltage of the photovoltaic panel, or called the working voltage). The present invention realizes MPPT by directly controlling the power variation of the converter. The converter is a DC/DC or DC/AC converter as shown in FIG. 1 . The specific implementation is: the converter control itself is a process of balancing power. In the case of a certain load, such as DC or AC bus, resistors, batteries and other structures, variables that characterize the power can always be found for indirect power control (such as when the load is a voltage bus, controlling the load current is to control the power) . Therefore, the power variation of the converter is directly controllable, which is the basis of the method.

Further, the structure of a common photovoltaic system can be expressed as the structure shown in Fig. 1 . The block PV is the solar photovoltaic panel, the block DC/DCorAC is the converter, which can be a DC/DC or DC/AC converter, the block Load is various types of loads, P _PV is the output power of the photovoltaic panel, and P _L is the power of the converter (regardless of the loss of the converter itself, it should be considered that the input and output power of the converter are equal, so the output power, input power and load power of the converter are collectively referred to as the converter power, and will not be described in detail), V _C is the output voltage of the battery board, and P _C is the charging power of the capacitor. The structure can cover almost all types of photovoltaic applications at present, so it has universality.

Further, the method implements MPPT by sampling the output voltage and current of the photovoltaic panel and controlling the increase or decrease of the power of the converter. The principle is as follows:

The characteristic curve of the photovoltaic panel is shown in Figure 2.

The P-V curve is a unimodal curve, and the maximum power point (MPP) divides the plane corresponding to the curve into two planes, left and right. In the left plane, the output power of the panel increases with the increase of the output voltage, and in the right plane, the output power of the panel decreases with the increase of the output voltage.

It can be seen from Figure 1 that equation (1) is always established:

P _L =P _PV -P _C (1)

Further, in combination with Fig. 3, analyze the working state of the system shown in Fig. 1 in the left and right planes of Fig. 2 .

In Figure 3(a), when the system works stably at a point P _pv on the left plane, if the control converter power P _L increases to P _L1 , at this time P _C <0, the capacitor discharges → the output voltage of the battery panel decreases → The output power of the battery panel decreases → the capacitor voltage further increases, eventually causing the output power of the battery panel to drop to 0 rapidly, and the circuit is out of control;

In Figure 3(b), when the system works stably at a certain point P _pv on the left plane, the control converter power P _L decreases to P _L2 , at this time P _C >0, the capacitor is charged → the output voltage of the battery board increases → The output power of the battery panel increases → the capacitor voltage further decreases, and finally the operating point of the battery panel will cross the MPP along the PV curve to reach the steady operating point SOP (Steady Operating Point, SOP) on the right plane, and the system enters a new steady state;

In Figure 3(c), when the system works stably at a point P _pv on the right plane, if the control converter power P _L increases to P _L3 , at this time P _C <0, the capacitor discharges → the output voltage of the battery panel decreases → The output power of the battery board increases → the system enters a steady state when P _pv =P _L3 is reached;

In Figure 3(d), when the system works stably at a point P _pv on the right plane, if the control converter power P _L is reduced to P _L4 , at this time P _C >0, the capacitor is charged → the output voltage of the battery board increases → The output power of the battery panel decreases → the system enters a steady state when P _pv =P _L4 is reached.

To sum up, it can be seen that through the method of power control, the photovoltaic panel cannot work stably in the left plane, but all power points in the right plane are completely controllable.

Further, from the above analysis, it can be seen that the photovoltaic cell panel has the following working characteristics in the left and right planes as shown in Table 1: ("↑" means increase, "↓" means decrease).

Table 1

The boundary between the left and right planes is the longitudinal corresponding line of the maximum power point MPP in Figures 2 and 3. Therefore, according to the above table, detect the corresponding variables and judge the plane where the system works. When the system working point is at the critical point of the left and right planes, it is the maximum power point.

Further, the specific MPPT implementation method can be summarized as:

A. The initial P _L → 0, so that the system works in the right plane near the open circuit operating point;

B. Increase P _L at a fixed period T ₁ , so that P _Ln = P _Ln-1 + ΔP ₁ ;

C. Fixed period T ₂ Detect V _c and P _pv according to Table 1 and judge whether the system is working on the left plane, if so, execute P _Ln =P _Ln-1 -△P ₂ , and return to step 2; otherwise, directly return to step 2 ; The system works in the left plane according to ΔP _pv ·ΔV _C >0.

Its logical block diagram is shown in Figure 4.

Further, it should be noted that steps B and C operate independently, have their own operating cycles, and there is no order, and each independently affects _PL . B period T ₁ should be much larger than C period T ₂ to ensure that the plane where the system operating point is located can be accurately detected during each disturbance process to avoid misoperation.

Further, the increase ΔP ₁ for _PL in B should be greater than the decrease ΔP ₂ for _PL in C.

Further, for the out-of-control phenomenon that may be mentioned in the above analysis, as shown in Figure 3a, a lower voltage limit V _off should be set, and the system will be restarted when it is detected that V _C is lower than the threshold voltage.

The invention is applied to the MPPT control of solar power generation, and proposes a new MPPT method. The principle of this method is simple and easy to implement. Compared with the traditional most commonly used P&O and INC method, the method of the present invention has the following advantages: 1. can work at the working point of the whole area of the right plane (traditional P&O and INC method can only work at the maximum power point); 2. directly Control the power of the converter, even if the light changes suddenly, it will not cause impact on the subsequent stage; 3. There is no energy loss caused by oscillation at the steady-state operating point; 4. There is no compromise between the disturbance step size and period. The method of the invention has the efficiency no less than that of the traditional method, has more superior performance and a more comprehensive working mode, and provides a new idea for the design of MPPT.

Claims (3)

1. A kind of 1. MPPT methods for solar power generation, it is characterised in that：

   A. converter power output P is controlled_LFor 0；

   B. each cycle T₁Increase converter power output, P_Ln=P_Ln-1+△P₁, P_Ln-1、P_LnBecome in the respectively adjacent forward and backward cycle Parallel operation power output, △ P₁For cycle T₁Interior converter power output value added；

   C. each cycle T₂Detect photovoltaic battery panel power output P_PVWith output voltage V_CIf Δ P_pv·ΔV_C＞ 0, then reduce conversion Device power output, P_Ln=P_Ln-1-△P₂, △ P_pv、△V_cRespectively adjacent forward and backward cycle photovoltaic battery panel power output increase Value, output voltage value added, △ P₂For cycle T₂Interior converter power output decreasing value.
2. 2. according to the method for claim 1, it is characterised in that：T₁Much larger than T₂, △ P₁More than or equal to △ P₂。
3. 3. method according to claim 1 or 2, it is characterised in that：Photovoltaic battery panel output voltage threshold value V is set_off, when Detect photovoltaic battery panel output voltage V_CDrop to V_off, converter power output is decreased to 0, system switchs to restart.