# 物理代写|光电技术代写Photovoltaic Technology代考|MEC2150

## 物理代写|光电技术代写Photovoltaic Technology代考|The Variable Step-Size MPPT Method

The fixed step algorithms, like perturbation observation method, INC, step size determines the tracking speed of the step length, the greater step length, the faster response, and vice versa. In order to obtain both speed and accuracy, the variable step-size algorithm arises at the historic moment. In PV MPPT schemes, the fixed step algorithm cannot meet both speed and accuracy, there are oscillation and miscalculation in fixed step P\&O method, the system cannot track the MPP accurately, which will cause the energy loss, so need to improve the fixed step-size method, and the variable step tracking algorithm is proposed. The basic principle of variable step disturbance observation method as: when the operation voltage in the area far away from the MPP, in order to improve the tracking speed and reduce the PV cell in low power output in time, the larger step size is adopted. When near the MPP area, using the small step to guarantee the tracking accuracy. Variable step-size algorithm usually includes two important parameters: the speed factor and the maximum step [11].

According to the PV traits, the slope of the PV $P-U$ curve is zero at the MPP, positive at the left side of the MPP, and negative at the right side. The slope of power versus voltage curves is asymmetric at MPP. As shown in Fig. 13, when $u \in\left[0, U_{\mathrm{m}}\right], P^{\prime}(u)$ will change slowly. While when $u \in\left[U_{\mathrm{m}}, U_{\mathrm{OC}}\right], P^{\prime}(u)$ will change rapidly. And the $P^{\prime}(u)$ can be expressed by
$$P^{\prime}(u)=I_{\mathrm{SC}}\left(1+C_1-C_1 \exp \left(\frac{u}{C_2 U_{\mathrm{OC}}}\right)\right)-\frac{C_1 I_{\mathrm{SC}} u}{C_2 U_{\mathrm{OC}}} \exp \left(\frac{u}{C_2 U_{\mathrm{OC}}}\right)$$

A good variable step-size algorithm requires both reasonable step size and acceptable step change rate $P^{\prime \prime}(u)$. A smaller step change rate could make contribution to faster dynamics when it is far away from the MPP, while a lager step change rate could do well to steady-state accuracy when it is around the MPP. $P^{\prime \prime}$ $(u)$ is a monotonically decreasing curve and the value of $P^{\prime \prime}(u)$ is always less than 0 .
$$P^{\prime \prime}(u)=-\frac{2 C_1 I_{\mathrm{SC}}}{C_2 U_{\mathrm{OC}}} \mathrm{e}^{\frac{u}{\tau_2 U_{\mathrm{OC}}}}-\frac{C_1 I_{\mathrm{SC}} u}{\left(C_2 U_{\mathrm{OC}}\right)^2} \mathrm{e}^{\frac{u}{\tau_2 \omega_{0 \mathrm{C}}}}$$

## 物理代写|光电技术代写Photovoltaic Technology代考|The Constant Voltage Tracking

Constant voltage tracking (CVT) method is one of the open-loop MPPT methods on basis of the output characteristic curve, which is the simplest one of all MPPT methods. As shown in the Fig. 25, under the condition of a certain temperature and the value of temperature change is not big, when the irradiance changing, both MPPs distributed in a vertical straight line on both sides, the output voltage of PV cells generally does not change. Therefore, the output voltage of the PV power supply control near the MPP of a constant voltage, the PV cells will work under approximate maximum power. Convert MPPT control to constant voltage control actually, which is the theory basis of constant voltage control.

The CVT transmission algorithm based on the mathematical model of PV cells, and find out the maximum output voltage of PV cell point, this method is simple, and the operation is convenient. But due to ignoring the influence of temperature on the PV cell output voltage, therefore, the larger temperature difference, the CVT method of MPPT error become greater. It cannot achieve the true sense of maximum power tracking. Although CVT method is difficult to realize MPPT accurately, but it has advantages as simple and quick close to the MPP, so the CVT method is often combined with other methods of closed-loop MPPT, which can adopt the CVT method in PV systems at the process of start to close the MPP voltage quickly, and then use the other closed-loop MPPT algorithm of MPP for further search. This combination of MPPT method can effectively reduce the power loss in the process of start, which is caused by the far away from the area of MPP to search. Voltage tracking method can be used in low price and simple system in which control requirements is not high in general.

## 物理代写|光电技术代写光伏技术代考|可变步长MPPT方法

$$P^{\prime}(u)=I_{\mathrm{SC}}\left(1+C_1-C_1 \exp \left(\frac{u}{C_2 U_{\mathrm{OC}}}\right)\right)-\frac{C_1 I_{\mathrm{SC}} u}{C_2 U_{\mathrm{OC}}} \exp \left(\frac{u}{C_2 U_{\mathrm{OC}}}\right)$$ 表示

## 物理代写|光电技术代写光伏技术代考|恒压跟踪

CVT传输算法基于PV电池的数学模型，并找出PV电池点的最大输出电压，这种方法简单，操作方便。但由于忽略了温度对PV电池输出电压的影响，因此，温差越大，CVT方法的MPPT误差就越大。它无法实现真正意义上的最大功率跟踪。虽然CVT方法难以准确实现MPPT，但它具有简单、快速接近MPP的优点，因此CVT方法常与其他闭环MPPT方法相结合，可在PV系统启动过程中采用CVT方法快速关闭MPP电压，然后再使用MPP的其他闭环MPPT算法进行进一步搜索。该方法与MPPT方法的结合可以有效地减少启动过程中由于MPP搜索区域较远而造成的功率损耗。电压跟踪法可用于一般控制要求不高、价格低廉、结构简单的系统中

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