KR102159334B1 - Method for control of MPPT using 3-point - Google Patents

Abstract

The present invention relates to a maximum power point following control method of a solar cell when an amount of solar irradiation rapidly changes. More specifically, the present invention relates to a solar cell maximum power point following control method using a three-point which can efficiently output changing maximum power from the solar cell in an environment in which the amount of solar irradiation rapidly changes and can output the maximum power by reducing an error.

Description

Solar cell maximum power point tracking control method using 3-point {Method for control of MPPT using 3-point}

The present invention relates to a maximum power point tracking control method of a solar cell, and more particularly, a 3-point capable of outputting a variable maximum power output from a solar cell in an environment where the amount of solar radiation rapidly changes, efficiently and with lower errors. The present invention relates to a method for tracking the maximum power point of the used solar cell.

Renewable energy does not use limited fuel and can minimize environmental pollution, so it is a power generation system that can prepare for environmental problems such as replacement of depleted fuels and global warming. According to the International Energy Agency (IEA), the proportion of renewable energy in meeting global energy demand is expected to increase to a fifth within the next five years from 2017, reaching 12.4% in 2023. Announced that it will see the fastest growth in the electricity sector, supplying electricity demand from 24% in 2017 to nearly 30% in 2023. In addition, the Korean government has announced a'Renewable Energy 3020' policy that raises the ratio of renewable energy generation to more than 20% by 2030. Among them, the power generation device using solar power is growing at a very fast rate, and it can be seen that it is a very important energy source. The module output of such a photovoltaic device is non-linear, and the output can vary greatly depending on various conditions (module temperature, insolation, shadow influence).

The power generated in the situation where the output between the modules of the photovoltaic device is unbalanced as shown in Fig. 1 due to various external conditions (surface contamination, partial shadows caused by clouds and trees, etc.) becomes smaller than the sum of the maximum power of each module. As a result, mismatch loss due to partial shading occurs. The undesired loss of the solar cell deteriorates the power generation efficiency of the solar power generation system and causes an increase in module temperature, which shortens the life of the solar module in the long term. In addition, in the case of a solar cell in a normal state, it has one local maximum point in the solar cell characteristic curve, but in this situation, it has a number of local points, so it cannot accurately follow the maximum power point. There is a serious problem in that output and efficiency are deteriorated.

Republic of Korea Patent Publication No. 10-2013-0025286 (20130311) Republic of Korea Patent Publication No. 10-2018-0129346 (20181205)

In order to solve the above problems, an object of the present invention is a stepped current (I) of a solar cell array according to the shadow effect of a solar cell in a power conversion device for solar power generation (a DC-DC converter and DC-AC inverter are combined) It is to provide a DC-DC converter for each string and a multi-MPPT control method to minimize the power loss of DC output from the )-voltage (V) curve and to improve the efficiency of the power conversion device.

In the solar cell's maximum power point tracking control method applying the P&O technique,

In order to determine whether the amount of solar radiation fluctuates, the solar output voltage (Vpv) value is fed back at a predetermined time, and the solar output voltage (Vold) value stored before the predetermined period, and the predetermined one week of solar power output A first step of storing a power (Pold) value stored before a time point, a solar power output power (Ppv) value obtained at the predetermined time point, and a flag value and a flag_B (flag_B) value stored at the predetermined time point, respectively;

A second step of obtaining a difference (dP) value between the Ppv value and the Pold value among the stored first step values, and a difference (dV) value between the Vpv value and the Vold value;

A third step of determining whether the flag value of the first step value is 0 or 1, or determining whether the flag_B (Flag_B) value is 0 or 1;

If it is determined in the third step that the flag value is 0, the output voltage of the solar cell is varied by the dP and the dV, and the solar output voltage (vref1) value and the power (P1) value before the predetermined time point; And performing a fourth step of storing the PV output voltage (vref2) value, the power (P2) value, and the flag value as 1 at the predetermined point in time.

If it is determined in step 3 that the flag value is 1, the solar output voltage (Vpv) value is the solar output voltage (vref1) value before the predetermined time point, and the solar output power (Ppv) value is the 5 steps of storing the PV output power (P2) value at a predetermined point in time and storing the flag value as 2 are performed;

If the flag_B (flag_B) value is determined to be 0 in step 3, the solar power output power (Ppv) value obtained at the predetermined time is stored as a power (P3) value, and the power obtained in steps 4 and 5 And comparing the values of P1>P2>P3 or P1<P2<P3, varying the value of the solar output voltage, and changing the value of the flag_B (flag_B).

In addition, when it is determined that there is no fluctuation in the amount of insolation, the power P1>P2 is compared to determine the solar power output voltage (vref1) value before the predetermined time point or the solar power output voltage (vref2) value of the predetermined time period before It is characterized in that it stores the output voltage of and repeats the step 6.

According to the present invention can have the following effects.

1. Using the Multi-MPPT method based on a 3-point algorithm with robustness against changes in external conditions, it has the advantage of outputting high efficiency even in situations where the amount of insolation changes rapidly.

2. Through the 3-point algorithm, the output voltage does not significantly deviate from the MPP (Maximun Power Point) voltage even when the insolation changes rapidly, and each array can output the maximum power even when a shadow occurs due to external conditions. have.

3. It can improve the utilization efficiency of solar power generation devices.

1 is an example of a situation in which the output between solar modules is unbalanced
2 is a module in a partial shading situation
3 is an output of the solar module according to the shadow effect
4 is a high-medium solar radiation graph (EN50530)
Figure 5 is a flow chart for the 3-point MPPT algorithm of the present invention
6 is a comparison between the conventional P&O technique and the 3-point MPPT algorithm according to the change of high-medium insolation
7 is an efficiency graph when the reference voltage increases or decreases when the amount of insolation increases
8 is a system configuration diagram including multiple individual inverters
9 is a system configuration diagram combining multiple converters and one inverter
Figure 10 is a simulation using three boost converters
11 is a system configuration for an experiment

Hereinafter, the present invention will be described in detail with reference to the drawings.

2 is a diagram showing a module in a partial shading situation. Modules are connected in series to increase the output voltage and parallel to increase the current. As shown in FIG. 2, when a module that does not produce power or produces less power occurs due to a shadow of a part of the module, the voltage or current decreases due to the module that does not produce power.

The 18kW photovoltaic array of Fig. 2 is divided into three parts by 6kW, and it is assumed that one part does not generate a shadow and the other two parts have a shadow. In the first stage of Figure 2, it has an open-circuit voltage of 650V and a short-circuit current of 12A, and the maximum power outputs 6kW. On the other hand, in the second stage of Fig. 2, it is the output when about 20% of shading occurs, and the open-circuit voltage is 416V and the short-circuit current is 12A like the solar array without shading, assuming that there is no shading in the parallel connected part. The power outputs about 80%, 4.8kW. Lastly, it is the output when about 40% of the shade occurs in the 3rd stage, and the open-circuit voltage is 330V and the short-circuit current is 12A like the solar array without shade, assuming that there is no shade in the parallel connected part, and the maximum power is 60. It outputs 3.6kW which is %. In a situation where there is an influence on the shadow, it develops with an output curve with several local points as shown in FIG. 3. In this situation, since there are three local points, there is a problem that the maximum power can be tracked at a low local point, and even if a high local point is found, the actual maximum power cannot be output.

The maximum power of the part without partial shading is 6kW, and the maximum power with partial shading is 4,8kW and 3.6kW, respectively. When these two are combined, the maximum power of this solar array is output only when 14.4kW is output. However, the maximum output point of the characteristic curve in Fig. 3 is 11.3kW. In the present invention, a new 3-point MPPT algorithm was presented to the boost converter of each stage array so as to output 14.4 kW, and simulations and experiments were performed to find out the performance of the Multi-MPPT incorporating 3 stages.

In Europe, evaluations are being conducted to measure the efficiency of MPPT under various conditions. FIG. 4 is a high-medium solar radiation graph (EN50530), which is a test for measuring how MPPT efficiency becomes when solar radiation is rapidly changed. In the case of the existing MPPT, the efficiency decreases when the amount of insolation changes rapidly. FIG. 4 presents a 3-point MPPT algorithm as shown in FIG. 5 using the solar array characteristic in which the voltage at the maximum power point does not change much when the amount of solar radiation changes. This algorithm periodically checks whether the amount of insolation changes, and if it changes, it maintains the magnitude of the voltage of the previous period, and if the amount of insolation does not change, it follows the MPP point. The operation sequence is as follows.

First, the current power and the power of the previous cycle are compared, and then the reference voltage is increased or decreased. Secondly, the reference voltage is output before increasing or decreasing the reference voltage. Third, it is divided into when insolation changes and when insolation does not change. When the amount of insolation changes, the current voltage and the voltage of the previous cycle are output alternately and repeated until it is determined that the amount of insolation does not change. If the amount of insolation does not change, the current voltage is compared with the voltage of the previous period, outputs a voltage point of higher power, and returns to the beginning.

Figure 5 is a three-point algorithm flow chart. The initial state is set to flag = 0, flag_B = 0. By default, flag == 0 starts true. Flag = 0 is an operation to change the voltage. At this time, the operation executes the P&O algorithm, stores power after voltage change, and stores 1 in the flag value. Next, it operates as if flag == 1 is true. Flag = 1 makes an operation comparison object for comparing the power before the voltage change and the power after the change. At this time, the power and voltage before the voltage change are output, and 2 is stored in the flag value. Since 2 is stored in the flag value, flag = 1 is false, and flag_B == 0 operates as true by the unchanged flag_B value in the initial state. The flag_B is an operation for confirming that the insolation fluctuates, and is operated by being divided into a fluctuating situation and a non-changing situation. If the insolation does not change, flag = 0 and flag_B = 0 are output, and the voltage point of higher power is output by comparing the power output from flag = 0 and flag = 1, and returns to the same state as the initial state. When the amount of insolation changes, the voltage output from flag = 0 and flag = 1 is repeatedly output. This operation uses the voltage that outputs the maximum power does not change significantly when the amount of insolation changes.

FIG. 6 is a simulation for comparing the P&O technique and the 3-point technique, which is a conventional MPPT technique used in general. This is a graph comparing the power output and the maximum output power when the slope of the high-medium solar radiation changes rapidly to 100 ^W/㎡/s . From the graph, it can be seen that the 3-point MPPT is close to the maximum output point. The situation in which the amount of insolation changes was simulated by referring to EN50530 of IEC, which is the standard regulation of solar efficiency. In EN50530, the P&O algorithm has a large difference in efficiency depending on whether the reference voltage increases or decreases when the solar radiation increases and decreases. In order to observe the exact efficiency, the efficiency was analyzed by dividing when the reference voltage increased and when it decreased. Tables 1 and 2 are tables showing the efficiency according to various insolation changes.

Table 1. Efficiency comparison of P&O technique and 3-point technique according to high-medium insolation change (when Vref increases)

Table 2. Efficiency comparison of P&O technique and 3-point technique according to high-medium insolation change (when Vref decreases)

7 is a graph showing the average value of the efficiency when the reference voltage increases and decreases when the amount of insolation increases. The left graph of Fig. 7 is a low-medium solar radiation situation, and Fig. 7(a) is a graph according to a high-medium solar radiation condition.

In general, if the photovoltaic device is installed in a place where shadows may occur due to external features such as buildings and trees, installing an inverter by dividing an array as shown in FIG. 8 can improve output. However, since it is expensive to use multiple inverters, it is common to drive a system that performs MPPT by arranging three boost converters in one inverter as shown in Fig. 9 so that MPPT can be performed instead.

Accordingly, the diagram of the present invention is composed of a simulation comparing the operation of one boost and the operation of three. When using one, it is implemented as a 3-point algorithm, and in the case of three, each boost converter is used as a Multi-MPPT algorithm. Is implemented to perform a 3-point algorithm. 'General MPPT' is MPPT using one boost converter,'MPPT considering partial insolation' is MPPT using hybrid MPPT, and'Multi-MPPT' is a total of three boost converters with one boost converter attached to each PV. Is the MPPT using. The MPPT efficiency was compared in Table 3 by simulating a situation where PV1 has no shading effect, PV2 has 10% shading effect, and PV3 has 0% to 40% shading effect.

Table 3. Output comparison of MPPT performance results through simulation

In the experiment, the results shown in Table 4 were obtained using two three-phase 12kW inverters and solar power generation simulation power supplies as shown in Figure 11 due to hardware limitations.

Table 4. Output comparison of MPPT performance results through experiment

In the experiment, PV1 does not receive the shading effect, and PV2 is an efficiency table with the shading effect changed from 0% to 40%. MPPT considering partial solar radiation is a Hybrid-MPPT technique. When the shading effect of PV2 is more than 20%, the output of PV2 is 0kW in general MPPT, while the MPPT considering partial solar radiation outputs 4.8kW. While the general MPPT suffers from a large power drop when it receives the shading effect, the Multi-MPPT is close to the maximum output power that PV2 can produce when PV2 is shaded, and the output is very high compared to the general MPPT. Confirmed high.

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Claims (2)

In the solar cell's maximum power point tracking control method applying the P&O technique,
To determine whether the insolation fluctuates,
The photovoltaic output voltage (V pv ) value is fed back at a predetermined time, and the photovoltaic output voltage (V old ) value stored before the predetermined period, and the power stored before the predetermined period of the photovoltaic output power A first step of storing a (P old ) value, a solar power output power (P pv ) value obtained at the predetermined time point, a flag (Flag) value and a flag_B (Flag_B) value stored at the predetermined time point, respectively;
A second step of obtaining a difference (dP) value between the P pv value and the Po ld value and a difference (dV) value between the V pv value and the V old value among the stored first step values;
A third step of determining whether the flag value of the first step value is 0 or 1, or determining whether the flag_B (Flag_B) value is 0 or 1;
If it is determined that the flag value is 0 in step 3, the output voltage of the solar cell is varied by the dP and the dV, and the solar output voltage (V ref1 ) value and power (P 1 ) before the predetermined time point value; And performing a fourth step of storing the predetermined time full-cycle solar output voltage (V ref2 ) value, power (P 2 ) value, and the flag (Flag) value as 1, respectively.
If it is determined in step 3 that the flag value is 1, the solar output voltage (Vpv) value is converted to the solar output voltage (V ref1 ) before the predetermined time, and the solar output power (P pv ) value 5 steps of storing the PV as a value of the PV output power (P 2 ) of the predetermined point in time, and the flag value being stored as 2;
If the flag_B (Flag_B) value is determined to be 0 in the third step, the solar output power (P pv ) value obtained at the predetermined time is stored as a power (P 3 ) value, and the fourth and fifth steps are obtained. Comparing the values of the power P1>P2>P3 or P1<P2<P3, varying the value of the solar output voltage, and changing the value of the flag_B (Flag_B);
Maximum power point tracking control method of a solar cell comprising a. The method of claim 1,
When it is determined that there is no fluctuation in the amount of insolation, the power P1>P2 is compared and the solar output voltage (V ref1 ) value before the predetermined time point or the solar power output voltage (V ref2 ) value of the solar cell during the predetermined time period is compared to the solar cell. The maximum power point tracking control method of a solar cell, characterized in that it stores as an output voltage of and repeats step 6.

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