KR20130025286A - Method for maximum power point tracking in mismatched solar cell - Google Patents

Abstract

PURPOSE: A maximum power point tracking control method of a solar energy generating device in a mismatch state is provided to track the maximum power point of the solar energy generating device in the mismatch state by including an additional maximum power point tracking step. CONSTITUTION: The cell voltage V(k) of a solar power module is calculated. I(K) is calculated by substituting the V(k) for the I-V curve of the solar power module. A proximity straight line about the maximum power point for solar radiation intensity is drawn. The I(k) according to the V(k) is calculated. An error range with the I(k) is measured. The error range is less than a predetermined value. The V(k) and the I(k) are determined as the correct maximum power state. The error range is greater than the predetermined value. The V(k) and I(k) are determined as a mismatch state. An additional maximum power point tracking step is performed. [Reference numerals] (AA) V_ref = V_op * 0.79 in an initial driving process; (BB) Calculating a 1(k) value through a solar power module equation

Description

Method for Maximum Power Point Tracking in Mismatched Solar Cell}

The present invention relates to a maximum power point tracking control method of a photovoltaic device, and more particularly, in a state in which a solar module is connected in series and in parallel, some cells do not operate normally or are not affected by shadows. Some parts that are not normal when the error between the maximum power point (I _tn ) and the measured maximum power current value (I (k)) derived through the straight line of the maximum power point when the current is output to the output is above a certain level. The present invention relates to a maximum power point tracking control method of a photovoltaic device that determines a mismatch situation in which a cell does not operate properly and performs an additional maximum power point following step.

Recently, due to concerns about environmental pollution and energy depletion, interest in renewable energy is increasing day by day, and multifaceted studies on this are being conducted. In particular, photovoltaic power generation is converting sunlight into electrical energy through solar cells, and it is growing rapidly due to its infinite energy and pollution-free advantages and growing in scale.

 In photovoltaic systems, the photovoltaic module changes its output characteristics according to solar radiation intensity and temperature, and has nonlinear characteristics. Therefore, to obtain the maximum power output, it is necessary to control the maximum power point tracking by the converter.

 In the photovoltaic power generation system, since the maximum power operating point varies according to the solar radiation intensity and temperature, as shown in FIG. 1, the maximum power tracking (MPP) is performed to operate the photovoltaic power system at the maximum power point. MPPT; Maximum Power Point Tracking (MPPT) control is required.

 The P & O control method (Perturbation and Observation), which has been widely used as the conventional MPPT control method, is as follows. The P & O control method is based on the calculation of the change of power by the product of the solar cell output voltage and the current, and is the most widely used control method in the actual solar power generation system because of the simple control and easy implementation. In addition, there is a feature that follows the maximum power point well where the amount of insolation gradually changes and maintains the operating point in the vicinity thereof. The change in power is determined by comparing the past voltage values of the present and previous periods by varying the reference operating voltage. As shown in the flowchart of FIG. 3, after calculating the difference between the power value of the previous period and the power value of the current period, when the calculated value becomes 0, the value is maintained without adjusting the array operating voltage. However, if a difference occurs in the power value, that is, a difference between the voltage value of the entire period and the voltage value of the current period, the maximum operating point is followed by changing the operating point by increasing or decreasing the array operating voltage.

First, the power is calculated by sensing the voltage V (k) and the current I (k) of the solar cell array. The value obtained by subtracting the previous period voltage V (k-1) from the current voltage V (k) becomes an increment dV (= V (k) -V (k-1)). The increment of power is also calculated as voltage, resulting in power increment dP (dP (k) -dP (k-1)). Next, we determine if dP is equal to zero. If dP is equal to 0, the power value of the current period and the previous period is the same, so it is determined as the maximum power point and the operation of the maximum power point following control algorithm is exited to maintain the reference value of the entire period. If dP is not equal to 0, the sign of the power increment dP value is determined. If dP is positive, the next conditional sentence, the sign of the voltage increment dV is determined, and if the voltage increment is positive, the reference voltage is increased, and if negative, the reference voltage is decreased. On the contrary, if the power increment dP is negative, the sign of the voltage increment dV is judged, and if it is positive, the reference voltage is decreased, and if it is negative, the reference voltage is increased.

This P & O control method has the advantage that the algorithm is simple to implement and the operating voltage can be increased or decreased to a fixed length in MPP tracking.Increasing the fixed length increases the tracking speed, but is stable. In the state, there is a problem that oscillation occurs around the MPP.

As a way to improve this problem is the MPPT control method according to the differential element.

The differential element control method is an algorithm that reduces the follow-up control time by using the existing P & O and Increment and Conductance techniques. The differential element value for dP / dV initially increases linearly and gradually decreases as it approaches MPP value. If this value is taken as an incremental value for the following operation voltage control (V _step ) value (V _step = V _step * (dP / dV)) It follows the maximum power value faster than the existing algorithm because it controls the voltage with variable size instead of fixed size. The dP / dV (differential element value) of the solar output curve is shown in FIG. 4, and the flowchart of the present algorithm is shown in FIG. 5.

This differential element control method has an advantage that the maximum power tracking speed is fast because the tracking speed is changed dynamically by changing the operating voltage value from fixed length to variable length.

However, in the conventional MPPT control method including the differential element control method, as shown in FIG. 2, the maximum power point is correctly tracked when all the cells are normally operated as shown in (a) as shown in FIG. Problem occurs when the maximum power point of the photovoltaic output power is changed to two or more points, and the maximum power following control operation for a plurality of MPP points occurs when is mismatched. do.

Therefore, a new maximum power point tracking control method by the converter is required to obtain the maximum power output in the mismatched state of the solar power generation system.

The present invention provides a control method capable of following a maximum power point of a photovoltaic device even in a mismatched state.

In order to solve the above problems, the present invention comprises the steps of obtaining the I (k) by substituting the cell voltage V (k) and the V (k) of the solar module to the IV curve of the solar module; V (k) is derived by deriving a near straight line with respect to the maximum power point for each solar intensity in the form of a first-order curve in a certain voltage section. According to I _Tn Obtaining and measuring an error range with I (k); And if the error range is less than the set value, determine the normal state, and determine the V (k) and the I (k) as the correct maximum power state which is the normal state, and if the error range is more than the set value, mismatched. Determining the state, characterized in that for performing the additional maximum power point following step.

In addition, the additional maximum power point following step is substituted by I (k) to the near straight line I _Tn Obtaining a; And wherein I (k) is in the solar module I _Tn If larger or smaller, determining the abnormal maximum power following control and performing the maximum power point following control again from a voltage point lower than the open voltage (V _op ) of the photovoltaic module.

In addition, the error range may be within 10% of the I (k) value when the maximum power point tracking control.

According to the present invention, a correct maximum power point can be tracked and controlled even in a mismatched state according to the external environment or the characteristics of a module in a solar power generation system.

1 is an output characteristic curve when all solar cell modules are in a steady state.
Figure 2 (a) is the output characteristic curve when all of the solar cell modules are in a steady state.
Figure 2 (b) is an output characteristic curve when a portion of the solar cell module does not operate normally mismatched state.
3 is a flowchart of a perturbation and observation (P & O) maximum power point tracking control algorithm in a photovoltaic system.
4 is a conceptual diagram of maximum power point following control using a differential element technique in a solar characteristic curve.
5 is a flowchart illustrating a maximum power point following control algorithm using differential element technique in a photovoltaic power generation system.
6 is an approximation of the maximum power point and the maximum power point according to the change in insolation intensity.
7 is a flowchart illustrating a maximum power point tracking control method algorithm for a photovoltaic device according to the present invention.
8 is an additional maximum power point tracking process when it is determined that the mismatch state in the maximum power point tracking control method of the photovoltaic device according to the present invention.
9 is a simulation circuit diagram of the maximum power point tracking control method of the solar cell apparatus according to the present invention.
Figure 10 is a maximum power tracking control waveform, respectively obtained according to the present invention and the prior art.

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

Current-voltage, power-voltage curve of the solar cell module is shown as a non-linear curve and the output characteristic curve of such a solar cell module can be expressed as Equation 1 below.

[Equation 1]

[ I _ph : Light generating current, I _out : Current through load, V _out : Output voltage, R _S : Internal output resistance, R _sh : Internal resistance, k : Boltzmann constant (1.38x10 ^-23 ), B : Material coefficient, q : Charge amount (1.6022x10 ^-19 ), I _max : Insolation intensity , K : coefficient (diode forward voltage), T : absolute temperature (273 + degreeC)]

Equation 1 the temperature coefficient (T) of the time assumed to be fixed at a constant value (25 ℃) and sikyeoteul gradually changing the solar radiation intensity at 0.05kW / m ² by 0kW / m ² up to 1kW / m ² for The maximum power point for the current-voltage characteristic of any one solar cell module with a 3kW output can be expressed as shown in the following [Table 1] by using the existing maximum power point following control method.

Irradiation V _mpp (V) I _mpp (A) P _mpp (P) 0.00 0.00 0.00 0.00 0.05 206.9 0.57 118.22 0.10 220.3 1.15 253.18 0.15 228.1 1.73 394.52 0.20 233.7 2.31 540.02 0.25 238.1 2.89 688.61 0.30 241.6 3.48 839.67 0.35 244.6 4.06 992.79 0.40 247.2 4.64 1147.68 0.45 249.5 5.23 1304.10 0.50 251.6 5.81 1461.89 0.55 253.4 6.40 1620.92 0.60 255.1 6.98 1781.07 0.65 256.7 7.57 1942.25 0.70 258.2 8.15 2104.38 0.75 259.5 8.74 2267.39 0.80 260.8 9.32 2431.22 0.85 262.0 9.91 2595.83 0.90 263.1 10.49 2761.16 0.95 264.1 11.08 2927.18 1.00 265.2 11.67 3093.85

In FIG. 6, the maximum power points are displayed by displaying data values of [Table 1] for the maximum power point which changes with respect to insolation intensity. Can be. With this in mind, it can be expressed by formulating a straight line near the maximum power point, which can be expressed as Equation 2 below.

&Quot; (2) "

(V _Tn : Maximum power point approximation curve equation, a: slope value of V _Tn , b: Offset value)

Based on the data values in Table 1 above, the maximum power point near straight line V _Tn Since the value of slope a for (265.2-259.5) / (11.67-8.74) = 1.87 and b value is 250, the equation of the maximum power point proximity curve of the photovoltaic module of 3 kW capacity is V _Tn = 1.87 I (k ) +250.

In the present invention, the maximum power point tracking control method of a photovoltaic device capable of compensating mismatch loss has a maximum I (k) defined in Equation 2 when the maximum power is tracked in a conventional power tracking control algorithm. It consists of an algorithm that considers this as a mismatch, not a solar cell with normal output, when I _{Tn has an} error over a certain range due to the line near the power point, and performs an additional maximum power point following step. do.

 For more accurate maximum power point tracking, the error range is preferably within 10%.

When the error range is greater than or equal to the set value, it is determined to be a mismatch state to perform an additional maximum power point following step, which is shown in FIG. 7 and the definition of the sign used therein is as follows.

V (k), I (k): Current control cycle voltage and current value of solar cell

V (k-1), I (k-1): Control cycle voltage and current values one cycle before the current of the solar cell

V _Tn , I _Tn : Approximate maximum power point when the solar cell is in a steady state without mismatch

dV, dI, dP: voltage, current, power difference between the current cycle and the previous cycle

V _step : Voltage control magnitude value during maximum power algorithm

V _ref : Voltage command value for tracking and judging by the maximum voltage

Hereinafter, FIG. 7 will be described in order.

① The cell is in a steady state by applying the VOC algorithm (an algorithm that generally estimates that 79% of the open voltage (V _op ) value is the maximum power voltage value) by reading the open voltage value of the solar module during the initial PCS _operation . In this case, set V _ref = V _op * 0.79 as the following value.

Calculate the I (k) value that the IV curve of the photovoltaic module can point to based on the photovoltaic module formula based on Equation 1 through the set V _ref value Substitute V _Tn Find the value. Since the value of V _Tn is substituted into the maximum power point proximity equation, this value is substituted for the reference value (V _ref ).

③ to put a V _ref value of the control value PV PCS, receiving sensing the actual I (k) value with respect to the V _ref matches the current value of the I _Tn value of the maximum power point in the value is already known and , I (k) value and I _Tn value of error is the current I (k) if I is less than 10% difference than the value predetermined in value and determined that the normal state, the open-circuit voltage (V _op) low voltage preferably of 20 The value corresponding to ˜30% is given as the setpoint (V _ref ). (In FIG. 7, a value of 30% is given as the setpoint.)

④ The maximum power following control method follows the differential element control method described above.

[Example]

In order to confirm whether the maximum power point tracking control method of the photovoltaic device capable of mismatch loss compensation according to the present invention correctly controls the maximum power point in the mismatch state, a simulation circuit diagram as shown in FIG. In order to compare with the algorithm P & O, it is confirmed whether the differential element technique and the following control method of the present invention correctly follow the maximum power point while mismatch loss is occurring. As a result, it can be seen that the maximum power point following control method proposed in the present invention is controlled to the maximum power point having a higher output power than the conventional algorithm as shown in FIG.

none

Claims (3)

Obtaining I (k) by substituting the cell voltages V (k) and V (k) of the solar module into the IV curve of the solar module;
V (k) is derived by deriving a near straight line with respect to the maximum power point for each solar intensity in the form of a first-order curve in a certain voltage section. According to I _Tn Obtaining and measuring an error range with I (k); And
If the error range is less than the set value, it is determined as a normal state, and the V (k) and the I (k) are judged as a correct maximum power state that is a normal state. The maximum power point following control method of the photovoltaic power generation system capable of compensating for mismatch loss, characterized in that to perform additional maximum power point following step. The method of claim 1,
The additional maximum power point following step
Substituting the I (k) into the near straight line, I _Tn Obtaining a;
The I (k) in the solar module I _Tn Determining the abnormal maximum power following control and performing maximum power point following control again from a voltage point lower than the open voltage V _op of the solar module;
Maximum power point following control method of the photovoltaic system capable of mismatch loss compensation, comprising a. The method of claim 1,
The error range is a maximum power point tracking control method of a photovoltaic power generation system capable of mismatch loss compensation, characterized in that within 10% of the I (k) value when the maximum power point following control.

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