WO2019062023A1

WO2019062023A1 - Variable-step-length, fixed-speed and limit-derating power mppt disturbance method

Info

Publication number: WO2019062023A1
Application number: PCT/CN2018/077723
Authority: WO
Inventors: 钟建朋; 刘永奎; 周洪伟
Original assignee: 特变电工新疆新能源股份有限公司
Priority date: 2017-09-29
Filing date: 2018-03-01
Publication date: 2019-04-04
Also published as: CN107491137B; CN107491137A

Abstract

A variable-step-length, fixed-speed and limit-derating power MPPT disturbance method, which involves: calculating a power derating coefficient α by comparing the amount of power change generated by the previous disturbance with a power variation calculated according to a power derating rate issued by an upper computer, and calculating, in real time and by means of the power-voltage relationship of a photovoltaic assembly, the relationship between the next disturbance voltage step length and the power derating coefficient and between same and the previous actual voltage change step length; giving the next voltage disturbance reference value by combining a current voltage measurement value; and when the difference between actual power and expected power is less than a certain set value, using fixed-step-length disturbance to reduce the power oscillation caused by disturbance and improve the disturbance precision. The method of the present invention can implement real-time step length calculation in a simple manner, thereby achieving fast and accurate power change requirements.

Description

Variable step size fixed speed derating limit power MPPT disturbance method

Technical field

The present invention relates to the field of maximum power tracking (MPPT) technology, and in particular to a variable step size fixed speed derating power MPPT disturbance method.

Background technique

In general, considering the influence of light intensity and temperature, in order to maximize the output power of the photovoltaic array, maximum power tracking (MPPT) control is required in the inverter system. However, in practical applications, distributed photovoltaic power plants often have limited power operation due to the limitation of load and grid capacity. At this time, the power station needs to reduce the active output to a certain expected value according to the requirements of the superior scheduling. In addition, with the gradual application and popularization of the microgrid system, the microgrid system operating in the island mode will also issue a limited power operation command to the distributed power generation unit in the system according to the actual load demand.

To this end, the inverter needs to disturb the output voltage of the PV array to the open circuit voltage in the MPPT control, similar to the MPPT control. At this time, the disturbance step of the voltage has a great influence on the limited power speed and accuracy, and it is necessary to select a suitable disturbance method. To ensure the speed and accuracy requirements of limited power operation.

Inverter limited power operation means that the PV array output voltage needs to be disturbed in the direction of the open circuit voltage, which is opposite to the voltage disturbance direction in the MPPT control. However, in terms of implementation method, it is similar to MPPT control, and the purpose of changing power is achieved by adding a disturbance voltage at the current voltage.

The existing MPPT control is mainly divided into fixed step size and variable step size. The common observation disturbance method and conductance method belong to the fixed step method. The advantage of the fixed-step algorithm is that it is simple, low-cost and easy to implement, but its shortcomings are also obvious. The adjustment flexibility of the fixed step size is poor. The smaller step size can ensure sufficient accuracy, but the adjustment time is too long. However, a larger step size results in poor power adjustment accuracy, which often results in a large fluctuation in power near the target value. For this reason, in the case where there is a high demand for adjustment speed and accuracy, a variable step size algorithm is often used. The main idea of the method is: when the working point is far away from the target power point, the larger step size is used for the disturbance, and when the power is close to the target value, the smaller step size is used for the disturbance. The method better solves the contradiction between the disturbance speed and the precision, but the disadvantage is that the method for determining the step size is complicated, and usually needs to collect a large amount of data than to perform complicated processing.

Summary of the invention

In order to solve the contradiction between the speed and the precision in the existing fixed step disturbance, and the complexity of the algorithm in the current variable step size algorithm, the object of the present invention is to provide a variable step size fixed speed derating power MPPT disturbance method, which can be simple Real-time step size calculations are achieved to achieve fast and accurate power change requirements.

In order to achieve the above object, the present invention adopts the following technical solutions:

A variable-step fixed-speed derating power-limited MPPT disturbance method. When the inverter is operating in a limited power mode, the current power value is P _current , and the target power value is P _aim ; if the inverter starts running initially In MPPT mode, the limited power mode means that the power is reduced, that is, P _aim <P _current , which usually causes the voltage to be disturbed to the open circuit voltage direction to reduce the power to achieve the target. Therefore, the limited power mode usually runs on the power voltage curve. The right side of the maximum power point;

If the power is expected to reach the target value within a certain time, the simplest method is to use the uniform speed drop method, set the power change rate to υ _p , the unit is W / s, and the MPPT disturbance period is T _m , then within a single disturbance period The expected power derating value is the limit

Δp _s =υ _p T _m (1)

If it can be ensured that the power reduction amount after each disturbance is approximately equal to the expected value Δp _s , then the power reaches the target value within the specified time, so that the step size of each disturbance is updated as the basis for selecting the disturbance step size;

The specific iteration rules are as follows:

When each disturbance moment arrives, the power change Δp at the previous voltage disturbance step is obtained by making the power of the current moment and the power of the previous disturbance period, so that it is quotient with Δp _s ,

Here, α is regarded as the power derating coefficient of the previous step; if α>1, it indicates that the previous disturbance did not generate enough power reduction, so the disturbance step should be increased in the next disturbance; on the contrary, if α< 1, indicating that the previous disturbance produced excessive power reduction, the next disturbance step needs to be reduced; and when α=1, it indicates that the power reduction caused by the disturbance just meets the expectation, so the next disturbance step is unchanged. ;

If the MPPT has been disturbed to the right of the maximum power point, the period is small enough, and after each disturbance, the voltage and power can reach the steady state quickly, then the power decrement generated by the two disturbances is considered to be approximately equal, that is, at the power voltage. The slope on the curve is approximately equal

Therefore, in the case where the above assumptions are true,

To make this step disturbance reach the desired power drop value, ie Δp _k+1 = Δp _s , then there is

Therefore, the iterative update of the disturbance step can be realized by using the power derating coefficient α; in practical applications, the step size of the previous MPPT disturbance can be utilized as the initial value of the step in the limited power mode. Here, the voltage disturbance step is the actual measured value.

The specific steps of the perturbation method are as follows:

Step 1: After the MPPT function is turned on, the inverter counts down according to the disturbance period T _m counter set by the system. Before the set disturbance period T _m arrives, the system remains in the waiting state until the disturbance period T _m reaches;

Step 2: After the disturbance period T _m arrives, calculate the current power value P _current according to the currently collected voltage value U _k and the current value I _k , and then store the previous power value P _k-1 and the previous voltage value stored by the system. U _{k-1 is} different from the current power value P _current and the current voltage value U _k , respectively, thereby obtaining the power variation amount Δp _k and the voltage variation amount Δu _k caused by the previous disturbance;

Step 3: According to the selection of the operating mode of the inverter by the host computer, if the inverter operation selects the non-limited power mode, the system will calculate the next voltage according to the current power value P _current and the current voltage value U _k according to the normal MPPT mode. Disturbance step

Step 4: If the inverter operation selects the limited power mode, it is determined whether the difference between the current power value P _current and the limited power target value P _aim exceeds a system-set power derating value, that is, a limit value Δp _s , if both If the difference does not exceed the limit value Δp _s , it indicates that the current power P _current is very close to the limit power target value P _aim . At this time, to ensure the disturbance accuracy, a constant voltage disturbance step is adopted;

Step 5: If the difference between the current power value P _current and the limit power target value P _aim exceeds the limit value Δp _s , the power derating value calculated according to the set power derating rate, and using the previous power calculated in step 2 The amount of change Δp _k and the power derating value Δp _{s are} calculated according to formula (2), and the power derating coefficient α is calculated by using the voltage variation Δ-u _k caused by the previous disturbance obtained in equation (5) and step 2. Output voltage change disturbance step;

Step 6: According to the current voltage value U _k and the voltage change disturbance step calculated in step 3 or 4 or 5, add the two to determine the next voltage reference value, as a voltage disturbance reference, and thus, a disturbance period The algorithm flow ends.

The points that the algorithm needs to pay attention to in the application:

First, in the limited power mode, if the system operation changes around the target power point, it will cause the sign of α in equation (5) to change, which may cause an error in the direction of the disturbance. The algorithm uses the absolute value of the calculated α, is determined by the relationship between P _aim P _current magnitude of the disturbance is determined to make the right direction;

Second, when the power changes to the vicinity of the target value, in order to avoid the power oscillation caused by the excessive disturbance step, the power oscillation is separated separately, and the current power and target power difference is less than the set rate, and the single period power derating value Δp _{In s} , the disturbance is performed with a fixed step size S _d .

Third, in order to prevent system problems that may be caused by excessive disturbance step, in practical applications, it is necessary to set a limit for the disturbance step, and the specific limit needs to be determined according to the derating rate.

Compared with the prior art, the present invention has the following advantages:

1) The variable step size disturbance method has a clear relationship with the expected operating power and derating rate indicators, and the disturbance step size can be continuously adjusted according to the set derating rate to ensure that the actual derating rate of the inverter follows the set value, thereby The derating operation of the specified power is reached within the time.

2) The step size determination method and logic of the variable step size perturbation method are simple, and only by judging the power and voltage difference of the last two samples, combined with the derating rate and the MPPT disturbance period, the perturbation step size can be obtained. Specific value.

3) The variable step size perturbation method does not use a fixed finite step size, but continuously adjusts the step size according to an artificially set desired value, and the step size adjustment idea is not limited to the MPPT perturbation only in the limited power operation mode. The same step size determination method and method are also applicable in normal MPPT control.

DRAWINGS

1 is a flow chart of a method of perturbing according to the present invention.

Figure 2 shows the voltage disturbance step variation and corresponding power variation of the inverter at different derating rates. Figure 2(a) shows the segment voltage disturbance at 50kW/s derating rate. Figure 2(b) shows The difference between power and voltage disturbance at 50kW/s derating rate, Figure 2(c) is the segment voltage disturbance at 100kW/s derating rate, and Figure 2(d) shows the difference in power and voltage disturbance at 100kW/s derating rate. value.

Detailed ways

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

As shown in FIG. 1, the specific steps of the variable step size fixed speed derating power MPPT disturbance method of the present invention are as follows:

Step 5: If the difference between the current power value P _current and the limit power target value P _aim exceeds the limit value Δp _s , the power derating value calculated according to the set power derating rate, and using the previous power calculated in step 2 The amount of change Δp _k and the power derating value Δp _{s are} calculated according to formula (2), and the voltage derating coefficient α is calculated according to formula (5) and the voltage change amount Δu _k caused by the previous disturbance obtained in step 2 Change the disturbance step size;

As shown in Fig. 2(a), Fig. 2(b), Fig. 2(c) and Fig. 2(d), in order to use the perturbation method of the present invention, the inverters respectively have voltage disturbances at different derating rates. Step size changes and corresponding power changes. It can be seen that the inverter voltage disturbance step size and power drop amplitude are significantly different under different derating rates.

It should be noted that the present invention analyzes the perturbation method by taking a uniform derating as an example. However, the algorithm is not limited to uniform derating, and its core disturbance step calculation method and method are also applicable to the limited power operation disturbance of other different derating methods, and the step determination in normal MPPT control.

Claims

A variable-step fixed-speed derating power-limited MPPT disturbance method, characterized in that: when the inverter is operating in a limited power mode, the current power value is P current , and the target power value is P aim ; When the machine is initially running in MPPT mode, the limited power mode means that the power is reduced, that is, P aim <P current . At this time, the voltage is usually disturbed to the open circuit voltage direction to reduce the power to achieve the target. Therefore, the limited power mode usually runs at The right side of the maximum power point of the power voltage curve;

If the power is expected to reach the target value within a certain time, the simplest method is to use the uniform speed drop method, set the power change rate to υ p , the unit is W / s, and the MPPT disturbance period is T m , then within a single disturbance period The expected power derating value is the limit

Δp s =υ p T m (1)

If it can be ensured that the power reduction amount after each disturbance is approximately equal to the expected value Δp s , then the power reaches the target value within the specified time, so that the step size of each disturbance is updated as the basis for selecting the disturbance step size;

The specific iteration rules are as follows:

When each disturbance moment arrives, the power change Δp at the previous voltage disturbance step is obtained by making the power of the current moment and the power of the previous disturbance period, so that it is quotient with Δp s ,

Here, α is regarded as the power derating coefficient of the previous step; if α>1, it indicates that the previous disturbance did not generate enough power reduction, so the disturbance step should be increased in the next disturbance; on the contrary, if α< 1, indicating that the previous disturbance produced excessive power reduction, the next disturbance step needs to be reduced; and when α=1, it indicates that the power reduction caused by the disturbance just meets the expectation, so the next disturbance step is unchanged. ;

If the MPPT has been disturbed to the right of the maximum power point, the period is small enough, and after each disturbance, the voltage and power can reach the steady state quickly, then the power decrement generated by the two disturbances is considered to be approximately equal, that is, at the power voltage. The slope on the curve is approximately equal

Therefore, in the case where the above assumptions are true,

To make this step disturbance reach the desired power drop value, ie Δp k+1 = Δp s , then there is

Therefore, an iterative update of the disturbance step size can be achieved by using the power derating coefficient α;

The specific steps of the perturbation method are as follows:

Step 1: After the MPPT function is turned on, the inverter counts down according to the disturbance period T m counter set by the system. Before the set disturbance period T m arrives, the system remains in the waiting state until the disturbance period T m reaches;

Step 2: After the disturbance period T m arrives, calculate the current power value P current according to the currently collected voltage value U k and the current value I k , and then store the previous power value P k-1 and the previous voltage value stored by the system. U k-1 is different from the current power value P current and the current voltage value U k , respectively, thereby obtaining the power variation amount Δp k and the voltage variation amount Δu k caused by the previous disturbance;

Step 3: According to the selection of the operating mode of the inverter by the host computer, if the inverter operation selects the non-limited power mode, the system will calculate the next voltage according to the current power value P current and the current voltage value U k according to the normal MPPT mode. Disturbance step

Step 4: If the inverter operation selects the limited power mode, it is determined whether the difference between the current power value P current and the limited power target value P aim exceeds a system-set power derating value, that is, a limit value Δp s , if both If the difference does not exceed the limit value Δp s , it indicates that the current power P current is very close to the limit power target value P aim . At this time, to ensure the disturbance accuracy, a constant voltage disturbance step is adopted;

Step 5: If the difference between the current power value P current and the limit power target value P aim exceeds the limit value Δp s , the power derating value calculated according to the set power derating rate, and using the previous power calculated in step 2 The amount of change Δp k and the power derating value Δp s are calculated according to formula (2), and the power derating coefficient α is calculated by using the voltage variation Δ-u k caused by the previous disturbance obtained in equation (5) and step 2. Output voltage change disturbance step;

Step 6: According to the current voltage value U k and the voltage change disturbance step calculated in step 3 or 4 or 5, add the two to determine the next voltage reference value, as a voltage disturbance reference, and thus, a disturbance period The algorithm flow ends.
The variable step size fixed speed derating power MPPT disturbance method according to claim 1, wherein the power derating coefficient α of the previous step is calculated by using an absolute value, because in the limited power mode, If the system is running at around near the target power point changes will lead to changes in the sign of α, may lead to the occurrence of the disturbance in the wrong direction; calculated using the absolute value of α, it is determined by the relationship between P aim P current magnitude, i.e., to make The correct judgment of the disturbance direction.
The variable-step constant-speed derating power-limiting MPPT perturbation method according to claim 1, wherein when the power is changed to the vicinity of the target value, the power oscillation caused by the excessively large disturbance step length is avoided. Separation, perturbation at a fixed step size S d .