CN111242371B - Photovoltaic power generation short-term prediction correction method based on non-iterative multi-model - Google Patents
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Abstract
The invention discloses a photovoltaic power generation short-term prediction correction method based on a non-iterative multi-model, belonging to the technical field of new energy. And then establishing a plurality of correction model correction prediction results through a non-iterative method. The method avoids the influence of accumulated errors, can reduce the prediction error by about 2-6%, effectively improves the prediction precision, and is favorable for the dispatching of a power system and the safe and stable operation of a power grid.
Description
Technical Field
The invention belongs to the technical field of new energy. In particular to a photovoltaic power generation short-term prediction correction method based on a non-iterative multi-model.
Background
In recent decades, due to the advantages of abundant solar energy resources, cleanness, no pollution and inexhaustibility, the photovoltaic power generation technology is rapidly developed, and the number of photovoltaic power stations is rapidly increased in the global scope. With access of large-scale photovoltaic power stations to a power system, characteristics of intermittency, random fluctuation and the like of photovoltaic power generation bring many difficulties and challenges to power dispatching work. Therefore, an accurate photovoltaic power prediction result has important significance for safe and stable operation of the power system under the condition of high-proportion photovoltaic access.
The photovoltaic power prediction can be divided into ultra-short term prediction, short term prediction and medium and long term prediction according to the predicted time scale. The short-term prediction is generally used for correcting a day-ahead plan curve, can be used for smooth control of photovoltaic output and maintenance and overhaul of components of a photovoltaic power station, is beneficial to processing of emergency states, and has important significance in operation and real-time scheduling of a power system. According to the adopted mathematical physics theory and the prediction output quantity thereof, the photovoltaic power generation short-term prediction method can be divided into two categories: (1) a direct prediction method (statistical method) for directly predicting the output power of the photovoltaic system; (2) firstly, the solar radiation is predicted, and then an indirect prediction method (physical method) of photoelectric output power is obtained according to photoelectric conversion efficiency.
Both the direct prediction method and the indirect prediction method are applied in different occasions, but the two methods cannot avoid the problem of large prediction error. Due to the random fluctuation of the photovoltaic output, when the photovoltaic output fluctuates greatly, an accurate result cannot be obtained only by depending on weather forecast values and historical data. Therefore, in order to reduce the negative impact of the error on the grid system, the power prediction error needs to be corrected. The correction method can improve the prediction precision and reduce the influence of uncertainty, and is an effective means for improving the engineering practicability of the photovoltaic power prediction system.
Disclosure of Invention
The invention aims to provide a photovoltaic power generation short-term prediction correction method based on a non-iterative multi-model, which is characterized by comprising the following steps of:
s1, processing original data, namely dividing historical environment data and numerical weather forecast data according to seasons;
s2, dividing meteorological data and photovoltaic output data according to seasons in the step S1, and establishing a prediction model of each season through training of input and output variables;
s3, forecasting and obtaining a result, namely selecting a proper model from the forecasting models of each season established in the step S2 to forecast, and obtaining the result;
s4, selecting correction data; selecting the required predicted power and the actually measured power from the result obtained in the step S3;
s5, establishing a correction model, and establishing correction models at different moments in a non-iterative mode;
and S6, correcting results, and synthesizing the outputs of different models to obtain correction power.
The collected original data in the step S1 comprise historical power data and historical meteorological data of the photovoltaic power station; the historical power data of the photovoltaic power station comprises historical irradiance, ambient temperature, humidity and wind speed data corresponding to the photovoltaic power station, and the numerical weather forecast data comprises numerical weather forecast irradiance, ambient temperature, humidity and wind speed data of the location of the photovoltaic power station; and classifying the collected historical power data and historical meteorological data of the photovoltaic power station according to seasons.
In step S2, a seasonal prediction model is established according to the classified data, and the method specifically includes the following steps:
s21, training input variables and output variables in different seasons by using a support vector machine, a BP neural network and an extreme learning machine respectively, and establishing a mapping relation between the input variables and the output variables in each season;
s22, obtaining prediction models of the support vector machine, the BP neural network and the extreme learning machine in four seasons of spring, summer, autumn and winter respectively.
In the step S3, the photovoltaic output power is predicted through the seasonal model, and the method specifically includes the following steps:
s31, judging the season of the prediction data, and selecting a proper seasonal model for prediction by using three algorithms;
and S32, obtaining a photovoltaic power prediction result.
In step S4, the training and prediction data of the correction part is selected, which specifically includes the following steps:
s41, selecting the predicted power and the actually measured power quantity during training of a correction part of models;
s42, selecting the predicted power quantity according to the needs to correct; during correction, the predicted power x (t + 1), x (t + 2), … x (t + L) and the actual power x ' (t-k + 1), … x ' (t-1) and x ' (t) are selected to be input into a model, and the corrected power at the corresponding moment is obtained; wherein, (x (t + 1), x (t + 2), … x (t + L) are the predicted power at time t +1, t +2, … t + L, respectively, and x ' (t-k + 1), … x ' (t-1), x ' (t) is the actual power at time t-k +1, … t-1,t, respectively.
In the step S5, the extreme learning machine is used to train the input and output variables, which specifically includes the following steps:
s51, training model parameters by using the selected training data through an extreme learning machine, and establishing a mapping relation between an input variable and an output variable;
is provided with M arbitrary data (x) i ,t i ) Wherein x is i =[x i1 ,x i2 ……x im ] T ∈R m ,t i =[t i1 ,t i2 ……t in ] T ∈R n Then, the output of the feedforward neural network with N hidden node excitation functions as G can be expressed as:
in the formula: a is i =[a 1i ,a 2i ,……a mi ] T The weight vectors of the ith hidden node and the input node are obtained; beta is a i =[β i1 ,β i2 ,……β in ] T The weight vectors of the ith hidden node and the output node are obtained; b i Is the bias of the ith hidden node; n is the number of hidden nodes; equation (1) can be abbreviated as:
Hβ=T (2)
in the formula: h is a hidden layer output matrix, and the ith column of H corresponds to an input x 1 ,x 2 ,……x n An ith hidden layer output vector; the output weight may be obtained by solving a least squares solution of the system of linear equations (3);
the least squares solution is:
β=H + T (5)
middle H + Is the Moore-Penrose generalized inverse of the hidden layer output matrix H.
S52, establishing L models by using a non-iterative method through an extreme learning machine.
In step S6, correcting the short-term prediction result specifically includes the following steps:
s61, inputting numerical weather forecast data into the seasonal model of S32 to obtain primary predicted power;
and S62, substituting the predicted power into the correction model in the step S52 to obtain the final photovoltaic correction power.
The photovoltaic power short-term prediction correction method has the advantages that: and the short-term prediction result of the power is corrected through the continuity between the output sequences, so that the prediction precision is improved. And effective information is provided for smooth control of photovoltaic output and maintenance and overhaul of components of the photovoltaic power station, including operation and real-time scheduling of the power system. The invention has the following characteristics:
1. the prediction method based on the seasonal model provides possibility for improving the adaptability of the model to the output under different seasonal conditions, and can obtain higher prediction accuracy.
2. And the continuity between photovoltaic output sequences is considered, and the actual value is introduced into the correction model, so that the prediction precision is improved to a great extent. Provides guarantee for dispatching and safe and stable operation of the power system
3. Compared with the conventional iterative method, the non-iterative method avoids the influence caused by accumulated errors and has higher prediction precision.
Drawings
FIG. 1 is a flow chart of short-term predictive correction of photovoltaic power generation;
FIG. 2 is a diagram of an ELM structure;
FIG. 3 is a non-iterative correction model;
FIG. 4 is a predicted power corrected using 6 actual values; wherein a is the relation between the actual power and the predicted power after the first 6 values are corrected; b is the error distribution condition after the first 6 values are corrected;
FIG. 5 shows predicted output power for different seasons; wherein, a spring, b summer, c autumn and d winter;
FIG. 6 shows an error distribution of different months; wherein, a, RMSE and b, MAE;
Detailed Description
The invention provides a photovoltaic power generation short-term prediction correction method based on a non-iterative multi-model. The photovoltaic power multi-model interval prediction method of the present invention is further described below with reference to the accompanying drawings and specific embodiments.
FIG. 1 is a flow chart illustrating short term photovoltaic power generation prediction correction; the photovoltaic power generation short-term prediction correction method based on the non-iterative multi-model comprises the following specific steps:
s1, collecting historical operation data, historical environment data and numerical weather forecast data of the photovoltaic power station.
The historical operation data comprises historical power data of the photovoltaic power station, the historical environment data comprises historical irradiance, environment temperature, humidity and wind speed data corresponding to the photovoltaic power station, and the numerical weather forecast data comprises numerical weather forecast irradiance, environment temperature, humidity and wind speed data of the location of the photovoltaic power station. Selecting the data of 2018 years of a new energy power system national key laboratory photovoltaic empirical test power station of North China Power university, wherein the power station consists of a photovoltaic power generation system with the capacity of 250Kw, a solar energy meteorological station, a photovoltaic power station monitoring system and a numerical weather forecasting system, and the data sampling time is 15 minutes. According to the method, 10KW photovoltaic arrays are selected for analysis, and photovoltaic historical data are classified according to seasons.
S2, establishing a seasonal prediction model according to the classified data, and specifically comprising the following steps:
and S21, training input variables and output variables in different seasons by using a support vector machine, a BP neural network and an extreme learning machine respectively, and establishing a mapping relation between the input variables and the output variables in each season.
S22, obtaining prediction models of the support vector machine, the BP neural network and the extreme learning machine in four seasons of spring, summer, autumn and winter respectively.
S3, predicting the photovoltaic output power through a seasonal model, and specifically comprising the following steps:
and S31, judging the season of the prediction data, and selecting a proper model for prediction.
And S32, obtaining a power prediction result.
The analysis of the prediction result shows that the frequency of the positive error is higher than that of the negative error, which indicates that the probability of the occurrence of the case that the predicted value is larger than the measured value is higher.
S4, selecting training and prediction data of the correction part, and specifically comprising the following steps:
and S41, selecting the predicted power and the actually measured power quantity during training of the correction part of the model.
And S42, selecting the predicted power quantity according to the requirement to correct. During correction, the predicted power x (t + 1), x (t + 2), … x (t + L) and the actual power x ' (t-k + 1), … x ' (t-1), x ' (t) are selected to be input into a model, and the corrected power at the corresponding moment is obtained.
S5, training input and output variables by using an extreme learning machine, and specifically comprising the following steps:
and S51, training model parameters by using the selected training data through an extreme learning machine, and establishing a mapping relation between the input variable and the output variable.
The principle of the extreme learning machine is shown in FIG. 2
Is provided with M arbitrary data (x) i ,t i ) Wherein x is i =[x i1 ,x i2 ……x im ] T ∈R m ,t i =[t i1 ,t i2 ……t in ] T ∈R n Then, the output of the feedforward neural network with N hidden node excitation functions as G can be expressed as:
in the formula: a is i =[a 1i ,a 2i ,……a mi ] T The weight vectors of the ith hidden node and the input node are obtained; beta is a i =[β i1 ,β i2 ,……β in ] T The weight vectors of the ith hidden node and the output node are obtained; b is a mixture of i Is the offset of the ith hidden node; n is the number of hidden nodes. Equation (3) can be abbreviated as:
Hβ=T (2)
in the formula: h is a hidden layer output matrix, and the ith column of H corresponds to an input x 1 ,x 2 ,……x n The ith hidden layer outputs a vector. The output weight may be obtained by solving a least squares solution of the system of linear equations (3);
least squares solution of β = H + T (5)
In the formula H + Is the Moore-Penrose generalized inverse of the hidden layer output matrix H.
S52, establishing L models by using a non-iterative method through an extreme learning machine.
When short-term prediction is performed, a rolling iteration method is often adopted, namely, a value obtained by each prediction is used as an actual value of the current moment and is input into a model. However, due to accumulation of errors, the prediction error increases with the increase of the prediction time, resulting in an increase in deviation of the prediction result. In consideration of the limitation of the iterative method, the iterative method is abandoned, and a non-iterative mode is selected.
Assuming that the photovoltaic output power needs to be corrected at time t for L future times, the input-output variables are shown in fig. 3.Actual power, model input value; x (t + 1), x (t + 2), x (t + 3), …, wherein x (t + L) is the predicted power, and the bold parts are input values of different models respectively; the corrected power portions x '(t + 1), x' (t + 2), x '(t + 3), …, x' (t + L) are the output values of the different models. The input data of training is a predicted power value at a certain moment and k real values before the predicted value. In correction, the input data are predicted values and/or based on x (t + 1) to x (t + L)>To>The output data is the correction value.
S6, correcting the short-term prediction result, which specifically comprises the following steps:
s61, inputting numerical weather forecast data into the seasonal model of S32 to obtain primary predicted power;
and S62, substituting the predicted power into the correction model in the step S52 to obtain the final photovoltaic correction power.
RMSE and MAE are selected as error evaluation indexes, and are as follows:
in the formula: p is installed capacity; n is the number of samples; e.g. of the type t Representing an error.
As shown in Table 1, RMSE and MAE are 4.4774 and 1.9372, respectively, when corrected with the past value. As the number increases, both the RMSE and MAE values begin to decrease, with RMSE and MAE reaching a minimum at the sixth value. After reaching this minimum point, the RMSE and MAE remain substantially stable and increase even slightly. That is, the best effect is obtained when the number of past values is 6. Here we select the top 6 value corrections.
TABLE 1 error table corrected by different values in the past
Fig. 4 is a graph of actual power versus predicted power and corrected power after correction using the first 6 values. Wherein a is the relation between the actual power and the predicted power after the first 6 values are corrected; it can be seen that the deviation between the predicted power and the actual power is large and the distribution is more obvious above the left of the dotted line y = x, i.e. the probability that the predicted power is greater than the actual power is higher than the probability that the actual power is greater than the predicted power. However, the points of correcting power are positioned at two sides of the dotted line, the distribution is concentrated, the integral deviation is not very large, only the individual value of the pole deviates far from the dotted line, and the error is large. b is the corrected error distribution for the first 6 values. As can be seen from the figure, the value of the prediction error is large, the distribution range is wide, and the prediction error has distribution in the range of [ -3000,4000 ]. The corrected error value has obvious large amplitude convergence, the error is greatly reduced, most of the error is in the range of [ -1000,1000], and a small part of the error exceeds the range. This shows that after correction, the error is reduced and the prediction accuracy is improved. Although non-iterative methods can reduce the accumulation of errors, the prediction accuracy also decreases with increasing time scale. Moreover, the predicted value is corrected by using the adjacent actual value, the close connection between the two values is required to be ensured, the continuity on a certain time sequence is ensured, and the higher the continuity is, the higher the precision is. Once the time scale that needs to be corrected is too long, the error becomes larger and larger. In table 2, the error of the first correction value is small, and as the time distance between the correction value and the actual value becomes larger, the RMSE increases almost 1% each time, and the MAE increases about 0.5% each time. In particular, the error of the seventh and eighth correction values is already greater than 10%, increasing the error by more than a factor of two compared to the first 4.3764%. Therefore, a suitable correction timescale needs to be selected. The time resolution of the photovoltaic data used this time was 15 minutes, and the four points were 1 hour. From the above table it can be found that it is relatively reasonable to correct 4 points, i.e. the time is one hour.
TABLE 2 error cases when correction values are different
And the table 3 shows the statistics of the prediction errors of the photovoltaic power station in 2017. RMSE of BP, SVM and ELM is about 10%, and MAE is about 5%. And the two errors of the correction model are 5.6795% and 2.5028%, which are almost reduced by one time compared with the errors of other models, and the accuracy of the correction model can be effectively improved.
TABLE 3 different model errors
As shown in fig. 5 a spring, b summer, c autumn, and d winter, relationships between predicted values, actual values, and corrected values for different seasons are presented. As can be seen from fig. 5, the predicted curves obtained by the three algorithms have larger deviation and are smoother than the actual output curve, and the fluctuation characteristics of the photovoltaic output cannot be well fitted. Compared with the prediction curve, the correction curve not only reduces the error, but also has higher fitting degree with the actual curve. Since the predicted value is corrected one hour at a time by a non-iterative method, although accumulation of errors due to iteration can be prevented, the accuracy also decreases with time. It can also be seen that the first value of each correction has a reduced error compared to the last value of the previous correction, and that the subsequent correction value has an increased error over time.
As shown in fig. 6 a, RMSE and b, MAE, the overall trend of the three verification algorithms is consistent, with only slight differences in the next few months. The correction algorithm is trending with other algorithms, but there are some differences in some places (e.g., months 2, months 7, etc.). The correction algorithm can reduce most errors by more than one time. In addition, when observing fig. a and b, both RMSE and MAE show a tendency of rising first in descending, and then rising and descending again. Furthermore, the maximum value of the error for the three validation algorithms occurs in July, while the maximum value of the correction error occurs in August. The minimum of the errors for the three validation algorithms occurs in december, while the minimum of the correction errors occurs in january. This means that the prediction error is not the smallest, and the correction error is necessarily the smallest, and the two are not in a definite relationship. Among the three algorithms, BP is high in prediction speed, small in fluctuation at the later stage and relatively stable. The SVM has slow prediction speed and difficult parameter selection, but the error in the first two months is obviously lower than that in the other two methods. The ELM converges quickly, but the error is sometimes large.
The invention considers the seasonal distribution characteristic of errors, establishes a seasonal prediction model, introduces an actual value into a correction model, and corrects the prediction result. In addition, compared with an iterative method, the non-iterative method used by the invention avoids the influence caused by accumulated errors and has higher prediction precision.
Claims (6)
1. A photovoltaic power generation short-term prediction correction method based on a non-iterative multi-model is characterized by comprising the following steps:
s1, processing original data, and dividing historical environment data and numerical weather forecast data according to seasons;
s2, dividing meteorological data and photovoltaic output data according to seasons in the step S1, and establishing a prediction model of each season through training of input and output variables;
s3, forecasting and obtaining a result, namely selecting a proper model from the forecasting models of each season established in the step S2 to forecast, and obtaining the result;
s4, selecting correction data; selecting the required predicted power and the actually measured power from the result obtained in the step S3;
s5, establishing a correction model, and establishing correction models at different moments in a non-iterative mode;
in the step S5, the extreme learning machine is used to train the input and output variables, which specifically includes the following steps:
s51, training model parameters by using the selected training data through an extreme learning machine, and establishing a mapping relation between an input variable and an output variable;
with M arbitrary data (x) i ,t i ) Wherein x is i =[x i1 ,x i2 ……x im ]T∈R m ,t i =[t i1 ,t i2 ……t in ] T ∈R n Then, the output of the feedforward neural network with N hidden node excitation functions as G is expressed as:
in the formula: a is i =[a 1i ,a 2i ,……a mi ] T The weight vectors of the ith hidden node and the input node are obtained; beta is a i =[β i1 ,β i2 ,……β in ] T The weight vectors of the ith hidden node and the output node are obtained; b i Is the bias of the ith hidden node; n is the number of hidden nodes; equation (1) is abbreviated as:
Hβ=T (2)
in the formula: h is a hidden layer output matrix, and the ith column of H corresponds to an input x 1 ,x 2 ,……x n An ith hidden layer output vector; the output weight may be obtained by solving a least squares solution of the system of linear equations (3);
the least squares solution is:
β=H + T (5)
in the formula H + The Moore-Penrose generalized inverse of the hidden layer output matrix H;
s52, establishing L models by using a non-iterative method through an extreme learning machine;
and S6, correcting results, and synthesizing the outputs of different models to obtain correction power.
2. The non-iterative multi-model-based photovoltaic power generation short-term prediction correction method according to claim 1, wherein the raw data collected in step S1 comprises historical power data and historical numerical weather forecast data of a photovoltaic power station; the historical power data of the photovoltaic power station comprises historical irradiance, ambient temperature, humidity and wind speed data corresponding to the photovoltaic power station; the numerical weather forecast data comprises numerical weather forecast irradiance, ambient temperature, humidity and wind speed data of the place where the photovoltaic power station is located; and classifying the collected historical power data and historical meteorological data of the photovoltaic power station according to seasons.
3. The non-iterative multi-model-based photovoltaic power generation short-term prediction correction method according to claim 1, wherein in the step S2, a seasonal prediction model is established according to the classified data, and the method specifically comprises the following steps:
s21, training input variables and output variables in different seasons by using a support vector machine, a BP neural network and an extreme learning machine respectively, and establishing a mapping relation between the input variables and the output variables in each season;
s22, obtaining prediction models of the support vector machine, the BP neural network and the extreme learning machine in four seasons of spring, summer, autumn and winter respectively.
4. The non-iterative multi-model-based photovoltaic power generation short-term prediction correction method according to claim 1, wherein in the step S3, the photovoltaic output power is predicted through a seasonal model, and the method specifically comprises the following steps:
s31, judging the season of the prediction data, and selecting a proper seasonal model for prediction by using three algorithms;
and S32, obtaining a photovoltaic power prediction result.
5. The non-iterative multi-model-based photovoltaic power generation short-term prediction correction method according to claim 1, wherein in the step S4, training and prediction data of a correction part are selected, and the method specifically includes the following steps:
s41, selecting the predicted power and the actually measured power quantity during training of a correction part of models;
s42, selecting the predicted power quantity according to the requirement to correct; during correction, the predicted power x (t + 1), x (t + 2), … x (t + L) and the actual power x ' (t-k + 1), … x ' (t-1) and x ' (t) are selected to be input into a model, and the corrected power at the corresponding moment is obtained; wherein, (x (t + 1), x (t + 2), … x (t + L) are the predicted power at time t +1, t +2, … t + L, respectively, and x ' (t-k + 1), … x ' (t-1), x ' (t) is the actual power at time t-k +1, … t-1,t, respectively.
6. The non-iterative multi-model-based photovoltaic power generation short-term prediction correction method according to claim 1, wherein in the step S6, the short-term prediction result is corrected, specifically including the steps of:
s61, inputting numerical weather forecast data into the seasonal model of S32 to obtain primary predicted power;
and S62, substituting the predicted power into the correction model in the step S52 to obtain the final photovoltaic correction power.
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