CN109299431B - Photovoltaic module performance consistency evaluation method based on Hilbert marginal spectrum characteristics - Google Patents
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Abstract
The invention relates to a photovoltaic module performance consistency evaluation method based on Hilbert marginal spectrum characteristics. The inconsistency of the performance of the photovoltaic module can cause the reduction of the photovoltaic power generation efficiency and even influence the stability of a photovoltaic power generation system, so that the consistency evaluation of the performance of the photovoltaic module is carried out, and the method has important application value. The invention considers the frequency domain linear superposition characteristic when photovoltaic modules form a photovoltaic array through series-parallel connection, firstly utilizes empirical mode decomposition and Hilbert marginal spectrum transformation on a photovoltaic array direct current volt-ampere signal, then screens a frequency range calculated by marginal spectrum consistency indexes, removes a frequency range corresponding to a marginal spectrum peak value reflecting the superposition characteristic, and provides a calculation method for evaluating consistency indexes of a marginal spectrum local frequency range, thereby realizing the online evaluation of the performance consistency of the photovoltaic modules. The invention can improve the operation efficiency and stability of the photovoltaic power station and is beneficial to the popularization and application of photovoltaic power generation.
Description
Technical Field
The invention belongs to the field of new photovoltaic energy, and relates to an online evaluation method for performance consistency of a photovoltaic module based on Hilbert (Hilbert) marginal spectrum characteristics.
Background
Traditionally, a photovoltaic module is tested by using solar simulated light in a standard laboratory environment, and finally performance consistency evaluation is given through comparison of a volt-ampere characteristic template. In addition, for simplicity of operation, the uniformity of performance is generally evaluated by directly using a certain point on the current-voltage characteristic curve, for example, single-point detection methods such as a maximum power method, an open-circuit voltage, and a short-circuit current. It must be pointed out that although the representative points of the voltammetric characteristic curve may characterize the whole curve to some extent, there are great limitations from the point of completeness of characterization. In addition, a method combining piecewise linear interpolation and a least square method also exists, although the limitation is solved, the method is mainly based on detection of branch current difference in the photovoltaic array, and hardware investment of an actual system can be remarkably increased; moreover, the method also requires that the illumination intensity and the temperature are in a standard state in a laboratory, so that the requirement of online evaluation on the performance of the photovoltaic module cannot be met. In the practical application of photovoltaic arrays, consideration must be given to: (1) The weather and solar radiation conditions can not meet the standardized requirements, and at the moment, even the volt-ampere characteristics of the same photovoltaic module can show certain fluctuation; (2) Due to the influence of unstable photoelectric conversion, the performance of the photovoltaic module changes accordingly, so that the online evaluation of the performance of the photovoltaic module aiming at practical application scenes is more meaningful.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a photovoltaic module performance consistency evaluation method based on Hilbert marginal spectrum characteristics.
The present invention contemplates: (1) In a standard laboratory environment, the method for evaluating the consistency of the performance of the photovoltaic module cannot meet the online requirement in practical application; (2) Considering that in practical application occasions, photovoltaic modules are connected in series and parallel to form a photovoltaic array, and the superposed volt-ampere characteristics of the photovoltaic array cannot reduce the volt-ampere characteristic curves of the photovoltaic modules, it is not feasible to attempt to use the differences of the volt-ampere characteristic curves of the photovoltaic modules to realize detection of inconsistent performance. Under the same meteorological and solar radiation conditions, the volt-ampere characteristic curve of the photovoltaic module with better performance consistency has better coincidence, and shows a linear superposition characteristic on a frequency domain, so that the performance consistency of the photovoltaic module can be represented by utilizing the consistency of frequency domain distribution; (3) Empirical mode decomposition enables frequency narrow-band decomposition of volt-ampere characteristics of the photovoltaic array, and Hilbert marginal spectrum transformation helps describe consistency of the photovoltaic array on frequency domain characteristics. Therefore, the invention provides the photovoltaic module consistency online evaluation method based on the Hilbert marginal spectrum characteristics by comprehensively considering the linear superposition effect corresponding to the marginal spectrum peak value.
The invention provides a photovoltaic module performance consistency online evaluation method based on Hilbert marginal spectrum characteristics, which comprises the following steps of:
aiming at a photovoltaic system in practical application, in order to ensure the relative stability of air temperature conditions and change the solar radiation amount by utilizing cloud layer shielding so as to enable the output of a photovoltaic module to fluctuate, randomly selecting a plurality of time periods with cloudy weather types, collecting photovoltaic array volt-ampere signals on the direct current side of an inverter, and interpolating by adopting a secondary spline to obtain a photovoltaic array volt-ampere characteristic curve.
And (2) decomposing the volt-ampere characteristic curve of the photovoltaic array into narrow-band regions with frequency bands as independent as possible, carrying out empirical mode decomposition on the narrow-band regions, and obtaining a group of intrinsic mode functions in a self-adaptive manner by utilizing the requirement that the remainder needs to meet a monotonous condition.
And (3) carrying out Hilbert transformation on the intrinsic mode functions obtained in the step (2), and calculating the instantaneous frequency of each intrinsic mode function.
And (4) calculating a Hilbert spectrum of the volt-ampere characteristic curve of the photovoltaic array on the basis of the Hilbert conversion result and the instantaneous frequency of the intrinsic mode function, and obtaining a corresponding Hilbert marginal spectrum.
And (5) removing the frequency range corresponding to the marginal spectrum peak value reflecting the superposition characteristic from the Hilbert marginal spectrum obtained in the step (4), and calculating a marginal spectrum consistency index in a local frequency range to represent the dispersion degree of the volt-ampere characteristic curve distributed on a frequency domain, so as to evaluate the consistency of the performance of the photovoltaic component on line.
The invention has the following beneficial effects:
1. the method for evaluating the consistency of the photovoltaic assembly in the standard environment of the traditional laboratory is changed. In practical application scenarios, the performance of the photovoltaic module will usually show a dynamic characteristic with the changes of weather, solar radiation and photoelectric conversion characteristics, so that a method for estimating the standard curve of the current-voltage characteristic in advance is not feasible. The method is characterized in that the consistency of the volt-ampere characteristics of the photovoltaic modules on the frequency domain distribution under the same meteorological, solar radiation and time conditions is obtained; according to the series-parallel connection relation of the photovoltaic modules forming the photovoltaic array, a linear superposition characteristic is shown on a frequency domain, the consistency of the performance of the photovoltaic modules is represented by the consistency of frequency domain distribution, and the requirement of online evaluation can be met.
2. And (3) carrying out empirical mode decomposition and Hilbert marginal spectrum transformation on the volt-ampere characteristic curve of the photovoltaic array, removing a frequency range corresponding to a marginal spectrum peak value reflecting the superposition characteristic, and calculating a marginal spectrum consistency index in a local frequency range, thereby realizing the online evaluation of the performance consistency of the photovoltaic module. The invention can improve the operation efficiency and stability of the photovoltaic system and is beneficial to the popularization and application of photovoltaic power generation.
Detailed Description
Aiming at a photovoltaic system in practical application, in order to ensure the relative stability of temperature conditions and change the solar radiation amount by utilizing cloud layer shielding, so that the output of a photovoltaic module generates obvious fluctuation in a short time, a plurality of periods with cloudy weather types are randomly selected, photovoltaic array volt-ampere signals on the direct current side of an inverter are collected, a quadratic spline is adopted for interpolation, and a photovoltaic array volt-ampere characteristic curve is obtained and is marked as v (i), wherein i is direct current, and v represents direct current voltage corresponding to i.
And (2) decomposing the volt-ampere characteristic curve v (i) of the photovoltaic array into narrow-band regions with frequency bands as independent as possible. Performing Empirical Mode Decomposition (EMD) on v (i) to decompose v (i) into n intrinsic mode functions v k (i) And 1 remainder r n (i) As shown in formula (1), wherein the remainder r n (i) A monotonous condition needs to be satisfied.
Step (3) calculating intrinsic mode function v k (i) The instantaneous frequency of (c). For each intrinsic mode function v k (i) Carrying out Hilbert transformation to obtain a transformation result u k (i) As shown in formula (2).
Wherein i max And i min Respectively representing the upper and lower boundaries of the dc current of the photovoltaic array.
Computing intrinsic mode function v k (i) Instantaneous frequency f of k (i) As shown in formula (3).
And (4) calculating a Hilbert marginal spectrum H (f) of the volt-ampere characteristic curve v (i) of the photovoltaic array. And calculating the Hilbert spectrum H (f, i) of v (i) as shown in the formula (4).
And calculating a corresponding Hilbert marginal spectrum H (f) as shown in the formula (5).
Where Am represents the amplitude calculation function.
And (5) calculating a consistency index of the Hilbert marginal spectrum H (f) to represent the discrete degree of v (i) distributed on a frequency domain, so as to evaluate the consistency of the photovoltaic component performance on line.
Firstly, defining the frequency points corresponding to the peak value of H (f) as f m M =1,2, \ 8230;, M, satisfying the conditions shown in formulas (6) and (7). Wherein formula (6) represents H (f) m ) Certain absolute amplitude requirements need to be met; and formula (7) reflects H (f) m ) The relative extremum requirement of a certain local area needs to be satisfied.
H(f m )＞H(δ(f m )) (7)
Wherein H represents the mean of the marginal spectrum H (f); gamma is the screening coefficient of the peak value, and can be generally set to be 0.1; delta (f) m ) Is expressed as f m Any one frequency point in a specific region, δ (f), as a center m ) The corresponding specific region is [ f m -Δ m ,f m +Δ m ]，Δ m Can be set to 0.2 xf m 。
Then calculating the mean value of the marginal spectrum H (f) after M peak value areas are removed
As shown in equation (8).
In the formula (f) max Denotes the cut-off frequency of v (i).
And finally, calculating a consistency index s of the Hilbert marginal spectrum H (f), as shown in a formula (9).
In the formula (I), the compound is shown in the specification,
the relative size of s reflects the consistency of the performance of the photovoltaic module. The smaller s is, the better the consistency of the current-voltage characteristics of the photovoltaic module in the frequency domain is shown, and the better the consistency of the performance of the photovoltaic module is.
Claims (3)
1. The photovoltaic module performance consistency evaluation method based on the Hilbert marginal spectrum features is characterized by comprising the following steps of:
the method comprises the following steps that (1) aiming at a photovoltaic system in practical application, a plurality of time intervals with a cloudy weather type are randomly selected, photovoltaic array volt-ampere signals on the direct current side of an inverter are collected, and a quadratic spline is adopted for interpolation to obtain a photovoltaic array volt-ampere characteristic curve; denoted v (i), where i is a direct current and v represents a direct voltage corresponding to i;
decomposing the volt-ampere characteristic curve v (i) of the photovoltaic array into narrow-band regions with frequency bands as independent as possible; performing empirical mode decomposition on v (i) to decompose v (i) into n intrinsic mode functions v k (i) And 1 remainder r n (i) As shown in formula (1), wherein the remainder r n (i) A monotonous condition needs to be satisfied;
step (3) of the intrinsic mode function v obtained in the step (2) k (i) Performing Hilbert transform to calculate instantaneous frequency f of each intrinsic mode function k (i)；
Calculating a Hilbert spectrum of a volt-ampere characteristic curve of the photovoltaic array on the basis of a Hilbert conversion result and instantaneous frequency of the intrinsic mode function, and obtaining a corresponding Hilbert marginal spectrum;
removing a frequency range corresponding to a marginal spectrum peak value reflecting the superposition characteristic from the Hilbert marginal spectrum H (f) obtained in the step (4), and calculating a marginal spectrum consistency index in a local frequency range to represent the dispersion degree of the volt-ampere characteristic curve distributed on a frequency domain, so as to evaluate the consistency of the performance of the photovoltaic component on line; the method specifically comprises the following steps:
firstly, defining the frequency points corresponding to the peak value of the Hilbert marginal spectrum H (f) as f m M =1,2, \ 8230;, M, satisfying the conditions shown in formulas (6) and (7); wherein formula (6) represents H (f) m ) Certain absolute amplitude requirements need to be met; and formula (7) reflects H (f) m ) The relative extreme value requirement of a certain local area needs to be met;
H(f m )＞H(δ(f m )) (7)
in the formula (I), the compound is shown in the specification,
represents the mean of the marginal spectrum H (f); gamma is the screening coefficient of the peak value and is set to be 0.1; delta (f) m ) Is represented by f m Any frequency point in a particular region, δ (f), centered on m ) The corresponding specific region is [ f m -Δ m ,f m +Δ m ]，Δ m Is set to 0.2 xf m ；
Then calculating the mean value of the marginal spectrum H (f) after M peak value areas are removed
As shown in formula (8);
in the formula (f) max Represents the cut-off frequency of v (i);
finally, calculating a consistency index s of the Hilbert marginal spectrum H (f), as shown in a formula (9);
in the formula (I), the compound is shown in the specification,
the relative size of s reflects the consistency of the performance of the photovoltaic module; the smaller s is, the better the consistency of the current-voltage characteristics of the photovoltaic module in the frequency domain is shown, and the better the consistency of the performance of the photovoltaic module is.
2. The Hilbert marginal spectrum feature-based photovoltaic module performance consistency evaluation method according to claim 1, wherein the evaluation method comprises the following steps: step (3) calculating intrinsic mode function v k (i) The instantaneous frequency of (d); for each intrinsic mode function v k (i) Performing Hilbert conversion to obtain a conversion result u k (i) Specifically, the formula is shown as a formula (2);
wherein i max And i min Respectively representing the upper and lower boundaries of the direct current of the photovoltaic array;
computing intrinsic mode function v k (i) Instantaneous frequency f of k (i) As shown in formula (3);
3. the Hilbert marginal spectrum feature-based photovoltaic module performance consistency evaluation method according to claim 1, characterized in that: calculating a Hilbert spectrum H (f, i) of v (i) in the step (4), wherein the Hilbert spectrum H (f, i) is represented by a formula (4);
calculating a corresponding Hilbert marginal spectrum H (f) as shown in a formula (5);
where Am represents the magnitude calculation function.
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| CN104330624A (en) * | 2014-09-15 | 2015-02-04 | 燕山大学 | Method for detecting closely spaced frequency components of non-stationary signals |
| CN107070568A (en) * | 2017-04-28 | 2017-08-18 | 广东工业大学 | A kind of frequency spectrum sensing method based on Hilbert-Huang transform |
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| CN104330624A (en) * | 2014-09-15 | 2015-02-04 | 燕山大学 | Method for detecting closely spaced frequency components of non-stationary signals |
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