CN114841584A - A Comprehensive Evaluation Method for Power Quality of Distribution Network - Google Patents

Abstract

The invention provides a comprehensive evaluation method for power quality of a power distribution network, which comprises the steps of firstly, comprehensively considering various influence factors such as voltage qualification rate, three-phase imbalance, harmonic waves, frequency, power factors and the like, and constructing a power quality multi-index evaluation matrix; then, acquiring and analyzing distribution transformation information to obtain data values of various influence factors required by the evaluation matrix, and performing normalization processing to facilitate subsequent uniform evaluation on the power quality; then, determining a weight coefficient of each influence factor based on expert experience and least square optimization; and finally, multiplying the normalized data value of each factor by the sum of products of corresponding weight coefficients to obtain the evaluation result of the power quality. The invention constructs a multidimensional evaluation matrix by comprehensively considering the factors such as the voltage qualification rate, the three-phase unbalance degree, the voltage and current harmonic waves, the frequency deviation, the power factor and the like of the power distribution network, comprehensively evaluates each power quality, realizes the comprehensive analysis of a plurality of different power quality indexes, can comprehensively, truly and naturally reflect the property of the power quality, and meets the requirements of power users.

Description

A Comprehensive Evaluation Method for Power Quality of Distribution Network

technical field

The invention relates to a comprehensive evaluation method for power quality of a distribution network, and belongs to the technical field of comprehensive evaluation of power quality.

Background technique

In the increasingly fierce market competition, power quality has received more and more attention from the power supply department and power users. With the vigorous development of my country's national economy, the power grid load has grown rapidly, especially the non-linear and impact load capacity has continued to grow, resulting in The widespread use of nonlinear equipment (such as various power rectification equipment, electric arc furnaces, large-capacity speed-regulating motors, electric traction locomotives, etc.) in the power system has caused serious pollution to the power quality of the power supply system. Quality problems have become an important task faced by electric power workers.

All countries in the world have formulated a series of standards for power quality (such as voltage, load, frequency, reliability, etc.), but these standards can only be used to determine whether the power supply is qualified or unqualified, but not the quality. Reflected by multiple indicators, the polarized quality standards cannot fully, truly and naturally reflect the nature of power quality.

At present, the research at home and abroad mainly evaluates the power quality according to each single index. The evaluation of the single index can be treated according to specific problems. Different evaluation methods are determined according to different problems, which is conducive to solving a certain power quality problem. However, The evaluation of a single index ignores the interconnection between different power quality problems and the sensitivity of users to different power quality problems.

With the development of the power system, the power quality of each node in the entire power grid varies. Power quality is composed of a number of quality contents, and it is necessary to comprehensively evaluate each power quality to understand the power quality status of the entire power grid. With the marketization of electricity and the transparency of power quality, several different power quality should be Comprehensive analysis of indicators to meet the needs of power users.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a comprehensive evaluation method for the power quality of the distribution network according to the current situation of the distribution network, comprehensively evaluate the quality of each power, and realize the comprehensive analysis of several different power quality indicators to meet the needs of power users. need.

The technical scheme adopted by the present invention is as follows:

A comprehensive evaluation method for power quality of a distribution network, characterized in that it includes the following steps:

S1. Build a multi-index evaluation matrix: comprehensively consider various influencing factors such as voltage qualification rate, three-phase unbalance, harmonics, frequency, power factor, etc., and build a power quality multi-index evaluation matrix;

S2, normalization processing: obtain and analyze the distribution transformer information, obtain the data values of each influencing factor required by the multi-index evaluation matrix constructed in step S1, and perform normalization processing, so as to conduct a unified evaluation of power quality in the future;

S3. Determination of weight: Determine the weight coefficient of each influencing factor based on expert experience and least squares optimization.

S4. Comprehensively evaluate the power quality: multiply the normalized data values of each factor by the sum of the products of the corresponding weight coefficients, which is the evaluation result of the power quality.

Further, the power quality multi-index evaluation matrix constructed in step S1 is a multi-dimensional evaluation matrix, and its formula is as follows:

R={U,B,UH,IH,F,C,O},

In the formula: R refers to the total influence result; U refers to the influence result of the voltage qualification rate; B refers to the influence result of the three-phase unbalance; UH refers to the influence result of the voltage harmonic; IH refers to the influence result of the current harmonic; The result of the influence of frequency; C refers to the result of the influence of power factor; O refers to the result of the influence of other factors.

Power quality is a multi-dimensional comprehensive index, and a single index cannot reflect the overall status of power quality. Power quality is generally determined by the voltage qualification rate, three-phase unbalance, voltage and current harmonics, frequency deviation, power factor and other factors;

The voltage qualification rate refers to the percentage of the sum of the time that the voltage of the monitoring point is within the qualified range and the total time of monthly voltage monitoring within one month of grid operation. In the distribution network, single-phase power supply 220V residential customer power receiving end: -10% to +7%, that is, the highest voltage is not higher than 236V, and the lowest voltage is not lower than 198V; three-phase power supply 10KV (6KV) dedicated line Customer or 380V client: -7%～+7%, that is, the highest voltage is not higher than 10.7KV (6.42KV) or 407V, and the lowest voltage is not lower than 9.3KV (5.58KV) or 353V. The present invention considers the influence result of the voltage qualification rate on the power quality, and quantifies it as the first dimension element U of the evaluation matrix.

Three-phase unbalance refers to the inconsistency of the three-phase current (or voltage) amplitude in the power system, and the amplitude difference exceeds the specified range. As well as economic benefits, it is one of the important indicators of power quality assessment. The present invention considers the effect of three-phase unbalance on power quality, and quantifies it as the second dimension element B of the evaluation matrix.

The degree of three-phase unbalance can be measured by the three-phase unbalance degree as follows:

In the formula: B refers to the three-phase unbalance; I _max refers to the maximum value of the three-phase currents of A, B, and C; I _min refers to the maximum value of the three-phase currents of A, B, and C.

Harmonic refers to the components of each order greater than an integer multiple of the fundamental frequency obtained by the Fourier series decomposition of the periodic non-sinusoidal alternating current. (50Hz) the same component. The Fourier expansion is as follows:

where:

The interference of higher harmonics is a major "public nuisance" affecting the power quality in the current power system. The present invention considers the influence results of voltage harmonics and current harmonics on power quality, and quantizes them as the third dimension element UH and the fourth dimension element IH of the evaluation matrix.

In my country, the frequency of the power system is 50Hz, and the allowable value of normal frequency deviation is ±0.2HZ. When the system capacity is small, the frequency deviation value can be relaxed to ±0.5HZ. Frequency fluctuation is one of the important indicators to measure the quality of power. A small frequency fluctuation may affect the normal use of electrical appliances, and a large frequency fluctuation will endanger the normal operation of the generator and cause great harm to the power grid. The present invention considers the effect of frequency on power quality, and quantifies it as the fifth dimension element F of the evaluation matrix.

During the operation of the power system, it is usually measured by the power factor. The efficiency of the grid operation and the size of the power factor reflect the effective utilization of the apparent power and the active power output by the power grid system. The level of power factor is an important issue related to the power quality and the safety and economic operation of the power grid. By rationally configuring reactive power compensation equipment, the power factor of the system can be improved, thereby achieving the purpose of saving power and reducing losses. The present invention considers the influence result of the power factor on the power quality, and quantizes it as the sixth dimension element C of the evaluation matrix.

At the same time, the present invention considers the effect of other factors on the power quality, and takes it as the seventh dimension element O of the evaluation matrix.

Further, the normalization processing method described in step S2 is specifically: performing normalization processing on the quantized result of each influencing factor to obtain a value between 0 and 1, so as to evaluate the overall power quality subsequently ,

For the voltage qualification rate, it refers to the percentage of the time when the voltage is qualified in one degree to the total time.

For the influence of the three-phase unbalance, it is quantified as the three-phase unbalance degree to express. According to the calculation formula of the three-phase unbalance degree, the value range of the three-phase unbalance degree is between 0≤B≤1, which satisfies Normalization is required, so no processing is required.

For voltage harmonics and current harmonics, since the harmonic content has multiple values, when quantizing harmonics, the total harmonic distortion THD is used to quantify them. The calculation formula is as follows:

In the formula: G ₁ represents the effective value of the fundamental wave; G _n represents the effective value of the nth harmonic, where n=2,3,...;

Considering that the distribution transformers are mostly three-phase transformers, the calculation will generate the THD values of the three-phase voltage and current of A, B, and C. Therefore, in order to better meet the strict requirements of power quality during grid operation, voltage harmonics and currents are respectively adopted. The maximum THD value in the harmonics is used to evaluate the power quality, so as to reflect the worst case, as shown in the following formula:

U _THD = max(THD _Ua , THD _Ub , THD _Uc ),

I _THD =max(THD _Ia , THD _Ib , THD _Ic ),

In the formula: THD _Ua , THD _Ub , THD _Uc represent the THD value of A, B, C three-phase voltage harmonics respectively; THD _Ia , THD _Ib , THD _Ic represent the THD value of A, B, C three-phase current harmonics respectively ;

Considering that the range of THD values of voltage harmonics and current harmonics exceeds the interval of 0-1, the arc tangent function is used to normalize them, and the arc tangent function is as follows:

y = arctanx,

在进行归一化操作时，还需对其进行绝对值以及乘以相关系数的操作，如下式所示：The value range of the independent variable x of the arctangent function is (-∞,+∞), so there is no need to care about the domain of the function. The value range of the dependent variable y of the arctangent function is

When performing the normalization operation, it is also necessary to perform the operations of absolute value and multiplication by the correlation coefficient, as shown in the following formula:

For the influencing factor of frequency, considering that the frequency of my country's power system is 50Hz, therefore, taking 50Hz as the benchmark, the deviation of the actual frequency and the standard frequency is used as the standard for measuring the power quality, and it is normalized according to the above method. As shown in the following formula:

In the formula: f is the actual frequency of the power grid; f ₀ is 50Hz;

For power factor, its value range is between 0≤C≤1, which meets the requirement of normalization and does not need to be processed;

For other factors, the value is normalized by the assessor based on experience.

Further, the method for determining the weight of each influencing factor in step S3 is:

1) According to expert experience, judge the importance of each index, and obtain the subjective index weight coefficient of the evaluation matrix, as shown below:

E=[e ₁ , e ₂ , e ₃ , e ₄ , e ₅ , e ₆ , e ₇ ] ^T ;

2) The objective index weight coefficient determined by the coefficient of variation method is:

V=[v ₁ , v ₂ , v ₃ , v ₄ , v ₅ , v ₆ , v ₇ ] ^T ;

3) In order to determine the optimal weight coefficient, a Lagrangian function is constructed, and the extreme value is solved according to the partial derivative:

4) Based on the least squares method, the weight coefficient after comprehensive optimization is:

W=[w ₁ , w ₂ , w ₃ , w ₄ , w ₅ , w ₆ , w ₇ ].

Further, the method for comprehensively evaluating the power quality in step S4 is:

Suppose the object to be evaluated is n, and the evaluation index in the present invention is 7, then based on the evaluation matrix and the weight coefficient, the evaluation value of the i-th evaluation object is obtained:

Compared with the prior art, the comprehensive evaluation method for the power quality of the distribution network provided by the present invention has the following advantages:

The invention comprehensively considers the distribution network voltage qualification rate, three-phase unbalance, voltage and current harmonics, frequency deviation, power factor and other factors to construct a multi-dimensional evaluation matrix, comprehensively evaluates the quality of each power, and realizes the integration of several Comprehensive analysis of different power quality indicators can comprehensively, truly and naturally reflect the nature of power quality, and meet the needs of power users; the present invention comprehensively considers the subjective index weight coefficient and the objective index weight coefficient, and optimizes to obtain the value of each influencing factor. The weights make the evaluation results more accurate.

Detailed ways

The present invention will be described in detail below with reference to specific examples.

Example

This embodiment provides a comprehensive evaluation method for power quality of a distribution network, including the following steps:

Step 1: Construct a multi-index evaluation matrix, comprehensively considering various influencing factors such as voltage qualification rate, three-phase unbalance, harmonics, frequency, power factor, etc. The constructed power quality multi-index evaluation matrix is a multi-dimensional evaluation matrix, and its formula is as follows :

R={U,B,UH,IH,F,C,O},

In the formula: R refers to the total influence result; U refers to the influence result of the voltage qualification rate; B refers to the influence result of the three-phase unbalance; UH refers to the influence result of the voltage harmonic; IH refers to the influence result of the current harmonic; The result of the influence of frequency; C refers to the result of the influence of power factor; O refers to the result of the influence of other factors.

Step 2, normalization processing, obtaining and analyzing the distribution transformation information, obtaining the data values of each influencing factor required by the multi-index evaluation matrix constructed in step S1, and performing normalization processing,

There is no need to deal with the voltage qualification rate, three-phase unbalance and power factor;

For voltage harmonics and current harmonics, they are normalized using the arc tangent function, which is as follows:

y = arctanx,

在进行归一化操作时，还需对其进行绝对值以及乘以相关系数的操作，如下式所示：The value range of the independent variable x of the arctangent function is (-∞,+∞), so there is no need to care about the domain of the function. The value range of the dependent variable y of the arctangent function is

When performing the normalization operation, it is also necessary to perform the operations of absolute value and multiplication by the correlation coefficient, as shown in the following formula:

For the influencing factor of frequency, considering that the frequency of my country's power system is 50Hz, therefore, taking 50Hz as the benchmark, the deviation of the actual frequency and the standard frequency is used as the standard for measuring the power quality, and it is normalized according to the above method. As shown in the following formula:

In the formula: f is the actual frequency of the power grid; f ₀ is 50Hz;

For other factors, the value is normalized by the assessor based on experience.

Step 3: Determine the weight:

1) According to expert experience, judge the importance of each index, and obtain the subjective index weight coefficient of the evaluation matrix, as shown below:

E=[e ₁ , e ₂ , e ₃ , e ₄ , e ₅ , e ₆ , e ₇ ] ^T ;

2) The objective index weight coefficient determined by the coefficient of variation method is:

V=[v ₁ , v ₂ , v ₃ , v ₄ , v ₅ , v ₆ , v ₇ ] ^T ;

3) In order to determine the optimal weight coefficient, a Lagrangian function is constructed, and the extreme value is solved according to the partial derivative:

4) Based on the least squares method, the weight coefficient after comprehensive optimization is:

W=[w ₁ , w ₂ , w ₃ , w ₄ , w ₅ , w ₆ , w ₇ ].

Step 4: Comprehensively evaluate the power quality, multiply the normalized data value of each factor by the sum of the products of the corresponding weight coefficients, which is the evaluation result of the power quality, and set the number of objects to be evaluated as n, the present invention There are 7 evaluation indicators in the middle, then based on the evaluation matrix and the weight coefficient, the evaluation value of the i-th evaluation object is obtained:

The invention comprehensively considers the distribution network voltage qualification rate, three-phase unbalance, voltage and current harmonics, frequency deviation, power factor and other factors to construct a multi-dimensional evaluation matrix, comprehensively evaluates the quality of each power, and realizes the integration of several Comprehensive analysis of different power quality indicators can comprehensively, truly and naturally reflect the nature of power quality, and meet the needs of power users; the present invention comprehensively considers the subjective index weight coefficient and the objective index weight coefficient, and optimizes to obtain the value of each influencing factor. The weights make the evaluation results more accurate.

The technical solutions of the present invention are not limited to the above-mentioned embodiments, and all technical solutions obtained by adopting equivalent replacement methods fall within the protection scope of the present invention.

Claims (5)

1. a comprehensive evaluation method for the power quality of distribution network, is characterized in that, comprises the following steps: S1, construct multi-index evaluation matrix: comprehensively consider the voltage qualification rate, three-phase unbalance, harmonics, frequency, power factor and other factors. Influencing factors, constructing a multi-index evaluation matrix of power quality; S2, normalization processing: obtain and analyze the distribution transformer information, obtain the data values of each influencing factor required by the multi-index evaluation matrix constructed in step S1, and perform normalization processing, so as to conduct a unified evaluation of power quality in the future; S3. Determination of weight: Determine the weight coefficient of each influencing factor based on expert experience and least squares optimization. S4. Comprehensively evaluate the power quality: multiply the normalized data values of each factor by the sum of the products of the corresponding weight coefficients, which is the evaluation result of the power quality. 2. The comprehensive evaluation method for power quality of a distribution network according to claim 1, wherein the power quality multi-index evaluation matrix constructed in step S1 is a multi-dimensional evaluation matrix, and its formula is as follows: R={U,B,UH,IH,F,C,O}, In the formula: R refers to the total influence result; U refers to the influence result of the voltage qualification rate; B refers to the influence result of the three-phase unbalance; UH refers to the influence result of the voltage harmonic; IH refers to the influence result of the current harmonic; The result of the influence of frequency; C refers to the result of the influence of power factor; O refers to the result of the influence of other factors. 3. The comprehensive evaluation method for power quality of a distribution network according to claim 1, wherein the normalization processing method described in step S2 is specifically: for the voltage qualification rate, three-phase unbalance and power factor, no treatment is required; For voltage harmonics and current harmonics, they are normalized using the arc tangent function, which is as follows: y = arctan x, The value range of the independent variable x of the arctangent function is (-∞,+∞), so there is no need to care about the domain of the function. The value range of the dependent variable y of the arctangent function is

When performing the normalization operation, it is also necessary to perform the operations of absolute value and multiplication by the correlation coefficient, as shown in the following formula:

For the influencing factor of frequency, considering that the frequency of my country's power system is 50Hz, therefore, taking 50Hz as the benchmark, the deviation of the actual frequency and the standard frequency is used as the standard for measuring the power quality, and it is normalized according to the above method. As shown in the following formula:

In the formula: f is the actual frequency of the power grid; f ₀ is 50Hz; For other factors, the value is normalized by the assessor based on experience. 4. The comprehensive evaluation method for power quality of a distribution network according to claim 1, wherein the method for determining the weight of each influencing factor in step S3 is: 1) According to expert experience, judge the importance of each index, and obtain the subjective index weight coefficient of the evaluation matrix, as shown below: E=[e ₁ , e ₂ , e ₃ , e ₄ , e ₅ , e ₆ , e ₇ ] ^T ; 2) The objective index weight coefficient determined by the coefficient of variation method is: V=[v ₁ , v ₂ , v ₃ , v ₄ , v ₅ , v ₆ , v ₇ ] ^T ; 3) In order to determine the optimal weight coefficient, a Lagrangian function is constructed, and the extreme value is solved according to the partial derivative:

4) Based on the least squares method, the weight coefficient after comprehensive optimization is: W=[w ₁ , w ₂ , w ₃ , w ₄ , w ₅ , w ₆ , w ₇ ]. 5. The method for comprehensively evaluating the power quality of a distribution network according to claim 1, wherein the method for comprehensively evaluating the power quality in step S4 is: Suppose the object to be evaluated is n, and the evaluation index in the present invention is 7, then based on the evaluation matrix and the weight coefficient, the evaluation value of the i-th evaluation object is obtained: