CN107069718A - The low-voltage distribution network line loss calculation method influenceed based on three-phase imbalance on line loss - Google Patents

Abstract

The invention discloses a low-voltage distribution network line loss calculation method based on the influence of three-phase unbalance on line loss. Calculate the equivalent resistance of the low-voltage distribution line by sub-meter electricity value; calculate the single-phase and three-phase unbalance degree according to the real-time collected data; calculate the line loss.

Description

Calculation method of low-voltage distribution network line loss based on the influence of three-phase unbalance on line loss

technical field

The invention relates to a method for calculating line loss of a low-voltage distribution network, in particular to a method for calculating line loss of a low-voltage distribution network based on the influence of three-phase unbalance on line loss.

Background technique

The line loss rate is an important comprehensive economic index for power supply enterprises. Due to the low voltage level of the distribution network, relatively speaking, the line loss rate caused by unit transmission power is high, and the accurate calculation of the theoretical line loss is directly related to the normal operation and scheduling of the distribution network. Due to the characteristics of many distribution network nodes, many branch lines, and many components, and most components do not have the conditions to measure operating parameters, the conditions for the distribution network line loss calculation based on the power flow algorithm are very harsh. A variety of simplified methods have been proposed in practical applications, such as artificial neural network theoretical calculation of line loss, but this method is not very mature in practical applications; it is suitable for the calculation method of distribution network line loss for automation devices, but this method is not suitable for automatic measurement The requirements are relatively strict, and the current situation of the domestic distribution network is that the degree of automation and accuracy of measurement is not very high; matching the concept of power flow, making full use of real-time measurement, has a certain contribution to state estimation and line loss calculation, but the load acquisition method Complicated, not easy to operate and the calculation process is complicated and the amount of calculation is large.

Three-phase unbalance is one of the main factors affecting the line loss of distribution network. The improved equivalent resistance method of three-phase unbalance is introduced to improve the accuracy of theoretical line loss and solve the accuracy problem of traditional algorithms.

Contents of the invention

The purpose of the present invention is to provide a low-voltage distribution network line loss calculation method based on the influence of three-phase imbalance on line loss, which simplifies line loss analysis and calculation work and improves the accuracy of theoretical calculation of low-voltage distribution network line loss.

In order to achieve the above object, the present invention is achieved through the following technical solutions:

A low-voltage distribution network line loss calculation method based on the influence of three-phase unbalance on line loss is characterized in that the method includes the following steps:

Calculate the average load current value at the head end of the line based on the data monitored at the load end;

Calculate the equivalent resistance of the low-voltage distribution line according to the monitored electricity value of the sub-meter;

Calculate single-phase and three-phase unbalance according to the data collected in real time;

Calculate line loss.

The head-end average load current value of the calculation data calculation line is:

In the formula, U represents the rated voltage of the power line, is the power factor, P _i represents the electrical degree of each sub-meter on a certain day, and P _pj,t is the total active power, which can be expressed by the total meter electrical degree.

The equivalent resistance R _eq of the described calculation low-voltage distribution line is:

in,

In the formula: Ι _{∑, jf} represent the total root mean square current of the distribution line at the head end of the station area;

Ι _{i, jf} is the current value of the distribution line on the i-th line in the station area;

P _{∑, jf} is the equivalent power converted from the electricity recorded by the typical daily total outlet watt-hour meter;

P _{i, jf} is the equivalent power converted from the electricity recorded by the i-th outlet watt-hour meter in a typical day;

U ₀ corresponds to the voltage at P _∑,jf and P _i,jf ;

_Req,i is the resistance value of a calculation line section, _Req,i =N _i R _i where N _i is the length of each calculation line, and R _i is the resistance value per unit length of the calculation line wire.

The calculation of single-phase and three-phase unbalance according to the data collected in real time includes:

Calculate the unbalance for each single phase:

i ∈ [A, Β, C] phase, t ∈ [0, T], T represents the time period, Ι _{i, t} represents the value of single-phase current at time t, Ι _{av, t} represents a certain phase current within t time average value;

And calculate the three-phase unbalance degree:

The expression of the three-phase unbalance degree function is:

The calculation model of the described calculation line loss is:

In the formula: where n is the calculation time period constant; Y _i is the instantaneous three-phase unbalance; N is the structure constant of the distribution line low-voltage outlet grid; K is the characteristic coefficient of the current line load curve; I _pj,t is the average load current ; _Req is the equivalent resistance; T is the calculation time.

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

1. It can accurately express the influence of each three-phase unbalance degree on the overall line loss rate, and can be directly used in line loss analysis work.

2. It is simple and effective, improves the accuracy of the theoretical calculation of the line loss of the low-voltage distribution network, and the calculation result is closer to the real value of the line loss, which has better engineering practicability.

Description of drawings

Fig. 1 is a flow chart of a low-voltage distribution network line loss calculation method based on the influence of three-phase unbalance on line loss in the present invention.

detailed description

The present invention will be further elaborated below by describing a preferred specific embodiment in detail in conjunction with the accompanying drawings.

As shown in Figure 1, a low-voltage distribution network line loss calculation method based on the influence of three-phase imbalance on line loss, the method includes the following steps:

Calculate the average load current value at the head end of the line based on the data monitored at the load end;

Calculate the equivalent resistance of the low-voltage distribution line according to the monitored electricity value of the sub-meter;

Calculate single-phase and three-phase unbalance according to the data collected in real time;

Calculate the line loss based on the above calculation results and the monitored parameters.

Introducing the concept of phase imbalance degree β

In the formula, Ι _cp is the average phase current of the three-phase load, Ι _cp = (Ι _A + Ι _Β + Ι _C )/3;

is the phase current value,

The line loss formula when the line is unbalanced is:

R is the resistance of A, B, C three-phase transmission line, Ι _N is the neutral point current.

The above calculated data calculates the head-end average load current value of the line as:

In the formula, U represents the rated voltage of the power line, is the power factor, P _i represents the electrical degree of each sub-meter on a certain day, and P _pj,t is the total active power, which can be expressed by the total meter electrical degree.

The equivalent resistance R _eq of the above-mentioned low-voltage distribution line is:

in,

In the formula: Ι _{∑, jf} represent the total root mean square current of the distribution line at the head end of the station area;

Ι _{i, jf} is the current value of the distribution line on the i-th line in the station area;

P _{∑, jf} is the equivalent power converted from the electricity recorded by the typical daily total outlet watt-hour meter;

P _{i, jf} is the equivalent power converted from the electricity recorded by the i-th outlet watt-hour meter in a typical day;

U ₀ corresponds to the voltage at P _∑,jf and P _i,jf ;

_Req,i is the resistance value of a calculation line section, _Req,i =N _i R _i where N _i is the length of each calculation line, and R _i is the resistance value per unit length of the calculation line wire.

The above calculation of single-phase and three-phase unbalance based on real-time collected data includes:

Calculate the unbalance for each single phase:

i ∈ [A, Β, C] phase, t ∈ [0, T], T represents the time period, Ι _{i, t} represents the value of single-phase current at time t, Ι _{av, t} represents a certain phase current within t time average value;

And calculate the three-phase unbalance degree:

The expression of the three-phase unbalance degree function is:

The above calculation model for calculating line loss is:

In the formula: where n is the calculation time period constant; Y _i Yi is the instantaneous three-phase unbalance; N is the low-voltage outlet grid structure constant of the distribution line, three-phase three-wire takes 3, three-phase four-wire takes 3.5, and two-wire takes 2; K is the characteristic coefficient of the current line load curve; I _{pj, t} is the average load current; R _eq is the equivalent resistance; T is the calculation time.

Based on the parameters and operating data of the power grid in a 0.38kV low-voltage distribution transformer station area in a community, the traditional equivalent resistance method and the improved equivalent resistance method are used to calculate the theoretical line loss.

On October 4, 2016, the total power of the station area was 483kWh, the power factor was 0.9, the line running time T was 24h, and the average phase voltage U was 0.38kV.

Step S1:

Calculate the head-end average load current value of the line based on the real-time monitored power parameters

Step two:

Calculate the equivalent resistance based on the monitored electricity value of the sub-meter

Table 1 sub-meter electricity value

Note: N is the electric box number, W is the electric quantity.

Step three:

Calculation of line unbalance (the data used in the formula is collected in real time by the load detector)

Table 2 Current values and three-phase unbalance parameter values

Using the β _A , β _Β and β _C parameter values in different time periods in Table 2, and then according to the three-phase unbalance degree function formula (4), the three-phase unbalance degree value Y _t in different time periods can be obtained. The data used for formula calculation is collected in real time by the load monitor, so the data is more accurate and can accurately reflect the line loss value.

Step 4: Calculate the line loss based on the above calculation results and the information in Table 1 and Table 2

The results are shown in Table 3:

Table 3 method comparison

According to the data of the terminal load of the distribution transformer area recorded by the metering automation system, the total metered power of the station area on October 4, 2016 was 483kWh, the sub-metered power was 421kWh, the line loss was 62kWh, and the line loss rate was 13%. Using the traditional equivalent resistance algorithm to calculate the line loss in this station area, the power loss is 33kWh, and the line loss rate is 7%. Using the algorithm in this paper to calculate the line loss in this station area, the power loss is 27kWh, and the line loss rate is 5.6%.

According to the above calculation results, it is shown that the functional relationship formula between the increase rate of line loss and the degree of three-phase unbalance deduced by the present invention can be directly applied to the calculation of line loss when the three-phase unbalance of the line is unbalanced, which simplifies the analysis and calculation of line loss. The traditional equivalent resistance method is improved, and the influence of three-phase unbalance on line loss is introduced into the calculation model. The improved calculation method is applied to the theoretical line loss calculation of the actual distribution network, which improves the accuracy of the theoretical calculation of the line loss of the low-voltage distribution network, and the calculation results are closer.

Although the content of the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description should not be considered as limiting the present invention. Various modifications and alterations to the present invention will become apparent to those skilled in the art upon reading the above disclosure. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims (5)

1. A low-voltage distribution network line loss calculation method based on the influence of three-phase unbalance on line loss, it is characterized in that, the method comprises the steps: Calculate the average load current value at the head end of the line based on the data monitored at the load end; Calculate the equivalent resistance of the low-voltage distribution line according to the monitored electricity value of the sub-meter; Calculate single-phase and three-phase unbalance according to the data collected in real time; Calculate line loss. 2. the low-voltage distribution network line loss calculation method based on the impact of three-phase unbalance on line loss as claimed in claim 1, wherein the head end average load current value of the calculation data calculation line is: <mrow> <msub> <mi>P</mi> <mrow> <mi>p</mi> <mi>j</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>=</mo> <munderover> <mo>&amp;Sigma;</mo> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <msub> <mi>P</mi> <mi>i</mi> </msub> </mrow> In the formula, U represents the rated voltage of the power line, is the power factor, P _i represents the electrical degree of each sub-meter on a certain day, and P _pj,t is the total active power, which can be expressed by the total meter electrical degree. 3. the low-voltage distribution network line loss calculation method based on the influence of three-phase unbalance on line loss as claimed in claim 1, is characterized in that, the equivalent resistance _Req of described calculation low-voltage distribution line is: <mrow> <msub> <mi>R</mi> <mrow> <mi>e</mi> <mi>q</mi> </mrow> </msub> <mo>=</mo> <mfrac> <mrow> <munderover> <mi>&amp;Sigma;</mi> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <msub> <mi>N</mi> <mi>i</mi> </msub> <msubsup> <mi>I</mi> <mrow> <mi>i</mi> <mo>,</mo> <mi>j</mi> <mi>f</mi> </mrow> <mn>2</mn> </msubsup> <msub> <mi>R</mi> <mrow> <mi>e</mi> <mi>q</mi> <mo>,</mo> <mi>i</mi> </mrow> </msub> <mi>T</mi> </mrow> <mrow> <mi>N</mi> <mrow> <mo>(</mo> <msubsup> <mi>I</mi> <mrow> <mi>&amp;Sigma;</mi> <mo>,</mo> <mi>j</mi> <mi>f</mi> </mrow> <mn>2</mn> </msubsup> <mi>T</mi> <mo>)</mo> </mrow> </mrow> </mfrac> <mo>=</mo> <mfrac> <mrow> <munderover> <mi>&amp;Sigma;</mi> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <msub> <mi>N</mi> <mi>i</mi> </msub> <msup> <mrow> <mo>(</mo> <mfrac> <msub> <mi>P</mi> <mrow> <mi>i</mi> <mo>,</mo> <mi>j</mi> <mi>f</mi> </mrow> </msub> <msub> <mi>U</mi> <mn>0</mn> </msub> </mfrac> <mo>)</mo> </mrow> <mn>2</mn> </msup> <msub> <mi>R</mi> <mrow> <mi>e</mi> <mi>q</mi> <mo>,</mo> <mi>i</mi> </mrow> </msub> </mrow> <mrow> <mi>N</mi> <msup> <mrow> <mo>(</mo> <mfrac> <msub> <mi>P</mi> <mrow> <mi>&amp;Sigma;</mi> <mo>,</mo> <mi>j</mi> <mi>f</mi> </mrow> </msub> <msub> <mi>U</mi> <mn>0</mn> </msub> </mfrac> <mo>)</mo> </mrow> <mn>2</mn> </msup> </mrow> </mfrac> <mo>=</mo> <mfrac> <mrow> <munderover> <mi>&amp;Sigma;</mi> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <msub> <mi>N</mi> <mi>i</mi> </msub> <msubsup> <mi>p</mi> <mrow> <mi>i</mi> <mo>,</mo> <mi>j</mi> <mi>f</mi> </mrow> <mn>2</mn> </msubsup> <msub> <mi>R</mi> <mrow> <mi>e</mi> <mi>q</mi> <mo>,</mo> <mi>i</mi> </mrow> </msub> </mrow> <mrow> <msubsup> <mi>NP</mi> <mrow> <mi>&amp;Sigma;</mi> <mi>j</mi> <mi>f</mi> </mrow> <mn>2</mn> </msubsup> </mrow> </mfrac> </mrow> in, In the formula: Ι _{∑, jf} represent the total root mean square current of the distribution line at the head end of the station area; Ι _{i, jf} is the current value of the distribution line on the i-th line in the station area; P _{∑, jf} is the equivalent power converted from the electricity recorded by the typical daily total outlet watt-hour meter; P _{i, jf} is the equivalent power converted from the electricity recorded by the i-th outlet watt-hour meter in a typical day; U ₀ corresponds to the voltage at P∑ _,jf and P _i,jf ; _Req,i is the resistance value of a calculation line section, _Req,i =N _i R _i where N _i is the length of each calculation line, and R _i is the resistance value per unit length of the calculation line wire. 4. the low-voltage distribution network line loss calculation method based on the influence of three-phase unbalance on line loss as claimed in claim 1, wherein the calculation of single-phase and three-phase unbalance according to the data collected in real time comprises: Calculate the unbalance for each single phase: <mrow> <msub> <mi>&amp;beta;</mi> <mrow> <mi>i</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>=</mo> <mfrac> <mrow> <msub> <mi>I</mi> <mrow> <mi>i</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>-</mo> <msub> <mi>I</mi> <mrow> <mi>a</mi> <mi>v</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> </mrow> <msub> <mi>I</mi> <mrow> <mi>a</mi> <mi>v</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> </mfrac> </mrow> i ∈ [A, Β, C] phase, t ∈ [0, T], T represents the time period, Ι _{i, t} represents the value of single-phase current at time t, Ι _{av, t} represents a certain phase current within t time average value; And calculate the three-phase unbalance degree: The expression of the three-phase unbalance degree function is: <mrow> <msub> <mi>Y</mi> <mi>t</mi> </msub> <mo>=</mo> <mfrac> <mn>1</mn> <mn>6</mn> </mfrac> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <mrow> <mn>3</mn> <msup> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>A</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mn>2</mn> </msup> <mo>+</mo> <mn>3</mn> <msup> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>B</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mn>2</mn> </msup> <mo>+</mo> <mn>3</mn> <msup> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>C</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mn>2</mn> </msup> <mo>+</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>A</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>B</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mo>-</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mrow> <mfrac> <msqrt> <mn>3</mn> </msqrt> <mn>2</mn> </mfrac> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>A</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>C</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mo>-</mo> <mfrac> <mrow> <mn>3</mn> <msqrt> <mn>3</mn> </msqrt> </mrow> <mn>4</mn> </mfrac> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>B</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <msub> <mi>&amp;beta;</mi> <mrow> <mi>C</mi> <mo>,</mo> <mi>t</mi> </mrow> </msub> <mo>)</mo> </mrow> </mrow> </mtd> </mtr> </mtable> </mfenced> <mo>.</mo> </mrow> 5. the low-voltage distribution network line loss calculation method based on the influence of three-phase unbalance on line loss as claimed in claim 1, is characterized in that, the calculation model of described calculation line loss is: <mrow> <mi>&amp;Delta;</mi> <mi>W</mi> <mo>=</mo> <mfrac> <mn>1</mn> <mi>n</mi> </mfrac> <msubsup> <mi>&amp;Sigma;</mi> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </msubsup> <msub> <mi>Y</mi> <mi>i</mi> </msub> <msubsup> <mi>NI</mi> <mrow> <mi>p</mi> <mi>j</mi> <mo>,</mo> <mi>t</mi> </mrow> <mn>2</mn> </msubsup> <msub> <mi>R</mi> <mrow> <mi>e</mi> <mi>q</mi> </mrow> </msub> <mi>T</mi> </mrow> In the formula: where n is the calculation time period constant; Y _i is the instantaneous three-phase unbalance; N is the structure constant of the distribution line low-voltage outlet grid; K is the characteristic coefficient of the current line load curve; I _pj,t is the average load current ; _Req is the equivalent resistance; T is the calculation time.