CN103715686A - Energy efficiency analysis method suitable for direct-current power distribution network circuits - Google Patents

Abstract

The invention relates to an analysis method in the field of power transmission and distribution systems in electrical engineering, in particular to an energy efficiency analysis method suitable for direct current distribution network lines. This method is suitable for medium-voltage distribution network, and is dominated by line analysis; in the line part, the cable line is the main research object, and the energy efficiency indicators of AC lines, unipolar DC lines, and bipolar DC lines are analyzed, and then carried out. Compare and judge which method is more energy efficient. For the three different lines of AC lines, unipolar DC lines, and bipolar DC lines, a specific calculation model for four indicators of active power loss, power supply efficiency, power loss, and line loss rate is given. This method can be widely used in different The calculation and comparison of the distribution network of the voltage level, the results and conclusions obtained by different voltage levels will be different. Through this method, the energy efficiency analysis and comparison conclusions of different AC and DC schemes of the distribution network can be obtained.

Description

An Energy Efficiency Analysis Method Applicable to DC Distribution Network Lines

technical field

The invention relates to an analysis method in the field of power transmission and distribution systems in electrical engineering, in particular to an energy efficiency analysis method suitable for direct current distribution network lines. the

Background technique

The power grid is divided into two forms: the transmission network and the distribution network. These two types of power grids are classified based on different voltage levels and roles in the power system. The transmission network is a power network that connects power plants and substations, and substations and substations through high-voltage and ultra-high-voltage transmission lines to complete power transmission. It is also called the main grid in the power grid. The distribution network is a power network that receives electric energy from the transmission network or regional power plants and distributes it to users locally or step by step through power distribution facilities. At present, the high-voltage DC transmission technology is relatively mature, and compared with the AC transmission method, it is very economical and has relatively high efficiency. It has been widely used in power transmission projects: the Three Gorges-Shanghai ±500 kV DC transmission project has a total length of 1048.6 kilometers. The transmission capacity is 3 million kilowatts. If the medium-strength all-aluminum alloy wire replaces the ordinary wire, under normal power, if the annual transmission hours are 4,000 hours, it can save 79,800 kWh/km of electricity, and the whole line can save electricity every year. 83.72 million kwh. the

In the future, the power load density in cities will gradually increase, and the power supply in high-load density plots such as large commercial centers, data financial centers, and high-end intelligent communities will have higher and higher requirements for power quality and reliability. At present, the power supply radius of my country's 10kV medium-voltage distribution network is small, and the power supply capacity is obviously insufficient. Switching to 20kV power supply involves huge investment in equipment transformation, resulting in poor adaptability of the current 10kV medium-voltage distribution network and difficulties in power supply. From the current point of view, the 10kV has not been upgraded to 20kV at the right time, and the problems of high network loss and insufficient power supply capacity of the 10kV medium-voltage distribution network are now difficult to solve. It can be predicted that in terms of energy saving and loss reduction, the DC distribution network will be a revolution in the future power grid. At present, the construction of DC distribution network has not yet started. Considering issues such as reducing power loss, improving power supply capacity, and improving power supply reliability and economy, it is of great significance to study DC power supply methods for medium and low voltage distribution networks. the

According to the theoretical principles of power output and the realization of DC transmission technology, we can predict that DC power distribution in the distribution network is also feasible. However, due to the limited speed of power grid development planning and transformation, the current distribution network in the traditional sense has not yet realized DC power distribution technology. At present, low-voltage DC power distribution technology has only been realized in small-scale power distribution systems such as aircraft and ships, and DC power distribution technology has not yet been put into high- and medium-voltage engineering examples. Therefore, it is necessary to provide one or several analytical methods to evaluate its energy efficiency before proposing the specific technical implementation of the DC distribution network, which can provide a feasibility basis for the construction of the DC distribution network and accelerate the pace of DC distribution network construction. the

Contents of the invention

Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide an energy efficiency analysis method suitable for DC distribution network lines, which is suitable for medium voltage distribution networks and is dominated by line analysis; The main research object is to analyze the energy efficiency indicators of AC lines, unipolar DC lines, and bipolar DC lines, and then compare them to determine which method has higher energy efficiency. Three different lines of polar DC lines are given, and the specific calculation models of four indicators of active power loss, power supply efficiency, power loss, and line loss rate are given. This method can be widely used in the calculation and comparison of distribution networks of different voltage levels. The results and conclusions obtained by different voltage levels will be different. Through this method, the energy efficiency analysis and comparison conclusions of different AC and DC schemes of distribution networks can be obtained. the

The purpose of the present invention is to adopt the following technical solutions to achieve:

The present invention provides an energy efficiency analysis method suitable for direct current distribution network lines. The improvement is that the method is suitable for medium voltage distribution networks, and the method includes the following steps:

A. Determine the energy efficiency analysis model with the line as the main analysis element: determine the energy efficiency analysis index of the line part with the cable line as the main body and the power equipment and load as the auxiliary;

B. Analyze and compare the energy efficiency analysis indicators of the cable lines, obtain the calculation results of the cable line mode, and select the cable line operation mode with the highest energy efficiency. the

Further, the medium-voltage distribution network includes a medium-voltage radiating DC distribution network, and the medium-voltage radiating DC distribution network includes two distribution network access lines; one of the distribution network access lines includes sequentially Connected AC power supply (1), AC transformer (2), AC/DC converter (3), DC/DC chopper (9) and (10); in DC/DC chopper (10) The output ends of the two outputs are respectively connected with two outputs, one of which includes a sequentially connected DC chopper (5) and a DC load; the other output includes a sequentially connected DC/AC inverter (8) and an AC load; the The output terminal of the AC/DC converter (3) is connected to the distribution network access terminal I;

Another distribution network access line includes two parallel inputs, one of which includes wind power/hydropower distributed energy, AC transformers (7) and AC/DC converters (4) connected in sequence; the other input includes sequentially connected The connected photovoltaic power generation/fuel cell distributed power supply and the DC/DC chopper (6); the output end of the DC/DC chopper (6) is connected to the distribution network access terminal II. the

Further, in the step A, the cable lines include three different lines: AC lines, unipolar DC lines and bipolar DC lines; the energy efficiency analysis indicators based on cable lines include active power loss, power transmission efficiency, Power loss and line loss rate. the

Further, the active power loss refers to the power loss generated by the resistance of the power line when the load current passes through the power line;

The active power loss divided by the cable line includes the active power loss of the AC line, the active power loss of the unipolar DC line and the active power loss of the bipolar DC line;

The line current expression for an AC line is:

Then the expression of the active power loss of the AC line is:

In the formula:

P—three-phase active power/single-phase active power consumed by the load;

U _N — rated line voltage/phase voltage of power line;

—负荷功率因数； 

- load power factor;

r—power line single-phase unit resistance;

L—power line distance;

Unipolar DC lines include earth/sea return line operation mode and metal return line operation mode;

The line current expression for a unipolar DC link is:

Then the expression of the active power loss of the unipolar DC line is:

In the formula: when the ground/sea water loop is running, R is the sum of the unit resistance of the power line and the ground unit resistance; when the metal loop is running, R is the sum of the unit resistance of the two wires, and the calculation assumes that the two wires are of the same type , then R=2r;

The bipolar DC system includes the bipolar two-end grounding neutral point grounding method, the bipolar one-end neutral point grounding method and the bipolar metal return line method;

The expression of the active power loss of the bipolar DC line is:

In the formula: r is the unit resistance in a single pole line. the

Further, the power transmission efficiency refers to the ratio of the output active power at the end of the power line to the input active power at the beginning of the power line, expressed in percentage;

The expression of transmission efficiency is:

In the formula: P ₁ - input active power at the beginning of the power line; P ₂ - output active power at the end of the power line.

Further, the method of directly obtaining the maximum load loss time τ _max by the maximum load utilization hours T _max is used, and then multiplied by the maximum load power loss ΔP _max of the power line as the final value of the power loss. The expression of the power loss is:

ΔW _z =ΔP _max ×τ _max <7>.

Further, the line loss rate refers to the ratio of the electric energy lost on the power line to the electric energy input at the beginning of the power line; when the ground conductance or corona loss is not considered, it refers to the loss of electric energy ΔW _z in the resistance of the power line and the power line The ratio of input electric energy W ₁ at the beginning; the line loss rate is expressed as a percentage, and the expression is:

In the formula: W ₂ is the electric energy output from the end of the power line.

Further, in the step A, the auxiliary energy efficiency index of electric equipment and load refers to the energy efficiency of transmission efficiency, and the expression of the transmission efficiency is:

In the formula: P ₁ —active power input at the beginning; P ₂ —active power output at the end.

Further, in the step B, when analyzing and comparing the energy efficiency analysis index of the cable line, select the YJV22 type cable and the single-phase cross-sectional area of 240mm ² cables, and set the following boundary conditions:

1) The unit resistance of the YJV22 cable with a single-phase cross-sectional area of ^240mm2 is 0.0985Ω/km when inputting AC power, and the unit resistance when inputting DC power is 0.0754Ω/km;

2) The line length is set to 3km;

3) The active power consumed by the load supplied by the line is 3MW, and the power factor is set to 0.9;

4) The voltage level of the line is set to five grades: 10kV, 15kV, 20kV, 25kV and 30kV;

5) When calculating the annual power consumption, the maximum load utilization hours T _max is selected as 3000h.

Furthermore, the calculation results of the cable line method include: the power efficiency of the bipolar DC line for cable transmission of electric energy is always optimal in the voltage range from 10kV to 30kV, and it has an advantage when compared with the AC line at the voltage level of 10kV Obviously; as the voltage level increases, the efficiency advantage of bipolar DC lines over AC lines decreases;

In the unipolar DC system that uses cables to transmit electric energy, the power efficiency of the earth/sea water return line is higher than that of the metal return line, and the loss is small; the power efficiency of the metal return line has no advantage over the AC line;

Among the cable lines of the DC system, the bipolar DC line has the highest power efficiency and the lowest loss, and the bipolar DC system is selected as the optimal cable line mode in terms of energy efficiency. the

Compared with prior art, the beneficial effect that the present invention reaches is:

1. The present invention selects the cable lines used in engineering as the carrier of distribution network line analysis, and the analysis process is clearer and simpler;

2. The line analysis method selected in the present invention is produced by referring to two theories of high voltage direct current transmission technology and medium and low voltage AC distribution network operation mode, and has a strong theoretical basis in the derivation process;

3. The AC and DC comparisons adopted in the present invention select a voltage level of 10kV for comparison, which is close to the actual engineering;

4. In addition to selecting the 10kV voltage level, the present invention also considers the energy efficiency of the AC and DC lines under the other 4 groups of voltage levels, and simulates the energy efficiency results and changing trends of the AC and DC lines under different voltage levels;

5. Four kinds of energy efficiency analysis indicators adopted in the present invention follow the basic principles of power system analysis, and analysis with them can be used as comprehensive evaluation results;

6. The present invention provides the implementation examples of the most extensive 240mm2 cables in engineering applications, and uses four energy efficiency indicators of active power loss, power supply efficiency, power loss, and line loss rate to analyze the AC and DC lines of the distribution network. The results tend to be consistent, and valid conclusions can be drawn. the

Description of drawings

Fig. 1 is the basic frame diagram of DC distribution network provided by the present invention;

Fig. 2 is the hierarchical structure diagram of distribution network energy efficiency analysis provided by the present invention;

Fig. 3 is the YJV22 cable power loss curve (power factor 0.9) of single-phase cross-sectional area 240mm provided by the present invention;

Fig. 4 is the YJV22 cable power transmission efficiency graph (power factor 0.9) of single-phase cross-sectional area 240mm provided by the present invention;

Fig. 5 is the YJV22 cable electric energy loss curve (power factor 0.9) of single-phase sectional area 240mm2 provided by the present invention;

Fig. 6 is the line loss rate curve of YJV22 cable with a single-phase cross-sectional area of 240mm2 provided by the present invention (power factor 0.9);

Fig. 7 is a flowchart of an energy efficiency analysis method applicable to DC distribution network lines provided by the present invention. the

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings. the

Grid energy efficiency refers to the efficiency of the entire system of electric energy from production to transportation to users, not just the efficiency of transmission. The energy efficiency analysis method proposed by the present invention is suitable for medium-voltage distribution networks. The energy efficiency analysis indicators on the line of the method refer to four energy efficiency indicators: active power loss, transmission efficiency, electric energy loss, and line loss rate. the

As shown in Figure 1, the present invention provides a basic frame structure of a medium-voltage radial DC distribution network, in which equipment 1 is an AC power supply, equipment 2 and equipment 7 are transformers, equipment 3 and equipment 4 are AC/DC converters Converter (to convert alternating current into direct current energy), equipment 5, equipment 6, equipment 9, and equipment 10 are all DC/DC DC choppers (DC transformers), and equipment 8 is a DC/AC inverter. The medium-voltage radial DC distribution network includes two distribution network access lines; one of the distribution network access lines includes AC power supply 1, AC transformer 2, AC/DC converter 3, and DC/DC direct current Choppers 9 and 10; two outputs are respectively connected to the output ends of the DC/DC chopper 10, wherein one output includes the DC chopper 5 and the DC load connected in sequence; the other output includes the DC chopper connected in sequence /AC inverter 8 and AC load; the output end of the AC/DC converter 3 is connected to the distribution network access terminal I; the other distribution network access line includes two parallel inputs, one of which includes Wind power/hydropower distributed energy, AC transformer 7 and AC/DC converter 4 connected in sequence; another input includes photovoltaic power generation/fuel cell distributed power and DC/DC chopper 6 connected in sequence; the DC The output terminal of the /DC chopper 6 is connected to the distribution network access terminal II. the

As shown in Figure 1, alternating current 1 is converted into low-voltage direct current at the access end I of the distribution network through transformer 2 and AC/DC converter 3; due to the different generation mechanisms of distributed energy such as wind power and fuel cell energy Different forms of electrical energy are generated. Wind farms and small hydropower will directly generate alternating current, which needs to be converted into direct current after passing through the AC transformer 7 and AC/DC converter 4, and then input to the distribution network access terminal II; photovoltaic power generation and fuel cells directly generate direct current, and pass through DC/DC The DC chopper (DC transformer) transmits the DC power to the distribution network access point II. The DC load side needs to install a DC/DC DC chopper (DC transformer) 5 on the line, this part is to directly transmit DC power to the DC load; the AC load line needs to install an inverter 8 on the line, and the DC power passes through the inverter , converted into AC power for AC loads. the

As shown in Fig. 2, the present invention proposes a distribution network energy efficiency analysis method (hierarchical model) for line energy efficiency for comprehensive analysis. the

After the above hierarchical structure is established, the distribution network lines can be evaluated according to methods such as Analytical Hierarchy Process (AHP) and Analytic Network Process (ANP), and the weights of all levels can be calculated, and finally the relative value of each index to the energy efficiency of the distribution network can be obtained. importance. the

According to the present invention, four indexes of active power loss, power supply efficiency, electric energy loss and line loss rate are respectively analyzed and compared for three different lines of AC line, unipolar DC line and bipolar DC line. The energy efficiency analysis model proposed by the present invention can assist in the analysis of the energy efficiency comparison results of the distribution network using AC and DC methods. Through the analysis process, the expected energy efficiency results after the distribution network uses DC power distribution technology can be obtained, which is the DC distribution network Provides a numerical result. the

The flow chart of the energy efficiency analysis method applicable to DC distribution network lines provided by the present invention is shown in Figure 7, including the following steps:

A. Determine the energy efficiency analysis model with the line as the main analysis element: determine the energy efficiency analysis index of the line part with the cable line as the main body and the power equipment and load as the auxiliary;

1. Line energy efficiency analysis:

1. Active power loss:

When the load current passes through the power line, power loss occurs on the resistance of the power line, that is, active power loss. the

This part of the active power loss mainly produces thermal effects. According to the thermal effect power formula ΔP=I ² R, and the size of the resistance R is proportional to the resistivity r and the line length L, it can be estimated that this part of the active power loss ΔP multiplied by the current I Square, resistivity r, line length L is a linear relationship.

(1) AC line active power loss

的比值，即线路电流

In the analysis theory of AC power system, the current I can be understood as active power P, line rated voltage U _N and load power factor

The ratio of the line current

Accordingly, the present invention combines the power thermal effect formula and the power system analysis formula to make a summary:

In the formula:

P—three-phase active power/single-phase active power consumed by the load;

U _N — rated line voltage/phase voltage of power line;

—负荷功率因数； 

- load power factor;

r—power line single-phase unit resistance;

L—power line distance;

This formula is suitable for all AC line engineering calculations. the

(2) Active power loss of unipolar DC line

Unipolar DC lines (currently this DC transmission method has been adopted in high-voltage DC transmission, and this invention proposes it as a method of DC power distribution) roughly has the earth (seawater) return line operation mode and the metal return line operation mode There are two ways. the

The operation mode of the earth (seawater) return line: use a wire and the earth (or seawater) to form a unipolar circuit on the DC side, and the converter stations at both ends need to be grounded, and the power transmission line can be simulated as a simple DC circuit. the

The current I in a DC circuit can be simply expressed as a simple ratio of power P to rated voltage U _N ,

Right now $I=\frac{P}{u_N}---<3>$

According to the thermal effect theory of DC circuit, the loss generated on the resistance of the power line:

$\mathrm{<span\; class="notranslate">\; \&Delta;P</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="u\; N"\; filenames="HDA0000454663420000011.png"\; state="\{\{state\}\}">u</figure-callout></span><figure-callout\; id="2"\; label="u\; N"\; filenames="HDA0000454663420000011.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; RL</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; <</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; ></span>$

Among them: when the earth (sea water) circuit is running, R is the sum of the unit resistance of the power line and the earth unit resistance. Since the earth resistance is very small, it can usually be ignored in the calculation; when the metal circuit is running, R is the unit resistance of the two wires In the calculation, it can be assumed that the two wires are of the same type, then R=2r. the

(3) Active power loss of bipolar DC line

The bipolar DC system can be divided into three operation modes: bipolar two-end grounding neutral point grounding mode, bipolar one-end neutral point grounding mode, and bipolar metal return line mode. When these three methods operate normally, the DC current path is the positive and negative pole lines, the current direction in the two pole lines is opposite, and the current in the return line is the current difference between the two pole lines, so it can be ignored in the calculation. Then the loss on the bipolar DC line resistance:

$\mathrm{<span\; class="notranslate">\; \&Delta;P</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="P"\; filenames="HDA0000454663420000011.png"\; state="\{\{state\}\}">P</figure-callout></span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; <</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; ></span>$

In the formula: r is the unit resistance in a single pole line. the

The above is the mathematical model (calculation formula) used in the calculation of active power loss proposed by the present invention. After calculation and verification, the formula is suitable for the analysis process of high-voltage transmission lines. In view of the difference between the transmission line and the distribution line only in the voltage level, the present invention summarizes and proposes these three formulas as the theoretical basis for calculating the power loss of the distribution line. the

2. Transmission efficiency:

Transmission efficiency refers to the ratio of the output active power at the end of the power line to the input active power at the beginning of the power line, usually expressed in percent. Right now:

In the formula: P ₁ - input active power at the beginning of the power line; P ₂ - output active power at the end of the power line.

3. Power loss:

The operating status of the power line changes with time, and the power loss on the line also changes with time. When analyzing the energy efficiency of a line or system, it is also necessary to calculate the power loss within a certain period of time (within a year). In engineering calculations, empirical formulas are usually used to calculate power loss. the

The present invention adopts the method of directly checking the maximum load loss time τ _max by the maximum load utilization hours T _max , and multiplies it by the maximum load power loss ΔP _max of the line as the final value of the electric energy loss.

ΔW _z =ΔP _max ×τ _max <7>.

4. Line loss rate:

The line loss rate refers to the ratio of the electric energy lost on the line to the electric energy input at the beginning of the line. When the conductance to ground or corona loss is not considered, it refers to the ratio of the electric energy ΔW _z lost in the line resistance to the input electric energy W ₁ at the beginning of the line. The line loss rate is also often expressed as a percentage.

In the formula: W ₂ is the electric energy output from the end of the power line.

2. Analysis of power equipment and load energy efficiency

Power equipment mainly refers to AC transformers, AC/DC converters, DC/DC choppers, UPS equipment, inverters and other equipment in the line transmission process, while load refers to AC loads, DC loads and some sensitive equipment. Various loads including loads. the

In the energy efficiency calculation process of power equipment and loads, energy efficiency mainly refers to the transmission efficiency of electric energy,

In the formula: P ₁ —active power input at the beginning; P ₂ —active power output at the end.

B. Analyze and compare the energy efficiency analysis indicators of the cable lines, obtain the calculation results of the cable line mode, and select the cable line mode with the highest energy efficiency. the

The implementation example in the present invention selects the medium-voltage distribution network of cable line transmission as an example, and calculates its active power loss, transmission efficiency, electric energy loss, and line loss rate in the line part AC, unipolar DC, and bipolar DC lines. According to a specific index, the energy efficiency comparison is made, and the analysis and comparison results of AC lines, unipolar DC lines, and bipolar DC lines are given. the

Example

The present invention selects the YJV22 model cable as the research object in the medium-voltage circuit, and studies the cable circuit. the

The example of the present invention selects the YJV22 model cable as a special case, selects the single-phase cross-sectional area 240mm ² cables, and carries out energy efficiency analysis to it, four energy efficiency indicators of active power loss, transmission efficiency, electric energy loss and line loss rate of these two types of cables Perform the calculation and compare the case of input AC current and input DC current.

1. Boundary conditions set

1) After calculation and looking up information, the unit resistance of the YJV22 cable with a single-phase cross-sectional area of ^240mm2 is 0.0985Ω/km when inputting AC, and the unit resistance when inputting DC is 0.0754Ω/km;

2) The line length is set to 3km;

3) The active power consumed by the load supplied by the line is 3MW, and the power factor is set to 0.9

4) The voltage level of the line is set to five grades: 10kV, 15kV, 20kV, 25kV, 30kV;

5) When calculating the annual power loss, the maximum load utilization hours are selected as 3000h. the

2. Calculation result:

(1) Power loss:

Table 1 Statistics of power loss data of YJV22 cable with single-phase cross-sectional area of ^240mm2 (unit: kW)

The results are plotted as a line graph, as shown in Figure 3. the

(2) Transmission efficiency:

Table 2 Statistics of YJV22 cable transmission efficiency with single-phase cross-sectional area of 240mm ² (unit: %)

The results are plotted as a line graph, as shown in Figure 4. the

(3) Power loss (annual):

Table 3 Statistics of power loss data of YJV22 cable with single-phase cross-sectional area of 240mm ² (unit: MWh)

The results are plotted as a line graph, as shown in Figure 5. the

(4) Line loss rate (annual):

Table 4 Statistics of line loss rate of YJV22 cable with single-phase cross-sectional area of 240mm ² (unit: %)

The results are plotted as a line graph, as shown in Figure 6. the

3. Calculation result analysis:

Figures 3 to 6 are the curve drawings of the maximum load power loss, transmission efficiency, power loss and line loss rate of the type YJV22 cable with a cross-sectional area of ^240mm2 . Through the curve drawings, the AC line and unipolar DC can be compared and analyzed. Power Efficiency Trends for Lines and Bipolar DC Lines. Through the analysis of these graphs, it can be concluded that the power efficiency of the bipolar DC line that uses cables to transmit electric energy is always optimal in the range of voltage levels from 10kV to 30kV, and it is compared with the AC line at the voltage level of 10kV The time advantage is very obvious. As the voltage level increases, the efficiency advantage of bipolar DC lines over AC lines gradually decreases. In the unipolar DC system that uses cables to transmit electric energy, the power efficiency of the earth return line is higher than that of the metal return line, and the loss is smaller; while the power efficiency of the metal return line has no advantage over the AC line. In some cases On the contrary, the loss is greater. Therefore, in the DC system cable lines, the bipolar DC line has the highest power efficiency and the lowest loss. If only energy efficiency is considered, the bipolar DC system is the best choice.

3. Summary and analysis:

Through the line energy efficiency analysis model proposed by the present invention, we have carried out energy efficiency analysis and calculation with the cable lines of the medium voltage distribution network. the

In terms of lines, medium-voltage lines are adopted, and the situation of cable power supply is analyzed. Under the condition of a specific power factor of 0.9, calculations are made, and it is concluded that in terms of lines, changing the distribution network to DC lines will help improve the energy efficiency of all aspects of the line, and from the perspective of energy efficiency On the one hand, the bipolar DC circuit in the DC circuit is better than the unipolar DC method. Through the analysis, it can be concluded that switching to a DC power distribution system can improve the working efficiency of the equipment in most cases, improve the working mode of the equipment, reduce redundant power equipment components, and improve energy efficiency. the

The present invention proposes an energy efficiency analysis method and calculation model suitable for distribution network lines, which can be widely used in the calculation and comparison of distribution networks of different voltage levels. The results and conclusions obtained by different voltage levels will be different. Through this method, it can be obtained Energy efficiency analysis and comparison conclusions of different AC and DC schemes for distribution network. the

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the present invention can still be Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention. the

Claims (10)

1. An energy efficiency analysis method applicable to DC distribution network lines, characterized in that, the method is applicable to medium-voltage distribution networks, and the method may further comprise the steps: A. Determine the energy efficiency analysis model with the line as the main analysis element: determine the energy efficiency analysis index of the line part with the cable line as the main body and the power equipment and load as the auxiliary; B. Analyze and compare the energy efficiency analysis indicators of the cable lines, obtain the calculation results of the cable line mode, and select the cable line operation mode with the highest energy efficiency. 2. The energy efficiency analysis method according to claim 1, wherein the medium-voltage distribution network comprises a medium-voltage radiating DC distribution network, and the medium-voltage radiating DC distribution network comprises a two-way distribution network Access line; one of the distribution network access lines includes AC power supply (1), AC transformer (2), AC/DC converter (3), DC/DC chopper (9) and ( 10); two outputs are respectively connected to the output ends of the DC/DC chopper (10), one of which includes a sequentially connected DC chopper (5) and a DC load; the other output includes sequentially connected DC /AC inverter (8) and AC load; the output terminal of the AC/DC converter (3) is connected to the distribution network access terminal I; Another distribution network access line includes two parallel inputs, one of which includes wind power/hydropower distributed energy, AC transformers (7) and AC/DC converters (4) connected in sequence; the other input includes sequentially connected The connected photovoltaic power generation/fuel cell distributed power supply and the DC/DC chopper (6); the output end of the DC/DC chopper (6) is connected to the distribution network access terminal II. 3. The energy efficiency analysis method according to claim 1, wherein, in the step A, the cable line includes three different lines of an AC line, a unipolar DC line and a bipolar DC line; The main energy efficiency analysis indicators include active power loss, transmission efficiency, power loss and line loss rate. 4. energy efficiency analysis method as claimed in claim 3, is characterized in that, described active power loss refers to when load current passes through power line, the power loss that the resistance of power line produces; The active power loss divided by the cable line mode includes the active power loss of the AC line, the active power loss of the unipolar DC line and the active power loss of the bipolar DC line; The line current expression for an AC line is:

Then the expression of the active power loss of the AC line is:

In the formula: P—three-phase active power/single-phase active power consumed by the load; U _N — rated line voltage/phase voltage of power line; - load power factor; r—power line single-phase unit resistance; L—power line distance; Unipolar DC lines include earth/sea return line operation mode and metal return line operation mode; The line current expression for a unipolar DC link is:

Then the expression of the active power loss of the unipolar DC line is:

In the formula: when the ground/sea water loop is running, R is the sum of the unit resistance of the power line and the ground unit resistance; when the metal loop is running, R is the sum of the unit resistance of the two wires, and the calculation assumes that the two wires are of the same type , then R=2r; The bipolar DC system includes two-pole grounding neutral point grounding mode, bipolar one-end neutral point grounding mode and bipolar metal return line mode; The expression of the active power loss of the bipolar DC line is:

In the formula: r is the unit resistance in a single pole line. 5. The energy efficiency analysis method according to claim 3, wherein the transmission efficiency refers to the ratio of the output active power at the end of the power line to the input active power at the beginning of the power line, expressed in percentage; The expression of transmission efficiency is:

In the formula: P ₁ - input active power at the beginning of the power line; P ₂ - output active power at the end of the power line. 6. The energy efficiency analysis method according to claim 3, characterized in that, the maximum load utilization hours T _max is used to directly check the maximum load loss time τ _max , and then multiplied by the power line maximum load power loss ΔP _max as electric energy The final value of the loss, the expression of the power loss is: ΔW _z =ΔP _max ×τ _max <7>. 7. energy efficiency analysis method as claimed in claim 3, is characterized in that, described line loss rate refers to the ratio of the electric energy of loss on the power line and the electric energy that power line beginning end inputs; Refers to the ratio of the power loss ΔW _z in the resistance of the power line to the input power W ₁ at the beginning of the power line; the line loss rate is expressed as a percentage, and the expression is:

In the formula: W ₂ is the electric energy output from the end of the power line. 8. The energy efficiency analysis method according to claim 1, wherein in said step A, the auxiliary energy efficiency index of electric equipment and load refers to power transmission efficiency energy efficiency, and the expression of said power transmission efficiency is:

In the formula: P ₁ —active power input at the beginning; P ₂ —active power output at the end. 9. energy efficiency analysis method as claimed in claim 1, it is characterized in that, in described step B, select YJV22 model cable and single-phase cross-sectional area ^240mm cable line when analyzing and comparing the cable line energy efficiency analysis index, and set The following boundary conditions: 1) The unit resistance of the YJV22 cable with a single-phase cross-sectional area of ^240mm2 is 0.0985Ω/km when inputting AC power, and the unit resistance when inputting DC power is 0.0754Ω/km; 2) The line length is set to 3km; 3) The active power consumed by the load supplied by the line is 3MW, and the power factor is set to 0.9; 4) The voltage level of the line is set to five levels: 10kV, 15kV, 20kV, 25kV and 30kV; 5) When calculating the annual power consumption, the maximum load utilization hours T _max is selected as 3000h. 10. The energy efficiency analysis method according to claim 1, wherein the calculation result of the cable line mode includes: the power efficiency of the bipolar DC line for the cable transmission of electric energy is always optimal within the range of the voltage level from 10kV to 30kV , at a voltage level of 10kV, it has obvious advantages compared with AC lines; as the voltage level increases, the efficiency advantage of bipolar DC lines over AC lines decreases; In the unipolar DC system that uses cables to transmit electric energy, the power efficiency of the earth/sea water return line is higher than that of the metal return line, and the loss is small; the power efficiency of the metal return line has no advantage over the AC line; Among the cable lines of the DC system, the bipolar DC line has the highest power efficiency and the lowest loss, and the bipolar DC system is selected as the optimal cable line mode in terms of energy efficiency.

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