The Loss Reduction Research of Unit Power Optimizing with Considering Economic Benefit

Two primary approaches, which can reduce network loss and improve economic benefit of electric power system, are optimizing active and reactive power. This paper studies actual power network and puts forward a method which can reduce network loss through optimizing active power. And this method increases the constraint condition of power stations attribution. It changes the opening mode of units to divert electric quantity between different units. The relationship between the run mode of units and the change of loss electric quantity is found according to analyze the situation of network loss of different opening modes. The result indicates that the method can achieve the goal of reducing primary network loss by optimizing unit power in the same group. The economic benefits of various groups are not damaged at the same time.


Introduction
With the development of the society and the increasing growth of the science and technology, electric power industry has become the pillar industry of the countries in the world, which supplies necessary energy sources and power for the entire society. Because of the existing of impedances in the electric power system, electric energy transform, transport and distribute, there will be a mass of inevitable loss in the process of the transformation, transmission and distribution of electric energy. The loss is termed electric power system network loss. It is an important comprehensive index of technology and economy to judge operation condition of electric enterprises. The primary measures used to reduce network loss are optimizing active power and optimizing reactive power. The specific measures include optimizing power grid operation mode, enhancing power grid economic dispatching, accelerating the reform of power grids and strengthening management of line loss [1][2][3].

Traditional methods of network loss reduction
Electric power network loss mainly consists of line loss, transformer loss, station using electricity loss and other loss. Station using electricity loss and other loss is 1.13% for all. Therefore, primary network loss in this paper means line loss L A  and transformer loss T A  . The following formula comes from RMS current method: R in the formula means element resistance. T means operation time. jf I means RMS current.
The common method of line loss calculation,which is used in traditional theoretical calculation, just uses several running data. For this reason, the error between theoretical calculation and practical calculation is so large that it is difficult to realize line loss management effectively [4]. For advancing the accuracy of theoretical calculation, various modified algorithms that based on OPF method have become mainly research method to calculate network loss. The methods which apply OPF to change the unit power are easily influenced by network loss sharing scheme. For example, network loss factor (network correction factor) is built by network loss tiny-increasing-rate [5][6][7] to correct objective function, and then, be corrected by sharing network loss [8]. This kind of method is limited by accuracy of calculation and practical applicability.
At present the research of energy saving and consumption reducingis mainly concentrated onreducing network loss by management measure and increasing the reliability of power grid by optimizing reactive power, thereby reforming the structure of power grid. Literatures [9][10][11][12] study the method which can reduce network loss by analyzing operation mode of electricity market, perfecting the mould of electricity market and optimizing dispatching mode. Literatures [13][14][15] put forward the method which can reduce network loss by installing SVC, storing energy by capacitance and improving the structure of power grid. This paper takes the structure of power grid as entry point. It analyses optimizing the unit power by how to change operation mode of power grid under the constraint condition of power stations attribution. Units power has been arranged reasonably based on the electricity sales of power generation groupunchanged.This method considers economic benefit and adds the constraint condition of power plant attribution. It can make the economic benefit loss of each power generation group minimally. The aim of primary network loss reduced is achieved effectively.

The loss reduction method of unit power optimizing based on economic benefit
This paper doesn't depend on theoretical calculation. It studies real power grid and puts forward a new method that could reduce network loss by optimizing unit power based on the structure of power grid and economic benefit. This method has been checked by simulation. The steps will be shown as follow. First, simulation data are adjusted and made sure the initial loss electric quantity in the simulation data, the same as the network loss data in the line loss monthly report. This step could confirm initial start-up mode of each power plant. Second, the influence rate of each power plant on network loss is confirmed. Third, the unit power transferring programs are built among different plants of the same group in all the power grid areas with constraints of the belonging group and power grid area of the plant. Fourth, the economic benefit of each program has been analyzed. Last, each program is simulated with electric power system simulation software of electric power system. The loss electric quantity of each scheme is gotten. The simulation results are compared with initial electric quantity , and then the regularity of optimizing unit power is found out.
If the problem of power plant attributionis not considered and the unit power is only changed, the economic benefit of power generation group will be affected. This paper adds constraint condition of power plant attribution to solve this problem. The benchmark price is calculated with 0.3723RMB/kWh. If group A transfers a hundred million kWh electric quantity to group B, it will loss 37.23million RMB. So it is good for the groups to keep their economic benefit and achieve loss reduction by adding constraint condition of power plant attribution.
The study objects of each program are six kinds of running mode of goal grid including winter heavy load, winter medium load, winter low load, summer heavy load, summer medium load and summer low load. The situation of heating period and non-heating period is considered generally. The data can be handled by following formula:

The influence analysis between unit power and network loss
In order to confirm the best program of optimizing unit power under the constraint condition, following formula can be used to express the influence of unit power on loss. Q is power loss. the same time. The start-up mode should be adjusted. In order to achieve the aim, large capacity and large influence units in middle grid and western grid should be closed when generated energy in eastern grid is increased. If program that transferring quantity from eastern grid to western grid and middle grid is arranged reasonably, network loss should be reduced. As shown in figure.2, figure.3, figure.4. Positive number means tendency of loss increase. Negative number means tendency of loss reduction.  figure.2 shows the situation of loss change that electric quantity is transferred from middle grid to eastern grid. Sampling dot 6 to 10 in figure.2 shows the situation of loss change that electric quantity is transferred from eastern grid to middle grid. Sampling dot 1 to 4 in figure.3 shows the situation of loss change that electric quantity is transferred from western grid to eastern grid. Sampling dot 5 to 8 in figure.3 shows the situation of loss change that electric quantity is transferred from eastern grid to western grid. Sampling dot 1 to 5 in figure.4 shows the situation of loss change that electric quantity is transferred from western grid to middle grid. Sampling dot 6 to 10 in figure.4 shows the situation of loss change that electric quantity is transferred from middle grid to western grid.

Figure5. Unit power change and corresponding network loss of four power generation group
Through comparison and analysis of simulation we can see that loss reduction could be achieved through optimizing unit power between the same power generation group and different groups. When electric quantity is transferred, network loss will be reduced more in the different groups than that in the same group, however, it will make some power generation groups loss income. That is why, on consideration of economic benefit principle, the constraint condition of power plant attribution should be added when unit power is optimized.

4.3.2.
The influence of electric quantity on network loss in the same area.Unit power is adjusted and the change of loss quantity situation is analyzed in the same area of aimed grid under the condition of power plant attribution. As shown inTable.1, positive numbers mean electric quantity transfer in, and negative numbers mean electric quantity transfer out.