The Method of Harmonic Source Identification in Power Supply System

Abstract: With the continuous expansion of the scale of the grid, more and more new electrical equipment and precision equipment access to electricity power system, and they have inject a large amount of harmonic power grid, the problem is very prominent. Since the system harmonic source numerous and dispersed, this make accurate measurement become a difficult problem, and in order to make a fair price in electricity charges, This is closely related to the harmonic source identification. In order to solve this problem, This paper proposes the method based on harmonic source identification, and through calculation and analysis of the fundamental and harmonic by separation. Analyzing the harmonic power flow in measurement point, and it can provide a basis for reasonable electricity charges in measure harmonics bilateral energy. This paper has create a digital simulation to show its correctness and feasibility.


Introduction
Power system harmonics are periodic functions by Fourier series decomposition, those components' angular frequency are n times to the fundamental angular frequency [1].Under the presence of harmonics in the power system, the linear load electricity metering points, and the power metering is the fundamental power plus the harmonic power.While in the non-linear load energy metering point, the power metering is the fundamental power minus the harmonics power.According to the different harmonic currents flowing judge this load branch harmonic power of positive and negative, and then judge the true harmonic power generation side and consumption side [2,3].In this paper, the relevant research lack of harmonic interference on electric energy metering, research on harmonic source identification, and propose the method based on harmonic source identification, and it can be able to solve complex power system energy metering which have large errors.And effectively identify sources of harmonic power in grid load branch, accurate identification of harmonic source.This method is feasible and effective.

The method of harmonic source's harmonic power identification
In order to analysis the nonlinear network of the distribution of power relations, using the harmonic power flow to distinguish harmonic source.Established linear load and nonlinear load equivalent network, as it shown in Figure 1.The network that only harmonic source work is shown in Figure 2. Tt

Figure 2. The network that only harmonic source work
There is a total of m branches in Figure 2, among them, the power supply system branch is TS branch and linear branch is T i (i = 1, 2, 3, ...,t) branch.And Z i = R i + jX i , nonlinear branch is T j (j= t,t + 1,...,m) branch.And T j branch where nonlinear equivalent harmonic source U j(k) and equivalent impedance Z jk (Z j = R j + jX j ), k represents the number of harmonics, and k=2,3,…,n.U ab(k) representatives the voltage between two points a and b in k harmonic.
According to circuit theorems. ( Branch current in power system: Its system power active power consumption: The current of linear branch T i (i = 1, 2, 3, ...,t): The linear branch active power consumption of each branch: The current of non-linear branch T j (j= t,t + 1,...,m): The nonlinear branch active power consumption of each branch: The system power working alone is shown in Figure 3.Among them, U s is system power, r represents branch, r=1,2,3,…,m.According to circuit theorems.
(1) (1) Active power consumption in power system U s branch: (1) cos( ) The current in other branch T r = (r = 1, 2,...,m): Active power consumption in other branch: In expression of active power consumption in linear load, P i(k) and P r(k) are positive.Description linear user consume fundamental power from the power system.And also absorb harmonic power; In linear load branch, harmonic power from the grid side to the user side.Active power consumption in nonlinear loads, P r(k) is positive, it means that nonlinear user consume fundamental power from the power system; P j(k) is negative, it means that nonlinear user feedback harmonic to the grid, thus, the branch T j have harmonic source.By analyzing these results, it can get further harmonic source identification method by harmonic power flow: Judging by the direction of harmonic power flow in measurement point, if the harmonic power flows from the power system to the load branch, the load branch is not have harmonic source.Conversely, if harmonic power from the load side to the power system, it is determined that the load branch have harmonic source [4,5].

The digital simulation of harmonic power identification
For ease of calculation, set the number of branches to four, the simplified model shown in Figure 4.Among them, Z s = R s + jX s is equivalent impedance of system source, Z 1 = R 1 + jX 1 is equivalent impedance of load branch 1, Z 2 = R 2 + jX 2 is equivalent impedance of load branch 2, Z 3 = R 3 + jX 3 is equivalent impedance of load branch 3; U 1k , U 2k and U 3k respective for the three branches of the load harmonic voltage; T 1, T 2 and T 3 are power metering points; U s is system source, only produce the fundamental voltage [6,7].Each module parameters in Figure 4 are as follows: sin(100 t) ; =100 sin(300 + ) ; =20 sin(500 ) ; =100 sin(300 + )+ 20 sin(500 ) ; =6 ; =2 ； =1 ; =1 ; =1 .
Modeling results of fundamental and harmonic current in each branches [8,9], are as it is shown in Table 1; And each of branches of power consumed by the fundamental and harmonic power [10,11], as it is shown in Table 2.According to Table 1 and Table 2, in the fundamental wave, the measuring point are T s , T 1 , T 2 , T 3 , T 4 .Only fundamental power in T s is negative, whilefundamental power in T 1 , T 2 , T 3 , T 4 are positive.Explanation only the branch T s have the fundamental source, other branch doesn't have the fundamental source.The sum of active power consumed by branches T 1 , T 2 , T 3 , T 4 is equal to active power issued by T s .In the fundamental source of the active power consumption in branches T s , T 1 , T 2 , T 3 , T 4 algebraic is the 0. In the 3rd harmonic, 3rd harmonic power in T 1 , T 3 are negative, while 3rd harmonic power in T s , T 2 , T 4 are positive.Explanation the branches T 1 , T 3 have 3rd harmonic source, T s , T 2 , T 4 doesn't have the 3rd harmonic source.And in the 3rd harmonic source of the active power consumption in branches T s , T 1 , T 2 , T 3 , T 4 algebraic is the 0. In the 5th harmonic, 5th harmonic power in T 2 , T 3 are negative, while 5th harmonic power in T s , T 1 , T 4 are positive.Explanation the branches T 2 , T 3 have 5th harmonic source, T s , T 1 , T 4 doesn't have the 5th harmonic source.And in the 5th harmonic source of the active power consumption in branches T s , T 1 , T 2 , T 3 , T 4 algebraic is the 0.Then, the comprehensive analysis, the T s branch only have the fundamental source, the branch T 1 only have the 3rd harmonic source, the branch T 2 only have the 5th harmonic source, the branch T 3 have both the 3rd harmonic source and the 5th harmonic source, the branch T 4 doesn't have any harmonic source or fundamental source.This is consistent with the parameters that have set, thus verifying the correctness of MATLAB simulation.

Conclusion
Conclude the analysis results of this paper, we can get the conclusions as follows: 1. Abandon the existing measurement methods based on the total distortion of the waveform to calculate active power consumed by the user, and separate the fundamental wave and harmonic.Refinement analysis of each harmonic power load, and identification system harmonic source.2. Analyzing the flow of harmonic power measurement point, if the harmonic power from the power grid to the load, the load branch doesn't have harmonic source.On the contrary, if the harmonic power from the load to the power grid, the load branch have harmonic source.3.In the linear load energy metering point, energy metering power is the fundamental power plus harmonic power.In the non-linear load energy metering point, energy metering power is the fundamental power minus harmonic power.

Figure 1 .
Figure 1.The equivalent model network of linear load and nonlinear load

Figure 3 .
Figure 3.The circuit diagram of power system working alone

Figure 4 .
Figure 4.The circuit diagram of harmonic power flow identification

Table 1 .
The fundamental and each harmonic currents in Measuring point

Table 2 .
Active power consumption of fundamental and each harmonic in Measuring point