Research On Harmonic Contributions Determination In Distribution Network

With the development of power system, a large number of nonlinear electric equipment is connected to the grid and inject harmonic current to the grid, thus the harmonic influence the normal operation of loads and devices in power grid. At the same time, photovoltaic power generation, as a kind of clean energy, the proportion of which is becoming more and more big in power grid. The harmonic current produced by photovoltaic inverter device has volatility and randomness, and it would make a new impact on power system harmonic distribution. Therefore both the nonlinear load and photovoltaic power generation systems are the harmonic source, and they affect the power quality of power system. In order to distinguish the harmonic contribution between utility and customer, to provide theoretical basis for harmonic treatment, the harmonic emission level and harmonic contribution of harmonic source must be calculated accurately. Harmonic contribution determination can provide the theoretical basis for harmonic rewards and punishment mechanism, and it is of great significance to coordinate disputes over power department and customer under electricity market environment.A model about estimating harmonic emission level was established and an equation about determining harmonic contribution was deduced in this paper. And it points out the key point of determining harmonic responsibility is calculating the harmonic self-impedance of the bus accurately which the harmonic source locate. Because traditional linear regressions only pay close attention to the harmonic voltage error, based on the orthogonal distance regression, a method for calculating harmonic impedance was proposed in this paper. This method pays attention to the harmonic voltage error and the harmonic current error, so it can give more reliable results. On this basis, the harmonic contribution on particular buses of harmonic source could be calculated. A case study based on the IEEE 14-bus test system was conducted, which shows that the proposed method could give more liable results by comparing with several traditional linear regressions.In view of current research only paid attention to a particular harmonic, in order to fully determine the influence of harmonic sources to the system’s harmonic pollution, a definition about "total harmonic contributions" was proposed in this paper. And a computational formula about total harmonic contributions determination was deduced in this paper. Firstly, a time domain equivalent model of power system was established. Using the voltage and current instantaneous value measured at point of common coupling to extract the harmonic component, based on the covariance of random variables is 0, equivalent resistance and inductance parameters of system side could be got. Secondly, calculate the harmonic voltage instantaneous value at the point of common coupling produced by harmonic source’s harmonic current to separate the background harmonic voltage and harmonic source’s harmonic voltage. Finally, Finally according to the definition and calculation formula proposed in this paper, the total harmonic contributions of harmonic source could be calculated. At last, based on the platform of LabVIEW, a prototype was manufactured to determine harmonic source’s total harmonic contributions. Through the simulation experiment, the method proved its validity.The proportion of distributed power in power grid becomes increasingly higher, the harmonic current of inverter device is random and fluctuating, the new impact on the power system harmonic distribution, it will have a new impact on the power system harmonic distribution. Take the distributed photovoltaic power supply for instance and analyze the harmonic contribution determination problem of the distribution network with distributed power supply. Aiming at the shortcomings of the existing algorithm, this paper proposes a two-step cumulant method to calculate the probabilistic harmonic power flow. This method turns the solution of harmonic voltage amplitude of each bus into the process of two step linear equation calculation, in the process of the two steps one cumulant calculation and one Gram-Charlier series expansion are calculated respectively, and the probability density function of harmonic voltage amplitude’s square could be got.95% probability value of all bus’s harmonic voltage amplitude and harmonic voltage ratio of all buses could be calculated. In distribution network with distributed photovoltaic power, first of all, based on two-step cumulant method calculate the harmonic voltage ratio of all buses, and then determine the harmonic contribution of the focused photovoltaic power in distribution network. Based on IEEE33-bus distribution system, simulation analysis is carried out.