AC System Harmonic Impedance Equivalence And Background Harmonic Analysis

The AC system equivalent impedance and background harmonic distribution characteristic are very important for powe system analysis and design, especially for HVDC system. Under the basic frequency, AC system equivalent impedance can be calculated by short-circuit capacity of the given bus and the short-circuit ratio can be derived to analyze some characteristic of power system. However, when designing the filter in HVDC system, only the AC system equivalent impedance under basic frequency is not enough. Not only the characteristic of AC system equivalent impedance during interest frequency range but also the background harmonic distribution in the HVDC convert station is needed. The change of power system load, generator and network operation state in each year are taken into account when calculating the characteristic of AC system equivalent impedance.NBVISCAN program, which are developed by TESHMONT Company in Canada, has been used for AC system equivalence in previous HDVC system projects. However, there are many disadvantages such as the time-consuming calculation, non-compatible with PSS/E data flile. On the other hand, at present, PSS/E and BPA are widely used in our country, which provide power flow calculation, short-circuit current calculation and stability analysis except for AC system equivalence and harmonic study. So, it is very necessary to develop an AC system harmonic impedance equivalence and harmonic voltage and total harmonic distortion calculation program based on PSS/E and BPA data, which has important significance for Chinese independent HVDC system design.This paper studies on how to build up proper network element harmonic model for AC system wide frequency range equivalence and calculate the background harmonic voltage and total harmonic distortion when the harmonic sources are given. In order to meet the HVDC system design demand, it describes the method for establishing proper network element harmonic model based on *.raw file of PSS/E and *.DAT and *.SWI files of BPA, which are commonly used power system analysis software. Futhermore, this paper describes how to scan and calculate AC system equivalent impedance and background harmonic voltage distortion using the analyzing-method of sinusoidal steady-state network under single frequency.During the preparation for this paper, I study and summarize power system element harmonic modeling method, the selection principle and background harmonic level analysis method. At the same time, this paper introduces the process of developing a program for AC system wide frequency range equivalence and harmonic calculation which possesses high practical value in HVDC system design project.In the end, a simple tested system is calculated by both this program and the PSCAD/EMTDC harmonic analysis function. These two results are very close, so this program can be a useful and reliable tool for the design of HVDC transmission systems. Then, the power flow data of East China Power Grid during summer of 2003 is also used by this program to calculate the system harmonic equivalent impedance characteristic.