Measurement Of Power System Harmonic Impedance

The AC system harmonic impedance and the distribution of background harmonic voltage characteristic are important in the domain of power system analysis and design, especially for the design of HVDC system. When designing the filter of HVDC system, only the AC system impedance under basic frequency is not enough. Both of the characteristic of AC system harmonic impedance during interest frequency range and the distribution of background harmonic in the HVDC convert station are needed to make sure that the resonance point of the HVDC system away from the basic frequency and second harmonic frequency. In modern power system there are many different equipments whose parameters are always changed as their operation conditions changed .So it's hard to get the harmonic impedance through the power system equivalent calculation .The measurement of harmonic impedance only need to know the voltage and the current of the measuring point ,so it can reduce lot of workload.In this paper ,the author propose three methods to measure the system harmonic impedance .They are network harmonic impedance measurement based on capacitor switching, network harmonic impedance measurement based on thyristor branch switching and network harmonic impedance measurement using harmonic current source.In the method of network harmonic impedance measurement based on capacitor switching, a trap filter is used besides the capacitor to eliminate the disturbance of background harmonics and spectrum leakage .Otherwise the noise is eliminated by the wavelet transform.In the method of network harmonic impedance measurement based on thyristor branch switching, because of the stabilization, high efficiency and convenience of trigger control ,the controllable thyristor is employed to get the accurate measurement and reduce the disturbance to the power grid. Meanwhile, when the state of power grid is changed, the condition of measurement can be improved via adjusting the turn-on time of controllable thyristor. In the situation when noise exists, through the control of the turn-on time of the controllable thyristor, the impregnation current can be increased and the influence of noise with respect to the measurement is reduced greatly. In the third method proposed ,a harmonic current source is employed to impregnate harmonic current to the measuring point .Because the harmonic frequency and component of the impregnation current from harmonic current source are easily controlled, the problem that the harmonic component in some frequency is too little which could disturb the measuring accuracy can be solved.The author also performed a lot of simulations based on PSCAD and Matlab to certify the effect of these methods. The proposed methods acquire the accurate measurement conclusion both in ideal condition and the case of existence of background harmonics and noise. Therefore, the accurate conclusion can be drived through the proposed methods .Because the measuring conditions of the three methods are different ,these methods can be used in different situation.