HVDC system DC loop resonance analysis based on time domain simulation

In HVDC systems, power converters are well known the generators of harmonics on both ac and dc side. Because of the interaction between ac and dc systems, some kinds of disturbances in the ac or dc side may cause the fundamental or second harmonic current on the dc side. When the dc loop of the HVDC system has natural resonant frequencies on the fundamental or the second harmonic frequencies, the low-order harmonic current oscillation will arise, which will damage the station equipment. In order to move the natural resonant frequencies at DC side far away from the fundamental and the second harmonic frequencies, the characteristics of the system harmonic impedance should be studied through varying the parameters of some elements of the HVDC system. In this paper, the DC loop impedance at any frequency is defined, and an approach to calculate the DC loop impedance is proposed, which is based on the test signal method and time domain simulation. Through a typical ± 500 kV HVDC sample system, the impacts of some important factors on the DC loop resonant characteristics are studied. Those important factors include the short circuit ratio of the AC systems, the capacity and type of the AC filters, the short circuit voltage drop of the converter transformers, the firing angle of the converters, the transmitted power of the HVDC system, the inductance of the smoothing reactor, the number of the DC filters, the length of the DC transmission line, as well as the operation mode of the HVDC system. Furthermore, based on the proposed approach, the resonant characteristics of the DC loop of the Nuozhadu-Guangdong ± 800kV UHVDC project is analyzed. The results are very useful when the DC loop resonant characteristics needs to be changed through varying the parameters of some elements of the HVDC system.