By drawing on the analytical method of sympathetic inrush current,the generation mechanism and variation characteristics of symmetrical inrush current caused by the energization of a set of two ultra-high voltage (UHV) converter transformers are dealt with.Based on an ultra-high voltage direct current (UHVDC) transmission project simulation model,the phenomenon of symmetrical inrush currents in the cases of UHV converter transformer energization are simulated.The reason for failure of the secondary harmonic restraint criterion to block the converter connection-transformer differential protection is explained,which provides a means for enhancing the operation reliability of the converter transformer differential protection for UHV converter stations.

Zero-sequence differential protection has become one of the foremost protective countermeasures for EHV and UHV power transformers due to the advantages of high sensitivity and very fast operation speed. It is the common knowledge that it has no need to be equipped with extra countermeasure besides percentage restraint criterion since it is immune to the inrush in theory. However, according to the waveform analysis of the recovery inrush resulting from removal of external fault of converter transformer, it is disclosed that the current bias component due to core saturation of converter transformer may lead to the accumulation of magnetic bias component within the core of TA. In the extreme conditions, this would lead to the distortion of the current coming from TA, and make the zero-sequence differential protection mal-operate. According to the theoretical analysis and simulation tests, the risk of zero-sequence differential protection mal-operation resulting from the inrush due to fault removal does exist in some specific fault conditions. In this case, additional restraint countermeasures are necessary to guarantee the operation security of zero-sequence differential protection.

VSC-HVDC is anticipated as a flexible and reliable solution for photovoltaic (PV) power transmission. In this paper, the model of PV grid paralleled in infinite grid through VSC-HVDC is built in PSCAD/EMTDC. And corresponding D-Q decoupled control strategy for the system is proposed. Both normal condition and short-circuit fault condition have been simulated. The results show VSC-HVDC can improve recovery performance greatly.

This paper analyzed current transformer DC bias research trend,research needs and analysis method of the DC basis,and established a simulation model to evaluate the relationship between the AC transformer neutral point DC bias and operation mode of high voltage direct current(HVDC) transmission system.Taking the Tian-Guang DC transmission system engineering as an example,CIGRE HVDC standard model control system with the actual parameters was modified.By using PSCAD/EMTDC electromagnetic transient simulation software,the ground return operation mode of Tian-Guang HVDC transmission system was built up.Based on circuit element equivalents,this model could simulate the relationship between converter transformer DC bias current and system operation.After the Simulation of converter transformers DC bias on a variety of operating conditions,the relationship of converter transformer neutral point current and the transmission power was established.