The spill current is generated when Xiangjiaba-Shanghai ±800 kV ultra-high voltage direct current (UHVDC) transmission system is in monopole-ground return operation mode. According to analysis

the cause is the passage of current is generated between the neutral bus of de-blocking pole and equipment of blocking pole through by-pass switch closing by control logic in the state of hot standby or blocking of poles. The effects of spill current are discussed. In order to eliminate the spill current

such countermeasures are put forward as operating

modifying sequence control or designing spill current longitudinal differential protection

all being beneficial to improving the reliability of UHVDC project. The results can provide reference for UHVDC system design

operation and maintenance．