RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

1.2.  DPFC distance protection

Fault on and near busbar is the most serious fault for electric power system transient state stability. Clearing such kind of fault quickly is key to keep system stability. As to line protection, main protection, i.e. pilot protection, cannot trip related breaker quickly enough with the limitation of signal transmission time delay. Operation time of distance zone Ⅰ should not be less than one cycle for the sake of security of itself. DPFC distance protection is a kind of distance protection using fault component, and has ultra fast operation speed along with high security. In fact, DPFC distance protection has set up a record of 3 ms operation time on site.

1.3.  DPFC current differential protection

Current differential protection based on DPFC principle bas been applied to line, busbar, transformer and generator successfully [9][11].

RCS-931 ultra high voltage series protection employed DPFC current differential protection with adaptive floating threshold, which can withstand system unbalance and disturbance. The measuring element can operate with ultra high speed and high security. The pickup element based the same principle has high sensitivity, and yet it will not pick up frequently under normal condition even with unbalance and disturbance thanks to floating threshold.

Inter-turn fault is the main kind of fault in electric power transformer. It is meaningful that employing sensitive and secure protection to identify inter-turn fault inside power transformer. In RCS-978 series power transformer protection, DPFC percentage restraint differential protection makes use of DPFC in phase current on each side of transformer and DPFC in differential current, with a high setting of percentage restraint factor. The unwanted operation caused by TA saturation during external fault can be prevented. The sensitivity during internal fault is not affected by pre-fault load current and is high enough to identify internal slight fault, such as phase-to-ground fault near neutral point and low percentage inter-turn fault on the same phase under heavy load.

When slight fault occurs in generator or power transformer, traditional current differential protection cannot identify such fault for the sake of sensitivity limited by normal load current. RCS-985 Series generator-transformer unit protection is equipped with transformer DPFC percentage restraint differential protection and generator DPFC percentage restraint differential protection. Comparing with traditional percentage restraint differential protection, the DPFC percentage restraint differential protection can identify the slight fault with higher sensitivity, because they are not influenced by normal load current at all and are affected by fault transient resistance slightly. Higher restraint percentage is selected to deal with TA saturation; floating threshold prevents unwanted operation during system swing and frequency shifting.

2 . Key technology in line protection

2.1.  Series-capacitor compensated line

Series-capacitor compensation equipment installed on long-distance transmission line can improve stability allowance, optimize load distribution between parallel lines and increase the transmission power capacity. But the line impedance and its’ distribution is change by series-capacitor, which will affect the operation characteristics of protection [8][10][14].

RCS-902XS series distance carrier protection and RCS-931XS optical fiber current differential protection make up the whole protection scheme for Series-capacitor compensated line.

Split-phase current differential protection is not affected by series-capacitor at all.

For zero-sequence directional comparison protection