RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

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, so-called compensated impedance setting is introduced to ensure comparing direction correctly during asymmetrical phase-to-ground fault ahead.

DPFC distance protection keeps unambiguous when installed on series-capacitor compensated line [14], but may overreach when fault occurs ahead after capacitor. Enhance the threshold by additional MOV protection-level voltage to prevent protection from overreaching, at the same time the fast operation speed needed when near and serious fault occurs is not affected at all.

For traditional distance protection, polarization voltage vector is introduced to prevent missing operation during fault at the very beginning of line after capacitor.  Logic combination of two distance relays with different attenuation time constant each is designed to prevent unwanted operation during backwards near fault after capacitor. So-called forwards protection-level voltage is introduced to protection. Operation curve of reactance relay is adjusted according to measured current in order to accommodate various power system operation modes. In this way, under-reaching distance protection will not overreach when fault occurs at the end of line or the beginning of the next line after series capacitor.

The whole scheme takes into account the influence caused by operation of protective devices of series capacitor unit, for example MOV break-over or protective gap break-over, and ensures correct operation of protection under any operating condition of MOV and gap.

Based on the information only from one line, the mentioned measures ensure correct operation of protection without compromise of operation speed, are appropriate for the line with series capacitor installed on it or on its’ neighboring line, no matter where TV installed.

RCS-902XS and RCS-931XS series protection devices have been applied to several series capacitor compensated lines and their neighboring lines. 转贴于 酷文网-论文下载中心 http://www.coolwen.net