RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

, 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.

2.2.  Double-circuit lines on the same tower

More and more double-circuit lines on the same tower appear in Chinese electric power network for their high transmission capacity and saving of line corridor. Protection for double-circuit lines on the same tower should identify any kind of faults especially those faults cross over double-circuit lines, select fault phase correctly and trip breaker as quickly as possible. Auto-reclosing logic for those important double-circuit lines on the same tower should be designed carefully to increase their transmission reliability, to improve auto-reclosing success percentage and reduce risks of reclosing onto serious permanent fault.

2.2.1. Protection

The whole protection scheme for double-circuit lines on the same tower consists of split-phase current differential protection or split-phase composite distance carrier protection and zero-sequence directional comparison protection as main protection, and DPFC distance protection, step zones distance protection and zero-sequence directional over-current protection as backup protection.

Split-phase current differential protection has excellent capability of fault phase selection no matter whether the fault occurs on the same line or cross over lines.

For composite distance carrier protection and zero-sequence directional comparison protection, phase selection results are exchanged between two ends of the same line, thus each end can recognize the fault phase whether the fault occurs on the same line or cross over lines.

Communication channel-independent DPFC distance protection, step zones distance protection and zero-sequence directional over-current act as backup protection. Their phase selection and trip logic can distinguish single, two or three phases fault correctly, so that only the fault phase is cut off.

《RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY(第3页)》