RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

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.

In order to meet the demand of adaptive auto-reclosure, only the fault phase of double-circuit lines on the same tower is cut off under any condition.

2.2.2 . Auto-reclosing

To prevent severe strike on electric power system cause by reclosing onto permanent fault or fault at the very beginning of line, novel adaptive auto-reclosure scheme is brought forward here.

Auto-reclosure of double-circuit lines on the same tower is designed as one device. After fault, only one phase of the double-circuit lines is closed first, then the left opened phases are closed in turn according to the determinate rules, each at one time.

Measures are researched to distinguish between transient fault and permanent fault in order to prevent blind re-closing. For transient fault, auto-reclosure will operate only after arc at the fault point has been extinguished. For permanent fault, auto-reclosure will not operate at all.

For fault at the very beginning of line, auto-reclosure at the remote end will operate first. If it succeeds, the local auto-reclosure will operate subsequently. If it fails, i.e. it re-closes onto permanent fault and all three phases are tripped by protection at the remote end of line, local auto-reclosure will not operate at all.

Such an auto-reclosing logic keeps links of electric power system at the greatest extent during fault, improves auto-reclosing success percentage and prevent sever strikes on electric power system caused by reclosing onto serious permanent fault.

Split-phase current differential protection RCS-931, split-phase composite distance carrier protection and zero-sequence directional comparison protection RCS-902C have been widely applied to 220kV and above lines in Chinese electric power system. RCS-902E, RCS-931E with the same protection scheme as RCS-931 and RCS-902C respectively, and with additional auto-reclosing logic for double-circuit lines on the same tower, have been applied to LongWan-HongGou transmission line in SiChuan, which is the first 500kV double-circuit lines on the same tower along the whole length in China.

2.3.  Power-swing-blocking relay

For distance protection, plenty attention must be paid on influence caused by power swing. To prevent unwanted operation of distance protection during power swing, power-swing-blocking relay must: a) block distance protection during power swing without internal fault; b) not block distance protection during internal fault; c) ensure correct operation of protection for fault during power swing; d) keep selectivity of distance protection during power swing period after fault.

The main protection of RCS-900 series protection devices, i.e. current differential protection, directional comparison protection and composite distance carrier protection don’t need power-swing-blocking logic in principle. On the other hand, step zones distance protection needs power swing blocking relay. Power-swing-block relay consists of four parts [4]: ①enabling element in short-term duty; ②enabling element for asymmetrical fault; ③enabling element for symmetrical fault; ④enabling element during phase discrepancy period. The power-swing-block relay can enable distance protection quickly for internal faults under any condition even during power-swing, and block distance protection during power-swing with or without external fault. 转贴于 酷文网-论文下载中心 http://www.coolwen.net