RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

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.

 

3.  Key technology in busbar protection

The main difficulty busbar protection facing is to prevent unwanted operation caused by TA saturation during external fault. Traditional mid-impedance and high-impedance differential protection inserts appropriate impedance in differential circuit, while traditional low-impedance differential protection makes use of rapid saturation prin

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