RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY(2)

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.

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.

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 principle to prevent unwanted operation. Digital busbar protection sample currents of each branch circuit separately so that only low-impedance percentage restraint current differential principle is available. Some digital busbar protection take countermeasures such as increasing percentage restraint factor or delaying operation after a short term enabling, to prevent unwanted operation caused by TA saturation during external fault, thus the sensitivity and operation speed of protection are harmed.

RCS-915 series busbar protection employs adaptive weighting busbar differential protection principle based on DPFC for the first time in the world [9], and takes steady state percentage restraint current differential protection with waveform identification element as assistant criterion. With ultra fast operation speed, high sensitivity and high anti-saturation ability, this protection keeps best balance between sensitivity and security. For external fault, unwanted operation will not appear even if TA is saturated as quickly as in no more than 2ms. For internal fault the protection can operate quickly.