RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

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.

4. Key technology in power transformer protection

For power transformer protection, a great challenge is to prevent unwanted operation caused by inrush current and to operate quickly when switching onto internal fault especially those slight ones.

RCS-978 series power transformer protection devices employ a novel differential current phase angle adjusting method. After phase angle adjusted, fault character of fault phase current is more obvious, and the current of left phases contains more component of inrush current. When switching onto internal fault, operation speed of fault phase protection will not be slowed by inrush current of healthy phases thanks to split-phase inrush current restraint. Inrush current that appears when switching on transformer with no fault, when external fault is cleared and when the parallel transformer is switched on can be distinguished more easily.

5.  Key technology in generator-transformer unit protection

The scheme of large generator-transformer unit protection must ensure the security of whole unit and reduce the damage in case of fault. The scheme must be perfect, reasonable, and not complex. RCS-985 series generator-transformer unit protection devices employ DPFC principle, variable slope percentage restraint current differential protection, asynchronous anti-TA saturation identifying criterion, zero-sequence transverses differential protection with current restraint and floating threshold, meet the requirements of protection for large generator-transformer unit [17]. DPFC principle has been described in the former part of this paper.

Percentage restraint current differential protection has a variable slope percentage restraint curve, which has a none-zero percentage at the beginning of curve. The curve well adapts to unbalanced differential current and the initial setting of current differential protection can be lower, thus the differential protection sensitivity is high when internal slight fault occurs even during unit start-stop process.

It was a misunderstanding that the unbalanced differential current in secondary coil of TA is low because the TAs installed on head and tail of generator are of the same type and both are P class, and the fault current passing through during external fault is low, thus the initial setting can be set lower properly to improve the sensitivity for internal slight fault. In fact, TA wiring cable lengths from head and tail of generator may be not the same though TAs are of the same type, TAs are of different type in some cases, the aperiodic components decays slowly though the whole amplitude of current is not high under external fault which leads to TA saturation. All these factors lead to no acceptable unbalanced current, which causes unwanted operation repeatedly.

Lifting differential setting or introducing differential protection with product- quantity restraint will lose sensitivity or even lead to missing operation. With identifying harmonic component and waveform character of differential current asynchronous anti-TA saturation criterion can distinguish between internal fault and TA saturation under external fault.

So far inter-turn fault protection, such as negative-sequence direction blocking longitudinal zero-sequence voltage protection, negative-sequence direction blocking second harmonic component protection, third harmonic component blocking longitudinal zero-sequence voltage protection, transverse differential protection still have some defects in sensitivity and security. Take transverse differential protection as an example, the setting must be higher than the maximum unbalanced current under three-phase fault at generator terminal in order to prevent unwanted operation during external fault or power-swing, thus the sensitivity is not enough for inter-turn fault. RCS-985 introduces longitudinal zero-sequence voltage inter-turn fault protection and high sensitive transverses differential protection for the first time; both have current restraint and floating threshold. Setting of zero-sequence voltage only need to be higher than maximum unbalanced fundamental harmonic voltage under normal operation mode of generator, and setting of transverse differential protection be higher than maximum unbalanced differential current under normal condition, thus the sensitivity is high for internal low percentage inter-turn fault without risks of unwanted operation under external fault. 转贴于 酷文网-论文下载中心 http://www.coolwen.net