RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

ciple 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 fact

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