RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

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.

6.    Integration of main and backup protec-

tion, duplicate allocation

DL/T769-2001, Technical guide for microprocessor-base protection equipment of power system [18] laid down the guideline for digital protection devices allocation. Digital protection may integrate main and backup protection of protected main equipment or line into one device, main and backup protection share the same DC power supply, TAs and TVs. For those lines and main equipments that need duplicate protection system, each system may be a digital protection device with main and backup protection integrated together.

Main protection is the protection to selectively clear fault on protected main equipment or line in definite time short enough to guarantee system stability and equipment security. Backup protection clears the fault when they cannot be cleared by main protection or breaker fails to trip. Backup protection falls into two categories, remote backup protection or near backup protection.

Based on information sharing and powerful calculation capacity of embedded system, one digital protection device can integrates main and backup protection together, so that different protection elements can coordinate with each other on sensitivity. For those lines or main equipments that need duplicate protection, two such devices can be installed, thus loss of protection will be prevented even if one device fails. In each device, two independent measuring and deciding circuits are installed, thus failure of any one circuit will not lead to unwanted operation. Integration of main and backup protection and duplicate allocation make it convenient to test, maintain, operate, and design the protection and it’s secondary winding.

Integration of main and backup protection and duplicate allocation have been achieved first years ago in ultra high-voltage line protection, which provides convenience for operation, management and maintenance of line protection and improve rate of correct operation largely. In recent years, this concept has been applied to main equipment prot