RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

6.1.  Line protection

In each device of RCS-900 series line protection, pilot protection such as differential protection, directional comparison protection and composite distance protection act as main protection, while step zones distance protection, over-current and zero-sequence over-current as backup protection. Double devices are installed on line of 220kV and above, and in some cases, three devices altogether.

6.2.  Busbar protection [12]

Traditionally, busbar protection and breaker failure protection are independent, each has its’ own TAs. RCS-915 series busbar protection integrates busbar protection and breaker failure protection together on the basis of correctly recognizing operation mode; two protections share the same TAs. Double devices are installed on busbar of 220kV and above, thus both busbar and breaker failure protections are doubled with no need of additional TAs. This scheme is of high reliability, simple secondary winding, flexile operation mode, i.e. maintenance on each protection device can be carried out without switching off the busbar.

6.3.  Transformer protection [15]

Each device of RCS-978 series transformer protection integrates all electric quantities protection for power transformer, including steady state percentage restraint differential protection, differential instantaneous protection, DPFC percentage restraint differential protection, zero-sequence or winding percentage restraint differential protection, combined voltage (low-voltage plus negative-sequence over-voltage) blocking directional over-current, zero-sequence directional over-current, over-excitation protection, phase-to-phase impedance, zero-sequence over-voltage, and gap zero-sequence over-current. Main and backup protection share the same TAs; the concept of integration of both in one device is achieved. Installing two such devices on one power transformer, thus the main and backup protections are doubled.

Integration of main and backup protection with duplicate allocation reduces the total number of power transformer protection devices, and the secondary winding is clear, simple and independent. Main protection has been enhanced, so the backup protection can be simplified. Duplicate allocation eliminates the case that failure or time scheduled maintenance of protection take power transformer out of operation. All electric quantities of power transformer can be sampled and recorded during fault by protection, it is meaningful to make use of the record to analyze the fault in details and find the fault causation quickly.

6.4.  Generator-transformer unit protection [16]

Traditionally, for 300MW generator unit, only differential protection of main transformer, generator and generator-transformer unit are doubled. The rest is of single arrangement. But some fault type can only be protected by one dedicated protection element, when the dedicated protection is out of operation because of failure of wiring or device itself, the generator has to be stopped or run with defects, thus the loss and risks are high.

RCS-985 series protection integrates all electric quantities protection for one generator-transformer unit together, i.e. main protection, backup protection and abnormal operation protection. The protected range includes main transformer, generator, high-voltage house-service transformer and exciting transformer or field exciter. For large generator-transformer unit, double protection devices are needed, each has its’ own TAs and independent trip outputs.

This arrangement results in convenience of operation, setting, testing and maintenance, high security and reliability, simple design and clear wiring. Main protection and backup protection share