RESEARCH AND APPLICATION OF PROTECTION TECHNOLOGY

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 protection, which brings convenience for design and operation.

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]转贴于 酷文网-论文下载中心 http://www.coolwen.net