摘要：Abstract: Research and application progress of NARI-RELAYS in protection based on DPFC were reviewed, key technology employed in line, busbar, transformer and generator protection were presented. It was pointed out that integration of main and backup protection in one device and duplicate allocation was reasonable and beneficial. Key words: Protection; DPFC; Main protection; Backup protection; Duplicate protection

关键词：Research

0. Introduction

Electric power network in China has step into a new stage with large-scale networks, super power plants and high-rating generators. By the end of year 2002, total installed generation capacity is 353000MW, total length of 35kV and above transmission lines is 806500kM, total installed transformer capacity is 1194000MVA, and generated energy of the year is 164×1010kW·h。

As an important part of electric power system, protection is the technology measures to ensure the security and stability of power networks. Development of electric power system requires new functions and higher performance of protection. New progress in microelectronics, computer and communication technology makes it possible to improve protection further more.

NARI-RELAYS Electrics Co., Ltd. is the main base of research on protection principle and technology, and among the leading suppliers of protection and automation products in China. She has made great progress in research and application of new protection technology, especially series protection based on Deviation of Power Frequency Component (DPFC). She also possesses many invention patents. The author summarized these research and application progress, presented his own viewpoint on protection allocation, and look forward to discuss this issue with experts in protection area.

1.  Application of DPFC protection

DPFC protection is a kind of fault components protection. Fault components have two characteristics: (1) Fault components only appear during fault and disappear under normal condition; (2) All fault components are produced by the same electromotive force at the fault point, so that fault components at fault point are larger than elsewhere. Normal load current does not influence protection based on DPFC principle at all. Fault transient resistance slightly affects DPFC protection. In many cases, DPFC protection has excellent performances.

The first DPFC protection, DPFC directional comparison protection was invented in 1982. In the following years, the whole DPFC protection system was set up successively, DPFC distance protection and DPFC current differential protection were developed, at the same time this technology migrates from line protection to busbar, transformer and generator protection. Large amount of real on-site operation records proved that DPFC line, busbar, transformer and generator protection are of high security, high reliability and fast operation speed, and play an important role in improving electric power system security, stability and transmission capacity.

1.1.  DPFC directional comparison protection

No matter what type of fault, which fault phase, whether or not a transferring fault, whether or not under phase discrepancy condition, the phase angle between voltage DPFC and current DPFC only depends on system impedance backward from where the protection installed, and has nothing to do with system electromotive force and fault transient resistance. Line fast protection based on DPFC directional comparison protection can distinguish between positive direction and negative direction unambiguously. This protection was proved reliable by practice and has been applied to electric power system widely and successfully [3][5][7].

1.2.  DPFC distance protection

Fault on and near busbar is the most serious fault for electric power system transient state stability. Clearing such kind of fault quickly is key to keep system stability. As to line protection, main protection, i.e. pilot protection, cannot trip related breaker quickly enough with the limitation of signal transmission time delay. Operation time of distance zone Ⅰ should not be less than one cycle for the sake of security of itself. DPFC distance protection is a kind of distance protection using fault component, and has ultra fast operation speed along with high security. In fact, DPFC distance protection has set up a record of 3 ms operation time on site.

1.3.  DPFC current differential protection

Current differential protection based on DPFC principle bas been applied to line, busbar, transformer and generator successfully [9][11].

RCS-931 ultra high voltage series protection employed DPFC current differential protection with adaptive floating threshold, which can withstand system unbalance and disturbance. The measuring element can operate with ultra high speed and high security. The pickup element based the same principle has high sensitivity, and yet it will not pick up frequently under normal condition even with unbalance and disturbance thanks to floating threshold.

Inter-turn fault is the main kind of fault in electric power transformer. It is meaningful that employing sensitive and secure protection to identify inter-turn fault inside power transformer. In RCS-978 series power transformer protection, DPFC percentage restraint differential protection makes use of DPFC in phase current on each side of transformer and DPFC in differential current, with a high setting of percentage restraint factor. The unwanted operation caused by TA saturation during external fault can be prevented. The sensitivity during internal fault is not affected by pre-fault load current and is high enough to identify internal slight fault, such as phase-to-ground fault near neutral point and low percentage inter-turn fault on the same phase under heavy load.

When slight fault occurs in generator or power transformer, traditional current differential protection cannot identify such fault for the sake of sensitivity limited by normal load current. RCS-985 Series generator-transformer unit protection is equipped with transformer DPFC percentage restraint differential protection and generator DPFC percentage restraint differential protection. Comparing with traditional percentage restraint differential protection, the DPFC percentage restraint differential protection can identify the slight fault with higher sensitivity, because they are not influenced by normal load current at all and are affected by fault transient resistance slightly. Higher restraint percentage is selected to deal with TA saturation; floating threshold prevents unwanted operation during system swing and frequency shifting.