e 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 pr

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