Theory Of Operation; Directional Principle; Current Directional Protection - GE T35 Instruction Manual

T35 Transformer Protection System
Chapter 9: Theory of operation

Theory of operation

This chapter outlines advanced information on operation.

9.1 Directional principle

9.1.1 Current directional protection

For better security, the T35 uses the current directional protection principle to dynamically supervise the main current
differential function. The directional principle is in effect permanently for low differential currents (region 1 in the Two
Regions of Differential Characteristic figure) and is switched on dynamically for large differential currents (region 2 in the
same figure) by the saturation detector (see the Saturation Detector section) upon detecting CT saturation.
The directional principle responds to a relative direction of the fault currents. This means that a reference signal, such as
transformer voltage, is not required. The directional principle declares that
• If all of the fault currents flow in one direction, the fault is internal, or
• If at least one fault current flows in an opposite direction compared with the sum of the remaining currents, the fault is
external
The directional principle is implemented in two stages.
First, based on the magnitude of a given current, it is determined whether the current is a fault current. If so, its relative
phase relation has to be considered. The angle check must not be initiated for the load currents as the direction is out of
the transformer even during internal faults. The auxiliary comparator of this stage applies an adaptable threshold. The
threshold is a fraction of the restraining current. The current from a particular feeder is used for bus directional
comparison if its magnitude is greater than 0.2 × I
Second, for the selected fault currents, the phase angle between a given current and the sum of all the remaining currents
is checked. The sum of all the remaining currents is the differential current less the current under consideration. Therefore,
for each, say the pth, current to be considered, the angle between the I
Ideally, during external faults, the said angle is close to 180° (as shown); and close to 0 degrees during internal faults.
T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
or it is greater than 2 times its CT rating.
restraint
and I
p
- I phasors is to be checked.
D p
9-1