Chapter 6 - Current Protection Functions
The setting I>1 tRESET determines the reset time for the DT characteristic
The outputs of the timer modules are the single-phase trip signals. These trip signals are combined to form a 3-
phase Trip signal.
The timer modules can be blocked by a Phase Overcurrent Timer Block (for example I>1 Timer Block).
For DT-only stages, the DT timer can be blocked by the Autoreclose function. An Autoreclose blocking signal is
produced by the DDB signal AR Blk Main Prot and the relevant settings in the I> Blocking cell.
If fault current can flow in both directions through a protected location, you will need to use a directional
overcurrent element to determine the direction of the fault. Once the direction has been determined the device
can decide whether to allow tripping or to block tripping. To determine the direction of a phase overcurrent fault,
the device must compare the phase angle of the fault current with that of a known reference quantity. The phase
angle of this known reference quantity must be independent of the faulted phase. Typically this will be the line
voltage between the other two phases.
The phase fault elements of the IEDs are internally polarized by the quadrature phase-phase voltages, as shown in
the table below:
Phase of protection
Under system fault conditions, the fault current vector lags its nominal phase voltage by an angle depending on
the system X/R ratio. The IED must therefore operate with maximum sensitivity for currents lying in this region. This
is achieved by using the IED characteristic angle (RCA). This is the is the angle by which the current applied to the
IED must be displaced from the voltage applied to the IED to obtain maximum sensitivity.
The device provides a setting I> Char Angle, which is set globally for all overcurrent stages. It is possible to set
characteristic angles anywhere in the range –95° to +95°.
A directional check is performed based on the following criteria:
-90° < (angle(I) - angle(V) - RCA) < 90°
-90° > (angle(I) - angle(V) - RCA) > 90°
For close up three-phase faults, all three voltages will collapse to zero and no healthy phase voltages will be
present. For this reason, the device includes a synchronous polarisation feature that stores the pre-fault voltage
information and continues to apply this to the directional overcurrent elements for a time period of a few seconds.
This ensures that either instantaneous or time-delayed directional overcurrent elements will be allowed to operate,
even with a three-phase voltage collapse.