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Functions
2.3 Directional Overcurrent Protection Ground 67, 67N
Figure 2-22
2.3.4
Inverse Time, Directional Overcurrent Element 67N-TOC
The inverse time elements are dependent on the device ordering version. They operate either according to IEC
or ANSI-standard. The characteristics and associated formulae are identical to those of the non-directional
overcurrent protection and are illustrated in the Technical Data. When configuring an inverse time curve, the
definite time elements 67N-2 and 67N-1 are also operative.
Pickup Behavior
The ground current is compared with the setting value 67N-TOC PICKUP. When a current value exceeds the
corresponding setting value by a factor of 1.1, the function will pick up and a message is sent provided that the
fault direction is identical to the direction configured. If the inrush restraint feature is used, either the normal
pickup signal or the corresponding inrush signal is issued as long as inrush current is detected. When the
element picks up, the trip time is calculated from the actually flowing fault current using an integrating measure-
ment method. The calculated tripping time depends on the selected tripping curve. Once this time has elapsed,
a trip signal is issued provided that no inrush current is detected or inrush restraint is disabled. If an inrush con-
dition is detected while the inrush restraint function is enabled, there will be no trip but a message will be gen-
erated indicating that the timer has elapsed.
The pickup value of the 67N-TOC element, the delay time and the Curve can be selected individually.
The element can work directionally or non-directionally.
Dropout Behaviour
With an ANSI oe IEC curve it can be selected whether an element will drop out instantaneously after the thresh-
old has been undershot or whether dropout is to be performed by means of disk emulation mechanism. "In-
stantaneous" means that the pickup will drop out when the value falls below 95% of the pickup value. For a new
pickup the timer is restarted.
The disk emulation evokes a dropout process (timer counter is decrementing) which begins after de-energiza-
tion. This process corresponds to the reset of a Ferraris-disk (explaining its denomination "disk emulation"). If
several faults occur in succession, the "history" is thus taken into consideration due to the inertia of the Ferraris-
disk and the time response is adapted. Reset begins as soon as the value falls below 90 % of the setting value
in accordance with the dropout curve of the selected characteristic. In the range between the dropout value (95
% of the pickup value) and 90 % of the setting value, the incrementing and decrementing process will pause.
Disk emulation offers advantages when the overcurrent relay elements must be coordinated with conventional
electromechanical overcurrent relays located towards the source.
The following figure shows the logic diagram of the 67N-TOC overcurrent element of the directional inverse-
time overcurrent protection.
92
Logic diagram of the dropout delay for 67N-1
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A5, Release date 11.2012
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