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51 Time-Delayed Overcurrent – Phase
Overview of Function
Time overcurrent protection is used to provide:
•
Short circuit detection in electrical equipment
•
Backup or emergency overcurrent protection in addition to other protection functions or devices
•
Coordinated operation with other devices using current and time graded setting
Structure of the Function
The time delayed overcurrent function has group dependent settings. These settings can be different in each
settings group – group numbers (Gn) 1 to 4.
This function corresponds to the logical node
A51PTOC*
in IEC 61850.
The function monitors the primary system current using the 3 phase CT inputs e.g. CT1/2/3.
The basic time overcurrent protection function (51) is summarized below:
•
Current measurement is based on either fundamental or RMS current
•
A number of time versus current operate curves are selectable based on IEC and ANSI standards
•
Alternatively the 51 elements can operate with a definite time delayed (DTL) characteristic
•
Outputs are provided for each phase. A general pickup and general 51 output are also provided
•
Phase indications are provided
The basic function is non-directional, it can be directionalized this requires 3 phase VT inputs e.g. VT1/2/3.
Each 51 element can be set to operate for forward current, reverse current or independently of current direc-
tion. Directionalized functionality uses the decision of the 67 element.
•
Provides information regarding flow direction of phase current for selective application of overcurrent
protection
•
Ensures selective fault detection for parallel lines or transformers with infeed at one end
•
Ensures selective fault detection in cable runs with infeed at both ends or in lines connected to form ring
topologies
Where directionalized overcurrent protection is used correct operation is dependent on valid information
being received from the VT inputs. Operation of each directionalized element will be affected if a VT fuse fails.
When a fuse failure is detected, 60VTS function, each element can be selected to either ignore the VT failure,
switch to non‑directionalized operation (which does not require VT inputs) or be inhibited.
When installed in power transformer circuits increased currents may be measured by the overcurrent function
during switch in of the transformer. Each element can be set to be inhibited when inrush current is detected,
(see
6 Supervision Functions , section 81HB2 Inrush Current Detection).
Elements can be mapped to start
79 Automatic Reclosing
by selection of the element as a
79 PF
Trigger
within the 79 function element menus. For advanced autoreclose applications each element can be
selected to be inhibited during delayed autoreclose shots of a multi‑shot sequence.
Where significant distributed generation is installed in the power system network, load current may flow in
the reverse direction to normal operation. This is not fault current but may be in excess of low directionalized
overcurrent settings. Each element can be set to be inhibited during periods of low reverse load current flow,
(see
5 Protection and Automation Functions , section 21LB Load Blinder).
Voltage Controlled Overcurrent
Reduced fault current may flow where the driving fault voltage is reduced e.g. due to system impedances. The
overcurrent setting can be automatically reduced by the
51V Multiplier
when low voltage is measured at
the VT inputs.
5.14
5.14.1
5.14.2
Protection and Automation Functions
5.14 51 Time-Delayed Overcurrent – Phase
228 Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019