Inverse Time Overcurrent Elements 51-1, 51-2, 51-3, 51-4 And 51N-1, 51N-2, 51N-3, 51N-4 - Siemens 7SC80 Manual

Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
Ferraris disk and the time response is adapted. Reset begins as soon as 90 % of the setting value is undershot,
in accordance to 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 the decrementing processes
are in idle state.
Disk emulation offers advantages when the overcurrent relay elements must be coordinated with conven-
tional electromechanical overcurrent relays located towards the source.
For recloser characteristic curves, an element drops out immediately after the value has fallen below a
threshold.
User-defined Characteristics
When user-defined characteristic are used, the tripping curve may be defined point by point. Up to 20 value
pairs (current, time) may be entered. The device then approximates the characteristic, using linear interpola-
tion.
The dropout curve may be user-defined as well. See dropout behavior for ANSI and IEC curves in the function
description. If no user-defined dropout curve is required, the element drops out as soon as the respective
current falls below approx. 95% of the set pickup value. When a new pickup is evoked, the timer starts at zero
again.
Inverse Time Overcurrent Elements 51-1, 51-2, 51-3, 51-4 and 51N-1, 51N-2,
2.2.5
51N-3, 51N-4
There are 4 inverse elements for Recloser. You can pre-define these 4 inverse elements in the overcurrent
protection independently without any type restriction, then assign them in each recloser cycle.
In Device Configuration, in Parameter address 112 Charac. Phase and 113 Charac. Ground, choose All
Curves in the drop-down list, then four inverse elements in Overcurrent Protection are available. The 4
elements follow all the features of the element in previous version, such as, inrush, CLP, etc. 51-1, 51-2, 51-3,
51-4 can also support 51- V, which is the unique feature for 51. The 4 elements can work independently. Only
the first inverse element is available for cold load pick-up in function 50/51 and 50N/51N, even though the
function is enabled All Curves and has 4 inverse elements.
Moreover, when the pickup value of any element is set to ∞, the element will be disabled. In addition, the 4
elements don't support the user-defined function.
Pickup Behavior
For each element, an individual pickup value 51-1 PICKUP, 51-2 PICKUP, 51-3 PICKUP, 51-4 PICKUP
or 51N-1 PICKUP, 51N-2 PICKUP, 51N-3 PICKUP, 51N-4 PICKUP is set. Apart from Fundamental, the
True RMS can also be measured. Each phase and ground current is separately compared with the setting
value per element. If a current exceeds 1.1 times of the setting value, the corresponding element picks up and
is signaled individually. If the inrush restraint function is used, either the normal pickup signals or the corre-
sponding inrush signals are issued as long as inrush current is detected. If the element picks up, the tripping
time is calculated from the actual fault current flowing, using an integrating method of measurement. 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 the inrush restraint function
is enabled and an inrush condition exists, no tripping takes place but a message is issued indicating when the
overcurrent element time delay elapses.
These elements can be blocked by the automatic reclosing feature (79 AR).
For ground current elements, the characteristic may be selected independently of the characteristic used for
phase currents.
Pickup values of elements and the relevant time multiplicators may be set individually
The following two figures show the logic diagrams for the 51-1 and 51N-1 inverse time overcurrent protection
respectively.
78 SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017