Backup Time Overcurrent Protection; General - Siemens SIPROTEC Manual

Functions

2.16 Backup Time Overcurrent Protection

2.16 Backup Time Overcurrent Protection
The 7SD5 device features an integrated time overcurrent protection function. This function can optionally be
used either as back-up overcurrent protection or as emergency overcurrent protection. Please note that this
protection function is available in addition to the already available main protection functions, such as differential
and distance protection, to provide even more security.
2.16.1

General

While the line protection 7SD5 with configured differential protection can only operate properly if each device
correctly receives the data of the other devices, and while a distance protection can only operate properly if the
correct measuring voltage is applied to the device, the emergency time overcurrent protection only requires the
local currents. The emergency time overcurrent protection is automatically activated when the data communi-
cation of the differential protection is disturbed and the measuring voltage fails (emergency operation). Both
differential protection and distance protection are then blocked.
This means that emergency operation replaces the differential protection and/or the distance protection as
short-circuit protection if protection data communication fails and the distance protection working in parallel
detects a failure of the measuring voltages from one of the following conditions:
• The „Voltage transformer mcb tripped" signal is received via binary input, indicating that the measured
voltage signal is lost, or
• One of the internal monitoring functions (e.g. current sum, wire break or „Fuse-Failure-Monitor") is activated,
see Section 2.24.1.3.
The overcurent protection has a total of four stages for each phase current and four stages for the earth current,
these are:
• Two overcurrent stages with a definite time characteristic (O/C with DT),
• One overcurrent stage with inverse time characteristic (IDMT),
• A further overcurrent stage which has an additional enable input.
These four stages are independent of each other and are freely combinable. Blocking from external criteria via
binary inputs as well as instantaneous tripping is possible. During energization of the object to be protected
onto a fault, any stage - or several stages - can be switched to instantaneous tripping. If not all stages are re-
quired, the ones not needed can be deactivated by setting the pickup value to ∞.
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SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011