Section 2 Application
2.1 General IED application
SEMOD121007-5 v8
The Intelligent Electronic Device (IED) is designed for the selective, reliable and fast differential
protection of busbars, T-connections and meshed corners in up to 6 zones. It can be used for
protection of single, double and triple busbar with or without transfer bus, double circuit
breaker or one-and-half circuit breaker stations. The IED is applicable for the protection of
medium voltage (MV), high voltage (HV) and extra high voltage (EHV) installations at a power
system frequency of 50Hz or 60Hz. It can detect all types of internal phase-to-phase and
phase-to-earth faults in solidly earthed or low impedance earthed power systems, as well as all
internal multi-phase faults in isolated or high-impedance earthed power systems.
Ordering of VT inputs inside of the busbar protection IED will allow integration of voltage
related functionality like under-voltage release, residual over-voltage, power functions,
metering and voltage recording during the faults. However attention shall be given to the fact
that inclusion of VT inputs will reduce number of available CT inputs (in total 24 analogue
inputs are the product limit). Consequently when VT inputs are ordered the busbar protection
IED will be applicable for buses with a fewer number of bays. Practically the number of
available CT inputs will limit the size of the station which can be protected.
The IED has very low requirements on the main current transformers (that is, CTs) and no
interposing current transformers are necessary. For all applications, it is possible to include
and mix main CTs with 1A and 5A rated secondary current within the same protection zone.
Typically, CTs with up to 10:1 ratio difference can be used within the same differential
protection zone. Adjustment for different main CT ratios is achieved numerically by a
parameter setting.
The numerical, low-impedance differential protection function is designed for fast and
selective protection for faults within protected zone. All connected CT inputs are provided
with a restraint feature. The minimum pick-up value for the differential current is set to give a
suitable sensitivity for all internal faults. The pick-up setting for the fault current must be less
than 80% of the lowest fault current that can occur on the protected bus bars. If the minimum
fault current is high enough, the set value should be set higher than the maximum load current.
This setting is made directly in primary amperes. The operating slope for the differential
operating characteristic is fixed to 53% in the algorithm.
The fast tripping time (minimum trip time is 5ms when SOM output module is used) of the
low-impedance differential protection function is especially advantageous for power system
networks with high fault levels or where fast fault clearance is required for power system
stability.
All CT inputs are provided with a restraint feature. The operation is based on the well-proven
RADSS percentage restraint stabilization principle, with an extra stabilization feature to
stabilize for very heavy CT saturation. Stability for external faults is guaranteed if a CT is not
saturated for at least two milliseconds during each power system cycle.
The advanced open CT detection algorithm detects instantly the open CT secondary circuits
and prevents differential protection operation without any need for additional check zone.
Differential protection zones in the IED include a sensitive operational level. This sensitive
operational level is designed to be able to detect internal busbar earth faults in low impedance
earthed power systems (that is, power systems where the earth-fault current is limited to a
certain level, typically between 300A and 2000A primary by a neutral point reactor or resistor).
1MRK 505 370-UEN D Section 2
Application
Busbar protection REB670 29
Application manual
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