ABB Relion REQ650, Applications Manual

Page: 53 / 340

3.1.5 Calculating settings for four step phase overcurrent
protection 3-phase output I> OC4PTOC
The purpose of the delayed phase overcurrent protection is:
• Backup protection for short circuits on the whole line length
• Backup protection for short circuits in the remote busbar
• Backup protection for short circuits in other lines from the remote substation or
the own substation, when possible
The reach of the phase overcurrent line protection is dependent of the operation state
and the fault type. Therefore, the setting must be based on fault calculations made for
different faults, fault points and switching states in the network. Although it is possible
to make hand calculations of the different faults, it is recommended to use computer
based fault calculations. Due to different practices, different time delay principles can
be used.
The following principle for the phase overcurrent protection is recommended:
• One delayed step used as line backup protection. It should be set to assure
selectivity to line protections. Directional function forward is used.
• One delayed step used as backup protection for the own busbar. It should be set
to assure selectivity to line protections. Directional function reverse is used.
• A long time delayed step covering further parts of the system. This step should
have a time delay longer than 1.0 s. Non-directional function is used.
3.1.5.1 Calculating general settings
1. Set GlobalBaseSel to 1 .
The settings are made in primary values. These values are given in the base
settings in Global base 1.
3.1.5.2 Calculating settings for step 1
The protection shall be able to detect all short circuits within the defined protected
zone. In this case, it is required that the protection shall detect phase-to-phase short
circuit at the most remote point of the protected line. A two-phase short circuit at the
remote line end and high source impedance, as shown in figure
13 , results in the
following fault current for the protection:
1MRK 505 291-UEN A
Section 3
REQ650 setting examples
47
Application manual