It will be used where normal overcurrent relay cannot be set properly.
Over load current, and fault current discrimination can be done more accuratly and precisely by using these type of relays between phase to neutral (R,Y,B) and neutral to ground connection, for large machines, equipments and power systems.
In some situations the short circuit current could be less than load current.
Consider a synchronous condenser (for simplicity non salient ignoring controls
such as field forcing and transients only steady state) the with synchronous
reactance xs we get ss operation leading power factor rated current
Eg = Vt - Ir*xs where Eg is generated voltage and Vt is network or Bus
voltage . Notice Eg is less than Vt. When a short occurs at the terminals
assuming Eg is kept constant the steady state Ishort =Eg/Xs is less than Ir
so we need to shift the operating characteristic of OCR to lower values.Of course
this is just an example and in an actual system there would be many other
aspects such as speed of CB's transient short cct currents etc to take into
account.
Cheers
- Badges
- Science topic
- Similar topics
- Biological Science
- Bioecology
- Systems Ecology
- Landscape Ecology
- Connectivity