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MV Switchgear and Soft Starter Protection Relays
Q: I have a situation here where there is both stall/locked rotor protection relays installed in the 6kV switchgear and the soft starter for the 6kV motor. Would it be appropriate that the 48/51LR relay of the MV switchgear be disabled because the soft starter already has that same protection for the motor? Or is it necessary that both LR be enabled? What could be the implications of each setup? I have some opinion on this but others could have experience on similar scenario.
Original design involved direct line starting but upon evaluation, soft starter was selected. We acknowledged that electronic starting introduces additional possible fail points or requires fine tuning to achieve coordination. We have an intention to make the switchgear protection relay as backup.
A: Whether or not to use the switchgear relay will ultimately be your choice, what I would take into consideration are the following:
As an older engineer, who grew up with direct-on-line, star delta, Korndorffer (closed transition auto transformer) and rotor resistance, I would question why use a soft starter have you considered the standard starters, the one that comes to mind for MV motors is Korndorffer, reading the various discussions on LinkedIn it seems to me to put soft start or any other electronic starting system is much more liable to failure than standard starters. The only time I would consider soft start is if the power supply was so weak that the standard starters could not be used without inflicting excessive volt drops on the supply network.
Original design involved direct line starting but upon evaluation, soft starter was selected. We acknowledged that electronic starting introduces additional possible fail points or requires fine tuning to achieve coordination. We have an intention to make the switchgear protection relay as backup.
A: Whether or not to use the switchgear relay will ultimately be your choice, what I would take into consideration are the following:
- Does the soft starter protection cause the switchgear to trip on fault, if not then I would incorporate the switchgear relay into the protection scheme.
- Is the switchgear protection connected to the soft start controls to prevent the soft starter trying to start when the trip has not been reset. If not affirmative then further system integration will be required.
As an older engineer, who grew up with direct-on-line, star delta, Korndorffer (closed transition auto transformer) and rotor resistance, I would question why use a soft starter have you considered the standard starters, the one that comes to mind for MV motors is Korndorffer, reading the various discussions on LinkedIn it seems to me to put soft start or any other electronic starting system is much more liable to failure than standard starters. The only time I would consider soft start is if the power supply was so weak that the standard starters could not be used without inflicting excessive volt drops on the supply network.
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