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Can Sepam be used to protect a motor that is powered by a variable speed or variable frequency drive (VSD or VFD)?

It is very common to see low-voltage and medium-voltage motors that are powered by a variable-speed or variable-frequency drive.

Customers have previously asked if Sepam relays can be used at the output of the VSD to protect the motor that is supplied by the drive.

Generally speaking, Sepam protection relays are designed to work at the nominal power system frequency.  This is a fixed setting of 50Hz or 60Hz that should not be changed once the relay has been commissioned and is in service.

Although the Frequency Measurement and Frequency Tracking algorithms are specified to work in the range from 25Hz to 65Hz, the performance of the individual protection functions (e.g., ANSI 50/51) is only tested for faults at the nominal power system frequency.

For this reason, we do not recommend to use Sepam at the output of a VSD or VFD, where the frequency of the voltage and/or current input signals may not be at the System Frequency.  Another situation to avoid is the case where the VT's of the Sepam are measuring a voltage that is always at Nominal Frequency, but the CT's are installed downstream of a VSD, so that the frequency of the measured current will sometimes not be at the Nominal Frequency.  In this case, unexpected behaviour of the protection functions may result.


To summarize, it is preferable to use a relay that is dedicated for this purpose, or to use the protection functions embedded within the VSD or VFD itself, instead of using the Sepam to protect the motor.  The Sepam can be used upstream of the VSD to protect against any high-magnitude faults resulting from damage inside the VFD.


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