123MB
Electrical
- Apr 25, 2008
- 265
Hi All,
I'm a mid level Electrical Engineer with mostly LV experience that's been asked to do a small amount of design work for a MV job.
The task is very simple - design of a feeder cable for supply of a 6.6kV VSD (Siemens Perfect Harmony drive)
I am struggling with overload protection of this cable. Ordinarily in LV design I am used to simply sizing the cable so that it is protected by the breaker. In this case there is no breaker - there is a contactor that is controlled by the VFD. The VFD opens the contactor when it's protection settings trip.
So the VFD will have some overload settings, and it will open the contactor when it trips. Is it common practice for me to size the cable overload protection on this overload setting? Is the combination of VFD + contactor equivalent to a circuit breaker in best practice?
What I would do for an LV drive normally is to size the cable to the VSD full load amps with some margin (%20)... but in some cases we are using large drives on small motors and it would not be accepted (from a cost point of view) to size the cable to the VSD full load amps. Even if I wanted to do it that way, I can't find what the full load amps of this damn VSD is! (Siemens perfect harmony drive)... All I can find in the manual is the recommendation that the cable be sized to the rating of the input transformer
Yeah so in summary I am confused. It doesn't make it any easier that here in Australia there aren't many standards for MV/HV installations.
If anyone can give me some advice I would appreciate it! Thanks
I'm a mid level Electrical Engineer with mostly LV experience that's been asked to do a small amount of design work for a MV job.
The task is very simple - design of a feeder cable for supply of a 6.6kV VSD (Siemens Perfect Harmony drive)
I am struggling with overload protection of this cable. Ordinarily in LV design I am used to simply sizing the cable so that it is protected by the breaker. In this case there is no breaker - there is a contactor that is controlled by the VFD. The VFD opens the contactor when it's protection settings trip.
So the VFD will have some overload settings, and it will open the contactor when it trips. Is it common practice for me to size the cable overload protection on this overload setting? Is the combination of VFD + contactor equivalent to a circuit breaker in best practice?
What I would do for an LV drive normally is to size the cable to the VSD full load amps with some margin (%20)... but in some cases we are using large drives on small motors and it would not be accepted (from a cost point of view) to size the cable to the VSD full load amps. Even if I wanted to do it that way, I can't find what the full load amps of this damn VSD is! (Siemens perfect harmony drive)... All I can find in the manual is the recommendation that the cable be sized to the rating of the input transformer
Yeah so in summary I am confused. It doesn't make it any easier that here in Australia there aren't many standards for MV/HV installations.
If anyone can give me some advice I would appreciate it! Thanks