123MB
Electrical
- Apr 25, 2008
- 265
Hi All
I am struggling to provide some installation recommendations for a project I have designed which includes 3-off 315kW VSD. The project is in Australia - utilising IEC standards. The relevant data for this problem is as follows:
1) 3-off 8 pole 315kW 570A 3 phase non-inverter rated new motors
2) Run length 60m from VSD to motor
3) The installer would like to use single core screened cables to the motor. These will be 2x150mm2 X-90 insulated PVC sheathed Al tape and braid screened per phase.
5) The installer would like to arrange the cables in trefoil groups - so there will be 6 trefoil groups - each group spaced by 1 cable diameter. The installer would like to install all 6 trefoil groups on the same cable ladder.
4) For each drive, the installer would like to use a single earth cable from the VSD to the motor
5) For each drive there will be an isolator between the drive and motor. The cable screen will be continuous through the isolator, probably clamped to a backplate which will be isolated from the safety earth.
I am concerned with:
1) Mutual inductance between each trefoil group and cables in the group inducing currents in the shields of the other groups and possibly inducing voltages in the conductors - the situation made worse by the absence of an overall shield on each trefoil group.
2) The radiated noise caused by each of the three single cores in each trefoil group and how the noise will not be cancelled in the screens of the singles of the proposed cables - like it would be in an overall screened 3C + 3E cable.
3) Currents being induced in the shield which will reduce the current carrying capacity of the conductors
4) The motor is not inverter rated - however it does have reasonably good insulation (1350V peak, dv/dt 1600V/us) and we have done a reflected wave simulation on the system that seems okay.
5) Increased common mode voltage and the affect this will have on the non inverted rated motor which does not have insulated bearings, etc.
I am struggling to give the installer some recommendations on how to install these cables in order to minimise the issues that I have described about (and I'm sure there are other issues I haven't thought about!). The installer has his mind set on using single cables (i.e. not a 3C+3E) and I do not think I can sway him on this issue unless I have some hard evidence there will be problems.
Things I have considered are:
1) Recommending the feed to each motor is on a separate cable ladder. There will be three ladders stacked at 300mm spacing
2) Recommending the installer lays the single trefoil groups in a pseudo symmetrical arrangement - i.e. 3 singles with 3 earths
3) Recommending the conductors are oversized (but how much)?
4) Recommending the HF noise return path to the VSD from the motor is excellent with good HF bonding practices being used
But that is all I have been able to think of - can anyone think of any more/better recommendations that are hopefully going to save this installation from what is threatening to be a pretty dire situation.
Cheers guys
I am struggling to provide some installation recommendations for a project I have designed which includes 3-off 315kW VSD. The project is in Australia - utilising IEC standards. The relevant data for this problem is as follows:
1) 3-off 8 pole 315kW 570A 3 phase non-inverter rated new motors
2) Run length 60m from VSD to motor
3) The installer would like to use single core screened cables to the motor. These will be 2x150mm2 X-90 insulated PVC sheathed Al tape and braid screened per phase.
5) The installer would like to arrange the cables in trefoil groups - so there will be 6 trefoil groups - each group spaced by 1 cable diameter. The installer would like to install all 6 trefoil groups on the same cable ladder.
4) For each drive, the installer would like to use a single earth cable from the VSD to the motor
5) For each drive there will be an isolator between the drive and motor. The cable screen will be continuous through the isolator, probably clamped to a backplate which will be isolated from the safety earth.
I am concerned with:
1) Mutual inductance between each trefoil group and cables in the group inducing currents in the shields of the other groups and possibly inducing voltages in the conductors - the situation made worse by the absence of an overall shield on each trefoil group.
2) The radiated noise caused by each of the three single cores in each trefoil group and how the noise will not be cancelled in the screens of the singles of the proposed cables - like it would be in an overall screened 3C + 3E cable.
3) Currents being induced in the shield which will reduce the current carrying capacity of the conductors
4) The motor is not inverter rated - however it does have reasonably good insulation (1350V peak, dv/dt 1600V/us) and we have done a reflected wave simulation on the system that seems okay.
5) Increased common mode voltage and the affect this will have on the non inverted rated motor which does not have insulated bearings, etc.
I am struggling to give the installer some recommendations on how to install these cables in order to minimise the issues that I have described about (and I'm sure there are other issues I haven't thought about!). The installer has his mind set on using single cables (i.e. not a 3C+3E) and I do not think I can sway him on this issue unless I have some hard evidence there will be problems.
Things I have considered are:
1) Recommending the feed to each motor is on a separate cable ladder. There will be three ladders stacked at 300mm spacing
2) Recommending the installer lays the single trefoil groups in a pseudo symmetrical arrangement - i.e. 3 singles with 3 earths
3) Recommending the conductors are oversized (but how much)?
4) Recommending the HF noise return path to the VSD from the motor is excellent with good HF bonding practices being used
But that is all I have been able to think of - can anyone think of any more/better recommendations that are hopefully going to save this installation from what is threatening to be a pretty dire situation.
Cheers guys