Selection of voltage level for MV cables | Eng-Tips

Hello,

When selecting cable for high current power supplies 500A+, and a specific voltage rating of e.g. 3.3kV.
The cable is between a converter and a large motor, so the system will have some switching dv/dT over voltages, but limited by a dv/dt filter.

Do you then select a 3,3kV cable or go up a size to 6,6kV to account for any over voltages in the system that could damage the cable insulation.
I've talked to both people who insist on choosing a voltage rating one larger than the rated voltage, and people who doesn't.

When looking at cable datasheets, the cables have been tested with a spark voltage test to a voltage 2-4x times higher than the voltage rating.
Is this test a guarantee of the cables over-voltage withstand capabilities?.

Preferably do you have any information/articles mentioning this? I cannot seem to locate any.

Thanks in advance!

Best regards Jens.
Jensdsds The most commonly used method (regardless whether it is "correct" or not) is to choose cable with the next higher commercially available voltage rating. Example 1 - maximum rated voltage at drive or motor end of connecting cable might be V. Next higher commercially available cable is 5 kV (although some places it might be V). Example 2 - maximum rated voltage (drive or motor) might be V. Next higher commercially available rating is 15 kV.

As a motor designer, I would choose to use cable (within my machine) rated for at least 1.7x the "nominal" drive output voltage, and preferably 2x. This is based on experience with the switching frequencies and dv/dt results at ALL operating speeds/frequencies (across multiple drive manufacturers). Drives are (usually) tuned to work well within a fairly narrow operating region; outside of that, the output waveform can be significantly distorted.

Converting energy to motion for more than half a century Gr8blu thanks a lot for your detailed answer and shared experience.
Your answer corresponds with what I've been hearing from other engineers. It could be great to figure out a way to calculate or show the reasons behind this choice. For an example when you have a 6.6kV system and the converter has a dv/dt filter limiting the dv/dt to <1.5kV/us. Or show what more could be done to ensure that a 6.6kV cable could be acceptable for a 6.6kV voltage rating.
Thanks in advance Jensdsds True - the converter filtering may limit max dv/dt to 1.5 kV/us. That means we could easily have 6.6 + (1.5) = 8.1 kV at any instant on the cable. Add in the likelihood of common mode voltage (which may be VERY significant) and we might end up around 9 kV or more.

As an example - I once had to diagnose insulation failure on a V motor. Turns out the drive was putting out about 1.5 pu "peaks" ... and the common mode voltage had the drive neutral plane about 14 kV away from the (solidly grounded) motor neutral. The insulation system within the machine was good for roughly 2x V - but it was exposed to upwards of 17 kV across the ground wall. No wonder the winding eventually let the smoke out!

As to cable rating: if you specify the drive output such that there is NO overshoot, the common mode voltage is a maximum of maybe 100 volts, and the recommended installation of the cable occurs without error ... maybe you get a V cable on a V drive/motor combination. Personally, I wouldn't be holding my breath waiting for it to happen.

Converting energy to motion for more than half a century

Good morning,

I am working on a project that involves sizing cable for a MV voltage motor (600 hp, v). When you are working on LV motors you can always find the current on the NEC table 430.50 and based on that you can select the cable size.

But there is no such a table for V and V motors. You can find vendor data for these motors but I am not sure if we can use these data for selecting cable size.

What do you guys recommend? Is there a full load amp table for MV motors to use for sizing cable similar to that of LV motors?

Thanks,

Good morning,

I am working on a project that involves sizing cable for a MV voltage motor (600 hp, v). When you are working on LV motors you can always find the current on the NEC table 430.50 and based on that you can select the cable size.

But there is no such a table for V and V motors. You can find vendor data for these motors but I am not sure if we can use these data for selecting cable size.

What do you guys recommend? Is there a full load amp table for MV motors to use for sizing cable similar to that of LV motors?

Thanks,

For cabling design, I always use the cable manufacturer's data and start to fit my choices from the cable manufacturer's published figures. Here are the important design checks I usually do when doing things above:

1. Check for the voltage drop at the motor terminals if acceptable. The efficiency of the motor is greatly impacted by the supply voltage.
2. Check for the voltage dip at the motor terminals during starting. Remember that the motor torque is directly proportional to the square of the voltage it receives.
3. Check for the operating temperature your selected cable, given the number of conductors per phase, ambient temperature of the cable location, the cable design temperature, the grouping factor and temperature derating factor.
4. Check if the cable insulation will not get damaged if ever an accidental short-circuit occurs at the motor terminals. This has something to do with how fast your protections are set to isolate your cable before the insulations melt.

Hope that helps.