690V Motor Cable Insulation off Microgrid 1

[HenryOhm]

Hi All,

I have worked with two large system integrators on 4-5 different projects the last few years. They have competing views of cable insulation for 690V motors operating off VFD. One recommends a specific VFD cable with 2000V rating, three symmetrical ground wires, shield, etc. The other recommends just 600/1000V rated cable. The latter also lists a variety of ground wire/shield options. These are their general recommendations irrespective of cable distance or other more specific factors. The latter does use VFD rated motors, of course, good for at least 2kV per the motor spec sheets.

It seems to me that VFD dV/dt issues, reflected wave, etc would be better understood by now. Are there standards, either IEEE or IEC that would better define some of this? Also, these projects are microgrid in nature with approx 1000VDC main bus. I believe these main buses may be either floating or high-resistance grounded (?) so I worry about motor cable that is rated 600V to ground.

There was some good info in this post (however it's 12 years old, not conclusive and with broken links):

https://www.eng-tips.com/viewthread.cfm?qid=277339

Thanks for any and all help in advance!

 

[Gr8blu]

HenryOhm: The problem lies in two things. First, the quality (and cost) of the drive. Second, the quality (and cost) of the cable. The drive has a switching frequency that is MUCH higher than line frequency - which means any conductor it passes power through has a MUCH higher impedance. This all boils down to the reflected wave phenomenon from mismatched impedance, by the way. A cheaper/noisier drive will require a better (and larger, and more expensive) conductor than a good drive.

Line voltage is not the only thing to worry about, either. There is also common mode voltage - which may be as much as two orders of magnitude higher than the actual line voltage. That, plus the "spike" energy from overshoot on the drive waveform creates the requirement for what looks like overkill on the conductor insulation.

In reality, the method of system grounding and the length of conductor from source to load need to be factored into the type of conductor used as well as its insulation method. For some things, cable (even multi-core/multi-phase) is not the best answer.

If the second guy is quoting you something that bare bones, chances are the specification wasn't as clear-cut as you'd like to believe.

Converting energy to motion for more than half a century