Voltage Drop Motor Starting

[maze02]

Hi All,

I'm turning to this forum hoping to get some opinions and hopefully learn something new.

I have a power system study I'm currently working on that involves determing the % voltage drop when starting the largest motor (125HP) in the system. I have done some in the past with no issues but this one has some weired results I'm hoping someone here might have an answer to or share their experience with a similar issue.

A brief summary of the system is as follows:

-There are two power sources via an ATS - 1 utility and 1 gen, the gen is a 320kW and the utility source is via a 750kVA XFMR and both are supplying the system @480V. The largest loads are 3 motors (a 125HP and 2 60HP) in the system fed from VFDs without by-pass contactors.:happyno:

In the past studies I've done, there were no VFDs involved just regular DOL starters and in one case a RVSS which I had to do a transient motor analysis rather than a static one.

Procedure/Issue:

The procedure to run the LF with motor starting is to first run the analysis with all motors running (depending on the system and specs) to determine the %VD at the gen or utuility bus and then adjust it to be close to the rated voltage as possible with the assumption that the utility is supplying power at rated voltage at the service entrance point (XFMR primary is my preference) or in the case of the gen that it has a perfect exciter and voltage regulator to maintain the volatge at 480V. In this case, the sofware (PTW) has options to adjust the "Initial Operating Conditions" to achieve this. However, when the initial conditions are adjusted such that the %VD at the gen bus/terminal is 0%VD, the motor switched to "starting", and analysis rerun the new %VD at majority of the busses are negative implying that the motor is generating some extra power and returning it back into the system. This is opposite to what one should expect. I can only attribute this to the VFD the motor is connected to since VFDs do not present the inrush issues associated with DOLS, RVSS, and Soft Starters but I'm not sure how the software handles this. I read a little bit on VFDs with Regen capabilities which the software also considers or at least gives you the option to select but this doesn't seem to change the results regardles of the option chosen. Another thing I noticed is that the software has an option to adjust the "Initial Operating Conditions" specific to the VFD on the VFD page options. However, when this is adjusted based on the calculattion ; pu=Bus voltage/480V (not sure if this is the right way) there's no signifiact improvement.

I'm relative new to this and not too familiar with VFDs but I'm also willing to learn. I'll appreciate all comments/suggestions I can get.

Thanks.

 

[topgone]

You said you have an ATS and the sources are a 320kW genny and a 750kVA trafo. Then you went forward telling us "both" are supplying your loads!:happysad:
Which is which? Your ATS will choose either source, not both. That's what ATS do-->automatic transfer switching!

 

[JRW 70]

Range of Inputs

Range of Inputs

If you set the v.d. To zero, I imagine that the
calculation goes out of range. Try 0.01 or some
small fraction. The calcs. will not blow up, and all
non superconductors will have v.d.
Try to reframe the objective here and go
with better assumptions. The resistance of copper
and aluminum are both widely available.

As a side note, your 750kVA transformer sounds good,
with that load the service should be very solid.

JR

 

[maze02]

You said you have an ATS and the sources are a 320kW genny and a 750kVA trafo. Then you went forward telling us "both" are supplying your loads!:happysad:
Which is which? Your ATS will choose either source, not both. That's what ATS do-->automatic transfer switching!

You can only have one source (either utility or gen) feeding the loads at any given time. Sorry if that wasn't clear earlier.

 

[kingpb]

I would select actual voltages to be viewed within the results and not percentages. Not saying this will change the results, but actual numbers may help pinpoint the problem area.

CSI drive should absorb power from motor to drive, it should not appear on the power system.

The next step would be to take the VFD out of the circuit, and start the motor DOL and see if the results match more closely. If not, you know something else is the issue.

Use a process of elimination by simplifying, then build back up to see where it blows up.

These programs are only as good as the information provided.

 

[jcormack]

Maze, I am not familiar with the software you listed, but the approach I would take is to simply look at voltage drop with a higher load - You stated that all three motor loads are VFD supplied - as you stated you really don't have the inrush issue with a VFD - but it typically has a current limit - what are the drive(s) current limits set at 150%? 125%? High torque -vs- variable torque app? You could simply model the system with all loads but the 125 hp @ 100% and put a 200 hp motor (slightly over 150%) at FLA in place of the 125 and do the load-flow again, this would tell you what the system would look like when the 125 hp VFD hits current limit. I would though be concerned about the THD when running on the genset though - 360 KW is smaller than I would use with that much VFD hp, the THD & voltage drop on the gen system at that VFD content may cause issues- although if your apps are low torque fans/pumps you may be OK - if you have hoists/elevators/conveyors, ie high torque apps with 180-200% current limits the generator may complain. I would normally size one at least twice the VFD total (500 kw in this case), or even higher depending on the load requirements. You did not state if these were just diode front-end or active front-end (full regen) drives - or, do they have dc-buss brake resistors?