VFD starting
VFD starting
(OP)
I am designing a pipeline project, where the system is very weak, so that we have to use VFD to start the biggest motor(5000KW).
Utility company informed the client that even VFD starting would cause voltage drop on the consumer bus and on the utility bus, which would exceed the flicker limits.
But I did load flow study. with all the loads running, the volatge drop is only 3%. There is no voltage drop on utility side.
Since while VFD starting, the power factor is 0.95 and current is limited only around 100%FLA. Theoretically, the voltage drop should be the same as under normal running condition.
However, ETAP gave me the different results. Whenever I started motor by 100% or higher, i always got much higher voltage dip on the system, which didn't make sense to me.
I presumed that ETAP cann't model VFD starting properly.
Any inputs would be appreciated.
Utility company informed the client that even VFD starting would cause voltage drop on the consumer bus and on the utility bus, which would exceed the flicker limits.
But I did load flow study. with all the loads running, the volatge drop is only 3%. There is no voltage drop on utility side.
Since while VFD starting, the power factor is 0.95 and current is limited only around 100%FLA. Theoretically, the voltage drop should be the same as under normal running condition.
However, ETAP gave me the different results. Whenever I started motor by 100% or higher, i always got much higher voltage dip on the system, which didn't make sense to me.
I presumed that ETAP cann't model VFD starting properly.
Any inputs would be appreciated.





RE: VFD starting
But I'm a little suspicious when you say that "there is no voltage drop on the utility side". There is always voltage drop when you have current flowing. Are you sure you are not looking at the swing bus? Its voltage will be artificially constant due to the limitation of the power flow reduction method.
If you are buying a 5000 kW AFD, the potential vendors will be happy to work with you to estimate the line current demand during starting and probably do the power flow calcs for you. Their estimates will be more accurate than the generic ETAP data you are using.
RE: VFD starting
RE: VFD starting
RE: VFD starting
I remodeled the system again and found out that the voltage dip while starting motor (by VFD) is the same as under normal running condition, which will exceed the utility flicker limits.
This means, the system is too weak to handle such a big load, unless the utility build another transmission line.
DPC. It is a centrifugal pump.
RE: VFD starting
RE: VFD starting
Actually it is not the case, when the system is weak, there is always a high impedance on Utility side, which will cause high voltage drop in the system even under normal running condition.
what else we can do to solve this problem?
RE: VFD starting
This is not a question of being "reliable" - it just the way the load flow calculation works. There has to be at least one bus that is designated to have a constant voltage and capable of providing/absorbing all necessary watts and vars.
Virtually all power flow programs work this way.
Are you talking about the ETAP problem or the actual problem?
RE: VFD starting
Thanks a lot. Could you do me a favor to verify the load flow calculation result? This is extremely important to us.
Our system is configued like this,
Utility supply, 25KV, 80MVA S-C
Transformer 15MVA: Z=%7 25V/6.9kV
Running load: 13MVA
It is a radial system. Any utility said they couldn't supply a strong power supply.
The voltage drop on 6.9KV bus I got is 20% under normal running condition.
RE: VFD starting
Like dpc, with such large size machine OEM's technical support unwaveringly helps you out on the mess. They'll provide you with what you are doing this time, accuratetly rather than ETAP.
The only problem that it remains unsolved is your load flow analysis.
RE: VFD starting
I ran it in EasyPower and got about 2.5% drop at a load pf of 0.95. As the power factor decreases, the voltage drop increases.
Are you sure you've got the transformer data entered correctly - like maybe MVA instead of kVA?
You can deal with the voltage drop by means of transformer taps, including possibly a load tap changer if necessary.
But your transformer may be a bit small if your load includes this big AFD.
Maybe it's time to call in some grey-haired folks who are used to dealing with this type of situation.
RE: VFD starting
If the VFD causes a 2.5% voltage drop while running, there will be significant voltage waveform distortion and this may cause other problems.
The starting current of a pump on a VFD should not exceed the running current except where there is extremely fast acceleration, so the issue is how the supply performs with the VFD under full load conditions.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: VFD starting
What I assuemd is the infinite swing bus(voltage constant)at the utility power supply side, not on the primary of the transformer. If you add another impedance after this utility bus, you will get a higher voltage dip because of S-C impedance from the utility.
We are in the process of investigating how weak the power supply is because our loading to see what the utility can do for us.
Tap changer cann't help for the primary side of the transformer.
RE: VFD starting
There's no way you are going to have 20% voltage drop for a normal running condition.
With all due respect, I think you need to get some direct assistance from a EE with some experience doing power system studies.
RE: VFD starting
Thanks for your suggestion.
The actual situation is, the power supply is so weak that we will see a higher voltage dip at the Tie-in side.
We will talk to the utility guys and get them to look at the possibilty of boosting their supply voltage level.
RE: VFD starting
Once synchronised the synchronous motor will stiffen the system, you can specifiy the motor to have a leading power factor and maybe sell them some vars if the system is that weak. It will cost more than an induction machine but will be much cheaper than any improvements to their system.
If they wont allow the AFD, you could use a pony motor and a fluid coupling with a synchronous motor.
RE: VFD starting
In our case, we don't have any synchronous motor.
We won't have any motor starting problem with VFD. The only problem is the voltage dip during normal running. We are trying to convince the utility company to elevate their supply volatge to suit the cutomer's need.
RE: VFD starting
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Sometimes I only open my mouth to swap feet...
RE: VFD starting
RE: VFD starting