Motor starting model
Motor starting model
(OP)
I am doing a motor starting study by ETAP. We have a 10MVA transformer with fixed Tap and on load tap. I am wondering if we can set Fix tap and automatic On load tap simutanneously. I remember that only one can function. If we set LTC, Fixed tap won't work. Is it right? Can they combine to work somehow, like ETAP allows?
Thank you in advance.
Thank you in advance.





RE: Motor starting model
A transformer with an LTC can either be set to a fixed tap position with no automatic response, or to an automatic control mode which controls the tap position based on the controlled voltage.
It cannot be set to both at the same time.
I apologize if I have misunderstood your question.
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RE: Motor starting model
That's what I modeled in ETAP.
RE: Motor starting model
You are better off modeling as a fixed tap based on its position determined by a power flow simulation without the motor running. This will correspond more closely to the actual situation.
RE: Motor starting model
RE: Motor starting model
The 'Fixed Tap' is the non-Load Tap Changer setting, which, in the actual unit, would require a transformer shutdown to change. Pretty much all transformers have this type of tap changer.
The 'LTC/Voltage Regulator' setting allows you to model a Load Tap Changer with a voltage regulator, specifying the min and max taps in percent, number of taps, voltage control bands, and time delays for Initial and Operating tap changes.
You can model both simultaneously, as both can exist in the same system simultaneously. But the 'Fixed' won't change at all, just like in the actual system. The LTC will change taps according to the modeled parameters.
RE: Motor starting model
RE: Motor starting model
If you use the correct parameters for the LTC, the model will respond the same way as the actual system. I.E., if the motor starting voltage drop (below the set voltage control band) lasts for longer than the set time of the LTC, then the LTC will change taps. This may or may not be healthy for the system, but that is why analysis software is used.
RE: Motor starting model
We have 8000 hp 13.2kv motors with flywheels that take approx 20 seconds to come up to speed.
More likely a given LTC would respond to that type of motor start than a 0.5-second motor start. If the model includes the LTC dynamics, then let the model figure it out.
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RE: Motor starting model
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RE: Motor starting model
RE: Motor starting model
You are right. ETAP allows us to model both a 'Fixed Tap' setting, and an 'LTC/Voltage Regulator' setting.
But in the real world,the 'Fixed' won't change at all, the LTC will change taps according to the modeled parameters.But if you set both, ETAP will give us different results. So we should be really watchful about the settings.
We shouldn't set up both at the same time.
RE: Motor starting model
The Fixed Tap is supposed to represent the actual fixed tap setting of the transformer.
RE: Motor starting model
RE: Motor starting model
Then the LTC (which can be set for either the primary or the secondary winding) will change on that basis.
RE: Motor starting model