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Motor starting model

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.

RE: Motor starting model

I don't do studies on ETAP and I probably don't undeerstand your question.

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

(OP)
Thank you.
That's what I modeled in ETAP.

RE: Motor starting model

The LTC will not help prevent the initial voltage dip caused during motor starting - it is too slow.  It's doubtful if it will do much at all, unless the motor starting time is long and the LTC response time is fast.  

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

An LTC fast enough to help in motor starting would have a life time measured in weeks or months, not decades.  The LTC will be part of what establishes your initial voltage, but from then on forget about the LTC.  If the motor is large enough compared to the rest of the load on the LTC, the LTC may respond to the voltage drop caused by the running current of the motor.

RE: Motor starting model

The transformer model in ETAP allows both a 'Fixed Tap' setting, and an 'LTC/Voltage Regulator' setting.

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

LTC is fine for a load flow study, or anything else that is at least quasi-steady-state.  For things like motor starting and fault studies, the LTC needs to be left where the load flow study set it.  A real LTC just won't respond in less than several seconds.

RE: Motor starting model

davidbeach may be correct, but just to reiterate, ETAP will allow you to model the complete system as it is designed (or in any revision or configuration of any settings).

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

Good point Dan.

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

by the way, that 20 seconds is a DOL start

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RE: Motor starting model

Ouch!

RE: Motor starting model

(OP)
DanDel,

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

What different results are you talking about? Obviously, if you set the Fixed Tap to something other than nominal, you will get different results.
The Fixed Tap is supposed to represent the actual fixed tap setting of the transformer.

RE: Motor starting model

(OP)
You were saying that Fixed Tap determines the actual tap setting of the transfomer,reagrdless of LTC setting?  

RE: Motor starting model

No, the Fixed Tap can be used along with the LTC setting. A transformer can have both. If the Fixed Tap is used for a off-nominal voltage setting, that is now the base transformer ratio.
Then the LTC (which can be set for either the primary or the secondary winding) will change on that basis.

RE: Motor starting model

(OP)
Thanks. Danbel

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