Transformer Tap setting 2

[SELEC]

Hi,Everybody,

I am running ETAP motor starting study. I set the fixed tap at -10% on Primary side,in the mean time, I set up Auto LTC at 100%. In this case, I got the minimum voltage dip compared to other cases. Does the fixed tap setting affect the voltage dip while the Auto LTC is also in service? According ETAP, it does. But I am not sure it is a real situation. I don't have much experience in transformer setting. Can anybody here explain this to me better?

Thanks a lot in advance.

 

[davidbeach]

In real life, an LTC won't enter the picture on motor starting. Motor starting transients are very fast compared to the deliberately slow response of an LTC. You've got to pick an operating point and stick with it for the motor starting study.

 

[SELEC]

I talked to the Transforemr manufacturer this morning. They told me thansition period of tap changing could be 2 seconds which would show on the motor starting picture.

 

[davidbeach]

Sure, after 2 seconds you aren't back down to FLA yet, but you are likely to be way past the worst of the inrush currents. Your long term results will be much better if you don't rely on a transformer LTC to help solve a motor starting voltage dip problem. If a transformer LTC does respond to a motor starting voltage dip, you then have an overvoltage condition following the motor start until the LTC comes back down again. Better that the LTC slow down a bit and ignore the motors.

 

[SELEC]

Under the normal conditions, we use VFD to start the motors. We just need to start the motors across the line under emergency conditions.In this case, I want to use Auto LTC and manual Tap changer in the mean time to achieve the minimum Voltage dip if possible.

 

[davidbeach]

You can't do it. You are looking for a slow process to help you out with a fast event. Unless you can tap into some form of stored energy, your motor starting will happen with one transformer tap. That said, if there is a higher voltage your system can handle for the longer term, and one that you can easily achieve, you can use that for your motor starting. But in any case, you should assume that the infinite bus behind your source impedance will maintain a constant voltage throughout the entire motor starting event and the motor current (added to what ever other load current there might be) through the source impedance and all the impedances between there and the motor will cause the voltage drop.

 

[stevenal]

What are you trying to achieve?
If you set the LTC to a high voltage, you can keep the value of the voltage during the dip higher. The extent of the sag is the same, but by starting at a higher value you end up at a higher value as well. This might be helpful if you're trying to help voltage sensitive equipment ride through the event. We've done this to help a voltage sensitive customer ride through fault induced voltage sags. They see fewer shutdowns with LTCs set to deliver voltage at 105% of nominal than they did at 100%.

Load and no-load taps do affect the impedance, but the effect is small and usually neglected.

 

[jghrist]

If you want to reduce starting current, you could manually set the LTC at a lower voltage during start to make a reduced voltage starting condition. This would make the voltage at other loads that much lower, however, which is what you may want to avoid by limiting starting current.

 

[SELEC]

Stevenal,

Thank you for the response.What I am trying to do is to reduce the possible voltage dip when starting a motor in the event that the system is weak.I did some simulation tests by ETAP software. With Manual LTC and Auto LTC working simutaneously, I can achieve the lowest voltage dip. But I am not sure about the time delays of LTC (initial and operating).What would the real picture look like?

 

[davidbeach]

Sounds like you need a soft starter. You can't rely on transformer tap changing to deal with motor starting; why can't you accept that?

 

[SELEC]

Davidbeach,

I do have a softstarter-VFD for 2 motors(5000 HP), but I am thinking about arcoss- line starting under emergency situations in the event of VFD failures.

 

[stevenal]

Why do you want to reduce the dip? Are you simply trying to assure sufficient torque to start the motor? Maybe you can start the motor unloaded. If not possible, and operation is critical, maybe you need to deal with the emergency by having back up equipment standing by.

Autotransformers typically have less impedance than 2 winding transformers.

 

[rcw retired EE]

SELEC - The other engineers have done a great job of listing your options. I'll try to be brief.

Tap changers are electro-mechanical devices operated by a hand wheel or by a motor – both inherently slow operations. An automatic tap changer controller has a dead band and a time delay that would prevent it from automatically compensating for the motor voltage drop. The controller could not sense the dip and get the transformer to react in time. You would need a response time of milliseconds, but have a system that responds in minutes.

If you are worried about starting without the VFD/Soft Start, buy two of them so you have a spare, or install two motors with two VFD's. Otherwise, just live with the voltage dip on across the line starting. Use the methods suggested above to minimize the dip. I once wrapped rope around a large fan’s shaft and had a tractor to pull the rope to get the fan spinning before we hit the motor with full voltage. That didn't reduce the initial voltage dip, but it shortened the acceleration time and length of the dip to within the motor’s limits.

Good lcuk with your design.

 

[ScottyUK]

I once wrapped rope around a large fan’s shaft and had a tractor to pull the rope to get the fan spinning before we hit the motor with full voltage.

Did you by any chance video this episode? I would love to see it: my safety department would be in apoplexy!


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Experience is what allows us make a completely new mistake every day!

 

[rcw retired EE]

Actually, I had suggested the rope-assisted start in jest, but the operators tried it anyway. They were desperate to get the plant running after a VFD failure had them down for 2 weeks. They hard wired around the VFD, direct from the 7.5 MVA, 600V isolation transformer to the ID Fan motor, using some temporary CT's and a relay for protection. They started the motor by closing a 12 kV breaker feeding the hard wired transformer/motor. We had adjusted taps on the Main and the motor transformers to compensate some for the anticipated voltage drop on starting.

Problem was, they wrapped the rope in the wrong direction, spun the fan backwards and tripped on inrush. Second attempt worked better. The rope trick reduced the acceleration time about 5 seconds, enough to keep below the rotor or transformer damage curves.

Would have made a good “Things Not To Do” video. Plant went out of business a few years ago. (Thank goodness!)

 

[davidbeach]

Ouch. Catching a reverse spinning motor is not something one really wants to do. Any chance that you captured any oscillography from the relaying or any fault recorders?

 

[SELEC]

Thank you for the valuable inputs from all of you above.I realized that we cann't rely on ETAP software or other software too much. We have to consider the real situations in order to conduct our engineering studies.