Inrush when starting induction generator

[norcalnewb]

We are looking at installing an induction generator into an industrial facility. A question has come up to the inrush of the machine if it is accelerated to synchronous speed by the prime mover before the generator breaker is closed. In my mind, this is essentially the same thing as energizing a transformer, but a third party ran a study as a typical motor start. When connected to the 15 kV bus, and when modeled as a typical motor start, the voltage drop exceeded 20% and obviously lasted for several seconds. The third party is now recommending to make the machine 6.9 kV with a transformer to limit the voltage drop on the 15 kV system. Pretty much all of my experience is with synchronous machines, so I thought I would reach out to see if anyone has any experience with something similar to this.

 

[waross]

There are two factors at play when a motor is started.
The first factor is the transformer like inrush as you surmised.
This is there but is so fleeting that it is often overlooked.
The duration may be measured in a few cycles rather than a few seconds.
This transient may be greater than the normally accepted 6 times rated current.
It is too fast to be seen with an analogue Ammeter.
The second and most familiar inrush is caused by the rotor impedance and the slip frequency.
The greater the slip frequency, the greater the current.
At synchronous frequency you can expect a reactive magnetizing current and a small real current component to supply I²R loses.

Mitigating the first factor:
1. If there is enough residual magnetism to detect with a phase check relay, closing when in sync may be similar to point on wave switching.
2. Pre-energizing the motor/generator through an impedance and shorting the impedance when up to speed may be more dependable than sync checking.
Beware:
You have a small RPM window to avoid inrush.
eg: If you energize a 1760 RPM motor at 1720 RPM you will see about a 200% inrush at a minimum., likewise at 1880 RPM the same 200& current.
Your window for an initial current at or below rated current is from the motor rated speed to the same amout of over speed.
1760 to 1840,
1750 to 1850,
1740 to 1860.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[norcalnewb]

Bill, thank you for the response. I don't think I am concerned about mitigating the first factor, as the duration should not cause any problems. Is there a significant duration to the second component?

 

[waross]

At less than rated speed the machine will be acting as an overloaded motor and will be drawing more than rated current.
At rated speed (eg: 1760) the machine will be a motor, running at full load and drawing full load current.
At synchronous speed (1800) the machine will be idling, neither motor nor generator, and drawing magnetizing current.
At maximum safe over speed (1800 + 40 RPM slip = 1840 RPM) the machine will be an induction generator and will be generating rated current.
At speed greater than synchronous speed plus slip speed (in our example 1800 RPM + 40 RPM = 1840 RPM) the machine will be operating as an overloaded synchronous induction generator and will produce more than rated current.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Um...er...always an induction generator no matter what, no? I can't see where synchronous suddenly came from...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

That was a senior's moment cr. Correction made.
Thanks for the heads up.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Actually, in your opening statement, you said that the generator would be accelerated BY the prime mover. While not typical, it is possible, so if that is indeed the case, then you would NOT model the induction generator as a motor when accelerating it, you would only energize it at synchronous or near super-synchronous speed, eliminating the starting current. In that case then, all you have left is the magnetizing current transient as Bill described as the first issue.

So before you begin, you need to clarify your use of the term “prime mover”. In a generator application, the prime mover is the source of rotational energy; the wind, water, or steam turbine, or engine shaft. The “prime mover” is NOT the induction machine itself.

In a lot of induction wind turbines, the shaft stresses on the machine when trying to accelerate it from a stop with just the wind is often too much for the gearbox. So they accelerate it with the induction machine as an induction motor, then just let the wind take it super-synchronous. In that acceleration process, they use Solid State Soft Starters or VFDs to bring them up to speed with as low of an impact on the grid as possible. If your situation is like that, and you need to avoid that 20% VD, you may want to consider that as an option.

As a plug for the VFD method, if you use a Line Regenerative Active Front End (AFE) drive, you can accelerate without exceeding the FLA, plus you can use it to capture energy at lower wind speeds.



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[norcalnewb]

This would be a steam turbine generator. So, it would be accelerated by the steam turbine.

 

[jraef]

Then if you get to, or slightly over synchronous speed with the steam turbine, there is only the magnetizing current to consider. The induction machine is not accelerating anything. Maybe your "third party" that did the study using the induction generator as a motor to accelerate it was unaware of this starting situation? Because it's either that, or they didn't know what they were doing...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[xnuke]

I once reviewed a system impact study for a geothermal plant with a self-excited induction generator. The study was done by a utility that made the same error in modeling. They said that the system failed their study because the starting current dropped voltage below their allowable tolerances - it turned out they modeled the unit as an induction motor starting from locked rotor. When I pointed out that the prime mover would have the unit at synchronous speed prior to synchronizing, they agreed that it likely wouldn't drop the voltage too low. I don't know if they ever did try to remodel the system with just magnetizing current to see the impact, though - it's possible they worked with the unit manufacturer to figure out the system impact of connecting near synchronous speed. They eventually did decide that the plant could be built and connected to their grid.

See also thread238-75824.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[norcalnewb]

That is why I was asking the question. I just wanted to make sure I wasn't incorrect in my thinking.

 

[waross]

xnuke: I wouldn't give that thread much credibility. There is some accurate information mixed with less credible statements.
That may be acceptable for you or me but may be confusing for someone trying to get up to speed on the subject.
Note: The magnetizing transient is random depending on the point on wave of the switching.
Given that the magnetizing transient, at the worst case, may be greater than the starting surge and given that the the thread linked by xnuke states "The inrush the utility is simulating with EMTP is in the order of 100 mS." it is reasonable to assume that the utility is concerned with a possible worst case transient rather than the starting surge of an induction motor.
Note: 100 ms = 6 cycles. Not many motors can come up to speed in 6 cycles. This time frame is more in line with an offset transient and the decay of the DC component.
There are techniques to mitigate and eliminate the magnetizing transient.
Note: An induction motor starting from rest has both the magnetizing transient for a very short time and the starting surge as the motor accelerates lasting relatively much longer.
One mitigation technique is to pre-energize the machine through current limiting resistors to establish an alternating magnetic field in sync with the grid and to then bypass the resistors when the machine is up to speed.


Bill
--------------------
"Why not the best?"
Jimmy Carter