Insane Motor Circuit 4

[ReliabilityTech]

Because I'm not an engineer, and I'm currently in heated discussions with an electrical engineer, I need some theoretical assistance with the following circuit.

13.8kV primary to 600V secondary, 600V secondary to 600V/4.16kV autotransformer to 4.16kV softstarter, softstarter to 4.16kV motor.

13.8kV-600V = 1MVA
600V-4.16kV = 330kVA (Actually a 4.16kV H-side, but reversed for whatever purpose rated at 45.8A)
4.16kV softstarter (designed for 600HP motor originally)
4.16kV motor (45.7FLA, 594RPM, 12 pole, 300HP)

Trying to convince them that this will not function, nor is it a conventional installation.
Currently they cannot get the shaft to turn due to loss of control voltage on the softstarter due to voltage drop incurred based on the inrush current.

It should be noted that, as designed, there was a 600V softstarter in the circuit in front of the autotransformer causing voltage collapse as well.

There are multiple issues with impedance as far as reversing an autotransformer in this fashion, as well as inrush conditions changing dramatically, but are there any expected values that I can present to this engineer that can backup my words rather than just saying "You can't do that"?

Even if they achieve spinning the shaft uncoupled, there's no way that this circuit will run loaded.

Any input would be great.

Thank you in advance.

 

[waross]

The biggest problem I see is insubordination in the ranks.
As long as you are trying to tell the engineer his job, you are distracting attention from the source of the problem.
There may be a bad connection, there may be a faulty soft starter, the motor may be jammed mechanically, the motor may be shorted, A connection may have been missed.
In Canada the standard voltage is 600 Volts but many plants have stuck with 480 Volts.
It is not uncommon for a plant to use both 480 Volts and 600 Volts.
480 Volt gen-sets are much more common than 600 Volt gen-sets.
I have worked on turn-arounds where auto transformers were used to boost a temporary generator output from 480 Volts to 600 Volts for plant equipment and then more auto transformers to drop the 600 Volts back down to 480 Volts to power rental equipment.
Dedicated transformers for motor loads are often oversized 25%, but I don't see a serious problem with a 45.7 Amp rated motor starting on a 45.8 Amp rated transformer.
The transformer may be a little overloaded at full motor load, but nothing that will prevent the shaft from turning.
By the Way:

There are multiple issues with impedance

Transformer impedance issues generally reduce the applied voltage enough to aid in reducing the starting current.
Best advice:
Sit down, shut up, let the engineer do his job.
Tough love but you asked for it.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ReliabilityTech]

Thank you very much for your reply! I appreciate the feedback no matter which direction it is coming from.

I've had a very difficult time finding any external references where a 600V source is used to drive a 4160V motor whether it's through an autotransformer or standard transformer. Stepping UP with an autotransformer is not recommended at ALL as far as multiple manufacturer recommendations are concerned from at least two sources I have come across.

Generally, with a HV source and a medium voltage load, the standard would be to size the transformer accordingly to the load. ie; 13800V to 4160V transformer, 4160V starter, 4160V load.

I'm not making an attempt to argue with the engineer for the project, so I apologize if I've come across as a brat, just curious (as the client) why this decision was made over standard installation practices.

I hope it DOES work for the contractor in the end as they haven't had much success electrically with this project and it would be great for them to get a win here. But, I'm losing faith in the installation.

 

[jraef]

13.8kV-600V = 1MVA
600V-4.16kV = 330kVA (Actually a 4.16kV H-side, but reversed for whatever purpose rated at 45.8A)
4.16kV softstarter (designed for 600HP motor originally)
4.16kV motor (45.7FLA, 594RPM, 12 pole, 300HP)

I take it this kluge job is because this is the equipment available on hand? Because otherwise it makes no sense to step the 13.8kV down to 600V and then back up to 4160V.

Highlighted numbers are the crux of your problem here. Even WITHOUT factoring in all the added impedance by having two transformers, when using a soft starter the quick and dirty rule-of-thumb to avoid more than a 5% voltage drop is that the MINIMUM source kVA is 2.5x the motor HP; you are at only 1.1x. It's never going to work. Even completely unloaded you can't accelerate a motor with a solid state starter without at some point exceeding 200% FLA and you don't have a transformer capable of delivering that.

You MIGHT be able to accelerate it with a 4160V VFD by current limiting at the transformer capacity and letting it take as long as it needs with that limited torque, but a 4160V VFD is probably going to cost you more than a bigger transformer, or the RIGHT transformer to just go from 13.8-4.16kV.

Just FYI it is not that unusual to step up from LV to MV for an MV motor, it's done quite often with deep submersible pumps in order to avoid voltage drop on the long down-hole cable lengths. But that's where the kVA being 2.5x the motor HP rule comes from. For that motor you would need a 750kVA transformer, minimum, unless you can live with a deeper VD.


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

 

[panter]

I didn't see what the mashine needed to start your motor because the functionality of the soft starter depends of torque characteristic. There are a lot of words and stories here about fan drives where even a 5 x more powerful transformer than the motor couldn't have been enough to made good joob .
Good luck !

 

[waross]

Respectfully Jeff, I have used a lot of open delta auto-transformers sized at 125% for DOL with never an issue.
The 5% voltage rule applies to transformers with a motor load shared with other loads.
When the transformer is dedicated to the motor, a much greater voltage drop may be allowed.
We haven't been told what the load is.
RT:
You may consider using a series impedance to limit the starting current.
If you are starting uncoupled, try feeding the controls from the 600 Volt source and going DOL.
You may be getting, at a rough estimate, a 50% voltage drop when the motor tries to start, DOL.
Calculations:
Enough assumptions to make this close to a WAG.
Rough numbers:
Auto-transformer impedance 17% Short circuit current = 6 times rated current. At 50% voltage drop, about 300% rated current.
Motor starting current = 600% of rated current.
At 50% voltage drop, about 300% rated current.
That will drop out any controls derived from the 4160V transformer but it should be enough to start an uncoupled motor.
As the motor accelerates the voltage will recover and the voltage drop will transition from impedance dominated to regulation dominated.
If this is a repurposed step down Auto-transformer you will probably need to adjust the voltage taps to bring the voltage up to rated voltage.

Step one: Repair relations with the engineer.
Step two: Derive your control source from the 600 Volt supply rather than from the 4160 source and see if the motor will start.
This may be an easy sell to your engineer.
Step three: Go DOL and let the auto-transformer act as a series starting impedance.
Point out that there is too much voltage drop, but there may be a way to turn this to your advantage.
eg: How much will the voltage drop limit the starting current if you go DOL. Is it worth a try?
Step four: Use a larger auto-transformer or a conventional transformer and accept my apology.
Note on auto-transformer starters.
Yes I know that this is a different application but;
Auto-transformer starters were short time rated and were undersized compared to an equivalent full time rated transformer.
As well as a voltage/current reduction from transformer action many old auto-transformer starters gave a further voltage reduction due to series impedance voltage reduction.


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

 

[LionelHutz]

This sounds strangely like an issue at work - with the 600V soft-starter that is. That setup had too much voltage drop to accelerate the motor and load to full speed.

 

[jraef]

Bill, that’s the issue. If the transformer went right to the motor, you are absolutely right, the VD is just a minor inconvenience that simply affects your acceleration time. But in this case because the soft starter is on the downstream side of that step up transformer, the VD is what is causing his soft starter to shut down.


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

 

[ScottyUK]

Possibly time to sell the two transformers and buy one of the right ratio and winding arrangement. You might even get an exchange if you're lucky - two-for-one.

I'm not sure what the soft start will make of the reversed 4.16/0.6kV transformer if it has a convention delta primary and star secondary but most soft starts will be designed to be supplied from an (impedance) earthed star source. It is an expensive piece of hardware to damage so probably worth checking.

 

[waross]

Thanks Jeff.
My point is that a try with a 600:120 Volt control transformer to isolate the controls from the voltage drop will be relatively inexpensive. The price of a control transformer and an hour or so to install the transformer.
For a quick test an extension cord plugged into a 120 Volt convenience receptacle will be adequate. No real expense other than an hour or so labour.
If the soft start won't get the motor turning, then trying DOL with the auto-transformer acting as a series impedance will cost an hour or so to change the connections. Again no expense, just a little labour.
If it is shown that the motor can not be started with the equipment presently on site, then the next step entails major expenditures and lots of labour.
There is such an extreme ratio between the cost of my suggestion and the cost in both lead time, labour and dollars of a larger transformer that it's worth a try.
Scotty, it's an auto-transformer so most likely a star connection.
There may be other 600 Volt loads.
Even with a free transformer, there will be a lot of transport, hardware and labour costs to change.
If the auto-transformer is composed of two transformers in open delta, be aware that the capacity of the bank will be less than the sum of the transformer capacities. I doubt that this is an issue with this installation.


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

 

[ScottyUK]

Missed that - thanks Bill!

 

[LionelHutz]

If the 600V soft-starter that was first tried was any good then the circuit has already been tried ATL.

 

[waross]

Starting at the begining.
300 Hp motor is about 330 KVA.
Capacity at 600 Volts = 1000 KVA.
Required capacity per jraef = 2.5 x 330 KVA = 825 KVA
The 1 MVA transformer appears adequate.
Voltage collapse; Where? At 13.8 kV, at 0.6kV, or at 4.16 kV?
This may be a very soft grid.
There may be a fault in the motor.
When reduced current start is desired, voltage collapse may not be a bad thing.
We had an issue with a soft start tripping out on unbalanced voltages.
There was nothing we could do about the primary voltage unbalance and the extra motor current was still within the motor RMS loading capability. We didn't have much available to work with, but a manual bypass solved most of the problems.
We have a problem here. It could get expensive.
How do we find the quickest, most inexpensive solution?
First define the problem. Stating "voltage collapse" or "voltage drop" alone is not enough.
Example: If the voltage drop is 10% but the soft start drops out at 6% voltage drop,
Then the fast and economical solution is to live with the 10% voltage drop and somehow allow the soft start to accept the 10% voltage drop.
So, is the problem actually voltage drop or is it the soft start tripping prematurely?
At this point, it may take less time than has already been spent arguing to try to supply alternate control voltage to the soft start.
If the motor still won't start, then I suggest that it will be quick and easy to try a DOL start.
Consider; If we take the conventional wisdom and buy a new transformer or transformers it may be an expensive wat to discover that the grid is too soft to allow a start without accepting greater than normal voltage drops.
It may be that the grid supply is so soft that the only way to get the motor turning is to install a VFD.
If the voltage collapse was affecting the control circuit of the 600 Volt SS, then quite possibly you have a very soft grid.
If that is the case, changing the auto-transformer may not be productive.
That will be expensive, but not nearly as expensive as buying and installing new transformers and then buying and installing a VFD.
Quite possibly the motor will start DOL.
If the motor will not start DOL, then possibly a VFD will start the motor with the existing transformers.
There has been a lot of good sound engineering advice given by all contributors here, but I submit that that advice has been based on assumptions.
The assumptions are reasonable but may not be accurate in this instance.
Is the motor in good repair?
Is the grid firm?
Is the voltage drop so much that the motor will not turn or only so much voltage drop that the soft starter trips out?
The expense of a 4.16 kV soft starter has been added to the the cost of a 600 Volt soft starter.
Let's do some testing with the equipment at hand before buying more equipment.
In regards to the transformer impedances; Yes the impedance of the 4.16 kV winding expressed in Ohms may appear high but the current is about 7 times lower than the 600 Volt current.
In any event, please let us know how this turns out.

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

 

[jraef]

Starting at the begining.
300 Hp motor is about 330 KVA.
Capacity at 600 Volts = 1000 KVA.
Required capacity per jraef = 2.5 x 330 KVA = 825 KVA
The 1 MVA transformer appears adequate.
Voltage collapse; Where?...

The problem is going from 600V back UP to 4160V, it is a 335kVA transformer feeding a 300HP 12 pole motor via the 4160V soft starter. The VD is going to happen on the 4160V side, which is what is feeding the soft starter.

IF the soft starter is given a steady state control power source derived from a CVS or UPS the 600V side of the circuit, it may be able to track its current limit and ramp process to the dropping line voltage, but not all of them are capable of it. That's a question to pose to the Soft Starter mfr.


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

 

[waross]

Understood Jeff.

Currently they cannot get the shaft to turn due to loss of control voltage on the softstarter due to voltage drop incurred based on the inrush current.
It should be noted that, as designed, there was a 600V softstarter in the circuit in front of the autotransformer causing voltage collapse as well.

This makes me suspect that there may be undiscovered issues.
Possibly a very weak grid, possibly a faulty motor or faulty auto-transformer.
But they are new, nothing will be wrong with them.
You and I have both been there and seen the time wasted because a new piece of equipment is faulty but others refuse to do a simple test;
"Because it's new."
If the controls were dropping out due to voltage drop on 600 Volts, fed from a 1 MVA transformer, there may very well be an issue with the supply at 600 Volts or at 13.8 kV.

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

 

[LionelHutz]

Ya, well the posting saying that the soft-starter was causing the voltage collapse pointed out the issue. Blaming the electronics instead of figuring out why the voltage was dropping, how much, where and the what voltage the motor actually needs to start would be more productive.

I highlighted that last one, because no matter what exact reduced voltage method you use in an attempt to start the motor, you MUST give the motor enough voltage that it can accelerate.

 

[ReliabilityTech]

Good points, gentlemen. Thank you very much with keeping up with this thread.

Where the electrical contractor is currently since my initial post last week:

A company projects representative/consultant newly appointed to this project has terminated any further testing by the contractor since contractually, they have to have a manufacturer representative present when attempting to start while the load is coupled.
This projects rep had the contract engineer and electrical supervisor open the motor junction box and show him the terminations which turned out to be hand-tightened split bolts under the tape.
After terminating with #6AWG ring connectors and properly torqued, the motor started uncoupled with the 4160V softstarter on the secondary side of the autotransformer.
They have since disconnected the 4160V softstarter and reconnected the 600V softstarter to the primary side of the autotransformer.
They are still unable to now test the motor as designed, but I visited the location to verify the nameplate information I was provided.
I now know that this is also a J class motor with 0.71 power factor. This drives up the inrush current considerably no matter how it is connected.

The contract engineer is now asking what we would prefer as the client and we're recommending an F class motor with 0.9 PF as well as a 13.8kV to 4.16kv isolated transformer. (The secondary of the 13.8kV to 0.6kV transformer at the source has multiple branch circuits from other secondary breakers).

I will keep this thread alive with updates after the manufacturer does dry commissioning with the contractor's current connected circuit to prove that it isn't feasible, and go from there.

I appreciate all of the suggestions in this thread.

 

[waross]

There may be a bad connection,

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

 

[LionelHutz]

Yet messing with the connections didn't actually change anything. Still can't start the load...

 

[waross]

A company projects representative/consultant newly appointed to this project has terminated any further testing by the contractor since contractually, they have to have a manufacturer representative present when attempting to start while the load is coupled.

I understood that this was the reason that they couldn't start the load.

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