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Help with the X''D for a wind turbine inverter

Help with the X''D for a wind turbine inverter

Help with the X''D for a wind turbine inverter

(OP)
One of my clients installed a small 75kW wind turbine to gain experience with local conditions.
The unit was used. The wrong instructions were provided.
This is an induction generator with an invertor.
Now, after four years, it is able to produce, but he is up against utility regulations before he can connect to the grid.
He has a list of information required by the utility before approval will be granted.The list is more suitable to a large co-gen operation in the mega Watt range.
The owner asked the engineer who presented the requirements to recommend an engineer to do the study. The engineer couldn't recommend anyone.
Cliet asked the engineer to do the study for a fee. The engineer said that he didn't understand the form and was not capable of doing a survey, his job was just to ask for the information.
Remember, this uses a DC link and an invertor.
Some of the information requested;
Capacity to absorb reactive power.
Capability curve.
xd, xq, xd', xq', xd'', xq''
Time constants; Td0', Tq0', Tq0''
Co-efficients of saturation for; V=1.0 and V=1.2
I am thinking that the invertor will not absorb KVARs
Is a capability curve applicable to an invertor?
The invertor output goes through three sets of transformers, all 3 x 25KVA. Is it reasonable to use the combined transformer impedances or reactances and state that as a limit eg; "reactance or impedance not to exceed".
Are the transient time constants applicable to an invertor?
Any links to online information on transient and sub transient calculations where I may educate myself will be gratefully accepted.
I will be trying to contact the manufacturer but as this was a used machine with the wrong instructions, my hopes are not high.
If i can put a reasonable case together with reasonable estimates and justifications, I will see my contact at the utility and try to negotiate the hierarchy to an engineer familiar with the requirements.
This will be a 75KVA inverter connected to a 34.5kV distribution line feeding a light industrial area including a small sea port.
The capacity of the plant is so small in relation to the capacity of the line that I feel that estimates based on the current let through of three transformer banks will be a safe and reasonable value maximum.
Thanks friends.
Yours
Bill

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

RE: Help with the X''D for a wind turbine inverter

Doesn't answer your queries, and is out of left field somewhat, but over here there are different regulations as its becoming quite popular to connect solar systems into the grid (i.e. solar panels and grid connect invertors).

Perhaps it'd be an option with the utility to look at the possibility of arguing that its an invertor system, and therefore the application should be assessed on that basis?

Apologies I can't be of more assistance.  

RE: Help with the X''D for a wind turbine inverter

(OP)
Thanks Freddy.
That's what I'm looking toward. I'm trying to get a better feel for typical values before going in to the utility..
 

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

RE: Help with the X''D for a wind turbine inverter

Bill:

If this is a true inverter, like in solar PV systems, your thinking is correct. (A different animal than DFIG unit).  But n that case, the inverter mfr has to provide the cetification that their product is certifiied for grid interconnection.

I appears that this is again a classic case of "green' project started without proper 'consent'(not approval) of all parties involved, especially the utility company,whose system the WTG was to be connected to.

Used model? And client wanted to gain "experience" in local conditions? At whose expense?

There are set rules for inverter interconnections and generator interconnections. Inverter connection reviews are also different for smaller (up to 10kW) and larger units.

75 kW is a grey area. It may require a full blown electrical system studies that may cost more than the cost of the installation. I guess only thing you can do is to talk to the utility company. If they are not cooperative to begin with, your client has a battle on his hand.  My experience: Utility company will not even begin the review until the "application" is submitted and accepted.

Wish you good luck.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Help with the X''D for a wind turbine inverter

(OP)
Thank you Rafiq.
The client is a successful business man and this project is at his own expense. He can afford toys such as a 75 KVA wind turbine.
Thinking outside the box, would it be acceptable engineering practice to install current limiting fuses and report the fault currents and transient time as not more than the fuse ratings?
A decade or so ago, I was involved with a failed project to install about 1000KVA of steam turbine co-gen with this utility. The utility PR department bragged about their co-gen program but when the project was abandoned after three years, not one piece of paper had been received from the utility.
Times have changed and a number of waste fired co-gen projects have been successfully completed.
Is it a reasonable assumption that the reactance as seen by the grid will be dominated by the reactances of the two transformer banks the energy must pass through?
The % impedances of the transformers are available. The X of the transformers may be closely estimated from the Z.

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

RE: Help with the X''D for a wind turbine inverter

Bill:
My reference to expense was not related to money but the system stability.   The transformer reactance may be dominating, the form data still needs to be filled out. Plus there would be a need for voltage fluctuation study, which probably will follow once the application is made. Just speaking from a particular experience, it may not apply in your case.

Rafiq Bulsara
http://www.srengineersct.com

RE: Help with the X''D for a wind turbine inverter

(OP)
Used model? And client wanted to gain "experience" in local conditions? At whose expense?
I am sure that this project has been financed "Out of pocket". After several years of running if it ever happens, he will have the data to present to a bank for financing a larger project.
I will be trying to meet with my contact at the utility later today. Thank you for your comments.
I am under the impression that the fault current that an inverter may develop is much less than a similar sized rotating generator. Is this correct?

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

RE: Help with the X''D for a wind turbine inverter

Bill,

If the only interface with the utility is the inverter output, then all the machine constants are really irrelevant to the utility.  X"d is a rotating machine parameter and not applicable to inverters.

The fault output current from the inverter should be pretty limited.  This is normally limited by the inverter control system to limit damage.  

Good luck.  

Dave

David Castor
www.cvoes.com

RE: Help with the X''D for a wind turbine inverter

I'm with Dave.  I'd love to see a good machine like model for inverter systems, at least until all the system modeling software knows what to do with inverters, but if there is no ac connection between the induction machine and the ac of the utility, it doesn't much matter what the machine parameters are.

If they really insist, figure out the maximum fault current the inverter can produce, generally in the 1.2-1.5 pu range and then provide a X"d value in the 0.833-0.667 range.  Taking that approach, I'd suggest an X'd value about half way between X"d and unity.  You're on your own for the time constants.

RE: Help with the X''D for a wind turbine inverter

(OP)
Thank you very much friends. It's good to have my thoughts supported.
I had a meeting with one of the distribution engineers today. (Also an Eng-Tips member.)
He agreed that the various factors were meaningless for an inverter system. He is forwarding an explanation of the system and an inquiry to the small generation specialist in the head office.
I suggested that the impact on the system, particularly back feeding to faults would probably be less than a 100HP motor.
Back feeding an open, lightly loaded line is an issue that will have to be addressed, but later for that one. First things first.

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

RE: Help with the X''D for a wind turbine inverter

Thank you for the feedback, Bill. Sounds like you found the right person to talk to. That back feed study relates to voltage fluctuation concerns, I alluded to. It should not be an issue for such a small unit.
 

Rafiq Bulsara
http://www.srengineersct.com

RE: Help with the X''D for a wind turbine inverter

(OP)
Thank you for the practical suggestions, David.  

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

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