×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

HV/MV transformer impedance

HV/MV transformer impedance

HV/MV transformer impedance

(OP)
It seems that a high transformer (HV/MV)impedance protects the MV busbars from short-circuits in the HV Grid by limiting the short-circuit current.
Accordingly a high impedance value should be desirable in all cases.

What are the parameters influencing the increasing of a transformer's impedance?
Why can't I find on the market (easily) 150kV/20kV transformers rated at 40-60MVA with an impedance of 25% or 30% ?

What is the financial and technical trade-offs in choosing high impedance tramsformers e.g. going from a 15% transformer to a 25% one with the same rating.

RE: HV/MV transformer impedance

Higher impedance results in poorer regulation of the secondary voltage which can cause problems during load changes such as motor starts. Higher impedance transformers usually require (expensive) on-load tap changers with high numbers of taps to maintain the secondary voltage at somewhere near setpoint.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: HV/MV transformer impedance

As Scotty points out, voltage regulation suffers if you go too high in impedance.  Once you try to load up a high impedance transformer, in addition to LTC, you probably need a fair amount of switched capacitor banks to be able to hold the voltage within normal limits.

Since transformer impedance is largely reactance, as you load up the transformer, you consume a lot of reactive load (I^2 X).

Admittedly the short circuit duty goes down with high impedance transformers, but the voltage control is more difficult and the need for more reactive supply increases.

RE: HV/MV transformer impedance

Look at air core reactors to limit your short circuit current. Higher transformer impedance means higher losses and poorer regulation.

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

RE: HV/MV transformer impedance

In addition to poor regulation, per Sankar's tutorial, "higher impedance gives more winding eddy and tank stray losses and also hot spot problems."

http://www.transformerscommittee.org/info/F01/Tutoria_sankarl.pdf

Transformer impedance is caused by flux linking only one winding and not the other. Normally the windings would be concentric tubes with the spacing between LV and HV determined by voltage stress,etc.  In a very high impedance transformer, the windings would need to be further apart to allow leakage flux. The larger diameter of the windings thus increase the resistance losses and increase the amount of materials needed and therefore increases the cost of the transformer.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources