Powerhouse transformer tap changer question

[sigshane]

Hi all. I am learning all about our main power transformers, and the no-load tap changers they are equipped with. I understand the tap changer - what it does, how it does it, and why. I would really appreciate some real-world examples of why the tap changer, which is on the high (230kV) side of the transformer - would be adjusted.

We are located on the Savannah River, and someone said that when the Olympics were in Atlanta, the taps were changed, but they don't remember to which setting they were moved, and cannot really explain to me WHY (from a power requirement perspective).

Every example or scenario will be GREATLY helpful.

Thanks in advance!

Shane
hydroelectric power plant electrician trainee
U.S. Army Corps of Engineers

 

[augie47]

Each time I have experienced the utility changing taps it has been to compensate for a voltage drop problem.
It is rare in this area, but occasionally due to seasonal loads the POCO voltages drop and the taps need to be changed on transformers to maintain the desired voltage level on the secondary side.

 

[transient]

Powerhouse transformer tap changer question

Very often it is required that the transformer secondary voltage be controlled in order to hold a pre-set
value within a certain tolerance. This is usually accomplished by the use of a special assembly which
causes a contact to move on a dial switch in a manner to place more or fewer transformer turns in the
circuit and thereby regulate the voltage in a step-change manner. This assembly is called a Load Tap
Changer (LTC) due to its ability to change switch positions (taps) with load current flowing.

Load tapchanging may first appear to be a trivial matter of switching a contact between taps. The first
question encountered is: Should the switch operate as break-before-make or make-before-break? That
is, should the switch contact part the first contact (including arc extinguish) before making on the second,
or should contact be established on the second before disconnecting from the first?

some means of bridging a portion of the transformer winding is required in the
operation of all load tap changers. This is necessary to avoid the momentary interruption of load during
tap transition. Consequently, all tap changers which operate under load accomplish a make-before-break
sequence. There are numerous ways by which this may be accomplished.

 

[Smart $]

Very often it is required that the transformer secondary voltage be controlled in order to hold a pre-set value within a certain tolerance. ...

I agree with your explanation but wanted to add that quite often a power plant will push the high end of the tolerance so they are outputting the maxium power they have available.

 

[sigshane]

These are all very good responses - thank you.

Our transformers have tap changers only on the high side - away from the generators. And they are No-Load tap changers - our transformers must be out of service before changing them.

The remark about seasonal fluctuations in grid voltage being a catalyst for changing the taps makes sense, and I would attribute that to the need to change them for the hosting of the Atlanta Olympic games.

Thank you all again for your insightful replies.

Shane

 

[jcormack]

As already posted the main reason the NLTC are used is to adjust your secondary voltage to fit your needs or tolerances. If the base load on the grid feeding you grows and your baseline voltage drops you can adjust the taps to "boost" your voltage back to closer to nominal - same thing in reverse. The POCO may make changes to his lines feeding you; adding capacitors, new loads, new parrallel feeds, etc that will impact your voltage - thus requiring the end user to make adjustments. Sometimes these adjustment are longer term, sometimes short term. Say, for example, that your site is feed by two lines in parrallel - the POCO has a problem & you are feed by the "weaker" B line alone & will be so for 2 weeks - you may choose to change the taps on your transformers to compenstate for the known drop in voltage for that time period.

NLTCs are typically on the higher voltage primary side as that is the lower amperage side (Less $$$$)