Operation of Transformers OLTC and Generators 1

[sparkview]

Hi all,

Our Powerplant is currently in the preliminary construction phase of a new Reciprocating Engine powerplant consisting of 6x17 MW engines.
This new powerplant totaling 102MW will be tied to our 60 kV system by three 50 MVA step-up transformers. The idea is currently to seek the possibility of integrating an On-Load Tap Changer (OLTC) into these new transformers. As I can understand the integration of OLTC controlled by an AVR can potentially have adverse interactions with the Generators AVR. All of the 6x17MW are supplying a 13.8 kV double bus system and then stepped-up by the three 50MVA transformers to the 60 kV double bus system upstream. As I can understand, the generator AVR and OLTC should operate in a carefully coordinated manner to avoid faster generator AVR and slower OLTC controls hunting each other.

I was wondering if anyone could share their opinion on this matter.

Jairo

 

[ScottyUK]

It depends to some extent on how demanding your transmission system operator is - what do they require? Don't over-complicate things if they don't need to be complicated, it makes for a much easier life for the plant operator!

We generally used to let the AVR's look after the machine terminal voltage and flex the reactive load against the GSU transformer. The GSU transformer OLTC's were operated in manual mode. We had a pair of 4+1 CCGT generating modules so it was fairly easy to trim the overall reactive despatch order on the module using tapchangers alone.

 

[crshears]

Most places like that that I've worked in would use an integrated AVR for the generating plant, and centralized manual control only for the ULTCs [ under load tap changers ].

The need for tap changes has typically proven to be rare to occasional, rather than frequent, therefore the cost and complexity of auto ULTC control has usually been found unwarranted.

BTW in my experience an OLTC refers to an Off Load Tap Changer.

CR

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

 

[ScottyUK]

Hi crshears,

In the UK we'd use the term 'off-circuit tap changer' or 'off-circuit tap selector'. 'OLTC' is exclusively reserved for the high speed tapchangers with diverter gear designed to change taps while under load.

If a tapchanger isn't an on-load type then it's almost always an 'electrically-dead-and-isolated tap selector'. 'Off-load' is an inaccurate and potentially dangerous description which is wide open to misinterpretation - the phrase 'off-load' can reasonably be interpreted as 'energised but not carrying current', but changing tap on an off-circuit tap selector with the transformer energised is likely to result in a cataclysmic failure because most tap selectors don't have a break-before-make action as the moving contact rotates between positions. I've seen the results of a tap selector incident first hand, I lost three friends that day and I would hate any other facility to go through what we did.

 

[crshears]

My first thought is how terrible that must have been for you...

My second is, "Terminology again..."

Thank you for that; in my utility here in Ontario Canada ULTC describes the "high speed tapchangers with diverter gear designed to change taps while under load."

Our system includes some high capacity tie line phase shifting transformers with two sets of tapchangers in series; one is the high speed tapchanger with diverter gear designed to change taps while under load, while the other is operated with the phase shifter 'energised but not carrying current.' The purpose of the latter is to alter the neutral point of the overall unit so that a wider range of control is available.

And there are also numerous autotransformers within our system that incorporate OCTCs; these are only very rarely operated, and our procedures stipulate doing tests after any change to ensure the correct ratio has indeed been arrived at internally, as well that contact resistance is acceptable.


CR

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

 

[RRaghunath]

I agree with the opinions expressed.
But, if you like to put it in Auto, you need to make sure that the dead band for OLTC-AVR is set as +/-5%. This way, you can avoid OLTC-AVR interfering with the exciter AVR (exciter AVR has regulation range of +/-5%, generally speaking).
Simple thing is to keep it in manual mode. I guess you may require to operate the OLTC-AVR only when the grid voltage is very low or high (to work within generator capability curve) or, during generator synchronisation for matching the voltages.

 

[ScottyUK]

Terminlogy indeed! I was careful not to say "You are wrong, crshears!" because it depends so much on how things are defined where you work. In context an 'off-load' tapchanger makes sense.

I'm not aware of any "off-load" tapchangers on our system, but it's geographically a much smaller and more tightly meshed system. That may explain why we don't have a broadly-accepted abbreviation for them.

 

[prc]

Now as per IEC 60076-1(2011) & IEEE C57.131-2012,the terms to be used are DETC (de-energized tap-changer) and OLTC (On-load Tap-changer),though IEEE allows an alternate term LTC. So let us stick to these as Scotty said in India also we had some accidents and we are careful to use these recognized terms only. Even in that phase shifter case, it would have been better to use OLTC Nos 1 &2 instead of calling it off -load.

 

[crshears]

Acknowledged, Scotty; these fora have served to broaden my horizons on many fronts, particularly in this case, hence my carefully chosen wording when I prefaced my comment with the codicil "in my experience"...I have also learned in my turn to not too quickly call others out as wrong.

As to OLTC's 1 & 2, prc, in our context this terminology would fail to provide the highly desirable immediate differential descriptive, since the same designation would be used for the tapchanger that is capable of being operated under load [ULTC] as for that which can only be operated while off load [OLTC]...

In the phase shifter case under consideration, operation of the off load tapchanger is interlocked with breaker position, meaning that one of the two CBs on either side of the shifter must be open or the OLTC will not operate. Nomenclating both tapchangers as OLTC would remove one of the mental/cognitive barriers to incorrect operation and therefore place greater reliance on the interlocks.

CR

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