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GRID OPERATIONS

GRID OPERATIONS

GRID OPERATIONS

(OP)
Many grid operators are requiring on load tap changers on generator step up transformers to provide voltage support outside normal generator voltage limits. Does anyone have any operating experience as to what the AVR settings are used? HV and EHV connections to a grid.   

RE: GRID OPERATIONS

Agree to some degree with your first sentence, but I'm not clear on the meaning of the second one w.r.t AVR settings. Can you expand a bit on what you're looking for?
  

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

RE: GRID OPERATIONS

In additional to Scotty's Q.
AVR..are you menat AVR of generator or TC control?
Are it industrial generator, about 60-70MVA, or utilities generator?
Are it block generator-transfromer w/o GCB or with GCB and some load on the generator bus.

Best Regards.
Slava
 

RE: GRID OPERATIONS

(OP)
I mean AVR automatic voltage regulator, tap changer control on large units 500 MVA step up transforners, grid requirement because generator voltage regulator can only provide plus or minus 5%. Setting are usually for band width and time like for a nomal on a load tapchanger. I think the answer varies depending on the grid voltage history. In a developing country the grids tend to have a lot more swings.Just wonder if anyone was operating some like this. Normal settings like on a step down transformer are not any good.     

RE: GRID OPERATIONS

In the UK many of the big plants are despatched on reactive power rather than to maintain a nominal voltage setpoint. OLTCs elsewhere on the system compensate for the high or low system voltage caused by the reactive despatch requirements on the generating plant. My old station we used to typically float on the bars in terms of reactive power for the majority of the time but was contracted to provide reactive support when required under an ancillary services agreement. Typically the nominal 275kV system was running at 280-285kV because of the presence of other major generation in the area and the relative lack of heavy loading on the local system, with a net power flow toward the south-east. Our GSU transformers were usually tapped well down toward the lower end of the scale so we could maintain machine terminal voltage near nominal.

Not sure how much that helps, still not exactly sure what you're looking for. smile
  

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

RE: GRID OPERATIONS

(OP)
Thanks. it seems to be a relatively new requirement to have OLTC on Generator Step Transformers and only required by some grid codes. I don't think the cases you mentioned had OLTCs. Largest Industrial plant for Q Gas has them installed om Frame 9 GSTUT'S but I think they use them on manual.I will see what I can find out in IEEE Power Society proceedings.      

RE: GRID OPERATIONS

In few projects I found OLTC with auto control on the GSU.
It's something new and used for the reactive power regulation.
Best Regards.
Slava

RE: GRID OPERATIONS

I can't think of a British generating site without a GSU equipped with an OLTC, and some of those stations were there since the early 1960's. The UK grid Code doesn't directly mandate an OLTC but a tapchanger of some form is certainly implicit in the code. I've never seen an off-circuit tap selector on a major GSU transformer because they are so limiting operationally. The handful of European and Middle-Eastern sites I'm familiar with have also all been equipped with OLTCs.
  

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

RE: GRID OPERATIONS

Scotty, I understand that Volt Amp reactive production is voltage dependent regardless of the source of the voltage difference. Could a transformer tap changer allow the production of an amount of VARs that would result in a generator operating outside its capability curve if a fixed transformer was used?
Said another way, a generator has a limit on the quantity of VARs that it may produce depending on the capability curve.
With a tap changer, the plant may raise the voltage supplied to the grid with a lower excitation level on the generator.
Is it possible to generate more VARs and still stay within the capability curve with the help of a tap changer?
(Note: VARs is an acronym for Volt Amps reactive, not to be confused with the symbol var which must be used when writing a formula.)

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

RE: GRID OPERATIONS

Hi Bill,

The tapchanger allows the generator to export (or import) vars over a wider range of operating conditions than would otherwise be possible.

The generator ultimately supplies all the vars, but it is common for grid conditions to make it virtually impossible for the generator to deliver its full reactive capability without the assistance of the tapchanger. For example, a high grid voltage immediately makes it difficult to export vars because the flow of vars is dependent on the difference between the internal machine voltage, usually denoted 'E', and the terminal voltage denoted 'V'. 'E' is inherent in the machine design and is a function of excitation current; there are well-defined limits on 'E' because of thermal limits on the rotor winding and on stator end heating due to the flux distribution at low field strengths. The use of a tapchanger effectively changes the value of 'V' for a given level of reactive power flow. For example, when a high reactive export is required the tapchanger is adjusted to force the generator terminal voltage down below nominal, and the AVR then boosts the field to bring the terminal voltage back to setpoint. In doing so the machine export reactive power to the grid.

The use of the tapchanger doesn't notably increase the reactive capability of the machine, although there are some second-order effects which modify the capability diagram slightly, but the tapchanger makes it possible for the machine to operate over a wider envelope than would otherwise be possible. Without a tapchanger it is likely that a limiter such as over-fluxing or under-excitation would come in to operation before the machine delivered its full reactive capability as shown on the capability chart.
  

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

RE: GRID OPERATIONS

(OP)
Scotty are your tapchangers on manual control?  

RE: GRID OPERATIONS


xxjohnh:

In some cases transformer on-load tap changers can avoided by designing synchronous generators for plus/minus 10% voltage range. This would pose no problems. Increase of generator price may be minimal (about 1%).

Best regards

Wolf
WWW.HYDROPOWER-CONSULT.COM  

RE: GRID OPERATIONS

Thanks for the confirmation Scotty.
Yours
Bill

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

RE: GRID OPERATIONS

xxjohnh,

They're on manual control. Theoretically they're on ganged parallel manual control but in reality they are invididual control.

Bill,

No problem,


Wolf,

Have you actually managed to get this done? If so I am quite impressed in these days where the big three manufacturers make their standard product and stubbornly refuse to modify their 'tried and proven design'. I guess it is easier to get a semi-customised design in the non-CCGT market.
  

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

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