changing the PF on my installation 1

[mottt]

here in Spain the generators must work depending on the time of the day on differents PF, from 0,95 capacitive to 0,95 reactive, but my generators are limited on the subex-cited side, cause of the capacitive curve, this is 5 MW ABB generators, 6600 V, and I was thinking on use reactances, but first question is: do a dynamic regulation, thinking on the same equipment used for correction of lag reactive, on installations, but insted of use capacitor i´ll use reactances, i´m thinking on change the current transformer signal to lie the regulator as it´ll try to go to 0,95 lag but really i´ll be on capacitive side. Do you know some equipment similar to this
about 1 MWr, on the other hand, i was thinking on blocks of reactances switched by a contactor, but the problem i see is the price, cause i have to do it on the high voltage side, my question is: could i do it with a voltage power transformer to reduce the voltage to low voltage and then do it on the low voltage side, to connect the reactances?
maybe i have to change the main circuit breaker cause of the summ of a new short circuit current to the circuit?

thank in advance.

 

[tommom]

Doesn't the voltage regulator have a power factor or VAR control? I assume you're paralleling this generator with the grid?

 

[mottt]

as i said my generators could go till 0,97 pf capacitive subexcited...

 

[dpc]

There are some terminology/language issues here.

An underexcited (subexcited?) generator consumes vars, similar to an induction motor. This machine is operating at a leading power factor.

An overexcited generator produces vars (similar to a capacitor, I suppose) and is operating at a lagging power factor.

If you wish to consume more vars, you can reduce the voltage produced by the generators (subject to the generator capability limits), turn off any power factor correction capacitors on the system, add more motor load, or add shunt reactors.

I'm not sure if I understood your question correctly.

 

[ScottyUK]

Yes, you can compensate on the LFV side of a transformer. Be aware that the transformer will absorb VARs and that you will need to allow additional capacitance to counter the VAR consumption of the transformer.

All synchronous generators are limited in their underexcited region. What is the limit defined on the capability curve for the machine? Can you post an image of the capability curve? The reactive capability of an underexcited synchronous machine improves as the MW load drops.

Does the generator have a transformer where it ties in to the network? Does this transformer have any tappings?


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

 

[mottt]

yes the transformer got taps, and the capacitve curve let me go to 0,97 subexcited, with the 100 % of kw.... i want to generate the 100 % kw and 0,94 subexcite, in this way i could get more money when i sell the power.
why do you ask me if i got taps?

 

[ScottyUK]

Please clarify what you mean by 'subexcited'. You are the first person I have heard use the phrase.

Industry standard terms are 'under-excited', when the generator field current is lower than that required to maintain a zero reactive load, and 'over-excited' when the field current is greater than that required to maintain a zero reactive load. A generator operating in the under-excited region has a leading power factor, and a generator operating in the over-excited region has a lagging power factor.

Tapchangers are frequently used to control reactive load on a generator. By changing tap to a numerically higher tap (tapping up) you will decrease the reactive load on the generator.


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

 

[slavag]

Hi.
I would like add only one recommendation.
Motorxplosion,firstly, check this issue with AVR ( I assume, that is also ABB )vendor.
Your tapchanger, I assume, is off-load type.
Regards.
Slava

 

[slavag]

Hi Motorxplosion.
I read again your posts.
Somthing sounds starange:
the capacitve curve let me go to 0,97 subexcited, with the 100 % of kw.... i want to generate the 100 % kw and 0,94 subexcite,
O.K., we assume that term subexcited is mean underexcited.
But 0.94Reactive power (p.u.) in underexcitation region
with 1.0 P ( p.u.) it's something strange ( for my pinion in this case Q=0).
What about:
1.Stator, Rotor and Core-End heating limit?
2.Zero-field current limit.
3.Practical stability limit.
Normal situation is 0.8-0.9P (p.u.) with about 0.4Q (p.u) in underexcitation region.
Are you have special winding for absorbe reactive power?
Get more money? May be it's Spain practic? You get money
for the Active power.
Regards.
Slava

 

[mottt]

i can´t reach the point of 0,94 lead power factor, cause my generator will overheat... i know but, cause of a selling law (since the last year) i´ll get more money for the same amount of KW/h if i change my powerfactor into the requierements of the buyer (the grid)... this is cause i was thinking on use a transformer to carry some reactances on the low voltage side.. and change the power factor.

 

[slavag]

O.K.
Intresting situation in Spain ( I assume, reason is overreactive energy on the night).
I think possible solutions are recommended at the DPC post:
1. Disconnect all capacitor banks.
2. Add motor load. ( of course if possible)
3. Add sunt reactor, but were and what size, this is a Q.
Regards.
Slava

 

[mottt]

yes the problem is not in a factory, the problem is in a hydro unit, and for me the shunt reactor is the best solution, but since the power is not constant, cause sometimes you got a lot of water and in other situations you got less... i was thinking on some dynamic control for the reactors, for this i was thinking on some equipment for regulate PF, modification must be done changing the capacitors for reactors, and changing the sense of the the current transformers that feed the equipment... how do you see it?, is there some issue I forgive? in example, how would you calculate the power transformer used for reduce the voltage..? V*I*square3 will give the KVA, but will you use some kind of factor to take into account?
other option will be the use of a fixed reactor with differents taps, and use contactor to conmutate, would you use some kind of factor to take into account when you calculate the transformer? or some special specification for it? And yes, the idea is to compensate overreactive power in the night. Thanks for your attention

 

[slavag]

Hi Motorxplosion.
I'm not familar with this issue. ( absorbe of reactive power). Shunt reactor it's standard solution for transmission lines. I heard about Static VAR's Compensator for small MVAR's.
For example:

http://library.abb.com/GLOBAL/SCOT/...98DD92DD66C1256E2F004D45E4/$File/Minicomp.pdf

Maybe better check this issue with your local ABB, Areva, Siemens.
Regards.
Slava

 

[waross]

There may be a solution that does not require a lot of additional expense. As I understand transformer theory and ScottyUK's post you may use your existing transformer as a reactor. If you shift the output voltage of the transformer down, it will absorb VARs.
Rather than reducing the generator excitation, drop your transformer tap so that the transformer absorbs magnetizing VARs from the system rather than the generator. Will your transformer safely handle the slightly greater current that will result? Are you willing to slightly overload it if required?
Have I got this right ScottyUK?
respectfully

 

[ScottyUK]

I think the first question which needs answering is why the generator has this restriction, or better still for the OP to post the capability curve of his machine and things will probably get a bit clearer. I expect that the generator probably has quite a generous operating area in the lagging (VAr exporting) mode and this may prove to be useful if used with a shunt reactor, allowing the generator to work well away from its stability limit.

Bill,

Yes, that is somewhere close. I usually find it is a tricky concept to put into words without the aid of a drawing - you've summed it up quite well. The ability to use this method of reactive power control depends on the relative sizes of the generator and the utility at the PCC. A 'large' generator on a 'small' utility connection will flex the system voltage significantly which may introduce other issues. In the reverse situation with a stiff utility the generator reactive load will vary with tap position even though system voltage and generator terminal voltage will remain fairly constant.


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

 

[slavag]

Hello.
Scotty, Bill, I totally agree with you.
Always need start from simple solutions. I attached this document for education, not for recommendation ( it's was intresting for me). Of course Motorexplosion need check
option of underexc. region of generator.
About tap control of xfr, I think it's off-load type, it will be problem change tap twice per day.
May be, underexc region of generator and erection ( sorry for term) current for the xfr. will enough for change of PF accorfing to grid request.
Regards.
Slava

 

[mottt]

whats xfr?

 

[slavag]

Hi
Transformer.
Regards.
Slava

 

[slavag]

Hi.
Please see attached capability curve. Of course it's not same type of generator, but I hope very close to your generators curve.
5- is core-end heating limit.
3- practical stability limit.
You saied, possible run PF=0.97 leading or "-".
Add to this reactive power absorbed by your transformer.
Where, you meas PF: on generator side or on HV transformer side.I know it's not so.. Q, but I see several options.
Are you have data sheet of your transformer?
What is a Cu and Fe losses of transformer, persent exciting current, persent impedance?
What is a HV of transformer?
What is MVA of transformer?.
Regards.
Slava.
Try meas PF in case of PF near to +0.95-0.98.