Relationship of Bus Voltage and (+or-) mvar After Grid

Hello,

Your question was not clear. I can give you clarification on what I understand.

Q=V*I*sintheta

The Q is nothing but the power needed to magnetise the inductance in the network which is supplied by your excitation system which is providing the magnetic effect necessary to deliver the voltage. If the inductive effect of the load is more then your voltage goes down and you excite more to get the necessary voltage.

Kindly check up if you are doing the reverse ie., overexciation and underexcitaion.

Regards,

S.Shyam Sundar

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mr sundar thank you for your interest

but you didnt understand my question ı know what you said. you said that about before synchronisation.you can change terminal voltage(14,4kv) of generator as changing exciter current.but you can do this only before synchronisation.

when your generator is in synchronisation you can change reactive power of generator as changing exciter current.

MY QUESTİON İS,

our main power transformer is 14,4kv-380kv.

our high side voltage value is 409kv.it should be 400-405kv.load controller center in turkey warned me and they wanted to decrease high side voltage from us. how can ı decrease bar voltage while we are in grid.?

if ı decrease exciter current ı can decraese reactive power,and with this way can ı decrease high side voltage? ı wanna know relationship.

It sounds like you need to change the tap of the No-Load Tap Changer (NLTC or De-Energized Tap Changer DETC) on the main transformer. This accessory is designed to provide 2 steps of 2.5% voltage adjustment above and below the nominal voltage. If you are getting high voltage on the primary for nominal secondary voltage, you can lower the tap one step to compensate. Then, your 14.4kV generator bus voltage will produce a lower stepped-up voltage to match the grid. The NLTC settings are usually labeled A,B,C,D,E or 1,2,3,4,5, and the ratios listed on the transformer's nameplate. You should take a look to see what it's set at. The NLTC switch is usually on the side of the transformer, often fairly low and easy to access.

WARNING! Only change the NLTC setting with the transformer de-energized! The NLTC is not designed for switching current, only for changing winding taps. If you try to change the setting with the transformer live, you could have a smoking pile of rubble left. If you are unsure what to do, hire a qualified HV electrical service company to do it for you. And while the transformer is energized, put a padlock on the NLTC tap control (it should have capability to do that) so that someone won't think they can just go ahead and switch it with the transformer live.

Can you not alter the tap position on the transformer and get the desired voltage.

Regards,

S.Shyam Sundar

But our tap changer is in auto mode.it is altering primer side voltage(14,4kv) automaticly when you need.

The simple relationship is that if the excitation system causes the generator terminal voltage to be higher than the system voltage then +MVARs will flow away from the generator. Conversely if the terminal voltage is lower than the system voltage -MVARS will flow into the generator, at unity PF there is no VAR flow and the voltages are matched. Unfortunately you need to know the equivalent impedance of everything from the generator terminals back to the metering point in Turkey, as well as the outputs of all generators, load characteristics, tap-changer settings, bus voltages, etc. in order to calculate this properly. The most accurate method is to do a load-flow analysis using commercially available s/w.

An interesting experiment is to hold the generator MW at a constant real power output, adjust the excitation current until you are at unity power factor, then start varying the excitation current and plot the MVARs and corresponding voltage levels. WARNING: Doing this improperly can lead to system instability, pole slipping, unacceptably high or low voltages elsewhere in the the system, and many other unintended consequences!!

To sum up this discussion this is a non-linear, time-varying problem that requires an iterative solution that unfortunately does not lend itself to an accurate simplified calculation.

ps- you need to be mindful of what is controlling the LTC operation since if it is sensing the generator voltage then you may have the AVR and the LTC trying to control the same point. Try putting the LTC in Manual and adjusting it to achieve your desired results.

"if the excitation system causes the generator terminal voltage to be higher than the system voltage then +MVARs will flow away from the generator. Conversely if the terminal voltage is lower than the system voltage -MVARS will flow into the generator"

this is the answer that ım looking for.thank you very much :)

ı have one question more.

why produce or consumpt the power transformers a mvar ? what is the meaning of +mvar and -mvar for transformers?

for example;produced mvar is negative.absorbed mvar is positive.

are these sentences right?

Transformers are consumers of Inductive VARs since they are basically coils of wires around a metal core.

Transformers are rated in MVA which is the vector sum of MW and MVAR, and when we do our vector math we find that the sign of the MVARS doesn't matter since we have done a squaring operation. What matters to the tranformer is the magnitude of the current passing through the windings since this contributes to the heating of the windings and the core.

The transformer doesn't really care which direction the MVARs are flowing, though as the system operator you do since it will affect the terminal voltages.

on connecting to the grid or synchronising to the grid the MVAr is increasing or decreasing has been explained .Kindly give the details of the location of the MVAr wehther on the grid or on the generator and also whether increase means export or import to the grid.

You never replied to the transformer tap change solution. Or you are not able to that if the MVAr change is dynamic.

My email is <email removed>

I work for CPRI,INDIA . Can I know where do you work.

Shyam

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which details do you want to know from me? can you say exactly?ım working in turkey.

tap changer is in auto mode forget it:)