Measuring VARs 3

[LearningEE]

Hi,

I am seeing if there is a forumla I can use to calculate the Auxiliary Reactive Load VARs and the Net Generator Reactive VARS. Currently, we meter the gross VARs of both generators in our combined cycle plant and meter the plant net vars also. Is there is a way to calculate them using transformer impedance. I currently have data on:
-Generator Gross Generation
-Geneartor Gross Reactive
-Generator Terminal Volt
-Aux Load
-Aux Bus Volt
-Trans bus volt
-Net Reactive to the Transmission System
-Transmission Bus Volt

 

[alehman]

Not unless you know net outgoing VAR's from each generator. VAR's add and subtract just like you were metering watts.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[waross]

VARs are mostly load dependent. The reactance of the transformers and generators is often so little in relation to the load reactance as to be negligible.

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

 

[ScottyUK]

Hi Bill,

The GSU transformer for a large generator will likely have an impedance of about 16 - 18% and absorb about 10% of its MVA rating as lagging vars. Not sure if this is negligible.


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[Eleceng01]

Depending on the type of protective relays that you have on the generators, you may be able to obtain the VARS from them. I assisted a client with doing this a couple of years ago using SEL relays. We were able to get the VARS from the Modbus map in the relays. Just a thought.

 

[waross]

I stand corrected Scotty. Thank you.
Yours
Bill

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

 

[LearningEE]

Eleceng01, I will check on that.

Scotty, how do I go about finding the impedance of the transformer and how much VARs it absorbs.


Sorry, I am young and right out of college. I am new to all of this

 

[jghrist]

The nameplate of the transformer will give the impedance, but will not say how much is resistance and how much is reactance. It will also not say what the exciting current is. The exciting current depends on the voltage applied, not on the load current and is part inductive, part resistive. To get these values, you need the transformer test report.

 

[LearningEE]

Could I use the Geneartor Gross Reactive (MVAR) and the Aux Load (kW) to obtain the Aux Reactive Load (MVAR)?

 

[LearningEE]

Correction from the above post****


Could I use the Geneartor Gross Reactive (MVAR), Aux Load (kW), and the transformer impedance to obtain the Aux Reactive Load (MVAR)?

 

[ScottyUK]

If you have good MVAr metering on the HV side of the GSU and on the generator terminals then you should have enough information to calculate the auxiliary MVAr load. Usually the generator terminal metering is not good enough to give really accurate results because the measurement uncertainty due to CT and VT errors is of the same order of magnitude as the auxiliary load. The HV metering is usually revenue class metering so it is about as good as you will get without getting involved in custom measuring solutions.


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[wolf39]

Scotty:

I'm a bit uneasy with your statement 9 Sep 09 2:15 where you respond to Bill (waross) regarding the VARs of transformers. In reality the VARs generated by a generator step-up transformer is negligible, as Bill mentioned.

I've got a generator step-up transformer test report of a 600 MVA unit, 21/400 kV, impedance 19%. The no-load (excitation) current at the 21 kV side has been determined in the test field to 5.9 A, this giving us an input, delivered by the generator, of 215 kVA. The transformer core loss of 196 kW is equivalent to an active current of 5.4 A. With these data one arrives at a reactive load of 88 kVAr, which indeed can be called negligible compared with the transformer throughput of 600,000 kVA. The reactive load of 88 kVAr on the 21 kV side is required to generate the nominal core flux by magnetizing the core lamination and the air gap butt joints of these laminations.

The core flux required to produce 400 kV at the primary side at 600 MVA (with an impedance voltage drop of 19%) is 101,8%, this increasing the reactive load slightly to about 90 kVAr. There is no other reactive power "generation" linked with this impedance of 19%. Transformers transmit active and reactive powers practically 1 : 1 from the primary to the secondary winding.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[electricpete]

I think Scotty is on the mark. A transformer equivalent circuit has two reactive elements which can both consume vars: there is magnetizing reactance in parallel which you have accounted for. There is also leakage reactance in series which absorbs a load-dependent quantity of vars that you have not accounted for.

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[electricpete]

Here is a rough idea of the calculation. It may be off by a factor sqrt(3) or so. Maybe someone can add any needed corrections
Qpu = sqrt(3)*Ipu * Xpu

At full load Ipu = 1
Qpu = sqrt(3) * Xpu

Xpu = 0.19*sin(theta) where theta is pf angle of the transformer impedance
Qpu = sqrt(3) * 0.19*sin(theta)

Convert to standard vs pu
Q = (Sbase/sqrt(3) * Qpu = 600MVA * 0.19*sin(theta)

theta between 0 degrees and 90, meaning sin(theta) between 0 and 1. But transformer impedance is predominantly inductive so theta fairly close to 90 and sin(theta) fairly close to 1

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[electricpete]

Correction in bold
Qpu = sqrt(3)*|Ipu|^2 * Xpu

Once you plug in Ipu=1, it doesn't make any difference

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[electricpete]

So I think as an order of magnitude:
the leakage reactance aborbs 100MVA at full load (see my calc)
the magnetizing reactance absorbs 0.1 MVA (see wolf's) calc)

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[electricpete]

Correction in bold:
So I think as an order of magnitude:
the leakage reactance aborbs 100MVAR at full load (see my calc)
the magnetizing reactance absorbs 0.1 MVAR (see wolf's) calc)

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[electricpete]

19% seems on the high side for a GSU impedance, isn't it?

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[electricpete]

I would say typical GSU fall more in the range 8%, so in the ballpark we may say a typical GSU at full load would consume vars equal to 10% of their MVA rating. To get more exact numbers consider the exact impedance of your transformer and if available phase angle of the impedance.

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[ScottyUK]

Here's a handful of GSU voltage, MVA and impedances from real plants across the globe, mainly Europe and Middle East:

400/26kV, 900MVA, 18%
420/21kV, 555MVA, 16%
132/11kV, 165MVA, 12%
132/11.5kV, 165MVA, 15.5%
220/19kV, 560MVA, 16.25%
220/20, 500MVA, 14%
275/16kV, 399MVA, 18.15%
275/15.5kV, 172MVA, 18.25%

The grid contribution to LV fault level from a big transformer of only 8% impedance would be large, especially on a dense and heavily interconencte dgrid such as that in the UK. Seems very low for a large GSU transformer.

Wolf,

My data is observed on a real plant, reading direct from the generator terminal VAR metering and the export revenue metering at the HV grid connection. The only plant between the metering points is the excitation transformer (2% of unit rating), the IPB, the GSU transformer itself, and the SF₆ CGIT; the unit auxiliaries are from a separate station aux transformer.

ElectricPete has run some illustrative numbers, but have a look at the grid metering and the gen bus metering to convince yourself next time you're on a plant. If the GSU transformers are very different in impedance then the VAR figures will be different also.


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