Measuring VARs
Measuring VARs
(OP)
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
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






RE: Measuring VARs
Alan
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"It's always fun to do the impossible." - Walt Disney
RE: Measuring VARs
Bill
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"Why not the best?"
Jimmy Carter
RE: Measuring VARs
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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If we learn from our mistakes I'm getting a great education!
RE: Measuring VARs
RE: Measuring VARs
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Measuring VARs
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 :)
RE: Measuring VARs
RE: Measuring VARs
RE: Measuring VARs
Could I use the Geneartor Gross Reactive (MVAR), Aux Load (kW), and the transformer impedance to obtain the Aux Reactive Load (MVAR)?
RE: Measuring VARs
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If we learn from our mistakes I'm getting a great education!
RE: Measuring VARs
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
WWW.HYDROPOWER-CONSULT.COM
RE: Measuring VARs
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RE: Measuring VARs
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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RE: Measuring VARs
Qpu = sqrt(3)*|Ipu|^2 * Xpu
Once you plug in Ipu=1, it doesn't make any difference
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RE: Measuring VARs
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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RE: Measuring VARs
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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RE: Measuring VARs
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RE: Measuring VARs
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RE: Measuring VARs
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!
RE: Measuring VARs
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RE: Measuring VARs
Tx Z%- ANSI transformers are usually specified on the impedance at the base (self-cooled) rating. An ONAN/ONAF/ONAF 100/133/166 MVA transformer might have an 8% nameplate impedance at 100 MVA which is 13.3 % at 166 MVA. IEC usually spec's the %Z at the top rating. (Same unit is listed as 13.3% 166 MVA.)
I would probably have a 150 MVA generator connected to that 166 MVA GSU transformer. Assume the generator is putting out 1.0 per unit load or amps = 1.0. The 8% transformer impedance is 12% on the generator base. Since VAR losses = I*I X, we can calculate the MVAR's "lost" in the transformer. (Neglect the transformer resistance and assume X=Z). MVARtx = (1.0)(1.0)(0.12)= 0.12 per unit or 18 MVAR's in this example.
If the generator was operating at 150 MVA 0.85 pf, MW = 150*0.85 = 127.5 MW. The generator MVAR's = 79 MVAR. Transformer output = 127.3 MW (wild guess losses = 200 kW), and MVARs = (79-18) = 61 MVAR. Total ouput to the utility = 141.2 MVA at 0.90 power factor.
That's why we spec 0.85 power factor generators to achieve 0.9 to the utility.
You can make the same estimate for the MVAR losses in the Auxiliary transformer. You probably have a meter reading the auxiliary loads. If you only have a kW reading, or just the amps and volts, assume 0.85 power factor for the loads and you will be in the ball park.
RE: Measuring VARs
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If we learn from our mistakes I'm getting a great education!
RE: Measuring VARs
Thank you folks:
I had the Grand Coulee G22 - G24 generators in mind when commenting on this topic. These 805 MW units have a power factor of 0.975, thus giving us an electrical output of 826.64 MVA. I know that these hydro generators and quite a few others in Washington state are practically running at unity power factors all the time.
By checking numbers on my 600 MVA generator step-up transformer example (19% impedance), I arrive at a reactive load of 114 MVAr. This impressive figure, however, "deteriorates" the power factor by about 0.02 only and this is almost negligible for generators with power factors of 0.9 or even 0.8.
I bow my head to Scotty and Pete.
Regards
Wolf
WWW.HYDROPOWER-CONSULT.COM
RE: Measuring VARs
You contribute far too much experience and expertise to have any reason to bow your head for any of us.
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If we learn from our mistakes I'm getting a great education!