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

 

[electricpete]

Thanks Scotty. The GSU's at my plant are 8%. Now I know we are way on the low side of "typical"

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rcw retired EE]

A couple of comments-

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.

 

[ScottyUK]

Good catch rcwilson - I'd forgotten that ANSI-land and IEC-land quote impedances slightly differently.


----------------------------------

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

 

[wolf39]

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

http://WWW.HYDROPOWER-CONSULT.COM

 

[ScottyUK]

Wolf,

You contribute far too much experience and expertise to have any reason to bow your head for any of us.


----------------------------------

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