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Plant Performance Q&A
This issue's question and answer are taken from an e-mail exchange between GP's Joe Nasal and one of our clients.
Question
Hi Joe,
Thanks for the VAR explanation! The debate around here is whether or not reactive power is free. If so, the argument goes that producers should just hand it over with their real product (power). Please comment on a scenario when a generator is operated at design MVA and unity power factor. It appears that the generator's rated capabilities in real power capability decrease as the lagging MVAR loading is increased due to armature heating and other generator limitations.
Wouldn't it be more profitable for the power producer to supply more true power and decrease reactive loading rather than producing reactive power and having to reduce true power output due to generator limitations?.
Thanks,
Fran Mara
Alliant Energy
Answer
Fran,
As you point out, reactive power is not exactly free, which is why most utility rate structures charge for VARs supplied to their industrial customers (residential and commercial customers typically do not pay for reactive power). To respond to your question, I will refer to two graphs. The first graph shows a typical "Estimated Reactive Capability" curve provided by GE, and the second graph shows a typical "Generator Losses" curve provided by GE for the same generator.
Let's have the generator initially loaded at unity power factor (no reactive load) to 500 MWe (or 500 MVA)—from the Generator Losses curve, it can be seen that the I2R losses (the curve does not include mechanical losses associated with windage, pumping H2, and bearing friction) are approximately 6.4 MWe. These losses are due to the heat generated in the windings of the generator (field & stator) due to current flow. This also represents the amount of heat energy that must be removed by the hydrogen coolers and/or water-cooled stator system (if provided).
Now let's add some more field current by increasing the DC voltage to the rotor field so that we 'pick up' (supply) 200 MWARs of reactive load, but we will not move the turbine control valves (i.e., shaft power from the turbine is not changed). Note that our apparent power is now about 538 MVA, which is a combination of the current flowing through our windings for the true power along with current for the reactive power we are now providing to our customers. When we reference the 'Generator Losses' curve at 538 MVA, the I2R losses have increased to about 6.8 MWe - an increase of 0.4 MWe in heat losses that must be removed by the generator cooling system(s). What this means in reality, is that gross generation has decreased to 499.6 MWe with the same shaft input power from the turbine (first law effect).
The key part of your question refers to decreasing generator capability primarily due to the cooling limits of the generator - in other words, efficiency is not really impacted that much (as shown above), but the ability to deliver power has now been curtailed as the generator 'picks up' reactive load and starts to create more heat due to the increased I2R losses. Fortunately, most generators are provided with enough copper and cooling capacity to increase reactive load considerably before they are limited. In the example given, the generator is rated for 728 MVA at .9 power factor; 655 MWe of true power and about 140 MVAR's of reactive power. However, it should be noted that the generator could in fact be over-excited to carry about 300 MVAR's at the rated 655MWe of true power and a hydrogen pressure of 60 psig.
Anything above that reactive load will most likely result in overheating, and a generator winding high temperature alarm will be annunciated. At that point, either reactive load (MVAR) or true power load (MWe) must be reduced to avoid damaging the generator.
There is also another aspect to reactive power that power plant operators shouldn't overlook - that is the impact on the transmission lines and distribution equipment. The current associated with providing reactive power is real current and has real I2R losses associated with it. For this reason, transmission lines with large reactive loads will have an adverse impact on transmission line capacity concurrent with overheating similar to the generator.
So in the end; our customers need reactive (magnetizing) current provided from somewhere in the system, and the power plant generators make a convenient source for providing this current at varying rates simply by adjusting field current (terminal voltage) versus tap changers on transformers or placing capacitor banks in service. All within the operating limits of the generator of course—which was your point originally.
Hope this answers your question,
Joe
Joseph R. Nasal, PE: Mr. Nasal is Vice President of Energy Services at GP. Over his 33 year career, Joe has helped thousands of power plant personnel assess and improve plant performance with his practical, hands-on approach.
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