Our machine is GE 9fa with MkVie controller.

Generally when we are importing or exporting the Vars Generator Control is in Var Control. But when the Var setpoint is zero (not exporting or importing Vars from the grid), I observed the Generator Control changed to Voltage Control. Does this Generator control changeover is automatic, if yes why its needed. Even though our Gas turbine have 3 controls(Var control, voltage control, Pf) all the time i observed generator control shifted to Var control when we are exporting or importing Vars to grid and if not its under voltage control. To my knowledge from the posts in control.com i came to understand that Generator control in Pf mode leads to maximum efficiency (when PF=1 as setpoint)..Can anyone please clarify about this...

As well as we have GE D11 steam turbine where we have only pf/var control only but no voltage control available.And all the time both pf and var control are in enable condition. I was unable to understand one thing Pf setpoint is .8 and when we are exporting or importing vars that time Pf is almost near to 0.97 lagging or leading ...but never i have seen PF value reaching to .8 (i think its not desirable to reach even I know) but as both PF and Var control enabled at a time i was in a confusion.

 

We have no idea from the information provided how your plant is configured. And we would need to be able to look at the Mark VIe to be a me to comment for certain.

If your plant control system (DCS or BOP) is capable of controlling the reactive power flow across the tie breaker to the grid then it is probably sending signals to the Mark VIes to control excitation to maintain some VAr setpoint across the tie breaker. If that's happening then the gass- and steam turbine generator exciters CANNOT be in automatic VAr or PF control when they are receiving signals for some external controller. And, depending on how the external controller is configured and programmed the VAr flow out of the two generators may or may not be equal to each other.

In general, if either generator is in automatic VAr or PF control and <b>EITHER</b> the operator clicks on VOLTAGE RAISE or -LOWER <b>OR</b> some external controller is enabled to send exciter RAISE/LOWER signals to the Mark VIes then automatic VAr or PF control is disabled--automatically.

So, without understanding how the tie-line VAr/PF control is accomplished it's really not possible for us to say how your plant is to be operated or if it's operating correctly.

As for the relationship between VArs and power factor, 0 VArs equals 1.0 (Unity) power factor <b>at the generator terminals or at the tie breaker.</b>. The most efficient operation is when the VAr flow is zero--which is the same as when the PF is 1.0 <b>at the generator terminals.</b>

But, if you have to maintiain a certain VAr setpoint (or PF setpoint) at the tie breaker, then it's probably not going to be possible to operate the generators at 0 VArs (1.0 PF) and maintain a certain setpoint at the tie breaker.

I have never seen any generator that does not have a voltage control mode--that's the normal generator exciter control mode. VAr or PF control are optional modes of generator (not turbine) control. (The confusion comes in because the signals to raise and lower generator terminal voltage (which affects BOTH VAr and power factor) are done through the Mark VIe turbine control and/or HMI.)

So, without a LOT more information and knowledge about your site, how it's configured and controlled, we can't say too much more.

 

Sriram,

>We have no idea from the information
>provided how your plant is configured.
>And we would need to be able to look at
>the Mark VIe to be a me to comment for
>certain.

Should have read:

We have no idea from the information
provided how your plant is configured.
And we would need to be able to look at
the Mark VIe to be <b>able</b> to comment for
certain.

What I also didn't say very well is that VAr/PF control is usually done on a "per generator" basis--meaning that each generator controls its own VAr or PF output (or input as the setpoint might be). When enabled on the HMI for a particular generator that is almost never for a tie-line breaker or some 'remote' location.

So, my presumption is that someone is trying to monitor or control VArs or PF at the point of connection (tie or tie-line breaker) with the larger grid or utility, and is trying to do so by enabling VAr/PF control at individual generators. And that's not going to work very well.

Or, a separate control system (say a DCS, or even the grid operator or utility) is sending signals to control the VAr/PF at the utility tie breaker, and someone at the plant believes the individual generator VAr/PF control needs to be enabled when that is happening. And, that's likely not true and likely not going to work well either.

Most all GE turbine-generator control systems send signals from the turbine control system to the generator exciter ("AVR") to raise or lower generator terminal voltage. If the turbine control system (yes, the Speedtronic turbine control system) has a generator control mode option with the appropriate sensing device (usually a VAr transducer) then the turbine control system (yes, the Speedtronic turbine control system) can be set to monitor the VAr "flow" <b>OR</b> the PF <b>at the generator terminals</b> and will send signals to adjust the generator terminal voltage to make the VArs or the PF equal to the setpoint when enabled.

For a plant with multiple turbine-generator systems, there is usually a single connection with the larger grid/utility, and the VAr "flow" or PF at that point in the system is not usually the same as at individual turbine generator terminals. To control the VAr flow or PF at the utility tie breaker, it would be necessary to control the VAr flows or PFs of all the turbine-generators producing power at that time. And, that will likely also depend on how the plant is configured (where the Main Step-up Transformer is located, or if each turbine-generator has it's own step-up transformer (sometimes called a GSU--Generator Step-up Transformer).

There are a lot of variables we would need to understand--but suffice it to say that if one is trying to control VAr flow or PF at a utility tie breaker when two or more generators are operating to provide power (real and reactive) to the utility/grid across (through) that breaker, then one would NOT likely enable the individual generator VAr/PF control modes to do so. It would most likely be done by a separate (third) control system providing signals to each generator to increase or decrease each generator's terminal voltage as necessary to control the VAr flow/PF across some common breaker (the utility/grid tie breaker).

Hope this helps!

(And, yes. It can be confusing when the turbine control system is sending signals to the generator exciter control system. It's my personal believe that most generator control systems nowadays are perfectly capable of controlling VArs or PF on their own, but because the turbine control system has been used to do so in the past for so many years (decades) the practice continues to this day. Which can be misleading to many when they first encounter the practice.)

PF can be thought of as the ratio of real power being produced to reactive power "being produced." If the reactive power being produced is zero, then the PF is 1.0--meaning that 100% of the energy being input to the generator is being converted to real power (watts), and none is being converted to reactive power (VArs). If 10% of the energy being input to the generator is being converted to VArs, then only 90% of the energy is being converted to real power (watts)--and the PF will be 0.9. PF is a measure of the efficiency of the generator, since we don't really want to produce VArs (there generally isn't a VAr-hour meter on the generator output used to generate revenue (money) for the turbine-generator operator. There's usually only a watt-hour meter for measuring the watts being produced and that's how the turbine-generator operator gets paid: by what the watt-hour meter reads. So, it's not really in the turbine-generator operator's (and the turbine-generator owner's) best interest to generate VArs AND watts (KW or MW)--because they are only getting paid for watts, and not VArs. So, by keeping the VArs low, and the power factor high (as close to 1.0 as possible) the energy the turbine-generator owner/operator is buying to produce electricity is going into producing as many watts as possible--and the power plant is usually only getting paid for producing watts. And because there is usually only a watt-hour meter being used to determine how much energy the turbine-generator is producing, by keeping the watt production at maximum (by keeping the VAr production to a minimum--which can be monitored using either the VAr meter or the PF meter) then the power plant owner can receive the most money for the power being produced versus the money spent for the energy to produce the power.

Many contracts with utilities require power plant operators to maintain a certain VAr output or PF, especially at certain times of the day or year. VArs are a necessary evil in AC power systems, and the utility doesn't want to have to produce all the VArs required all the time--that costs money to do so, and so they try to get others to share in the cost of producing VArs. And one of the ways they do so is by writing into the purchased-power agreements (contracts) some requirement to produce so many VArs or maintain a certain PF at certain times of the day or year.

Again, except at the biggest customers (oil refineries; metal-working foundries/mills; very large factories) utilities don't charge their customers for VArs--even though almost everyone uses VArs (most all AC motors "consume" VArs--so refrigerators and fans and air conditioners all "consume" VArs). (The cost of VArs is factored into the watt-hour rate people pay for electricity, but there's almost never a VAr-hour meter--only a watt-hour meter.)

In the same way if you have two turbines producing watts and supplying power to a grid through a tie breaker, and you want to control the amount of power being sent to the grid through the utility tie breaker you will usually set one turbine to produce a relatively constant output of watts and use the second turbine to produce a variable output to control or limit the amount of watts going out to the utility through the tie breaker, VArs are similar. In fact it's exactly the same.

The difference here is that the utility voltage can--and does--go up and down during the day, and throughout the year. So, that means it's a little tougher to maintain a constant VAr flow or PF at the utility tie breaker at all times. But, one would still likely set one generator to produce a constant VAr flow or PF, and then use the second to produce a variable VAr output or PF in order to be able to control the VAr flow or PF at the utility tie breaker.

Watts and VArs can both be thought of being produced and consumed. The difference is, one you get paid for and one you don't. But, you can't ignore the one you don't get paid for! If you produce too many VArs, then you won't be able to produce as many watts.

 

Dear CSA sir,

Firstly my sincere apologies to you for late reply. You had given a very great explanation, but i am finding it hard to digest in one time reading. I will go through your explanation and comes up with more doubts....Hope you can help me even further...

Sriram

 

Dear CSA sir,

Our Plant is configured into 3 blocks. Each block constitutes 2 Gas turbines with 2 HRSG's and one Common Steam turbine. Each Turbine has its own Generator with its own corresponding Generator Step up transformer. So..Total 9 Generators...(6 Gas turbine+ 3 Steamturbine).

Gas turbine having 52G as synchronizing breaker which lies b/w generator terminals and corresponding step up transformer..(52L all the time in close condition corresponding to Gas turbine), where as steam turbine having 52L (line breaker) as synchronizing breaker.

Generally Grid operators used to call us to increase or decrease Grid KV by 1 or 2 kv like that...So according to that we give var setpoint to each and every generator equally (sharing) almost....

I felt tie breaker u r telling is nothing but LIne breaker...

To control the grid voltage we are adjusting the vars flow of all turbine generators producing power at that time...