hello

We have 2 turbines generators MS 6001B control System MKv TMR <I> connected to the grid running in Droop mode.

My question is what are the risks of keeping the turbine in Preselected load (e.g 37 Mw)in case of surges or huge hunting loads. Since as far as I know the preselected load mode is disabled only(?) when i operate the governor manually.

Thanks very much in Advance.

 

What causes the distrubances? Frequency excursions? Fuel supply disruptions? Dirty oil?

Preselected Load Control causes the Speedtronic to adjust the fuel to try to maintain the Preselected Load Setpoint. If the instability is caused by frequency excursions, then the Speedtronic will NOT be properly responding to frequency excursions as a unit running in Part Load ("manual") control would. Preselected Load Control will over-ride the speed error caused by the frequency excursion to try to maintain the Preselected Load setpoint, which is not what one really wants to happen during frequency excursions.

But there is no risk involved in operating a unit at Preselected Load Control--except for the issue of frequency response as described above.

If the surges or swings are caused by other factors, then operating on Preselected Load Control might help, but it might also make the load swings more exaggerated. It all depends on the frequency and the cause.

So, without understanding the causes of the surges and swings, which are NOT normal and should not be considered normal, its very difficult to respond concisely.

If you have specific concerns based on experiences, please provide the details.

 

Thanks for responding this post.

The question is about the behavior of the generator against big disturbances of frequency in the National grid [Argentina] where we are connected to, being in Preselected Load Control.

So as you explain is not convenient to operate the turbine on that mode since the Speedtronic Control system is not going to react as should be. Is that right?

PS: We were told to keep the generator on this mode just to provide a fix rate of load to the grid, which i was not very sure if was for better doing it in that mode.

Thanks very much again

 

Preselected Load Control is really just a <b>lazy man's</b> way of not having to manually make small adjustments to the load.

If an operator manually loads a unit to a desired value, or even if it's loaded to a set point using Preselected Load Control and then Preselected Load Control is canceled effectively leaving the unit in straight Droop Speed Control (just as during manual loading/unloading) there will be very little drift from the value. The amount of load will be affected by ambient conditions (changes in ambient temperature), but that's about all. The variance from the load it was "parked" at will be usually be quite small and
and negligible.

No; operating a GE-design heavy duty gas turbine with a Speedtronic turbine control system at Part Load with Preselected Load Control enabled is not a wise way to operate units which are expected to respond in a normal manner to grid frequency disturbances.

GE will be glad to sell you a fix for their little 'oversight' in this matter; it's called (inappropriately) 'Primary Frequency Response.' (Pretty cool, huh; getting owners to pay for fixing errors in code.)

 

Seek permission to summaries what I can gather from the post.

1. Preselected Load (PSL)- the active power will be maintaining at the value keyed in by the machine operator.The value wont be changing during grid instability.

2. PSL is not desirable by the grid operators as this can contribute to severe instability if many machines are operating in PSL.

3. From machine operator point of view, this is good as not to interrupt with their power scheduling.However the machine frequency also will be affected if grid instability is too severe. (please correct me for this).

4. During minor instability and of course normal stable operation, machine operator will not worry of the machine to over producing as the value is fixed.

5.When standard droop is active, the power feedback for PSL will not be active, the loop will active as normal.

 

gustavo_marcelo,

Permission granted.

>1. Preselected Load (PSL)- the active power will be
>maintaining at the value keyed in by the machine
>operator.The value wont be changing during grid
>instability.

The value the operator keyed in (the Pre-Selected Load Control reference or setpoint) remains active and will not change during grid instability. BUT, during a grid disturbance (frequency excursion/deviation), what WILL happen is the actual load will oscillate (hunt) because the inner- and outer loops (Droop Speed Control and Pre-Selected Load Control, respectively) will be fighting each other.

>2. PSL is not desirable by the grid operators as this can
>contribute to severe instability if many machines are
>operating in PSL.

Yes; it's worse (grid instability, that is) if there are many machines with Pre-Selected Load Control active during a grid frequency disturbance.

>3. From machine operator point of view, this is good as not
>to interrupt with their power scheduling.However the machine
>frequency also will be affected if grid instability is too
>severe. (please correct me for this).

Grid operators will forgive load excursions when there is a grid frequency disturbance--believe me, they WANT the support, AND they DO NOT want the oscillation/hunting that Pre-Selected Load Control (without Primary Frequency Response) causes! Yes; the machine's output will be hunting or will be more or less than the scheduled power if the grid frequency is also hunting while operating on only Droop Speed Control--the magnitude of the load swings will be proportional to the frequency swings. If Pre-Selected Load Control is active without Primary Frequency Response the load swings can be mild to severe depending on the severity of the frequency excursions, also. AND, when Pre-Selected Load Control is active AND Primary Frequency Response is also active, load swings will also occur, and will be proportional to the severity of the frequency excursions.

The difference is: When Pre-Selected Load Control (without Primary Frequency Response) is active during a frequency event the Mark* will not let the load go to the value it needs to go to to support grid stability and will instead be oscillating be hunting and oscillating trying to return the load to the Pre-Selected Load Control reference/setpoint--which is what the grid operators DON'T want. Droop Speed Control only ("pure" Droop Speed Control or Constant Settable Droop Speed Control) will allow the load to go to the value it needs to go to to support grid stability. And, if the unit has Pre-Selected Load Control AND Primary Frequency Response and both are active during a grid frequency disturbance, then the unit will also allow the load to go to the value needed to support grid frequency.

The power plant operators, their supervisors and Management almost universally believe their power plant load MUST remain stable during a grid frequency disturbance. And, if for no other reason other than they think the grid operators don't want the import/export to deviate from the schedule (or because they think their unit should NOT experience load swings when the grid is experiencing load swings--which just shows how little they know about AC (Alternating Current) power systems). The grid operators WANT the support from units operating on Droop Speed Control or FGMO or Pre-Selected Load Control with Primary Frequency Response active. Most grid operators know how AC power systems work--and they know that loads are going to swing in response to frequency excursions, and the system needs that to happen. (That's why some grid operators and regulators are "banning" the use of Pre-Selected Load Control. It's probably coming soon to a grid near you, especially if the grid is prone to frequency disturbances, but even if it's not!)

It's VERY difficult for people to understand one of the most important aspects of AC power systems (for some reason): When units are <b>synchronized</b> together on a grid of any size, magnetic forces inside the synchronous generators keep them all spinning at their synchronous speed (which is related to the grid frequency and the number of poles of the generator field (rotor)). Full stop. Period. No single unit--not two units, not eight units, not 23 units--can run at anything OTHER THAN their synchronous speed when <b>synchronized</b> to a grid with other generators and their prime movers. Everyone thinks that the unit accelerates to rated speed, and then a really complicated thing called synchronization happens where speed (and voltage to a certain extent) are VERY critical, and then once the breaker closes the speed of that machine changes as the unit is loaded. And it just is not so.

To get 50 Hz out of a socket or to an electric motor EVERY generator synchronized to the grid supplying that socket or that electric motor has to be running at 50 Hz. There's no "averaging" of frequency device or magic that happens on an AC power system that allows generators to run at whatever frequency (speed) they want to run at. And yet everyone just assumes that there is, and that, without any investigation or proof their machine runs at something other than the grid frequency (the machine's synchronous speed). It just is not so. No matter how much anyone wants it to be; it's just not so. Never has been. Never will be.

Almost every generator and prime mover synchronized to a grid with other generators and their prime movers uses Droop Speed Control when not operating at Base Load (rated power output,) so that as the speed error changes the energy flow-rate into the prime mover changes, and that as the torque produced by the prime mover and applied to the generator rotor changes the amperes flowing in the generator stator change, the power being produced by the generator will change. But the speed will not change. Yet, most people don't know this--and haven't considered it. They just assume that because the unit speed changes while accelerating, that after <b>synchronization</b> the unit speed is free to do whatever it wants while <b>synchronized</b> to the grid with other generators and their prime movers. Which it is not. Because it is <b>synchronized</b> to the grid--it is rotating at its synchronous speed, which is a function of grid frequency (and all the generators see the same grid frequency!) and the number of poles of the generator field (rotor). Because of magnetic forces at work inside the generator keeping the generator spinning at a speed that is <b>in synchronism</b> with the frequency of the AC flowing in the generator stator.

I know this is probably very confusing. But, AC power generation has several very important fundamentals which most people do not know, have never been told, and have never considered. They just assume. They just close the light switch and expect the lights to come on, and even most power plant operators don't really know how the lights come on--except that there are some wires with electricity in them connected to the switch and to the light fixture. Or to the TV, or to the tea kettle, or to the espresso machine. Or to the refrigerator. And without knowing and understanding these very basic principles it's very difficult to understand how generators and prime mover governors work and how they should work and how to troubleshoot them if they're not working correctly.

>4. During minor instability and of course normal stable
>operation, machine operator will not worry of the machine to
>over producing as the value is fixed.

Yes, if Pre-Selected Load Control is enabled that what's most operators and their supervisors believe. BUT, the same thing will happen if Pre-Selected Load Control is disabled once the unit is at the desired output--because the speed reference won't be changing. So the speed error won't be changing. So, the energy flow-rate into the prime mover won't be changing. So, the generator output (amperes) won't be changing. BUT, you will NEVER convince the overwhelming majority of power plant operators that if they disable Pre-Selected Load Control that the plant won't trip (eventually) because the turbine control system isn't actively controlling load! They just refuse to believe it--because that's how they were told to operate the machine, and that's how they believe the machine operates in their mind (and they haven't really thought about--that's just their perception): that it MUST have a continuous load reference or it will just drift aimlessly at any load (and speed!) and they will lose their job if that happens. And, it's just not so.

>5.When standard droop is active, the power feedback for PSL
>will not be active, the loop will active as normal.

For machines with GE Mark* turbine control systems using Constant Settable Droop Speed Control there IS a load component to the Droop Speed Control function. But, for units with "pure" Droop Speed Control you are correct--there is no power feedback to "pure" Droop Speed Control. It's strictly speed feedback.

Sorry; I've been on the phone this morning with power plant operators and their supervisors and plant mechanical engineers who are insisting their puny 25 MW turbine, <b>synchronized</b> to a gigawatt grid, was operating at 96% of grid frequency for an extended period of time (almost an hour) while <b>synchronized</b> to the grid and with the exciter regulator working normally--and they refuse to believe otherwise. The laws of physics and electricity and magnetism are suspended at their plant. They have no graphs or data to support their position. But they KNOW it was (operating at 96% of synchronous speed, while the grid was at rated frequency, at nearly full load on a cold day, with no excitation or voltage alarms). But, it must be. Nobody even took a picture with their phone of the screen that supposedly said the unit was not at rated speed while synchronized.

But it's not.

I'm not trying to scare anyone away; it's just a fact that most people operating power plants genuinely believe that unless the GE-design heavy duty gas turbine has a continuous load reference/setpoint it will drift aimlessly through time and eventually trip--and they will lose their job when that happens. It seems so logical, right? There have been several posters to control.com who have promised they will write back with the results of canceling Pre-Selected Load Control--but no one ever has.... (I don't even wonder why any more.)

I have no problem being, and remaining, the crazy person in the room. (I just need to learn to let it go. Let it go. Let it go. Let it drift off and not bother me.)

This is not droopspeedcontrol.com.

 

Thank you CSA, you are the expert.