I have taken over the operation of a cogeneration plant with 2 10mW steam turbine generators. One with a Woodward 505 and he other with a GE MarkV governor. When we break parallel with the utility grid, our system puts the Woodward into isoch and the MarkV into Power-extraction mode. In addition by the time we make these mode changes, we are already in the process of shedding load.

With the MarkV in this mode it will not change its loading and drives the frequency up. The Woodward on the other hand drives the other generator right off line.

How can I force the MarkV to droop and have it change its loading when the frequency changes?

What would be the down side of going to isoch on reverse power in the tie breaker if we do not separate from the grid?

 

I suggest you get someone familiar with Mark V steam turbine control applications to come and investigate what can be done. You say "...our system..." puts the two controls into specific modes. Is it some signal from a DCS? What mode is the Mark V being operated in just before going into Power-extraction mode?

Usually, when these kinds of control mode changes are implemented, it's because of some requirement which was expressed by the plant designer or the plant commissioning personnel during the original installation and commissioning of the steam turbine.

It may also be a requirement of the steam turbine and/or the steam "host" or the condenser. Without being able to understand your plant's configuration and operating requirements, including the type of turbine (direct condensing vs. back-pressure control, etc.) it's really difficult to say what can be driving the control system or why it might be necessary to be in one mode or another at least for some brief period of time. That's why it's suggested that someone come and look over the installation and look at the sequencing to see what might be causing the mode switch and then to determine what can be done and implement the change(s) is possible.

Let us know how this progresses.

 

It is the nature of a generator running in isoch to consume load to maintain frequency. Think of a isoch as a frequency standard or atomic clock.

 

I've been thinking about this some more, and I have a few doubts.

Isoch units are unloaded and loaded in one of two ways: the first is by changes in the load connected to the unit (motors stopping or starting; lights going off or on; etc.), and the second is by increasing or decreasing the load of droop units connected in parallel with the Isoch unit. So, I don't know how one could put a turbine in Isoch control and have it "...drive the other generator right off line...."

One cannot (usually) load or unload an Isoch unit manually like a Droop unit can be loaded or unloaded ; it's reference is a speed setpoint (which is usually 100%) and it's feedback is actual turbine speed and it has very "tight" integral control. If one changes the speed setpoint, then the frequency will change, but the load doesn't change. The load is a function of the number of motors and lights and such being driven by the Isoch unit, and the power output of the unit will change as the number of motors and lights and such changes--or, if a unit or units being run in Droop control are "loaded" or "unloaded". The only change an operator can make to an Isoch unit which is driving a load is to change the frequency of the output, not the load, except by changing the "load" on Droop "units" which are being operated in parallel with the Isoch unit.

I'm really not clear about the sequence of events. And I know Woodward governors have some pretty unique features, but I'm not sure how any control system in Isoch mode can be used to drive another generator off line.

 

I've been thinking about this even more, and re-reading and then re-reading the original post again. I still can't make out if you're describing one scenario or two, and I'm still not really clear about the sequence of events. We just really don't have a lot of information.

But, if you're unloading the Woodward-equipped unit (which is in Isoch mode) by shedding load and doing nothing with the Mark V-equipped unit's power setpoint, then it's conceivable how the Mark V-equipped unit would not be reducing load and could be causing the frequency to increase once the Woodward-equipped unit reached zero load and was taken off line and more load was shed. What's happening to steam pressure during this event? How is the steam pressure and flow from the "boiler(s)" being reduced? What is being used to generate the
steam: some kind of boiler or some kind of heat recovery steam
generator on a combustion turbine exhaust? Are there other generators connected in parallel to the two steam turbines? Does each steam turbine have its own steam generator/boiler or are they fed from a a common line from one or more boilers?

I would suggest that the Mark V-equipped unit is already in Droop mode, and that the load on the unit must be reduced first before the Isoch unit can control frequency. There should be some kind of power setpoint and/or extraction pressure setpoint that the Mark V-equipped unit is trying to control to that should be reduced to reduce the load on the Mark V-equipped unit.

You say you break parallel with the utility; is this to shut the units down? Is it because the facility is being disconnected or tripped off the grid? What is the facility load that you need to support while separated from the utility? How long can you remain separated from the utility? How do you parallel with the utility when you can?

And, most importantly, how did they do things before you took over operations? Is this something new for the operators? Did they have this same problem during these separations before? Has something changed in how the plant is being operated?

In the case where you need to reduce load to less than the output of one generator and keep that generator on line (sometimes referred to as "island mode"), one suggestion would be to reduce the power/extraction setpoint(s) on the Mark V-equipped unit as load is being shed until the generator breaker can be opened, and let the Woodward-equipped unit run in Isoch mode to control frequency as you continue to shed load to keep the frequency at rated. The Isoch unit, if capable, should be used to control frequency, and if necessary the other unit(s) be operated in Droop when paralleled with the Isoch unit.

Again, if you're not doing anything to reduce the load on the Mark V-equipped unit as load is shed, then it will increase the frequency once the Isoch unit is unloaded and taken off-line and load continues to be shed. If the Isoch unit is taken off line without any other unit switching to or being switched to Isoch mode, then frequency control will only be possible by manually controlling the Droop unit(s) (and that is a possibility, it's just not easy depending on the nature of the load(s) being driven by the Droop unit).

Lastly, only one unit should be in Isoch mode at a time. Multiple units can be operated in Droop mode, but there will be no automatic response to frequency excursions. If the frequency increased, the operators could reduce the load setpoint on the Droop unit(s) to lower frequency, or vice versa if frequency decreased. But, without a unit operating in Isoch mode there will be no automatic frequency control in response to load changes, which is what you seem to be describing.

If it's not possible to change power- or extraction setpoints on the Mark V-equipped unit, then something is blocking the setpoint changes or there is some atypical control mode active n the unit. Again, these kinds of oddities usually occur in response to problems experienced or perceived to have occured during initial start-up and commissioning.

But, we still don't have enough information about the plant and what is the ultimate goal of load shedding: to remain on line at some reduced load or to shut down all the generators after separation from the utility.

 

Just guessing!

When they disconnect from the grid, the frequency on the droop generator is slightly lower that the isoch.

Since the are islanded at that point, the isoch consumes all of the available load and causes reverse power relays to open the breaker on the droop machine. It is the nature of a isoch to consume load while maintaining frequency at the governor setpoint.

Sound plausible?

CTTech

 

CTTech,

Think synchronous generators. Synchronism. If two (or more) synchronous generators are operating in parallel, they are operating at the same frequency. (Speed is proportional to frequency: F = (P * N)/120, where F=Frequency (in Hz); P=Number of poles of the generator; and N=Speed of the generator rotor, in RPM.)

The originator said the Mark V-equipped unit wasn't changing its loading, which means that as load is being shed the energy being admitted to the steam turbine it's controlling isn't changing, which is why the frequency is increasing. There's too much energy for the load that's remaining, which means the torque output of the steam turbine exceeds that required to keep the generator rotor spinning at synchronous speed and because it's in Droop mode (straight proportional control) the speed isn't being controlled which means the frequency is increasing. Even if the Isoch unit were on line, it would be driven to reverse power because it couldn't absorb the extra energy from the Mark V-equipped unit.

But, there's still something we don't know about this event. (And, it appears we won't know because we're not getting any input from the originator to provide any of the requested information.) Originally, he said "our system" changes the modes of the two steam turbine governors, and then he says that by the "time we make these changes" they are shedding load. I originally took the "system" statement to mean some kind of automatic control mode change, such as from a DCS; but now I'm not so sure that was correct. System might have meant their operating procedures ("system").

And then there's the part about the Woodward driving "the other generator" right off line. If the Woodward is truly in Isoch mode, the operator can't (shouldn't be able to) change it's load/power output because that is controlled automatically in response to frequency changes as load is shed. So, I don't know if he's referring to the generator connected to the steam turbine with the Woodward governor or the generator connected to the steam turbine with the Mark V. If they are shedding load and the Woodward is truly in Isoch mode, and the Mark V-equipped unit isn't shedding load (because it's Power-Extraction setpoint isn't changing or being changed), then as load is shed the Woodward will reduce the energy input to its steam turbine until its generator breaker opens on reverse power. At that point, the Mark V-equipped unit, with its non-changing Power-Extraction setpoint, will start to drive frequency up as more load is shed.

I don't understand if they're trying to keep the plant on line at some small load until they can resynchronize to the grid, or if they are trying to shut the plant down completely. If they are unloading the plant as described in the paragraph above, then they are *not* going to be able to control frequency once the Isoch unit is off line, *unless* they can reduce the Power-Extraction setpoint of the Mark V as they shed load. But, even then, the Mark V is in Droop control mode (probably) and that's not the best mode to try to control frequency.

But, I'm not going to spend any more time on this thread without some feedback and input from the originator to clear up the information originally posted and to provide the information requested.

 

"In addition by the time we make these mode changes, we are already in the process of shedding load."

Only a droop can shed load, an isoch must be "pushed around".