answer for the problem

[GoldDigger]

It is not completely clear what the OP described.
But, except for a very short transient period, short enough that the two (or more) sources do not drift far out of phase (certainly less than 1/4 period), the frequency of all units in the interconnected system must be the same.
The nature of current flow between the paralleled generators would be such that the faster unit (leading phase, at least at first) will see a heavier load and the lagging unit will see a lowered load or even a motor drive.
That power balance shifting will have the effect of pulling the units to a common frequency even as the governors are trying to achieve a steady state mutually agreed final frequency.

As noted in the referenced paper, separate elements in the network are responsible for effectively tweaking the governor action to reach the target system frequency and even phase. (For example, long term network performance to maintain the right number of ~60Hz cycles in a 24 hour period.)

 

[Ingenieur]

It is not completely clear what the OP described.
But, except for a very short transient period, short enough that the two (or more) sources do not drift far out of phase (certainly less than 1/4 period), the frequency of all units in the interconnected system must be the same.
The nature of current flow between the paralleled generators would be such that the faster unit (leading phase, at least at first) will see a heavier load and the lagging unit will see a lowered load or even a motor drive.
That power balance shifting will have the effect of pulling the units to a common frequency even as the governors are trying to achieve a steady state mutually agreed final frequency.

As noted in the referenced paper, separate elements in the network are responsible for effectively tweaking the governor action to reach the target system frequency and even phase. (For example, long term network performance to maintain the right number of ~60Hz cycles in a 24 hour period.)

the relationship (as in Z) to the load and generator also come into play

 

[Besoeker]

apparently you missed the point
sure they do
during the transient period of load change
exactly as the op described
the 2 machines operate at different freq until the governors restore synch/equilibrium


Speed governors vary prime mover output(torque) automatically for changes
in system speed (frequency). The speed sensing device is usually a flyball assembly
for mechanical-hydraulic governors and a frequency transducer for electro-hydraulic
governors. The output of the speed sensor passes through signal conditioning and
amplification (provided by a combination of mechanical-hydraulic elements,
electronic circuits, and/or software) and operates a control mechanism to adjust the
prime mover output (torque) until the system frequency change is arrested. The
governor action arrests the drop in frequency, but does not return the frequency to
the pre-upset value (approximately 60 Hz) on large interconnected systems.
Returning the frequency to 60 Hz is the job of the AGC (Automatic Generation
Control) system. The rate and magnitude of the governor response to a speed
change can be tuned for the characteristics of the generator that the governor
controls and the power system to which it is connected

"until the system frequency change is arrested."
Not individual frequencies.

 

[Ingenieur]

"until the system frequency change is arrested."
Not individual frequencies.

Wrong

the answer is 2 speeds up initially (1 slows down)

initially
both synch'ed fi
apply delta load 100% to unit 1
1 slows down say 4% initially
2 speeds up but at a lower rate, maybe 1%
during this period the f's differ
1 will stabilize slightly lower than fi (droop) say recovers to 2%
2 will drop and synch at 2% after the delta load is shared
both the same again at steady state but but a bit slower than pre load application

 

[Besoeker]

Wrong

the answer is 2 speeds up initially after steady state (1 slows down)

initially
both synch'ed fi
apply delta load 100% to unit 1
1 slows down say 4% initially
2 speeds up but at a lower rate, maybe 1%
during this period the f's differ
1 will stabilize slightly lower than fi (droop) say recovers to 2%
2 will drop and synch at 2% after the delta load is shared

My post was quoted verbatim from your link.
If you think your source is wrong that's fine.

 

[Fnewman]

Two generators connected to a POCO line cannot operate at different frequencies. As a result, the governors of such systems generally operate on 'load' rather than 'speed' control. If two generators are operating in parallel isolated from the POCO, one is usually set to operate on speed control while the other on 'droop' (power output swings in response to load changes). But in all cases frequency is the same. Very common application in large industrial plants with at least some self-generation.

 

[peter d]

If they are connected in parallel they can't run at different frequencies.

Reminds me of a story told by a British engineer at the old ECN forum long ago. He worked for BT and described a paralleling gone wrong at one of their remote sites. The generator was started and then brought online while not synced either to the supply or another generator (I forgot which.) He said they were pulling pistons and engine parts out of the ceiling of the generator room.

 

[truck41trouble]

2 powerplants are very rarely operating at exactly 60hz. Turbines never spin at exactly 3600 or 1800rpm. There will always be some oscillation between units. Also, an over or under frequency event received by a plant computer will almost always trip the unit instead of trying to adjust the frequency. At the power plant I worked at the protective relays were timed based on the severity of the event. 58.5 hz sustained for over 45 seconds will activate the protective relays and the operators will be in for a bad night. Same thing with 62.5 hz. Very bad for the bearings.

Sent from my VS987 using Tapatalk

 

[JFletcher]

Okay, let me refine my question (mods, feel free to move if its too far off topic):

At the moment of transition from POCO to internal (generator) power, doesnt the plant/site generator HAVE to be at a slightly higher frequency to 'pick up' the load from the grid? The plant I worked at, we had a synchronizer switch that rotated ever so slightly clockwise, and it had to be thrown at 12 o'clock, as mentioned before. At that point, the plant generator and POCO power are in perfect synchronicity, however, since the genset is running at a very slightly higher frequency, it takes the load vs leaving it on the grid. Is this grossly incorrect? Is it phase angle and not frequency that determines what source supplies ALL the power?

I'm not talking about 2+ sources running in parallel for any amount of time, I'm talking about switching loads from one source to another, in entirety. And iirc, our plant genset was an EMD 710 20cyl with ~4500HP and could take a 2.1MW load right off the grid without so much as a hiccup.

 

[topgone]

Okay, let me refine my question (mods, feel free to move if its too far off topic):

At the moment of transition from POCO to internal (generator) power, doesnt the plant/site generator HAVE to be at a slightly higher frequency to 'pick up' the load from the grid? The plant I worked at, we had a synchronizer switch that rotated ever so slightly clockwise, and it had to be thrown at 12 o'clock, as mentioned before. At that point, the plant generator and POCO power are in perfect synchronicity, however, since the genset is running at a very slightly higher frequency, it takes the load vs leaving it on the grid. Is this grossly incorrect? Is it phase angle and not frequency that determines what source supplies ALL the power?

I'm not talking about 2+ sources running in parallel for any amount of time, I'm talking about switching loads from one source to another, in entirety. And iirc, our plant genset was an EMD 710 20cyl with ~4500HP and could take a 2.1MW load right off the grid without so much as a hiccup.

Very small generators usually don't really matter much when synchronizing unto the "system". The synchronization point maybe off by a fraction of a cycle but never too much or flying objects will be seen!

The idea running the incoming unit a wee bit faster than the in-service power is actually a good measure to make sure your generator will not suck incoming power in when the unit is connected to the system. Once the synchronizer sees the synch values in phase/same magnitude, the breaker can be closed. Good synchronization requires the frequencies and voltages to be just within allowed limits at the moment your unit closes its breakers.

Then it is necessary to speed up the incoming generator some more, to pull-in the required load it wants to take, while care is also done in slowing down some of the generators in the system to bleed some load-->the same amount of load the newly connected genny takes in.

 

[Besoeker]

Two generators connected to a POCO line cannot operate at different frequencies. As a result, the governors of such systems generally operate on 'load' rather than 'speed' control. If two generators are operating in parallel isolated from the POCO, one is usually set to operate on speed control while the other on 'droop' (power output swings in response to load changes). But in all cases frequency is the same. Very common application in large industrial plants with at least some self-generation.

The voice of reason. Thank you for that.

 

[iwire]

At the moment of transition from POCO to internal (generator) power, doesnt the plant/site generator HAVE to be at a slightly higher frequency to 'pick up' the load from the grid?

No

Think of it as meshing gears on a transmission, ideally the gears RPM will be identical. Once you are latched in you can put the hammer down.


I too had to manual bring generators on line with one of those clocks but I think I had from say from the 11:30 to 12:30 position to flip the switch.

 

[Fnewman]

Then it is necessary to speed up the incoming generator some more, to pull-in the required load it wants to take, while care is also done in slowing down some of the generators in the system to bleed some load-->the same amount of load the newly connected genny takes in.

Not exactly; once the breaker has been closed the frequency no longer changes. Using the same control that was used to match the frequency of the POCO system, you are then adding power instead of speed which translate to increased loading on the generator. The reason you synchronize the incoming a little 'fast' is so that you don't trip the reverse power relay as soon as the breaker is closed.

 

[Ingenieur]

My post was quoted verbatim from your link.
If you think your source is wrong that's fine.

partially and selectively quoted
when the load increases the machine will slow down
it will eventually recover in steady state

 

[Besoeker]

partially and selectively quoted
when the load increases the machine will slow down
it will eventually recover in steady state

Think about what synchrounous means and read what others are telling you.

 

[Ingenieur]

2 powerplants are very rarely operating at exactly 60hz. Turbines never spin at exactly 3600 or 1800rpm. There will always be some oscillation between units. Also, an over or under frequency event received by a plant computer will almost always trip the unit instead of trying to adjust the frequency. At the power plant I worked at the protective relays were timed based on the severity of the event. 58.5 hz sustained for over 45 seconds will activate the protective relays and the operators will be in for a bad night. Same thing with 62.5 hz. Very bad for the bearings.

Sent from my VS987 using Tapatalk

good point
machines may operate at slightly different speeds but the grid at a composite
this is at steady state

during a fault, step load change or load transfer/sharing, ie transient conditions, the delta may increase but will return close to nil at steady state

no control system is instantaneous

 

[Ingenieur]

Think about what synchrounous means and read what others are telling you.

I suggest you pull out Fitzgerald's and review and convince yourself

synchronous for A GIVEN MACHINE mech speed = elec speed
DIFFERENT machines can have different mech, and hence, elec speeds
the controls minimize this in steady state
during transients different story

 

[Besoeker]

I suggest you pull out Fitzgerald's and review and convince yourself

I don't need convincing.
Others here have explained better than I.
Accept and move on.

 

[Bugman1400]

I think there is still a bit of confusion from the OP's question and the responses and actual technical fact. To preface, I've operated and sync'd many large and small generators (15MW to 1050MW). I think what the paper from the previous link is trying to teach is for large or small systems that may have a frequency excursion due to some event. Smaller grids, such as small countries or islands, commonly have this problem. But, the Northeast had the same issue back in 2003 that affected +50Million!
It is typical for any grid (large or small) to have a frequency range between 59.95 and 60.05 Hz.....yes, this is just like having a range for voltage of 0.95 to 1.05pu. And this frequency changes throughout the day.
As generation units are sync'd to the grid, the idea is to match the unit frequency with the grid frequency. You also need to match the voltage magnitude and the phase relationship. The 12 o'clock reference pointed out previously is in reference to the phase relationship. Ideally, you would like them to match before sync'ing to mitigate the transition when the breaker is closed. If the generator phase is behind the grid phase when the breaker is closed then the grid will "pull" the generator into sync with the grid. If the generator phase is ahead of the grid phase then the grid will "push" the generator into sync. If the grid has to pull the generator then Amps will flow from the grid into the generator during this transition and vice versa if the generator is pushed.

To the OP's question, when two generators are cut loose from the grid and have to carry an isolated load, several things can happen. If the load is excessive, it will swamp the generation and the frequency will continue to decline until the protection or control system removes the units (opens the breaker). If the load is matched to the generation capability and the governor is equipped to run in an isochronous mode (island), it will try to react to the capability of the prime mover control system. A governor system that is controlling two units in an isochronous mode is very difficult. Obviously, feedback loops are involved from each unit into the governor controller. I have seen where two units hunt back and forth against each other trying to maintain a stable frequency.

 

[topgone]

Not exactly; once the breaker has been closed the frequency no longer changes. Using the same control that was used to match the frequency of the POCO system, you are then adding power instead of speed which translate to increased loading on the generator. The reason you synchronize the incoming a little 'fast' is so that you don't trip the reverse power relay as soon as the breaker is closed.

No need to split hairs there!.
I am trying to explain how loading up a unit is done right after synchronization -->turning the speed control knob/ clicking on the increase speed button on the speed control.
On the reverse power protection, I have no quarrel with that--> it's just that it will be better understood if one explains why that certain protection actuates, that is because your unit is taking in power instead of exporting. That simple.