Openning switchyard Breakers...

[Lenz81]

Gentelmen

Two generators are feeding the same line through two different transformers and one common switchyard breaker, if I open the Breaker for that line both Gens will be off the grid and technically they should trip their 52G breakers because of either overspend or over frequency, but as soon as they go off grid they will push power toward each other ,can it cause any damage to them before tripping their 52G breakers?
I hope my question is clear enough.

Thanks in advance.

 

[TheBlacksmith]

If the governor is properly matched and working correctly, the unit should take a 100% load dump without overspeeding. At least the military units I work with can. Generators don't store power, they make it, if both units are in speed droop and voltage droop as they should be for parallel operation, neither should be a significant load on the other and the voltage regulater should sharply cut excitation and the governor should sharply cut power. If the energy in the transformers magnetic fields or the line causes the breaker to restrike it will get interesting.

 

[odlanor]

I only know hydropowerplant:
1-With overspeed there is a hydraulic shutdown:
-open gen, CB,open field CB , stop machine and close intake gate.

2-With overfrequency is not very used. Sometimes there is a partial stop:
-open gen. CB,open field CB ,put gen. to unload position.

3-underpower is a problem of turbine not generator. In this case , Quantitatively speaking, there is no problem.

 

[davidbeach]

There will always be an overspeed associated with a 100% load rejection. It may be that the overspeed stays within acceptable limits, but there will be an overspeed; no place else for the input power to go and no throttle is fast enough to prevent it. Excitation has nothing to do with speed.

 

[rbulsara]

What type (prime mover) and size of generators are this? What type of application? Prime gen? standby/emergency? The consequences of sudden loss of loads are different for different systems. (I presume you are talking about accidental loss of full load).

Regardless, the set up you describe is pretty common, in fact that I can't think of any other way to connect a multiple gen plant to a grid.

A properly designed (engineered) and tested system should work just fine, meaning the overshoot will be within safe limits.







Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

"Excitation has nothing to do with speed."

If I may qualify that statement a bit, the rotor speed does respond to loss of excitation, i.e. speed increase abruptly upon loss of excitation. But that's a different story.
Lenz81, loss of load on generators will result to overspeeds, the degree of which depends on your inertia and control responses. Once your generator senses loss of load, protections will act to shut your units down:
[ul]1. With zero load; main steam stop valves full-open and governors at the level prior to load loss, the turbine speed will rise abruptly.[/ul]
[ul]2. Overspeed relay activates and sends trip signal to turbine and generator controls[/ul]
[ul]3. MSV goes closing, generator breakers opening, etc. [/ul]
In the process, you cannot expect both of your units to be pushing each other as main breakers could have isolated each generator unit upon loss of load.

 

[Lenz81]

Thanks gentlemen for good explanations.

 

[odlanor]

Another question is:
Overspeed commands stop valve at same time with open breaker
or
Overspeed commands stop valve, and at no load position open the breaker ?

 

[ScottyUK]

An overspeed can not occur with the breaker closed so the second case can not occur. The breaker opening initiates the overspeed event and the governors or main stops respond to the overspeed by closing to regain control of the machine. On a Westinghouse designed machine the MSVs remain open during a load dump and the governors slam closed until the speed comes down, then re-open to idle the machine ready for re-sync.


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If we learn from our mistakes I'm getting a great education!

 

[odlanor]

ScottyUK,
A load rejection could happened very far from CB , submiting
generator suddenly without any load AND with CB closed!
Ovespeed OCCURS because turbine is transmiting hidraulic power to generator without load ,independent of any governor!

 

[ScottyUK]

Odlanor,

A synchronous machine connected to the system can not overspeed while the GCB is closed unless the machine either loses its field completely, in which case it becomes an induction generator, or the field weakens to the point where pole-slipping occurs. If the grid local to the machine separates itself from the rest of the system then possibly what you describe could happen, or if the system is very small and has only one or two major loads. I guess that situtation may exist on an island or somewhere similar but I've never heard of it happening in an established and interconnected grid.


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If we learn from our mistakes I'm getting a great education!

 

[odlanor]

1-At empty load the wicket-gate are open less then 10%. At full load the wicket gate are open 100%.
If you running at full load with wicketgate completely open, and if you suddenly remove the load , it will occur overspeed into turbine. It is independent of excitation.
2-Your governor should respond to this overspeed in order to
bring wicketgate to empty position(<10%). They SHOULD guarantee this when you loose load at normal condition.
3-If you made load rejection tests using CB generator you will have a governor perfomance different then when you remove load remotelly.

 

[waross]

The speed of a generator is controlled by the grid when the generator is connected to the grid.
The speed of a generator is controlled by the governor when the generator is off the grid.
If the grid connection is lost on a loaded generator, the load is lost. The generator reverts to governor control but the prime mover is producing power so the generator increases speed and frequency. The initial rate of speed increase is determined by the mechanical inertia of the rotating elements of the generator. The governor sees the increase in speed/frequency and reduces the power to the prime mover until the speed is correct.
Now it depends on the inertia of the rotating mass, the response time of the governor and the setting of the over-frequency trip as to whether you get an over-frequency trip when the load is lost. Some units never trip, some will always trip and some will sometimes trip.
I have had issues with small standby generators sometimes tripping. When grid power is returned, these units transfer the load and then shutdown. These units run automatically and are unattended. From time to time the load dump would trip them out on overspeed. They would have to be manually reset, but as the load was back on the grid there would be no indication of a problem until the next grid failure when the set would fail to start.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

Bill,
How big is your "small" standby generators? The last unit I was involved with were design to be unattended too. I am thinking your issues could occur with that last install we did!

 

[waross]

Typically 17 KA to about 50 KVA. Typical no load frequency setting is + 3% or 61.8 Hz. (51.5 Hz for 50Hz sets.) If the no load speed is a little high, spurious overspeed trips may occur on shutdown. You may set the speed a little lower but beware of UFRO. At 2 Hz or 3 Hz below nominal frequency, "Under Frequency Roll Off" will start to reduce the voltage setting. The solution is usually a compromise that is application specific. (Small adjustments on site)
Notes; Most sets work well with the factory settings, I suspect that on some, the factory settings are off a little. I have installed several identical sets and only one had overspeed trips on shutdown.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[byrdj]

Two generators are feeding the same line through two different transformers and one common switchyard breaker, if I open the Breaker for that line both Gens will be off the grid and technically they should trip their 52G breakers because of either overspend or over frequency, but as soon as they go off grid they will push power toward each other ,can it cause any damage to them before tripping their 52G breakers?

The more I though of this, the more interesting this question became as I though back to all the utility steam turbine generators I have worked on. I don’t recall any 52 logic that was purely speed frequency related.

If the prime mover torque does not equal the generator load, there will be a change in speed, no mater how small. The generator load is dependant of the system current usage.

First, all the turbine generator controls for this were design to respond to a full load rejection, remain in service carrying station load and return to a condition that allowed immediate reclosure to grid. The test was have the unit at full load with station service transformer feeding and open the switch yard tie to grid. Upon the loss of generator load, the turbine torque causes the unit to accelerate at 10% per second. The CVs are quickly closed by the governor before the speed exceeds 105%, but due to entrained energy, the unit speeds peaks about 110% a couple seconds later and then coast back down. The energy in the steam generator is expended by the unit with the speed about 106%. This can take several minutes, once the steam generator energy being generated equals the steam energy for the turbine, the unit will be running near rated and ready to reclose.

While the turbine generator controls can easily provide this control, the steam generator would normally trip since rods and recirc could not be runned back to prevent a high flux scram or feedwater causing a high level scarm or boiler MFT. In the event the governor fails and the speed exceeds 110%, the emergency governor will trip the stop valves. The generator 52 logic has a trip when SV closed AND reverse current, open the generator breaker. This allows any counter torque from the generator magnetic field to add in reducing the turbine acceleration.

Now what would happen with two units is where it becomes very dependant on a lot more of the equipment design. I first though bout how a cross compound unit responed. A cross compound is two separate turbine generators (a HP section and LP section) with one governor. The two generators are synchronized while on barring gear and those HP generator motors the LP generator during roll to rated speed. Looking at a load rejection, the low inertia high energy density HP section will motor the LP generator during the intial acceleration, but during the steam generator blow down the LP will motor the HP. For a generator to be motored, it must be designed to do so. This is more copper, more cooling, etc and thus more cost. Very few units are designed to motor for long periods of time.

Back to the original question…and it still greatly depends upon the equipment. For example lets say the station has a newer unit and an older low inertia low pressure simple cycle unit. In this case the acceleration rate of the newer unit will greatly exceed the older and the older will be motored to stay in synch with the new. If the motoring event exceeds the reverse current settings, then the 52 logic will open its breaker.

Another possibility, The emergency governor settings can be very different with the possibility the older unit is below 110% and thus the ewer unit take it to its emergany trip. If the accelerating motoring did not trip the older units’ breaker, it will now motor when the newer unit returns to rated speed.

in areas with a lot of hydro (NE US and E Canada), I do recall some older were equiped to motor. I even recall a new unit going through the factroy for a Canadian Nuclear designed to provide turbine reverse cooling flow and generator motoring

As I see it, this would be a very interesting system to model but a lot more information is needed.

 

[odlanor]

waross,
....The governor sees the increase in speed/frequency and reduces the power to the prime mover until the speed is correct. ....
I would say there is different response of governor for overspeed resulted from grid lost OR CB opened!.
For the later, governor return more quickly to no load speed.
It must be observed that governor has 3 regulation mode: opening regulation, speed regulation and power regulation.
I don know exactly how they operate during this condition

 

[waross]

There is a setting on a basic governor called the set point. If the speed is above the set point, the governor reduces the power to the prime mover. If the speed is below the set point the governor increases the power input to the prime mover.
A load control panel is more sophisticated and may monitor the load so as to give faster response to block changes in loading. Normally a generator would be taken off line by first reducing the loading. Dropping two generators off line with one switch must be seen as an unwanted event. If the generators are tripping on over speed/frequency there may be a problem with the governor, with the control valves or with the protection settings. Has the installation always done this?If so, review the protection settings. If the installation has just started doing this, check for binding linkage in valve control mechanisms, check the calibration of the protection relay and look for any change in operating parameters.

Bill
--------------------
"Why not the best?"
Jimmy Carter