Isolated Generator and Droop Governor

G

Thread Starter

gshan

I need clarification regarding Can an isolated generator use droop governor?

Is it both isochronous governor and droop governor are simultaneously installed with a generator.

because my question is when in parallel mode a generator was sharing load with other based on droop control. after sudden islanding then governor control should switch over to isochronous control from droop to maintain its own frequency?

or droop governor is capable of giving stable operation in isolated case too?
 
gshan,

First of all, a governor may or may not be capable of both Isochronous and Droop Speed Control. It depends on the governor manufacturer, and if the governor is capable of both then there must be a method for switching between the two (since both governors modes cannot be active at the same time in the same governor).

In my experience, most all of the governors I have worked with or been exposed to were all capable of Droop Speed Control, and most of those were also capable of Isochronous Speed Control. I can only recall one instance of a governor that was only capable of Isochronous Speed Control and not Droop speed control, and that was on a shipboard application on an emergency generator that could only be operated if none of the other ship's generators were on-line producing power.

Second, the purpose of Isochronous Speed Control is to adjust the power output of the prime mover driving a generator to vary prime mover load in order to maintain speed--which is directly proportional (related) to frequency. When other generators and their prime movers are synchronized together with an Isochronous machine if the Isoch machine is doing its job properly the speeds of all the machines synchronized together will not change as load changes (the number of motors and lights and computers and computer monitors connected to the grid being supplied by all the generators and their prime movers).

As the load changes (the number of motors and lights and computers and computer monitors) for a grid with an Isoch unit and one or more Droop units the amount of load being carried by the Isoch unit will change in order to maintain the frequency of the entire network of synchronized machines. Presuming there is no automatic load-sharing or power management system controlling the loads of any of the prime movers and generators synchronized together in this example the loads of the Droop units will NOT change if the Isoch unit is doing it's job properly. (This means that human operators also have to be knowledgeable about keeping the load on the Isoch unit in the controlling range of the Isoch unit's prime mover--which is THE problem in many places.)

If one Droop machine and some of the grid load (some of the motors and lights and computers and computer monitors) gets separated from the other machines and the load is fairly stable when that happens (it's not changing very much) then the Droop governor will be able to control speed AS LONG AS THE AMOUNT OF LOAD REMAINS AT THE EXACT SAME LEVEL AS WHEN THE UNIT WAS STILL SYNCHRONIZED TO THE GRID WITH THE ISOCH UNIT.

If the amount of load (the number of motors and lights and computers and computer monitors) on the "islanded" Droop machine is higher than it was when the separation occurred, then the frequency will decrease. If the amount of load (the number of motors and lights and computers and computer monitors) on the "islanded" Droop machine is lower than it was when the separation occurred, the the frequency will increase. A conscious and knowledgeable may be able to manually increase or decrease the energy input to the prime mover to return frequency to normal and even maintain frequency as load changes (presuming the amount of load does not exceed the prime mover's rating).

Droop Speed Control will not increase or decrease it's fuel to try to maintain frequency. If the load (the number of motors and lights and computers and computer monitors) increases on an islanded Droop machine and the operator takes no action (and there is no external automatic control scheme providing signals to the governor) the load on the generator and prime mover will increase--but the frequency will decrease. If the load (the number of motors and lights and computers and computer monitors) decreases then the load on the generator will decrease--but the frequency will increase.

As long as the load (the number of motors and lights and computers and computer monitors) connected to the islanded Droop unit does not exceed the rating of the generator prime mover as the load changes (the number of motors and lights and computers and computer monitors) the load of the Droop unit will change--but if there is no operator or external automatic control scheme to make additional adjustments to the governor the frequency will NOT remain constant.

Now, if the islanded unit automatically switched to Isoch Speed Control when the separation occurred, then the unit would automatically adjust the energy flowing into the prime mover to maintain rated frequency while also providing only the power required for the number of motors and lights and computers and computer monitors connected to the islanded unit--without any operator intervention or external automatic control scheme.

The answer to your last question really involves how much the load (the number of motors and lights and computers and computer monitors) differs from that which was being produced before the separation occurred, and how much the load is changing after the separation occurs. If the load on the islanded Droop unit after the separation is more or less than before the separation then the frequency will be less or more than rated (respectively).

And, if the load (the number of motors and lights and computers and computer monitors) is changing very quickly after the separation then the frequency is likely not going to be very stable unless the operator is very quick to respond to changes, and the governor is quick to increase or decrease energy the energy flow-rate into the prime mover in response to the operator's commands.

An islanded Droop unit is capable of stable operation (maintaining a steady power output at a relatively stable frequency) as long as the load (the number of motors and lights and computers and computer monitors) is not changing very quickly, or if the operator can make changes to the governor as quick as the number of motors and lights and computers and computer monitors is changing AND the governor is also capable of changing energy flow-rate very quickly in response to the operator's commands.

Most Droop governors are relatively slow to respond to operator commands to change load, while most Isochronous governors are very, very fast to change the energy flow-rate to the prime mover in response to changes in load--BUT Isoch units aren't loaded and unloaded by operators, it's all in response to changes in the number of motors and lights and computers and computer monitors).

Islanded units can be operated in Droop speed control, but depending on how much the load changes (the number of motors and lights and computers and computer monitors) the stability of the frequency may not be very good.

Hope this helps!
 
It is a really very elaborative response from CSA and i believe this subject is very close to his heart and same is with me also.

Here i want to share my experience, we have 3 generators sets running in parallel without connected to grid. These turbines are operating in droop speed mode, however some time we need to operate one turbine in isolation with other two due to load management. During isolated operation also we operate in droop speed control, however i always recommend to site people to operate in isochronous speed control mode. During isolation operation with droop speed mode, any load change in the system, causes change in speed of the turbine as the reference speed in this condition will remain constant. Depending upon the rejection / addition of load capacity the speed either goes up or down. This change in speed corresponds to droop % and hence human intervention will be required to bring down / up the speed to suit to the desired frequency. Hence i feel it is better if we operate in droop speed mode when in parallel and ischronous mode when in isolated condition.

Regards
Rangcharya
 
Dear CSA Sir,

I am very much thankful to you for your wonderful explanation. It really helped in making myself more clear on this topic.

Related to Rangacharya Sir's reply... i want to ask that there are 3 generators running in parallel and their turbine are in droop control mode without connected to grid and if i connect a 4th generator with isochronous gov (assuming this unit is maintaining rated frequency at some load) to that 3 gen system then will there be any effect on the load sharing by other 3 gen.

If the load on iso-unit increases while being connected to other 3 units then that increased load on iso-unit will fully be shared by rest three droop units or all four will participate in sharing.

Regards,
gshan
 
Thank you very much Sir, for clarification.

Regards,
gshan

> It is a really very elaborative response from CSA and i believe this
> subject is very close to his heart and same is with me also.
 
gshan,

Presuming the load (the number of motors, lights, computers and computer monitors) connected the grid being supplied by the three generators with their governors operating in Droop mode was stable and the grid frequency was stable and therefore the grid frequency was stable, when the fourth generator was synchronized to the grid with three generators already supplying the load there would be a negligible effect on the three generators. The newly synchronized unit, whose governor was operating in Isoch mode, would operate at 0 watts/KW/MW as long as the load (the number of motors and lights and computers and computer monitors) remains unchanged.

Now, let's presume some load was added to the grid (more motors, more lights, and more computers and computer monitors). The initial effect on the grid would be for the grid frequency to decrease BUT the Isoch unit would immediately increase its power output to keep the grid frequency at rated. The power output of the Droop units WOULD NOT CHANGE. As long as the grid frequency remains at rated--and the Isoch unit is supposed to do that by adjusting its power output to maintain frequency as load changes (which would change frequency)--the Droop units will remain at their current power output indefinitely. (And in this example, there is NO external load control system or controller, and the three Droop units are operating in Droop speed control mode with NO load reference or "pre-selected load" control function enabled or active.)

Now, let's presume the loads on the four units are as follows:<pre>
Droop Unit 1 Droop Unit 2 Droop Unit 3 Isoch Unit
10 MW 12 MW 5 MW 6 MW</pre>
Now, let's further presume all the prime movers in this example are capable of producing 20 MW (nameplate rating), and that all are in good mechanical condition, the ambient conditions are very near nameplate rating, and the fuel being burned is very close to specification.

As it is approaching evening, the load on the grid is expected to drop by more than 10 MW (as people go to sleep, and the need for power to run the water treatment plant pumps and air conditioners and TVs and radios and computers and computer monitors decreases). If the operators of the units did nothing in advance of this expected load decrease, then the load on the Isoch unit would decrease to zero MW, and as the load continued to decrease further it's likely the Isoch unit's generator breaker would open on reverse power, and the Droop Units would continue to try to produce power at their current levels. That power, being in excess of what was required by the grid would result in an increase in prime mover speed, which would result in an increase in frequency. That increase in frequency would result in the loads of the Droop units decreasing to match the grid load, but at an increase frequency.

However, the power plant operators of the units in this example are knowledgeable and well-trained and experienced, and are aware of the typical load decrease of the grid in the night and in anticipation of the load decrease, they lower the loads of two of the Droop units, 1 and 2, by approximately 10 MW total. The loads on the four units would look like this:<pre>
Droop Unit 1 Droop Unit 2 Droop Unit 3 Isoch Unit
5 MW 7 MW 5 MW 16 MW</pre>
Note that the load on the Isoch unit increased by the same 10 MW which was "taken off" Droop Units 1 & -2. That's because as load on a Droop unit is reduced, the grid frequency would tend to decrease--but the Isoch unit will sense the change in frequency/speed and increase its output to keep frequency/speed at rated.

Note also that the power plant operators did NOT change the load on the Isoch unit--that happened automatically. Note also the load on Droop Unit 3 did not change. All the power plant operators did was lower the load on Droop Units 1 & -2, and the load on Droop Unit 3 remained unchanged and the load on the Isoch Unit increased by the same amount the load on Droop Units 1 & -2 decreased. There is no communication of any kind between the four units--other than speed sensing! And since speed and frequency are directly proportional (as frequency changes, so will speed), the governors of the four units all work together to continue to supply the required power while maintaining the desired frequency. And, synchronous AC generators and their prime movers have been doing this for more than a century--before there was electrical or even electronic governors!

Now, when the load on the grid decreases by approximately 10 MW as anticipated, the loads on the four units will be:<pre>
Droop Unit 1 Droop Unit 2 Droop Unit 3 Isoch Unit
5 MW 7 MW 5 MW 6 MW</pre>
But, something happens at the local refinery which is also powered the four units in this example, and the load on the grid suddenly reduces by about 9 MW. This was unanticipated by the power plant operators, and the load on the four units changes:<pre>
Droop Unit 1 Droop Unit 2 Droop Unit 3 Isoch Unit
4 MW 6 MW 4 MW 0 MW</pre>
And, at the same time the loads on the four units changes the frequency on the grid (and the speeds of the three Droop units) increases to slightly more than rated. Note that the load on the Isoch unit went to 0 MW; that's because its generator breaker opened when the load went below zero MW. The Isoch unit's prime mover is still running at rated speed, but it is not synchronized to the grid and is not providing any electrical power to the grid.

Note that the three Droop units all reduced their loads by the same amount--1 MW each. That's because the four units in this example all have the same rating and most likely have the same Droop setpoint (we'll presume they all have the same Droop setpoint in this simple example, and in real life that's not unlikely if the prime movers are all of similar type, say gas turbines or steam turbines).

The power plant operators, being very knowledgeable and well-trained and experienced, quickly adjust the governors of the three Droop units to bring the grid frequency (and prime mover speeds) back to rated. The resulting loading is as follows:<pre>
Droop Unit 1 Droop Unit 2 Droop Unit 3 Isoch Unit
4 MW 6 MW 4 MW 0 MW</pre>
The operators have contacted the local refinery are have been told the refinery operators will be re-closing their grid tie breaker in a few minutes, which means the load on the grid will increase as soon as the grid tie breaker closes. The power plant operators re-synchronize the Isoch unit to the grid, just in time for the additional load which is being added to the grid by the reclosure of the local refinery grid tie breaker. The loading is as follows:<pre>
Droop Unit 1 Droop Unit 2 Droop Unit 3 Isoch Unit
4 MW 6 MW 4 MW 9 MW</pre>
Note that the additional load from the local refinery after the closure of its grid tie breaker was about 9 MW, and that's the exact amount of the load on the Isoch unit--and the loads on the three Droop units remained unchanged as the local refinery was reconnected to the grid.

That's a very basic example of grid operation with a single Isoch unit and multiple Droop units without any external power management or load control or frequency control scheme, and without any load control enabled or active in any of the prime mover governors. Just pure speed control--Droop and Isochronous speed control. The prime mover governors "communicate" with each other by sensing frequency (speed) changes and adjust their outputs depending on how their governors are configured (Droop or Isochronous).

The take-aways from this example are:

1) When an Isoch unit is being operated between minimum and maximum power output and the load changes do not exceed the minimum or maximum power output of the Isoch unit, the grid frequency will remain at rated and the loads of any Droop units synchronized with the Isoch unit will NOT change--unless manually changed by operators (presuming there is no external power management system or load controller and that the governors of all the units are operating in simple speed control).

2) It is the operator's responsibility to ensure that expected load changes do not cause the power output of the Isoch unit to exceed the minimum or maximum power output of the Isoch unit's prime mover to maintain grid frequency. The Isoch unit's prime mover governor cannot decrease the Isoch unit's prime mover load below minimum (usually zero) or above rated, and therefore anticipated load changes must be considered by operators in order to maintain grid frequency at rated by adjusting the load on Droop units to keep the load on the Isoch unit in the operating range (within the rating) of the Isoch prime mover.

3) The load on an Isoch unit CANNOT be directly controlled by operators. It is a function of grid frequency, and the loads being carried by any Droop units synchronized with the Isoch unit. If it is desired to reduce the load on an Isoch unit, the load on one or more Droop units must be increased. It it is desired to increase the load on an Isoch unit, the load on one or more Droop units must be decreased. And, the Isoch unit will automatically change it's load without any operator intervention with the Isoch unit's governor.

It is possible to calculate exactly how much grid frequency will change when the Isoch unit is unable to increase or decrease it's output. Grid operators try to do this all the time to be prepared for expected grid load changes, some with more success than others.

This is a very simple example of some very stable loads and load changes, and also presumes the grid is geographically small and the distances between generators are not long and there are sufficient impedances between generators to prevent large reactive power swings and voltage changes. As such, this example is very ideal, and in the real world there would likely be other impacts on the grid with the load changes. But, we are only interested in the real power aspects of a small grid in this example and ideal conditions are fine for the purposes of this example.
 
Respected CSA Sir,

Upon seeing your reply to some of the post made that you were earlier associated with GE.

Sir, i am working on microgas turbine as a distributed generation.

In the literature it is reported two types of turbine single shaft (Rowen's model) and split-shaft (GE GAST model).

Is it in GAST model also all three control (acceleration, speed and temperature control) exists as it is used to be with single-shaft??

In most of the research paper, authors has not included the temperature and acceleration controls...though temperature control blocks exists but it does not able to model turbine behaviour at higher load levels where the control is to be done based on exhaust temp.

It is also said that GAST model is no longer WECC compliant (Western Electricity Coordinating Council) and it has been superseded by other model.

Is that GE still manufacture GAST model gas turbine with all three control block intact??
 
gshan,

Although I'm familiar with the Rowen model, I'm not familiar with the GAST model.

To the best of my knowledge, GE still produces gas turbines--and controls--with the same speed, acceleration and exhaust temperature control. Most gas turbine manufacturers have some similar types of controls.

A lot of research papers are written by people who don't have a lot of practical experience. Mr. Rowen is a very experienced person, and very respected in his field. Most gas turbine manufacturers develop their own methods of control, and I'm sure Mr. Rowen was responsible for many of the heavy duty gas turbine control philosophies and practices developed while he worked for GE, and which he documented in many ASME papers.

As for WECC compliance, I haven't been following that situation of late. I do know that NERC is working on droop testing and compliance regulations and I believe, smartly, GE is also participating (so as to have some "control" and input into the situation).

Best of luck with your project.
 
CSA Sir,

I was modeling Rowen's model of Gas microturbine (30 KW)coupled with PMSG (permanent magnet synchronous generator) with turbine shaft speed taken as 96000 rpm.

Sir, I wanted to ask...
1) what is the efficiency of 30 Kw turbine at full load in general?

2) PMSG input is torque in Nm where as Rowen's model torque output in in p.u. How to calculate Torque in N-m? i have taken Tbase as 2.98 Nm(= 30000/10048(speed in rad/s)). Is it correct what i have done? I have used 2.98 Nm, and at no load my speed of the turbine is not able to achieve 96000 rpm in simulation. I am able to understand why it is happening?
 
1) You would have to ask the manufacturer or consult the documentation regarding the efficiency of any prime mover, regardless of size or rating.

2) P.U. means "Per Unit". For example, of the rating of the prime mover was 30 KW, then 0.5 p.u. would be 50%, or one half of rated, or 15 KW. That's true of any engineering unit of measure; p.u. is effectively the percent of rated, where 1.0 p.u. is equal to 100% of rated--be it Nm or lb-ft or KW or MW or joules.

The generator rating is not the important factor. Most gas turbines drive generators that have a higher output rating than the gas turbines--because under certain conditions (cool/cold ambients) gas turbine power output can actually exceed rated. Gas turbines are usually rated at a particular ambient temperature and pressure, and when the machine is in a clean condition, and the inlet air filters are clean, and the exhaust back-pressure is normal, when the ambient air temperature is less than the nameplate rating the gas turbine power output will be higher than rated. So, the generator needs to be capable of a higher output that the nameplate rating of the gas turbine--or else there must be some kind of limiter on the gas turbine to prevent over-driving the generator and damaging the generator, or even the coupling between the generator.

Synchronous generators, when synchronized to a properly operated grid with other synchronous generators, spin at a constant speed--loaded or unloaded. In other words, full load speed is the same as 50% load speed, is the same as 0% load speed (when the generator breaker is closed). It's even the same speed at -5% load!

Now, the microturbine might rotate at varying speeds because the microturbine is not mechanically coupled to the generator rotor--but generator rotor speed doesn't change when synchronized. Just as in the example above. Some combustion turbines have two or even three shafts which rotate at various speeds and only one of the shafts is coupled to the synchronous generator--and that shaft must rotate at synchronous speed. Full stop. Period.

Hope this helps!

I think you are missing some very important aspects of prime movers and generators and speeds and ratings. Prime movers produce torque, which generators convert into amps. Prime movers are typically rated in units of torque (N-m, for example; or lb-ft). But, those ratings can be converted to watts (or KW or MW as the case may be) when prime movers are used to drive generators to produce electrical power. The formulae are very straightforward to convert from N-m or lb-ft or horsepower to watts or KW or MW.
 
hi please put your input on below problem. This is related to shipboard problem.

AE#1/2/3- AE#1/2 can be parallel and sharing equal load.
AE#3 when paralleling with AE#1 or 2 -AE#3 taking full load and other generator taking minimum load i.e-AE#1 or AE#2
Droop setting is as follows-Droop setting of AE1/AE2/AE3:-1/+3/+2.5.

What can be the solution?
Droop should be same or different...i think if Governor is same make, it should be same.

Even if droop is different then how come AE#1 and AE#2 are sharing the load equally as their droop is also different?

CSA can u pls put light on this...its urgent for me.

[email protected]----my email id-prefer if u reply on this
 
>Thank you very much Sir, for clarification.

> Regards,
> gshan

>> It is a really very elaborative response from CSA and i believe this
>> subject is very close to his heart and same is with me also.

as he is with me
 
I'm really not clear on the problem.

The common description of droop as "sharing the load" can be very confusing if sharing is not properly defined.

The definition of sharing when discussing droop means that if the frequency drops the droop machine will "share" (take; accept; produce) some of the additional load which is causing the frequency to drop, thereby helping to prevent a complete collapse of the frequency. The amount of load a droop machine will pick up when the frequency drops (or increases) depends on the amount of droop the governor has.

The other possibility is an Isochronous machine--which does <b>NOT</b> share load. If the frequency begins to drop an Isochronous unit will take all the load that is causing the frequency to decrease. That's why it's not possible (without some kind of Isochronous load sharing method/scheme) to have more than on machine operating in Isochronous mode--they will NOT share any increase (or decrease) in load that is causing the frequency to deviate from normal.

It is entirely possible that two machines with completely different droop settings can have the same amount of load when the frequency is at rated. It just depends on how the operators are controlling the machines.

It would seem <b>from the information provided</b> that AE#3 might be in Isochronous mode after it is synchronized with the other two machines. That would explain why--if the frequency were a little bit low--AE#3 would take as much load as possible and the other two machines would go to some low ("minimum"?) load.

We just don't have enough information to be able to help with this problem. I don't even know what a droop setting of -1 is....

I don't like to take postings off-line. The benefit of this forum is that many people can see the solutions, and most importantly, if people respond to say the information provided was helpful or not then the information is most helpful as others reading the postings can see what works and what doesn't. So, while taking a post off-line (to direct emails) helps one person, it doesn't help anyone else--and, again, that's the real benefit here. Many people can reads these threads and find information and answers, and if feedback is provided they can see if the information was helpful or not.

Best of luck with your problem!
 
Very nice explanation,

You have explained the concepts of droop and isochronous mode of operation of a governor. But I am not sure about how practically the droop settings of a governor are adjusted for the change of load.

I want to know how the droop settings are adjusted practically.....???

It would be very helpful for me to understand the concept clearly.

Thank you.
 
iqbal,

Droop is not a field-adjustable or tuning parameter. Most utilities and grid regulators <b>REQUIRE</b> droop to be maintained at a particular value because that's how they can attempt to calculate how the grid will respond to disturbances.

Because droop speed control is so widely misunderstood and there is so much bad documentation available people mistakenly make adjustments to droop speed control to try to alleviate problems caused by other stimuli.

If droop is changed from 4% to 5% for every 1% change in speed error the load will only change by 20% of rated load instead of 25% of rated load.
 
M

MarktheSecond

The answer is that you don't change the droop settings with a change of load. The setting remains the same (e.g. 4%) for all loads on your machine.
 
Markthesecond,

Thanks for directly answering the question!

I was under the presumption that if iqbal had read the post that it was clear the droop setpoint didn't change as load changed, that the droop setpoint is multiplied by the error between the speed reference and the actual speed to change the load.
 
Top