Generator Droop / Isochronous Mode

[NickParker]

1. How does droop control ensures load sharing of gensets in Parallel?
2. Why is it necessary to run the generators in parallel with loads in proportional to their rating? What happens if we don't?
3. If there four generators, 3 generators to be run in droop mode while the other has to be run in Isochronous mode? Why? What happens if all the generators are to be run in droop mode?
4. By increasing the steam/fuel, why don't the speed increase during the droop mode? Instead, only the power output increases.

 

[waross]

1.- For a 60 Hz standby set, with 3% droop, the no load frequency will be 61.8 Hz. As the frequency drops, the set picks up more load.
As the load increases, the frequency and speed drop slightly.
As the frequency drops below 61.8 Hz the sets all pick up their share of the load in proportion to their ratings.
2.- It is not necessary to run the sets with loads proportional to their settings.
3.- They can all be in droop mode. We ran a micro utility with all sets in droop.
No-one ever noticed the slight frequency variation.
4. The speed is controlled by the grid frequency. If a single set has enough capacity relative to the grid load to slightly change the frequency, the swing set will drop load to correct the frequency.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Residential grade UPS type power supplies are very frequency sensitive and many will cut in within the normal droop speed range. Also, plug-in alarm clocks run fast but who uses those anymore?

 

[waross]

Also, plug-in alarm clocks run fast but who uses those anymore?

Most of our digital alarm clocks plug in and are not frequency sensitive.
The last time (Actually the last 3 or 4 times.) that I used a synchronous motor clock was to check the frequency of a small generator.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[catserveng]

Maybe you will find this helpful in answering your questions, Woodward Application manual attached.

MikeL

 

[ScottyUK]

Hi Bill,

Slight aside but over here intruder alarms seem to use a mains cycle counting clock rather than a quartz oscillator. I have no idea why, particularly as there's a uP onboard with a crystal of its own. Weird!

 

[crshears]

OP wrote:

1. How does droop control ensures load sharing of gensets in Parallel?

To which waross responded:

1.- For a 60 Hz standby set, with 3% droop, the no load frequency will be 61.8 Hz. As the frequency drops, the set picks up more load.

I'd reverse the two parts of that last sentence, viz., "as the set picks up more load, the system frequency drops in directly reverse proportion." This assumes the governor to be set for speed droop operation only without any superimposed isochronous elements. I'd go even further and state "as load is applied to the unit" [ verb transitive ] instead of "as the set picks up more load" [ verb intransitive ] , since most units are not capable of picking up load themselves absent some sort of automated computer or PALC enabled to perform the necessary switching functions.

OP also wrote:

2. Why is it necessary to run the generators in parallel with loads in proportional to their rating? What happens if we don't?

To which waross responded:

2.- It is not necessary to run the sets with loads proportional to their settings.

I totally agree with this statement.

I would however add an answer to the question at the end, viz., "What happens if we don't?" by saying that as the load increases, the more highly loaded units may / will eventually run out of capacity as they reach full load, with the result that as those units have run out of maneuvering room they will no longer provide any frequency stabilization to the system. One way to address this requires lowering the speeder setting on the maxed out unit so that it is carrying less load but is once again able to contribute to frequency stabilization. In this instance, the overall system frequency will drop slightly and all of the other units connected to the system will pick up the load that the single unit in question stopped carrying.

The other way to address this scenario is to pick up load on some or all of the other units supplying the system; the slight increase in system frequency will be detected by the governor of the maxed-out unit, and it will decrease its energy input so as to satisfy its present governor setting, while simultaneously re-enabling it to contribute to system frequency stabilization.

Which approach is taken depends upon whether the system frequency is higher or lower than nominal; if the frequency is right where it should be, both operations would be performed together.

Another thing the OP asked was:

4. By increasing the steam/fuel, why don't the speed increase during the droop mode? Instead, only the power output increases.

Bill responded:

4. The speed is controlled by the grid frequency. If a single set has enough capacity relative to the grid load to slightly change the frequency, the swing set will drop load to correct the frequency.

Again completely correct.

In the other scenario Bill suggested when he wrote

3.- They can all be in droop mode. We ran a micro utility with all sets in droop. No-one ever noticed the slight frequency variation.

, if there is NO "swing set" in isochronous mode on your system, the unit undergoing

increasing steam/fuel

WILL in fact increase the system speed/frequency, however this change will be very slight and, as previously noted, directly proportional to the amount of system load that unit took on.

I hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Something that we haven't mentioned is the action of Central Load dispatch, load control panels or plant operators.
As the load on a system is increasing, one of the above authorities will call for more generation to be added on-line.
It is generally inefficient to run all of the generating sets regardless of the load.
The sets may be run at 80% or 90% of rated load.
As the load on the system increases, more sets are added to keep the load on the individual sets near the target load.
As the system load is dropping, sets are taken off-line.
In our small, all droop system, the operators recorded the parameters and manually trimmed the frequency every 15 minutes.
In a large system with a swing set, the load on the swing set is monitored to keep the swing set at a power level that allows for adjustments for either positive or negative load swings.
An exception:
95 Years ago this power house was British Columbia's largest source of hydro-electric power.
The old Stave Falls Power House
In the 70's this power house was under manual control.
The generators ran at either 10% output to keep the sets warm and turning or at 90% output when the power was needed.
I lived about 2 miles away at the time and dropped in one day.
The one operator on duty was happy to have a visitor and give me a tour of the station.
The plant was running at 10% output.
While I was there, the phone rang.
It was central dispatch requesting 90% output.
The operator started to advance the set-point of a droop governor to pick up load.
The operator would stop from time to time and manually adjust the field current to bring the PF up.
He did this about three times before he reached 90% output.
Then he repeated the process for the other four generators.
When the afternoon peak was past, the phone would ring and the sets would go back down to 10%.

Frequency control:
A load change is shared by all the generators in the system and has little effect on the frequency.
The very small drop in frequency is detected by the swing set which increases its output to equal the increased load and corrects the frequency back to 60 Hz.
At 5% droop;
If 100% output is required, the governors of the sets will be set to 63 Hz.
If 50% output is required, the governors will be set to 61.5 Hz. (60 Hz + (50% x 3 Hz))
If 10% output is required, the governors will be set to 60.3 Hz. (60 Hz + (10% x 3 Hz))


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

From the Wiki article linked above.
The Stave Falls Power House as it looked when I toured it.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Thanks for that, Bill. Presumably "central dispatch" had some means of determining the river flow throughout the year, and would adjust the number of units in service so as to pass the amount of water available while avoiding both inefficient operation and spill...

The turbines look very much like horizontal Francis units; the design of these is such that they can tolerate extended periods of operation @ 10% gate [ or was it 10% of rated output? ] without sustaining damage due to cavitation.

Operating generators in stations and cows in barns in the winter can both have the same effect: keep the structure and the earth surrounding it from freezing temperatures, dramatically extending its life.

For sure there are scaling factors that come into play as the sizes of grids increase; central dispatch will not only match power generation to load, but determine which generating unit, or even an entire generating station, to use as a "swing set" or, in my case, swing plant.

Automatic generation control [ AGC ] is employed in interconnected systems, and will ramp the output of such plants or large units up and down based on a combination of system frequency and tie line bias, i.e., if a sudden drop in grid frequency is accompanied by a corresponding increase in flow from your system to neighbouring systems, there's clearly a problem external to your system which the central dispatchers of that system must address; for "your" central dispatchers to increase generation in this instance without request or consultation would be big-league counterproductive.

I do like smaller systems a lot, though, due to their often rudimentary nature; when analyzing them and their behaviour one can strip away a lot of those other complications and simply think things through.

Understanding the basics of how small or isolated systems operate can stand you in good stead when it comes to dealing with the more complex ones.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Central load dispatch responds to fluctuations in system load in real time by calling more or less generation on-line.
There was another department that forecast water availability and snow packs in the mountains.
This department looked ahead a year or more and instructed central dispatch as to which generation units to call up.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!