CR4 - The Engineer's Place for News and Discussion®

Previous in Forum: Class 1 Division 1 24 VDC On/Off Switch   Next in Forum: Small Converter Efficiencies
Close

Comments Format:






Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







8 comments
Associate

Join Date: Jun 2009
Location: Singapore
Posts: 28

Diesel Genset Droop Speed Control And Isochronous Speed Control

04/24/2012 8:43 PM

Hi Guys,

I would like to know more about how does droop speed control and isochronous speed control works.

From what I know, when a genset is operating at isochronous speed, the frequency will be constant irregardless of the amount of load there is . Having this in mind, can I safely say that a genset which is operating at isochronous speed when running in parallel with other genset is unable to control the amount of load it takes?

Take for instance, there are two gensets , one operating in isochronous speed ( @ 1000MW ) and the other operating in droop mode (@500MW) running in parallel and there is a total load of 1200MW. If the speed setting is @ 50hz, the isochronous genset will just take the amount of loan required to keep that frequency?

Similarly, for droop speed control , from what I know , the speed of the prime mover / genset will change according to the load. So , this would mean that we can control the amount of load that the genset operating in Droop mode ?

With all these in mind, whenever we talk about load sharing capability , we are actually just sharing the load between all the generators running in droop mode and not sharing the load with the generator operating in isochronous mode?

I am assuming power generation in island mode , with no connection to the power Utility.

Lastly, I do not quite understand how can a genset operate at let's say 1500RPM at all load conditions.

I understand that if there is an increase in load, more fuel will enter the prime mover. If there is more fuel , shouldn't there be more mechanical energy being produced by the prime mover, i.e shouldn't the prime mover move faster which will directly cause the alternator to more faster as well , since they are all coupled together.

I thought the amount of power produced by the genset is determined by the alternator which is determined by the amount of excitation applied to the rotor. Since Lenz's law states that a current will be induced in an opposite direction to oppose the change in magnetic flux (nothing to do with speed ), having this in mind, changing the speed will not affect the amount of current induced?

So sorry for the lengthy question. I have been in the diesel genset industry for almost 5 years and I am quite ashamed of how little I know about genset . I just wish to have a clear understanding of the machine i am dealing with everyday.

Thanks for your patience in reading this !

__________________
There is no limit towards how much one can learn.
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Associate

Join Date: Jun 2009
Location: Singapore
Posts: 28
#1

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

04/25/2012 10:40 AM

Guys any help please ~~

__________________
There is no limit towards how much one can learn.
Register to Reply
Participant

Join Date: Apr 2012
Posts: 2
#2

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

04/25/2012 12:36 PM

Droop versus Isochronous.

First - More Fuel = more power = more watts out of generator and/or more speed.

To get the diesels to share load they need to be in droop, then more load = less speed. If in Isochronous then load will wildly go back and forth between diesels assuming isolated unless you have a fancy governor that can share load without dropping speed.

Regulator = Voltage = VARs NOT EQUAL to POWER has nothing to do with power. We have some operators that are confused by this also since the governor control says speed.

On a utility generator the governor has a speed component included so if the speed is too high the valves close, if too low, the valves open (unless they are wide open anyway). Since it can be considered to be connected to an infinite bus then more energy into turbine == more MW out. Normally lower frequency on the load means lower watts consumed and higher frequency means more watts consumed.

Register to Reply Score 1 for Good Answer
Associate

Join Date: Jun 2009
Location: Singapore
Posts: 28
#4
In reply to #2

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

04/25/2012 8:34 PM

Hi There,

Thanks for your explanation !

So , more fuel = more mechanical power generated by prime mover = more real power generated by alternator?

But, I thought the mechanical power generated by prime mover is in the form of rotational speed of the flywheel ( since the cylinders in the engine is generating more energy and this is actually being converted to rotational movement of the flywheel) which in turns turn the shaft of the engine which is coupled to the alternator and in turn, increase the speed of the alternator?

I have read some somewhere that when the load demand increases, the voltage at the stator windings of the generator will decrease and therefore, the AVR has to supply more excitation current to the rotor of the generator to bring up the voltage to its rated voltage value.

Why will the voltage decrease when the load demand increases?

So sorry for the plentiful questions.

__________________
There is no limit towards how much one can learn.
Register to Reply
Participant

Join Date: Apr 2012
Posts: 2
#7
In reply to #4

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

04/26/2012 8:17 AM

the mechanical power generated by prime mover is in the form of rotational speed of the flywheel

Not exactly - it is more torque gives more power. Think of an auto on a flat highway going 1500 rpm. You need more gas/torque to keep the same rpm going up a hill. Similar priinciple for a generator and load.

when the load demand increases, the voltage at the stator windings of the generator will decrease and therefore, the AVR has to supply more excitation current to the rotor of the generator to bring up the voltage to its rated voltage value.

Correct - If one looks at the current in a rotor for the same voltage & power factor as the load goes up the current goes up to combat the "back emf, Lenz's law" caused by more current flow in the stator. If you did not have an automatic voltage regulator then the voltage would sag.

Register to Reply
Guru
Popular Science - Cosmology - New Member Engineering Fields - Civil Engineering - New Member Engineering Fields - Nuclear Engineering - New Member United States - Member - New Member

Join Date: Aug 2010
Posts: 717
Good Answers: 37
#3

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

04/25/2012 12:49 PM

Take a look here. It should help clear things up for you.

__________________
Sometimes my thoughts are in a degree of order so high even I don't get it...
Register to Reply Score 1 for Good Answer
Associate

Join Date: Jun 2009
Location: Singapore
Posts: 28
#5
In reply to #3

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

04/25/2012 8:37 PM

Very useful piece of information indeed !

Thanks !

But I wish to know in detail some of the theories discussed ( like how does AVR affect the VAR of the generator , how does a governor increase the real power of the generator when it is in parallel with other generators etc... )

__________________
There is no limit towards how much one can learn.
Register to Reply
Guru

Join Date: Apr 2010
Posts: 1538
Good Answers: 95
#6
In reply to #5

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

04/26/2012 7:12 AM

Here is a simple explanation, and I apologize if it's to simplistic. The field windings generate a magnetic field that is rotating. The armature is also an electromagnet. The armature shaft can either be connected to a mechanical load (and it behaves as a motor) or it can be driven by an engine (acting as a generator). As a motor, the greater the mechanical load, the more the armature field lags behind the rotating field. Likewise the more power delivered by the engine, the more the armature field leads the rotating field. When you parallel two generators, then the amount of power each delivers to the common load is determined by how much each armature field leads the rotating field, and this is determined by the throttle setting on the engine. In essense, the generator shafts are connected together with a flexible magnetic coupling.

The governor increases the engine power when the rotation rate is too slow, and decreases it when it is too fast. In isochronous, this control is very tight. Not much speed error is tolerated before the governor adjusts the engine. In contrast, in droop mode, more error is tolerated. If you parallel two engines in isochronous mode, the one that is set slightly higher will take all the load. It is difficult to get the throttles set close enough together for the load to be shared.

The first generator to be connected to the load should be set to isochronous and its throttle adjusted so that the power frequency is correct. Subsequent generators paralleled with the first should be set in droop mode and the throttle adjusted to accept their share of the power (Kilowatts, not amperes).

If the voltage regulators are not set properly, there will be cross currents between the paralleled generators. This extra current is not delivered to the load and only serves to heat up the generator windings. You will notice that the sum of the currents times the voltage is greater the the power reading. Cross current compensation in the voltage regulator detects this condition and adjusts the excitation to the armature to minimize it.

Register to Reply
Power-User

Join Date: Oct 2008
Location: Meherrin Virginia
Posts: 315
Good Answers: 6
#8

Re: Diesel Genset Droop Speed Control And Isochronous Speed Control

05/06/2012 2:22 PM

This particular question is more in the realm of Speed Regulation, if we design a governor for 10% droop or regulation;

Speed at 100% rated is equivalent to 0% load and essentially 0% valve/fuel opening.

Speed at 90% rated is equivalent to 100% load and 100% valve/fuel opening.

Speed at 95% rated is equivalent to 50% load and 50% valve/fuel opening.

Let me explain each of those conditions;

In the first instance, this describes any prime mover waiting to be synchronized. Whatever you are using for a speed sensing device is nulled or has found its place in space.

The second instance describes a unit that is isolated from a system and while running at full speed no load, a load of 100% is applied. The valves/fuel have to open to 100% to satisfy the load. The system speed dropped because the system was designed to droop 10% from no load to full load.

In the third instance and we are still speaking of running in an island mode, 50% load was applied and required 50% valve/fuel opening to satisfy the load. The speed changed 5% because it was designed for 10% speed regulation.

Taking that a step further let's say we have two units side by side both designed for 100 MW and 10% speed regulation. Both units are on an isolated system and a system load of 200 MW is placed on the bus. Neither unit has the capability to pick up the 200 MW so both units decay in speed to 90% and both units are running valves/fuel wide open. If this load is reduced to 100 MW, both units will increase in speed to 95% and the valves/fuel will be 50% open.

This is all assuming you are loading the governor up and not touching it from that point. In actuality you would be biasing the governor up to keep the system speed up. In fact if your governor has 10% regulation the amount of input to the governor to make up for the speed decay would be present in case of a breaker opening and the turbine would increase speed to 110% and require resetting to full speed no load.

One thing that I find interesting about the whole concept is that if you were at full speed, full load, valves wide open and you increased speed to 110% you would drive the valves shut.

__________________
If you fail to follow through, you will fail.
Register to Reply
Register to Reply 8 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

ChaoticIntellect (1); liukawa (3); Older-I-get-Less-I-know (2); otha (1); Rixter (1)

Previous in Forum: Class 1 Division 1 24 VDC On/Off Switch   Next in Forum: Small Converter Efficiencies

Advertisement