Hello guys,

I understand isochron and droop control modes, I also know droop must be set around 3~5%. but I can't seem to pull them all together in my mind.

can anyone explain droop setting for following cases without load sharing.

1. island operation. one isochronous genset with other droop gensets. all droop setting of gensets must be equal? all gensets size must be equal?

2. island operation. only droop gensets. what about droop setting and gensets size?

3. mains parallel operation. the droop can be set as what you want between 3~5%?

 

Hello,

So, would you please tell us what you understand droop speed control to be--what it does and how it does it? Just the basics--no load sharing, no isochronous just the basics of droop speed control. I'm looking for two answers, minimum--because droop speed control does many things, but two of them are the most important functions. You can list as many as you think droop speed control provides, but you must list at least two.

1. No. When gensets are synchronized together either on an island or an infinite grid they are all acting as one generator supplying one load (all of the motors and lights and televisions and computers and computer monitors). And, gensets of just about any size and type (from reciprocating engines, to hydro turbines, to wind turbines, to steam turbines, to gas turbines can all be synchronized together--as long as they "play nicely together" when synchronized. The most common way to ensure gensets "play nicely together" is to use droop speed control. But, gensets don't have to be the same size nor have the same droop to be synchronized together and "play nicely together." Gensets can be of any size, and have any droop setting which promotes stability when synchronized with other gensets.

2. It's entirely possible to synchronize two, three, four or more gensets together with all of them operating in droop speed control mode. No problem. BUT, someone or something (like a PMS--a Power Management System) must manually adjust the governor settings of the units to maintain frequency control as the load changes (the number of motors, lights, televisions, coffee makers, tea kettles, computer and computer monitors increases or decreases). [That's what the Isochronous machine does--it automatically adjusts its output to maintain frequency as load(s) change. Without someone to do that, or something (like a PMS), frequency control is going to be erratic especially if the load changes a lot or very quickly.

>3. mains parallel operation. the droop can be set as what
>you want between 3~5%?

3. Yes, and, no. Most types of genset prime movers have typical droop settings. For example, many steam turbines have 5% droop, and many gas turbines have 4% droop. Some diesel engine-driven gensets have 3% droop, and so on. The problem with changing droop from what the prime mover manufacturer specifies is two-fold: First, it may negatively impact genset stability (cause oscillation/hunting of the prime mover, and hence the generator output). And, second, most grid regulators (including small island operations) like to know what the droop settings of the various gensets which are synchronized to the distribution system are, for planning purposes and for stability calculations. Changing the droop setting of an individual genset governor can have serious consequences--not only to the unit operation, but also to the grid during a grid disturbance event. So, best not to be changing droop settings without careful consideration of all the knock-on effects, as well as notifying the grid/system operators/regulators (who may not like the suggested change at all!).

Hope this helps! And looking forward to hearing how you explain droop speed control!

 

Dear CSA,

Thank you very much for your reply.

so far as I know, droop expressed as the percentage reduction in speed that occurs when the load increases from zero. So in the speed control the target speed decreases in the same percentage as the droop setting, while the load increases.

droop speed control provides:

1. stable fixed power control. when gensets are synchronized together on an infinite grid, each droop setting are adjusted to generate a fixed amount of power.

2. indirect load sharing among gensets in parallel are proportional to their power rating.

 

cbcdiligence,

On an AC (Alternating Current) power system of any size--the frequency of the system is related to the speeds of the generators (and their prime movers). And when synchronous generators are synchronized together on an AC power system of any size no single (nor multiple) generator(s) can go any faster or any slower than the speed dictated by the system frequency per the formula:<pre>F=(P*N)/120</pre>So, this idea that speed changes when load changes when a machine is operating in droop speed control is balderdash--pure, unadulterated balderdash.

This means that on a 50 Hz system, some generators can't run at 49.67 Hz while others run at 50.13 Hz and still others run at 50.02 Hz and still others run at 49.94 Hz, and so on. EVERY generator--and its prime mover--runs at the speed that is proportional to the number of poles of the generator and the frequency of the system. (Some prime movers don't operate at synchronous speed, but rather operate at a speed that is, usually, higher, and are coupled to the generator rotor through a reduction gear to convert the prime mover speed to synchronous speed.)

If there is no frequency control on the system, or if there is a single generator operating in droop speed control mode supplying a load and the load changes without the operator taking any action--then, yes, the frequency will change as the load changes.

Droop speed control is about how much the load changes for a given change in the speed error--the difference between the prime mover speed reference and the prime mover actual speed (which is governed by the number of poles of the generator it is driving and the frequency of the system with which the generator and prime mover are synchronized. So, a machine with 4% droop will change load by 25% of prime mover rating for each 1% change in the speed error. A machine with 5% droop will change its load by 20% for each 1% change in speed error.

Droop speed control relies on the fact that some other "entity" (governor; operator; control system) is controlling grid frequency and keeping it stable. In this way, the prime mover/generator actual speed remains stable--and constant. By changing the speed reference, the error between the reference and the actual speed will change--which will change the energy flow-rate into the prime mover which will change the electrical load of the generator. <b><i>But the speed of the prime mover and generator doesn't change--it's fixed by the system frequency with which it is synchronized.</b></i>

Droop speed control is the governor mode that allows multiple generators to be synchronized together to supply a load that is larger than any single generator could supply--and do so stably and without large frequency oscillations (presuming the system operators are astute and anticipate and respond to load changes appropriately).

Droop settings are almost always determined by the prime mover manufacturer, and are "approved" by the system regulators of the AC power system the genset will be synchronized to.

And, finally, because speed error is the difference between speed reference and actual speed, if the speed reference is stable (which it usually is when a unit is producing power) and the actual speed changes (meaning the system frequency changes) then the output of the genset changes--and <b>should</b> change. That's necessary to support system stability, and while most power plant operators and their supervisors and management don't think their genset output should change when the grid frequency is changing they're wrong--flat wrong.

Anyway, your description wasn't bad--but it fails to account for real-world operation. It's regurgitated from ivory tower text- and reference books which don't properly state the conditions of the description. No AC power system would stay in operation for very long--nor the operators have their jobs for very long--if the speed (frequency) changed when load changed. That's just not how AC power systems are supposed to be operated--they are supposed to be operated to maintain system frequency as close to rated as possible (well, in most parts of the world, anyway).

One more thing which might be contributing to your misunderstanding--and that's the term "droop setting." To me, droop setting, or droop setpoint, is the amount of droop the genset's prime mover is programmed to have. So, for example, 4% or 5%--and this DOES NOT change while the machine is operating.

What DOES change while the machine is operating is the prime mover speed reference--which <i>might</i> be termed the "droop speed control setpoint" or "-setting." When an operator wants to increase the load of a machine with 4% droop from 37.5% of rated power output to 62.5% of rated power output (for a machine rated at 20 MW and operating at 7.5 MW and who's load is to be increased to 12.5 MW) even though the operator is watching the MW meter as load is changing what's really happening is that the speed reference is changing by 1% (25% of machine rating, or 5 MW for a 20 MW machine). The change in speed error because of the change in speed reference is what causes the energy flow-rate into the prime mover to change which changes the torque being produced by the prime mover <b>at a constant speed</b>, which the generator then converts into amperes to increase the MW.

So, be sure of your terms and meanings and don't let authors confuse you by improper use of terms, or failure to specify terms. Droop speed control is about the change in energy flow-rate (torque; amperes; watts) for a change in the speed error of the prime mover--which is the difference between the speed reference (droop speed control speed reference) and the actual speed (which is a function of system frequency when synchronized to a system with other generators).

Hope this helps!

P.S. Remember--the amount of power being produced by a system of synchronous generators must exactly equal the load being supplied by the system (the total number of motors and lights and televisions and computers and computer monitors) in order for the system frequency to be at rated. Increase the power being produced by any one of the generators without reducing the power being produced by any other of the generators will result in a frequency increase--if there is no isochronous machine controlling frequency, and no one or no system is monitoring system frequency and adjusting the load(s) of the gensets to maintain system frequency. It's all a very delicate balancing act--made even more difficult as load on the system changes, and it does change as motors are started and stopped, lights are turned on and off, televisions are turned on and off, and computers and computer monitors are turned on and off. Usually in a very uncoordinated manner--people being people.

 

Dear CSA,

still a question:
you said, Droop settings are almost always determined by the prime mover manufacturer, and are "approved" by the system regulators of the AC power system the genset will be synchronized to. Most types of genset prime movers have typical droop settings. For example, many steam turbines have 5% droop, and many gas turbines have 4% droop. Some diesel engine-driven gensets have 3% droop, and so on.

Is droop the mechanical characteristics of prime mover or it is something what we programmed in the speed controller? why droop in different types of genset are different?

 

cbcdiligence,

Yes; it's somewhat related to the mechanical characteristics of the prime mover and associated equipment. Most steam turbines can't be loaded as quickly as many gas turbines (well, actually, the steam turbines can be loaded quickly but in some cases steam production from the boiler can't be ramped up as quickly so pressure and temperature will drop which will eventually reduce steam turbine load until the boiler can "catch up"), and some hydro turbines can't be loaded very quickly at all (there is either too much inertia on the slowly rotating machine, or the flow through the penstock can't be increased very quickly). And some reciprocating engines can be loaded very quickly. So, the choice of droop setpoint, or sometimes called droop regulation, is somewhat dependent on the type of prime mover and it's characteristics as relates to it's ability to be loaded quickly or not.

Some of it is just historical precedence, too, I believe (that's my own personal opinion). "We've always done it that way," is a common refrain in the power generation industry--not a great reason or answer, but it just seems to be how some things are done. (Thankfully, digital control systems and young engineers without much historical perspective or tradition are changing some of those things. (Too many, in some cases--but one has to take the good with the bad, eh?))

Droop setpoint is something programmed into the governor (speed controller), but the choice is somewhat related to historical precedence, mechanical characteristics and system requirements (when possible).

But the key is that, once determined, the droop setting of a genset is not to be changed--while it's usually adjustable, it isn't (adjusted). It doesn't vary automatically during normal operation--that's the speed reference that is varied to change load (or the actual speed when there's a system frequency disturbance). And system regulators factor the droop setting of a machine into their system stability calculations--so changing the droop setting of a machine can have very serious knock-on effects under some circumstances.

Hope this helps!

 

Dear CSA,

Thank you very much! very helpful!