Droop, Isochronos speed and Recirculating Current

If you understand the terms droop and isochronous speed, you should understand that if you have zero droop you have isochronous governing since both terms (zero droop and isochronous governing) mean that there is no difference between maximum r.p.m. at no load and full load.

Exactly correct, there are few on this planet that fully understand that statement. You are one of the few!! Many thanks.

Droop is a way to share load between two paralleled generators and has nothing to do with recirculating current. It does have something to do with one unit trying to motor the other, meaning that if two machines don't have the same droop setting one will try to motor the other and/or take more load than the other. As mentioned, Isochronous means zero droop under load; i.e If 1500 RPM no load = 1500 RPM full load then you have zero droop = isochronous operation.

Recirculating current is due to voltage difference. So if one is set at 400 VAC and the other 390 VAC then some current will pass between the machines (recirculate) and heat up the windings without going out to the load. Cross current compensation regulators with a cross current compensation current transformer on phase B are used to control this. Older regulators had droop also, and there was a droop slide resister to correct it.

A simplification is this:

Speed difference = load sharing differences and power (kW) transferring between machines (motoring or reverse power)

Voltage difference = recirculating current between machines

25 years ago we perfected electronic governors and all units operate at isochronous mode and still share the load since the governors talk to each other via 'parallel lines', 2 small wires daisy chained to each governor, and the CTs connect to the governors as well and keep a balance on watt sharing. A 100 kW machine can perfectly share load with a 1000 kW machine in proportion.

With older hydraulic droop governors, you could still load share a 100 kW with a 1000 kW but you had to get both droop characteristics the same by adjusting the droop slide inside the governor head to have each unit react the same % droop under full load. I can tell you how to do this, but it will add 3 looooong paragraphs to this long post and Andy as already fallen asleep 2 sentences ago !

*******

If you have recirculating current on a single machine then you have a low power factor load with recirculating magnetism going from the load to the generator and not doing any work. It heats up the windings, nothing else. Wattless power, so to say, causing heating issues. So a 0.4 PF load will have much more recirculating current than a 0.95 PF load. Pure resistive loads like a electric stove element or 1.0 PF (Unity) meaning the voltage and current wave forms line up on top of each other in Unity. Inductive loads like induction motors cause these two sin waves to split apart and one lags the other by a 'factor', the power factor, and you have recirculating current.

For low power factor loads (low numbers like 0.3 PH but high amps) the generator guy builds the generator different. He used more copper in the stater to limit temperature rise in the stater (due to amperage is much higher) and also puts more copper in the rotor as low power factor loads heat up the rotor more than the stater as the field current (in rotor) goes high to help keep the voltage stable.

Average building load power factor is 0.8 with a mix of resistive and reactive loads so generator manufacturers standardised on rating the generator amperage output at 0.8 PF. For my drilling rig generators that use DC motors and a SCR system (AC to DC) I specify 0.6 PF generators and also oversize the watts 15% to get good stater winding temperatures and rotor temperatures and stable voltage drops under shock loads since all this copper is a type of magnetic accumulator.

Up to now, I have never fallen asleep reading CR4, ESPECIALLY not when reading your interesting posts!!!

This was also a good post from you!!!

Signed,

Totally awake,

Andy

Andy, thanks for your comment.

Re previous. I always thought one used lamps or a synchroscope when paralleling alternators which ensures correct voltage on each phase and correct phase sequence. The needle on your synchroscope (once speed adjustments are carried out on the oncomming alternator) is slowly turning in a clockwise direction and when the needle reaches 12 o'clock alternators are paralleled. NO DROOP!!!!

I tend to agree with you but I also know that my 3 phase knowledge in this area is getting a bit long in the tooth. I worked on 300MW 440 Volt 3 phase generators in my time in the RN......60s & 70s, and due to the design and build time, even before they went the first time to sea, they were old fashioned....

I do believe that there are electronic systems available today which will do all the fancy stuff for you in this area......I could be wrong of course (would not be the first time!)

"I worked on 300MW 440 Volt 3 phase generators"

Off course it was not a single alternator.

That was why I put an "s" on the end of Generator....this means more than one in English.....or were you just trying to be sarcastic?

If you wish further infos, please read on......

If I remember correctly on one particular ship, we had eight steam driven Generators and 2 Diesel driven for emergency and speed. (It takes some hours to bring a steam boiler up to pressure and if you have to leave harbour in a hurry using gas turbine main engines.......!)

We could parallel and feed using any combination of generators, but usually we let each Gen power its own section, or maybe a number of sections (but never the whole ship on one Genny), the reason being that in an emergency, you did not want the whole ship to be powered from either one source or even 2 sources in parallel. Most important pumps etc, could take power from at least two different sections with automatic switchover if we lost a generator. That happened extremely rarely anyway....

Plus the fact that when running in parallel, only one could be in automatic control (its 40 years ago or more!!), the other just took a static fixed load......also not a good situation for long term use.

Hola Andy and PetroPower. Most people have trouble understanding that an Isochronous Generator requires a 0 droop ( Iso= the same speed) and that a droop governing mode is the ability of the Generator to change load (in a synchronus electyrical system, 60 HZ) following a load curve with a gradient (%) slope equal to the droop value (%), actually you are tricking the governor to think he is loosing speed (up power) or gaining speed (release power) by changing the speed changer... and that the droop slope allows you to change load in a very short span say (1% droop ) than in a longer span (10% droop)... if you have worked with old electromechanical hydraulic governors you may have learn the differences on operation.

I am saving this thread for future references, most of the time there are different ways of saying or explaining something and there are many good aproches to the same problem...

The only problem I have is that this thread seems that have started somewhere else and I will like to read it from the begining, any help?

Regards ,Luis

You can see the entries in one of (at least!) two ways, as they happened in terms of who, when and how or the correct chronological order. I find the first better...

Either way can be heavy going, so if I were you, I would try both and see what is best for you personally.

With regards to droop, you appear to be talking about generator droop, not governor, am I correct?

My friend Samak patently did not understand it at all, in fact he got niggled, then rude and then right humpy.....

Hola Andy The governor has the droop and it´s adjustable. When you are governing a generator in parallel to other units, if all the generators are of the same capacity is required for all of them to have the same adjustment on the governor droop, in case one of the generators is quite larger than the others then is recomended to have his governor droop adjusted to lessers amount of droop so this unit may take a greater percentage of the system load changes.

In old electro-mechanical hydraulic governors the droop adjustment was an excentric cam (adjustable from 0% to 5%) conected from the wicket gates acting on the speed changer (decresing the strengh of the spring conected to the governor head and to the speed changer). This arrangement was called Equalizer " The purpose of the Equalizer is to cauce two or more units operating in parallel at the same speed to each take their proportionated or desired share of the load." " when two or more units are operated in parallel, a drooping of the speed ( inherent speed droop ) as the gates open is necessary to assure proper division of load"

In modern digital governors they call it droop or speed droop ( off line and on line) and this value may be changed from 0% up to 10% tipically being the theory the same.

Thanks for your advise . I never fall asllep reading this thread too... Regards Luis

My knowledge is quite old, pre-digital even......

I do remember that the governors on the Gennies we had were flat till in overload, then governor droop came in, I do not remember the percentage, but I would imagine that your 5% was probably not far from reality. It was not user adjustable with a cam or anything.....or at least, we had no access.....

I ought to mention that the gennies were manufactured to accept a 25% overload without any problems whatsoever....quite normal in a RN marine environment for most equipment....

Andy my knowledge is pre-pre-digital from the analogic computers time, but the concept is what matters...

you are right some of them where not adjustable cams set to 2% ( old Woodward Governors) later they made 0-5% adjustable ones...

Early in my apprentice years I worked for 4 years at the Miraflores Power Plant at the Canal Zone, Panama Canal ( U.S. Goverment, Pre-treaty Time) and them move to the Gatun and Madden Dam Hydro Electric Stations and have been around for the last 29 years...

At Gatun Hydro we had 3 Hydraulic Generators rated to 3MW (Francis Hydraulic wheel desing aluminun-bronce Runners) and three hydraulic Generators rated 5MW Steel runners... At Madden Hydro We have 3 Allis-Chalmers Hydraulic Turbines (Francis type S.S. runners) Rated at 12 MW... some Pelton Governors and some Woodward Governors... Today all of them are digital but the theory is still the same... Overspeed limited to 15 %... But strong enough to hold the station on heavy electrical system disturbances...

I guees your system was quite complex and you use to have your instability problems... thanks to this I have learned most of what I know... I mean troubleshooting is a good teacher...

Thanks for your memories... by the way I had to read back againg some of the old manuals to write this entry just to avoid inacuracies...

Regards... Luis

Do you have a mail so I can share you some of my Photo Galleries?

Drop me an email via CR4, I will reply with my own email address....

Your info is very helpful,thanks. If I may, I'd like to use an example and ask a question : asuming three 3ph/ac gensets, all w/electronic gov, operating in parallel, on 1 bus( therefore voltage is the same for all three- a) can this be assumed ? ), and in isochronous mode ; gen 1 = 100kw/125kva, gen 2= 200kw/250kva and gen 3 = 400kw/500kva ; therefore, the total gens = 700kw/875kva ; the load is = 656kw/875kva ( pf = 0.75). b) How is the load shared among the three gens ? c) do the electronic governors & voltage regulators & cross-current compensators all automatically communicate and adjust the proper engine speeds and alternator voltages? d) does the alternator reactance X play any role in load sharing ?

In days of old(!) once the two generators were running in parallel, one would be put into auto mode and the other in manual. The manual would be adjusted to take aproximately half the load and the auto would handle the changing load. This is still a bit dangerous as if the load changes dramatically, the auto generator could actually drop off, if the total load went below 50 or the original, so a careful watch needs to taken....

Its not really to be recommended.

There are I believe modern systems available which will share the load equally between the generators.....

Droop is designed into a generator (RN was 45%) so that you could control the output. Theoretically a generator can be designed without droop, that maintains the voltage irrespective of the load.....I have personally never heard of one actually being built like that myself, but it was said (accurately or not) that power station generators for the national grid have very little droop....how true I cannot say, sorry.

"Theoretically a generator can be designed without droop, that maintains the voltage irrespective of the load.....I have personally never heard of one actually being built like that myself, but it was said (accurately or not) that power station generators for the national grid have very little droop"

Sorry Forum's friend, but I'm afraid, again, you are confused concerned the fact of the governor droop.

Droop is a relation between the output active-power of a generator and its frequency (for parallel operation: fuel/steam/water valve-position instead of frequency). http://www.canadiancontrols.com/documents/technical/Speed%20Droop%20and%20Power%20Generation.pdf

Obviously, there is not any relation between governor droop and the voltage as you can find in the above URL.

However, If I'm wrong in this post, fortunately, nobody will be electrocuted .

Regards...

Samak

You wrote:-

However, If I'm wrong in this post, fortunately, nobody will be electrocuted

.

Which is true thankfully.....

But in this case you should be, because it appears that your training did not cover the needed knowledge to understand Generator droop! Governor droop is a seperate subject. In RN situations, such governor droop was designed in once the normal maximum load of the generator had been passed.

It is possible also, when designing a generator, to set the droop of the Generator itself as a whole, from no load to full load, without any changes to the voltage control system eg. Auto = off.

Usually, RN generators were built to have a natural droop of 45%. I believe that values of this nature are fairly typical for such units.

BUT, as my professor taught us, it is possible to design the Generator droop to have other characteristics, theoretically, you could design one with no droop or even a rising voltage with load, if you so wished, though it has little practical value as far as I can tell in most situations.......

This droop, if I remember correctly, is a function of both the gap between the stator & rotor as well as their designed (electrical) shapes/sizes/etc.. Being so many years away from school does not help me much in remembering such fine details.....

Mr Andy Said,

"But in this case you should be, because it appears that your training did not cover the needed knowledge to understand Generator droop! Governor droop is a separate subject".

Governor droop is the main subject of this thread. At least I thought.

May you S.O.S - Thread "Transformer neutral": Post #11.

I never tried to be sarcastic. Excuse me, check your self!.

Sorry;

Samak

You said:-

I never tried to be sarcastic. Excuse me, check your self!

No, you were just rude, I was just trying to give you a gentlemen's way out, the point of which of course you seem to have missed completely.....!

If you still did not understand it I can put a full explanation here if you wish, though as you are trying to stay rigidly on the "point"! It would probably be considered off topic....

I personally see generator droop as learning something new.....for you anyway....I learnt it more years ago than I care to remember, I would not be surprised if in the Unis today, they do not have time or interest to cover such areas fully....

I would appreciate hearing fully what you understand as governor droop as well if you have time in this blog, thanks in advance. Surely that must count as "on-Topic" with you?

I guess you are online 24/7...!

You said,

"I was just trying to give you a gentlemen's way out"

You mean, you was just trying to prove your rudeness+sarcasm.

You said,

"you were just rude, I was just trying to give you a gentlemen's way out, the point of which of course you seem to have missed completely.....!"

Successfully, you did.

Think about the Newton's third law, and you'll find the above is fair and normal.

Please, reply to our game in thread "Transformer neutral": Post #11"

Good Luck...

Your best friend , I guess.

Samak

I guess what you meant really was:-

Game, set & match to Andy from Germany!

Hooray, hooray, hooray.

What I see from you is a fast backward move to get out of shooting range as quickly as possible!!!!!

I don't blame you in the slightest......in your position I would too!!

I wish you a great day where ever you are and may your God go with you.

Welcome ovide brudo,

Answer of a):

The voltage of the bus-bar is off course the same, the difference can be found in the induced voltage of each generator (E) even they are connected to the same bus-bar. Such a difference affect the power factor of each generator.

Answer of b):

If the droop of each gen. is zero (isochronous) then:

theoretically, gen 1 carries 656/3 kw as well as gen 2 and gen 3

practically, gen 1 will trip due to overload or excessive heat up to failure, dropping out its share of load to the other two. gen 2 will follow the same scenario as gen 1 as gen 2 is required to carry 656/2 kw, also dropping out its share of load to the last one. Gen 3 also will follow the same scenario. Finally, complete power failure.

Answer of c):

There is no need for any communication between governors and voltage regulators in island or parallel operation, however both do sensing the generator output current in parallel operation. Such a parallel operation, off course, will be achieved with a certain droop as there is no parallel operation with isochronous.

Answer of d):

The alternator reactance X does Not play any role in active-load sharing, but it affects the reactive load sharing only.

Regards

Ahmad Samak

Thanks Samak : If I may ask supplementary question --- a) it seems that the load sharing, as you describe, is due to "isochronous operation" ? b) if I want each gen to share the load on this common bus based on the ratio of each gen's rating to the total load, i.e. : gen1= (100/700)(656kw), gen2= (200/700)(656kw), gen3= (400/700)(656kw) , is this correct ? and how do we set/adjust the gens operating parameters? c) how, under this new operating condition, is the reactive part of the load ( which I calculate to be 578kvar), shared ? I guess I need to operate these gens this way, in order to avoid all that tripping, which you rightly described . Let's assume that X'= .16 for gen1, X'=.15 for gen2 and X'= .14 for gen 3 .

Dear ovide,

a) No, the load sharing, as I described, is due to "same droop for each gen", remember, isochronous is a droop of 0%. My scenario is based on the equality of generators' droop whatever the droop is (even isochronous).

b-1) Yes, this is correct to share the loads as you described above.

b-2) Just identify two points on an xy-plan (x is for active power, and y is for frequency):
point 1: (generator rated power , generator or desired rated frequency)
point 2: (zero active power i.e. no load, desired no load frequency)
Calculate the governor droop in % as follows:
Droop (%)= (delta Frequency/rated active power of a generator)x100.
delta frequency = (no load frequency) - (full load frequency)

The above data or other (sure, can be concluded/determined) should be passed to the governor of each gen whatever type it is (Read your instruction manual for details).

c) The above will not affect reactive power sharing in presence of AVR or VAR regulator.
Reactive power sharing is fully controlled by AVR, power factor/VAR regulator.
One of two you may have:
1-(AVR + power factor/VAR regulator) or
2-(AVR only).
In case of (1), which is highly recommended, just select VAR control instead of power factor control, set the reference value for each VAR controller to achieve the required VAR ratio (i.e. 1:2:4).
In case of (2), each AVR has to sense bus bar voltage and generator current, such that it evaluates the required increment/decrement of excitation voltage to compensate/minimize the circulating current between the three generators.

Regards...

Samak

Please allow me to answer several posts at once.

First, we can't confuse phase rotation and phase synchronization. Before paralleling generators, during installation, one must do a phase rotation check and be sure the line side of all generator breakers are phase A, B, C with a phase rotation meter and physically switch cables accordingly until all are alike. Then we can begin the paralleling explanation:

First, synchronizing:

Sync scopes and lamps do not guarantee voltage level differences, they only guarantee phase alignment by using potential differences between phases as a means of looking at the two generators NOT YET connected to each other (a way of looking in advance of combining them before doing so). Phase alignment comes with 2 things, speed and simple alignment. Imagine 2 children, each with their own drum in separate rooms, with a closed connecting door separating them momentarily, beating their drums independently. One is beating boom . . . boom . . . boom at a certain speed. The other, boom,boom,boom at a different speed. First we must get them BOTH on the same speed. Boom .. boom .. boom. But if we want alignment for a concert, speed is one 'must' but alignment is the other 'must' so both children hit the first beat and subsequent beats together, same speed, in sync.

How it works is you have one unit on line (one child in one room), and the other unit is started and producing voltage and frequency (the other child in the other room) but the tie breaker (the door) is not connecting the two units yet. So we can put a voltmeter (listen) across the tie breaker with leads on the same phase of the breaker, such as both leads on phase A. An old analog voltmeter is more visual. Put the leads across the breaker, one lead on the line side and one lead on the load side of the same phase. When the phases of one machine cross by the phases of the other, there is potential difference. So on a 400 volt system, when the phases criss-cross and momentarily line up, the difference is zero (the voltmeter goes to zero or the sync lamps go dark), but when the phases are at the maximum out of alignment the difference is 800 volts (the voltmeter goes to 800 volts and the lamps would glow very bright). If the phases are criss-crossing fast, the lamps blink very fast (speeds are very different); and if using a voltmeter the needle is going from zero to 800 VAC very fast. So we must get the speeds closer, so you adjust the speed of the oncoming machine and you will notice the lamps slow down in blinking as the phases become aligned and leave alignment. The idea with sync lamps is to get them to blink 3-4 times a minute. If the speed matches exactly, the lamps will stay in a steady state wherever they are, glowing bright, glowing half bright, glowing dim or not glowing. If there is any sign of glowing, but they are steady state, then the speeds match, but they are out of phase by the amount that represents a potential difference equal to the brightness. If they are steady, but dark, you are in sync (no potential difference with phases so you must be aligned one on top of the other (you have the two children lined up on each other when they hit the drum . . . . same speed . . . same timing). Then you can close the tie circuit breaker (open the central door in the children's rooms to connect the two drummers together in concert).

If the lamps are blinking too fast, it is difficult to time closing the tie breaker precisely when they are dark.

If there is voltage difference between two machines, let's say one at 400 VAC and the other at 390, then this would make the lamps not as bright (790) as they would be if they were at 400 VAC and 400 (800), but they would still blink with a speed difference, and still go dark when phases were lined up.

The sync scope is a fancy device like lights but tells you which machine is going too fast (CW or CCW needle movement) and also tell you the phase angle difference by where the needle is. So you connect the two generators when the needle is no more than 5* of zenith and moving slow enough you have time to close the breaker.

***********

Load sharing:

In Ovide Brudo example, they all will share load automatically the same. The electronic governors 'talk' to each other and adjust the amount of power (throttle) to each engine, and it is this available extra torque that causes one machine to take additional load even though the bus frequency remains constant. To explain this scientifically requires vector diagrams of torque vs active power and Andy might can explain it, but I too dumb actually

The voltage droop is taken care of by the regulator automatically adjusting the field current against the target line voltage. Due to the reactive capacity of generators, they have inherent voltage droop under load, so we must compensate for that with a precision voltage regulator. For about 20+ years, the newer analog regulators can hold 1/2%. The new digital ones are also about ½% or maybe even a bit better, but they are very fast, sometimes too fast.

Once paralleled, the bus voltage is constant, but individual generators can be slightly different voltage and then current will flow between units. A protective relay will not allow paralleling unless the voltage is within the limits of the cross current compensation CT circuit. Normally a difference of 5% voltage between machines will lock out the permissive voltage segment of the "Sync Check" relay. So modern regulators have the cross current compensation design, when cross currents (actually recirculating currents) are detected between machines it produces backwards current flow only on the one phase where you mounted the CCCT . . . . then voltage is adjusted (field current is increased by the regulator) until there is no reverse current flow. How we did this manually in the old days was to adjust each generator's voltage until the lowest amount of amps was indicated on each generator ammeter. And like above with speed droop, we would use a slide resistor to get the voltage droop the same in a manual paralleling system

When a large amount of current is detected on all phases, we have a reverse power situation meaning we are motoring one unit from the other unit due to one governor giving more fuel to a unit (more torque) faster than the bus load can accept it. Then the reverse power relay trips the breakers.

Reactance assists with voltage dip, waveform distortion and recovery time for different types of loads. The problem with going with low sub transient reactance (<12% x"d), which is good for motor starting and reactive loads, is that the short circuit current is very high and you may need more expensive switchgear bus and bus bracing. Higher x"d reactances of 20% or so limit short circuit capacity and you can use smaller bus bars and less bus bracing since short circuit amps are significantly reduced (they literally wind the generator with special high resistance wire that has more impedance under load and limits ampacity).

I hope this clears up some misunderstandings and keep in mind I am an idiot so only above half of the above is technically accurate.

Hello Petropower : You are a mountain of knowledge, so keep up the good work ! For me ,examples with numbers is needed to help me understand ( that way I don't have to try to understand language--math is better than any language). So how do my three different rating gens share the common load , or in what proportion,both KW's and KVAR's ?

You asked:

I'd like to use an example and ask a question : asuming three 3ph/ac gensets, all w/electronic gov, operating in parallel, on 1 bus( therefore voltage is the same for all three- a) can this be assumed ? ), and in isochronous mode ; gen 1 = 100kw/125kva, gen 2= 200kw/250kva and gen 3 = 400kw/500kva ; therefore, the total gens = 700kw/875kva ; the load is = 656kw/875kva ( pf = 0.75). b) How is the load shared among the three gens ? c) do the electronic governors & voltage regulators & cross-current compensators all automatically communicate and adjust the proper engine speeds and alternator voltages? d) does the alternator reactance X play any role in load sharing ?

I'll answer:

When tied to a common bus, the bus voltage is constant and the frequency is constant. If properly adjusted isochronous electronic load sharing governors are utilised, the load will be shared proportionately equal to their individual capacity. As 656 kW load is 93.7% of 700 kW capacity each unit will take on 93.7% of it's capacity, so #1 will take 93.7 kW, # 2 will take 187.4 kW, # 3 will take 374.8 kW load. The VARs are shared the same way as they will follow the watts of each machine providing each generator has the same 'electrical' characteristics and the amps will be kept in balance by the voltage regulator that will constantly adjust line voltage against the primary preset target (let's say you set each one to 400 VAC) without allowing current to reverse flow from one generator to the other. I have never paralleled a 5% subtransient reactance machine (x"d) with a 30% x"d unit to know if they will share VARS proportionately during a heavy load transient as the watts do but I don't think so (some smart person help me here). But I do think they will share VARS equally in proportion once the bus load stabilises and the regulators have readjusted to hold line voltage target of 400 VAC and adjusted to eliminate any reverse current flow.

*******

How electric load sharing governors 'know' each machines capacity is that you set them up to 'know'. Three phases of CTs and PTs (VTs) connect to each governor board so it can convert things to active power (watts) inside its brain.

You take the # 1 generator and put a 100 kW load on it with a dummy resistive 1.0 PF load bank. The load sharing governor board has an adjustable gain signal potentiometer, so you connect a hand held voltmeter to the gain terminals and adjust the gain to maximum; let us say the maximum obtainable gain signal was 9.5 volts DC. Then you stop unit #1, start unit # 2 and load it to 200 kW. You adjust #2's gain to 9.5 VDC also, at 200 kW load. #3 the same; 400 kW adjust the gain to 9.5 VDC. If you cannot get 9.5 volts from one of the load sharing boxes at that individual full load (let us say # 2 could only give 9.3 volts DC), then you go back to unit # 1 and # 4 and at full individual load you adjust to 9.3 VDC. i.e. all gains are set at the highest value possible when under full resistive (watts) individual load, and all equal.

Then there are "parallel" wires, 2, that connect each governor board to each other via auxiliary contacts of the individual feeder breakers. So unit # 1 running alone will take load up to gain signal 9.3 VDC which will equal 100 kW if you have 100 kW of load. If you start # 2 and parallel it with # 1, and you have 100 kW bus load, when you close the breaker the governors will 'talk' to each other and at 100 kW load on a 300 kW bus each gain signal will be proportional; i.e. unit # 1 will be gain signal 4.75 VDC (half of the 9.5 VDC full load setting) and number 2 will be at 2.375 VDC 1/4 of its full load setting of 9.5 VDC. The fuel actuators on an electronic governor are proportional (they know what fuel rack position they are in with respect to % of full load travel) so a gain signal of x volts represent that unit's full fuel load position (torque capacity). A dip in frequency (load applied) is managed by the governor's normal speed bias correction circuit to apply as much fuel as required to get back to the set frequency of 50 Hz, but the gain signal also does that in concert with the speed bias correction circuit to keep all fuel racks adding fuel (torque capacity) in proportion so one unit does have more torque capacity than the other even though it is holding frequency steady once stabilised after the load change.

Finally, the reactance plays no role in load sharing (at my simple level of education for every day applications and setting up a paralleling switchboard . . . . I may be wrong)

Very very clear now . . . clear as mud. Ahemmmmm.

Maybe someone smarter than me (Dell ? Andy? Samak? Powertothepeople ? . . . . is there anyone named "Reactivepowertothepeople" ? we need someone like that to comment) can take the above words and put it into equations using all the Greek alphabet. I only use the Greek alphabet to write a Baklava recipe !

Cheers

Hi Petropower : you blew me away with that recipe !! Even though it is very sweet, I need supplementary clarification; please , your utmost patience is vital ! Going back to #14 and #9 responses, Samak made it clear to me ( in #9), I thought, that with isochronous operation, each of the three gens would trip out , one by one, due to overload; so, in #14, I agreed to avoid parallel operation in isochronous mode; I believe you're telling me that these three gens can share the common bus load proportionately, according to each gen rating, even with isochronous mode electronic governors , without any overloading or tripping. Is my interpretation correct ?? ( maybe I need some souvlaki with my baklava ?) --Cheers.

My dear pal Ovide (remember Ovide the Platypus of Ovide and the Gang ?)

Yes, you can parallel and load share in isochronous mode to any isolated bus (our discussion) if you have a load sharing governor board (the thing in the switchgear) and proportional actuator (the thing on the engine).

You cannot parallel in isochronous mode to an infinite bus (the grid) as you cannot influence the grid with your little machine so the governor will be at full fuel (if the grid is slightly below your machine frequency) or no-fuel (if the grid is slightly above your frequency), and nothing in between. Droop mode is required for grid use.

Go have a look at publication 02303 (put in search box) from Woodward. Now don't go read this and become smarter than I am as I have Andy and Dell fooled that I'm intelligent ! (and my mom also).

It has been 20 years since I personally paralleled generators, and all the above I wrote from my "old man memory". I was 15* out of phase when my avatar picture was taken. Reading the Woodward bulletin, the load gain signal is set at 6 VDC if you can get 6 VDC at full load; an example of my memory loss and hair loss.

If you parallel Baklava with Souvlaki, YOU will be in droop mode as you sit on the toilet.

Some good stuff here and some fond memories. Many thanks.

One extra warning:-

If you only have only two gennies, there is not much that can go wrong. On RN ships, you might have 10 different generators, they are selected for paralleling by two large selector switches to select the lamps and voltage indications.

God help you if you mistakenly selected a wrong generator and paralleled up to another one out of phase!! I have heard that it can send a Gennie through the side of a ship....heard only!!!!

You had better believe it Andy. On a patrol boat that had two gen sets the "Engineer" broke the crankshaft of the on-coming alternator by not watching the synchroscope; turned the on-coming alternator into a motor......results........as you correctly said...not good.

With all this talk of droop, my experience has been that gen sets have about 1,25% droop otherwise your rpm range is too high and voltage and hertz are not stable which can effect sensitive electronic equipment and throw it off the board. Whereas a main engine requires about 9% droop so that we have a fairly large dead band and the governor is not so sensitive and with continual load changes, due to sea state, the engine will not hunt.

Andy you are also correct in saying that this is all done automatically now. Takes the thrill out of the job what with all the other electronic "stuff."

Petropower says it quite well...in fact better than a layman .

In reading this thread it seems that the title "Droop, Isochronous Speed, and Recirculating Current" kind of threw off some of the answers. The three don't go together like that. 'Droop' (reactive) and recirculating / cross current go together as a regulator issue. 'Droop' (speed) and/or Isochronous are governor issues.

Petropower...excellent post but you kind of threw me with this though... "A protective relay will not allow paralleling unless the voltage is within the limits of the cross current compensation CT circuit". I got what you meant reading all of your post but someone else might not. A 25 sync relay is just for paralleling up and is usually a 1% or 5% voltage diff and many have a settable slip rate but a 25 but doesn't have anything to do with the cross current comp circuit. Typically...at least on DG units in the states the manufacturers like CAT or Cummins supply the CCCT with the gens, mount them in the peckerhead, and do not want anything else in that circuit. 50/51, metering, var/pF, etc. all go on seperate CTs mounted in the switchgear. Regarding 'droop' ... most of the modern DG regulators like CAT's dvr or VR6 can operate in cross current compensation (zero droop) or reactive droop mode. You can usually get up to about 10% reactive droop if you are running that mode. Running in zero droop is most common for paralleled gens that aren't up against the utility. The regulators need balancing resistors...most are built into the regulator...to balance out minor CT differences. I've seen many times where a guy can't get all of his load on and sure enough there's 400 or 500amps running between the gens before applying any load.

Anyway...watdefac...when you got your gens together, assuming you're not up against the utility...run your gens and carefully match freqs and volts individually while not paralleled. Parallel the units and with NO LOAD look for current / kw on either engine. If no current you are fine. This is a different animal than mismatched speeds causing one unit to take more load or push power at the other gen. Mismatched speeds push current/kw. Mismatch volts push vars.

Well, given the readers may not understand paralleling perfectly I was trying to stay away from super technical explanations of device 25, 50 and 51s as this leads to "What is a 25 device Sync Check" and a whole other spin off. I was trying to keep it 'global and conceptual'. You busted me !

I'm not sure if the readers have automatic gear, manual gear, or are just curious (I think the latter).

I'm so old I dissembled the first UG8 D in my crib when it first came out (I think Columbus used a UG8 L on the Santa Maria).

I know a CCCT circuit cannot throttle the field current infinitely so I like to be within 'range' of the CC capacity before I parallel, so I set the permissives to be a bit tight so the operator doesn't get lazy when people play with rheostats on doors.

I've dangerously double wrapped CCCTs to improve sensitivity when I was young and stupid. Now I'm old and stupid . . . . . and I have droop also now but that's due to heavy load without much recover throttle left in the old bones.

But anyway, I just wanted to define voltage droop, speed droop, (and what they affect) and isochronous paralleling.

If I get hits on what all this slang means, you step in ! My fingers hurt.

Cheers