7/23/2019 Isochronous vs Droop_mode

http://slidepdf.com/reader/full/isochronous-vs-droopmode 1/4

1. Great explanation on isochronous vs

droop

Original thread : http://www.control.com/thread/1026221805

Whenever two governors are in Isoch mode (withot some !ind o" Isoch #oad $haring

scheme% the& will '"ight each other to control the "re)enc&. *hat+s their natre, and

the&+re not disposed to sharing the load. -ach governor wants to control "re)enc& and

so each will respond to an& change in "re)enc& (which reslts "rom a change in load,

s&stem load%.

n Isochronos #oad $haring scheme will allow the two nits to 'split or 'share theload etween them depending on how the scheme/controller is programmed. It will

allow more than one governor to e operated in Isoch mode and will redce and

 possil& even eliminate "re)enc& swings as a reslt o" load changes. Whether or not

two governors need to e operated simltaneosl& in Isoch mode is a "nction o" man&

"actors, and to cover all the possiilities wold ta!e the e)ivalent o" several chapters

in a tetoo!.

Imagine two people riding on a tandem ic&cle, each with pedals to contrite to the

tor)e re)ired to move the ic&cle and an& load the& might e carr&ing. Imagine the&

have to maintain a constant speed (which in an s&stem speed is directl&

 proportional to "re)enc&%. *he two riders are each prime movers providing tor)e to

the ic&cle, st as trines provide tor)e to generators. It ta!es a certain amont o"

tor)e to maintain a certain speed (on a "lat and level road% when there is no load on

the ic&cle (pac!ages, etc.% and more tor)e to maintain that same speed when there

are pac!ages and possil& a passenger on that same "lat and level road.

#et+s "rther sa& that the nmer and weight o" the pac!ages can e variale, that the&

can e dropped o"" the ic&cle and added to the ic&cle at random times and intervals

while the ic&cle is moving, meaning that the tor)e re)ired to maintain a constant

speed is going to change as the nmer o" pac!ages and the weight o" the pac!ages

change.

I" oth riders are each wor!ing to independentl& control the speed o" the ic&cle as the

load changes, it+s ver& li!el& that the speed o" the ic&cle won+t e ver& constant. I"

the& oth appl& more tor)e as a pac!age or pac!ages are added to the ic&cle the

speed might increase aove the desired rate, and i" the& oth decrease their tor)e at

the same time to slow the i!e down then the speed might decrease elow the desired

rate.

3t, i" the& decided that one rider will pedal with a "airl& constant tor)e and the other 

rider will var& his tor)e otpt to maintain the desired speed as the nmer and

weight o" the pac!ages changes, then it+s more li!el& that the speed o" the ic&cle (andit+s load% will remain "airl& constant.

7/23/2019 Isochronous vs Droop_mode

http://slidepdf.com/reader/full/isochronous-vs-droopmode 2/4

We+ll call the rider who+s maintaining a relativel& constant tor)e '4roop and the

rider who+s var&ing his otpt as a "nction o" load 'Isoch.

#et+s sa& that 4roop is sppl&ing a constant tor)e to the pedals e)al to 50 o" his

ailit& while the ic&cle is moving and pac!ages o" var&ing weight are eing dropped

o"" and pic!ed p. *his means that Isoch+s otpt is var&ing as necessar& to maintainthe desired speed as the load on the ic&cle is changing. 3*, i" the load eceeds

Isoch+s ailit& to prodce tor)e, then the speed o" the ic&cle will slow down,

 ecase 4roop is onl& ptting ot 50 o" his otpt. $o, Isoch can as! 4roop to

increase his tor)e to help get the ic&cle ac! to desired speed. nd in so doing, as

4roop increases his tor)e the amont o" tor)e that Isoch is prodcing will e

decreased as the desired speed is reached. 4roop shold increase his tor)e otpt

sch that an& anticipated load increase can e handled & Isoch so that the speed will

remain constant as epected load changes occr.

 7ow let+s sa& that the load on the ic&cle has dropped signi"icantl& (a lot o" the

 pac!ages have een dropped o"" the i!e%, and the amont o" tor)e that Isoch has to provide is down to near ero (he+s st 'coasting% to maintain the desired speed. I"

even more pac!ages are removed "rom the i!e, then the speed will start to increase.

nd, so 4roop will have to decrease his tor)e otpt to redce the speed o" the

 ic&cle. 4roop shold redce his tor)e otpt sch that Isoch has to increase his

tor)e otpt aove that re)ired "or the loss o" an& additional pac!ages in order to

maintain the desired speed.

generators (alternators% with prime movers are identical to this eample. *he&

have to operate at a constant speed ("re)enc&% and the& have to var& their tor)e

otpt in order to control speed ("re)enc&% when load is changing when the& are

 eing operated 'o""9grid, or in 'island mode. 3t, i" two governors are oth tr&ing to

control the speed ("re)enc&% the& will generall& "ight each other which will case the

"re)enc& to e nstale, and the loads on the two generators to e nstale.

We have no wa& o" !nowing what the load is when the two nits are separated "rom

the grid. #et+s presme the two nits are rnning at 10 W and 20 W in parallel

with the tilit&. nd sddenl&, the tie rea!er opens, and the 'island load drops to 12

W. #et+s "rther presme that one (onl& one% o" the 505- controllers will switch to

Isoch mode when the tie rea!er opens, and let+s sa& it+s the nit rnning at 20 W

(which we+ll call '3ig%. $o, when the tie rea!er opens 3ig will switch to Isoch mode

and the other nit (we+ll call it '#ittle% sta&s in 4roop control mode.

3ecase the power otpt o" the two nits at the instant the tie rea!er opens eceeds

that re)ired "or the 'island load, the "re)enc& will start to increase ver& )ic!l&,

and 3ig will redce it+s load (hope"ll& ver& )ic!l&;% to aot 2 W, which in

comination with the load on #ittle (10 W% shold reslt in a stale "re)enc&.

(What will reall& happen is that the "re)enc& will increase and #ittle+s load shold

decrease ecase o" 4roop action and as 3ig+s load comes down #ittle+s load will

increase ac! to 10 W as "re)enc& retrns to normal.%

<ope"ll&, when 3ig closes the control valves ver& )ic!l& it doesn+t case a sdden

spi!e in inlet steam pressre and ma&e even li"t a sa"et& valve or two; (*he oiler

7/23/2019 Isochronous vs Droop_mode

http://slidepdf.com/reader/full/isochronous-vs-droopmode 3/4

control also has to redce "iring rate prett& )ic!l& to tr& to help redce steam pressre

 ecase o" the redced steam "low to 3ig.%

3ig, eing the Isoch nit, will now respond to an& load changes in the 'island load.

(*his presmes #ittle is in 'straight 4roop speed control with no load setpoint.%

<owever, i" the island load decreases & more than two W, 3ig can+t redce itsotpt elow 0 W, and so what will happen is that the load on #ittle will decrease

 t the "re)enc& o" the island will increase.

 7ow, i" someone wants to increase the load on 3ig the operator needs to decrease the

load on #ittle.

=resming #ittle+s 505- is operating in 'straight 4roop speed control, it+s load will

remain "airl& constant as long as "re)enc& remains "airl& constant. I" #ittle+s 505- is

tr&ing to maintain a speci"ic load setpoint and 3ig has an& di""iclt& responding to

load swings and the "re)enc& starts to var&, then #ittle won+t help tr& to maintain

"re)enc&>it will actall& ma!e the "re)enc& prolem worse, ecase instead o"changing it+s otpt to help maintain the "re)enc& setpoint & increasing or

decreasing load as necssar&, it will & tr&ing to !eep its load constant which will ma!e

the "re)enc& prolem worse.

I" &o want to operate oth nits in Isoch mode (and there ma& e compelling reasons

to do that, we don+t and can+t !now that "rom the in"ormation &o have provided% &o

shold seriosl& consider investing in an Isoch #oad $haring modle. 3t, it sholdn+t

 e necessar&, nless &or operators can+t respond to load changes and adst the otpt

o" #ittle to !eep 3ig in a range o" otpt that wold allow it to control "re)enc& as

load changes.

$team trines don+t generall& ma!e great Isoch machines, ecase the "iring rate o"

the oiler can+t sall& e changed ver& )ic!l& i" load changes )ic!l&. It+s not that

the& can+t e tned to e good Isoch nits, it+s st that it sall& ta!es tning o" oth

the governor and the oiler control to achieve smooth "re)enc& control, and then onl&

"or epected load changes. I" the load changes mch more than epected "or some

reason, then the& generall& won+t respond as )ic!l& as necessar&.

=osted 2 &ears ago 

Ingat 5 =er!ara $eelm 5 =er!ara. $ihat $eelm $a!it. da $eelm

*a. #apang $eelm $i!. ?a&a $eelm is!in. <idp $eelm ati.

7/23/2019 Isochronous vs Droop_mode

http://slidepdf.com/reader/full/isochronous-vs-droopmode 4/4

• s! me an&thing

• rchive

op&right 201@>2016