innovative steam turbine and engine control system for

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Innovative Steam Turbine and Engine Control System forLoad Sharing Application in Pulp and Paper Mills

Ashish Kukrety

Woodward Governor India Pvt. Ltd.,23/6, Mathura Road, 8allabgarh-121 004 (Haryana)

•ABSTRACT

Certain changes that are required to be carried out by existing Pulp and Paper plants in order to integratethemseleves with the changing requirements and the plant needs, are suggested in this paper.

INTRODUCTION

Technological changes have ensured that the upcomingnew' plants are being designed under considerationthat all turbines will share load and if required theywill' have an integrated cogeneration plant and willexport the surplus power to the grid. For this it isessential that each turbine has an electronic governor.What is important is that the number of new plantscoming up is comparatively very small to the existingplants. What the existing Paper plants need to do, toensure that they are able to share the load on allexisting turbine/engines IS to upgrade the existinggoverning with electronic governor.The Need for Electronic Governors

There has been a steady growth in demand for failsafecontrols for turbines, which have the increasedversatility of load control, process control, load sharingand synchronizing besides the basic speed control.This has created the need for electronic governors.For load sharing application to have the facility ofchanging the parameters based on the plant need andthe steam availability, the electronic governor is bestapt for the situation.

Most of the existing Paper plants are having twoor more power turbine, which are working in isolation.Each turbine is catering to certain plant load. Thereare times when one turbine is running at 50% loadand the other turbine trips because of overload. Thishappens in spite of the fact that the installed capacityis say 9 MW and the total plant requirement is 8MW. In such situations the best possible solution isto ensure that the total plant load is distributed onall the turbines based on the generation capacity andload carrying capacity of each turbine. To understand

this, let us talk little about load sharing.Droop

Droop is decrease in speed or frequency, proportionalto load, i.e. as the load increases, the speed or frequencydecreases (Fig. 1). This reduction in speed isaccomplished with negative feedback. As the systemis loaded, the feedback signal increases in magnitudeand in opposite polarity to the speed set voltage. Thefeedback signal is subtracted from the speed set voltagelowering the speed reference in direct ratio to theincreases in Ioad. Droop is expressed as the percentthat the speed drops below no load speed when thesystem is fully loaded. With a given droop setting,an engine generator set will always produce the samepower output at a particular speed or frequency.Isochronous

Iso in Greek means single or constant and Chronousmeans time; Isochronous means repeating at a single

52

50-"CQ,IQ,IC.

rJ:J'-'N= 47

0%

.- -.~.- - .

100%Load

Fig. 1 Droop Mode

IPPTA Vol. 14, No.3, Sept 2002 83

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rate or having a fixed frequency or period. In otherwords, an engine-generating set, operating in theIsochronous mode will operate at the same speed/frequency regardless of the load it is supplying, upto the full load capabilities of the turbine-generatorset (Fig. 2). This mode can be used on one turbine-generator set running by itself in an isolated system,

Load

Fig. 2 Isochronous Mode

Concepts of load sharing

There can be two possibilities;1. You are sharing the load in Isolation from theUtility or the. state grid.2. You are sharing the plant load being tied to thegrid. In this mode you can automatically export/importpower to/from the grid based on plant requirements.

RESULTS AND DISCUSSION

Load sharing for an isolated system

Droop/Isochronous Load Sharing. Droop andIsochronous combine the first two modes. All turbine-generator sets in the system are operated in the droopmode except for one, which is operated in theisochronous mode. It is known as the swing machine.In this mode, the droop machine will run at the speed/frequency of the Isochronous unit. The drooppercentage and speed setting of each droop unit areadjusted so that it generates a set amount of power.The output power of the swing machine will changeto follow variation in the load demand whilemaintaining constant speed/frequency of the system(Fig. 3).

84 IPPTA Vol. 14, No.3, Sept 2002

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Generator A 52 Generator B

_~I-:_: I_'~=-_,""" I I

tI

I

1~ 75% 50% 25% o 25% 50% 75% 10'0

Fig. 3 Droop / Isochronous mode

Maximum load for this type of system is limitedto the combined output of the swing machine andthe total set power output of the droop machines. Aload above this maximum will result in decrease inspeed/frequency. The minimum system load cannot beallowed to decrease below the combined output setfor the droop machines. If it does, the system frequencywill increase and the swing machine can be motorized.

The machine with the highest output capacity shouldnormally be operated as the swing machine so thatthe system will accept the largest load changes withinits capability. This is not a hard and fast rule. Selectionof the swing machine will depend on such things asefficiency of different engines and the amount theload that is expected to change. In order to increasethe load swing capabilities of a Droop/Isochronousload sharing system, several Isochronous load sharingturbine-generator sets can be connected in parallel torespond as a single swing machine. Other turbine-generators can then be run in droop with these swingmachine.Droop/Droop Load Sharing

In this case all the units are set in droop mode. Ifthe droop set is equal in all the turbines the individual

•

GeneratorGenerator

•

•~ 15% '!OO%L,:r:i1B

Fig. 4 Unequal Droop/Droop Load Sharing

I.

•

Fig. 5 Isochronous Load Sharing

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speed references will very with the same amount tomaintain an even percentage of load. Load will staybalanced under all load conditions but the frequencywill not stay constant with load variation. In case ofunequal droops (Fig. 4) the load sharing will beunequal. For example if the percentage droop form/c, A is 3% and for mlc, B is 5% for the samefrequency the percentage carried by each m/c will bedifferent.

If all engine generator sets in a droop system havethe same droop setting, they will each share loadproportionally. The amount of load each carries willdepend on their speed settings. If the system loadchanges, the system speed/frequency will also change. ,A change in the speed setting will then be requiredto offset the effect of droop and return the systemto its original speed/frequency. In order for eachengine generator set in the system to' maintain itsproportion of the shared load, the operator will needto adjust the speed set point equally for each turbine-generator set.

If all turbine/generators in a droop system do nothave the same droop setting, they will not share loadsproportionally with the same speed settings. If thesystem load changes, the system speed/frequency willalso change but the percentage of load on each turbine-generator set will not be changed proportionately.The operator will need to adjust speed set pointdifferently for each turbine-generator set to makethem carry their proportional share of the load.

This could result in running out of speed set pointadjustment on an turbine-generator set before it isfully loaded and limiting the system load sharingcapability. It is best to have the same percentage ofdroop set on each turbine-generator set.Isochronous Load Sharing

For Isochronous load sharing, the controls of allturbine-generator sets in a system are operated in the

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isochronous mode. Load sharing is accomplished byadding a load sensor to each electronic isochronousgovernor. The load sensors are interconnected by theload-sharing lines (sometimes called paralleling lines).Any imbalance in load between units will cause achange to the regulating circuit in each governor inthe system, causing each unit to produce itsproportional share of the load to rebalance the loadsignals. While each unit continues to run at isochronousspeed, load changes force each machine to supply its

GeneratorBIsoc:nronol..lS

Generator l~tsoch Base LOt1C

50.0

100% 75% .sb% 25% 0 25% 50"-4 75% 100P'('

Load(%)Fig. 6 Isolated Bus with Isochronous base load

proportional share of power to meet the total loaddemand on the system. If the turbine-generators areof unequal output ratings, the output of each generatorset will be proportional to its rated output (Fig. 5).Isochronous base load

Isochronous base load is a method of setting a baseor fixed load on an engine generator operating in theisochronous governing mode. This is accomplishedby using an isochronous load-sensing control andconnecting an external bias signal across its load-sharing lines (Fig. 6).

The external bias signal impressed on these linesappears to governor as a load imbalance. The Governorwill force the generator output to increase or decreaseuntil the output of the load sensor is equal to thebias signal on the load-sharing lines.

At this point, the system is balanced. This methodcan only be used where other turbine-generator setsare producing enough power to meet the change inload demand. This base loading method is ideal foreither soft loading additional units into an isochronoussystem, for derating or unloading a turbine-generator,or for setting a fixed amount of load on a turbine-generator that is in parallel with other units (Fig. 7).Implementation of Concept of Load Sharing forPulp and Paper Mill Application

The concept of isochronous load sharing has beenaccepted by the various captive power plants inFertilizer, Sugar and Petrochemical Industries. The


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•...•..•.,.....~

Fig. 7 Scheme for Isochronous load sharing for an Isolated system

PLANT BUS

pulp and paper Industries is also realizing the potentialof Isochronous load sharing as it gives the flexibilityof loading the various generators at desired ratingand at the same time not sacrificing the frequencyof the system. Let us assume that a pulp and paperplant has a combination of three turbines and enginesof 2.5 MW each. Each Turbine/engine is loaded to90% of its installed capacity l.e. 2.25 MW and 250KW is kept as safety margin for sudden kick loads.This means that total of 750 KW is kept as safetymargin. Using concept of load sharing that mlcs canbe loaded to their maximum installed capacities as allkick load will be shared by all turbines and engines.For this it is essential that each turbine is fitted withan electronic governor and has its own DSLC (DigitalSynchroniser and Load Control). The DSLC performsthe function of a synchronizer as well as load controller.Each Turbine has its independent DSLC, whichcommunicates with the speed controller to match thefrequency and with the AVR for voltage. Each DSLCcommunicates to other DSLCs on its own network(LON). The DSLC receives CT and PT inputs to knowthe generation and the total load requirement. AllDSLCs are aware of the individual turbine generatingcapacity (programmed into the system). The variationin the load is distributed among all the turbines basedon generating capacity and the percentage of loadrequired to be shared by each turbine (programmedinto each DSLC). This will enusre that no turbine isoverloaded within the generating capacities of all thethree turbine. Further the load sharing will beIsochronous i.e. the plant frequency will be maintained.This scheme is totally suitable for pulp and paperplants, which are isolated from the grid (Fig. 8).


The limitations of the hydromechanical controls areeliminated in this kind of scheme using digital controlsystem. The Power plant is now able to achieve thefollowing features, which would have been difficultto achieve through a hydromechanical control.

• Manual/Semi AutomaticlAutomatic start up can nowbe programmed in the system depending upon thecold hot start-up curves. These curves are configuredin the system at the time of detailed engineering.

• Critical speed avoidance is programmed in thesystem. As soon as the controller senses the criticalspeed band, the controller crosses the critical bandat a faster ramp rate as programmed in the system.The same ramp rate will be followed duringcontrolled shutdown.

• Idle running and acceleration rate adjustments canbe programmed.

• Isochronous load sharing between the three STGslI'

Generator ADroop

52GeneratorSIsochronous

51

so

250"" 50% 75% 100100% 75% 50% 25°"" 0

Fig. 8 Droop unitfixed or base load

Grid (SwingMachine)

•

••

•.

I·

•

Generator AIsoch. Base LoadGenerator B

Isochronous

25% 50% 75% 1000,(,

exception that the utility or grid will control thefrequency and act as does the swing machine,absorbing any change in load. The speed and droopsettings are adjusted so that the turbine-generator setsupplies a fixed, or base amount, of power to theutility (Fig. 9).

When the turbine-generator supplying power tothe utility is separated from the utility. the speed!frequency of the now unloaded isolated unit willincrease to the speed set point on the control whereit was advanced to load the turbine-generator whiledrooped against the utility.Isochronous base loadIsochronous base load for a turbine-generator feedingan utility is the same as isochronous base load foran isolated system. Any difference between the speed!frequency setting of the isochronous base loadedmachines, and that of the utility, will show up asa change in the base load. This is why it is soimportant to have the speed/frequency setting of theoncoming unit as close as possible to that of theutility. As many as 15. turbine-generators can beisochronous base loaded by applying the isolated biassignal across their common system load-sharing lines.The advantage of isochronous base loading over droopbase loading is that when separating from an utility.there is no frequency change. Simply removing thebias signal from the load-sharing lines returns theengine-generator to Isochronous speed (Fig. 9).

~~Trw.IAIOI!I:

Fig. 10 Scheme for load sharing with the system tied to Grid

son

_-- sn:"ATE (lRIl)

I

I

I

Load : SwingsI

I

o1000,(, 75% SO°,4 25%

Load (%)

Fig. 9 Grid (Swing Machine)Isochronous base load machine

(Fixed or base load)

Engine generator will be achieved through DSLCsin island mode. The system will be provided withthe facility of overspeed testing.

• Scan time has been configured between 5 msec to80 msecs rate groups depending upon the criticalityand nature of I/O as per Woodward Standard.

• The digital controller communicates with plantdistributive control system (DCS) on Mod Bus.

Systems tied to Utility grids .In case the plant is also tied to utility there are againtwo of load sharing possibilities.• Droop base load sharing• Isochronous base load sharingBase LoadDroop base load is the same as droop, with the


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Import-Export to the GridWhen an individual pulp and paper plant has surplusMW, then the same can be exported t~ the grid aftermeeting the plant requirement. This Tmport/Exportcontrol action will be initiated by MSLC (MasterSynchroniser and Load Control) after the TG setshave been synchronized with the grid. Once the plantis synchronized with the grid then MSLC will initiateimport/export function. In this mode of import/exportcontrol, the MSLC measures the real power flow onthe incoming feeder with help of CT and P'Ivinputs.It then determines the power delivery required ofeach STG set and communicates this requirement tothe individual DSLC(Digital Synchroniser and LoadController). The individual DSLCs will control to itspercentage of rated loads, and the MSLC will adjustthe load up or down to achieve the proper importlexport level set (Fig. 10). Import Control functionwill be achieved from MSLC when total generationis less than the total plant load. Export control willbe achieved when the Captive generation is more thenthe plant load. In case the plant load increases thenthe controller will increase the reference to maximum

generating capacity of the plant and it will keepexporting, till the maximum set load level is reached.In case the plant load goes beyond the maximumplant load limit level, import will start to meet theplant load.

CONCLUSION

For maximum production it is essential thatuninterrupted power is available and the load on allthe turbines/engine generator is shared in the sameproposition as the generating capacities of individualturbine/engine. This will ensure that all the turbines/engines will share any sudden load coming in thesystem and no individual turbine/engine will getoverloaded. This will have an added advantage topaper mills that they need not keep the safety marginsof 10% on individual turbine/engine. This effectivelymeans that on an average adopting the load-sharingconcept will make each paper mills utilize 500 to 600KW of energy, which they normally keep as safetymargins for sudden loads.

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innovative steam turbine and engine control system for

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