l.

Innovative Steam Turbine and Engine Control System for

Load 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 integrate
themseleves with the changing requirements and the plant needs, are suggested in this paper.

this, let us talk little about load sharing.

INTRODUCTION
Droop
Technological changes have ensured that the upcoming Droop is decrease in speed or frequency, proportional
new' plants are being designed under consideration to load, i.e. as the load increases, the speed or frequency
that all turbines will share load and if required they decreases (Fig. 1). This reduction in speed is
will' have an integrated cogeneration plant and will accomplished with negative feedback. As the system
export the surplus power to the grid. For this it is is loaded, the feedback signal increases in magnitude
essential that each turbine has an electronic governor. and in opposite polarity to the speed set voltage. The
What is important is that the number of new plants feedback signal is subtracted from the speed set voltage
coming up is comparatively very small to the existing lowering the speed reference in direct ratio to the
plants. What the existing Paper plants need to do, to increases in Ioad. Droop is expressed as the percent
ensure that they are able to share the load on all that the speed drops below no load speed when the
existing turbine/engines IS to upgrade the existing system is fully loaded. With a given droop setting,
governing with electronic governor. an engine generator set will always produce the same
The Need for Electronic Governors
power output at a particular speed or frequency.
Isochronous
There has been a steady growth in demand for failsafe
controls for turbines, which have the increased Iso in Greek means single or constant and Chronous
versatility of load control, process control, load sharing means time; Isochronous means repeating at a single
and synchronizing besides the basic speed control.
This has created the need for electronic governors.
For load sharing application to have the facility of 52
changing the parameters based on the plant need and
the steam availability, the electronic governor is best
apt for the situation.
50 .- -.~.- - .
Most of the existing Paper plants are having two
or more power turbine, which are working in isolation.
Each turbine is catering to certain plant load. There
-
"C
Q,I
Q,I
C.
rJ:J
are times when one turbine is running at 50% load '-'

= 47
N
and the other turbine trips because of overload. This
happens in spite of the fact that the installed capacity
is say 9 MW and the total plant requirement is 8
MW. In such situations the best possible solution is 0%
to ensure that the total plant load is distributed on 100%
all the turbines based on the generation capacity and Load
load carrying capacity of each turbine. To understand 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 other

words, an engine-generating set, operating in the Generator A 52 Generator B

_~I-:_: I_'~=-_
Isochronous mode will operate at the same speed/
frequency regardless of the load it is supplying, up
to the full load capabilities of the turbine-generator
set (Fig. 2). This mode can be used on one turbine-
generator set running by itself in an isolated system, ,""" I I

t
I

..
I

o 25% 50% 75% 10'0

1~ 75% 50% 25%

Fig. 3 Droop / Isochronous mode

Maximum load for this type of system is limited

to the combined output of the swing machine and
the total set power output of the droop machines. A
load above this maximum will result in decrease in
speed/frequency. The minimum system load cannot be
allowed to decrease below the combined output set
for the droop machines. If it does, the system frequency
will increase and the swing machine can be motorized.
The machine with the highest output capacity should
normally be operated as the swing machine so that
Load the system will accept the largest load changes within
Fig. 2 Isochronous Mode
its capability. This is not a hard and fast rule. Selection •
of the swing machine will depend on such things as
efficiency of different engines and the amount the
Concepts of load sharing load that is expected to change. In order to increase
the load swing capabilities of a Droop/Isochronous
There can be two possibilities; load sharing system, several Isochronous load sharing
1. You are sharing the load in Isolation from the turbine-generator sets can be connected in parallel to
Utility or the. state grid. respond as a single swing machine. Other turbine-
2. You are sharing the plant load being tied to the generators can then be run in droop with these swing
grid. In this mode you can automatically export/import machine.
power to/from the grid based on plant requirements. Droop/Droop Load Sharing
In this case all the units are set in droop mode. If
RESULTS AND DISCUSSION the droop set is equal in all the turbines the individual

Load sharing for an isolated system Generator

Generator
Droop/Isochronous Load Sharing. Droop and
Isochronous combine the first two modes. All turbine-
generator sets in the system are operated in the droop
mode except for one, which is operated in the
isochronous 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 droop
percentage and speed setting of each droop unit are
adjusted so that it generates a set amount of power. •
The output power of the swing machine will change ~
L,:r:i1B
15% '!OO%

to follow variation in the load demand while

maintaining constant speed/frequency of the system
(Fig. 3). Fig. 4 Unequal Droop/Droop Load Sharing

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

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isochronous mode. Load sharing is accomplished by

adding a load sensor to each electronic isochronous
governor. The load sensors are interconnected by the
load-sharing lines (sometimes called paralleling lines).
Any imbalance in load between units will cause a
change to the regulating circuit in each governor in
the system, causing each unit to produce its
proportional share of the load to rebalance the load
signals. While each unit continues to run at isochronous
• speed, load changes force each machine to supply its

GeneratorB Generator l~
Isoc:nronol..lS tsoch Base LOt1C
Fig. 5 Isochronous Load Sharing
50.0

speed references will very with the same amount to

maintain an even percentage of load. Load will stay
balanced under all load conditions but the frequency
will not stay constant with load variation. In case of
unequal droops (Fig. 4) the load sharing will be
100% 75% . sb% 25% 0 25% 50"-4 75% 100P'('

unequal. For example if the percentage droop for Load(%)

m/c, A is 3% and for mlc, B is 5% for the same Fig. 6 Isolated Bus with Isochronous base load
frequency the percentage carried by each m/c will be
different.
proportional share of power to meet the total load
If all engine generator sets in a droop system have demand on the system. If the turbine-generators are
the same droop setting, they will each share load of unequal output ratings, the output of each generator
• proportionally. The amount of load each carries will set will be proportional to its rated output (Fig. 5).
depend on their speed settings. If the system load
Isochronous base load
changes, the system speed/frequency will also change. ,
A change in the speed setting will then be required Isochronous base load is a method of setting a base
to offset the effect of droop and return the system or fixed load on an engine generator operating in the
to its original speed/frequency. In order for each isochronous governing mode. This is accomplished
engine generator set in the system to' maintain its by using an isochronous load-sensing control and
proportion of the shared load, the operator will need connecting an external bias signal across its load-
to adjust the speed set point equally for each turbine- sharing lines (Fig. 6).
generator set. The external bias signal impressed on these lines
If all turbine/generators in a droop system do not appears to governor as a load imbalance. The Governor
have the same droop setting, they will not share loads will force the generator output to increase or decrease
proportionally with the same speed settings. If the until the output of the load sensor is equal to the
system load changes, the system speed/frequency will bias signal on the load-sharing lines.
also change but the percentage of load on each turbine- At this point, the system is balanced. This method
generator set will not be changed proportionately. can only be used where other turbine-generator sets
The operator will need to adjust speed set point are producing enough power to meet the change in
differently for each turbine-generator set to make load demand. This base loading method is ideal for
them carry their proportional share of the load. either soft loading additional units into an isochronous
.. This could result in running out of speed set point system, for derating or unloading a turbine-generator,
adjustment on an turbine-generator set before it is or for setting a fixed amount of load on a turbine-
fully loaded and limiting the system load sharing generator that is in parallel with other units (Fig. 7).
capability. It is best to have the same percentage of Implementation of Concept of Load Sharing for
• droop set on each turbine-generator set. Pulp and Paper Mill Application
Isochronous Load Sharing The concept of isochronous load sharing has been
For Isochronous load sharing, the controls of all accepted by the various captive power plants in
turbine-generator sets in a system are operated in the Fertilizer, Sugar and Petrochemical Industries. The

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PLANT BUS

•...•..•.
~
,.....
Fig. 7 Scheme for Isochronous load sharing for an Isolated system

pulp and paper Industries is also realizing the potential The limitations of the hydro mechanical controls are
of Isochronous load sharing as it gives the flexibility eliminated in this kind of scheme using digital control
of loading the various generators at desired rating system. The Power plant is now able to achieve the
and at the same time not sacrificing the frequency following features, which would have been difficult
of the system. Let us assume that a pulp and paper to achieve through a hydro mechanical control.
plant has a combination of three turbines and engines • Manual/Semi Automaticl Automatic start up can now ••
of 2.5 MW each. Each Turbine/engine is loaded to be programmed in the system depending upon the
90% of its installed capacity l.e. 2.25 MW and 250 cold hot start-up curves. These curves are configured
KW is kept as safety margin for sudden kick loads. in the system at the time of detailed engineering.
This means that total of 750 KW is kept as safety
margin. Using concept of load sharing that mlcs can • Critical speed avoidance is programmed in the
be loaded to their maximum installed capacities as all system. As soon as the controller senses the critical
kick load will be shared by all turbines and engines. speed band, the controller crosses the critical band
For this it is essential that each turbine is fitted with at a faster ramp rate as programmed in the system.
an electronic governor and has its own DSLC (Digital The same ramp rate will be followed during
Synchroniser and Load Control). The DSLC performs controlled shutdown.
the function of a synchronizer as well as load controller. • Idle running and acceleration rate adjustments can
Each Turbine has its independent DSLC, which be programmed.
communicates with the speed controller to match the • Isochronous load sharing between the three STGsl
frequency and with the AVR for voltage. Each DSLC I'
communicates to other DSLCs on its own network
(LON). The DSLC receives CT and PT inputs to know Generator A GeneratorS
52 Isochronous
the generation and the total load requirement. All Droop
DSLCs are aware of the individual turbine generating 51
capacity (programmed into the system). The variation
in the load is distributed among all the turbines based so
•.
on generating capacity and the percentage of load
required to be shared by each turbine (programmed
into each DSLC). This will enusre that no turbine is
overloaded within the generating capacities of all the
three turbine. Further the load sharing will be 100% 75% 50% 25°"" 0 250"" 50% 75% 100

Isochronous i.e. the plant frequency will be maintained. Fig. 8 Droop unit Grid (Swing
This scheme is totally suitable for pulp and paper fixed or base load Machine)
plants, which are isolated from the grid (Fig. 8).

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

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exception that the utility or grid will control the

Generator B Generator A
Isoch. Base Load frequency and act as does the swing machine,
Isochronous
absorbing any change in load. The speed and droop
son settings are adjusted so that the turbine-generator set
I
I
I
supplies a fixed, or base amount, of power to the
Load : Swings utility (Fig. 9).
I
I When the turbine-generator supplying power to
1000,(, 75% SO°,4 25% o 25% 50% 75% 1000,(, the utility is separated from the utility. the speed!
frequency of the now unloaded isolated unit will
Load (%)
• increase to the speed set point on the control where
Fig. 9 Grid (Swing Machine) it was advanced to load the turbine-generator while
Isochronous base load machine drooped against the utility.
(Fixed or base load) Isochronous base load
Isochronous base load for a turbine-generator feeding
Engine generator will be achieved through DSLCs an utility is the same as isochronous base load for
in island mode. The system will be provided with an isolated system. Any difference between the speed!
the facility of overspeed testing. frequency setting of the isochronous base loaded
• Scan time has been configured between 5 msec to machines, and that of the utility, will show up as
80 msecs rate groups depending upon the criticality a change in the base load. This is why it is so
and nature of I/O as per Woodward Standard. important to have the speed/frequency setting of the
• The digital controller communicates with plant oncoming unit as close as possible to that of the
distributive control system (DCS) on Mod Bus. utility. As many as 15. turbine-generators can be
Systems tied to Utility grids . isochronous base loaded by applying the isolated bias
In case the plant is also tied to utility there are again signal across their common system load-sharing lines.
two of load sharing possibilities. The advantage of isochronous base loading over droop
• Droop base load sharing base loading is that when separating from an utility.
• Isochronous base load sharing there is no frequency change. Simply removing the
Base Load
bias signal from the load-sharing lines returns the
engine-generator to Isochronous speed (Fig. 9).
Droop base load is the same as droop, with the

_-- sn:"ATE (lRIl)

~~
Trw.IAIOI!I:

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

IPPTA Vol. 14, No.3, Sept 2002 87

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Import-Export to the Grid generating capacity of the plant and it will keep
When an individual pulp and paper plant has surplus exporting, till the maximum set load level is reached.
MW, then the same can be exported t~ the grid after In case the plant load goes beyond the maximum
meeting the plant requirement. This Tmport/Export plant load limit level, import will start to meet the
control action will be initiated by MSLC (Master plant load.
Synchroniser and Load Control) after the TG sets
have been synchronized with the grid. Once the plant CONCLUSION
is synchronized with the grid then MSLC will initiate
import/export function. In this mode of import/export For maximum production it is essential that
control, the MSLC measures the real power flow on uninterrupted power is available and the load on all ..
the incoming feeder with help of CT and P'Ivinputs. the turbines/engine generator is shared in the same
It then determines the power delivery required of proposition as the generating capacities of individual
each STG set and communicates this requirement to turbine/engine. This will ensure that all the turbines/
the individual DSLC(Digital Synchroniser and Load engines will share any sudden load coming in the
Controller). The individual DSLCs will control to its system and no individual turbine/engine will get
percentage of rated loads, and the MSLC will adjust overloaded. This will have an added advantage to
the load up or down to achieve the proper importl paper mills that they need not keep the safety margins
export level set (Fig. 10). Import Control function of 10% on individual turbine/engine. This effectively
will be achieved from MSLC when total generation means that on an average adopting the load-sharing
is less than the total plant load. Export control will concept will make each paper mills utilize 500 to 600
be achieved when the Captive generation is more then KW of energy, which they normally keep as safety
the plant load. In case the plant load increases then margins for sudden loads.
the controller will increase the reference to maximum

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