Reduction of Emissions by Optimal use of Gas Turbines · PDF fileReduction of Emissions by By...

Reduction of Emissions by

By Terence G. Hazel Senior Member IEEE

Optimal use of Gas Turbines

Summary

...................................................................................... p.1

Introduction..................................................................................................p.2

Basic.concepts.............................................................................................p.4

Frequency.control.&.Dry€Low€NOx. operation .............................................. p.8

Voltage.control............................................................................................p.10

Implementation.of.algorithms......................................................................p.12

Case.study.................................................................................................p.13

PMS.implementation..................................................................................p.17

Conclusion p.20

References.................................................................................................p.21

Executive€summary

.................................................................................................

of€Emissions€by OptimaI.use.of.Gas.Turbines.Reduction

COM-POWER-WP0�EN Rev1 | 1

Executive summary

Large.facilities.are.often.powered.by.gas.turbines..In.many.cases.turbines.

having.different.ratings.and.operating.characteristics.are.used.at.the.same.

site..The.decision.regarding.which.turbines.to.use.and.when.is.not.an.easy.

one.to.make.since.several.constraints.must.be.taken.into.account...

Some.of.these.are:

maintain .sufficient.spinning.reserve

operation.at.low.NOx.emission.levels

poor.dynamic.response.of.larger.sized.turbines

us . .Heat.Recovery.Steam.Generators

sufficient.power.for.all.production.islands.

Some.of.these.criteria.can.be.contradictory..For.example,.operation.at.low.

NOx.emission.levels.requires.a.high.load.on.the.turbine,.but.high.load.on.

turbines.often.means.lower.spinning.reserve.and.unequal.load.sharing,.both.

of.which.can.be.detrimental.to.system.stability.

The.paper.will.explore.the.different.aspects.that.must.be.taken.into.account.

in.the.optimal.selection.of.which.turbines.to.use.under.different.operating.

conditions€to€ .It .also.describes

.the.minimum.requirements .of .the.Power .Management .System. (PMS).

that

.

is

.

necessary.to.implement.this.optimization.

•••••

enhance€the€ .


ing

ing

reliability€of€the€suppfly€of€power

COM-POWER-WP0�EN Rev1 | �

Introduction

Gas.Turbines.(GTG).and.Steam.Turbines.(STG).often.provide.much,.if.not.all.

of.the.power.necessary.for.the.process.at.petrochemical.plants..They.are.

therefore.very.important.pieces.of.equipment.and.must.be.carefully.selected.

in.order.to.fulfill.the. requirements. . .:

sufficient.power.at.rated.frequency.for.all.loads

correct.voltage.at.all.load.busbars

robustness.to.ride.through.system.disturbances.

In.addition.there.are.several.environmental.constraints.that.must.be.met,.one.

of.which.is.the.emission.level.of.Nitrogen.Oxides.(NOx)..Modern.GTGs.use.

combustion.methods.which.limit.NOx.emission.without.the.use.of.water.or.

steam.injection,.a.process.called.Dry.Low.NOx .combustion (DLN).

This. .consists .of .mixing . . gas.thus. recording .

the.formation.of.NOx..This.type.of.combustion.however,.requires.

that.the.GTGs.operate.at.certain.minimum.power.output.levels.which.are..

a.function.of.the.air.inlet.temperature.

Another.environmental.constraint.is.the.increase.in.efficiency.in.order.to.

produce.more.MW.per.unit.of.gas..Cogeneration.is.a.method.commonly.used.

and.consists.of.producing.steam.from.the.exhaust.gas.of.the.GTGs.to.power.

steam.turbines..While.this.increases.efficiency,.it.makes.the.reliable.operation.

of.the.GTGs.even.more.important.since.the.loss.of.a.GTG.will.not.only.reduce.

the.available.power.of.the.gas.turbine,.but.also.of.the.associated.steam.

turbine.running.off.of.the.heat.of.the.GTG.exhaust.gas.

It.is.thus.very.important.that.extensive.engineering.studies.be.carried.out.

during.the.Front.End.Engineering.Design.(FEED).to.ensure.that.the.correct.

number.and.size.of.GTGs.are.selected..This.will.depend.on.the.plant.load.but.

also.on.the.design.of.the.electrical.distribution.system..As.will.be.shown,..

the.use.of.distributed.generation.can.make.the.designing.of.power.

systems.quite.complicated..Since.the.steam.supply.is.very.important.when.

cogeneration.is.used,.it.must.be.decided.if.independent.firing.means.are..

to.be.used.in.addition.to.the.heat.recovery.systems.

For.illustration.purposes.in.this.paper,.the.system.frequency.is.50.Hz.and..

40.MW.and.90.MW. . .are.considered..There.is.no.difference.when.

60.Hz.is.used.or.GTGs.with.other.power.output.ratings.

•••

GTGs

following

better of air and


ensuring

Optimal use of Gas Turbines

Reduction of Emissions by


Robust Control

A.system.is.“robust”.when.it.can.withstand..

severe.disturbances.and.keep.operating.within.

acceptable.limits..A.power.system.must.be.robust.

in.order.to.be.able.to.provide.power.to.loads.even.

when.faults.occur.such.as.short-circuit.conditions.

or.the.loss.of.a.GTG.or.STG..Since.electrical.

energy.that.is.produced.is.immediately.consumed.

without.any.intermediate.storage,.a.sudden.loss..

of.power.will.result.in.an.immediate.decrease..

in.system.frequency..A.short.circuit.causes..

a.decrease.in.active.power.consumption.due.to..

a.loss.of.voltage.at.the.fault.location..This.results.in.

an.immediate.increase.in.system.frequency...

A.robust.system.must.react.very.quickly.to.such.

disturbances.by.changing.the.operating.points

..

of.the.turbine.governors,.as.well.as.the.generator

.

and.transformer.AVRs..The.primary.regulation

.

associated.with.this.equipment.can.provide.

the.rapid.response.required.provided.that.the.

power.system.is.designed.accordingly.as.will.be.

discussed.hereafter.

The .primary .regulation .must .be .able.to.rapidly.

restore.the.balance.between.generation.and

.load..It.operates.with.no.consideration.for.the

.location.of.the.initial.imbalance..Primary.regulation

.can.be.implemented.in.several.manners.–.it.is.the.

responsibility.of.the.power.system.engineer.to.

determine.which.mode.is.correct..The.goal.of.

primary.regulation. . . . .keep ..the.frequency.

within.±0.5.Hz.and.. ..±5.%.

after.a.disturbance.has.occurred...The

.frequency. steady-state.error.

. less.than.. 150.mHz.

Basic concepts

Secondary.regulation.operates.at.a.global.

level..After.the.transient.conditions.following.

a.disturbance.have.subsided,.the.secondary.

regulation.will.send.±.set.point.changes.to..

the.GTG.and.STG.governors.to.bring.the.

frequency.back.to.its.nominal.value,.and.to..

ensure.correct.load.sharing.among.the.turbines..

It.will.also.send.±. set. point. changes. to. generator.

and.transformer .AVRs.in.order.to.ensure.correct.

voltage.levels.throughout.the.system..Secondary.

regulation.will.also.ensure.reactive.power.sharing.

The.key.points.of.robust.control.are.therefore:

separate ..global.&.local.control.functions.(Fig..1)

local.control.implement .in.primary.regulation

global.control.implemented.in.a.PMS

communicatie.between.control.layers.using.

incremental.values.(±.set.points).

Secondary regulation,set point changes

Optimizing equipmentoperating points

Global controllayer

Incremental set-point changes

Primary regulation,governor, AVR

Local controllayer

Turbine, generator,transformer

Stand-aloneregulation of electrical equipment

Power generation &transmission equipment

Figure 1 – Robust control philosophy

••••


is€to and

voltage deviation

of

should€be

provide

Un


The.local.controllers.should.implement.control.

based.on.droop.characteristics..Turbines.should.

all.operate.at.a.4%.frequency.droop..The.use.of.

droop.characteristics.will.ensure.that.all.turbines.

react.in.the.same.manner.to.disturbances.and.this.

greatly.enhances.the.ability.of.the.power.system.to.

recover.correctly..Fig..2.shows.2.GTGs.operating.

in.frequency.droop.and.sharing.the.load.–.each.

GTG.supplies.the.same.percentage.power.based.

on.its.maximum.rated.output..When.the.plant.load.

increases,.the.GTG.primary.regulation.will.detect.a.

decrease.in.system.frequency.and.will.increase.the.

GTG.power.output.to.compensate...Due.to.the.4.%

.droop.setting,.the.new.operating.point.will.be.at.a

.slightly.lower.but.still.acceptable.frequency.as.

shown.in.Fig..3.

FrequencyHz

51

50 Hz

49

MW

90 MW GTG

7560

40 MW GTG

20

20 MW 45 MW

40 90

Figure 2 – Primary regulation in frequency droop

Isochronous.or.fixed.power.modes.should.not.be.

used.except.for.very.specific.operating.conditions..

As.will.be.discussed.later,.the.use.of.DLN.turbines.

introduces. additional. operation. constraints.

When .generators.are.directly.connected.to.. the. same. busbar,. their. AVRs. should. operate.

at..a.5.%.voltage. droop.

40

FrequencyHz

51

50

49

MW

90 MW GTG

7560

40 MW GTG

20

49.75 Hz

25 MW 56.3 MW

90

Figure 3 – Operating points after load increase

When.generators.are.connected.to.busbars.via.

step-up.transformers,.the.transformer.impedance.

ensures.the.voltage.droop.characteristic,.and.it.is.

recommended.to..set.these.generator.AVRs.to.

fixed.voltage...In.some.cases,.busbars.will.be.also

.be.supplied.by.transformers.which.import.power.

from

.

other

.

sources.in.the.system.(Note:.

Transformers.can.control.the.flow.of.reactive.

power.but. do. not. produce

.

any.).. The. AVRs. of.

these.transformers.should.also.operate.at.a.5.%.

voltage.droop.thus.contributing.to.maintaining.

the.busbar.voltage.in. the.same.manner.as.the.

generators.

[1]...

This.

avoids.conflicts. that.can.occur.when. .

.with€have€.different .operating.modes.and.

characteristics

.

try

.

to. control. the. voltage. on.. the. same.

busbar..It.is. also.possible.in.some.cases.to.allow.the

.transformer.to.supply.the.reactive.power.necessary.

to.maintain.the.busbar.voltage.and.to.operate.the.

generators.at.fixed.power.factor..Generators.

operating

.

at

.

fixed

.

power. factor. however,. are. not.

able.to.contribute.to.maintaining.system.voltages.

after.a.disturbance.has.occurred.

Primary Regulation of GTGs and STGs

generators and tansformers which



Fast Load Shedding

In.some.cases.it.is.not.possible.for.the.primary.

regulation.to.keep.the.power.system.within..

the.limits.defined.above.after.a.disturbance.has.

occurred..One.reason.can.be.the.limitation.of.

the.dynamic.release.of.reserve.power.from.large.

turbines.as.will.be.discussed.in.a.later.section...

The.only.means.of.preventing.a.collapse.of..

the.power.system.in.such.cases.is.to.shed.

sufficient.load.quickly.enough.that.the.generation.

can.continue.to.supply.the.priority.loads...

Plant-wide.fast.load.shedding.is.one.of.the.typical.

functions.implemented.in.a.PMS.

The.level.of.security.is.defined.taking.into.

account.credible.contingencies,.called.“normative.

incidents”,.for.which.the.power.system.is.

supposed.to.remain.within.the.limits.defined.above..

Primary.regulation.cannot.ensure.that.nominal.

values.of.frequency.and.voltage.are.present.

throughout.the.power.system..Global.control.is.

required.and.is.provided.by.the.PMS...

The.PMS.measures.the.system.frequency,.busbar.

voltages.and.power.output.of.the.turbines.and.will.

change.the.set.points.of.the.primary.regulation.in.

order.to.ensure:

correct.frequency

correct.busbar.voltages

DLN.operation.where.possible

equal.sharing.of.load.as.a.percentage.of.rated.

power.

The.secondary.regulation.measures.the.frequency.

and.output.power.of.each.GTG...

When.the.frequency.deviates.from.the.set.point,.

the.secondary.regulation.will.adjust.the.position.of.

the.droop.curves.of.each.GTG.such.as.to.adapt.

the.output.power.to.correct.the.frequency...

The.droop.curves.of.all.GTGs.are.adjusted.by..

the.same.amount.which.guarantees.automatic.

sharing. of. the. loads.. Fig.. 4. shows. the.modified.

droop.curves..

••••

FrequencyHz

51

50 Hz

49

MW756020

Droop curves moved upwardsto maintain 50 Hz at new load.

90 MW GTG40 MW GTG

25 MW 56.3 MW

40 90

Figure 4 – Secondary regulation frequency adjustment

Frequency.control,.load.sharing.and.DLN.

operation.require.set.point.changes.in.turbine.

governors..It.is.very.important.that.the .PMS .

algorithms.consider.frequency.control . be.the.main.

criteria.and.ensure.that.the.steady.state.frequency.

will.never.be.far.from.the.set.point.even.if.it.is.not.

possible.to.ensure.correct.load.sharing...

A.typical.normative.incident.is.the.loss.of.a.GTG..

For.contingencies.larger.than.these.normative.

incidents,.such.as.the.simultaneous.loss.of.several.

GTGs,.a.defense.plan.is.required.that.will.allow.the.

controlled.shutdown.of.the.installation.under.the.

best.possible.conditions.(defense.plans.are.not.

covered.in.this.paper)...

.

To.implement.fast.load.shedding,.the.PMS.

normally.calculates.what.load,.if.any,.must.be.shed.

for.each.normative.incident..When.that.particular.

normative.incident.occurs,.the.predefined.loads.

are.immediately.tripped..Underfrequency.load.

shedding.is.also.used.as.a.backup..Should.for.any.

reason.insufficient.load.have.been.shed.and.the.

frequency.continue.to.decrease,.additional.circuits.

will.be.tripped.as.a.function.of.frequency.and.time.

Secondary Regulation


to

position€of€the


As.an.example,.consider.a.governor.of.a.lightly.

loaded.turbine.not.responding.to.set.point.

changes.from.the.PMS.due.to.some.internal.fault.

condition..In.order.to.achieve.load.sharing,.the.

PMS.must.increase.the.load.of.this.turbine.but.at.

the.same.time.decrease.the.load.on.other.turbines.

in.order.to.avoid.any.changes.in.frequency..Since.

the.faulty.governor.does.not.respond.to.set.

point.increases,.it.will.not.increase.its.load.whereas.

the.other.turbines.will.respond.to.the.PMS.set.

point.changes.and.decrease.their.power.output..

The.result.will.be.a.decrease.in.frequency..Should.

the.frequency.decrease.below.the.dead-band.value,.the.PMS.must.immediately.cease.attempts.

to.share.the.load.since.that.would.result.in..

an.unacceptable.decrease.in.system.frequency.

Secondary.regulation.does.not.react.quickly.and.

should.not.react.quickly..It.is.used.to.“fine.tune”.

the.power.system.by.bringing.frequency,.voltage.

and.power.values.within.steady.state.tolerances..

Attempting.to.change.state.values.too.quickly.

could.cause.instabilities.and.power.outages...

In.addition,.the.reaction.of.primary.regulation.to.set.

point.changes.is.inherently.slow.and.even..

if.the.secondary.regulation.tried.to.quickly.change.

set.points,.it.couldn’t.do.so.

The.basis.for.power.system.design.is.knowing.

how.the.system.will.react.to.contingencies...

For.green.field.projects,.or.for.large.extensions.to .

existing.facilities,.simulations.must.be.carried.out.

since.it.is.not.possible.to.make.measurements..

The.simulations.must.calculate.the.transient.and.

steady.state.frequency,.active.and.reactive.power.

flow,.and.busbar.voltages.immediately.after.

contingencies..In.many.cases.it.is.necessary.to

.conduct.simulations.over.a.long.period.of.time.to.

ensure.that.the.power.system.remains.stable.after.a.major.disturbance..Short-time.simulations.may.

show.that.the.power.system.recovers. correctly.

after.a.contingency.only.to.find . .

after

.

several

.

seconds,.the.power. system. actually.collapses.

The.transfer.functions.of.the.turbine.governor,.

and.the.generator. and.transformer.AVRs.must.

be.well.known.in.order.to.perform.such.

simulations...This.information.is.unfortunately.

difficult.to.get...As.will.be.discussed,.the.dynamic.release.of.reserve.power.is.often.much.

less.than.would.be.expected.and.must.be.

correctly.simulated.[2].

System Calculations and Simulations

at€the site€out

actually

that€



Frequency control & DLN operation

Frequency.control.and.active.power.sharing.

among.turbines.is.very.important.and.often.

referred.to.as.P/f.control..Implementation.of.

DLN.operation.adds.to.the.complexity.of.the.P/f.

control.algorithms..In.order.to.reduce.the.NOx.

concentrations.in.exhaust.gas,.the.air/gas. . .must€be€homogeneous .This.can,.however,..

only.be.achieved.when.the.turbine.output.exceeds..

a.certain.percentage.of.its.nominal.rating.as..

a.function.of.the.air.inlet.temperature..Fig..5.shows.

the.relationship.between.the.DLN.limit.and..

the.maximum.power.output..Both.depend.on.

the.air.inlet.temperature.(shown.on.the.abscissa)..

The.ordinate.is.in.percent..From.Fig..5.it.can.be.

seen.that.for.DLN.operation,.the.turbine.output.

must.generally.exceed.60.%.of.the.nominal.rating..

Since.operation.close.to.the.DLN.limit.can.result.in.instability,.it.is.necessary.to.ensure.that.the.turbine.

output.is.either.well.above.or.well.below.the.DLN.

limit..This.constraint.does.not.apply.to.turbines.

that.do.not.have.the.capability.of.DLN.operation.

nor. to.steam.turbines.

As.mentioned.in.the.previous.section,.equal.

sharing.of.the.load.as.a.percentage.of.nominal.power.is.important.since.it.enhances.the.capability.of.the.power.system.to.respond.correctly.to.

contingencies..Equal.sharing.is.always.possible.when.no.turbines.operate.in.the.DLN.mode.

since.there.are.no.constraints.regarding.output.power.other.than.exceeding.the.minimum.or.

maximum.power.output..It.is.easy.to.meet.

these.requirements.since.these.points.are.at.the.extremes.of.the.turbine.output.capability.curves..When.DLN.operation.is.used,.it.is.not.possible.to.

ensure.equal.loading.among.all.turbines.

44244-16-3600

25

50

75

100Maximum Output

DLN limit

Figure 5 – DLN limits as function of temperature

As.an.example,.consider.several.non-DLN.GTGs.

supplying.the.load..As.shown.in.Fig..2,.3.and.4,.

operation.in.4.%.frequency.droop.will.automatically.

ensure.that.load.is.shared.equally.as.a.percentage.

of.nominal.power..Should.the.plant.load.increase,.

the.steady-state.frequency.will.decrease.and..

the.PMS.will.send.the.same.set.point.adjustments.

to.all.governors.to.increase.the.GTG.output.thus.

bringing.the.frequency.back.to.its.nominal.value.

and.ensuring.correct.load.sharing.at.all.times...

This.is.true.for.all.values.of.output.power.

Should,.however,.one.of.the.GTGs.operate.in.

the.DLN.mode,.it.is.no.longer.possible.to.have.

identical.load.sharing.among.the.machines.since.

it.is.necessary.to.ensure.that.the.DLN.machines.

operate.far.from.the.DLN.limit..Secondary.

regulation.is.used.to.ensure.operation.far.from.

the.DLN.limit..This.is.achieved.by.considering.two.

different.values.of.minimum.output.for.the.DLN.

machine..At.low.load,.the.minimum.output.will.be.

a.few.%.of.the.nominal.rating..Operation.of.the.

DLN.GTG.can.then.be.at.the.same.%.output.as.

all.other.GTGs.since.at.light.load.the.output.will.be.

far.below.the.DLN.limit.which.is.close.to.60.%.of.

rated.power..When.the.plant.load.is.high.however,.

the.DLN.GTG.must.be.made.to.operate.well.above.

the.DLN.limit..For.this.case,.the.minimum.output.

used.is.the.DLN.limit..Secondary.regulation.will.

ensure.that.the.%.load.within.this.range.(DLN.limit.

to.maximum.output).is.the.same.as.the.%.load.of.

all.other.GTGs,.their.range.being.between.the.

minimum.and.maximum.values..This.is.shown.in. Fig..6..

FrequencyHz

51

50 Hz

49

MW756020

DLN Limit

90 MW GTG

40 MW GTG

12.2 MW

40

65 MW

90

Figure 6 – Operation above the DLN limit

%Pn

Co

mixture.



The.dashed-line.portion.of.the.droop.curve.of.the.90.MW.GTG.corresponds.to.output.values.below.the.DLN.limit..The.GTG.is.not.to.operate.there.at.high.load..This.range.is.from.0.MW.to.54.MW.(60.%.of.the.rated.output)..The.solid-line.portion.of.the.droop.curve.of.the.90.MW.GTG.is.the.permissible.operating.range.and.equals.36.MW.(90.MW.–.54.MW)..The.%.loading.“x”.is.based.on.the.total.power.required.“P”. . .

P.=.(x(36).+.54).+.x(40)€;.. x.=.P.-.5476

The.power.supplied.by.the.GTG.is.65.MW.which.exceeds.the.DLN.limit.by.a.comfortable.11.MW...11.MW.is.equal.to.30.5.%.of.the.permissible.36.MW.operating

.range.

..The. other. GTG. shall. also. operate. at.

30.5.%.of.its.range,.but.its.range. is. between. 0. MW.and

.40

.MW.

.Its.operating.point.is..12.2.MW...Thus

.although.the.%.loading.within.the.operating.ranges.are.the.same,.the.%.loading.based.on.the.total.output.capacity.is.not..The.DLN.GTG.is.operated.at.a.higher.%.load.than.the.other.GTG.

When.a.load.step.occurs,.the.primary.regulation.of.all.turbines.will.react.to.increase.the.power.output.but.at.a.slightly.lower.frequency..As.shown.in..Fig..7,.primary.regulation.will.change.the.operating.point.along.the.droop.curves.without.taking.into.account.DLN.operation..

FrequencyHz

51

50

49

MW756020

DLN Limit

90 MW GTG40 MW GTG

17 MW

76 MW

40 90

Figure 7 – Reaction of DLN turbine to step load

The.DLN.turbines.are.intially.too.heavily.loaded.–.76.MW.corresponds.to.61.%.of.the.DLN.operating.range.whereas.17.MW.corresponds.to

.only.42.5.%.of.the.non-DLN.operating.range..

It.is.the.secondary.regulation.that.will.ensure.that.the.correct.operating.positions.of.all.DLN.and..non-DLN.turbines. Fig..8.shows.the.operating.points.after.secondary.regulation.has. .modified

.the.governor. set. points. of. all. turbines.. The.output

.of.the.DLN.

turbines.

was. decreased. and.that.of.the.non-DLN.turbines.increased..The.new.loading.factor.is..51.3.%.for.all.turbines.

FrequencyHz

51

50 Hz

49

MW756020

DLN Limit

90 MW GTG40 MW GTG

20.5 MW

72.5 MW

40 90

Figure 8 – Correction of DLN turbine loading

Even.if.all.GTGs.operate.in.DLN.mode,.there.may.be.situations.where.load.sharing.cannot.be.achieved..In.the.previous.example,.consider.that.both.GTGs.could.operate.in.a.DLN.mode..Should.plant.load.be.close.to.78.MW,.which.is.60.%.of.the.total.generation .capacity, .it .would .not€be .possible .to .

equally

.share.the.load.since.this.would.result.in.the.

GTGs.

operating.at.their.DLN.limit. ..One.GTG.mustbe.selected.to.operate. at. a. higher. rated.

load

.

and

.the.other. at. a. lower. rated. load.. Both. GTGs

.will.thus. be.operating.far.from..their.DLN.limits.but. .

.will

.not

.equally

.share

.

the

.

load.

.During .disturbances,.the.primary.regulation. of. all.turbines .will. react. based. on. the. 4. %. frequency.droop.curve.only,.as.shown.in.Fig..2...The.minimum.value.used. by. the. PMS. to. determine. the. operating.points.of.DLN.GTGs. has. no. influence. on.the.dynamic.response. of. the. turbines... Simulations.should.be.made.to.check.that..the.power.system.remains.stable.when.some.GTGs.are.more.heavily.loaded

.than

.others

.due.to. the.DLN.constraints..

After.the.transients.have.died.down.(and.possibly.load.shedding),.the.PMS.will.recalculate.the.power.output.that.each.GTG.should.have.and.send.the.required.incremental.set.point.changes.to.each.GTG.


is:

therefore


Voltage control

Voltage.control.and.reactive.power.sharing.is.often

.

referred.to.as.Q/V.control..Voltage.control.in..

a.power.system.is.very .different .from .frequency .

control..The.frequency.is.the.same.throughout.

the.whole.power.system.and.is.not.influenced.by.

.transformers..The.primary.regulation.of.all.

turbines.therefore.react.to.the.same.state.variable

.

no.

matter

.

where.a.turbine.is.located. ..Voltage,. on

.

the.other.hand,.is.a.local.state.variable.and.can.

differ.greatly.throughout.the.power.system..It.is.

the.impedances.in.the.power.system.(lines.&.

transformers).that.decouple.the.voltage.levels.at

.

different.locations..Frequency.control. is. made.

by.adjusting.the€power. output.of.the.turbines..

Voltage

.

control.requires.adjusting.the.reactive.

power

.

output.of.the.generators.but.also

.changing.the .tap.positions.of.transformers.

When.all.power.sources.are.connected.to..

the.same.busbar,.the.generator.AVRs.will.ensure

.

that.the.voltage.on.that.busbar.is.correct.by.

adjusting

.

the

.

reactive.power.output. ..The.PMS.will

.

fine

.

tune

.

the

.

voltage. to. ensure. that. it. remains.

within.the.steady-state.tolerance..Transformers.

supplying.

load.busbars.may.have. on-load.tap.

changers.(OLTC).and.their.associated.AVRs.will.

ensure.the.correct.flow.of.reactive.power.to.

downstream.busbars..When.distributed.

generation.is.used,. .it.is.often.necessary.to.

change.the.tap.position.of.the.transformers.on.

the.interconnection.lines.in.order.to.move.reactive

.

power.from.one.location.to.another..This.can.be

.

achieved.in.primary.regulation.by.implementing.

.

a.voltage.droop.characteristic.in.the.transformer

.

AVR.as.shown.in.Fig..9..When.the.transformer.voltage.droop.has..the.same.%.value.as.the.generator.AVR.droop,.then.all.generators.and.all

.

transformers.connected.to.the.same.busbar.

will .supply .the.same.percentage.of.reactive

.power.to.support.the.busbar.voltage..An.

increase.in.load.will.also.increase. the.reactive.

power.consumption..Additional.reactive.power.will

.

be.supplied.by.both.the.generator.and.the.

transformer...Transformers.can.supply.additional

.reactive.power.only.when.there.is.a.surplus.

elsewhere.in.the.power.system..Reactive.power.

is.produced.by.generators.–.transformers.control.

only.the.flow.of.reactive.power.

VoltagekV

7.2

6.6 kV

Mvar

100 MVATranformer

806020

6

40

40 MW GTG

18 Mvar

63 Mvar

100

Figure 9 – Reactive power sharing

A.40.MW.GTG.with.a.generator.that.has..

a.power.factor.capability.of.0.8.will.be.able.to.

supply.up.to.30.Mvar..The.100.MVA.transformer.

could.supply.100.Mvar.provided.there.was.no.

active.power.flowing.through.it..Both.voltage.droop.

curves.are.shown.in.Fig..9..In.reality.the.transformer.

voltage.droop.curve.is.not.linear... . .Changes.in.

reactive

.

power

.

flow

.

are. due. to. tap. position.

changes

.

which. are. step. functions,. not. linear. as.in. the. case. of. the. reactive. power. output. of. a.

generator..Also. the

.

generator.AVR.will.act.much.more.quickly.than.the.transformer.AVR..

The.steady.state.conditions.will .be.in .accordance.with.the.voltage.droop.curves.shown.in.Fig..9.but.not.during.transient.conditions..


Since.it.is.not.

necessary.to.have.perfect.sharing.of.reactive.

.

power,.a.linear.transformer.voltage.droop.curve.

having.the.same.dynamic.response.as.the.

generator.AVR.can.be.assumed.for.most.

calculations.

Since.voltage.is.a.local.variable,.reactive.power.

sharing.makes.sense.only.for.generators.

connected.to.the.same.busbar..Generators.

connected.to.different.busbars.may .have .very .

different .reactive .power .outputs.. Attempting .to .all.generators.produce.equivalent.amounts..

of.reactive.power.can.have.very.detrimental.effects.

or

.

voltage

.

levels. throughout. the. plant... The. fact. that.

the.power.factor.in.GTGs.and.STGs.connected.at.

have

.

different.locations.in.the.plant.may.be. very.

different

.

has.no.consequence.on.system.stability

.nor.on.correct.operation.

Voltage.control.is.independent.of.frequency.

control,.DLN.operation,.and.active.power.sharing..

This.allows.the.PMS.to.have.2.sets.of.algorithms,.

one.set.for.P/f.control.and.another.set.for.Q/V.

control..Since.these.control.algorithms.make.set.

point.adjustments.to.different.devices, ..

they.are.implemented.independently.of.each.other.

.

The.only.validation.required.is.to.ensure.that..

the.apparent.power.flowing.through.equipment.or.

in.an.interconnection.does.not.exceed.acceptable.

limits..For.example.in.Fig..9.it.is.necessary.for..

the.PMS.to.ensure.that.the.active.power.through.

the.100.MVA.transformer.does.not.exceed.78.MW.

Since.changes.in.reactive.power.output.occur.

more.quickly.than.active.power,.often.the.PMS.

will.introduce.time.delays.such.that.the.transition.

speed.resulting.from.set.point.changes.sent.to.

governors.and.AVRs.will.be.approximately.equal.

P/f and Q/V algorithms,,,,,,,,,;: -,

When.the.power.system.is.islanded,

.the.P/f. algorithm.will.determine.the.total.power.

required

.

to

.

maintain.the.system.frequency.at.its.

nominal.value.

.

When .however, .the .power

.

system

.

is

.

connected

.

to.the.local.utility,.the.frequency.is.

fixed .by .the. utility. .The . . .the .frequency

.because€it

.

fixes

.

.

has.thousands. of.installed.MW.of.power

.

generation.whereas.the. industrial.plant.only.a.few

.

hundred..

The.

PMS.

will.

now. determine. the. total.power

.required.to.ensure .that.the.import/export.

of.active.power.is.within. acceptable.limits..

After.having.determined.the.total.power.

required,.the.PMS.now.determines.how.this.

power.is.to.be.shared.among.the.GTGs.and.

STGs.in.operation,.also.taking.into.account.DLN.

operation.if.required..The.result.of.these.calculations.

will.be.set.point.adjustments.that.will.be.sent.to.

each.turbine.governor.as.described.above..At.the.

end.of.each.PMS.control.time.interval,.new.set.

point changes will be calculated and executed.

utility

As.mentioned,..voltage.is.a.local

.condition. .The.PMS.will. therefore. measure. voltage

.levels .at .different .parts .of .the .plant .in .order .to .determine..the. reactive. power. requirements...At.

each

.

location.where.voltage.control.is.possible,.the.PMS.will.determine.the.required.set.point.

changes.for.generator.and.transformer.AVRs...

Reactive.power.sharing.among.the.devices .controlling.the.voltage.at.a.specific.location.will..

also.be.implemented.by.the.PMS..

In.the.same.manner

.

as

.

for

.

the

.

implementation

.

of

.the.P/f.algorithms, .set.point.changes.corresponding.to. 50. %.of.the.

calculated.values.will.be.sent.to.the.AVRs..Since.

changes. in. reactive. power. normally. occur. much.

quicker.than.active.power,.the.PMS.will.add.time.

delays.in.the.Q/V.set.point.changes.such.that.the.

rate.of.change.of.P/f.and.Q/V.state.variables.will.

be.approximately.the.same.

Load shedding calculations

P/f€algorithms:

Q/V€algorithms:


Load.shedding.calculations: .are.based.on..the.

dynamic.spinning.reserve.of.all.turbines.in.operation..

Should.distributed.generation.be.used,.the.spinning.

reserve.for.each.viable.island.(generation.+.load).is.

calculated.

.

Should

.

there

.

not.be.sufficient.spinning.

reserve,.the.PMS.will. determine.which.loads.in.that.

island.are.to.be.shed.should.that.part.of.the.plant.

actually.be.islanded. from.the.rest..The.PMS.will.

warn.the.operator. that .load .shedding .occurs€after€a After .each .viable .island

.configuration

.

has .been

.

calculated,. the. PMS. will.

then .perform .the. same. calculations. for. the. whole.

plant. .Should .there .not. be. sufficient. spinning.reserve,

.the.PMS.will. determine .which

.

loads

.

are

.

to

.be .shed. should. .a. normative. incident. occur.. The.

loads

.

to

.

be. shed.are.calculated.based.on.process.

requirements.

only,.and.do.not.take .into

.

account

.which.normative .. leads. to. load. shedding

.execution..When.the.normative.incident.occurs,.

load.shedding.is.immediately .executed .based .on .

the.previously.executed.calculations.

time

disturbance .

any.

described in€the€next section,

the

These

incident


COM-POWER-WP0�EN Rev1 | 1�

The.PMS.will.perform.several.calculations.for.

determining.the.set.point.changes.required..

by.the.P/f.and.Q/V.algorithms..The.magnitude.of.

the.set.point.changes.will.depend.on.the.difference

.

between.the.actual.state.of.the.power.system.and.

what.is.should.be..Since.it.is.important.for.system.

stability.to.make.changes.in.a.controlled.manner, .

the.PMS.will..send.a.set.point.change.that.

corresponds.at.most.to.50.%.of.the.calculated.

total.set.point.change.as,.illustrated.in.Fig..10.

PMStimeintervals

P

P0

1 2 3 4 5

P1

P2P3

P4

dTmaxdT’2

dT’’2

Figure 10 – PMS execution of set-point changes

The.abscissa .Fig..10.shows.the.PMS.control.

time.intervals..After.each.time.interval,.the.PMS.

calculates.all.set.point.changes.and.will.send..

the.appropriate.control.signals.to.the.governors.

and.AVRs..These.signals.will.be.executed.during.

the.next.PMS.control.time.interval..Fig..10.

shows.the.P/f. calculations.to.illustrate.the.control.

principles .that .are .implemented..Pn.is.the.GTG.

output

.

power

.

that

.

should.be.produced.after.the.

nth.PMS.time.interval. .Since.the. response. to.

secondary

.

regulation.incremental.set.point.

changes. is. a. straight. line. with. a. fixed. slope,. the.

distance.between.the.actual.value.of.P.and.the

.next.value.is.shown.by.a.line,.the.length.of.which

.is.(Pn+1-Pn)/m.where.Pn+1-Pn.is.

Implementation of algorithms

the.amount..of.MW.

the.power. output. is. to. change€by,. and. m. is.. the.slope. of. the. response. in. MW. per. second..The

.result.of .the .calculation.is.the.time.dT..the.control.signal

.

is .to. be. applied. to. achieve. the. required.change. in.output.power,.and.this.is.illustrated.by

.the.line. joining.points.Pn+1.and.Pn...The.PMS.will,.

however,.only.execute.a.maximum.of.50.%.of.

the.change.and.this.is.illustrated.by.splitting.the

.

line

.joining.Pn+1.and.Pn.into.2.parts,.a.solid.line.and.a

.dashed.line..The.solid.line.represents.the.control.

signal

.

actually. sent,. and. the. dashed. line. shows.

what

.

control.signal.is.still.required. to. achieve. the.

change.in.power.output.

dT’.is.the.time.required.to.apply.the.control.signal.

to.produce.P1.MW..The.signal.will.only.be.applied.

for.dT’/2.however..Since.dT’/2.is.shorter.than..

the.PMS.control.time.interval,.after.dT’/2.seconds,.

the.control.signal.will.be.removed.and.the.power.

output.will.remain.unchanged.for.the.remainder.of.

the.PMS.calculation.interval..This.is.shown.by.the.

horizontal.curve.just.after.dT’/2..P2.shows.a.lower.

power.output.is.required,.and.the.same.control.

sequence.is. implemented,.this.time. .using .

a.negative.slope..P3.has.the.same.value.as.P2.and.

thus.no.control.signal.need.be.applied.as.shown.

by.the.horizontal.output.curve..

.

The.time.to.get.from.

P3.to.P4.exceeds.twice.the.PMS.control.time.interval..Thus.the.maximum.duration.for.this.control.

signal.will.be.the.PMS.calculation.interval.itself,.

which.is.less.than.50.%.of.the.calculated.time.

.Thus.at.no.time.do.the.PMS.control.signals.

achieve.full.correction.–.they.just.successively.bring.

the.system.closer.to.the.desired.operating.condition..

The.farther.away.the. desired. value,. the. longer. the.

control.

signal.

is.

applied...

This.type.of.control.is.very.common.

PMS control - signal duration

in



The.case.study.illustrates.many.of.the.concepts.that.have.been.developed.in.the.previous.sections.

.

The.plant.shown.in.Fig..11.has.2.main.process.areas.and.the.total.load.is.200.MW..The.total.installed.generation.is.340.MW..

The.90.MW.GTGs.are.to.operate.in.the.DLN.mode..The.40.MW.GTGs.have. .standard .combustion..The.40.MW.GTG.shown.in.dotted.lines.indicates.a.machine.in.maintenance...There.is.a.connection.to.the.local.utility,..the.purpose.of.which.is.to.be.able.to.import.power.during.contingencies.in.order.to.help.keep..the.plant.power.system.stable..The.power.system.can.thus.operate.in.the.connected.mode..(utility.circuits.are.closed).or.in.the.islanded.mode.(utility.circuits.are.open)..In.normal.operation..the.exchange.of.active.and.reactive.power.with..the.utility.should.be.kept.close.to.zero..

As.can.be.seen.in.Fig..11,.the.turbines.are.not.all.connected.to.the.same.busbar..In.both.process.units.there.are.sufficient.turbines.to. .

loads

.

Case study

. .

supply€the

.should.that.part.of.the.plant.be.islanded.from

.the

.utility

.and.from.the.other.process.unit..

This.

requires. that. the. load. shedding. algorithms.also.ensure.sufficient.generation.in.each.process

.area. to. avoid. loss. of. production. should. any.islanding.occur.

2x20 MVA

Utility G G 2x90 MW

110 kV

G G G G

33 kV

3x40 MW

2x100 MVA 2x60 MVA

80 MW+55 Mvar

120 MW + 70 Mvar

The.design.criteria.for.the.power.system.and.PMS.are:

implement.the.concepts.of.robust.controlmaintain .correct .frequency .when .islanded

import/export.of.active.&.reactive.power

...close.to.zero

maintain.correct.busbar.voltages.in.both..the.islanded.and.connected.modes

DLN.operation.is. .. .when.possibleimplement.active.power.sharing.among.DLN.

operated.turbinesimplement.active.power.sharing.among.other.

turbinesimplement.reactive.power.sharing.among.

generators.connected.to.the.same.busbarsensure.correct.transfer.of.reactive.power.

between.groups.of.generation

•••

•

••

•

•

•

ensure.sufficient.spinning.reserve.in.each.process.area.and.plant.wide

optimize.the.number.and.type.of.each.GTG.throughout.the.plant€to€reduce. .

provide.fast.load.shedding.in.each.process.area.and.plant.wide.In.some.cases.it.is.not.possible.to.meet.all.of..the.design.criteria..It.is.very.important.to.determine.up.front.which.criteria.are.the.most.important..After.the.essential.criteria.have.been.met,.the.others.will.be.implemented.based.on.their.importance...For.example,.frequency.and.voltage.control.are.more.important.than.active.and.reactive.power.sharing..DLN.operation.is.also.more.important.than.load.sharing..Under.no.circumstances.however,.should.the.concepts.of.robust.control.be.compromised.

•

•

•

Description of the power system

Figure 11 – Case study single-line diagram

System design criteria


use

keep€theemissions


Steady state operation

Normal.operation.is.in.the.connected.mode.as.

shown.in.Fig..12..Import/export.of.active.and.

reactive.power.is.close.to.zero.so.all.loads.are.

supplied.from.the.GTGs..The.larger.turbines.

operate.in.the.DLN.range.meaning.that.their.

proportional.loading.is.higher.than.the.other.

smaller.turbines..The.DLN.GTGs.are.connected..

to.HRSGs.and.provide.process.steam...Sufficient.

turbines.are.to.be.in.operation.to.ensure.

adequate.spinning.reserve.for.the.whole.power.

system.as.well.as.for.the.process.unit.islands...

Due.to.the.amount.of.load.in.the.various.process.

units,.active.and.reactive.power.will.flow.from..

the.process.area.with.the.larger.turbines.to..

the.process.loads.associated.with.the.smaller.

turbines..A.dead.band.is.associated.with.each.

state.variable.such.a.frequency,.voltage,.and.active

.&.reactive.power.output..When.a.state.variable.is

.within.the.dead.band,.no.corrections.. .will.be.

required.

In.the.connected.mode,.exchange.of.active.and.

reactive.power.with.the.utility.is.to.be.kept.close.

to.zero..The.P/f.algorithm.measures.the.active.

power.flowing.through.the.utility.transformers.and.

calculates.what.the.total.generated€power. should.

be

.

in

.

order.to.keep.this .value.close .to .zero...The.P/f.

algorithm.also.checks.the.correct.sharing.of.load.

among.the.smaller.turbines.and.between.the.2.

larger.ones.which.are.operating.in.the.DLN.range..

Based

.

on.these.2.sets.of.values.(total.power.

required

.

and

.

load. sharing). the. PMS. calculates.

the

.

set.point.adjustments.required.for.each

.turbine. ..It.then.sends.set.point.corrections.

equivalent

.

to

.

50. %.of.the.calculated.values.as.

described

.

in

.

the

.

previous.section...These

.algorithms.are.executed.continuously

.

and

.

ensure

.

correct

.

operation

.

at

.

all

.

times.

.In.a.similar.manner.the.Q/V.algorithms.are.

executed..In.the.connected.mode,.the.utility.tie.is.

not

.

strong .enough.to.guarantee.that.the.power.

system

.

voltage

.

will

.

be

.

correct.

0 MW + 0 Mvar

G G 2x71.5 MW+46 Mvar

G G G G

2x28.8 MW+16.5 Mvar

80 MW+55 Mvar

120 MW + 70 Mvar

2x31.5 MW+18.5 Mvar

Figure 12 – Case study steady-state power flow

This

.

is

.

due

.

to

.

the

.

lack.of.reactive.power.reserves.on.the.utility.side.and.is.the.reason.for.keeping.reactive.power.

import.close.to.zero..The.generators.are.used.to.

maintain.correct.system.voltages..

.

Thus.the.per.

unit.voltage.on.the.plant.side.of.the.utility

.transformers.will.be.close.to.the.nominal.

value,.but.the.per-unit.voltage.on.the.utility.side.of.

these.transformers.could.be.quite.different.from.

the.nominal.utility.voltage..The.only.way.to.ensure.

that.the.flow.of.reactive.power.through.the.utility.

transformers.is.close.to.zero.is.to.ensure.that..

the.turns.ratio.of.the.utility.transformer.equals..

the.ratio.of.the.voltages.on.the.utility.and.plant.

sides..This.will.cause.the.voltage.drop.across..

the.utility.transformers.to.be.very.small.leading..

to.a.very.low.flow.of.reactive.power.through..

the.transformers..In.order.to.achieve.the.correct.

turns.ratio,.the.PMS.will.send.voltage.set.point.

adjustments.to.the.utility.transformer.AVRs...

These.set.point.adjustments.will.result.in.the.tap.

position.being.changed.until.such.time.that..

the.correct.turns.ratio.is.achieved..In.accordance.

with.the.principles.of.robust.control.described.

earlier,.only.voltage.set.point.changes.are.sent.to.

the.transformer.AVRs.–.no.attempt.is.made.by..

the.PMS.to.directly.control.the.tap.position.of.the.

utility.transformers.



It.is.also.necessary.to.ensure.correct.voltage.

throughout.the.plant..For.load.busbars.supplied.

only.via.step-down.transformers.(not.shown.in.

Fig..11),.this.is.achieved.by.the.transformer.AVRs.

without.any.assistance.from.the.PMS...For. load

.busbars.that.are.also.supplied.by.GTGs..(the.33.

kV.bus.in.Fig..11),.it.is.necessary.to.ensure.that

.reactive.power. .provided.both.from..

the.generators,.and.from.the.step-down.

transformers .connected.to.the.same.busbar...

This.is.achieved.by.implementing.the.same

.voltage.droop.characteristic. in. the. transformer.

AVR

.

as.in.the.generator.AVRs..Should.the.busbar.

voltage.differ.from.the.nominal.value,. the. PMS.

will.send.voltage.set.point.changes.to.the.

transformer.AVR.and.reactive.power.output.

set.point.changes.to.the.generator.AVRs...Should.

insufficient.or.excessive.reactive.power.flow.

through.the.transformers,.the.PMS.will.make.

further.transformer.AVR.set.point.changes.to.

ensure.correct.flow.of.reactive.power.between.both.

process.units.

Load.shedding.calculations.are.made.for.each.

process.unit.individually.and.then.for.the.whole.

plant.in.both.the.connected.and.islanded.mode..

Load.shedding.tables.corresponding.to.every.

normative.incident.for.each. ..configuration ...

are

.

made.

.

Should

.

there.not.be.sufficient.spinning.

reserve.for.any.of.the.configurations,. the. PMS. will.

warn.the.operators. and.also.suggest.which.GTG.

should.be.started..

.If.there.is.too.much.spinning.reserve,.the.PMS.

will.also.warn.the.operator.and.will. .suggest .

which.GTG.should.be.stopped.

Seconds

130125120115110105100-4

0

4

8

12

16

MW step load

Figure 13 – Dynamic & static spinning reserve

The.fast.load.shedding.calculations.must.be.based.

on.the.dynamic.spinning.reserve.[2]..Thus.even.

though.there.is.sufficient.generation.capacity,.load.

shedding.may.be.required..Fig..13.shows.what.

often.happens.when.a.GTG.is.subjected.to.a.large.

load.step,.in.this.case.14.MW.for.a.90.MW.GTG..

Initially.the.governor.responds.by.opening..the.gas.

valve.which.results.in.a.very.quick.increase.in.

output

.

power..This.is.of.course.the.desired.result.

–.the.ideal.response.is.shown.in.Fig..13.with..

the.solid.line..The.increased.amount.of.gas,.

however,.increases.the.exhaust.gas.temperature .

until.such.time.as.sufficient.air. . .available..

The.opening.of.the.air.inlet.vanes.is,. however,.very.

slow.in.comparison.with.the.gas.valve...Thus.the.

exhaust.gas.temperature.will.initially.rise.and.the.

exhaust.gas.temperature.regulation.will.decrease.

the.flow.of.gas.until.the.air.inlet.vanes.have.

opened

.

sufficiently..This.explains.the.dip.in.

the.output.power.curve.shown.in.a.dotted.line...

The.dynamic.spinning.reserve.is.only.4.MW...

Full.power.is.available.only.after.20.seconds.

.

GTG dynamic response & fast load shedding


is

becomes

It.is.possible.however,.to.consider.2.types.of.

load.shedding..Some.process.loads.will.accept.

outages.of.several.tens.of.seconds.and.can.be.

restarted.afterwards.and.the.process.continues..

Other.process.loads.when.switched.off,.even.for.

a.very.short.period.of.time,.will.require.manual.

intervention.prior.to.restarting..Process.loads.

accepting.outages.could.be.shed.first,.and.then.

restarted.gradually,.at.about.the.same.rate.as..

the.opening.of.the.air.inlet.vanes..Loads.not.

accepting.outages.should.be.shed.only.if.

absolutely.necessary..In.this.manner.it.is.possible.


to.make.use.of.the.“static”.spinning.reserve.as.

well.at.the.dynamic.spinning.reserve..It.requires,.

however,.categorizing.loads.based.on.acceptable.

outage.times.and.not.just.on.importance.as.is.

normally.done.when.defining.load.shedding.tables.

Sudden.islanding.of.a.process.unit.will.of.course.

have.consequences.for.that.process.unit...

The.consequences.can.often.be.limited.by..

the.use.of.fast.load.shedding..There.are.however .

also.consequences.for.the.other.process.unit.

when.the.other.one.is.islanded..If.the.process.unit.

associated.with.the.40.MW.GTGs.is.islanded,..

the.other.GTGs.will.suddenly.lose.much.of..

the.load..Primary.regulation.will.ensure.that..

the.GTGs. .reduce.their.power.output,.but.

the.main.problem.will.be.that.they.can.no.longer.

operate.in.the.DLN.region..It.is.important.that..

the.consequences.of.normative.incidents.be.

reviewed.for.all.parts.of.the.plant,.and.not.just..

the.part.most.directly.affected.

In.this.plant,.much.of.the.process.steam.is.

produced.from.HRSGs.associated.with..

the.larger.turbines..The.power.output.of.these.

GTGs.is.however.determined.by.electrical.and.

DLN.considerations,.and.not.by.process.steam.

requirements..This.would.also.be.the.case.should.

STGs.use.the.steam..Since.it.is.not.possible.to.

regulate.the.GTG.output.based.on.process.steam.

requirements,.it.is.necessary.to.fit.the.boilers.with.

auxiliary.burners.[3]..Thus,.steam.can.be.produced.

independently.of.the.operation.of.the.turbines..

This.decouples.the.steam.system.from.the.power.

system..Thus.the.PMS.can.define.the.required.

power.output.of.the.turbines.based.on.electrical.

considerations.only,.and.does.not.need.to.take.

into.account.steam.system.constraints..

Process Steam


Case Study Remarks

There.are.advantages.to.using.distributed.

generation..A.single,.large.contingency.cannot.

cause.a.total.loss.of.generation.and.thus.

production..Also.having.loads.close.to.generation

.

reduces.transmission.losses..The.down.side.is..

the.difficulty.in.correctly.controlling.the.power.

system,.especially.the.voltage.

Operation.of.some.GTGs.in.DLN.mode.will.result.

in.very.unequal.load.sharing..The.DLN.turbines.

will.generally.be.more.heavily.loaded.since.they.

must.operate.well.above.the.DLN.limit.to.ensure.

stable.firing..The.dynamic.response.of.the.turbines.

will.not.be.the.same.since.they.are.operating.at.

different.percentage.loads..In.some.cases.this.can.

lead.to.nuisance.tripping.after.a.contingency.has.

occurred. The.very.limited.dynamic. response.

capability.of. large.turbines.is.often.a.surprise..

Fast.load. shedding.must.be.triggered.even.

though.there

.

is.enough.generation.capacity.

on.paper...From

.

a.dynamic. spinning. reserve.

viewpoint,..it. is. better .to. have. more. smaller.

turbines.than.fewer

.

larger.ones.

The.use.of.DLN.turbines.has,.however,. .positive

.

points..

The.

emissions.

are.

greatly.

reduced.

and.

this.is.very.important.today.since.more.and.more.

regulations.are.coming.into.effect.on.this.subject..

As.shown.in.the.case.study,.the.effective.loading..

of.the.DLN.turbines.is.higher.than.the.non-DLN.turbines.since.they.must.operate.well.above.

the.DLN.limit..The.DLN.turbines.are.modern.

machines.which.have.a.higher.efficiency.than.

older.machines..Thus.operating.the.more.efficient.

turbines.at.a.higher.output.reduces.energy.consumption.and.CO2.emissions...This,.however,.

can .be.achieved.only .using.secondary.regulation.



PMS implementation

A.Power.Management.System.is.an.integrated.set.

of.sensors,.actuators,.communication.devices.and.

networks,.control.logic.and.operator.interfaces.

.

The.purpose.of.the.PMS.is.to.provide.real-time.

control.of.the.industrial.power.system.by.means.of.

the.following.functions,.many.of.which.have.been.

discussed.in.the.sections.above:

monitor.the.power.system

remote.control.the. power.system.switching.

devices

control.power.system.frequency

control.utilization.voltages

ensure.good.power.quality

provide.immunity.to.power.system.disturbances

control.power.import.&.export.

••

•••••

Although.the.PMS.relies.heavily.on.Information.

Technology.to.perform.many.of.the.functions,.

the.conceptual.design.must.be.made.by.power.

system.engineers..Both.disciplines.must.work.

closely.together.to.design.a.PMS.that.meets

..the.requirements.

Fig..14.gives.a.very.general.overview.of.a.PMS...

It.shows.the.connections.to.the.electrical.

distribution.system.which.are.made.via.intelligent.

devices.and.communication.networks,.as.well.as.

hard-wired.digital.&.analog.signals..The.

operator

.

workstations . . . .The.

redundant.PLCs .required.for.power.

management. functions. . .including. their. interface.

to.the.governors. and. AVRs.. This. interface. shows.

the.use.of.incremental.set.point.changes,

.one.of.the.basic.principles.of.Robust.Control.

Description of a Power Management System (PMS)

provide€the interface.

are

G G

GGGG

Utility

PLC

I/O Rack

PLC

AVR

G

AVRGovernor

+- pow

er+- voltage

+- v

olta

ge

Figure 14 – PMS Overview

Data Integrity Validation

The.PMS.actuators.will.send.incremental.set-point.

changes.to.gas.turbine.governors.and.generator..

&.transformer.AVRs..These.set-point.changes.control.

the.system.frequency.and.voltage.and.thus.are.

very.important.for correct.power. system. operation..

All

.

decisions

.

made

.

by.the.PMS.are.based.on.the.

data

.

that

.

has

.

been

.

collected.from.the.various.

devices.connected.to..the. power. system.. It. is. thus.

very.important.that

.

only. valid. data. be. used.. Bad.

data.can.cause.the.PMS. to. send. incorrect. set-point.

changes.and.drive.the.power.system.to.an. abnormal

.operating.condition.

.


.

One.of.the.most.important.functions.of..

the.PMS.is.Data.Integrity.Validation..This.consists.

of.a.number.of.measures.to.check.that.the.data.

being.used.is.coherent..Should.discrepancies.

occur,.an.alarm.will.be.set.and.the.PMS.functions.

which.depend.on.this.data.will.be.suspended..

until.the.discrepancies.have.been.eliminated...

The.philosophy.of.robust.control.ensures.

satisfactory.operation.of.the.power.system.without.

the.PMS...Data.Integrity.Validation.ensures.that.PMS

.will.not.execute.functions.based.on.corrupt. data.

.


These.are.the.most.common.functions.of..the

..PMS.and.consist.in.showing.the.operator..

.status.of.the.power.system.and.allowing.him.to.

open.and.close.circuit.breakers.and.thus.change.

the.power.system.configuration..Remote.control.is.

usually.from.a.central.control.point..All.operators.

have.their.own.user.name.and.password.and.only.

operators.who.have.the.correct.authorization.are.

allowed. to. execute. orders. of. any. kind. Any. time.

remote.control.is.implemented,.whether.by.the

.operator.or.from.the.PMS.itself,.redundant.

communication.systems.are.normally.required.

This.prevents.the.failure.of.any.single.device.from.

propagating.through.the.system.

Monitoring & Remote Control

Power€Management€functions

voltage

control the€frequency

€and €and provide immunity€to€power€system

disturbances. Data Integrity Validation is of at€mostimportance€for€these functions. These€are

.

..

automatically.executed.and.the.control.logic.is.

performed.by.Programmable.Logic.Controllers.

(PLCs)...The.PLCs.are.normally.provided. in.a.

hot-standby .configuration.in.order.to.be.able.to.execute.their.

.

functions.even.should.a.fault.occur.

in

.

one

.

device.

.

The.PLCs.are.connected.to.the.PMS.

communication.system.in.such.a.manner.as.to.

have.direct.access.to.the.information.needed.to.

implement.the.functions..The.PLC.outputs.are.

connected.directly.to.the.primary.regulation.control.

devices.

the

Operator.work.stations.provide.the.information.

necessary.to.the.operator.for.correct.control.of..

the.power.system..The.information.must.be.

organized.in.a.very.clear.manner.in.order.that.the.

operator

.

immediately

.

sees. what.is.important.and.

what.needs.his.attention.. Generally. there. is. a.

main

.

PMS.operator.work.station.in.the

.

central

.control.room,.near..the.process.control

.work.stations. .The.main.work.station.normally.has

.dual.screens. allowing.permanent.visualization.

of

.

the

.

status

.

of

.

the.power.system.while.allowing.

the.operator.to.work.on.the.other.screen...Often.

there.are.local .PMS.work.stations.in.each.

substation,

.

these

.

work.stations. being. primarily.

used

.

when.

maintenance.work.is.being.carried.

out.



Testing a PMS

The.testing.of.a.PMS.is.always.a.challenge.since.only.

part.of.the.system.can.be.tested.prior.to.installation.

at.site..During.the.Factory.Acceptance.Tests.(FAT),.

the.complete.PMS.communication.system,.all.

operator.work.stations,.all.PLCs.and.other.PMS.

devices.are.interconnected.and.powered..Interfaces.

to.the.power.system.devices.such.as.intelligent.relays

and.meters.are.simulated.in.order.to.demonstrate.

correct.communication..All.PMS.functions.are.tested

during.the.FAT.but.it.is.not.possible.to.demonstrate

that.all.performance.criteria.are.met.since.only.few.

of.the.many.devices.are.connected.

It.is.only.at.site,.after.the.PMS.has.been.installed

&.connected.that.the.compliance.with.the.PMS.

performance.criteria.can.be.demonstrated...

There.are.many.control.functions.in.the.PMS.and

each.control.function.depends.on.many.variables

and

.

dead

.

bands.

.

During

.

the

.

commissioning

.

these

variables.are.optimized.in.order.to.provide.the.best

performance.of.the.system..This.“fine.tuning”.is..

a.very.important.step.and.requires.access.to..

the.GTGs.during.commissioning,.something.that

is.often.difficult.to.obtain.

The.PMS.should.be.designed.to.allow.testing.at.

site.without.risk.to.the.power.equipment...

One.means.of.achieving.this.is.to.provide..

a.“gain”.for.each. function... When. the. gain. is.

0.%,.the.PMS.will.not.send.any control.

signals.but.the.commissioning.engineer.can

.check.that.the.signal.that.the.PMS.would.have

.sent.is.correct.or.not..After.validation.of.the.

function.at.0.%.gain,.the.gain.is.slowly.increased.

and.the.functions.checked..When.all.checking.

has.been.done.and.the.variables.and.dead.bands.

optimized,.the.gain.will.be.at.100.%.and.the.PMS.

fully operational.

Training

It.is.quite.difficult.to.provide.adequate.training.for.

PMS.operators.for.several.reasons..One.reason.

is.simply.the.fact.that.any.orders.sent.by.PMS.

operators.will.have.important.consequences.on.

the.operating.point.of.the.power.system...

Trainees.cannot.be.allowed.to.actually.execute.

orders..The.other.problem.is.that.a.well.designed.

robust.power.system.normally.operates.correctly.

for.long.periods.of.time.without.any.operator.

intervention..Some.functions.will.be.used.only.very.

infrequently...

When.operators.don’t.use.the.system.for.long.

periods.of.time,.they.often.forget.what.they.have.

to.do.

Training.of.new.operators,.and.repeat.training.

of.experienced.operators.can.best.be.done.by.

means.of.a.training.simulator.[4]...

The.training.simulator.has.the.same.operator.

interface.as.the.PMS.and.allows.the.operator.to.

execute.orders.and.see.their.consequences...

The.orders.are.simulated.–.they.are.not.really.

executed.on.the.power.system.

COM-POWER-WP0�EN Rev1 | �0

Conclusion

The.use.of.DLN.combustion.is.becoming.more.

common.due.to.stricter.regulations..The.additional.

constraints.that.it.brings.regarding.the.operation.

of.the.power.system.should.however,.not.be.

underestimated..It.is.necessary.to.carry.out.in.

depth.studies.and.simulations.of.all.possible.

operating.modes.of.the.power.system.in.order.to.

be.sure.that.the.loads.can.be.supplied.at.the.right.

frequency.and.voltage..This.is.especially.the.case.

when.there.are.turbines.having.different.operation.

modes.(DLN.and.standard).at.the.same.facility.

The.dynamic.response.of.the.turbines.should.be.

carefully.reviewed.with.the.turbine.supplier...

This.should.be.taken.into.account.in.defining..

the.number.and.rated.power.of.the.turbines.to.be.

purchased.for.the.plant.power.supply..Although.

there.are.economical.reasons.for.having.fewer,.

larger.turbines,.these.should.be.weighed.against.

the.costs.of.poor.power.system.performance.

When.STGs.are.used,.or.when.the.GTGs.are.

associated.with.HRSGs,.close.collaboration.

between.the.electrical.and.steam.system.

engineers.is.required..This.will.help.ensure.

that.each.system.is.capable.of.satisfying.the.

requirements.independent.of.the.operation..

of.the.other.system.


COM-POWER-WP0�EN Rev1 | �1

References

[1]..Nick.Hiscock,.Terence.Hazel,.Jonathan.

Hiscock;."Voltage.Regulation.at.Sites.with.

Distributed.Generation,".IAS.Transactions.on.

Industry.Applications,.Vol.44,.No..2,..

March-April.2008,.pp.445-453.

[2]..Roy.Hamilton,.John.Undrill,.Paul.Hamer,.Scott.

Manson,.“Considerations.for.Generation.

in.an.Islanded.Operation,”.Causes.of.Long.

Interruptions,.IEEE.PCIC.Conference.Record,.

2009.

[3]..Graeme.Peck,."Challenges.in.Using.Waste.Heat.

Recovery.and.DLE.Combustion.to.Reduce.

CO2.and.NOx.Emissions".in.PCIC.Europe.

Conference.Record,.2009.

[4]..Terence.Hazel,.Isabelle.Condamin,.Fabrice.

Audemard,.“Facilitating.Plant.Operation.&.

Maintenance.Using.an.Electrical.Network.

Monitoring.&.Control.System.Simulation.Tool”,.

IEEE.PCIC.Conference.Record,.2004.

Vita

Terence. Hazel. graduated. from. the. University. of.

Manitoba.Canada.with.a.BScEE.in.1970...

He.worked.for.one.year.as.a.power.coordination.

engineer.in.Perth.Australia.and.for.several.years.

in.Frankfurt.Germany.as.a.consulting.engineer.for.

construction.and.renovation.of.industrial.power.

distribution.systems..Since.1980.he.has.worked.

for.Schneider.Electric.(formerly.Merlin.Gerin).in.

their.projects.group.where.he.has.provided.team.

leadership.for.several.major.international.projects.

involving.process.control.and.power.distribution..

His.main.interests.are.in.power.quality,.and..

the.reliability.of.electrical.distribution.systems...

Mr..Hazel.is.a.senior.member.of.IEEE.and.is.author.

of.several.IEEE.papers.and.tutorials..He.is.also.

Secretary.and.Technical.Chair.of.the.Petroleum.&.

Chemical.Industry.Committee.Europe.which.holds.

an.annual.technical.conference..

(www.pcic-europe.eu)


Schneider Electric Industries SAS35,.rue.Joseph.MonierCS.30323F-.92506.Rueil.Malmaison.Cedex

RCS.Nanterre.954.503.439Capital.social.896.313.776.€www.schneider-electric.com

12-2009COM-POWER-WP04EN

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.

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