IFAC – Cigré Symposium onPower plants & power systems control

Brussels, April 26-29, 2000

Session onControl concepts for conventional and non-conventional power plants

Black-startup simulation of a repowered thermoelectric unit

A. Borghetti * G. Migliavacca ^ C.A. Nucci * S. Spelta ^

* Department of Electrical Engineering, University of Bologna, Italy

^ CESI S.p.A. (ENEL Group) Milan, Italy

A typical ENEL repowering projectIntroduction

to atmosphereby-pass

Gas turbine

Steam power plant

compressor

GT

CC

synchronousgenerator

HP

MP

CondenserUP boiler

Rh

Sh1

Eco

regenerativeheat exch.

Sh2synchronousgenerator

120 MW gas turbine

320 MWonce through UP boiler

The problem to be studied

• Possibility to use a local repowering TG to perform a black start operation

on a UP-steam unit without feeding auxiliaries from the network. This could:

1. Spare precious time if load rejection on SPP fails

2. Avoid SPP auxiliaries feeding from network, complex operation involving

personnel on large geographical areas.

3. Allows to the group TG/SPP to feed at last other power stations, helping

to restore the network as fast as possible

The simulatorThe simulation environment

CESI has developed a real time simulation environment, named ALTERLEGO.

Its core consists of:• an efficient and reliable implicit solver of large sets of algebraic-differential

equations, • CAD-like user-friendly tools for building and managing models • a large library of mathematical models in the field of energy production processes.

A run-time executive enables to run together the various parts and the MMI.

ALTERLEGO man machine interface

Simulator structureThe simulator

Steam turbine

UP boiler with start-up circuit

Steam section control system

GTsynchronous

generator

Gasturbine

SPP load demandGT load demand

SPPsynchronous

generator

Electrical auxiliaries system and

external network

SPP

Load scheduler

• The temperature control is not represented and it is approximated by reducing the maximum fuel limit.

• The fuel valve positioner has two different actuation speed.

• Valve position 22% balance between gas turbine and compressor power

GT load

programmer

1

2

11+sT

sT1+sT

KsT1

1/droop

1+sT1

1+sT1

1+sT1

1sT

1

+

+

+

+

+

-

-

-

Frequency local integrator

Speedgovernor

GT loaddemand

Max acc.

P

cdfG

l l1

d1

d

d

f1

mech

Acceleration sensitive governor

ff n

0.1 0.3

1

d

Fuel valvepositioner

Fuel system Turbine

Compressordischarge

0,25 0,22 1

1

Gas turbine and its control

GT

Speed governor

FLI

The simulator

TG = 0.05 s speed governor time constantTVAL = 15 s valve time constant (opening)Tf = 0.1 s fuel system time constantTcd = 0.2 s compressor time constantKd = 10Td = 0.1 s accel. control transfer function ceff.Td1 = 1 sTl = 10 s FLI transfer function coeff.Tl1 = 4 s

GTmodelvalidation

The simulator

GT model validationThe simulator

• Thermal balance equations are not represented: loadconnections generate ∆Tmax=5-10 C

• Feedwater flow rate proportional to fuel flow rate at each load

• Steam production proportional to feedwater flow rate; time constant greater at diminishing loads

• Neglected F.T. drainings (but oil was increased byprogram at low load)

• Mass accumulation has been considered in: SH1, FT, SH2

UP Boiler: main assumptionsThe simulator

UP BoilerThe simulator

HP steam valve

From feedwater system

FLASH TANK

To deaerator

Boiler

V207

SH1

V205

V200 SH2

V240

Condenser

V270

HPRH

MP/BP

UP Boiler

1+sT1

sT

1sT

1sT

1+s1

1+0.5s1

k

k

k

k

205 noreturnvalve

k

boiler superheater 1

superheater 2

270 valve position

240 valve position

1

207 valveposition

Main steampressure

Fuel(steam

equivalent) b sh1

flash -tankpressure

200 valve steamflow rate

HP steam flowrate

SPP loaddemand

+ +

- -

-

+

+

++

-

+

- -

+

+

+

-

-+

ft

sh2

270

240

207

1+sTs

flash-tank

Relief valve

Relief valve

Steam flow rate toflash tank

1

195.5 204

1

182 190.4

Qft

Qsh1

x

sh2 presssurelosses

dd

sh1 outputpressure

flash into theboiler

Qsh2

Tb boiler time constantTsh1 = 7,5 s superheater 1 time constantTsh2 = 2,8 s superheater 2 time constantTft = 3 s flash tank time constant

Start-up circuit

SH2The simulator

SH1

boiler

Steam turbine and electrohydraulic controlThe simulator

ff n

1

21-y

11+sT

11-ysT

1sTΠ

+ +

+

+ +

+-Π+

--

-CH

HP steamchest

270 valve flowflow

RH

reheater

COFlp

HP control valvepositioner

intercept valvepositioner

y

crossover

Fhp

Impulse stagepressure regulator

Main steampressure

mechanicallosses function

Pmech

EHC loadreference

-

Tpr = 0,1 s pressure reg. time const.Tcv = 0,8 s control valve time const.TCH = 0,15 s HP chest time const.TRH = 11 s RH time const.TCO = 0,4 s crossover time const.y constant (prh,nom./pch,nom)Fhp = 0,4 fraction of total power generatedFlp = 0,6 by HP and LP sections

Qlp

Qhp

pCH

pRH

40%

60%

RH

Intercept

Admission

React. stages

Steam chest

Start-up control mode

fuel adjustment

SPP loaddemand

fuel programme

Fuel(steam equivalent)

+

-

+

+

+ + +

+-

1/droop

synchronisationadjustment

active power output

EHC loadreference

bias

0.1(1+1/10s)

80 MW

2.36

kg/s

kg/s

2.36

8.06

110 MW

-∆f

Control system

SPP model validation

The simulator

SPP smaller time contantbetween θamm and power

Great “inertia” betweenfuel and steam production

TG: greater time constantbetween fuel and power

The power is immediatelyavailable

Additional control system during

restoration Using TG to take load rapidly and bring to zero the frequencyerror (FLI)

When frequency and boiler pressure are correct, unload TG infavor of SPP so that TG preserves enough margin for subsequentload connections

Control system

Additional control system during

restoration

GTSPP

Control system

Auxiliaries’ electrical systemThe simulator

Models of the auxiliaries

4

4.5

5

5.5

6

6.5

0 10 20 30 40 50 60 70 80 90s

Bus

vol

tage

in

kV

4

4.5

5

5.5

6

6.5

0 10 20 30 40 50 60 70 80 90s

Bus

vol

tage

in

kV

The simulator

a)

b)

a) Start-up of 2 FW pumps

b) Residual voltage bus transfer

Motors: 3 order model

LV auxiliaries: static model

Time in s

0

1

2

3

4

5

6

7

2 4 6 8 10 12 14s]

Bus

vol

tage

kV

0

1

2

3

4

5

6

7

2 4 6 8 10 12 14s]

Bus

vol

tage

kV

Time in s

380/220 kV network near the power plant

Rossano Calabropower station

Laino

MontecorvinoTusciano

Rotonda

380 kV

380 kV

250 MVA 400 MVA

380 kV

220 kV

220 kV

100 MVA100 MVA

400 MVA

SPPGT

150 kVnetwork

121.2 km

126.5 km6.7 km

73.6 km 73.6 km

3 km

30MW

30MW

30MW

30MW

20MW

30MW

30MW

30MW

30MW

250 MVA

The simulator

150 kV network near the power plantThe simulator

RossanoCalabro

power station

5 MW

15 MW

15 MW

Scanzano

Nova Siri

Cirò

Crotone

15 MW

Acri

10 MW 30 MW 30 MW

Cosenza Cosenza Feroleto

5 MW

30 km 45 km 10 km 50 km

40 km

50 km

60 km

60 km

Ballast loadThe simulator

48 MW

13 MW

5 s

300 s

1,7 s

5.4

5.5

5.6

5.7

5.8

5.9

6

6.1

6.2

0 20 40 60 80 100 120 140 160 180 200 220Time s

Aux

iliar

ies’

bus

volta

ge k

V

with OLTCwithout OLTC

5.4

5.5

5.6

5.7

5.8

5.9

6

6.1

6.2

0 20 40 60 80 100 120 140 160 180 200 220Time s

Aux

iliar

ies’

bus

volta

ge k

V

with OLTCwithout OLTC

-45

-35

-25

-15

-5

5

15

25

35

45

55

1 21 41 61 81 101 121s

Mva

rTG reactive output with OLTC

SPP reactive output without OLTC

SPP reactive output with OLTC

TG reactive output without OLTC

-45

-35

-25

-15

-5

5

15

25

35

45

55

1 21 41 61 81 101 121s

Mva

rTG reactive output with OLTC

SPP reactive output without OLTC

SPP reactive output with OLTC

TG reactive output without OLTC

The simulator

OLTC model

Discrete

Inverse-time delay

a) OLTC of the start-up transformer(±16; 0.625%)

b) OLTC of Rotondatransformer(±8; 1.9%)

a)

b)

Model of the excitation limitersThe simulator

VP

0rifV Voltageregulator

oexV

+

-

oTs1+

-

uTs1+

-

limfI -

uexV

0

0

rifVfV

+

+

rif0Q

rifQ

β

VQ

fI

V

0vrifQ

P

α

UEL

β = tan α

Q

Under excitation limiter (linear characteristic)

- Generators and power plant auxiliary system protectionsabnormal voltages and frequenciesloss of excitationinadvertent energisation

Pick-up of a 30 MW loadsimulations

SPP alone

Pick-up of a 18 MW loadsimulations

SPP alone

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

time in s

0

0.2

0.4

0.6

0.8

1

1.2

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700 800 900 1000 1100 120046

47

48

49

50

51

FrequencyandSPP outputs

Pick-up of a 30 MW load(after other four loads) simulations

SPP alone

10 5

15

15

30

loads in MW

Control valve positionandFT pressure

simulations

Pick-up of a 30 MW load(after other four loads)

SPP alone

FrequencyandSPP outputs

Control valve positionandFT pressure

Pick-up of a 30 MW loadsimulations

SPP and GT synchronized

Power plant rampingsimulations

SPP and GT synchronized

Frequencyand

SPP outputs

GT outputs

Control valve position

andFT pressure

Conclusions

The study carried out has shown that

• The GT section can effectively help the SPP section

under the start-up and ramping phases

• A load scheduler control system is crucial to coordinate

the load requests to the GT and the SPP generators

during the manoeuvre

• The repowered thermoelectric power unit can therefore

assume the role of "early-restoration plant“.