island operation igsgemfinalreport.fi/files/sgem samuelsson island ig.pdf– new small-scale plants...

Island operation withinduction generators

Olof Samuelsson

Industrial Electrical Engineering and Automation

Lund University/ LTH/ Measurement Technology and Industrial Electrical Engineering/ IEA 2

Why induction generators?

• Hydropower– New small-scale plants often use IG– At renovation IG often replaces synchronous generator

• Windpower– Many Fixed-Speed Induction Generators installed

• Original question: Can IG contribute to island operation?• Modified question: Can island operation be based on IG alone?


Forsa power station

• Semi-Kaplan turbine 275 kW• Induction generator 315 kW, 400 V• Capacitors 3x50 kvar• Transformer 0.4/10 kV, 500 kVA• Customers connected to the transformer low voltage side


Grid connected operation

• Starting sequence– Gate starts to open– During opening

• Generator accelerates• Close breaker near synchronous speed• Generator energized

– Gate stops at fully open

• Power control by runner blade angle• Minimum output power is 80 kW

Gate


Reducing gate opening for low-load operation

0 200 400 600 80060

80

100

120

Time [s]

Act

ive

pow

er [k

W]

0 50 100 150Power [kW]

Runn

er p

ress

ure

H/H

max

Gate control not possible use dump load for low load operation

Angle increases


Black start of self-excited IG

• increases– generator is magnetized– voltage rises

• New method gives U and I without fixing

0

0

20

40

60

Spee

d [5

0(n/

n s)]

0 5 10 15 20-500

-250

0

250

500

Vol

tage

[V]

Time [s]

IG With loadC


Mechanical analogy

xkFL

i 1

dvFR

ui 1

Stator resistance neglected

capacitor - mass

load - damper

resistance – damper

voltage - speed

current - force

Rotor with inertia

dtdvmF

dtduCi

inductance - spring

flux - distance

power - power

load

capacitor


Black start• Remanence

– Offset Rr and m• increases

– Im(is) increases• 2x > kx

– mass moves outwards– Rr moves outwards– us increases

• k increases with x– Stable operating point


Loading and voltage control• No-load

– Re(is) = 0• Connecting load with constant

– slip increases– Re(is) increases– Im(is) decreases– mass moves inwards– us decreases

• m increases (STATCOM)– us increases


IG voltage and frequency coupled

• Slow voltage controlbetter than fast

• STATCOM affects bothspeed and voltage

• Voltage and frequencycontrol need to becoordinated

44

46

48

50

Spee

d [5

0(n/

n s)]

200

220

240

Vol

tage

[V]

900

1000

1100

Gate

[W]

0 20 40 60 80 100400

600

8001000

QS [v

ar]

Time [s]


Earthing of MV network

• Petersen coil earthing of MV busbar• Earthing at MV/LV transformer lost at islanding unearthed

CB

IGIf

IPetersen ICap

CB

RN+jXPet


Self-excited IG behavior at earth faults

• IG does not demagnetize

• Selective protection possible– Normal settings

• Unselective protection– More cable reduces V0– Upper limit to cable length– New method gives V0 in network at IG with D/Y transformer

0 2 4 6 8

x 10-6

0

0.5

V0 [p

u]

0 2 4 6 8

x 10-6

0

1

2

Ctot [F]I n

(%)

Rf = 5 k


Self-excited IG behavior at short-circuit• IG demagnetizes at any short-circuit• Fault current decays quickly – use undervoltage protection• STATCOM does not affect short-circuit current much

0.96 0.98 1 1.02 1.04 1.060

2

4

6

I a (p.u

.)

Three-phase short-circuit

0.96 0.98 1 1.02 1.04 1.060

0.5

1

time (s)STA

TCO

M C

urre

nts

(p.u

.)

ISTATCOM

IqId

without STATCOMwith STATCOM


Earth fault in LV network – normal situation

10kV

400V

400V10

kV


Earth fault in LV network – island operation

Reduced fault currentGrid connected - SEIG in island

Longer melting time

Undervoltage relay mayhave time to trip

Unselective disconnectionof SEIG for LV fault

0 50

0.5

1Voltages

V A [p

.u.]

0 2 4 60

0.5

1

V A [p

.u.]

time [s]

0 50

10

20

I f [p.u

.]

Currents

0 50

0.5

1

1.5

I f [p.u

.]

time [s]

ZLPh

+ZLPE

=0

ZLPh+ZLPE=0.8


References

J. Björnstedt, O. Samuelsson, ”Voltage and frequency control for island operated inductiongenerators,” CIRED seminar 2008: SmartGrids for Distribution, Frankfurt, 23-24 June 2008.

J. Björnstedt, O. Samuelsson, “Semi-Kaplan Turbine Controlling an Island Operated InductionGenerator,” NORDAC 2008 8th Nordic Distribution and Asset Management Conference,Bergen, Norway, 8-9 September 2008.

J. Björnstedt, “Island Operation with Induction Generators - Frequency and Voltage Control,”Licentiate thesis TEIE-1058, 2009, www.iea.lth.se/publications/publice.html

F. Sulla, O. Samuelsson, “Estimation of the zero sequence voltage on the D-side of a Dytransformer using one voltage transformer on the D-side,” Development in Power SystemProtection 2008, Glasgow, United Kingdom, 17-20 March, 2008.

F. Sulla, O. Samuelsson, “Analysis of Island-Operated Distribution Networks with DistributedInduction Generation under Fault Conditions,” UPEC 2008, 43rd International UniversitiesPower Engineering Conference, Padova, Italy, 1-4 September 2008.

F. Sulla, O. Samuelsson, “Short-Circuit Analysis and Protection of a MV Network in IslandOperation with Induction Generators,” NORDAC 2008 8th Nordic Distribution and AssetManagement Conference, Bergen, Norway, 8-9 September 2008.

F. Sulla, “Island Operation with Induction Generators - Fault Analysis and Protection,” Licentiatethesis TEIE-1059, 2009, www.iea.lth.se/publications/publice.html

island operation igsgemfinalreport.fi/files/sgem samuelsson island ig.pdf– new small-scale plants...

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