A Modular Series Connected Converter for a 10 MW, … · A Modular Series Connected Converter for a...

A Modular Series Connected Converter for a 10MW, 36 kV, Transformer-Less Offshore WindPower Generator Drive

Sverre Skalleberg Gjerde

Supervisor: Tore M. Undeland

Jan. 19, 2012

www.ntnu.no Sverre Skalleberg Gjerde

2

OutlineIntroduction

Proposed converter solution

Control systemStructure

Simulation resultsThe simulation modelCase I: no DC-droopCase II: DC-Droop

Conclusion

10 MW Reference turbine


3

Introduction-IElectric drive train for offshore wind turbine

Full converter

Direct Drive

Geared solutions

Geared solutionsAsynchronous

generator

PMSG

PMSG

DFIG

Variable speed


3

Introduction-I

Electric drive train for offshore wind turbine

0AC

DC///

AC

DCLV/MV

///


4

Introduction-II - 10 MW turbine

Challenges when going for a 10 MW wind turbine— Weight of generator— Low voltage => high currents— Location of transformer vs. cabling size.

Why 10 MW turbine?

The cost of energy from offshore wind power is high.Cost is driven both by total rating and number of turbines.


5

Introduction-III

Motivation for Transformer-Less Offshore Generator Drive:

— Reduce weight of nacelle— Opens possibilities for modularity— Facilitates operation and maintenance


6

Converter topology

0

AC

DC

AC

DC

DC grid

AC

DC

AC

DC

///

///

///

///

Module 1

Module 9

+

4 kV

-

+

4 kV

-

+

4 kV

-

+

36 kV

-

+

4 kV

-

Module 8

Module 2

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV

— Modular construction— Standard three phase

VSC modules— Series connected on

DC-side— Medium voltage

stress on windings


6

Converter topology

0

AC

DC

AC

DC

DC grid

AC

DC

AC

DC

///

///

///

///

Module 1

Module 9

+

4 kV

-

+

4 kV

-

+

4 kV

-

+

36 kV

-

+

4 kV

-

Module 8

Module 2

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV

— Modular construction

— Standard three phaseVSC modules

— Series connected onDC-side

— Medium voltagestress on windings


6

Converter topology

0

AC

DC

AC

DC

DC grid

AC

DC

AC

DC

///

///

///

///

Module 1

Module 9

+

4 kV

-

+

4 kV

-

+

4 kV

-

+

36 kV

-

+

4 kV

-

Module 8

Module 2

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV


VSC modules

— Series connected onDC-side



6

Converter topology

0

AC

DC

AC

DC

DC grid

AC

DC

AC

DC

///

///

///

///

Module 1

Module 9

+

4 kV

-

+

4 kV

-

+

4 kV

-

+

36 kV

-

+

4 kV

-

Module 8

Module 2

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV



DC-side



6

Converter topology

0

AC

DC

AC

DC

DC grid

AC

DC

AC

DC

///

///

///

///

Module 1

Module 9

+

4 kV

-

+

4 kV

-

+

4 kV

-

+

36 kV

-

+

4 kV

-

Module 8

Module 2

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV

Vll 2.45 kV



DC-side— Medium voltage

stress on windings


7

FeaturesAdvantages:

— No distribution transformer— Modularity— Redundancy— Behaviour as 9 standard 3-phase drives

Disadvantages:— DC short circuit protection not inherent from topology— Communication between modules— DC-bus balance needed

— Not standard system


7

FeaturesAdvantages:— No distribution transformer

— Modularity— Redundancy— Behaviour as 9 standard 3-phase drives




7

FeaturesAdvantages:— No distribution transformer— Modularity

— Redundancy— Behaviour as 9 standard 3-phase drives




7

FeaturesAdvantages:— No distribution transformer— Modularity— Redundancy

— Behaviour as 9 standard 3-phase drivesDisadvantages:— DC short circuit protection not inherent from topology— Communication between modules— DC-bus balance needed



7

FeaturesAdvantages:— No distribution transformer— Modularity— Redundancy— Behaviour as 9 standard 3-phase drives




7


Disadvantages:

— DC short circuit protection not inherent from topology— Communication between modules— DC-bus balance needed— Not standard system


7


Disadvantages:— DC short circuit protection not inherent from topology

— Communication between modules— DC-bus balance needed— Not standard system


7


Disadvantages:— DC short circuit protection not inherent from topology— Communication between modules

— DC-bus balance needed— Not standard system


7





7


Disadvantages:— DC short circuit protection not inherent from topology— Communication between modules— DC-bus balance needed— Not standard system


8

Main control systemStructure

Slave: DQ-controller+DC-bus balancingMaster controller

Wind turbine

controlω_ref Cω Iqref

ΣVdc,i

9Vdc,ref

Module

controllers

Slave

control 1

Slave

control 2

Slave

control 9

Sysmon signal[1:9]

C(Id)

C(Iq)

ωLd

ωLq

Iq*_i

Id_ref

Id

Iq

ψ

PWM

Iq_ref

ω_est

Vdc_ref

Vdc,mes,i

Cdroop

Iq.bal,i

Iq,droop,i

δ(droop)

— Main control strategy - Controlling power (speed)— Additional objective: Maintain all 9 DC-bus voltages equal


9

DC-bus balancing controlC(droop) – module # i

Vdc,msr

PI(dc,i)Vdc_ref,i Iq.bal,iVdc,ref

δdroop I(droop,i)

— PI-regulator for dynamic control— Droop control for static regulation— Controller output: Addition to torque reference


10

Droop characteristics

−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 10

0.5

1

1.5

Converter current [pu]

DC

−bu

s vo

ltage

[pu]

1,1

0,9

— Limit: 1 ± 0.1 pu (DC-voltage)— Idroop = 0→ Vdc,re f ,i = Vdc,nom


11

Simulations in PSCAD

The following assumptions were made for the simulation model— Each module of the generator can be represented by a

3-phase generator model— Stiff mechanical shaft— Semiconductors→ ideal switches— The DC-grid can be represented by a DC-voltage source

(infinite bus) and a resistor for the losses


12

Case I: PI-controller

0 10 20 30 400

1

2

3

4

DC

bu

s vo

ltag

e [k

V]

Time [s]

a

0 10 20 30 40−0.5

0

0.5

1

b

Bal

anci

ng

ref

. [p

u]

Time [s]

0 10 20 30 400

0.2

0.4

0.6

0.8

1c

Gen

. sp

eed

an

d r

ef. [

pu

]

Time [s]0 10 20 30 40

0

0.2

0.4

0.6

0.8

1

d

Cu

rren

t re

f. [

pu

]

Time [s]


13

Case II: Droop

0 10 20 30 400

1

2

3

4

DC

bu

s vo

ltag

e [k

V]

Time [s]

a

0 10 20 30 40−0.5

0

0.5

1

b

Bal

anci

ng

ref

. [p

u]

Time [s]

0 10 20 30 400

0.2

0.4

0.6

0.8

1c

Gen

. sp

eed

an

d r

ef. [

pu

]

Time [s]0 10 20 30 40

0

0.2

0.4

0.6

0.8

1

d

Cu

rren

t re

f. [

pu

]

Time [s]


14

Conclusion

— Modular converter for high voltage transformer-less generatordrive

— Control system proposed— Droop controller introduced for removal of steady state

deviation of DC-bus control— Simulation results shows the effectiveness of the droop


14

Conclusion


— Control system proposed

— Droop controller introduced for removal of steady statedeviation of DC-bus control

— Simulation results shows the effectiveness of the droop


14

Conclusion



deviation of DC-bus control

— Simulation results shows the effectiveness of the droop


14

Conclusion



deviation of DC-bus control— Simulation results shows the effectiveness of the droop


15

10 MW reference turbine

Electrical definitions

— Wind farm grid structure— Generator design base— Converter design base— Transformer definitions


16

Definition of collection grid-I

— 36 kV AC— 50 Hz— Grid code to apply:

UK


17

Definition of collection grid-II

— 36 kV DC— Grid code to apply:

UK


18

Generator design base

— Permanent magnet— Stator nominal

voltage: 4 kV— Frequency range: 15 -

30 Hz— Estimated weight:

310 tons

Additionally: Defined efficiency curve for both generator and drive,as function of loading.


19

Converter design base

— Back-to-backconfiguration

— 3-level NPC topology— DC-choppers for

protection— Switch technology:

IGBTsFor a DC grid: The inverter and transformer is replaced with aDC/DC converter


20

Transformer definitions

The transformer definitions are only valid for the AC-grid option

— Transformation ratio of 1:9— Estimated weight: 30 tons— Liquid/Oil filled

Located in the nacelle, due to cabling issues and maintenance.


Thank you for your attention


A Modular Series Connected Converter for a 10 MW, … · A Modular Series Connected Converter for a...

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