A Modular Series Connected Converter for a 10 MW, … · A Modular Series Connected Converter for a 10 MW, 36 kV, Transformer-Less Offshore Wind Power Generator Drive Sverre Skalleberg
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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
www.ntnu.no Sverre Skalleberg Gjerde
3
Introduction-IElectric drive train for offshore wind turbine
Full converter
Direct Drive
Geared solutions
Geared solutionsAsynchronous
generator
PMSG
PMSG
DFIG
Variable speed
www.ntnu.no Sverre Skalleberg Gjerde
3
Introduction-I
Electric drive train for offshore wind turbine
0AC
DC///
AC
DCLV/MV
///
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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.
www.ntnu.no Sverre Skalleberg Gjerde
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.
www.ntnu.no Sverre Skalleberg Gjerde
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.
www.ntnu.no Sverre Skalleberg Gjerde
5
Introduction-III
Motivation for Transformer-Less Offshore Generator Drive:
— Reduce weight of nacelle— Opens possibilities for modularity— Facilitates operation and maintenance
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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 onDC-side
— Medium voltagestress on windings
www.ntnu.no Sverre Skalleberg Gjerde
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 voltagestress on windings
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
— Not standard system
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
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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
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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
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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
www.ntnu.no Sverre Skalleberg Gjerde
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]
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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]
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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
www.ntnu.no Sverre Skalleberg Gjerde
14
Conclusion
— Modular converter for high voltage transformer-less generatordrive
— Control system proposed
— Droop controller introduced for removal of steady statedeviation of DC-bus control
— Simulation results shows the effectiveness of the droop
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
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
www.ntnu.no Sverre Skalleberg Gjerde
15
10 MW reference turbine
Electrical definitions
— Wind farm grid structure— Generator design base— Converter design base— Transformer definitions
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16
Definition of collection grid-I
— 36 kV AC— 50 Hz— Grid code to apply:
UK
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17
Definition of collection grid-II
— 36 kV DC— Grid code to apply:
UK
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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.
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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
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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.
www.ntnu.no Sverre Skalleberg Gjerde
Thank you for your attention
www.ntnu.no Sverre Skalleberg Gjerde
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