grid connection of wind farms - mpoller

GTZ-TERNA Expert Workshop 2009: Grid and System Integration of Wind Energy, 10.11.2009-12.11.2009, Berlin/Germany

Grid Connection of Wind FarmsMarkus Pöller and Oscar Amaya/DIgSILENT GmbH


Grid Connection

• Impact on thermal loading of lines/transformers• Impact on voltage during normal operation• Short circuit currents• Power Quality Aspects

– Voltage dips because of WTG switching

– Voltage dips because of transformer inrush– Continuous Flicker

– Harmonics

• Behaviour during grid faults/Fault ride through requirements


Impact on Thermal Loading


Options for network connection

HV

MV

AB

C

A Distributed/Embedded

B MV Substation

C HV Substation


Impact on Thermal Loading of Lines/Transformers

• Additionally required transmission lines must be planned based on well defined scenarios, considering size and location of planned wind farms.

• Load flow studies required for combinations of:– Load level (High-/Low-load)

– Wind speed level (High-/Medium-/Low wind)

• For avoiding investments in new transmission lines which are only required for a few hours per year, probabilities should be assigned to the studied cases.

• Assessment of potential of dynamic line rating recommended because of the good correlation of transmission line capacity and wind speed.


Impact on Thermal Limits – Example

To DROERIVIER

Voltage Levels

400, kV220, kV132, kV66, kV33, kV

Wind Park

150 MW

Continued onBacchus 132kV diagram

BOSKLOOF 1

BOSKLOOF 2

LAINGSBURG

LAIN132 WF

LAIN0.69 WF

LAIN33 WF

LAIN132B1

LADIS13

WELTV1_2

SWART22

SWART1

BUFPT22

WELTV1_1

BUFPT1

QUARY1_2

QUARY1_1

PIETM1_2

BAVIANSK

BANTM2_2

BANTM1_1

WHITH1_2

WHITH1_1

BAVIN1_2

LEEUG22

BAVIN1_1

LEEUG1_2

LEEUG1_1

PIETM1_1

GEELBEK

GEELB1_2

GEELB1_1

RUITK1_2

RUITK1_1

ANTJIESKKOUP

LAIN132B2

KOUP1_2

KOUP1_1

GEMSB1_2

GEMSB1_1

BANTAM

ANTJK1_2

ANTJK1_1

BOTES_2

BOTES_1

Laingsburg WF

LAIN

WF

Tr

1LA

IN W

F T

r 2

2181

WO

LF12

,41

2181WOLF9,40

lod_

7089

2..

Ladismith

M1311CH28,11

2161WOLF6,12

WP51WOLF6,00

2161

WO

LF14

,45

lod_72962_1 lod_72932_1

trf_

7068

2..

lod_70686_1

lod_72852_1

lod_70732_1

lod_

7096

2_1

lod_70922_1

lod_

7089

2_1

lod_70842_1 lod_70762_1

2181WOLF12,93

2181WOLF12,93

2181WOLF11,05

lod_70682_1

2181WOLF11,05

2181WOLF13,16

2181WOLF13,16

2181WOLF9,64

2181

WO

LF9,

40

2181WOLF9,64

2181WOLF14,20

2181WOLF14,20

2181WOLF24,09

2181WOLF24,09

2181WOLF21,81

2181WOLF21,81

lod_

7280

2_1

2181

WO

LF12

,40

lod_

7288

2_1

2181WOLF9,96

2181

WO

LF11

,45

2181

WO

LF11

,45

2181

WO

LF2,

08

2181

WO

LF3,

87

2181

WO

LF0,

51

2181WOLF74,80

WP

51W

OLF

14,4

5

2181

WO

LF2,

63

2181

WO

LF2,

07

2181WOLF11,41

2181WOLF11,41

2181WOLF9,96

2181WOLF22,49

2181WOLF22,49

2181WOLF11,17

2181WOLF11,17

lod_74006_1lod_73006_1

trf_

7400

2..

trf_

7300

2..

2181WOLF74,80


Impact on Thermal Limits – Example

Wind Park

150 MW

To DROERIVIER


BOSKLOOF 1

BOSKLOOF 2

LAINGSBURG

LAIN132 WF

LAIN0.69 WF

LAIN33 WF

LAIN132B1

LADIS13

SWART22

SWART1

BUFPT22

BUFPT1

QUARY1_2

QUARY1_1

PIETM1_2 BANTM2_2

BANTM1_1

WHITH1_2

WHITH1_1

BAVIN1_2

BAVIN1_1PIETM1_1

GEELB1_2

GEELB1_1

RUITK1_2

RUITK1_1

LAIN132B2

KOUP1_2

KOUP1_1

GEMSB1_2

GEMSB1_1

ANTJK1_2

ANTJK1_1

BOTES_2

BOTES_1

180,

00 M

VA

86,0

1 %

2,50

MV

A81

,36

%

2181

WO

LF

12,4

1 km

0,00

%

2181WOLF 9,40 km31,04 %

Ladismith

M1311CH 28,11 km7,06 %

2161WOLF 6,12 km14,59 %

WP51WOLF 6,00 km14,48 %

2161

WO

LF

14,4

5 km

15,9

7 %

lod_72962_1 lod_72932_1

lod_72852_1

lod_70922_1 lod_70842_1

2181WOLF 12,93 km20,17 %

2181WOLF 12,93 km22,62 %

2181WOLF 11,05 km19,67 %

2181WOLF 11,05 km20,68 %

2181WOLF 13,16 km18,27 %

2181WOLF 13,16 km20,32 %

2181WOLF 9,64 km17,76 %

2181

WO

LF

9,40

km

31,0

4 %

2181WOLF 9,64 km17,88 %

2181WOLF 14,20 km17,12 %

2181WOLF 14,20 km17,53 %

2181WOLF 24,09 km16,59 %

2181WOLF 24,09 km16,33 %

2181

WO

LF

12,4

0 km

120,

99 %

2181WOLF 9,96 km115,69 %

2181

WO

LF

11,4

5 km

20,8

3 %

2181

WO

LF

11,4

5 km

22,9

7 %

2181

WO

LF

2,08

km

0,95

%

2181

WO

LF

3,87

km

1,37

%

2181

WO

LF

0,51

km

0,39

%

2181WOLF 74,80 km0,00 %

WP

51W

OLF

14

,45

km15

,72

%

2181

WO

LF

2,63

km

6,05

%

2181

WO

LF

2,07

km

3,36

%

2181WOLF 11,41 km0,00 %

2181WOLF 11,41 km112,44 %

2181WOLF 9,96 km0,00 %

2181WOLF 22,49 km111,37 %

2181WOLF 22,49 km0,00 %

2181WOLF 11,17 km115,48 %

2181WOLF 11,17 km0,00 %

10,0

0 M

VA

60,6

8 %

10,0

0 M

VA

14,0

0 %

2181WOLF 74,80 km110,86 %

• 120% overload off


Impact on Thermal Limits - Example

General mitigation options if thermal limits are ex ceeded:

• Build a new line

• Limit wind farm output to 80% during all times (80% of rated output)

• Limit wind farm output in case of actual line failure (manual or automatic inter-trip).

• Consider dynamic line rating systems.


Violation of Thermal Limits – Cap Wind Farm Output

0,500 1,500 2,500 3,500 4,500 5,500 6,500 7,500 8,500 9,500 10,50 11,50 12,50 13,50 14,50 15,50 16,50 17,50 18,50 19,50 20,50

12,50

10,00

7,50

5,00

2,50

-2,50

0,000

x-Axis: Windpark Analysis: Wind Speed in m/sWindpark Analysis: Probability in %

100,0380,0360,0340,0320,030,03

160,00

120,00

80,00

40,00

0,00

-40,00

x-Axis: Windpark Analysis: Cummulative Probability in %Windpark Analysis: Generated Power in MW

Y =120,000 MW16.624 %

DIGSILENT High Load Plots

Voltage at Laingsburg Wind Farm Connection Point PV-Curve

Date: 7/23/2009

Annex: 1 /3

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1454 h/year


Violation of Thermal Limits – Cap Wind Farm Output

Not Delivered Energy depends on:• Wind conditions (average wind speed)• Site-specific aspects• Power curve of turbines

Rough cost estimates (example):• vw=7m/s:

– Energy not delivered around 5% of potential energy– 150 MW wind-farm: 19 000MWh not delivered -> 23 750 000 R/year

• vw=8m/s: – Energy not delivered around 7,5% of potential energy– 150 MW wind-farm: 37 000 MWh not delivered -> 46 250 000 R/year

• Must be compared to annualized costs of required line upgrade


Violation of Thermal Limits – Cap Wind Farm Output u nder Contingency Situations

More cost effective solution:

• Limitation of wind farm output only in situations in which one circuit is available (planned outage, unplanned outage)

• In case of minor overloads (below emergency rating): – Manual action of system operator

• In case of major overloads (above emergency rating):– Automatic inter-trip scheme


Dynamic Line Rating - Potential

• Thermal loading of overhead lines depends on:– Ambient temperature

– Wind speed -> correlation with wind generation

• Wind-generators:– cut-in wind-speed: 2.5...4m/s, rated: 12...16m/s– But: height, environment etc. must be considered too!

Ambient Temperature

Line Rating expressed in MVA at 66 kV*

Wind Speed = 0.5 m/s Wind Speed = 3.0 m/s Wind Speed = 5.0 m/s

30 ºC 22.6 39.9 49.5

35 ºC 16.5 32.9 41.5


Impact on Voltage Variations


Voltage Variations

• Distribution Grids: Considerable voltage variations for varying MW because of low X/R ratios (large R)

• Transmission Grids: Substantially less voltage variations for varying MW becaus of high X/R ratios (low R). Contingency cases are more relevant.

• Mitigation Options: – Q(P)-Characteristic (open-loop voltage compensation)

– Voltage control (voltage feed-back)


Voltage Variations - Procedure

• Step 1 - System Operator: Identify required reactive power range at connection point

• Step 2 – Wind farm planner: Design the reactive power capability for complying with reactive capability requirements.

- Step 1 might be defined by a general Grid Code requirement -


Example 1: Connection to Distribution/Subtransmission Gri d

To DROERIVIER

Voltage Levels

400, kV220, kV132, kV66, kV33, kV

Wind Park

150 MW


BOSKLOOF 1

BOSKLOOF 2

LAINGSBURG

LAIN132 WF

LAIN0.69 WF

LAIN33 WF

LAIN132B1

LADIS13

WELTV1_2

SWART22

SWART1

BUFPT22

WELTV1_1

BUFPT1

QUARY1_2

QUARY1_1

PIETM1_2

BAVIANSK

BANTM2_2

BANTM1_1

WHITH1_2

WHITH1_1

BAVIN1_2

LEEUG22

BAVIN1_1

LEEUG1_2

LEEUG1_1

PIETM1_1

GEELBEK

GEELB1_2

GEELB1_1

RUITK1_2

RUITK1_1

ANTJIESKKOUP

LAIN132B2

KOUP1_2

KOUP1_1

GEMSB1_2

GEMSB1_1

BANTAM

ANTJK1_2

ANTJK1_1

BOTES_2

BOTES_1

Laingsburg WF

LAIN

WF

Tr

1LA

IN W

F T

r 2

2181

WO

LF12

,41

2181WOLF9,40

lod_

7089

2..

Ladismith

M1311CH28,11

2161WOLF6,12

WP51WOLF6,00

2161

WO

LF14

,45

lod_72962_1 lod_72932_1

trf_

7068

2..

lod_70686_1

lod_72852_1

lod_70732_1

lod_

7096

2_1

lod_70922_1

lod_

7089

2_1

lod_70842_1 lod_70762_1

2181WOLF12,93

2181WOLF12,93

2181WOLF11,05

lod_70682_1

2181WOLF11,05

2181WOLF13,16

2181WOLF13,16

2181WOLF9,64

2181

WO

LF9,

40

2181WOLF9,64

2181WOLF14,20

2181WOLF14,20

2181WOLF24,09

2181WOLF24,09

2181WOLF21,81

2181WOLF21,81

lod_

7280

2_1

2181

WO

LF12

,40

lod_

7288

2_1

2181WOLF9,96

2181

WO

LF11

,45

2181

WO

LF11

,45

2181

WO

LF2,

08

2181

WO

LF3,

87

2181

WO

LF0,

51

2181WOLF74,80

WP

51W

OLF

14,4

5

2181

WO

LF2,

63

2181

WO

LF2,

07

2181WOLF11,41

2181WOLF11,41

2181WOLF9,96

2181WOLF22,49

2181WOLF22,49

2181WOLF11,17

2181WOLF11,17

lod_74006_1lod_73006_1

trf_

7400

2..

trf_

7300

2..

2181WOLF74,80


Voltage Variations/Step 1 – Example 1: cosphi constant (= 1)

207,50167,50127,5087,5047,507,50

1,08

1,05

1,02

0,99

0,96

0,93

x-Axis: Laingsburg WF: Active Power in MWLAIN132 WF: Voltage in p.u. - Base CaseLAIN132 WF: Voltage in p.u. - Lain132kV_Laingsburg_OffLAIN132 WF: Voltage in p.u. - Laingsburg_Boskloof_OffLAIN132 WF: Voltage in p.u. - Laingsburg_Droerivier_Off

Y = 1,050 p.u.47.697 MW58.375 MW

66.198 MW

X =150,000 MW

1.065 p.u.

1.070 p.u. 1.074 p.u.

1.050 p.u.

136.500 MW 1.074 p.u.

DIGSILENT High Load Voltage

Voltage at Laingsburg Wind Farm Connection Point PV-Curve

Date: 7/24/2009

Annex: 1 /2

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Voltage Variations/Step 1 – Example 1:cosphi(P)-characteristic

200,00160,00120,0080,0040,000,00

1,075

1,050

1,025

1,000

0,975

0,950

x-Axis: Laingsburg WF: Active Power in MWLAIN132 WF: Voltage in p.u. - Base CaseLAIN132 WF: Voltage in p.u. - Lain132kV_Laingsburg_OffLAIN132 WF: Voltage in p.u. - Laingsburg_Boskloof_OffLAIN132 WF: Voltage in p.u. - Laingsburg_Droerivier_Off

Y = 1,050 p.u.

X =150,000 MW

1.037 p.u. 1.038 p.u. 1.044 p.u.

1.050 p.u.

DIGSILENT High Load Voltage

Voltage at Laingsburg Wind Farm Connection Point PV-Curve - cosphi(P)-characteristic

Date: 7/24/2009

Annex: 1 /2

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Voltage Variations- Example 1: Summary

• High voltages in case of cos(phi)=1

• Small voltage variations if cos(phi) adjusted to actually generated power (absorbing vars for compensating increasing voltage)

• Voltage control (with voltage feed-back) at wind farm connection point is possible but not required in this particular case because:– Only small voltage steps in case of contingencies– Only small voltage variations in case of different operational scenarios

(high/low load)– No voltage stability issue


Example 2: Large Wind Farms at Transmission Level


Voltage vs. Reactive Power – Voltage Stability


Voltage Variations- Example 2: Summary

• Small Voltage Variations in function of active power variations (large X/R ratios)

• High Voltage Variations in case of critical contingencies

• Voltage control (with voltage feed-back) at wind farm connection point is required for maintaining voltage stability

• Required reactive power range can be determined by analyzing QV-curves


Voltage Variations/Step 2 – Wind Farm Design

• Wind farm design must consider reactive power requirements.

• Reactive power capability at grid connection point is limited by:– Reactive power capability of wind turbine generators (WTGs)– Thermal ratings of cables in the wind farm collector system.

– Voltage variations at the LV-nodes (voltage range of operation of WTGs)

• Requirement for additional reactive power compensation devices (STATCOM, switched shunts) must be taken based on:– Required reactive power capability– Required dynamic performance of voltage/reactive power control.


Reactive Power – Voltage Control

p

q

power factor limit

q

const var limit (recommended)

cos(phi)=0,95cos(phi)=0,95


Voltage Variations - Wind Farm Design

• Wind farm design must consider grid requirements

• Reactive power capability at grid connection point is limited by:– Reactive power capability of wind turbine generators (WTGs)

– Thermal ratings of cables in the wind farm collector system.– Voltage variations at the LV-nodes (voltage range of operation of WTGs)

• Requirement for additional reactive power compensation devices (STATCOM, switched shunts) must be taken based on:– Required reactive power capability

– Required dynamic performance of voltage/reactive power control.


Voltage Variations – Wind Farm Planning Studies

DIGSILENT

PowerFactory 14.0.513

Windfarm Red Sunset

CFE/GTZ/DIgSILENT

Project: Example

Graphic: Red Sunset

Date: 7/27/2009

Annex:

/115 kV

/20 kV

Windpark Analysis

PVUset=1,12

Tr

Tra

fo-T

yp

0

Shunt/Filter max. no.: 3 act. no.: 3 7,00 Mvar

3

Line

(4)

N

2XS

2Y 1

x..

0,80

km

Line

(3)

N

2XS

2Y 1

x..

1,00

km

WTG 13

WTG 5

WTG 12

WTG 11

WTG 10

WTG 4

WTG 3

WTG 2

WTG 1

WTG 14

WTG 15

WTG 16

WTG 6

WTG 7

WTG 8

S4 NA2XS(F)2Y 1x185RM 12/20kV ir

1,00 km


1,00 km


1,00 km

S1

N

A2X

S(F

)2Y

1x1

85R

M 1

2/20

kV ir

2,

00 k

m

S13

N

A2X

S(F

)2Y

1x1

85R

M 1

2/20

kV ir

0,

80 k

m


0,80 km


0,80 km


0,80 km

S5

N

A2X

S(F

)2Y

1x1

85R

M 1

2/20

kV ir

2,

50 k

m


0,80 km


0,80 km


0,80 km


0,80 km


0,80 km


1,60 km

Tr1

0LV

-Trf

0

Tr5

LV-T

rf0

Tr6

LV-T

rf

0

Tr7

LV-T

rf

0

Tr8

LV-T

rf

0

Tr4

LV-T

rf

0

Tr3

LV-T

rf

0T

r13

LV-T

rf

0

Tr1

4LV

-Trf

0

Tr1

5LV

-Trf

0

Tr1

6LV

-Trf

0

Tr1

2LV

-Trf

0

Tr1

1LV

-Trf

0

Tr2

LV-T

rf

0

Tr1

LV-T

rf

0

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Voltage Variations – Wind Farm Design Studies

-5

0

5

10

15

20

25

30

35

40

45

50

-30 -20 -10 0 10 20 30

P [MW]

Q [Mvar]


Short Circuit Contribution


Short Circuit Contribution of Wind Farms

• Calculation of max. short circuit currents:

– Impact on short circuit ratings of existing components (substations, CB-ratings, cable-/line ratings, transformers etc.)

– Impact on new components, inside the wind farm

• Calculation of min. short circuit currents:

– Verification of protection settings


Short Circuit Contribution of Wind Farms

0,300,200,100,00-0,10 [s]

0,30

0,20

0,10

0,00

-0,10

-0,20

-0,30

Tr2: Phase Current A/HV-Side in p.u.Tr2: Phase Current B/HV-Side in p.u.Tr2: Phase Current C/HV-Side in p.u.

Fault Clearedip

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Fault Current of DFIG with Crow-bar protection


Short Circuit Contribution - Modelling

• DFIG and WTGs with fully rated converter are devices with controlled currents.

• DFIG is usually equipped with protection mechanisms (Crow-bar, Chopper), which make short circuit behavior highly non-linear.

• Difficult to model for steady state short circuit analysis, which is typically based on Thevenin-equivalents.

• No special consideration of WTGs given in IEC 60909.

• Proposed approach:– „Equivalent Synchronous generator“ approach: Characterizing WTG short

circuit currents by subtransient and transient parameters.– Approach suitable for planning studies but not for highly accurate studies.


Power Quality


Power Quality

• Impact on Flicker– Continuous flicker– Flicker following switching actions (WTGs, Inrush)

• Impact on Harmonics– Harmonic injections

– Impact on harmonic impedance


Continuous Flicker

Caused by

• Turbulences

• “Rotational sampling”:turbulence variation across the rotor

• Tower Shadow

• Torsional oscillation

Applicable Standards:

• IEC 61000-3-6, IEC 61400-21

Mexican Grid Code: Pst<0,35Plt<0,25


Continuous Flicker - Example

70.60656.46542.32328.18214.041-0.1000 [s]

4.00

2.00

-0.00

-2.00

-4.00

-6.00

Rotor-Turbulence: vt0

70.60656.46542.32328.18214.041-0.1000 [s]

-3.20

-3.60

-4.00

-4.40

-4.80

-5.20

T3WT1: Total Active Power/HV-Side in MW

6.255.003.752.501.250.00 [Hz]0.001

0.01

0.1

1

10

100

T3WT1: Total Active Power/HV-Side, Magnitude in MW

DIgSILENT Turbulence, Electrical Power and Spectrum of a 5MW Variable Speed Turbine Plots(3)

V0=13.6 m/s

Date: 10/5/2003

Annex: /1

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Flicker due to Switching Actions

• WTGs automatically synchronize to the grid if vw>vcutin

• Variable speed WTGs: Very smooth synchronisation• Fixed speed WTGs: Considerable voltage dip. Mitigation: soft cut-in

• Wind farm energization causes more considerable voltage dips:– Switching of WTG step-up transformers

– Switching of main transformer– > only during wind farm energization, not repeting events.


Switching of Fixed Speed Induction Generator

ASM

Typical start-up procedure

• Turbine pulls up the rotor to0.9 ..1.1 nnominal

• Breaker is closed

V0

Z’’n

X’’

IG’’ • Approximate Formula

''''''

n

rGiGn

S

SkIZu ==∆


Switching

0.200.150.100.05-0.00-0.05 [s]

1.025

1.000

0.975

0.950

0.925

0.900

0.875

415V Machines: Voltage Phasor, Magnitude in p.u.

0.200.150.100.05-0.00-0.05 [s]

6000.00

4000.00

2000.00

0.00

-2000.00

-4000.00

-6000.00

WG 315kW: Phase Current A in AWG 315kW: Phase Current B in AWG 315kW: Phase Current C in A

0.200.150.100.05-0.00-0.05 [s]

1.003

1.000

0.997

0.994

0.991

0.988

WG 315kW: Speed

0.200.150.100.05-0.00-0.05 [s]

0.90

0.60

0.30

-0.00

-0.30

-0.60

-0.90

415V Machines: Line-Line Phase Voltage A in kV415V Machines: Line-Line Phase Voltage B in kV415V Machines: Line-Line Phase Voltage C in kV

DIgSILENT Wind Power Integration Training WG

Asm cut-In, directly on line EMT-simulation

Date: 10/5/2003

Annex: 1 /1

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With Soft Cut-In

0.210.170.130.090.05 [s]

0.90

0.60

0.30

-0.00

-0.30

-0.60

-0.90

415V Machines: Line-Line Phase Voltage A in kV415V Machines: Line-Line Phase Voltage B in kV415V Machines: Line-Line Phase Voltage C in kV

0.210.170.130.090.05 [s]

1.0250

0.995

0.965

0.935

0.905

0.875

415V Machines: Voltage Phasor, Magnitude in p.u.

0.210.170.130.090.05 [s]

1.0100

1.0040

0.998

0.992

0.986

0.980

Motor 315kW: Speed

0.210.170.130.090.05 [s]

200.00

100.00

0.00

-100.00

-200.00

Motor 315kW: Phase Current A in AMotor 315kW: Phase Current B in AMotor 315kW: Phase Current C in A

DIgSILENT Wind Power Integration Seminar Motor

Soft cut-in

Date: 10/5/2003

Annex: 1 /1

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Switching of Variable Wind Generators

DASM

1

2

SynM


Connection of Variable Speed WTG

0.060.040.02-0.00 [s]

0.20

0.10

0.00

-0.10

-0.20

PWM Grid Side: Current, d-Axis in p.u.PWM Grid Side: d-Axis Current Reference in p.u.

0.060.040.02-0.00 [s]

1.025

1.000

0.975

0.950

0.925

0.900

Point of Interconnection: Voltage Phasor, Magnitude in p.u.

0.060.040.02-0.00 [s]

0.04

0.02

0.00

-0.02

-0.04

2-Winding Transformer: Phase Current A/HV-Side in kA2-Winding Transformer: Phase Current B/HV-Side in kA2-Winding Transformer: Phase Current C/HV-Side in kA

DIgSILENT Wind Power Training Plots

Cut-In of Variable Converter driven synchronous machine

Date: 10/5/2003

Annex: 1 /3

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Harmonic injections caused by:

• Power electronics converters

• modern PWM converters produce high order harmonics

• Saturation effects (Generator, Transformer)

Inter-Harmonic injections caused by:

• PWM with switching frequency different from multiples of networkfrequency

Effect

• Voltage distortion depending on network impedance

• Resonance problems

Standards: IEC 61000-3-7, IEC 61400-21

Harmonic and Inter-Harmonic Injections


Self Commutated Converter

Udc Uac

ACUDCU


Self Commutated PWM Converter

0.080.060.040.02-0.00 [s]

0.20

0.10

0.00

-0.10

-0.20

2-Winding Transformer: Phase Current A/HV-Side in kA

6400.5120.3840.2560.1280.0.00 [Hz]

0.15

0.12

0.09

0.06

0.03

0.00

2-Winding Transformer: Phase Current A/HV-Side, Magnitude in kA

900.000 Hz 0.004 kA

1100.000 Hz 0.003 kA

1950.000 Hz 0.004 kA

2050.000 Hz 0.004 kA

DIgSILENT Wind Power Training Currents

PWM-converter

Date: 10/5/2003

Annex: 1 /4

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Impact on Harmonic Impedance

• Voltage source converters:– define a path via the coupling reactance to earth for high frequency

harmonics.

– At low frequency harmonics: Controller transfer function needs to be considered too.

• Effect: – Shift of resonance frequencies (towards higher order).

– Increased harmonic damping

• Cable capacitance of wind farm-internal cables.• Effect:

– Shift of resonance frequencies (towards lower order)

– Amplification of harmonic background distortion.


Impact on Flicker and Harmonics - Summary

• Analysis of Flicker and Harmonics using IEC 61400-21 data sheet of a typical variable-speed wind generator.

• Flicker generally low in case of large wind farms because Flicker-relevant turbulences within a wind farm are only weekly correlated

• Harmonics of modern wind turbines (with IGBT-converters) very low. Almost no harmonic current injections.

• WTGs can have a positive influence on harmonic impedance characteristics (improved damping, increased resonance frequencies)


Behaviour During Grid Faults – FRT Requirements


Example: Converter Driven Synchronous Genenrator

0.600.400.200.00 [s]

60.00

40.00

20.00

0.00

-20.00

Cub_1\PCC PQ: Active Power in p.u.Cub_1\PCC PQ: Reactive Power in p.u.

0.600.400.200.00 [s]

1.20

1.00

0.80

0.60

0.40

0.20

HV: Voltage, Magnitude in p.u.MV BusBar: Voltage, Magnitude in p.u.

0.600.400.200.00 [s]

1.15

1.10

1.05

1.00

0.95

0.90

Generator: speed

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FRT Requirements: Summary

Basic FRT-Requirements:

• WTGs must not disconnect in case of voltage dips

• WTGs must deliver active power shortly after a voltage dip

Advanced FRT-Requirements:

• WTGs must inject reactive current during a fault (voltage support, protection excitation)

• WTGs must not absorb reactive power during voltage recovery


Thank You

Markus Pöller Oscar Amay

[email protected] [email protected]

DIgSILENT GmbH

Heinrich-Hertz-Str. 9

72810 Gomaringen

www.digsilent.de

grid connection of wind farms - mpoller

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