Importance of advanced simulations of electrical system in wind turbinesApril 2010

Structure of the presentation

1. Introduction: Importance electrical transients

2. DFIG wind turbine• Mechanical operating regions• Rotor voltage limit and power factor• Rotor current variation• Effect on mechanical loading

3. Transient modeling of a fully rated converter wind turbine• LVRT• Torque control• Braking resistor• Combination of torque control and braking resistor

4. Summary

Until recently, electrical dynamics has not always been fully considered during the design of wind turbines.In order to account for the full impact of electrical dynamics, advanced computer models are being developed.

1. Importance electrical transients

Time [s]

Voltage [%]

85

90

95

100

105

110

115

Power factor [-]

Vol

tage

[%]

UnityInductive Capacitive

Grid Code requirements

LVRT requirement

Voltage/PF requirement

When designing a wind turbine all four systems have to be considered together

Aero dynamics

Structural

Controller

Electrical

B2

B1

B3

Electrical dynamics is part of a wind turbine

Grid code requirements are increasingly important and wind turbine needs to comply with these requirements.

Electrical dynamics is part of a wind turbine and there are interactions between the electrical and mechanical dynamics.

2. DFIG wind turbine

DFIG

Torque control

Voltage or PF control

Pitch controller

Wound rotor induction generator

IGBT PWM converters

• Rotor side converter controls the generator torque and power factor of the generator

• Converter has limitations such as voltage limit and current limit at low frequency

• The wind turbine controller tries to keep the wind turbine operating point along the maximum power curve• However during turbulent wind conditions, the operating point of the turbine is shifted from the desired power curve

Generator speed

Ge

nera

tor

torq

ue

Rated speed and torque

Torque speed curve of a variable speed wind turbine

2.a. Mechanical operating regions

Operating points of a DFIG variable speed wind turbine operating at maximum power during three different turbulent wind conditions. Generator needs to operate far away from the nominal operating point.

Operating regions of a variable speed wind turbine

1800

10610

20001600Generator speed [rpm]

Ge

ne

rato

r to

rqu

e [

Nm

] Rated speed and torque

TS

haft

[Nm

]

Speed [rpm ]

0

2000

4000

6000

8000

10000

12000

1000 1200 1400 1600 1800 2000 2200

Main powercurve

Operatingenvelop

Torque speed envelop is defined for the electrical system to operate. Main power curve ensures that the turbine operates at its maximum aerodynamic efficiency. Operating envelop is defined by generator speed tolerances, maximum generator torque limit and maximum generator power limit. Speed tolerance depends on the turbine control performance.

2.b. Rotor voltage limit and power factor

The rotor voltage changes with grid frequency, network voltage and power factor requirement. The worse case condition occurs at

Minimum grid frequency Highest network voltage Capacitive power factor.

At the top of the operating speed range, the converter voltage limit will force the generator to draw reactive power from the grid during normal operation, and significant VArs during gust transients. Grid code requirement for power factor above the rated speed is not satisfied due to converter voltage limit.

Rotor voltage curves of a DFIG

Generator stator power factor

Capacitive PF Inductive PF

PF

[-]

Speed [rpm ]

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1000 1200 1400 1600 1800 2000 2200

Rotor voltage increases with the slip speed until it reaches converter maximum voltage of 759 V.Limit on maximum speed occurs at minimum grid frequency and operating with a capacitive power factor.Since the rotor voltage is clamped at high rotor speed, the power factor has to be changed from capacitive to inductive

DFIG

Reactive power

85

90

95

100

105

110

115

Power factor [-]

Vol

tag

e [%

]

UnityInductive Capacitive

Grid code requirement

DFIG

Grid code requirement for reactive power is not satisfied !

Rated speed of the generator should be reduced to avoid reaching the converter limit. This means the mechanical design of the gearbox (gearbox ratio) has to be changed

TS

haft

[Nm

]

Speed [rpm ]

0

2000

4000

6000

8000

10000

12000

1000 1200 1400 1600 1800 2000 2200

Main powercurve

Operatingenvelop

In order to avoid absorbing reactive power from grid above 1800 rpm the rated generator speed is reduced to 1600 rpm by changing the gearbox ration by a factor of 8/9.

2.c. Rotor current variation

Thermal stress of IGBT at low frequency

aITa1

Ta1

Due to thermal high thermal stress IGBT converters have current limitations at low fundamental frequency. This results in rotor current variation entering high IGBT thermal stress.

2.c. Effect on mechanical loading

TS

haft

[Nm

]

Speed [rpm ]

0

2000

4000

6000

8000

10000

12000

14000

800 1000 1200 1400 1600 1800 2000

Pre

ven

ted

are

a d

ue

toIG

BT

th

erm

al s

tres

s

TS

haft

[Nm

]

Speed [rpm ]

0

2000

4000

6000

8000

10000

12000

1000 1200 1400 1600 1800 2000 2200

Pre

ven

ted

are

a d

ue

to v

olt

age

limit

Generator rated speed is 1800 rpm and it absorbs large amount of reactive power at high speed With rated speed of 1600 rpm, the converter needs to operate within its high thermal stress region. Both these cases are not acceptable for a wind turbine design

-2.00E+07

0.00E+00

2.00E+07

4.00E+07

6.00E+07

50 60 70 80 90 100

Time [s]

Tow

er b

otto

m M

z [N

m]

-2.00E+07

0.00E+00

2.00E+07

4.00E+07

6.00E+07

50 60 70 80 90 100

Time [s]

Tow

er b

otto

m M

z [N

m]

Loads that are related to thrust force suffers the most by tightening the turbine controller

TS

haft

[Nm

]

Speed [rpm ]

0

2000

4000

6000

8000

10000

12000

14000

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

The rotor speed now is set to 1700 rpm and then the pitch controller is tightened to keep the operating points close to the rated speed. Tightening the pitch controller has consequences on the mechanical loading

Relaxed pitch control Tightened pitch control

MzMy

3. Transient modeling of a fully rated converter wind turbine

Phase-to-phase generator fault

Grid fault

Grid code requirements

DC link voltage

Torque

Chopper heat loss

B

0H

Critical knee

Increasing temperature

Recoil

Irreversible demagnetization

Electrical faults such as grid faults and generator short circuits produce high amplitude, rapid electrical transients and wind turbine designers increasingly need to take them into account.

3.c. LVRT

Powerto grid

JAerodynamic

power

Kineticenergy Power to

DC-link

dcv

Torque control

Powerto grid

JAerodynamic

power

Power toDC-link

Heatloss

Chopper

30

100

15

00 0.15 1500

Time [s]

Voltage [%]

385 ms

• Response of a wind turbine to a grid fault is of increasing concern for turbine designers and network operators• During a grid fault, a wind turbine goes through heavy transients and the turbine could reach any of its design limits• In order to investigate the response of a wind turbine to a grid fault, appropriate electrical models have to be used

3.d. LVRT with torque control

Maintaining the DC link voltage below the upper limit during a fault is to reduce the generator power. The generator power can be rapidly reduced by means of reducing the generator torque.

3.d. LVRT with torque control

Dropping the generator torque from rated to zero has two effects Increases the rotor speed Excites drive train oscillation

The rotor accelerate because of the generator provides zero reaction torque while the turbine rotor generate aerodynamic torque. Wind turbine rotor is made up of flexible blades and flexible shafts and it is subjected to structural oscillation. Therefore step change in t he generator torque induces generator shaft oscillation As the speed tripping limit is reached the turbine controller shut down the turbine Not satisfying the grid code requirement.

3.e. LVRT Braking resistor

IEC 61400 Ed-3, Section 6.5 Electrical power network conditions “Auto-reclosing cycles – auto-reclosing cycle periods of 0,1 to 5 s for the first reclosure and 10 s to 90 s for a second reclosure shall be considered.

By using a braking resistor, the generator reaction torque can be maintained. Thus avoid any speed increases nor oscillation during grid fault. However in this successive three fault case, the chopper needs to be operating for about 7.5s

Three successive grid faults are applied within 40s

3.f. LVRT Combination of torque control and braking resistor

Instead of using braking chopper alone or torque control alone, a combination of both methods can be used.

Torque ramp off instead of torque step down

Generator speed is kept under tripping limit

Braking chopper is only used for about 2s in total for three successive grid faults

4. Summary

Wind turbine dynamics consists of aerodynamic, structural dynamics, controller dynamics and electrical dynamics. When designing a wind turbine all four system has to be considered together

Grid code requirements are becoming increasingly demanding ad they have direct influence on the design process of a wind turbine.

Electrical limitations determines the range of operation of a wind turbine, therefore electrical dynamics should be taken into account when designing a wind turbine

Converter voltage limitation could forces the generator to draw reactive VArs from the grid and not to satisfy the grid code requirements

IGBT converters have limitations at low frequency, this important characteristics has to be taken into account

During a grid fault, a wind turbine goes through heavy transients and the turbine could reach any of its design limits

Combination of torque control and braking chopper can be used to ride through three successive faults efficiently.