Importance of advanced simulations of electrical system in wind turbines April 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

1. Importance electrical transients 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. Grid Code requirements 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. LVRT requirement Voltage/PF requirement When designing a wind turbine all four systems have to be considered together

2. DFIG wind turbine Torque speed curve of a variable speed wind turbine 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

2.a. Mechanical operating regions Operating regions of a variable speed wind turbine 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. 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 Generator stator power factor Rotor voltage curves of a DFIG Reactive power Grid code requirement for reactive power is not satisfied ! Grid code requirement 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 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. 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.

2.c. Rotor current variation 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 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.

Thermal stress of IGBT at low frequency 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 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

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 My Loads that are related to thrust force suffers the most by tightening the turbine controller Tightened pitch control Relaxed pitch control

3. Transient modeling of a fully rated converter wind turbine 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 Torque control Chopper 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 Torque control Chopper

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. Three successive grid faults are applied within 40s 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

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. Generator speed is kept under tripping limit Torque ramp off instead of torque step down 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.