1. Introduction of Types Of WTGs With Respect to Grid Interconnection in Pakistan Hassan Jafar Zaidi CEO, Power Planners International Member IEEEPak Seminar at WAPDA House July 18, 2012

2. Power Generation by WTG Power coefficient Tip-speed ratio slower rotational speed Bigger turbine Wind speed changes from 7 to 13 m/s Generator speed has to be changed from 1000 to 2000 rpm Available power

3. Types of Wind Turbine Generators (WTGs) Type-1:Directly connected conventional squirrel cage induction generator Type-2: Directly connected wound rotor induction generator with variable rotor resistance Type-3: Directly connected doubly-fed induction generator Type-4: A generator connected to the grid via full size converter Type5:Synchronous machine with variable‐speed mechani cal coupling / gearbox

4. Wind Turbine Technologies: Type-1 FSIG wind turbine (Single Fed) Small change in speed causes large torque change Fixed speed operation, therefore maximum power production is impossible Aerodynamic fluctuation is transferred to the grid High mechanical stress Absorbs reactive power 600kW, 690V No method to control the generator torque

5. Wind Turbine Technologies: Type-2 SFIG with OptiSlip: A generator system that allows both the rotor and the generator to vary their speed by up to 10% during wind gusts, maximising power quality and reducing the strain on turbine components. Optispeed: A further development of the OptiSlip® system. OptiSpeed® allows the speed of revolution of both the rotor and the generator to vary by up to approximately 60 per cent and optimises energy production, especially in modest winds. OptiTip: A microprocessor- controlled pith regulation system which ensures continuous and optimal adjustment of the angles of the blades in relation to the prevailing wind. Vary Slip by varying rotor resistance External resistors to rotor External electronic control system operates the resistors Resistors are connected through brushes and slip rings

6. Type – 3 ( Doubly Fed) DFIG wind turbine Move the curve by altering rotor voltage Speed can be changed by 40 %, therefore maximum power production is achievable Generator torque is fully under control of the electronic converter Very fast torque control, torque response time is 5 to 50 ms depending on the type of control system Gearbox loading can be minimized Magnetizing current is supplied through the rotor terminal Aerodynamic fluctuation can be filtered before entering the generator Maintenance requirements of slip rings Wind Turbine Technologies: Type-3

7. Type – 4 ( Full Converter AC/DC/AC) Full converter wind turbine DC-link totally decouple the generator from the grid Grid frequency is decoupled, wind turbine can operate at any rotor speed Grid voltage is decoupled, change in grid voltage does not affect the generator dynamics Converter rating should be the rating of the turbine while the rating of DIFG converter is around 25 to 30 % of the turbine rating Converter has only a partial control over the DIFG, this cause difficulties during system disturbance DFIG stator is directly connected to the grid, therefore the grid determines the synchronous speed and the flux linkage DFIG: Voltage control / reactive power support has to deal with the generator dynamics. Full converter: Voltage control / reactive power support can be offered to the grid without disturbing the generator dynamics. Full converter: Converter has total control over the fully converter turbine, better controllability than DFIG Gearbox can be avoided if a multi-pole synchronous generator is used, Ex: Enercon turbines with 64 poles Wind Turbine Technologies: Type-4 Machine can be of any type –Induction –Synchronous –Permanent magnetic –Exotic –high ‐ pole count PM machine for direct ‐ drive

8. Type – 4 ( Full Converter AC/DC/AC) Wind Turbine Technologies: Type-5 Type 5 turbines consist of a typical WTG variable-speed drive train connected to a torque/speed converter coupled with a synchronous generator. The torque/speed converter changes the variable speed of the rotor shaft to a constant output shaft speed. The closely coupled synchronous generator, operating at a fixed speed (corresponding to grid frequency), can then be directly connected to the grid through a synchronizing circuit breaker. The synchronous generator can be designed appropriately for any desired speed (typically 6 pole or 4 pole) and voltage (typically medium voltage for higher capacities). This approach requires speed and torque control of the torque/speed converter along with the typical voltage regulator (AVR), synchronizing system, and generator protection system inherent with a grid-connected synchronous generator.

9. Comparison of WTG Technologies WTG TypeSteady StateFault/Voltage-DipRecovery after Fault Clearance Type-1 SFIG Fixed Speed Absorbs VARs (from locally connected Caps) Reduction of electromagnetic torque Rotor accelerates Absorbs VARs(transient) Rotor continues to accelerate till the over-speed protection trips the WTG Absorbs VARs (transient) Type-2 SFIG with Controls Limited Variable Speed due to Optislip, OptiSpeed, OptiTip and Pitch Control Absorbs VARs( local caps ) Reduction of electromagnetic torque Rotor accelerates Absorbs VARs (transient) Rotor deccelrates and comes back to original slip due to OptiSlip and other controls if supported by LVRT Absorbs VARs (transient) Type-3 DFIG Variable speed VAR control between 1.0 and ±0.95 PF Reduction of electromagnetic torque Rotor accelerates Absorbs VARs (transient) DC link overvoltages and rotor converter overcurrents can be reduced by crowbars Rotor deccelrates and comes back to original slip due to AC/DC/AC converter and some strength in converter to support LVRT Does not Absorb VARs Type-4 Full Converter (SG) Variable speed(can operate at any speed) VAR control between 1.0 and ±0.95 PF Reduction of electromagnetic torque Rotor accelerates Does not Absorb VARs DC link overvoltages and rotor converter overcurrents can be reduced by crowbars Rotor deccelrates and comes back to original slip due to AC/DC/AC converter and some strength in converter to support LVRT Does not Absorb VARs

10. A Typical Wind Farm Layout

11. Grid Interconnection Issues Frequency Control & MW Reserve Margins WTG Type Capability Type-1, Type-2 For the squirrel cage induction generator, follows the grid frequency with a difference corresponding to the slip. Optispeed WTGs may help to maintain Freq. Sensitive Mode 49.8-50.2 HZ Type-3 Type-4 Type-3 and Tyupe-4 WTGs (variable speed machines) with power electronics converters control the rotor speed and hence the frequency. In Type-4, the rotor speed and grid frequency are totally independent. In case of Type-3, the rotor speed and grid frequency are not independent but the rotor speed can be controlled by the converter connected to the rotor: in case of a loss of generation on the network and hence decrease in the grid frequency the slip of the wind generator is increased to maintain the rotor speed. Type-5Like a conventional synchronous generator, governor action will control the speed and hence the frequency

12. Grid Interconnection Issues Reactive Power & Voltage Support WTG Type Capability Type-1, Type-2 WTGs can typically not control voltage. Instead, these WTGs typically use power factor correction capacitors (PFCCs) to maintain the power factor or reactive power output on the low-voltage terminals of the machine to a setpoint. Our Grid Code Addendum criteria for PF= ±0.95 at interconnection point Type-3 Voltage is controlled by changing the direct component of the rotor current (this is the component of the current that is in-line with the stator flux. WTGs typically have a reactive power capability corresponding to a power factor of 0.95 lagging (capacitive) to 0.90 leading (inductive) at the terminals of the machines. Some Type 3 WTGs can deliver reactive power even when the turbine is not operating mechanically, while no real power is generated. Type-4 WTG voltage control is achieved by varying the quadrature (reactive) component of current at the grid-side converter. To allow voltage control capability, the grid-side converter must be rated above the rated MW of the machine. Some Type 4 WTGs can deliver reactive power even when the turbine is not operating mechanically, while no real power is generated. Type-5Since a synchronous generator is used in a Type 5 WTG, an automatic voltage regulator (AVR) is typically needed. Depending on the design of the generator, operating power factor ranges at rated output can vary from 0.8 leading to 0.8 lagging. A range of 0.9 leading and lagging is more typical.

13. Grid Interconnection Issues Fault Ride Through or Low Voltage Ride Through It is capability/duration a machine should stay connected under fault conditions. (Not mentioned in the present Grid Code But Grid Code mentions about Fault Clearance Criteria; – Three phase fault, normal clearing in 5 cycles (100 ms) – Single Phase fault; normal clearance in 5 cycles (100 ms) – 3-ph or 1-ph faults; breaker failure (stuck breaker); clearing time is 9 cycles (180 ms) This provides us basis for fixing duration for fault ride through or low voltage ride through (LVRT). Three Phase Fault Single Phase Fault Voltage Time 5 Cycles/ 9 cycles 0.3 PU 5 Cycles/ 9 cycles

14. LVRT Support WTG TypeCapability Type-1, Have limited VRT capability and may require a central reactive power compensation system to meet wind power plant VRT capability. Type-2 Type-3 Type-4 Many of the Types 2, 3, and 4 WTGs have VRT capabilities that may meet the requirements of International Grid Codes, and NTDC Grid Code Addendum No.1 (i.e., withstand a three-phase fault for 9 cycles at a voltage as low as 0.30 p.u measured on the high side of the substation transformer). Most WTGs are expected to ultimately meet the International Grid Codes which are more stringent than NTDC. Type-5 The VRT of a Type 5 WTG is very similar to that of standard grid- connected synchronous generators, which are well understood.

15. Other Comparisons of Type-3 and Type-4 FactorsType-3Type-4 Converter’s sensitivity to temperature: Maximum rated at 40 o C, derated output beyond Less sensitive because 30 % output comes from converter More sensitive because full output is coming from converter Fault current contribution in terms of strengthening the network Fault Current = 4 times the rated current, Helps in strengthening the system Fault current = 1-1.5 times the rated current Does not help much in strengthening the system Coupling between Aerodynamic side and Grid Side Partially decoupled.Fully decoupled: No impacts travel from either aerodynamic or grid side EfficiencyMore power output because both the rotor and stator feeds in power Slightly less power output due to converter losses

16. Population of WTG Types in USA July 2011 AWEA Statistics

17. Population of WTG Types in NPCC July 2011 AWEA Statistics NPCC: North East Power Coordinating Council NYISO: New York Independent System Operator

18. Pakistan’s Grid Code and Suitability of Types of WTGs Grid Code Addendum No.1 for wind power requires as follows: – Reactive Power Control & Power Factor = ±0.95 at point of common coupling (PCC) – Active Power & Frequency Control: Exempted – LVRT: WTG must withstand a voltage dip down to 30% Retained Voltage for a duration of at least 100 ms for a normal fault clearing case and at least for 180 ms in case of Stuck Breaker Type-3 and 4, both are most suitable and all the Wind Power developers in Jhimpir and Gharo are bringing either of the two

19. Grid Interconnection Issues Power Quality Harmonics Flicker Voltage dip IEC has set the standards (61400) for WTGs that may be complied provided if the Grid System at the point of connection is not very weak in terms of short circuit strength Grid Code Addendum has made it mandatory to comply the above mentioned IEC standard

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