1. AES Buffalo Gap Wind Farm Buffalo Gap 2 - 232.5 MW 155 – GE 1.5 sle Presented August 22, 2008 By Robert Sims AES Wind Generation

2. AES Buffalo Gap Wind Farm Three Phases totaling 524.3 MW Summary:  Located in Taylor and Nolan Counties SW of Abilene.  Buffalo Gap 1 – 67 Vestas V-80, 1.8 MW turbines totaling 120.6 MW, COD late/05  Buffalo Gap 2 – 155 GE 1.5 sle, 1.5 MW turbines totaling 232.5 MW, COD early/07  Buffalo Gap 3 – 74 Siemens 2.3 MW turbines totaling 170.2 MW, COD mid/08.  Interconnected to AEP Bluff Creek at 138 kV via project owned 12 mile 138 kV T/L.

3. AES Buffalo Gap 2 GE 1.5 sle Wind Turbine Physical characteristics & Rating  Three blades, rotor diameter of 77 M (253 ft.), swept area 4657 sq meters (50,128 sg. ft.)  80 meter tubular steel tower (262.5 ft.)  Variable speed operation, main rotor =10-20 rpm  Power regulation via adjustable blade pitch (electric) & electronic generator torque control  Main gearbox ratio 1:78 for variable generator speed of 870 to 1600 rpm with nominal power at 1440 rpm.  Rated 1.5 MW and 575 volts 3 phase  Over 5000 installed worldwide

4. GE 1.5 MW sle Turbine

5. GE 1.5 sle Turbine

6.  Method of starting When the wind in the area increases above the cut in speed (3.5 m/s) the turbine blades are pitched from “feather” to “power” and the wind begins to turn and accelerate the rotor. Once the rotor achieves operational speed of 10-11 rpm the turbine is softly connected to the line with a ramp up of the power electronics.  Method of controlling power output Output power is regulated through a combination of variable blade pitch for control of rotor torque, and variable frequency control of the generator to control drive train speed and torque. During a wind gust the rotor frequency is increased and the drive train is allowed to accelerate while reduging torque to “store” the gust power spike as kinetic energy in the rotor like a flywheel. Once the gust passes the controller extracts the power in the rotor and slows it back to the nominal speed.  Method of stopping For normal shutdown the rotor blades are pitched to feather. For an emergency shutdown the blades are pitched to feather (battery back-up if required) and a disk brake is engaged on the high speed shaft.

7. GE 1.5 sle Turbine  Reactive Devices as Part of the Turbine The GE 1.5SLE 60Hz wind turbine uses a power converter system that consists of an electronic converter connected to the generator rotor, a DC intermediate circuit, and a power inverter on the grid side. This system functions as a pulse- width-modulated variable frequency converter in 4-quadrant operation. The converter its self consists of an insulated gate bipolar transistor (IGBT) power module and the associated electrical equipment. Variable output frequency of the converter allows a rotational speed-module operation of the generator within the range of 870 rpm to 1600 rpm. The Standard GE 1.5SLE 60Hz Wind Turbine is designed with a selectable power factor. At 1.0 pu voltage (575 V) and full power (1500 kW), a power factor of 0.95 overexcited (reactive power delivered by the wind turbine) to 0.90 underexcited (reactive power absorbed by the wind turbine) as measured at the terminals of the trubine. The power factor is settable at each WTG or controlled remotely and dynamically by the wind farm SCADA system (more on this later). Optional Reactive Capability: 0.90 overexcited / 0.90 under-excited at 1.0 pu voltage (575 V) and full power (1500 kW) is available at an additional cost.

8. GE 1.5 MW sle Turbine

9. GE 1.5 sle Turbine  Grid Frequency Tolerance Continuous operation in the frequency range of 57.5-61.5 Hz. Trips as the frequency drops below 56.5 Hz or exceeds 62.5 Hz.

10. GE 1.5 sle Turbine  Voltage Tolerance (At the 575 volt terminals of the turbine!) Voltage limits for the GE 1.5 MW, 60 Hz wind turbine are as follows:

11. AES Buffalo Gap 2  Site Selection Criteria The Buffalo Gap project location was selected based on it’s high elevation resulting in high average wind speeds, rural location with compatible land use, and proximity to transmission lines.  Cabling Connections to collector substation & Electrical characteristics The GE turbines have a Padmounted type transformer located at the base of each turbine to raise the voltage from the 575 volt turbine terminal voltage to 34.5 kV for the site power collection system. A combination of underground and overhead power lines based on standard utility practices carry the power to the central step up substation. Typically 20-30 MW of generation is connected to each substation 34.5 kV breaker positions.  Operational Issues The only operational issues of moderate significance has been disturbances resulting from lightning strikes to overhead portions of the system.

12. AES BG 2 34.5 kV System

15. AES BG 2 Control System  Location of Control Rooms / Facilities The project has both local supervision & control, along with remote supervision & control 24/7 from the AES Wind Generation central control facility in Palm Springs, California.  SCADA The project has separate turbine SCADA systems for the 3 types of turbines associated with the 3 phases of the project (Vestas, GE, & Siemens) along with a separate SCADA system for the interconnection substations.  Turbine Power Control Turbine power is generally controlled locally by each turbine individually to maximize energy capture. However, the local control can be overridden for the fleet by the turbine SCADA control and capped at lower max power levels.

16. AES BG 2 Control System  Reactive Control Individual turbine reactive power is controlled centrally from the GE SCADA system. Voltage and power factor are monitored at the main collector substation. The system can operate on a voltage set point, or a power factor set point, and remotely adjusts the power electronic converters in each of the turbines along with automatically switching the 34.5 kV substation capacitor banks.  Switch / Breaker Control Substation control can be local from the substation using either traditional control devices on the switchboard or a SCADA terminal in the substation control house, or remote supervision and control via the substation SCADA.  Special Protection Schemes (SPS) The Buffalo Gap 3 project is being fitted with a short term SPS to reduce generation in the event of the loss of one of the 345 kV lines to Bluff Creek during maximum wind power generation. This SPS will be required for a few years until area transmission improvements are completed.

17. Thank You robert.sims@aes.com