Experiences with Wind Plants and Stability Studies in Isolated Systems International Wind Diesel Conference, Girdwood, AK March 9, 2011 John DL Hieb

Background EPS – specialize in planning and operational studies for small, isolated, electrical systems Numerous clients in Alaska and Hawaii Involved in a number of Wind IRS’s in Alaska and Hawaii Kodiak Study – focus of this presentation

Kodiak System Isolated on Kodiak Island Peak load of 25 MW (11MW minimum) Goal: Produce 95% of energy sales with cost effective renewable power by the year 2020 80% of energy supplied by one hydroelectric plant with two 11.25 MW units Remaining generation is wind and diesel The three newly installed 1.5 MW wind turbines have reduced the yearly diesel energy production from 20% to 11%.

Existing Pillar Mountain Wind Farm

Wind Expansion Study for Kodiak Evaluating the expansion of the existing wind plant from 3 to 6 turbines (all 1.5 MW turbines) Determine the impact on line fault events The hydroelectric plant, Terror Lake, is outside of town and cannot provide much voltage support in town during peak load times. The wind plant is in town near a main load substation. Ride through becomes a significant issue. Determine the impact of additional wind turbines on the frequency regulation of the system, especially with only wind and hydro generation online

Kodiak Frequency Regulation Question – How much worse will the frequency regulation become if the existing wind plant is expanded from three turbines today to possibly six turbines in the future? What measures can be taken to mitigate the impact on frequency? If the hydro plant is expanded to three units, how much improvement in frequency response can be expected?

Measuring Frequency Regulation Kodiak provided data from a “typical” 15 minute time period from SCADA Frequency, wind plant output, and hydro unit output data was available at one to two second sampling rates The wind was exhibiting “typical” variability.

Benchmarking the Model EPS conducted a 15 minute transient stability simulation using the SCADA data to assess the ability of the simulation to reproduce the actual system frequency variability. If successful, the simulation could then be used to evaluate the impact of adding more wind turbines and to evaluate mitigation techniques.

Benchmarking (cont.) The Terror Lake plant had recently undergone a governor replacement that resulted in significant improvements in response rates and regulation capability. However, the tunnel system is some 26,000 feet in length and therefore the water time constant is large.

Benchmarking (cont.) The simulation was run as follows. The plants outputs were used to determine / estimate the gross loads and system losses. The wind plant was not modeled with a traditional wind turbine model. Instead the plant output was used as a direct injection into the grid using the sampled data. The Terror Lake model was used in full detail. The Terror Lake output and grid frequency are computed by the simulation.

Load Terror Lake Wind Output

61 Hz Black is simulation Red is actual 59 Hz

Results Frequency response from simulation reasonably matches the actual frequency. How do we assign a metric to the frequency so that we can compare alternatives? We looked at some simple statistical measures.

From the actual frequency

From simulation

Assessing Alternatives Follow up simulations were conducted looking at expanding the wind turbines from 3 to 6. Kodiak also wanted to evaluate the benefit of expanding the Terror Lake plant from 2 to 3 units. The powerhouse was originally designed for 3 turbines. Kodiak will determine the availability of additional energy from the plant.

Two Terror Lake units, Six wind turbines

Three Terror Lake units, Six wind turbines

Maximum Wind Penetration The current operations are near acceptable frequency regulation limits Adding 3 wind turbines and an additional hydro turbine/generator is worst than current system. How many new wind turbines can be installed before frequency regulation is worse than current operations. Simulate system with 3, 4, 5, 6 wind turbines and 3 hydro turbines and compare

Maximum Wind Penetration Table is sorted from best to worst frequency regulation Benchmark of current operations is shown in red text Number of Turbines Standard 1 Std Deviation F 3 Std Deviation F Wind Hydro Deviation Low High 3 0.112 59.89 60.11 59.66 60.34 4 0.132 59.87 60.13 59.61 60.40 2 0.145 59.86 60.15 59.57 60.44 5 0.155 59.85 60.16 59.54 60.47 6 0.176 59.82 60.18 59.47 60.53

Maximum Wind Penetration If a third hydro turbine is added, the system frequency regulation will be improved even with 4 wind turbines installed However, a 5th wind turbine will result in decreased frequency regulation capabilities

Results from Kodiak The frequency statistics provide the utility with some measure of the impact of the wind on their frequency regulation. Kodiak and EPS will continue to work to quantify the frequency variability and the impact wind has on frequency (more data). This study is the first step in evaluating the operational impact of additional wind turbines.

Questions? John DL Hieb EPS, Inc. Redmond, WA jhieb@epsinc.com