Background

1. Background • Alberta is a single balancing authority area, AESO is the BA and operates wholesale energy and operating reserves markets. • Market participants (generators, importers and exporters) submit hourly price offer pairs for energy dispatch. Collectively, all price offer pairs are listed in descending price order. Within our regulating reserves range, the AESO dispatches the merit order as often as necessary to maintain supply demand balance including hourly interchange ramps. • Regulation and Contingency (Spinning and Supplemental) reserve providers can change on an hourly basis. Market participants are accountable to coordinate energy and operating reserves offers. Standby reserves are made available • AESO procures between 140 to 240MW regulating reserves hourly using a formula that takes forecast load change and forecast interchange change. • The Alberta BA is normally synchronized to WECC via three lines, a single 500kv line and 2 138kv lines. These three lines terminate in BC and are effectively a single radial connection to the Western Interconnection. All load and generation imbalance and variability is reflected on the three lines until AGC makes adjustments • AESO is a member of NWPP Reserve Sharing group

2. Path 1

3. Operations without the 500kv tie • There are more than 20 generators greater than 150MW (up to 450MW). A generator trip of more than 150MW is certain to trip the 138kV tie. • Other sudden supply load imbalances are reflected on the tie until AGC adjusts for the changes. • The 2 remaining lines are left in service to provide frequency stability to Alberta and add more reliability to local loads • The 138kV interconnection capacity is 140MW, however zero ATC is posted when the 500 kV tie is out and the entire 138kV capacity absorb all of the Alberta load and generation variability • Note - from 1973 to mid eighties, Alberta was connected via the 138kV tie. The 500kV was added in the late eighties.

4. Pre – Event Information • Applies to Sept 2nd, 4th, 7th and 9th events • Pre-disturbance system conditions were the 1201L/5L94 Cranbrook-Langdon 500-kV line was out of service for semi-annual maintenance from September 02, 2009 through September 21, 2009 • There were no scheduled transfers between AESO and BCTC • Sept 2nd 138kv tie trip resulted from a mis-match in ramp rates of generators dispatched up and generators dispatched down • Sept 4th, 7th and 9th events resulted from Alberta generator trips. • Wind generation was not a factor for these events, however ramp rates are expected to be a major concern as intermittent (wind) generation capacity increases in relation to conventional generation

5. Sept 2 Event Information • Regulation Reserves • HE 23 - 212 MW • HE 24 - 200 MW • Load change • 22:50 - 7990 MW • 23:10 - 7903 MW (87MW decrease) • Energy dispatches made at 22:56 • Dispatch up - predominantly 2 large coal units, • Dispatch down - almost entirely gas/hydro units with volume was spread evenly amongst them. • Tie tripped at 23:03 • The operators indicated there was not enough time to catch the ramp mismatch before the tie tripped. • Contingency reserves were directed after tie trip to recover frequency to re-sync

6. Operating Tools • Conventional SCADA/EMS displays for ACE, generation, load, etc. • A dispatch tool provides access to the energy and operating reserves merit orders and electronic dispatch messages are exchanged with market participants. This system handles hundreds of dispatches on a daily basis • Wind forecasting has been implemented • A dispatch decision support tool (DDST) is in prototype as we recognize a need a high level view of supply demand balance in a future time period based on variables including load, interchange, ramping of generation including variable generation and price responsive loads. The DDST provides what if scenarios based on real time system conditions.

7. EMMO & System marginal dispatch System Changes DDST screen capture ACE and AGC Wind Load Tie EMMO ramping Net Demand