NREL Wind Integration Workshop By Electric Power Systems, Inc. June 28-29, 2010

Wind Integration in the Railbelt Power Flow Results - Few power flow issues introduced by wind - Few turbines have voltage regulation - Turbines with constant PF require close coordination and study to optimize PF setting for voltage control - Some turbines with PF control can be changed in operation, others cannot Slide 2

Wind Integration in the Railbelt Energy Issues - Kenai wind energy exported to Anc/Fbks reduces Bradley energy availability - Regulation on Kenai for wind energy in Anc/Fbks reduces Bradley energy availability due to transmission reservations for regulation - Transmission regulation may warrant uneconomic dispatch and unit commitments for regulation Slide 3

Wind Integration in the Railbelt Short Circuit Results - With no unit de-commitments, no SC decrease issues - Inverter based systems limited by SC availability in certain areas - Protection requirements impacted due to limited SC current from WTGs - No uniform SC model for ASPEN, PSS/E or other programs available Slide 4

Wind Integration in the Railbelt Short Circuit Results - SC coordination requires multiple fault simulations for accurate scenario based on fault location/duration - Fault current very close to WTG results in 5 X unit SC current (varies with WTG) - Fault current drops to almost unity within 5 cycles (varies with WTG) - Faults external to WTG plant result in 1.2 X fault current (varies with WTG) Slide 5

Wind Integration in the Railbelt Short Circuit Results - In some WTGs, fault current is constant PF - Some WTGs fault current angle varies considerably through the fault duration - Impedance based relays difficult to use from WTG end - Overcurrent relays difficult to use from WTG end - LCD good solution, but requires communication Slide 6

Transient Stability Impacts No unit de-commitments – no problem De-committed units require inertial control on WTG unit Frequency variability will increase Depending on WTG, voltage stability will decrease Slide 7

Transient Stability Limits WTG amounts limited by transient stability by Railbelt area - Kenai – 30 MW - Anchorage -150 MW - Fairbanks – 100 MW - Total Railbelt penetration as defined by transient stability – 280 MW Slide 8

Transient Overvoltages Inverter based WTG islanding results in transient overvoltages of up to 2.0 pu Level and duration of transient overvoltage dependent upon system connections and controls Transient overvoltages presents problems with insulation coordination Existing arrestors, transformer insulation requires analysis Slide 9

Transient Overvoltages EMTP Simulation of overvoltage following separation Slide 10

Transient Overvoltages EMTP Simulation of overvoltage following separation Slide 11

Voltage Control Methodologies Grounding transformer BESS Arrestor coordination All require EMTP analysis to ensure correct operation Inverter based energy solutions are limited by SC capacity of Railbelt and its operating islands Slide 12

Transient Frequency Stability Following separation from power system WTG islanded system frequency varies from 45 to 90 Hz depending on power factor & load match prior to separation Slide 13

Voltage Ride Through LVRT capabilities vary a great deal between WTG manufacturers No agreement on “single” vs “multiple events” such as reclosing FERC defines an event requirement, but does not define the event as single fault or single fault followed by restoration and reclose event FERC standard not applicable in Railbelt Many islanded systems define reclosing as a required LVRT event Sequential events are not considered LVRT events Slide 14

Voltage Ride Through - GE Slide 15

Voltage Ride Through To avoid LVRT shutdown, reclosing sequence on all Chugach 35 kV lines must be changed, including those from University station Distribution reclose sequences do not result in LVRT event at ITSS/FIWF, but do in other parts of the Railbelt Distribution LVRT events are problematic and will force Railbelt utilities to make reclosing and protection changes Transmission faults throughout the Railbelt result in LVRT events Unbalanced faults must also be analyzed and specified for ride-through requirements 0 V ride thru must be required Slide 16

Wind Analysis Wind data analysis is the most important item for integration Wind analysis defines the amount of regulation required to integrate wind Highest cost integration piece in Railbelt Many different methods for evaluation No clear method for islanded power systems All introduce some risk to utilities Slide 17

Wind Analysis Data recorded in 10-minute averages Short –term ramp rates are generally a nuisance, but not a costly or technical concern WTG cut-out is defined by high winds or rapid changes in wind direction that force a wind turbine to shut down 10-minute data typically uses wind speed, not associated with change in direction in determining cutout occurrences Slide 18

Regulation Requirements Upward Regulation - Amount of regulation required to cover largest expect loss of wind power - Regulation can be split between hydro and thermal generation - Hydro cannot supply all of regulation Downward Regulation - Amount of regulation required to back down following the loss of the largest probable load or transmission line during export - Downward regulation required to prevent widespread loss of generation following system disturbance Slide 19

Regulation Requirements How much regulation is required? No clear answer Independent control areas increase difficulty and regulation requirement per control area Evaluated changes over 10-minute period, assumed to be fastest scheduling change for AGC controlled hydro Slide 20

Regulation Requirements Evaluated 30-minute changes – fastest start time for gas turbine Evaluated 60-minute changes – fastest gas nomination schedule change Slide 21

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Regulation Requirement Regulation is a system issue, not a wind project issue Regulation requirements for one WTG farm, impacts all wind farms on system Wind forecasting should incorporate all WTG projects to determine impact on system as opposed to individual control areas Slide 29

Regulation Requirement Final regulation requirement not determined Appears extremely variable dependent upon assumptions for regulation response time Minimum appears to be 15 MW for 10 minute case To cover 60 minute case, appears to be MW Slide 30

Hydro-Thermal Coordination Slide 31

Hydro-Thermal/Wind Coordination Slide 32

Hydro-Thermal Coordination Wind energy will require hydro energy to be used non-optimally to provide regulation support for wind variability Water “ponded” by storing wind energy must equal water required for regulation scheduling to break even More ponded water than regulation requirement = benefit Less ponded water than regulation requirement = cost Does not consider cost of wind Slide 33

Hydro-Thermal Coordination Additional regulation and hydro schedule requirements will decrease efficiency of combined cycles Wind may be able to back down simple cycle enough to “provide its own regulation” during certain times Due to gas restrictions – thermal regulation capacity may not be able to be resident at one plant or common supply system location Slide 34

Hydro-Thermal Coordination Hydro units must be dynamically scheduled to supply regulation Dynamic scheduling will increase probability of transmission restrictions on Kenai All Kenai hydro and thermal regulation must reserve transmission capacity 20 MW of hydro regulation reduces energy transfer limit to 55 MW for scheduled energy Slide 35

Hydro-Thermal Coordination Positive benefit of transmission regulation reservation is availability for spinning reserve Dynamic scheduling will require revision to contracts and operating agreements Dynamic scheduling will require modification to spinning reserve compliance Dynamic scheduling of Bradley by multiple control centers difficult due to transmission constraints Slide 36

Control Agreements Control area interchanges must be dynamically adjusted if sales are made between control areas unless host utility is supplying all required regulation Control area deviations may require new allowable standards of deviation between control areas Slide 37

Frequency Ride Through Without Inertial control, wind addition can lead to increased load-shedding for unit trips WTGs must ride meet frequency capability of existing generation Slide 38

Fuel Impacts Gas Schedules - $30/mcf penalty for variances over defined amount - Wind variance of only 300 MWh/day results in variance of 3,700 mcf - Gas penalty - $111,000/day - Gas penalty difficult to distribute between load variance and wind variance - Gas nomination changes concentrated at one plant may not be supported by gas delivery/supply system Slide 39

Curtailment Issues Economic Curtailment - During off-peak hours, curtailment of either wind or combined cycle plants required - Wind curtailment could be used to provide its own regulation resource - Curtailment priority among renewable resources Slide 40

Curtailment Issues Transmission Curtailment - Curtailment required due to transmission constraints (either energy or regulating capacity) - Curtailment required due to transmission outages - Curtailment required due to contract path interruption Slide 41

Curtailment Issues Generation Curtailment - Curtailment required due regulation shortage of generation - Curtailment required due to generation stability (turn- down) Slide 42

Curtailment Issues System Curtailment - Curtailment required due to system upset (load restoration, storms etc) Curtailment costs must be defined Curtailment methodology must be defined Curtailment through multiple control areas must be defined Use of curtailed energy must be defined Slide 43

Frequency Control Frequency deviations will increase Frequency variability will increase if units are de-committed Frequency impacts can be simulated Slide 44

Kodiak Hz – Simulation vs Actual Slide 45

WTG “Options” Inertial Control - Simulates machine inertia on power system - Increases frequency stability - Maintains load-shed probability for unit trips Wind Farm Management System - Provides for single point control for curtailment - Provides power ramp control Important for restoration Curtailment transitions Mitigate ramp rates Droop control (if used for regulation) Slide 46

WTG “Options” LVRT Ride Through - Option for extended VRT ride thru available on some units Voltage Control - Provides for voltage control mode as opposed to PF control or constant MVAr mode - New standard in some non-US grids - Not available in all turbines makes/models in US due to patent restrictions Slide 47