Tongxin Zheng and Jinye Zhao - PowerPoint Presentation

Tongxin Zheng and Jinye Zhao Business Architecture and technology VRWG Meeting Wind Dispatch Using Do-not-Exceed Limit Sep 29, 2014

Outline The need for do-not-exceed (DNE) limit for wind dispatch Wind dispatch process DNE Limit problem and its solution methodologyExamples of wind dispatch through DNE limits 2

Motivation More wind resources are being integrated into the system operation.Different from conventional generators, wind resources are Variable Increased level of uncertainty in the real-time operationNon-dispatchable Wind generation can be only curtailed when reliability issues ariseLow operating costNegative marginal cost How do we better utilize the low cost wind resources while recognizing their variability? 3

Existing Real-time Wind Dispatch Practice Manual DispatchFixed at SCADA values Do not set real-time prices Curtailment through phone calls in the event of transmission violationAutomatic Dispatch Expected output forecasted by the system operator or participants Dispatch between 0 and the expected outputUtilize economic offers in the dispatch and pricing Automatic curtailment as long as basepoint or desired dispatch point (DDP) is less than the expected output Electronic dispatch with basepoint and/or curtailment flag Allow a wider deviation range when no curtailment is activated 4

Issues with Existing Practice The dispatch signal does not provide a clear guideline of dispatch following for wind resources. They do not know whether additional wind generation beyond the basepoint/DDP will cause any reliability problems. The curtailment action is ex post and may not be efficient. Curtailment is implemented when system has already experienced reliability problems. 5 Reliability limit Expected Output Wind Fluctuation

ISO NE Proposal: Do not Exceed (DNE) Limit Send a do-not-exceed limit to each wind unit Do-not-exceed limit is constrained by reliability and economics.The DNE limit is the maximum amount of economical wind generation that system can accommodate without causing any reliability issues. Reliability: Capacity and Transmission Uncertainty: Any realization Benefits of DNE limit: Provide a dispatch guideline for wind resources Ensure reliable system operation Allow low cost wind resources to provide as much energy as possible Wind resources are eligible for setting prices 6

Real-time Wind Dispatch Framework CA – Contingency Analysis ED - Electronic Dispatch System DDP – Desired Dispatch Point DNE – Do-not-Exceed Limit 7 RT Dispatch DNE Limit Calculation Wind CA Forecast Network Transmission Constraints ED DDP for non-wind resource DNE Limit DDP Offer LMP

Wind CA N-1 Contingency AnalysisLoss of line Loss of generator Enhancement with wind dispatch Security under expected wind generationContingency analysis with expected wind output scenario Constraints are generated for the real-time dispatchSecurity under extreme wind realization Zonal basis Extreme wind generation Constraints are generated for the DNE limit calculation 8 RT Dispatch DNE Limit Calculation Wind CA ED

Real-time Dispatch Wind generation submit Offer curve Real-time high operating limit (RTHOL)Generation forecastMeteorology data Outage information ISO forecasts the expected generation (EG) of each wind resource.In the dispatch, each wind unit isDispatchable (eligible for setting price) Dispatched between 0 and its EG Dispatched against its energy offer 9 RT Dispatch DNE Limit Calculation Wind CA ED

Real-time Dispatch 10 RT Dispatch Wind EG Network Wind Offer Non-wind offer Non-wind operating status DDP LMP DNE Limit Calculation Wind CA ED Transmission Constraints DDP for non-wind resource DNE DDP ≤ EG ≤ RTHOL

DNE Limit Calculation Produce dispatch instruction for each wind resourceDNE Limit = Maximum economical output level that the system can accommodate without sacrificing system reliability 11 RT Dispatch DNE Limit Calculation Wind CA ED RTHOL DNE 0 RTHOL DNE 0

DNE Limit Calculation 12 RT Dispatch LMP DNE Limit Calculation Wind CA ED Transmission Constraints DDP for non-wind resource DNE DDP RTHOL Unit Status Network DDP ≤ DNE ≤ RTHOL DDP ≤ EG ≤ RTHOL DNE is calculated immediately after RT dispatch RTHOL takes the most updated value

DNE Limit Problem DNE Problem is an optimization problem that finds the maximum amount of economical wind energy the system can support without sacrificing system reliability. Major factors affecting DNE limitsOperating CriteriaCorrective Control ActionsVariable DNE Resources Tie-breaking Rule 13

System Response under Wind Over-generation 14 t=0 t=10 min t=15 min Non-wind: Receive DDPs Follow DDPs DDP Re-dispatch point AGC : Receive DDPs Provide frequency control DDP Reg lower limit Reg upper limit AGC capability restored Tie line: Finalize transactions Deviate from schedule Scheduled tie line Scheduled tie line Tie line upper limit Tie line lower limit Wind : Receive DNE limits Actual output Actual output 0 DNE EG

System Response Under Wind Over-generation (Cont.) At t=0DDPs are determined and sent to dispatchable units Between time t=0 and t=10 minSudden increase in wind generation above expected generation level Tie flow changes, and AGC units are dispatched to correct frequency deviation Transmission flow could violate its STE (15min) or even drastic action limit (5min) At t=10 Dispatch non-wind units to restore frequency, net tie schedule, AGC requirement, and balance future load. The dispatch may not be feasible due to the lack of ramping capability. 15

Operating Criteria Wind generation under DNE limit should not any create system reliability problems. Ramp Limitation (C1)The system should have enough ramping capability to handle wind power fluctuation within DNE limits. Ramping capability – AGC capability and Ramp. At the current level of wind penetration, the system has enough ramping capability to accommodate the wind fluctuation. Transmission Limitation (C2) In responding to the wind generation fluctuation, the system control actions should not cause any transmission violations. 16

Corrective Control Actions Corrective control actions are used to balance the wind fluctuation AGC units (internal and external) Conventional units in system redispatchAt the current wind penetration level, only external units are considered as corrective control units. 17 then wind units are allowed to produce 50 MW more than their forecast If corrective control units are capable to reduce 50 MW DNE Limit = EG + 50 MW

Variable DNE Resources for DNE Limit Calculation 18 In the economic dispatch, a wind resource will be dispatched to the following two situations DDP = EG (Variable DNE Resources) Additional wind power could lower the cost of the system. DNE limit will be computed. DDP < EG (Fixed DNE Resources) No additional wind power is needed, since the system could be constrained. DNE limit is set to the DDP.

Tie-breaking Rule 19 In the event that multiple DNE limits exist, the DNE limits will be determined based the ratio of available generation between DNE limit and DDP over potential wind generation capability beyond DDP. MOL is the maximum operating level, which is the minimum of the RTHOL and the maximum amount of economic energy can be produced below LMP.

DNE Limit Problem Formulation The DNE limit problem is formulated as a robust optimization problem described below: Maximize: DNE1+ DNE2+ … + DNEn Subject toSystem is able to maintain energy balance for any wind output varying between [DDP, DNE] <-Robust Constraint The flow on any transmission line is below its limit for any wind output varying between [DDP, DNE] (operating criteria c2) <-Robust Constraint Corrective control action must be subject to the corresponding physical limits (operating criteria c1) DDP ≤ DNE ≤ MOL 20

Assumptions in the DNE Limit Calculation System has enough capacity to replenish the missing MW if wind resources go below their DDPs. The output of variable DNE resources are assumed to vary between DDP and DNE. Non-Variable DNE and non-corrective control resources are assumed to perform according to the dispatch instruction and their output are fixed at their DDPs. Each corrective control unit’s response to wind fluctuation is assumed to be linear with respect to the total wind fluctuation. is corrective control unit (CCU) i ’ s response to the total wind fluctuation is set to be proportional to CCU’s ramping, regulation capability or current production level w is the index of wind resources 21

Solution Methodology Given the above assumptions, the robust constraints can be transformed into linear constraints. The DNE robust optimization problem thus becomes a linear programming problem and can be solved efficiently by existing commercial solvers. 22

A Basic Example: Dispatch Solution 23 ~ ~ Non-wind: EcoMax = 500 MW Offer = $35/MWh Load = 1200 MW Wind: RTHOL = 200 MW EG = 100 MW Offer = $0/MWH ~ CCU: 1000 MW with ± 1000 MW Capability Line limit = 300 MW DDP = 100 MW DDP = 100 MW DDP = 1000 MW LMP = $35/MWh LMP = $35/MWh CCU: Corrective control unit Line flow = 100 MW

A Basic Example: DNE Limit - Unconstrained 24 ~ ~ Non-wind: EcoMax = 500 MW Offer = $35/MWh Load = 1200 MW ~ CCU: DDP = 1000 MW Line limit = 300 MW Wind: RTHOL = 200 MW EG = 100 MW Offer = $0/MWH DNE = 200 MW = DDP + 100 MW Move Down = 100 MW DDP = 100 MW LMP = $35/MWh DDP = 100 MW Gen = 900 MW LMP = $35/MWh Worst flow = 200 MW

A Basic Example: DNE Limit - Constrained by Transmission Limit 25 ~ ~ Non-wind: EcoMax = 500 MW Offer = $35/MWh Load = 1200 MW ~ CCU: DDP = 1000 MW Line limit = 150 MW Wind: RTHOL = 200 MW EG = 100 MW Offer = $0/MWH DNE = 150 MW = DDP + 50 MW Move Down = 50 MW DDP = 100 MW LMP = $35/MWh DDP = 100 MW Gen = 950 MW LMP = $35/MWh Worst flow = 150 MW

A Basic Example: DNE Limit - Constrained by Offer 26 ~ ~ Non-wind: EcoMax = 500 MW Offer = $35/MWh Load = 1200 MW Wind: RTHOL = 200 MW EG = 100 MW ~ CCU: 1000 MW with ± 1000 MW Capability Line limit = 300 MW DDP = 90 MW DDP = 110 MW DDP = 1000 MW LMP = $35/MWh LMP = $35/MWh 90 MW 200 MW $100/MWH $0/MWH DNE = 90 MW <- Maximum Economical Output LMP = $35/MWh Line flow = 90 MW

3-Bus System: Basic Information 27 ~ ~ ~ L 2 = 80 MW ~ Load = 1200 MW L 1 = 30 MW L 3 = 120 MW A B C Wind A: RTHOL = 200 MW EG = 200 MW Offer = $10/MWh Wind B: RTHOL = 200 MW EG = 30 MW Offer = $0/MWh CCU: 1000 MW with ± 1000 MW Capability Non-wind: EcoMax = 500 MW Offer = $35/MWH Each line has the same reactance, but different limits.

3-Bus System: Dispatch Solution 28 ~ ~ ~ L 2 = 80 MW ~ Load = 1200 MW L 1 = 30 MW L 3 = 120 MW A B C Wind A: RTHOL = 200 MW EG = 200 MW Offer = $10/MWH Wind B: RTHOL = 200 MW EG = 30 MW Offer = $0/MWH CCU: 1000 MW with ± 1000 MW Capability Non-wind: EcoMax = 500 MW Offer = $35/MWH DDP = 30 MW = EG DDP = 120 MW < EG LMP = $10/MWH LMP = $60/MWH DDP = 50 MW DDP = 1000 MW LMP = $35/MWH 30 MW 90 MW 60 MW Wind A is constrained!

Generation Shift Factors (GSF) Line AB flow constraint is binding at the limit in the dispatch. Flow AB = 1/3*Wind A -1/3 Wind B = 1/3*120-13/*30 = 30 MW. Wind fluctuation Wind A is a harmer to line AB flow constraint. If we allow wind A to generate one more MW and CCU’s output is reduced by 1 MW, line AB flow will violate the limit by 1/3 MW. DNE limit for wind A is fixed at DDP, or 120 MW. Wind B is a helper to line AB flow constraint. One MW generation from B will relieve line AB flow by 1/3 MW. More generation from wind B is desirable. However, over generation from B potentially overloads line AC and BC, due to its positive GSF to them. 3-Bus System: Transmission Constraints 29 Bus GSF Line AB Line AC Line BC Bus A 1/3 2/3 1/3 Bus B -1/3 1/3 2/3 Bus C 00 0

3-Bus System: DNE Limit Solution 30 ~ ~ ~ L 2 = 80 MW ~ Load = 1200 MW L 1 = 30 MW L 3 = 120 MW A B C Wind 1: RTHOL = 200 MW EG = 200 MW Offer = $10/MWH Wind 2: RTHOL = 200 MW EG = 30 MW Offer = $0/MWH CCU: 1000 MW with ± 1000 MW Capability Non-wind: EcoMax = 500 MW Offer = $35/MWH DDP = 50 MW DDP = 1000 MW LMP = $35/MWH DNE = 120 MW DNE = 60 MW DDP = 120 MW LMP = $10/MWH DDP = 30 MW LMP = $60/MWH

3-Bus System: Extreme Wind Realization at DNE 31 ~ ~ ~ L 2 = 80 MW ~ Load = 1200 MW L 1 = 30 MW L 3 = 120 MW A B C 20 MW 100 MW 80 MW Move Down = 30 MW Output = 1000– 30 = 970 MW Wind 2: RTHOL = 200 MW EG = 30 MW Offer = $0/MWH Wind 1: RTHOL = 200 MW EG = 200 MW Offer = $10/MWH Fix at DDP = 120 MW CCU: 1000 MW with ± 1000 MW Capability DNE = 60 MW = DDP + 30 MW Non-wind: EcoMax = 500 MW Offer = $35/MWH Fixed at DDP = 50 MW DDP = 30 MW

Summary The wind dispatch framework contains three components: Wind CA Economic Dispatch DNE limit CalculationWind resources receive the DNE limit rather than the DDP as electronic dispatch signal. The DNE limit represents the maximum amount of economical wind generation that the system can safely accommodate. 32

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