The Smart Grid: Distributed optimization & control challenges Kameshwar Poolla UC Berkeley June 24, 2011 HYCON2 – Trento, June 2011
Objectives � Cover essential background on power systems � Tell you about what is changing - Renewables - Distribution automation - Markets - Communication and sensing � Discuss some opportunities - Distributed optimization - Distributed control � Describe our vision of Grid2050 HYCON2 – Trento, June 2011
A. The Legacy Grid 1. Components 2. Complications 3. Fact & Figures 4. Basic Power Systems Engineering 5. Basic Power Systems Economics HYCON2 – Trento, June 2011
Power System Components � Generation system: power source ideally with fixed voltage and frequency � Load or demand: consumes power ideally constant and resistive � Transmission system: transmits power ideally as a perfect conductor � Distribution system: local reticulation of power � Control equipment: many functions coordinate supply with load, regulate voltage and frequency, protection, handle component failures HYCON2 – Trento, June 2011
A simple power system generator load transmission HYCON2 – Trento, June 2011
Complications � Loads vary a lot, not purely resistive not known in advance day-ahead forecast accuracy 2-4% � Generators have constraints: ramping, capacity � Transmission lines are reactive, have capacity constraints � Everything is connected in a complex network of heterogeneous elements � Must be robust to component failure � Must deliver power economically through markets HYCON2 – Trento, June 2011
Load variability � Aggregate loads in CA on a hot day in 1999 � Peak is 2-6pm [system can be very stressed] � Variation is 50% of peak load HYCON2 – Trento, June 2011
Generation, T&D HYCON2 – Trento, June 2011
Generators � Limited capacity � Contain control systems to regulate frequency and voltage � Many different types with different costs, cold-start times, ramp rates, inertia Coal, gas, nuclear, hydro, wind, solar, � Other points - Stability issues are important for thermal generators - Start-up times require unit commitment in advance - Require to be taken off-line for maintenance/repair HYCON2 – Trento, June 2011
Transmission � Operate at high voltages to reduce resistive line loss [765, 500, 345, 230 kV] � Mainly 3 phase AC � HV DC for long lines and undersea cables � T & D losses in 2007 ������ � Moving electricity across 1000 Km is cheap 0.005 – 0.02 $/kW hr [not counting capital cost] Key fact: aging transmission infrastructure is causing many problems in the US HYCON2 – Trento, June 2011
HYCON2 – Trento, June 2011
It’s a Complex Network � Distribution network - has tree structure - Serving millions of customers - Changing topology � Transmission network - has loops, node degree < 5 - Enables routing of power from multiple generators - 4000-12000 buses � Network aspect can cause problems - Cascading failures - Islanding - Stability issues HYCON2 – Trento, June 2011
Operations � Too complex, so broken into control areas � Each area handled by a Balancing Authority/System Operator � Inter-area power transfer for control area imbalances � Balancing functions of System Operator - Ex ante: Economic Dispatch Schedule generation to meet forecast demand - Real time: Monitoring For system failures, power quality, line congestion - Ex post: Reserve management Schedule generation to handle unexpected events HYCON2 – Trento, June 2011
North America Interconnections Geographic segmentation for easier management of electricity HYCON2 – Trento, June 2011
Electricity Markets � Intimately connect to power systems engineering � Electricity is different from bananas � Bilateral Contracts 65% of power is sold in long-term bilateral contracts � Bulk-power markets Multiple time-scales: Day-Ahead, Hour-Ahead, Real-Time � Markets are partially regulated Price caps, subsidies, extra-market mechanisms � Ancillary Services Markets for other stuff Reserves, Inertia, HYCON2 – Trento, June 2011
Economic Dispatch � Scheduling generation to meet forecast demand � Done by SO in advance through electricity markets Forecast the demand 1. Get bid stacks from generators 2. Select generation to minimize cost 3. � Constraints Existing bilateral contracts Transmission line limits Power system security � It is a centralized optimization problem � Output: schedule of committed generation and prices HYCON2 – Trento, June 2011
Power System Security � Must deliver power even when a component fails - Worry about 1 failure - Called [N-1 contingency] � SO buys reserve capacity - to handle single largest failure - Margins are 7% in CA, 11% in Texas � Feasibility of power flow places constraints on economic dispatch HYCON2 – Trento, June 2011
Robustness & Reserves � Power system security - Contingency Reserves � Demand forecast errors - Operating Reserves - When load rises too sharply to schedule other resources � Keep power quality [frequency 60 ± 0.25 Hz] - Regulation reserves � Management of reserves is complex, centralized HYCON2 – Trento, June 2011
Facts & Figures HYCON2 – Trento, June 2011
US Generation Sources HYCON2 – Trento, June 2011
Generation Sources in California B#;.% A6"&'"29'68& /, >, ?@9%6 //, 1.2.3"4&.' 5)6&"%7'38297' :.6#;.%<"&= />, !"#$%"&'(") !$-&."% *+, /0, C6$%-.D'EC'FGH7'F&.-#%8-' Oregon is 57% Hydro, I63.%'H22$"&'>JJ+'K C#"#.' Washington State is 70% Hydro L"#"'M"4&.) HYCON2 – Trento, June 2011
Generation Sources in Illinois Renewable 0.0% Nuclear 43.6% Coal Hydroeletric 55.1% 0.0% Petroleum Gas 0.6% 0.6% HYCON2 – Trento, June 2011
HYCON2 – Trento, June 2011
Demand side facts � Power will be defined carefully later Installed U.S. generation capacity ������ GW [about 3 kW per person] Average load of UCB about 25 MW Average load of California about 34 GW � Annual U.S. electric energy consumption ������������������������ MWh per person [on average each American uses 1.5 kW of power] HYCON2 – Trento, June 2011
Retail Electricity Prices HYCON2 – Trento, June 2011
AC Power � 60 Hz in US, 50 Hz in US � Phasors � Complex power absorbed by a load ! " � Complex power delivered by a source ! � Conservation of complex power " HYCON2 – Trento, June 2011
Power Flow on a Line � Real power flow controlled by phase difference � Reactive power flow controlled by voltage difference HYCON2 – Trento, June 2011
Reactive power � What is it? - Power that is borrowed and returned each cycle - Not consumed in net � If reactive power is supplied remotely, we have HYCON2 – Trento, June 2011
Reactive power compensation � Best to supply reactive power locally � Need to adjust compensation as load varies � Can be done at load bus by - Adjustable shunt capacitance - Load-tap-changing transformer - Power electronics devices HYCON2 – Trento, June 2011
Three Phase � 3 conductors instead of 6 for same power transfer � Sum of powers across phases is constant HYCON2 – Trento, June 2011
Balanced 3-phase Operation � Symmetrical loads � Symmetrical generation � 120 o phase shift from one phase to next � Allows analysis one-phase-at-a-time � Steady-state frequency = 60 Hz � Quasi-steady-state frequency �� 60 Hz Changes slowly, so can still use phasors for analysis Time-varying magnitudes and angles HYCON2 – Trento, June 2011
Generator Models � Per-phase � Includes voltage regulation loop � More complex models for transient analysis [Swing equation] � Still more complex models to analyze field/armature effects, frequency swings, rotor vibrations, etc ! " HYCON2 – Trento, June 2011
Transmission line Models � Per-phase � Medium to long lines: � -model � Short lines (< 150 miles) � Line loadability Short lines: thermal limits Long lines: stability limits � Decent power system model sparse circuit driven by variable sources serving uncertain loads HYCON2 – Trento, June 2011
Power Quality � Voltage 1 ± 0.02 per unit � Frequency 60 ± 0.25 Hz Frequency oscillations are an early indicator of system stability Voltage collapse is the result Modest voltage excursions result from poor reactive power support Result is inefficiency HYCON2 – Trento, June 2011
Dispatch � Selecting generation to meet forecasted demand � Centralized - System operator communicates with generators - Schedules setpoints � Day Ahead - Forecast load ± 3% - Schedule generation in DA markets � Hour Ahead - Better load forecast ± 0.5 % - HA is an adjustment market � Real-time Balancing - Real time is actually 5-10 minute ahead - Reissue set-points for select generators HYCON2 – Trento, June 2011
Primary Generation Control � Also called Load-frequency control � To account for minute imbalances between supply and demand � Completely decentralized � How it works - Frequency drop: add generation - Frequency rise: decrease generation � Details - Done by adjusting governor in thermal generation - Time scale 0.1 seconds HYCON2 – Trento, June 2011
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