RES Integration Challenges: Market and Grid Problems and Potential Solutions

RES Integration Challenges: Market and Grid Problems and Potential Solutions

October 11, 2013, Thessaloniki, GreeceDr. Alex Papalexopoulos, CEO and Founder, ECCO International, San Francisco, CA

Power Market Challenges & Opportunities

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Where is the Problem? Load is stochastic, variable and uncertain

PV solar output is also stochastic, variable and uncertain

Supplies can also be stochastic

Need to know size, probability and duration of any shortfalls in both capacity and ramping capability

System needs flexible capacity to deal with the increased uncertainty and variability

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Where is the Problem?The penetration of Solar PV will continue to increase as more countries adopt Renewable Portfolio Standards (RPS) and continue to enforce more stringent targets

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The Anatomy of the “Duck”

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Implications for the Power Market Solar PV complicates the power market

clearing process (Day-Ahead, Hour-Ahead, Real-Time)

Solar PV suffers from lack of dispatchability

Current practices treat Solar PV energy outside the market process

Solar PV puts substantial downward pressure on market clearing prices (the number of negative prices is increasing)

The transmission grid is becoming increasingly congested

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Key Tools Available to the Power Market

Change the Power Market design rules to accommodate solar PV

Invest in flexible generation (gas fired power plants)

Implement demand response

Develop storage facilities

Curtailment of Solar PV

Improve transmission planning and expansion

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Power Market Design Rule Changes Develop Ancillary Services products for better

balancing, better price signals, better incentives (Performance based Frequency Regulation service, Ramping products, Load Following, etc.)

Allow very large negative bids to clear the market

Develop better forecasting tools for load, solar PV, ramping requirements, etc.

Develop Intra-Hour Scheduling financially binding Markets (every 15 minutes)

Develop centralized Capacity Markets that reward flexible generation to ensure security of supply (i.e., we cannot rely on scarcity pricing)

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Performance based Frequency Regulation Traditional approaches typically include

a capacity payment (usually based on shadow price)

an energy payment (for the net energy injected/withdrawn in/from the system)

The new market design

a capacity payment (usually based on shadow price)

Mileage Payments adjusted for accuracy

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Performance based Frequency Regulation We replace the net energy payments by a mileage

payment for the ACE correction provided

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Incorporating Flexibility Requirements Introduction of an Expected Flexibility Deficiency (EFD) function

To determine the anticipated amount of un-served energy caused by a lack of flexibility in the generating fleet

The EFD is a function of the ramp and reserve policies in any given region

The EFD is computed before executing the MIP-Based Unit Commitment It is derived from historical system load/renewable data, as well as the

forecasts associated with a given unit commitment window

Ramp and reserve policies must be defined, in order to determine the EFD

Expected Flexibility Deficiency Function

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Sample EFD calculation:An example of a ramp policy is that the average ramp of

the system is equal to the forecasted ramp plus some constant, x [MW/min].

An example reserve policy might be that y% of the forecasted net load is held in reserves

For these policy formulations, the EFD surface is built as a function of x and y

Note that the x and y variables are optimized within the MIP-Based Unit Commitment problem

Expected Flexibility Deficiency Function

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Expected Flexibility Deficiency Function

The EFD surface is built as a function of ramping and reserve policies. These are optimized within the MIP-based Unit Commitment Scheduling problem

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Flexible Deficiencies

Computing Flexible Deficiencies using Historical Net Load Data

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Day-Ahead Market Formulation:Standard Constraint EquationsEquations are used to enforce all standard equality and

inequality constraints, such as: Energy Balance (generation = load) Unit output limits Spinning Reserve Requirements Regulation Reserve Requirements Ramp rate limits (units, hydro, imports) Unit temporal constraints (min up, min down, min run, …) Hydro, Imports, and Pumped Hydro Energy Limits

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Network Constraint Modelling

Optionally, the network constraints may be included in the simulation

Monte-Carlo dispatch model iterates with full power flow model (AC or DC) to enforce network constraints, including contingency constraints

Zonal model may also be used to enforce flow constraints

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Other Constraints & Flexibility Mitigation Strategies Modelling All relevant constraints are modeled (energy

balance, over-generation, curtailment limits, capacity, UC constraints, ramping rates, hydro, imports/exports, EFD constraints, etc.)

Relative cost penalties impose flexibility mitigation strategy “loading order”

Costs will depend on specific system and applicable policies

Assuming that all renewables must be delivered is equivalent to placing an infinite penalty on curtailment and over-generation

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Solution Methodology of the Flexibility Problem

Power FlowPower FlowPower FlowPower FlowOptimizationEngineOptimizationEngine Power FlowPower FlowPower FlowPower FlowPower FlowPower FlowPower FlowsPower Flows

Schedules

PTDFs

Loss marginal rates

Separate power flows for each time interval from the economics

Iterate with optimization engine

Execute modified Monte-Carlo simulations using minute-by-minute Solar PV data

The math here is very complicated

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We need an Modified Monte-Carlo Simulation

Unit Outages are simulated using random draws of

outages based on unit MTTF and MTTR

We need other profilesLoad profiles Wind profilesSolar profiles

They are all selected by Monte-Carlo draws from selected bins

Monte Carlo Simulation Modeling

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Example Draw: High Load Weekday in August

Day-Type Bins - WindDay-Type Bins - Load Day-Type Bins - SolarLow Load High

Load

WeekdaysWeekends/Holidays

JanFebMarAprMayJunJulAugSepOctNovDec

JanFebMarAprMayJunJulAugSepOctNovDec

Low Load High Load

JanFebMarAprMayJunJulAugSepOctNovDec

Low Load

High Load

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Example Draw: High Load Weekday in August

Within each bin, choose each (load, wind, and solar) daily profile randomly, and independent of other daily profiles

Wind BinLoad Bin Solar Bin

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Three Market Simulations Day ahead, hour ahead and real-time markets

are simulated sequentially

Load forecast inaccuracy of the day ahead market vs hour ahead is also simulated via Monte-Carlo draws

In hour ahead simulation only short start units may be committed

In real-time simulation, only units that were on-line in the HA market may be re-dispatched

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MIP Based Flexible SCUC Results Flexibility violations that may occur, because the

penalty cost of these violations is less than the commitment of additional resources

Optimal levels of reserves and ramp-rate capability based on ramp/reserve policy in each Power Market determined by the Regulator and policy makers

Economic “pre-curtailment” of Solar PV that avoids flexibility violations and/or commitment of excessive fast-ramping generation

Requirements for flexible capacity

Optimal Procurement decisions

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Solar PV Curtailment Could Play a Significant Role

Power FlowPower FlowPower FlowPower FlowOptimizationEngineOptimizationEngine Power FlowPower FlowPower FlowPower FlowPower FlowPower FlowPower FlowsPower Flows

Schedules

PTDFs

Loss marginal rates

Scheduled curtailment of Solar PV can help position conventional resources to meet ramping requirements How does the cost of curtailment compare to the cost of procuring new flexible resources?

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Proposed Metrics with High Solar Penetration Resource Adequacy metrics:

LOLP, LOLE, EENS

Flexibility Deficiency metrics: Expected Ramp Not Served (ERNS) Expected Regulation not Served, etc. Flexibility Shortage Induced Curtailment

How does the cost of curtailment compare to the cost of procuring new flexible resources?

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Demand Response: Power Markets in Pain

No Price-Sensitive Demand -> Inefficiency, Everyone Pays For

Price Marginal Wholesale Rate

…If we could use just 5% less power for the current hour….

Power Demand (MW)

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Demand Response: Energy Demand Cloud

Energy Demand CloudEnergy Demand Cloud

Individual WirelessControllers

Home/Facility Management

Systems

Smart Buildings,Commercial & Industrial

DR client is ~10kb—virtually any embedded device can run it

Special Programs

Reliability SignaledReliability Signaled

Price Sensitive

Renewable ChoiceDistributed Generation

Energy StorageElectric Vehicles

Electric VehicleChargers

Smart Appliances

Distributed Energy & Storage

Demand Monitoring & Feedback over Internet Broadband/CellularDemand Monitoring & Feedback over Internet Broadband/Cellular

Affinity Programs

TODAY FUTUREFUTURE

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Demand Response Software in Devices

Today - Retrofit Future - EmbedExternal Load ControllerOEM Products to Seed Market

Internal Load Controller

WiFi Router

INTERNETINTERNET

80% of US householdshave broadband (as of 2011*)

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Storage Technologies Storage is the game changer

Pumped Hydroelectric Storage is important but is highly site-constrained

Other technologies that have shown promise are a) Compressed Air Electric Storage (CAES), flywheels, hydrogen electrolysis

Plug-In-Electric Vehicles in Vehicle-to-Grid (V2G) mode (could serve as a major distributed storage resource)

Problem: What is the value proposition?

Develop incentives mechanisms to account for risk and reward sharing (need a regulatory framework)

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Transmission Capacity New transmission capacity is required

Implement technologies to permit increased utilization of the existing transmission infrastructure

Dynamic Thermal Rating

Power Flow Controls (FACTS devices)

Fault current controllers

Intelligent protection systems (adaptive relaying)

Advanced stochastic modeling and planning tools

Increased reliance on DC links

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Conclusions High penetration of RES creates major power market

challenges

The issues involved can be viewed as a coordination problem at multiple scales in both space and time

The problems are solvable but the solutions are neither trivial nor cheap

The infrastructure upgrade costs in the legacy power system and the public’s willingness to socialize these costs could emerge as an important issue

The power market response involves solutions including a) power market design changes, b) demand response, c) Storage technologies, d) PV curtailment, e) flexible market products

Grid Integration Challenges & Opportunities

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Solar PV Grid ChallengesGrid Stability & ReliabilityGrid Stability & Reliability

Msec to Minutes Years

Power Systems Planning & Design

Power Systems Planning & Design

Hours to Days

Load BalancingLoad Balancing

Milliseconds to Minutes

Grid Stability & ReliabilityGrid Stability & Reliability

CONCERNCONCERN

Develop smart inverters to mimic the intrinsically stable inertial behavior of a rotating machine

Develop Low-Voltage or Fault Ride Through Inverters

Develop plant controllers to react to grid frequency

LEARNINGLEARNING

The effects of large additions of Solar PV generation on the ac grid stability and system oscillations are not well understood

They could exacerbate pre-existing wide-area stability problems

Solar PV erodes the mechanical inertia

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Modern Solar PV Plants Need to Contribute to the Reliability to the Grid

Voltage, VAR control and/or power factor regulation

Fault Ride Through (FRT)

Real power control, ramping, and curtailment

Primary frequency regulation

Frequency droop response

Short circuit duty control

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Automatic active power frequency control: What is Needed

Potential Issue: The frequency must be kept constant within strict limits

What is needed:

A plant controller to react to grid frequency increase by an automatic active power reduction

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Dynamic Grid Support

Potential Issue: When a grid failure occurs many PV plants may be disconnected immediately

What is needed:

Inverters with dynamic grid support functions to act within milliseconds in such events

Devices with full LVRT or FRT behavior (Low-Voltage or Fault Ride Through) can feed reactive power into the grid during grid voltage drops

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What Makes a PV Plant “Grid Friendly”?

Potential Issue: When a grid failure occurs many PV plants may be disconnected immediately

What is needed:

Critical for Managing Grid Reliability & Stability

Regulates power factor and plant voltage/VAR controlsReactive Power Capability

Curtails active power when necessary Active Power Regulation

Limits the ramp rate from variations in irradianceRamp Rate Control

Prevents faults and other disturbances Ride Through Capability

Monitors, tracks, and reacts to changes in grid frequencyFrequency Droop Control

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Hours to Days

Load BalancingLoad Balancing

Solar PV Grid Challenges

Solar Generation is not fully dispatchable

Adds variability and uncertainty . . . complicates daily dispatch

Years

Power Systems Planning & Design

Power Systems Planning & DesignGrid Stability & ReliabilityGrid Stability & Reliability

Msec to Minutes Hours to Days

Load BalancingLoad Balancing

CONCERNCONCERN

Integrate forecasting into daily operation

Improved operating procedures – balancing area, frequent updates, ramping support

LEARNINGLEARNING

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Single Location

20 Bundled Locations

One-minute Global Irradiance (W/sq.m)

One-minute Global Irradiance (W/sq.m)

Aggregation Effect Between Plants Reduces Variability

Source: “Implications of Wide-Area Geographic Diversity for Short-Term Variability of Solar Power”; Andrew Mills and Ryan Wiser, Lawrence Berkeley National Laboratory, September 2010

Source: Hoff et al. 2008

Mathematically, if the short term ‐output time series of N locations experiencing a similar level of variability are uncorrelated (i.e., if they vary independently from each other), the resulting variability of the ensemble should be 1/√N times that of a single location

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Grid Integration Challenges & Opportunities: Key Lessons Learned

Grid Stability & ReliabilityGrid Stability & Reliability

Milliseconds to Minutes

Need grid controls and smart inverters to support reliability and grid security

Hours to Days

Load BalancingLoad Balancing

Integrate forecasting into daily operation

Improved operating procedures – balancing area, frequent updates, etc.

Need flexible capacity

Years

Power Systems Planning & DesignPower Systems Planning & Design

Adopt diverse resource portfolio to increase flexibility and reduce risks

Ned flexible capacity

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Protection The legacy protection equipment were no designed for

the presence of Solar PV (or for DG)

Solar PVs complicate protection coordination:

Reverse power flow: A fault must now be isolated not only from the substation power souce but also from the Solar PV

Fault current contribution: Until a fault is isolated, Solar PV contributed a fault current that must be modeled and managed

Relay desentitization: The presence of Solar PVs may delay of prevent the actuation of protective devices

Need transfer trip strategies to allow communication between devices

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Islanding Current practices do not allow power islands supported

by Solar PV (very conservative policy to ensure safety, power quality, cultural issues, etc.)

Current interconnection rules for Solar PV simply require that Solar PV shall disconnect in a specific time (e.g., 10 cycles) in response to disturbances

This conflicts with the ability of the Solar PV to offer benefits to the grid if it is equipped with LVRT

Our industry needs a new set of rules, regulations and procedures to allow solar PV to support power islands

There is substantial research now in smart grids and microgrids to allow effective coordination of Solar PV with storage devices and intelligent controls so that heterogeneous power quality is achieved

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Conclusions High penetration of RES creates grid integration

challenges

The issued involved can be viewed as a coordination problem at multiple scales in both space and time

The problems are solvable but the solutions are neither trivial nor cheap

The infrastructure upgrade costs in the legacy power system and the public’s willingness to socialize these costs could emerge as an important issue

The grid integration challenges can be resolved by deploying smart inverters that enable Voltage/VAR control , Fault Ride Through, Real Power Control, Ramping, and Curtailment, Primary Frequency Regulation, smart grid and microgrid concepts, etc.

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Thank you for your Attention

[email protected]