integration of renewable energy resources · 2020. 6. 17. · •typical range of stability...

© 2020 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m

Aidan Tuohy, [email protected] Manager, Bulk System Renewables/DER Integration ProgramSouth Africa Institute Of Electrical EngineersJune 16, 2020

Integration of Renewable

Energy ResourcesEPRI Program 173: Bulk System

Renewable/Distributed Resource Integration

http://www.epri.com/

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© 2020 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m2

Electric Power Research Institute (EPRI)

▪ Founded in 1972 as an independent, non-profit center for public interest energy and environmental research

▪ Collaborative resource for the electricity

sector

– $425M annual R&D funding in 2018

– 1,000+ members in more than 30 countries

– International is ~30% of funding

Independent

Collaborative

Nonprofit



Market Operations & Design

Long-Term Planning Real-Time Operations

Resource

Adequacy

Transmission

Planning

Operations

Planning

Operations

Scheduling

Real-Time

Operations

System Protection

EPRI Transmission Ops & Planning R&D Program Strategy



Impacts of Variable and Distributed Energy Resources

Variability/Uncertainty

▪ Output varies across seconds, minutes, hours, days, weeks

▪ Some correlation between resources

▪ Not perfectly predictable or dispatchable

▪ Zero marginal costs

Location

▪ Connected to grid through power electronics

▪ Displaces traditional sources of inertia, active/reactive support, short cct, etc.

▪ Can be controlled to provide quick responses and various services

▪ Can be far from load and require additional transmission and system strength

▪ Can be distributed and provide visibility/controllability issues

▪ Often used to provide multiple services at dist level

Inverter Based

New models, methods and tools have been and are being developed to manage these issues



Generic wind/PV/storage models and

incl 100% inverter based and weak grid

Bulk System Renewables Integration – Key Activities

System Flexibility and Resource Adequacy

Assessment

Slip rings

Gearbox

Grid

Stator power

Rotor-Side

Converter

Grid-Side

Converter

Rotor power

Step down

transformer

Wind

turbine

Crow-bar

Vdc igir

IL , PL

Modeling: DER in Transmission Planning

and Transmission Hosting CapacityMarket Operations and Design



Bulk System Renewables Integration – Key Activities

Forecasting: Distributed PV/Load Forecasting

and Advanced Solar Forecasting w/Sky Imaging

Frequency Response and

Reserve Determination Tools

Variability/Uncertainty: Advanced Production

Cost Modeling and Reserve Requirements

Risk Based Transmission Planning

7,2

50

8,0

00

8,7

50

9,5

00

10,2

50

11,0

00

11,7

50

12,5

00

13,2

50

14,0

00

14,7

50

15,5

00

16,2

50

17,0

00

17,7

50

18,5

00

19,2

50

20,0

00

20,7

50

21,5

00

1

10

100

1000

0

800

1,600

2,400

3,200

4,000

4,800

5,600

6,400

Region Load (MW)

Frequency of Occurence

Renewable Output (MW)

100-1000

10-100

1-10

HILF

Events

Average

System

Conditions

HILF

Events


© 2020 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m77 . 7 . 7 .

Renewables

instantaneous

penetration

records in

North America

April 2019: 78%

Jan. 2020: 57.9%

April. 2020: 73.2%

Dec. 2018: 25%

Feb. 2019: 14.65%

March 2019: 10%

2019: 94% energy carbon-free resources



Checklist/curve for experiences

Interconnection reqts

Models (pos seq, generic)

Forecasting

Reactive and active control

Reserves/flexibility

Coordination across regions

‘Duck curve’

Inertia/frequency

Weak grid

Stability assessment

Seasonal storage

Grid forming

Demand side

Zero to low

Low (few % annual)

Medium (low 10s % annual)

High (instantaneous >50%)

Very high (40%+ annual)

Very few left Many regions ERCOT, SPP, NGUK Denmark, Hawaii islands, etc.

Each system will be somewhat different, general trends in

challenges

Need to plan for these issues well before we reach ops



Relative Reliability Contributions for Various Resources

• Must Ensure Reliability

when considering new

Resource Mix

• Not all Resources equal

in Reliability Capability

• Synchronous resources

broader & deeper ability

to support reliability

• Reliability is not only

consideration: Diversity,

Economics, Emissions,

and others…

EPRI whitepaper (2015):

Contributions of Supply & Demand

Resources to Required System

Reliability Services (3002006400)

Update coming soon



Modeling, Planning and Protection



Planning/Protection Work Streams

Renewables ProtectionDER/Load Models Transmission Hosting Cap.

Renewables Weak Grid Model Grid Strength AssessmentRenewables Dynamic Models

M

M

M

Electronic

UVLS

69 kV115 kV138 kV230 kV

UFLS

1-F AC

Static

R-DER

U-DER

Composite Load Model

12.5 kV13.8 kV

Slip rings

Gearbox

Grid

Stator power

Rotor-Side

Converter

Grid-Side

Converter

Rotor power

Step down

transformer

Wind

turbine

Crow-bar

Vdc igir

IL , PL

173.09173.03



Modeling Challenges – Transmission Connected

Renewables

PV

To

system m

od

el

R, X, B Equivalent Feeder Model

EquivalentGenerator

Step-up Transformer

SubstationTransformer

SubstationMSCs

Reality – many PV array inverters connected through a collector system

Simulation – simplified equivalent representation.Generic models are commonly used for simulation



RENEWABLE ENERGY SYSTEM MODELS

• The most recent publicly available renewable energy system (RES) models are

the 2nd generation generic models developed through the Western Electric

Coordinating Council (WECC) Renewable Energy Modeling Task Force

(REMTF) effort

• They allow for modeling:

• Wind Turbine Generators (WTG)

• Photovoltaic (PV) Generation

• Battery Energy Storage Systems (BESS)

• Complex plants RES plants

• IEC TC88 WG27 standard models are very similar, but slightly more detailed;

the plant-level models not yet completed



VALUES OF GENERIC MODELS

• Validation: numerous validation cases demonstrated

• Portability across software platforms: implemented and tested in major

commercial tools, and consistent across the tools

• Transparency & Documentation: standard, generic, public and open

with documentation/specifications that are available to all

• Publicly Available: avoid this issue of being able to share models.

• Modeling the Future: generic models are useful for performing futuristic

studies where the actual equipment to be used is not yet known



LIMITATION OF GENERIC MODELS• Positive-sequence: cannot be used for accurate assessment of unbalanced conditions.

• Typical range of stability studies: consider dynamics in the typical range of stability

studies (0.1 – 3 Hz); all other models are only truly good within this range of frequencies.

The control loops within the models (e.g. closed-loop voltage control) may also consider

frequencies ranging up to 10 Hz.

• Constant wind speed/ solar irradiation: the generic models assume that wind speed

(and solar irradiation) is constant during a stability simulation.

• Weak Systems: not intended for detailed local studies associated with control tuning and

design for the interconnection of wind/PV plants to very weak systems (i.e. short-circuit

ratios below approximately 2 or 3)

• Specialized Studies: The models cannot be used for detailed studies that relate to

phenomena such as potential torsional interactions between the wind turbine generator

and the electrical power system (e.g. nearby series capacitor).



2nd Generation Renewable Energy System (RES) Models

Generator

Id

Iq

Vt

Drive-Train

wt1g

wt1t

Pitch-Control

wt1p_b

w

Pm

Generator

Id

Iq

Vt

Drive-Train

wt2g

wt1t

Pitch-Control

wt1p_b

w

Pm

External

Resistor

Control

wt2e

Rext

Generator/

Converter

Model

Iq

Ip

Iqcmd

Ipcmd

Current

Limit

Logic

Vt

Pqflag

= 1 (P priority)

= 0 (Q priority)

Q Control

P Control

Iqcmd’

Ipcmd’

Qref

(or Qext)

Qgen

Pref

(or PExt)

Drive-Train

spd

reec_a

regc_a

wtgt_a

Pord

Torque

Control

wtgtrq_a

Pe

Pref0

Pitch-Controlwtgpt_a

wref

qAero

wtgar_a Pm

Plant Level Control

repc_a

Vref/Vreg or

Qref/Qgen

At plant levelFreq_ref/Freq and

Plant_pref/Pgen

Generator/

Converter

Model

Iq

Ip

Iqcmd

Ipcmd

Current

Limit

Logic

Vt

Pqflag

= 1 (P priority)

= 0 (Q priority)

Q Control

P Control

Iqcmd’

Ipcmd’

Qref

(or Qext)

Qgen

Pref

(or TExt)

Plant Level Control

reec_a

regc_a

repc_a

Vref/Vreg or

Qref/Qgen


Plant_pref/Pgen

Generator/

Converter

Model

Iq

Ip

Iqcmd

Ipcmd

Current

Limit

Logic

Vt

Pqflag

= 1 (P priority)

= 0 (Q priority)

Q Control

P Control

Iqcmd’

Ipcmd’

Qref

(or Qext)

Qgen

Pref

(or TExt)

Plant Level Control

reec_b

regc_a

repc_a

Vref/Vreg or

Qref/Qgen


Plant_pref/Pgen

WTG Type 1 WTG Type 2

WTG Type 3

WTG Type 4

PV

https://www.epri.com/research/prod

ucts/000000003002006525

Model User Guide for Generic

Renewable Energy System Models

•Latest renewable energy

models available in

commercial platforms for

dynamic studies (positive

sequence)


https://www.epri.com/research/products/000000003002006525


Wind Inertial-Based Fast-Frequency Response Model

(WTGIBFFR_A) – Technical Specifications

▪WTGs can provide “emulated” inertial response using auxiliary controls without maintaining any reserve (add-on feature)

▪Auxiliary Control Model in 2nd generation RES models

▪Model technical specifications initially proposed in 2018 (PID: 3002013641)

▪2019 Work: –Finalize model specifications –Develop prototype model for testing

–Engagement with vendors through WECC REMTF for model implementation commercial platforms

–Pursue validation using field measurements



Inverters and Weak Grids

▪Wind/Solar interconnections in weak systems are becomingmore common

▪Contingencies or scheduled outages (e.g. lines undermaintenance) may reduce system strength below typical levelsexpected under normal operating conditions.

▪Additionally, if such a network situation coincides withramping up of the plant, the location can become even weaker

▪Converter controller instabilities might occur under weak gridconditions

Source: “Deploying Utility-Scale PV Power Plants in Weak Grids”, First Solar, 2017

PES General Meeting, Chicago, IL, July 2017

Source: “Integrating Variable Energy Resources into Weak Power Systems”, NERC, June 2017

▪ No clear industry standard on metrics & associated thresholds to identify a weak area of the system

▪ Low SCR doesn’t necessarily imply converter control instability



Description of the Grid Strength Assessment Tool…

Value

Provide insights on

• Network locations/conditions which could be susceptible to issues related to weak grid conditions

• The need for detailed studies

Evaluates

• Generic SCR

• Weighted SCR

• Composite SCR

• Advanced EPRI metric

Compatibility

• Siemens PTI PSS®E v33 and v34

• GE-PSLF™ v19 and v21

• DIgSILENT PowerFactory 2017 SP7, 2018 (SP1, SP5, SP6), 2019 (SP1, SP2, SP3)

▪ EPRI’s advanced metric

– Completely analytical, and no requirement of a dynamic run

– Uses few dynamic data values (e.g. controller gains, time constants) of the inverter-based resource model to identify potential converter instability

– Is expressed as critical clearing time before converter instability

▪ Critical clearing time is from the perspective of converter controller and NOT of a synchronous machine.

The advanced metric can be used along with clearing time of existing protection schemes to determine system stability



Generic Model REGC_C

▪Representation of PLL and inner current control loop

▪Suggested to be used for weak grid studies before conducting detailed three-phase EMT studies

▪Model approved by WECC REMTF

REGC_C parameter values can be used as inputs for GSAT Advanced Metric Calculation



Inverter Based Resource Integration



Relatively high average penetrations can result in very high

instantaneous

Need to be able to manage ramping/variability, but also issues like frequency and voltage

Source: Drake Bartlett, PSCO, 2018



Inverter Interface Resource Considerations

System Protection

• Reduced short circuit

• Different fault contribution

Transmission Reliability

• Displaced inertia/PFR

• Inverter controls/capability

• Dynamic behavior

-- disturbance response

-- voltage/freq ride-through

Transmission Planning

• Validated dynamic models

• Modeling DER in Trans Plan

E, Ela et al., Active Power Control from Wind Power: Bridging the Gaps, NREL Technical Report, December 2013.

Load Shedding!

More realistic Governor Participation

60%

50%

40%



Concept of System Non Synchronous Penetration

▪ Ireland’s All Island Grid Study in 2008/2009 showed significant penetration levels could be technically possible from transmission planning and production cost perspective

▪ Further studies commissioned by TSOs to study stability issues – Facilitation of Renewables study (2010)

▪ Report concluded that that the two key issues that may limit increased instantaneous penetration

– Frequency stability after the loss of the largest infeed

– Frequency and transient stability after a severe network fault

▪ Developed System Non Synchronous Penetration metric

𝑆𝑁𝑆𝑃 =𝑊𝑖𝑛𝑑(𝑜𝑟 𝑜𝑡ℎ𝑒𝑟 𝑖𝑛𝑣𝑒𝑟𝑡𝑒𝑟 𝑏𝑎𝑠𝑒𝑑) + 𝐼𝑚𝑝𝑜𝑟𝑡𝑠

𝐷𝑒𝑚𝑎𝑛𝑑 + 𝐸𝑥𝑝𝑜𝑟𝑡𝑠

Originally limited to 50%, now at 65%



Interaction Between Synchronous Inertia, FFR, FCR, RR

In Ireland, new inertial response measures have been added to EIRGrid’s reserve planning scheme, as part of the coordinated plan to increase the penetration of renewable resources.

EirGrid: Frequency Response System ServicesSIR: Synchronous Inertial ResponseFFR: Fast Frequency ResponsePOR: Primary Operating ReservesSOR: Secondary Operating ReservesTOR1: Tertiary Operating Reserves, Level 1TOR2: Tertiary Operating Reserve, Level 2RR: Replacement ReserveRamping: Ramping Margin



ERCOT – recent developments with high penetrations of non-

synchronous resources

▪ Single synchronous system covering most of Texas

▪ Peak demand of 70 GW, minimum 25 GW

▪ Over 20 GW of wind and 1 GW of utility scale solar by end 2017

▪ Significant buildout of new transmission to West Texas reduced wind curtailment and improved integration of renewables

▪ Inertia is increasingly a concern and part of planning and operations activities



Trends in inertia in recent years

▪ ERCOT examined actual inertia online over past few years

▪ Clear downwards trend

▪ Wind seems to be part of the trend, with higher wind resulting in lower inertia

▪ Online nuclear units, responsive reserves and private networks provide approximately 78 GW.s so may reach situations in future where additional generators need to be brought online



ERCOT

• Inertia floor of 100 GWs

• Online monitoring

• Proposed ancillary service market redesign, SIR, FFR, PFR Ireland

• Inertia floor 23 GWs

• Minimum unit constraint

• RoCoF upgrade project to 1 Hz/s

• Ancillary services market redesign with SIR

• Auction-based market for SIR

NG UK


• RoCoF upgrade project to 1 Hz/s

• Ancillary service market redesign with EFR similar to FFR (storage)

Australia

• Inertia floor of 6.2 GWs

• Minimum unit constraint

• Online monitoring, inertia, and stability

NORDIC


• Online monitoring

• Redispatch design contingency nuclear unit for inertia constraint

Worldwide Systems with Inertia Constraint Characteristics

Systems with

Inertia Constraint

Recent public reportsEPRI IDs: 3002015131, 3002015132

https://youtu.be/ZCa2LHxq9C8


https://youtu.be/ZCa2LHxq9C8


Variability and Uncertainty Issues



Variability and Uncertainty Considerations

Scheduling & Dispatch

• Increase operating reserve

• Masking of load (DER)

Resource Adequacy

• Planning reserve margin

• Dispatchable Gen revenue

• Operational flexibility

Ops Planning & Real-Time

• Outage scheduling

• Changing flows & SOLs

Transmission Planning

• Which power flow cases?



The “Duck Curve” is Real – and all over the world

Pumped hydro and exports very helpful in managing rampFlexibility from conventional generation would be exhausted, relying on others



Operating Reserves and Other Essential Reliability

Services

EPRI developing tools for ensuring sufficient reserves, while minimizing costs

*Inertia is not a reserve but part of the instantaneous event correction process.

Planning Reserve

ICAPFlexible Capacity

Volt/Reactive Control Reserve

Static Dynamic

Black Start

Restoration

Short Circuit

Contribution

Op

era

tin

g R

eserv

e

Event

Contingency Reserve

Inertia*

Fast Freq. Response

Primary

Secondary

Tertiary

Ramping Reserve

Secondary

Tertiary

Non-Event

Flexibility Reserve

Regulating Reserve

Instantaneous events (e.g., contingencies)

Reduce ROCOF

Stabilize frequency

Return frequency to

nominal and/or ACE to zero

Bring back to N-1 secure

stateLonger duration events

Return frequency to

nominal and/or ACE to zero

Bring back to secure state

Correct the anticipated ACE

Manual (part of optimal dispatch)

Correct the current ACE

Automatic (within optimal dispatch)

Reduce nadir, avoid UFLS

Recent project in Hawaii showed

significant reduction in costs when

optimizing reserves ($20m/year)

https://eprijournal.com/a-win-win-for-grid-

operators-and-customers/


https://eprijournal.com/a-win-win-for-grid-operators-and-customers/


Traditional "Generic Capacity" Metrics New "Flexible Capacity" Metrics

What “Type” of Capacity is Needed?

Resource adequacy methods and tools need to be adopted

LOLEGENERIC-CAPACITY

Traditional metric to capture events that occur due to

capacity shortfalls in peak conditions

LOLEMULTI-HOUR

New metric to capture events due to system ramping

deficiencies of longer than one hour in duration

LOLEINTRA-HOUR

New metric to capture events due to system ramping

deficiencies inside a single hour

20,000

30,000

40,000

50,000

1 5 9 13 17 21

41,500

42,500

43,500

44,500

45,500

10:00 10:10 10:20 10:30 10:40 10:50 11:00

0

10,000

20,000

30,000

40,000

50,000

60,000

1 3 5 7 9 11 13 15 17 19 21 23

MW

Hours

Load Generation

Source: California Public Utility

Commission Workshop



Flexibility Will Become More Valuable

EPRI working on flexibility tools and metrics to assess long term resource adequacy impacts

▪ Increasing variability and uncertainty will require flexibility on all time scales and at different spatial scales

▪ Different resources may contribute– DER, storage and inverter

based resources may provide some of the needed flexibility services

– Retrofits and altered operational practices

▪ Wind/PV flexibility (with or without storage) increasingly important

More Information:

Metrics for Quantifying Flexibility in Power

System Planning, 3002004243, 2014 (EPRI)



Recent week in Ireland – up to 65% penetration

Forecasts, control of renewables and flexible system all important

0%

10%

20%

30%

40%

50%

60%

70%

0

1000

2000

3000

4000

5000

6000

5/31/2020 6/1/2020 6/2/2020 6/3/2020 6/4/2020 6/5/2020 6/6/2020 6/7/2020

Inst

anta

neo

us

Pen

etra

tio

n

Dem

and

/Gen

erat

ion

/Win

d (

MW

)

Date

ACTUAL DEMAND(MW) FORECAST WIND(MW) ACTUAL WIND(MW)

ACTUAL GENERATION(MW) Instantaneous Data from Eirgrid.com

Difference between generation and

demand is import/export

Limited renewables to 65% of generation

(and curtailed when over this, resulting in

forecast error)

Average penetration of 35% of generation,

40% of demand



Integration can Improve Reliability, Increase Efficiency, Create

New Opportunities, and Expand Customer Choice (ien.epri.com)



Together…Shaping the Future of Electricity


integration of renewable energy resources · 2020. 6. 17. · •typical range of stability...

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