IV.A Recent Planning

Studies

Shun Hsien (Fred) Huang

February 24, 2020

ERCOT GTC Workshop

Public

Most Common Stability Challenges

2

2018 LTSA Stability Assessment

Long Distance

Large Power

Transfer

Weak Grid

Public

Long Distance Large Power Transfers

3

1900.0

300.0

2BUS 2

0.9316.4

SW

-336.4

3BUS 3

1.0355.3

-900.0

36.4

927.1

358.8

463.6

179.4R

463.6

179.4RA Typical 345 kV

Circuit in West Texas

Significant reactive

losses and system

reactive need.

Continue adding

reactive support

may not be the best

solution.

300 600 900

Line Q Losses -60 80 395

Angle Separation 8.5 17 29

0

5

10

15

20

25

30

35

-100

0

100

200

300

400

500

An

gle

Sep

arat

ion

(d

egr

ee)

Lin

e Q

Lo

ss (

MV

AR

)

Line Flow (MW)

Large angle

separation limits the

transfer capability.

Public

Long Distance Large Power Transfers

4

Generator speed (Hz)

Time (sec)0.00 2.00 4.00 6.00 8.00 10.00

60.00

60.40

60.80

61.20

61.60

62.00

Syn. Gen

Syn. Gen

Angular instability Inter-area Oscillation

Robust exciters

improve the angular

instability.

Power system

stabilizers improve

the damping.

Public

Weak Grid

5

PMU measurement of a wind

plant connect to a weak grid in

West Texas.

Fault

location

Broader and larger voltage dips during

and after faults. Challenges for voltage

recovery.

Public

Weak Grid

6

Steady State Voltage Instability Dynamic Voltage Instability

Voltage Collapse

point above 0.95 pu

Tools/Models Adequacy?

Public 7

• Algebraic equation with no time step

• Solved in < 1 sec

• 60 Hz, steady state

• Tools: PSS/e, Powerworld, PSAT

• Parameters: ~tens• Differential equation with time step of 1 ~ 4 ms

• Simulation finished in 1 ~ 10 mins

• Tools: PSS/e, Powerworld, TSAT

• Parameters: tens ~ hundreds

• Differential equation with time step of

10 ~ 50 us

• Simulation finished in 10 mins ~

hours

• Tools: PSCAD

• Parameters: tens ~ hundreds

Steady StateDynamic

(Electromechanic)Transient

(Electmagnetic Transient)

12

00

21

BB

SE

S_

UN

IT1

1.1

19

.3

L1

27

0.0

23

5.6

R

-26

9.7

-21

4.2

1

1

27

0.0

23

5.6

12

00

22

BB

SE

S_

UN

IT2

1.1

19

.2

AL

0.0

0.0

27

0.0

23

5.6

R

33

80

BIG

BR

N_

_5

1.0

35

6.0

31

33

RIC

HL

ND

2_

5

1.0

35

6.3

31

34

RIC

HL

ND

1_

5

1.0

35

6.0

29

3.0

-12

.7

-29

2.7

8.3

All the GTCs are identified in the dynamic studies.

Accurate and good quality models are critical.

Public

Recent Planning Studies

• 2018 Panhandle and South Texas Stability and System

Strength Assessment

• 2018 Dynamic Stability Assessment of High Penetration of

Renewable Generation in the ERCOT Grid

• 2019 Panhandle Regional Stability Study

• 2020 Panhandle Regional Stability Study (ongoing)

• 2020 Dynamic Stability Assessment of High Penetration of

IBRs in West Texas (ongoing)

• 2019 RTP GTC Exit Alternatives Evaluation

8

Public

Panhandle Studies

9

4300

55365182 5223

126

1356

3174

4992

0

1000

2000

3000

4000

5000

6000

2016 Study 2018 Study 2019 Study 2020 Study

Gen

erat

ion

Cap

acit

y (M

W)

IBRs Capacity (MW) Met PG6.9

Panhandle IBRs (MW) Nearby Panhandle IBRs (MW)

Inverter-Based Resource (IBR): A Resource that is connected to the

ERCOT System either completely or

partially through power electronic

converter interface. For example, wind,

solar PV, and battery.

Public

Nearby

Panhandle WGRs

5182

MW

SCs

ERCOT

Grid

3174

MW

• Identified stability limitations in Panhandle

– Oscillatory/angular stability in normal operation (no planned outages)

– Voltage stability under planned outage condition (modified thresholds)

• Key takeaways:

– Nearby Wind Generation Resources (WGRs) provide voltage support

along transfer path

– Nearby WGRs drive larger angles in Panhandle

– Lubbock integration improves the stability issues

10

2019 Panhandle Study

Panhandle WGRs

Public

Aggregated MW in

Panhandle/Nearby Panhandle

MW of a synchronous

generator in the Coast region5MW

~180MW

Out ofPhase

Observed Oscillation

• Oscillatory responses are observed during high power

transfer

• Synchronous condensers identified as primary participant

11

Public

2018 Study: High Penetration of IBRs

• Based on studied 2031 LTSA scenario

• ~70% Penetration of Inverter-Based

Resources (IBRs, like wind and solar)

• Significant active and reactive power

losses

• IBR controls require sufficient system

strength for reliable operation or more

robust inverter control capability is

required, grid forming (?)

• Synchronous condensers are subject to

synchronous machine instabilities (inter

& intra area oscillations & angular

instability)

• Additional Transfer Paths between West

Texas and Central Texas Were

Beneficial

12

Public

2020 Study: High Penetration of IBRs

• 2022 DWG HWLL case with the inclusion of planned IBRs

(met PG 6.9) in West Texas.

• Average IBR Dispatch: 83%

• IBR Penetration: ~58% (historical penetration record)

13

Area IBR Capacity (MW) IBR Output (MW)

West Texas 2,4373 20,166

South Texas 6,841 5,615

Total 31,214 25,781

Public

2019 Regional Transmission Plan (RTP)

• The 2019 Regional Transmission Plan

(RTP) economic analysis enforced the Lobo

to North Edinburg GTC (N-1 conditions)

• The Panhandle GTC was not enforced in

the 2019 RTP based on the results from

updated Panhandle stability studies

– These updates were presented at the August

2019 ROS meeting

14

Public

2019 Regional Transmission Plan (RTP)

• The Lobo to North Edinburg GTC was modeled and

enforced in the 2019 RTP economic analysis

• The interface did not experience enough congestion to

justify the multiple 345-kV system improvements that make

up its GTC exit alternative

15

Study YearCongestion

Rent ($M)

% of Hours

Congested

2021 3.8 1.6

2024 6.7 2.4

Public

Ongoing Evaluation of GTC Exit Alternatives

• As appropriate, ERCOT will continue to evaluate GTC exit

alternatives against the economic planning criteria during

the RTP process

• Based on recent stability studies, ERCOT expects that

more GTCs will be modeled in the 2020 RTP

• ERCOT will consider the viability and usefulness of

processes for the economic review of GTC exit alternatives

outside of the RTP

16

Public

Takeaway: Evolving stability challenges

• Example

17

PanhandleNearby

PanhandleWest Texas

Export

Synchronous Generators

Wind Plants PV Plants Battery/DER…

Location and technology are important

Public

Takeaway: Dynamic model availability, accuracy,

and quality are critical

18

Dynamic Models

Manufactures/Consul

tants

Resource Entities/Developers

Utilities/

System Operators

Stability Studies

Typical time for IBRs from

initial planning to physical

interconnection

18-24

MonthsNumber of IBRs

currently planned

beyond 2022

0

2.6

1.61.1

2.6 2.8

1.3

0

0.5

1

1.5

2

2.5

3

2017 2018 2019T

ime D

iffe

rence (

Year)

Year of Meeting Modeling Requirements

Wind Solar

Average duration of planned projects

between meeting modeling requirements and

projected commercial operation date

Dynamic models can be available only ~8 months prior to COD

Public

Takeaway: Mitigation Considerations

• Better stability management

• Dynamic performance review and improvement

• Better reliability support

– damping support, robust control under weak grid,…

• Grid enhancement

– AC circuit, DC circuit, reactive support, synchronous condenser,

FACTs,…

19