1

Lessons Learned from the Texas

Synchrophasor Network

by

Prof. Mack Grady and Mr. Moses Kai

U.T. Austin

Presented at the

Schweitzer Engineering Laboratories

Modern Solutions Power Systems Conference

Chicago

June 7, 2012

Why are Synchrophasors

Important?

Here’s All You Need to Remember

Average Power Flow P Through a Mostly Inductive

Transmission Line (138kV and above)

)sin( 2121

X

VVP

11V 22V

jX

+

+

ground

These are

about 1pu

For small x,

sin(x) = x

Charles Steinmetz,

developed phasor

theory to analyze

AC circuits

Stability limit is when the angle difference reaches 90 degrees

Does anybody

have a GPS

antenna?

2

Bus 2

DFW

Bus 4

SA

Bus 5

VAL

Bus 6

COAL

Bus 7

WIND

300 km

250 km

150 km300 km

400 km

500 km400 km

Bus 3

UT

Bus 1

HOU

Positive-sequence line constants for

each 345kV circuit:

• R = 0.06 Ω/km per conductor

• L = 1 µH/m

• C = 12 pF/m

• Rating = 800 A per conductor

• From the L, XL (500 km) = 188.5 Ω

• For 345 kV, 100 MVA base, Zbase =

1190 Ω

• For each 500 km circuit, XL = 0.158 pu

• Thus, four parallel circuits have XL =

0.040pu, and six parallel circuits have

XL = 0.026 pu

Relatively small phase angle

variation in central ERCOT

McDonald

Observatory

PMU

UT Austin

PMU

Bus 2

DFW

Bus 2

DFW

Bus 4

SA

Bus 4

SA

Bus 5

VAL

Bus 5

VAL

Bus 6

COAL

Bus 6

COAL

Bus 7

WIND

Bus 7

WIND

300 km

250 km

150 km300 km

400 km

500 km400 km

Bus 3

UT

Bus 3

UT

Bus 1

HOU

Bus 1

HOU

Positive-sequence line constants for

each 345kV circuit:

• R = 0.06 Ω/km per conductor

• L = 1 µH/m

• C = 12 pF/m

• Rating = 800 A per conductor

• From the L, XL (500 km) = 188.5 Ω

• For 345 kV, 100 MVA base, Zbase =

1190 Ω

• For each 500 km circuit, XL = 0.158 pu

• Thus, four parallel circuits have XL =

0.040pu, and six parallel circuits have

XL = 0.026 pu

Relatively small phase angle

variation in central ERCOT

McDonald

Observatory

PMU

UT Austin

PMU

U.T. Pan Am

PMU

Synchrophasor Homework Network for ERCOT

U.T. McDonald Observatory

PMU

360º

+60º −30º

Small oscillations in the relative phase angles give an indication grid stability.

Essentially, these oscillations provide an “EKG” for large power grids.

UT Pan Am

Austin

McDonald

Observatory

400 miles

300 miles

The Texas Synchrophasor Network Monitoring 120V Wall Outlet Voltages and Communicating Through the Public Internet

Austin

Majority of load is in the green

area, i.e. “Central ERCOT.”

Little phase angle difference

across Central ERCOT (i.e.,

“one solder blob”)

120V wall outlet at U.T.

Austin, and 69kV 3Φ

monitor at nearby Austin

Energy Harris Sub

Wind Country

• Funding provided by EPRI

• Equipment provided by

Schweitzer Engineering

Labs

Waco

Our PMUs send back

•Vrms,

•Voltage Angle,

•Frequency,

•DFDT

30 times per second.

3

370 miles

280 miles

100 miles

Jaime Ramos, U.T.

Pan Am, South Texas

oscillations

Rex Barrick, McDonald

Observatory, West Texas

wind oscillations

Andrew Mattei, Brazos

Electric Power Coop,

Waco, North-South

interchange indicator

Scott Bayer, Austin

Energy, 69kV grid

benchmark

6

Cloudcroft, NM, Phasor Measurement Unit (PMU). GPS antenna mounted on a

PVC pipe and Christmas tree stand. Two views of the GPS antenna.

LOCATION

Latitude Longitude Altitude

N 32.52.54 W 105.39.10 7583 feet

Satellite Signal Strengths (Min for Lock, 3 Sats > 3.0)

12.0 ; 7.0 ; 5.2 ; 6.0 ; 3.6 ; 5.2 ; 1.8 ; 15.2 ; -.- ; -.- ; -.- ; -.-

4

7

The Cloudcroft, NM, Station, Piggy-Backed onto Standard DSL Internet Service

“The Grid Monitor”

(a power strip)

Cloudcroft

GPS Synchronized

Clock

Spare Antenna

SEL 421 Relay/PMU

DSL Modem and Wireless Router

On the Receiving End,

The Schweitzer Synchrophasor Vector Processor, and PC

for Data Logging

and

• Schweitzer’s Modal Analyzer (uses 10-second intervals)

Plus some of my own programs,

• Offline Daily Screener for data checking and integrity

• Offline Waveform Analyzer to screen for events

• A web scraper program to fetch ERCOT 1-minute wind and

load data

• Plus a number of smaller programs for graphs and reports

5

Thur Fri

West Texas voltage

phase angle often

swings nearly 100º and

back with respect to

U.T. Austin in about 24

hours

Because wind

generation in West

Texas varies

dramatically

McDonald Observatory Voltage Phase Angle with respect to U.T. Austin

March 25, 26, 2010

-60

-40

-20

0

20

40

60

24 36 48 60 72

36 Hours Beginning 12am, March 24, 2010

De

gre

es

Wind Generation in ERCOT - MW

March 25, 26, 2010

0

1000

2000

3000

4000

5000

6000

24 36 48 60 72

MW

48

Lesson 1. Every Day Has

Synchrophasor Surprises

59.9

60.0

60.1

0 2 4 6 8 10 12 14 16 18 20

Tuesday, March 31, 2009, 20 Minute Window Beginning 12:53 AM CDT Superimposed Frequency Measurements Taken at

UT Austin (in black) and McDonald Observatory (in red)

20-minute window

Our First Nugget

with strange 0.067 Hz oscillation

Forwarded this to ERCOT chief. Got some attention! Turned out to be throttle valving problem

on generator under test. Slow frequency variation had gone unnoticed.

6

McDonald Observatory Voltage Phase Angle w.r.t U.T. Austin 120V and Harris 69kV

-14

-13

-12

-11

-10

1600 1700 1800 1900 2000 2100 2200

Sample (30 samples per second)

De

gre

es

w.r.t U.T. Austin

w.r.t Harris 69kV

10 seconds

• The fixed net multiple of 30 degree phase shift between U.T. Austin 120V and Harris 69kV has been

removed. The variable but steady 0.1 degree power flow phase shift through the substation

transformer has also been removed.

• Can you see any significant difference between the waveform seen by the 120V wall outlet (in black)

and the waveform seen by the 69kV, 3Φ grid monitor (in red)? (The correct answer is NO!)

Voltage Ringdown at McDonald Observatory Observed at the Following Two Locations in Austin: a

120V Wall Outlet on Campus, and the Harris 69kV Substation that Feeds the Campus

Lesson 2. 120V Wall Outlets Work for Synchrophasors

U.T. Pan Am Voltage Phase Angle w.r.t U.T. Austin 120V and Harris 69kV

-40

-39

-38

-37

-36

-35

-34

-33

-32

1600 1700 1800 1900 2000 2100 2200

Sample (30 samples per second)

De

gre

es

w.r.t U.T. Austin

w.r.t Harris 69kV

10 seconds

• The fixed net multiple of 30 degree phase shift between U.T. Austin 120V and Harris 69kV has been

removed. The variable but steady 0.1 degree power flow phase shift through the substation

transformer has also been removed.

• Can you see any significant difference between the waveform seen by the 120V wall outlet (in black)

and the waveform seen by the 69kV, 3Φ grid monitor (in red)? (The correct answer is NO!)

Voltage Ringdown at U.T. Pan Am Observed at the Following Two Locations in Austin:

a 120V Wall Outlet on Campus, and the Harris 69kV Substation that Feeds the Campus

Wall Outlets Work. Of Course, We Prefer Grid Monitors.

7

0.52% drop

0.45% drop

1.78% drop

1.07% drop

U.T. Austin

120V wall outlet

Austin Energy

Harris Sub 69kV

3Φ positive seq.

McDonald Observatory

120V wall outlet

U.T. Pan American

120V wall outlet

Vrms plot

Vrms plot

Vrms plot

Vrms plot

Wall Outlets Work. Of Course, We

Prefer Grid Monitors.

2-minute window of a switching

event. June 23, 2010. All four

windows are Vrms readings, 30

samples per second

120V wall outlet voltages are

clearly more noisy than 69kV grid,

but nevertheless 120V wall outlets

together with Schweitzer relays

provide reliable phase angle

measurements

cleaner

14

Start Sec Stop Sec A t1 B Tau1 t2 C Tau2 Wdamp 2010/10/04 18:18:00.000 8-2

15 30 -41.60 21.38 -45.65 0.19 21.37 4.28 1.95 3.82 PMUs = McDonald 1P

Fdamp Zeta Angle reference = Austin

0.608 0.134

UT Pan Am Voltage Angle with respect to U.T. Austin, 2010/10/04 18:18:00.000

-49

-48

-47

-46

-45

-44

-43

-42

-41

-40

15 20 25 30 35

Measured

Curve Fit

Healthy response to a unit trip

Lesson 3. Modal Analysis of Synchrophasor Data Allows Us to Compute Grid

Damping, for Big Events and for Ambient Data. An “Unhealthy” Ringdown Would

Continue Much Longer Than the “Healthy” Ringdown Shown Below.

8

Grady, Brooks, NREL, 1/27/11 15

• 1 hour of modal

analysis, 11am- Noon,

Jan. 22, 2011

• One calculation per 10

seconds, Shows the 3

most significant modes

• Calculations performed

by Schweitzer

Synchrophasor Vector

Processor

Main mode is 0.76

Hz. Damping ratio

ranges from about

0.3 to 0.1

0.1 is threshold of significance

rated

s

s

m

rated

ss

rated

s

Pdtd

P

P

J

P

J

H/2

1

2

12

1 2

dtdfP

fP

dtdf

f

P

P

dtdP

PH

srated

sm

s

s

rated

m

s

s

rated

m

/2/2

2

2/2

1

H has the units of seconds. The correct interpretation is that the kinetic

energy in the equivalent system machine corresponds to H seconds of rated

power. Thus, the machine could provide rated power for H seconds, at which

time it would have spun down to zero RPM.

EPRI Study. Purpose – to compute ERCOT System Inertia Constant H From

Frequency Response During 42 Unit Trips Having 0.1 Hz or Greater Freq.

Drop.

Lesson 4. Wind Generation Does Not Appear to Reduce System Inertia

(but no generator operating at max power can contribute to governor

response)

9

August 2010, Event 7, cont.

Event 7

Voltage

Phase

Angle Event 7, 8/24/2010 07:31:00 PM GMT Minute

59.82

59.84

59.86

59.88

59.90

59.92

59.94

59.96

59.98

60.00

56 57 58 59 60 61 62 63 64 65 66

Second of the Minute

Fre

qu

en

cy UT Austin

Harris 69

McD

UT PanAm

Inertia

slope

Nadir

slope

Zoom

Estimated System H versus % Wind Generation

(42 Unit Trip Events, June through November 2010)

0

2

4

6

8

10

12

0 5 10 15 20

Wind Generation - % of Total Generation

Esti

mate

d H

42 Major Unit Trips, 0.1 Hz or Greater. Any Correlation Between System

Inertia and Wind Generation (% of Total Gen)?

10

Estimated System H versus Wind Generation (MW)

(42 Unit Trip Events, June through November 2010)

0

2

4

6

8

10

12

0 1000 2000 3000 4000 5000 6000 7000

Wind Generation - MW

Esti

mate

d H

42 Major Unit Trips, 0.1 Hz or Greater. Any Correlation Between System

Inertia and Wind Generation (MW)?

Start Sec Stop Sec A t1 B Tau1 t2 C Tau2 Wdamp 2010/10/04 18:18:00.000 8-2

15 30 -41.60 21.38 -45.65 0.19 21.37 4.28 1.95 3.82 PMUs = McDonald 1P

Fdamp Zeta Angle reference = Austin

0.608 0.134

UT Pan Am Voltage Angle with respect to U.T. Austin, 2010/10/04 18:18:00.000

-49

-48

-47

-46

-45

-44

-43

-42

-41

-40

15 20 25 30 35

Measured

Curve Fit

Healthy response to a unit trip

Lesson 5. Wind Generation Does Not Appear to Impact System Damping or

Damped Resonant Frequency

11

Ringing Frequency vs Wind Generation (% of Total), Sep 2009 - Feb 2010

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10 12 14 16 18 20

Wind Generation (% of Total)

Fre

qu

en

cy (

Hz)

Ringdown Analysis of More Than 100 Unit Trips Yields No Clear

Relationship Between Wind MW and Damped Resonant Frequency

Does Wind

Generation

Impact Grid

Stability?

Damping Ratio vs Wind Generation (% of Total), Sep 2009 - Feb 2010

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16 18 20

Wind Generation (% of Total)

No

rmal

ize

d D

amp

ing

Rat

io

Ringdown Analysis of More Than 100 Unit Trips Yields No Clear Relationship

Between Wind MW and Normalized Damping Ratio

Does Wind

Generation

Impact Grid

Stability?

12

Bus 7

WIND

Bus 7

WIND500 km

Local conventional

Pload,conv − Pgen,conv

Pexport

Thevenin Impedance

jXTH

Central ERCOT

Vangle = 0

Vangle = δ

Texas Nodal Market Caused Grid Oscillation. We Used it to Compute the Thevenin

Equivalent Impedance from West Texas to Central ERCOT

)sin( 2121

X

VVP

Solve

Lesson 6. Synchrophasor Data Can Be Used to Compute Thevenin

Equivalent Impedances Between Grid Regions

McDonald Observatory Voltage Phase Angle Relative to

Central ERCOT

-20

-10

0

10

20

30

40

0 1 2 3 4 5 6 7 8 9 10 11 12

Hour of March 11, 2011

De

gre

es

ERCOT’s

graph of West-

to-North P flow

Texas

Synchrophasor

Network’s graph

of West-to-

Central ERCOT

Voltage Angle

Grid Oscillation

Induced by ERCOT

Nodal Market Price

Signals with Positive

Feedback

Price went down, then wind gens responded, then

price went up, then wind gens responded, • • •

13

ERCOT Wind Generation MW, Last Two Weeks of May 2012

0

1000

2000

3000

4000

5000

6000

7000

8000

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Day (Starts at Midnight CDT)

MW

ERCOT Wind Generation % of Total, Last Two Weeks of May 2012

0

5

10

15

20

25

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Day (Starts at Midnight CDT)

Pe

rce

nt

MW

Percent

ERCOT Wind Generation MW, Last Two Weeks of May 2012

0

1000

2000

3000

4000

5000

6000

7000

8000

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Day (Starts at Midnight CDT)

MW

McDonald Voltage Phase Angle, One Minute Averages, Last Two Weeks of May 2012

-60

-40

-20

0

20

40

60

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Day (Starts at Midnight CDT)

De

gre

es

MW

Degrees

LOCALPX

VVP )sin( 21

21

About 3000 MW

14

2 Hz

Cluster

March 18, 02:00 – 03:00

Wind Generation > 20%

March 12, 02:00 – 03:00

Wind Generation ≈ 2%

March 12, 02:00 – 03:00

Wind Generation ≈ 2%

Big Wind (20%) with 2 Hz Cluster Small Wind (2%) without 2 Hz Cluster

Lesson 7. A 2 Hz Mode in Ambient Oscillation Sometimes Forms with High

Wind Generation

1 Hour of Damping Plot. 1.8 Hz Mode, Frequent During Spring 2012.

Small. Starts and Stops. Wind Related?

1-Hour,

April 13,

2012

15

Perhaps More Troubling – Sometimes Have Very Low Damping in Main Mode

(but it doesn’t appear to be wind related)

1-Hour,

April 13,

2012

On 01/21/12, a sudden loss of generation occurred at 13:34 totaling 584

MW. Frequency declined to 59.752 HZ, ERCOT load was 31,275 MW.

59.7

59.8

59.9

60.0

60.1

0 1 2 3 4 5 6 7 8 9 10

Minutes

Lesson 8. It is amazing what the loss of 584 MW can do when there is little

reserve. More fallout from the Texas Nodal Market?

Frequencies this low used to be less than once per year. Now ???

16

Lesson 9. There is Still Much to Learn About Event Screening and Analysis

? Freq. gives no

direction clues

DC importing

tie trip?

22 sec

22 sec Pan Am

Waco

McD (stems off North TX)

Heave to support south

Freq. gives no

direction clues

17

Event 5. April 3, 2012. 07:08 GMT.

ERCOT Load = 28,020 MW,

Generation Tripped = 730 MW, Wind

Generation = 1,085 MW.

Freq. gives no

direction clues

2-minute window

10-seconds

Voltage angles

Heaves to support

North & West

Unit trip up north?

A 1-minute window of voltage phase angles. A

small ordinary event is picked up by the PMU

Waveform Analyzer.

Each curve has 1800 points (i.e., 30 points per

second)

A small event, May 29, 2012, probably a

small unit trip, serves as an example.

Not on anybody’s radar screen.

18

Zoom-in of the frequencies

Tiny

Zoom-in of the voltage

phase angles. Swings are

significantly large

Modest West

Texas Wind,

perhaps 50%

of max No significant

change in

steady-state

angles

19

Waco

UT Pan Am

McDonald

Harris 69kV

2.6 peak-peak

≈ 1 peak-peak

1.9 peak-peak

≈ 0.1 peak-peak w.r.t. wall

outlet shows that wall

outlet works well

All freq rings are about 00.03 Hz

peak-peak (tiny, fourth significant

digit), and have no direction

finding characteristics

Degrees wrt U.T. Austin Hz

Hz

Hz

Hz

Degrees wrt U.T. Austin

Degrees wrt U.T. Austin

Degrees wrt U.T. Austin

This plot has two curves – Harris

69kV sub, and UT Austin wall outlet,

identical

Initial angle

drops

Initial angle

drops to reduce

N-S flow

Initial angle rises

to send power

• A small unit trip in North (or West) Texas?

• Would need to also examine the Vrms – it

might be a switching event

• A switching event would diminish in

magnitude according to distance

20

370 miles

280 miles

100 miles

Jaime Ramos, U.T.

Pan Am, South Texas

oscillations

Rex Barrick, McDonald

Observatory, West Texas

wind oscillations

Andrew Mattei, Brazos

Electric Power Coop,

Waco, North-South

interchange indicator

• Idaho National Lab,

equipment going in July

2012, Scott McBride

• Colorado Springs, DOD

affiliate, David Fromme

• Los Alamos National Lab,

115kV sub, working out

firewall details, Mark Hinrichs

• Cloudcroft Cabin, equipment

operating, needs new coax

feed

• Nebraska NPPD, two units

operational, Don Veseth,

Daniel Brooks (EPRI)

• Texas Tech, working out

firewall details, Mike

Giesselmann

• Washington D.C. area,

DOD affiliate, operational,

Mike Bollen

More People are Joining. We’re Not Just Texas

Anymore, and May Need a New Name Soon

Scott Bayer, Austin

Energy, 69kV grid

benchmark

Nebraska Ringdowns Look About the Same as ERCOT

Feb 11, 2012. 17:39 GMT

21

Questions?

Thank you!