Post on 21-Aug-2020
Cascade-Based Planning Analysis
Robert W. Cummings
NERC Senior Director of Engineering and Reliability Initiatives
1
WHAT SHOULD BE STUDIED
2
3
Tenets of Cascading Analysis – Why do it?
Learn from History
• What is a credible combinations of events?
• What should I think might get involved?
• Be imaginative – Murphy is!
Be a Student of the System!!
• Constantly observe how your system behaves normally and when under stress
• How does your system interact with the rest of the Interconnection?
• Practice simulating actual events – Good for model validation
– Helps avoid complacency
4
Some Trends in Event Analysis
1. Protection system misoperations (39)
2. Unexpected Gen. Turbine Control Action (33)
3. Transmission equipment failures (18) (most initiating)
4. Voltage sensitivity of gen. aux. power systems (13)
5. Human Error (12)
6. Near-term load forecasting error (6)
7. Wiring errors (incidental) (5)
8. Relay loadability (5)
9. Inter-area oscillations (5)
10. SPS & RAS Misoperations (5)
5
Two Top Disturbance Elements • 33 – Unexpected generator turbine control actions
– 29 in 8 events
– 2 units CAUSAL in a system separation event
• 13 – Voltage and/or frequency sensitivity of generation auxiliary power systems (not included above) or plant / Unit Digital Control Systems
• Problem: THESE BEHAVIORS ARE NOT MODELED OR STUDIED – Boiler and Turbine controls are not modeled
– Understand roll of controls action – Power-Load Unbalance (PLU)
– Typical dynamic analysis – only analyzing t = 0 to t = 20 seconds – control actions can go well beyond that
– What about fuel system controls?
6
Planning a Cascading Study Be imaginative!
• You may have to design study methodology to mimic past disturbances to see if the system is still susceptible
• Think in bus-breaker mode – Bus-line model thinking will not get you there
– Break-to-breaker study needed to analyze protection system misoperations
• Know your protection system configurations – Directional distance, reach, differentials, breaker failure – local or
remote, Remedial Action Schemes, transfer tripping, etc.
• Expect the analysis to piece-wise – slices of time to mimic the developing overloads or potential voltage collapses
7
Planning a Cascading Study (cont.) • What parameters do you have to monitor within the study to
tell you what would be the next step of the cascade? – Look for overloaded lines – How overloaded are they?
– Voltages below 0.9 per unit – How low?
– Results of one run flow into another
– Reactive flow into both ends of a line?
Think about controls!!
• Inverter controls play a part too!
• Behavior during protracted faults – breaker failure (to operate) emulation
– SLG faults transition to multi-phase in 20-30 cycles
– Longer faults evolve into 3-phase faults
8
Planning a Cascading Study (cont.) • Protracted faults mean protracted low voltages
– UVLS schemes?
• Inverter behavior during low voltages – Inverter blocking – current injection?
– Inverter tripping?
• Know your loads!
– Know your station load composition – residential, commercial, industrial, etc. • Helps characterize load behavior under abnormal conditions
– Loads with high-quality power requirements may “leave the system”
– Voltage-sensitive or frequency sensitive loads must be monitored during the study
9
Study Practices Cardinal Rule of forensic event analysis – Everything happens for a reason!!
• There is no “sympathy in the power system”
• Think like a protection system or plant control system – What the heck did it see?? Why did it react the way it did?
– Take advantage of all available disturbance monitoring data (PRC-002)
• Leave no questions unanswered
Don’t be myopic in the scope of your analysis!
• Examine the Interconnection to ensure there were no wider area impacts
Timing is everything!
• Creating a detailed sequence of events is crucial!
10
Study Practices (Continued) Cascading typically starts slow and progresses as multiple things go wrong
Know the cascade players
• Triggering event – lightning arrestor failure
• Causal events – Breaker fails to operate – protection system failure
• Contributory events – Protection system miscoordination widens impact
• Coincidental events – It was a dark and stormy night
• Resultant events – fault progressed from single-phase to multi-phase
EXAMPLE 1 NORTHEAST BLACKOUT
AUGUST 14, 2003
11
2003 Blackout Signature
12
-2000
-1000
0
1000
2000
3000
4000
16:10:38 16:10:40 16:10:42 16:10:44 16:10:46 16:10:48
Time - EDT
MW/MVAr
0
50
100
150
200
250
300
kV
MW
MVAr
kV
Major Path to Cleveland Blocked
13
ONTARIO
4:08:59 - 4:09:07
PM
ONTARIO
Generation Trips
8
Cascade Moves into Michigan
4:10:36 PM
9
Power Transfers Shift 4:10:38.6 PM
10
North of Lake
Superior
4:10:43 – 4:10:45
PM
Northeast Island Separates from EI
11
End of the Cascade
12
Area affected by blackout
Service maintained in isolated pockets
57
58
59
60
61
62
63
64
16:10:30 16:10:40 16:10:50 16:11:00Time
Fre
qu
en
cy
(H
z)
Lambton ONT-MI
NY-West
NY-East
-3,000
-2,000
-1,000
0
1,000
2,000
3,000
4,000
5,000
16:10:30 16:10:40 16:10:50 16:11:00Time
Pow
er Flo
ws (M
W)
New York into Ontario
PJM into New York
Ontario into Michigan
New York into New England
Sta
rt o
f split
betw
ee
n
East
and W
est
MI
16:10:36
Detr
oit, C
leve
land s
ep
ara
ted f
rom
W.
MI.
16:10:38
Cle
ve
lan
d c
ut
off
fro
m P
A
16:10:38.6
NY
separa
tes f
rom
PA
16:10:39.5
Cle
ve
lan
d s
ep
ara
tes
from
Tole
do,
isla
nds
16:10:41.9
NY
separa
tes f
rom
NJ
16:10:45.2
NY
and N
ew
Engla
nd s
ep
ara
te
16:10:48
Split
com
ple
te b
etw
ee
n
East
and W
est
NY
16:10:49
16:10:30
to
16:11:00
1630 M
W D
etr
oit g
enera
tion t
rips
16:10:42
On
tario s
plits
fro
m W
est
NY
16:10:50
On
tario r
econ
nects
with W
est
NY
16:10:56
2003 Blackout Analysis
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0
20
40
60
80
100
120
140
Outages
% o
f N
orm
al R
ati
ng
s (
Am
ps
)
Sammis-Star
345 kV
CantC-Tidd
345 kV
Star-S.Cant
345 kV
Hanna-Jun
345 kV
Hard-Chamb
345 kV
Can
tC X
fmr
Bab
b-W
.Ak
138 k
V
Hard
-Ch
am
b
345 k
V
Han
na-J
un
345 k
V
Sta
r-S.C
an
t
345 k
V
Clo
v-T
orre
y
138 k
V
E.L
ima-N
.Lib
138 k
V
W.A
k-P
V Q
21
138 k
V
E.L
ima-N
.Fin
138 k
V
Ch
am
-W.A
k
138 k
V
W.A
k 1
38 k
V
Bkr F
ailu
re
Dale
-W.C
an
138 k
V
2003 Blackout Simulations 3 Months to Build
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Some Key Elements of the Cascade • Vegetation management
• Relay loadability
• Miscoordination of generator controls and system protection
• Generator underfrequency protection and under / over speed controls not coordinated
• Pole Slipping
• UFLS failures – Constrained by extreme undervoltage
– Time delays too long
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What to look for in the study
• Slow cascading – not all cascades happen at dynamic speeds
• Slow, progressive Voltage collapse
• Increasing overloads of key elements over time
• Loading above Surge Impedance Limits (SIL)
Reactive power entering the line from both ends
• Excessive bus angles across transmission lines
Potential for parts of the system separating by out-of-step conditions
If it trips can you reclose it?? Doubtful for angles above 45 degrees
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Angular Separation Analysis
-170
-160
-150
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
15:05:00 15:32:00 15:44:00 15:51:00 16:05:00 16:06:01 16:09:05 16:10:38
Time (EDT)
Rela
tive P
hase A
ng
le
Cleveland West MI
Normal Angle ~ -25º
Reference:
Browns Ferry
16
24
24
Detroit
Cleveland
Western MI
NJ
NY Ontario
15
Degrees
Muskingum – Ohio Central - Galion
East Lima – Fostoria Central
Sammis - Star
16:05:50 to 16:08:52
25
16:08:50 to 16:10:50 Western MI
Detroit
out of
Synch
NJ NY
Ontario Cleveland
Cleveland
Separation
NY-PJM Separation
40
Degrees
18
EXAMPLE2 FLORIDA SYSTEM DISTURBANCE
FEBRUARY 26, 2008
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Event Overview
• Delayed clearing of 138 kV SLG fault progressed to a 1.7 second 3-Ø fault
• Loss of 1,350 MW load near fault
• Loss of 2,500 MW of generation near fault
• Loss of 2,300 MW more load by UFLS program
• Loss of 1,800 MW more generation across the Region
• Oscillatory effects across interconnection
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Key Elements of Cascade • Protection system turned off
• Single Ø fault progressed to three Ø fault – 1.7 seconds
• Nuclear units tripped on undervoltage as designed
• Eight turbines unexpected trips; – auxiliary bus voltage protection
– rate of frequency change -- burner lean blowout phenomenon
• Proper UFLS action – prevented system separation
• Inter-area oscillations – 1,000 MW swings in TVA 500 kV system
– 600 MW swings in New England 345 kV system
– 12 kV swings on Ontario 230 kV system
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Florida UFLS Activation
• Event Summary – Impact contained within
FRCC
– Majority of load restored within 2 hrs.
– No major equipment damage reported
– No thermal O/L
– 2 Nuclear units • (tripped as designed)
FRCC
RC Footprint
FRCC RC Visibility
Actuation of
UFLS
Location of
138 kV – 3θ fault
Generation Trips
Turkey Point (FPL)
Dorsey (MH)
Calloway
/ Rush
Island
TVA
Interconnection-wide Impact 29
What to look for in the study
• Localized voltage collapse
• Low voltages on generators
Motor controllers often drop out for voltages below 0.87 per unit
• Potential inter-area oscillations
• Excessive bus angles across transmission lines
Potential for parts of the system separating by out-of-step conditions
If it trips can you reclose it?? Doubtful for angles above 45 degrees
31
EXAMPLE 3 PACIFIC SOUTHWEST DISTURBANCE
SEPTEMBER 8, 2011
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CAISO Freq/ACE 1635-1705 MST
CISO Freq/ACE 1635-1705 MST
From RA Tool
1-Min. CAISO Freq. 1500-0530 MST
From RA Tool
System Separation & SONGS Trip
Loss of San Onofre Gen.
Loss of SDGE
Load
From RA Tool
Initial FNet FDR Angular Plot
AZ-NM-CO
California
FNET FDR Locations
Event A
Event A Detail
Event B
Event C
Event D
What we learned with FNET • The frequency shows four main events
A. The initial separation around 22:27:39 (UTC), resulting in a ‘slow’ frequency dip of about -30 mHz over about 25 seconds
B. A frequency ramp beginning around 22:32:10 increasing frequency +30 mHz over about 15 seconds
C. A frequency drop around 22:37:55 of over -40 mHz (B-A) over about 12 seconds
D. A frequency jump around 22:38:21 of over +150 mHz (C-A) in less than 5 seconds, settling at around +80 mHz (B-A) in about 20 seconds
Components of the Outage
• Over 30 ‘major’ element operations over the course of 11 minutes
Line and transformer trips
Generator trips and runback
Load shedding
Over 50 additional ‘minor’ operations such as capacitor and reactor switching
• Over 6 GB of data of different qualities and resolution
Operator logs, PI historian, SCADA, PMU, DFR, relay records
What to look for in the study • Think like a phasor measurement unit (PMU)
But don’t be fooled by phase-jumps at inception of a fault, clearing of a fault, or significant reactive switching
• Think Multi- dimensionally! Don’t fixate on single reading
Frequency, voltage, current, time – multi-dimensional plots
• Potential for inter-area oscillations
• Recognize tripping of various system elements
Line trip, Transformer trip, Load trip
• Excessive bus angles across transmission lines
If it trips can you reclose it?? Doubtful for angles above 45 degrees
45
Phase 1 – Pre-Disturbance • Hot, shoulder season day; some
generation and transmission
outages
• High loading on some key
facilities: H-NG at 78% of normal
rating; CV transformers at 83%
• 44 minutes before loss of H-NG,
IID’s RTCA results showed loss
of CV-1 transformer would load
CV-2 transformer above its relay
trip point
• 15:27:39: APS technician
skipped a critical step in isolating
the series capacitor bank at
North Gila substation; H-NG trips
Phase 2 – Trip of H-NG 500 kV
• H-NG 500 kV trips at
15:27:39
• APS tells WECC RC line
expected to be restored
quickly
• H-NG flow redistributes: 77%
to SCE-SDGE (Path 44);
remainder to IID, and WALC
• CV transformers immediately
overloaded above relay
settings
• Path 44 at 5,900 amps; 8,000
amp limit on SONGS
separation scheme
15:27:39 – 15:28:16
Initiating Event – Voltage Divergence Hassayampa – North Gila 500 kV Trip
Series Capacitor Bypass Switch Arcs
Over
Hass. – N. Gila 500 kV Line Trip
Hassyampa –N. Gila 500 kV
line trip
CCM Unit 1 generator trip
South of SONGS Current
Phase 3 – Trip of CV Transformers • 15:28:16 – CV-2 and CV-1
230/92kV transformers trip on
overload relays
• Severe low voltage in WALC
161 kV system
• Loading on Path 44
increases to 6,700 amps
15:28:16 – 15:32:10
Coachella Valley Transf. Trip
Coachella Valley 230/92 kV
transformers trip
South of SONGS Current
Phase 4 – Ramon Transformer Trip • 15:32:10 Ramon 230/92kV
transformer trips on overload
relay
• 15:32:13 Blythe-Niland
161kV line trips
• 15:32:15 Niland – CV 161kV
line trips
• IID undervoltage load
shedding; loss of generation
and 92 kV transmission lines
• Severe low voltage in WALC
161 kV system
• Loading on Path 44
increases to 7,800 amps;
settles at 7,200 amps
15:32:10 – 15:35:40
Ramon Transformer Trip
Ramon 230/92 kV transformer trip
Multiple line, generator and load trips
Voltage collapse in pocket, followed by load tripping
South of SONGS Current
Voltage in Northern IID 92 kV System
Ramon 230/92 kV Transformer Trip
Trip of Over 400 MW in Northern IID 92 kV Load
Over-Voltage Trip of 92 kV System
Capacitors
Blythe 161 kV Voltage
Trip of Coachella Valley 230/92 kV
Transformers
Trip of Hassayampa – North Gila 500 kV
Line
Ramon 230/92 kV Transformer Trip
Trip of Over 400 MW in Northern IID 92 kV Load
Yucca 161/69 kV Transformers 1 and
2 Trip
El Centro – Pilot Knob 161 kV Line
Trip
Phase 5 – Yuma Separates
15:35:40 – 15:37:55
• Yuma AZ Separates from IID
and WALC when Gila and
Yucca transformers trip
• Yuma load pocket isolated on
single tie to SDG&E
• Loading on Path 44
increases to 7,400 amps after
Gila transformer trip; to 7,800
amps after Yucca
transformers and generator
trip
Yuma Separation
Gila 161/69 kV transformers trip
YCA generating units trip
Yucca 161/69 kV transformers trip
Pilot Knob 161/92 kV
transformers trip
South of SONGS Current
Phase 6 – High-Speed Cascade • El Centro – Pilot Knob 161kV
line trips; all IID 92 kV system
radial from SDG&E via S-Line
• WALC 161 kV system voltage
returns to normal
• Path 44 exceeds 8,000 amp
setting and timer starts
15:37:55
El Centro – Pilot Knob 161 kV line
trip
Imperial Valley – El Centro 230 kV
“S” line tripCLR generating units trip SONGS
separation
Phase 6 – High-Speed Cascade South of SONGS Current
SONGS Separation Frequency Impacts
59.85
59.95
60.05
60.15
60.25
60.35
60.45
60.55
60.65
60.75
15:38:13.920 15:38:18.240 15:38:22.560 15:38:26.880 15:38:31.200 15:38:35.520 15:38:39.840 15:38:44.160 15:38:48.480
Ault (Denver) Mead (Las Vegas) Tesla (Sacramento) Palo Verde
Grand Coulee SONGS Recalculated Frequency Devers Recalculated Frequency
60.730 Hz Phase
Jump Transient
~60.210 Hz System
Zenith (Point C)
~60.050 Hz System
Response (Value B)
59.977 Hz Pre-Event (Value A)
SONGS generators ring down
SONGS separation
SONGS Unit 3 GSU trip and transfer of
aux load
SONGS Unit 2 GSU trip and transfer of
aux load
SONGS aux load on startup
transformers
UFLS Operations in the Island
Phase 4 Example
Two GTs
& UVLS
Blythe-Niland
Colmac GT
CV-Niland
UVLS
Motor stalling
Phase 5 Example
transformers
trip
generators
trip
Phase 6 Example
‘A’ line trip
Blythe RAS
‘S’ line RAS
generators
‘S’ line RAS
generators
Devers SVC Output
2 1 3 4 5 6 7
Capacitor Switching
capacitor switching candidate signatures
location confirmed by comparing voltage
Questions?
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