Download Willow Glen Short Circuit Study

“Importance of Reactive Power Management,
Voltage Stability and FACTS Applications in
today’s Operating Environment”
Sharma Kolluri
Manager of Transmission Planning
Entergy Services Inc
Engineering Seminar
Organized by IEEE Mississippi Section
Jackson State University
August 20, 2010
Outline
•
•
•
•
•
•
Introduction
VAR Basics
Voltage Stability
FACTS
Applications at Entergy
Summary
.
Voltage Profile during Aug 14th Blackout
• Voltages decay to almost 60% of normal voltage. This is probably the
point that load started dropping off.
• However, the recovery is too slow and generators are not able to
maintain frequency during this condition.
• Many generators trip, load shedding goes into effect, and then things just
shut down due to a lack of generation.
A “Near” Fast Voltage
Collapse in Phoenix in
1995
North American Electric
Reliability Council, System
Disturbances, Review of
Selected 1995 Electric System
Disturbances in North
America, March 1996.
Recommendation#23
• Strengthen Reactive Power and Control
Practices in all NERC Regions
“Reactive power problem was a
significant factor in the August 14
outage, and they were also important
elements in the several of the earlier
outages”
-Quote form the outage report
Reactive Power
Laws of Reactive Physics
• System load is comprised of resistive current (such as lights,
space heaters) and reactive current (induction motor reactance,
etc.).
• Total current IT has two components.
– IR resistive current
– IQ reactive current
– IT is the vector sum of IR & IQ
– IT = IR + jIQ
IT
IQ
IR
North American Electric Reliability Corporation
Laws of Reactive Physics
• Complex Power called Volt Amperes (“VA”) is comprised of
resistive current IR and reactive current IQ times the voltage.
– “VA” = VIT* = V (IR – jIQ) = P + jQ
VA
Q
• Power Factor (“PF”) = Cosine of angle between P andP “VA”
– P = “VA” times “PF”
• System Losses
– Ploss = IT2 R (Watts)
– Qloss = IT2 X (VARs)
North American Electric Reliability Corporation
Reactive Physics – VAR loss
• Every component with reactance, X: VAR loss = IT2 X
• Z is comprised of resistance R and reactance X
–
–
–
–
On 138kV lines, X = 2 to 5 times larger than R.
One 230kV lines, X = 5 to 10 times larger than R.
On 500kV lines, X = 25 times larger than R.
R decreases when conductor diameter increases. X increases as the
required geometry of phase to phase spacing increases.
• VAR loss
– Increases in proportion to the square of the total current.
– Is approximately 2 to 25 times larger than Watt loss.
North American Electric Reliability Corporation
Reactive Power for Voltage Support
VARs flow from High voltage
to Low voltage; import of
VARs indicate reactive
power deficit
Reactive
Loads
Reactive Power Management/Compensation
What is Reactive Power Compensation?
• Effectively balancing of capacitive and inductive components of a power
system to provide sufficient voltage support.
– Static and dynamic reactive power
• Essential for reliable operation of power system
– prevention of voltage collapse/blackout
Benefits of Reactive Power Compensation:
•
•
•
•
Improves efficiency of power delivery/reduction of losses.
Improves utilization of transmission assets/transmission capacity.
Reduces congestion and increases power transfer capability.
Enhances grid reliability/security.
Transmission Line Real and Reactive Power
Losses vs. Line Loading
Source: B. Kirby and E. Hirst 1997, Ancillary-Service Details: Voltage Control,
ORNL/CON-453, Oak Ridge National Laboratory, Oak Ridge, Tenn., December 1997.
Static and Dynamic VAR Support
• Static Reactive Power Devices
– Cannot quickly change the reactive power level as long as the voltage
level remains constant.
– Reactive power production level drops when the voltage level drops.
– Examples include capacitors and inductors.
• Dynamic Reactive Power Devices
– Can quickly change the MVAR level independent of the voltage level.
– Reactive power production level increases when the voltage level drops.
– Examples include static VAR compensators (SVC), synchronous
condensers, and generators.
Voltage Stability
Common Definitions
• Voltage stability - ability of a power system to maintain steady voltages
at all the buses in the system after disturbance.
• Voltage collapse - A condition of a blackout or abnormally low voltages
in significant part of the power system.
• Short term voltage stability - involves the dynamics of fast acting load
components such as induction motors, electronically controlled loads,
and HVDC converters.
• Long term voltage stability - involves slower acting equipments such
as tap-changing transformer, thermostatically controlled loads, and
generator limiters.
What is Voltage Instability/Collapse?
• A power system undergoes voltage collapse if
post-disturbance voltages are below
“acceptable limits”
– voltage collapse may be due to voltage or angular
instability
• Main factor causing voltage instability is the
inability of the power systems to “maintain a
proper balance of reactive power and voltage
control”
Voltage Instability/Collapse
• The driving force for voltage instability is
usually the load
• The possible outcome of voltage instability:
– loss of loads
– loss of integrity of the power system
• Voltage stability timeframe:
– transient voltage instability: 0 to 10 secs
– long-term voltage stability: 1 – 10 mins
Voltage stability causes and analysis
• Causes of voltage instability
– Increase in loading
– Generators, synchronous condensers, or SVCs reaching reactive power
limits
– Tap-changing transformer action
– Load recovery dynamics
– Tripping of heavily loaded lines, generators
• Methods of voltage stability analysis
– Static analysis methods
– Algebraic equations, bulk system studies, power flow or continuation power flow
methods
– Dynamic analysis methods
– Differential as well as algebraic equations, dynamic modeling of power system
components required
Over-excitation Limit
- Per unit MVAR
(Q) +
Leading
(Under-excited)
Lagging
(Over-excited)
Generator Capability Curve
0.8 pf
line
MW
Stator Winding Heating Limit
Normal Excitation
(Q = 0, pF = 1)
Turbine Limit
Under-excitation Limit
Stability Limit
P-V Curve
Q-V Curve
Q-V Curve with Detailed Load Model
Peak Load with Fixed Taps
120
200
100
80
60
Mvars
40
Base Case
20
Contingency
0
-20
0.5
0.6
0.7
0.8
0.9
1
1.1
-40
-60
-80
Voltage (p.u.)
1.2
1.3
1.4
1.5
Key Concerns
Limit UVLS
activation
Voltage
(pu)
Minimize
motor
tripping
Possible Solutions for Voltage Instability
• Install/Operate Shunt Capacitor Banks
• Add dynamic Shunt Compensation in the form of
SVC/STATCOM to mitigate transient voltage dips
• Add Series Compensation on transmission lines in the problem
area
• Implement UVLS Scheme
• Construct transmission facilities
Voltage Collapse
Fault Induced Delayed Voltage
Recovery (FIDVR)
• FIDVR Definition
• Load Models
Fault Induced Delayed Voltage
Recovery (FIDVR)
• What is it?
– After a fault has cleared, the voltage stays at low levels
(below 80%) for several seconds
• Results in dropping load / generation or fast voltage collapse
• 4 key factors drive FIDVR:
– Fault Duration
– Fault Location
– High load level with high Induction motor
load penetration
– Unfavorable Generation Pattern
Load characteristics
• The accuracy of analytical results depends on modeling of
power system components, devices, and controls.
• Power system components - Generators, excitation systems,
over/under excitation limiters, static VAr systems, mechanically
switched capacitors, under load tap changing transformers, and
loads among others.
• Loads are most difficult to model.
– Complex in behavior varying with time and location
– Consist of a large number of continuous and discrete controls and protection
systems
• Dynamics of loads, especially, induction motors at low voltage
levels should be properly modeled.
Induction motor characteristics
• Impact of fault on transmission
grid
• With fault clearing
Square-law load torque
Torque -per unit
– Depressed voltages at distribution feeders
and motor terminals
– Reduction of electrical torque by the
square of the voltage resulting in slow
down of motors
– The slow down depends on the
mechanical torque characteristics and
motor inertias
Electric torque
Constant load torque
Speed – per unit
1.0
Fig. 1 Induction motor characteristics
– Partial voltage recovery
– Slowed motors draw high reactive currents, depressing voltage magnitudes
– Motor will reaccelerate to normal speed if, electrical torque>mechanical torque
else, the motors will rundown, stall, and trip
– The problem is severe in the summer time with large proportion of air conditioner
motors
Air conditioner motor characteristics
• Characteristics
– Main portion (80-87%) consumed by compressor motor
– Electromagnetic contactor drop out between (43-56%) of the nominal
voltage and reclose above drop out voltage
– Stalling at (50-73%) of the nominal voltage
– Thermal overload protection act if motors stall for 5-20 seconds
– The operation time of thermal over load (TOL) protection relay is
inversely proportional to the applied voltage at the terminal
• Air conditioner should be modeled to analyze the short term
voltage stability problem
• Quite important for utilities in the Western interconnection
Load modeling
• Old models – Loads are represented as
lumped load at distribution feeder
• Does not consider the electrical
distance between the transmission bus
and the end load components
• The diversity in composition and
dynamic behavior of various electrical
loads is not modeled
• Modeling
–
WECC interim model
– 20% of the load as generic induction motor
load
– 80% constant current P and constant
impedance Q
Transmission Bus
OLTC
Distribution Bus
Distribution
Capacitor
Lumped Load
(ZIP load)
Fig. 2 Traditional load model
Composite load modeling
Transmission Bus
• Representation of distribution
equivalent
Bus 1
OLTC
– Feeder reactance
– Substation transformer
reactance
Distribution Bus
Bus 2
Substation
Capacitor
Distribution Feeder
Bus 3
• Parameters of various load
components
–
–
–
–
–
Discharge lighting
Electronic Loads
Constant Impedance loads
Motor loads
Distribution Capacitor
Feeder Equivalent
Dynamic Loads
(Small motor, Large
motor, trip motor loads)
Static Loads
(Constant impedance,
constant current, constant
impedance loads)
Fig. 3 Composite load model structure
Distribution
Capacitor
FACTS
What is FACTS?
Alternating Current Transmission Systems
Incorporating Power Electronic Based and
Other Static Controllers to Enhance
Controllability and Increase Power Transfer
Capability.
•power semi-conductor based inverters
•information and control technologies
Major FACTS Controllers
• Static VAR Compensator (SVC)
• Static Reactive Compensator (STATCOM)
• Static Series Synchr. Compensator (SSSC)
• Unified Power Flow Controller (UPFC)
• Back-To-Back DC Link (BTB)
FACTS Applications
Power Flow Control
System Reliability
Inter-area Control
Inter-tie Reliability
Inter-connected
RTO System
S/S
BTB
UPFC
Power Generation
Improved
Power Quality
Voltage Control
Power System Stability
S/S
Load
Load
STATCOM
Enhanced
Import Capability
STATCOM
Increased
Transmission Capacity
Inter-connected
Power System
Load
S/S
BTB
SSSC
Static VAr compensator (SVC)
• Variable reactive
power source
• Can generate as well
as absorb reactive
power
• Maximum and
minimum limits on
reactive power output
depends on limiting
values of capacitive
and inductive
susceptances.
V
I
TCR
Firing angle
control
XC
XL
Fig. 4 Schematic diagram of an SVC
Static compensator (STATCOM)
• Voltage source converter
device
• Alternating voltage source
behind a coupling reactance
• Can be operated at its full
output current even at very
low voltages
• Depending
upon
manufacturer's
design,
STATCOMs
may
have
increased transient rating both
in inductive as well as
capacitive mode of operation
System bus
V
Transformer
I
X
E
DC-AC switching converter
Cs
Vdc
Fig. 5 Schematic diagram of STATCOM
Technology Applications at
Entergy
Technology Applications at Entergy to
Address Reactive Power Issues
•
•
•
•
•
•
•
Large Shunt Capacitor Banks
UVLS
Series Compensation
SVC
Coordinated Capacitor Bank Control
DVAR
AVR
Determining Reactive Power Requirements in the Southern
Part of the Entergy System for Improving Voltage Security – A
Case Study
Sharma Kolluri
Sujit Mandal
Entergy Services Inc
New Orleans, LA
Panel on Optimal Allocation of Static and Dynamic VARS for
Secure Voltage Control
2006 Power Systems Conference and Exposition
Atlanta, Georgia
October 31, 2006
Areas of Voltage Stability Concern
North Arkansas
Mississippi
West of the Atchafalaya Basin
(WOTAB)
Southeast Louisiana
Western Region
Amite South/DSG
Study Objective
• Identify Voltage Stability Problems in the DSG
area
• Determine the proper mix of reactive power
support to address voltage stability problem
• Determine size and location of static and
dynamic devices.
Downstream of Gypsy Area - Critical Facilities
Little Gypsy-South Norco 230kV line
Ninemile Units
Michoud Units
1 - 50 MW
2 - 60 MW
3 - 128 MW
4 - 740 MW
5 - 750 MW
1 - 65 MW
2 - 240 MW
3 - 515 MW
Waterford-Ninemile 230kV line
115 kV
230 kV
115 kV
- 230 kV
DSG Issues
• Area load growth
• 1.6% projected for 2003 - 2013
• Weather normalized to 100º F
• Projected peak load – 3800 MW
• Area power factor - Low
• 94% at peak load
Michoud
Ninemile
• Worst double contingency
• Loss of the Waterford to
Ninemile 230 kV transmission
line and one of the 230 kV
generating units at Ninemile or
Michoud
New Orleans area voltage profile
on June 2, 2003
(with 2 major generators offline)
• Area Problems
• Thermal overloads of underlying 115 kV and 230 kV
transmission system
• Depressed voltages throughout New Orleans metro area
potentially leading to voltage collapse and load shedding
Various Steps Used for Determining Reactive Power
Requirements
• Step 1 – Problem identification
• Step 2 – Determining total reactive power
requirements
• Step 3 – Sizing and locating dynamic devices
• Step 4 – Sizing and locating static shunt
devices
• Step 5 – Verification of reactive power
requirements
Tools & Techniques Used
• Various tools and techniques used for analysis
purposes
–
–
–
–
PV analysis using PowerWorld
Transient stability using PSS/E Dynamics
Mid-term stability using PSS/E Dynamics
PSS/E Optimal Power Flow
• Detailed Models used
– Motor models and appropriate ZIP model for dynamic
analysis
– Tap-changing distribution transformers, overexcitation
limiters, self-restoring loads modeled in mid-term stability
study
Criteria/Requirements
Improve
post-fault
voltage
Voltage
(pu)
Minimize
motor
tripping
Steady State Analysis
Results
PV Curve
Ninemile Unit 4 out-of-service
Trip Ninemile Unit 5 and Waterford – Ninemile 230 kV line
cc
Without Waterford - 9Mile 230kV line 3of5 & Michoud unit 3 PV curves
1.05
1
Berman 230kV
Voltage (p.u.)
Market 230kV
0.95
Tricou 230kV
Almonaster 230kV
PARIS 230kV
Gretna 115kV
0.9
Delta 115kV
9mile 230kV
0.85
0.8
3300
3350
3400
3450
3500
3550
Load in DSG (MW)
3600
3650
3700
3750
3800
Dynamic Analysis
Stability Simulation
Ninemile Unit 4 out-of-service
Trip Ninemile Unit 5 and Waterford – Ninemile 230 kV line
Process for Determining Reactive
Power Requirements
• Approx 700 MVAr of reactive power shortage identified in the
DSG
– How much static and how much dynamic?
• Criteria for determining static and dynamic requirements
–
–
–
–
Voltage at critical buses should recover to 1 pu in several seconds
Voltage at critical buses should recover to 0.9 pu within 1.5 - 2 seconds
Voltage should not dip below 0.7 pu for more than 20 cycles
Generator reactive power output should be below Qmax
• Factors considered in sizing static/dynamic devices
– Short circuit levels, size & location of the stations, number and existing
size of cap banks, back-to-back switching, etc
SVC
Size and Location
• Sites considered
– Ninemile 230 kV
– Gretna 115 kV
– Paterson 115 kV
• Size
– 300 MVAR
– 500 MVAR
Optimal size and
location
Steps to locate Static Shunt Devices
• Static shunt requirements – 400 MVAR
approximately
• Options available to locate the static shunt
devices on the transmission or distribution
systems
• OPF Program used to come up with size and
location of shunt devices
OPF Application
• PSS/E OPF Program used
• Objective Function – Minimize adjustable
shunts
• OPF simulated for critical contingencies
List of Shunt Capacitor Banks Banks
Recommended
Number
Name
Voltage
Cap Size
[kV]
[MVAr]
1
Destrehan
230
64.8
2
Behrman
230
86.4
3
Waggaman
230
64.8
4
Poydras
115
21.6
5
Paterson
115
43.2
6
Snakefarm
230
64.8
7
Napoleon
230
64.8
8
Kenner
230
64.8
Simulation Results with the Capacitors and SVC
Ninemile Unit 4 out-of-service
Trip Ninemile Unit 5 and Waterford – Ninemile 230 kV line
SVC Performance
Ninemile Unit 4 out-of-service
Trip Ninemile Unit 5 and Waterford – Ninemile 230 kV line
Summary
• Process for determining static and dynamic
reactive power requirements discussed
• OPF program utilized for sizing/locating static
shunt capacitor banks
• Results verified using mid-term stability
simulations
• Study recommendation – 400 MVAR of static
shunt devices and 300 MVAR of dynamic shunt
compensation
Ninemile SVC Configuration
3AC 60Hz 230kV
SN = 300 MVA, uk = 9.5 %
3AC 60Hz 15.5kV
CTSC1
CTSC2
LTSC3
LTSC1
VR1
V1
LTSC2
VR2
V3
VR3
V2
CTSC3
TSC 1 = 75 MVAr
TSC 2 = 75 MVAr
TSC 3 = 150 MVAr
External Device Control
Single line diagram of SVC and MSC
Ninemile 230kV
Different
voltage
levels
(115 kV)
SVC system
.................
MSC 1
MSC 10
TSC 1-3
SVC Ninemile
SVC Ninemile
Porter 0/+300Mvar SVC
SVC Topology: 2 x 75MVAr TSC & 1 x 150MVAr TSC
Porter Static Var Compensator (SVC)
Maintains
system
voltage by
continuously
varying VAR
output to meet
system
demands
Controls
capacitor
banks on the
transmission
system to
match reactive
output to the
load
requirements.
Porter SVC
Series Capacitor –
Dayton Bulk 230kV Station
The Capacitor
offsets
reactance in
the line,
making it
appear to the
system to be
half of its
actual length.
Power flows
are redirected
over this larger
line, unloading
parallel lines
and increasing
transfer
capability.
DSMES Unit
Stores Energy in a
superconducting coil
Automatically releases energy to the
system when needed to ride through
voltage dips caused by faults. This
unit improves power quality and
reduces customer loss of production.
Industry Issues
• Coordination of reactive power between regions
• No clearly defined requirements for reactive
power reserves
• Proper tools for optimizing reactive power
requirements
• Incentive to reduce losses
Summary
• The increasing need to operate the transmission system
at its maximum safe transfer limit has become a
primary concern at most utilities
• Reactive power supply or VAR management is an
important ingredient in maintaining healthy power
system voltages and facilitating power transfers
• Inadequate reactive power supply was a major factor
in most of the recent blackouts
Questions?
Under Voltage Load Shed Logic Western Region
T&D Planning
April 2010
Western Region – Overview
≤ 230 kV Tie Lines
Generation
Load Center
Load Projection
• 2010 peak: 1770 MW
• 2012 peak: 1852 MW
Sample PV Curve Result
Lewis Creek Unit 1 & China-Porter 230kV Out - 2010
1.1
200
1.05
150
1
100
0.95
50
0.9
0
0.85
-50
0.8
-100
1350
1450
1550
1650
1750
1850
1950
2050
2150
Dayton
Rivtrin
Conroe
Poco
Jacinto
Cleveland
Huntsville
Goslin
Lewis Creek
Pelican Road
Cypress
Frontier
2010 Summer PV Curve Analysis
P Limit
(MW)
With 3% Margin
(MW)
Voltage (4/8
Buses) (pu)
Lewis Creek U1 out
2385
2313
0.84 – 0.89
Lewis Creek U1 + China-Jacinto out
2260
2192
0.83 – 0.89
Lewis Creek U1 + Grimes-Crockett
out
2230
2163
0.86 – 0.91
Lewis Creek U1 + China-Porter out
2065
2003
0.85 – 0.93
Scenarios
Approved Construction Plan Projects included:
*Relocate Caney Creek 138kV
Dynamic Analysis Results
Results: 2010 case without load
shed
Case 3 Voltages (pu): Goslin: 0.810;
Conroe: 0.855; Cleveland: 0.909; Jacinto:
0.924; Dayton: 0.944; Huntsville: 0.944
Case 4 Voltages (pu): Goslin: 0.757;
Conroe: 0.800; Dayton: 0.913; Huntsville:
0.928; Cleveland: 0.928; Rivtrin: 0.941
2010 Summer Conditions - Dynamics Analysis
• Lewis Creek Unit 1 outaged in the base case
• 50% induction motor load is modeled
• Result: Shed Load Block 1 (183 MW)
Observations for 2010 Summer Peak Conditions
• Existing load shed logic in Western Region OK for 2010
Summer conditions
• Voltage at some critical buses drop below 0.7 pu for more
than 20 cycles – Potential of motor load tripping
Conclusions for 2010 Summer
• Reducing load shed blocks to 180 + 70 MW in Western
Region has no negative impact
Results: 2010 case with load shed (Block
1)
Case 3 Voltages (pu): Goslin: 0.872;
Conroe: 0.902; Cleveland: 0.934; Jacinto:
0.948; Dayton: 0.966; Huntsville: 0.968
Case 4 Voltages (pu): Goslin: 0.827;
Conroe: 0.855; Dayton: 0.939; Cleveland:
0.951; Huntsville: 0.954; Jacinto: 0.964
Conclusions and Recommendations
• Retain the exiting UVLS logic
• Change the load blocks
– Block one: 180 MW
– Block two: 70 MW (existing size 111 MW)
Proposed Load Shed Logic
Voltage @
4/8 buses
<0.90 pu
Armed all
time
Drop load
One or more
Lewis Creek
units inservice?
OEL at
Lewis Creek
units
Voltage @
4/8 buses
< 0.92 pu
Time
Delay 3
seconds
Monitored Buses:
Metro 138kV
Goslin 138kV
Alden 138kV
Oakridge 138kV
Huntsville 138kV
Rivtrin 138 kV
Poco 138 kV
Conroe 138 kV
Load Blocks:
Block 1: 175 MW
Block 2: 75 MW
Load Blocks:
Block 1: 175 MW
Alden: 50 MW
Metro: 35 MW
Oakridge:30 MW
Goslin: 60 MW
Block 2: 75 MW
In the vicinity of Block 1
The above conditions need to be met for 3 scans to trigger load shedding
Reset the
Process for next
LVSH block