1
On-site Low Voltage Determination of
Zero Sequence Impedances for Station
Auxiliary Transformers
Mariana Kamel, Haytham Saeed, Abdelrahman
Karrar, Ahmed Eltom, Mark Bowman, Tamatha
Womack, Preston Cooper
University of Tennessee at Chattanooga & TVA
2
Background
• Motivation for this research was the incident of
January 30, 2012 at Byron Station NPP which
involved an open-phase condition on the primary of
two SATs.
– This research describes a new method for finding
the zero sequence parameters for typical SATs.
– The method cuts the costs of prevailing methods,
particularly for on-site measurements.
– Initial simulations were promising. Method was
then validated using actual measurements.
3
Background - 2
• In the standard IEEE/ANSI method, at least one
measurement must be carried out from the high
voltage side. This measurement is shown in figure as
Z1No
• The usual setting for this
test would be a high
voltage laboratory,
where suitable test
voltages are available.
Z1Ns or Z2Ns
Zp-0
Z1No
Zm-0
Zt-0
Zs-0
Z2No
4
Background - 3
• Onsite measurements however would require
renting high voltage mobile laboratories at a high
cost.
• Our method eliminates the high voltage test, and
replaces it with the low voltage test shown below.
• Only a low voltage (230 – 400 V) three phase supply
is required.
primary phase A
opened
Ia
Ib
Ic
primary phase B
and C shortcircuited
A
Vab V
A
Vbc V
A
secondary
neutral point
disconnected
Three-phase low
voltage source
V Vca
5
Background - 4
• Our test configuration requires application of a three
phase voltage to the secondary under the following
conditions:
1. Secondary neutral is disconnected.
2. Two primary (high voltage) terminals are shorted to the
neutral, with the remaining terminal left open.
• This connection creates conditions involving all three
sequence component voltages and currents.
6
Methods
• Our method exploits the primary open phase
sequence model.
• Two methods were proposed. A simplified
“approximate “ method and an iterative based
“exact” method.
I1
I2
I0
Zps-1
Zps-2
Zp-0
Zt-0//Zm-0
Zs-2
V2
Zs-1
V1
E1
7
Methods - 2
• The simplified method makes use of
symmetry resulting from neglecting the
transformer resistive components.
• This assumed symmetry allows to
calculate positive and negative
sequence currents from line currents Ia,
Ib and Ic .
𝐼𝐼𝑏𝑏̅ = 𝐼𝐼𝑐𝑐̅ = 𝐼𝐼
𝐼𝐼1 =
−2
3
𝐼𝐼 cos(𝜃𝜃 + 30°)
𝜃𝜃 = cos−1
𝐼𝐼2 =
2
3
𝐼𝐼𝑎𝑎̅
−
2𝐼𝐼
𝐼𝐼 cos(𝜃𝜃 − 30°)
𝐼𝐼0̅ + 𝐼𝐼1̅ + 𝐼𝐼2̅ = 0
Ic
θ
I2
-θ
I1
Ia
Ib
Vc
Vca
V1
Vbc
Va
Vab
Vb
ϕ
V2
8
Methods - 3
• The exact method includes transformer resistive
components and involves using the six current and voltage
measurements Ia, Ib, Ic, Vab, Vbc and Vca in addition to the
sequence network model to solve for real and imaginary
values of I1, I2, V1 and V2 . From this solution Io is deduced
and used to find the real and imaginary components of
the zero sequence impedance.
• The equations are non-linear and further overdetermined
by one. Thus, a non-linear least square estimation
method, for example the Newton-Gauss method is used
for a solution.
9
Results
• Summary of results for a test carried out on a TVA 18 MVA SAT
at a test site in Virginia.
10
Conclusions
• The test is expected to be valuable to those
seeking to determine zero sequence
parameters not available on many legacy SAT
units with affordable costs.
• Method is extendable to transmission type
transformers – authors are developing model.
• Field testing has confirmed the high accuracy
offered by the method.
1
Paper No: 16PESGM1489
Analysis of Open Phase Fault Events Using
ETAP Unbalanced Load Flow Module
Preston O. Cooper III
Tennessee Valley Authority
pocooper@tva.gov
2
Background
• Multiple events across the nuclear power industry
• Existing schemes did not provide adequate protection
• Need for greater understanding of what happens during an open phase fault (OPF)
3
Event
• OPF above transformer
• Magnetic circuits reconstruct missing phase
• Configuration affects low side unbalance
4
Why Steady State?
• Worst case V1 and V2 should not trip instantaneous motor protection (current)
• I2 creates additional heating in motors
• Motors have thermal mass
5
Methodology
IEQ
Equivalent Current
I1
Positive Sequence Current
I2
Negative Sequence Current
IFL
Motor Full Load Current
ILR
Motor Locked Rotor Current
6
7
8
Results
• Greater transformer loading yields greater unbalances
• Less impedance to ground yields greater unbalances
• Lack of a stabilizing winding yields greater unbalances
• Two different transformers in the same system yield equal maximum unbalances
9
Protective Devices
• Possible
– Transformer Phase Overcurrent
– Transformer Neutral Overcurrent
– Transmission Line Negative Sequence
• Discounted
– Transformer Differential
– Degraded Voltage
10
Summary
• Additional heating due to V2 and I2 is a significant consequence of an OPF
• Transformer construction is crucial
• Steady‐state calculations are viable
• Three dimensional surfaces
– Visualize the effects of an open phase fault
– Validate existing or develop new protection
1
Open-Phase Study in Nuclear
Power Plant
Presented by: Dr. Zia Salami
Associate Professor, Electrical Engineering
2
Outline
•
•
•
•
•
•
•
Acknowledgment
Introduction
Equivalent 3-Phase Power System Model
Open-Phase Scenarios Studied
Sample of Results
Notable Results & Conclusion
Questions
3
Acknowledgment
• Mr. Joel Mathewson,
• Mr. Mario Poujol,
• Mr. Volodymyr Habovda
– Graduated in 2014, UNC Charlotte
• Mr. Lee Easter,
– PowerC, LLC Vice President
4
Introduction
Equivalent 3-Phase Power System Model
(in EMTP)
Transformer Data/Model
• 3 winding transformer (i.e. 3x1 phase)
• Ratings: 345/6.9/4.16 kV, 40/35/5 MVA
• Yg-Yg-Yg connected (primary solidly grounded, resistive ground on
secondary 6.6 ohms, and tertiary 4.0 ohms, 600A)
7
Complete Equivalent 3-Phase Power
System Model
8
Open-Phase Scenarios Studied
Sample of Results
10
11
Sequence current (A), ES Bus,
Normal Operation:
•
•
•
Scenario 1, Case 0 (base)
Scenario 1, Case 1 (single)
Scenario 1, Case 4 (double)
12
Sequence current (A), BOP
Bus, 7000 hp starts:
•
•
•
Scenario 7, Case 0 (base)
Scenario 7, Case 1 (single)
Scenario 7, Case 4 (double)
13
Notable Results & Conclusion
• I2/I1 are minimum in Cases 3 and 4 (i.e. solid grounded open-phase
on the high voltage side of transformer and double open-phase). This
value can be used to set open-phase pickup to be less than this
minimum value.
• I1 & I2 maximum are in case 2 (i.e. line to ground open-phase from
the grid side). This value can be used for blocking the open-phase
logic in case of any short-circuit fault in the system.
• Longer motor acceleration times during double open- phase
condition.
• Bottom line, there is no single bullet to identify open-phase worst
case scenario. It depends on system configurations, loadings,
equipment, grid conditions, and more. I believe all possible plant’s
permissible scenarios/ configurations should be studied and analyzed
to identify the worst case open-phase condition.
14
QUESTIONS?
1
Influence of Zero Sequence
Impedances of Station Auxiliary
Transformers on Equipment
Performance under Open-Phase
Faults
2
AGENDA
•
•
•
•
•
Introduction
Open Phase Fault Network
Test Results
Conclusion
Questions
3
INTRODUCTION
• Primary open-phase faults on SATs result in voltage
imbalance at the auxiliary equipment level.
• Primary to ground zero sequence impedance has a
major role in balancing the secondary voltages
during open phase faults.
• This study investigates the impact of voltage
imbalance on the performance of station auxiliaries.
4
OPEN PHASE FAULT NETWORK
Single Line Diagram for
System Studied with Primary
Open Phase Fault
Corresponding Sequence
Network
5
SERVICE TRANSFORMER DATA
SAT 1
Sequoyah
33 MVA (base)
SAT 2
Sequoyah
18 MVA (base)
SAT 3
J. M. Farley
26 MVA (base)
Z1 P-S (%)
10.26
8.6
14.6
Z0 P-N (%)
8.75
10.37
32.41
6
TEST RESULTS
• The analysis performed to answer:
– Is the motor able to accelerate successfully to rated speed
under open-phase conditions?
– If yes, what is the acceleration time required?
– What is the voltage unbalance ratio V2/V1 during and after
attaining steady state operation?
7
TEST RESULTS
V2/V1 (%)
• Voltage imbalance was 8.6% at the instant of starting
and 1.2% at steady state conditions
10
5
0
0
2
4
6
8
10
Time (sec)
12
14
16
18
20
0
2
4
6
8
10
Time (sec)
12
14
16
18
20
0
2
4
6
8
10
Time (sec)
12
14
16
18
20
V1 (%)
200
100
0
V2 (%)
10
5
0
8
TEST RESULTS
• Voltage imbalance increased the starting time to 17 s
(3 seconds delay).
Ia (%)
1000
500
0
0
2
4
6
8
10
Time (sec)
12
14
16
18
20
0
2
4
6
8
10
Time (sec)
12
14
16
18
20
0
2
4
6
8
10
Time (sec)
12
14
16
18
20
T (%)
500
0
wr (rad/sec)
-500
500
0
-500
9
TEST RESULTS
• Results for all three transformers under base (actual
data) conditions
SAT 1
Sequoyah
33 MVA
SAT 2
Sequoyah
18 MVA
SAT 3
J. M. Farley
26 MVA
Start
Run.
Start.
Run.
Start.
Run.
Accel. time Normal (s)
14
-
15
-
17
-
Accel. time Open –Phase (s)
17
-
24
-
45
-
V2/V1 %
8.6
1.2
16
2.3
28.4
4.15
Ia current increase %
-15.3
-8.64
-27.51
-16.13
-44.2
-28.36
Ib , Ic current increase %
-4.3
5.6
-6.06
11
-9.03
20.87
Total heating increase %
-15.1
2.16
-23.66
5.58
-34.5
14.5
10
TEST RESULTS
• Results under unified MVA and positive sequence
impedance conditions:
SAT 1
Sequoyah
33 MVA
SAT 2
Sequoyah
18 MVA
SAT 3
J. M. Farley
26 MVA
Start
Run.
Start.
Run.
Start.
Run.
Acceln. time (s)
17
-
18
-
28.5
-
V2/V1 %
8.6
1.2
10
1.4
25.4
3.64
Ia current increase %
-15.31
-8.64
-18.34
-10.04
-40.5
-25.49
Ib , Ic current increase %
-4.31
5.6
-4.27
6.65
-8.56
18.43
Total heating increase %
-15.05
2.16
-16.68
2.8
-32.46
12
11
TEST RESULTS
• Results under base conditions with Two SATs in
Parallel:
SAT 1
Sequoyah
33 MVA
SAT 2
Sequoyah
18 MVA
SAT 3
J. M. Farley
26 MVA
Start
Run.
Start.
Run.
Start.
Run.
Acceln. time (s)
15.5
-
19.6
-
28.5
-
V2/V1 %
4.6
0.65
8.8
1.24
17
2.39
Ia current increase %
-9
-4.6
-16
-8.8
-20.7
-16.65
Ib , Ic current increase %
-2.3
2.96
-3.9
5.5
-10.9
11.17
Total heating increase %
-8.77
1%
-14.85
2
-26.1
5.56
12
CONCLUSION
• lower values of primary to ground zero sequence
impedance → better voltage imbalance → better
starting and running performance.
• A maximum voltage unbalance (V2/V1) of less than
10% during starting allowed adequate acceleration of
RCP.
• Delayed motor acceleration, or possibly failure to
start could be expected if the voltage imbalance goes
beyond 10%.
13
CONCLUSION
• Accurate determination of zero sequence
impedances is necessary to assess the response of
substation auxiliaries and accordingly decide on the
appropriate measures in the event of an open phase
fault on the primary SAT level.
1
Open – Phase Detection
Considerations for Nuclear Power
Generating Station
Electrical Systems
Tony Sleva, LSM - IEEE
Senior Engineering Consultant
Altran
2
Typical Nuclear Generating Station
One Line Diagram
3
Typical Nuclear Generating Station
Positive Sequence Diagram
4
Typical Nuclear Generating Station
Negative Sequence Diagram
5
Typical Nuclear Generating Station
Zero Sequence Diagram
6
Typical Nuclear Generating Station
Combined Sequence Diagram
7
Observations
• Sequence Models are functions of Connected Load
• Load Impedance is a function of Motor Speed
• Positive Sequence Motor Impedance is different
than Negative Sequence Motor Impedance
• Positive Sequence Torque = Accelerating Torque
• Negative Sequence Torque = Decelerating Torque
• No Zero Sequence Torque
8
Observations
• Sequence Voltage and Current are functions of
Connected Load
• When Zero Sequence Impedance is High,
Zero Sequence considerations are minimal
• When Zero Sequence Impedance is Low,
Negative Sequence considerations are
minimal
9
Observations
• High Zero Sequence Impedance
(Zero Sequence considerations are minimal.)
• Motors Rotating during open phase conditions:
o Positive sequence torque is reduced
o Negative sequence torque is low
o Motors will continue rotating
o Motors will overheat (minutes)
10
Observations
• High Zero Sequence Impedance
(Zero Sequence considerations are minimal.)
• Motor Acceleration during open phase conditions:
o Positive sequence torque is reduced
o Negative sequence torque is high
o Motors will not accelerate to rated speed
o Motors will overheat (seconds)
11
Observations
• Low Zero Sequence Impedance
(Negative Sequence considerations are minimal.)
• Motors Rotating during open phase conditions:
o Positive sequence torque is reduced
o Negative sequence torque is minimal
o Motors will continue rotating
o Motors will not overheat
12
Observations
• Low Zero Sequence Impedance
(Negative Sequence considerations are minimal.)
• Motor Acceleration during open phase conditions:
o Positive sequence torque is reduced
o Negative sequence torque is minimal
o Motors will Accelerate
o Motors will overheat slowly
13
Observations
• Traditional Open Phase Detection Schemes are
applied on a per Motor basis
• Non-Nuclear Electrical Systems are designed for
staged load increases
• Nuclear Power Generating Station Electrical
Systems are designed for Rapid Step Loading
14
Conclusions
• Understanding Zero Sequence Impedance is KEY
• Open Phase Detection with Microprocessor Based
Protective Relays is achievable
• Utilize Open Phase Watchdogs
• Consider “Voting Logic” Applications
15
Conclusions
Open Phase Watchdog Parameters
 Monitor for Negative Sequence Current
 Monitor Total Current
 Monitor Positive Sequence Phase Angle
Negative Sequence Current, Large Phase Angle (~700),
High Total Current
Motor Acceleration – Quick Trip
Negative Sequence Current, Moderate Phase Angle
(~400), Moderate Total Current
Motors Rotating – Slow Trip
1
Open-Phase Fault (OPF)
Modeling and Analysis
in ETAP
Salman Kahrobaee
Senior Electrical Engineer
ETAP
Tanuj Khandelwal
Senior Principal Engineer
ETAP
2
Agenda
•
•
•
•
•
•
Unbalanced System
OPF Study Objective & Requirements
OPF Analysis and Related Indexes
General Element Modeling
Examples
Conclusion
3
Real Unbalanced Power System
• Distribution System
– 3-phase, 2-phase, and single-phase network
components
– Unbalanced loads
• Transmission System
– Untransposed long transmission line
– Unbalanced loads (e.g., electrical railway traction
motors)
• Abnormal condition with open-phase fault
4
Impact of System Unbalance
• Overheating of equipment
• Stalling of running motors
• Longer motor acceleration times
• Nuisance tripping of protective relays
• Increased real power losses
• Lifetime reduction
5
OPF Study Objectives
• Identify the level of unbalance (voltage and current)
throughout the plant
• Analyze system performance during OPF
• Determine whether existing protective systems will
detect the OPF condition
• Determine the protective device settings to avoid
excessive heating
6
Unbalance Factors/Indexes
• System unbalance is considered as a disturbance
• Voltage/Current unbalance factors
• Ratio of negative sequence to positive sequence
• Ratio of zero sequence to positive sequence
• Ratio of maximum deviation from average value
to average value
7
OPF Study Requirements
• Positive, negative and zero-Sequence Impedances
• Mutual coupling of overhead lines
• No-load losses for Buried delta windings for 2&3
winding transformers
• Transformer winding connections
• 3-phase, 2-phase and single-phase network
components
• Internal impedance of electric machines
8
OPF Study Method
• The OPF analysis should consider
– Operating scenarios (events/loading)
– Configurations
– Amount of potential voltage unbalance in the incoming
plant power supplies (grid unbalance)
• Overall analytical method is a steady-state load flow
technique
• Current Injection (CI) method
9
Buried Delta Winding
• Assumption:
– Symmetric
– Linear – no saturation
• 3 independent variables/impedances: 2-winding
– 3 sets of test data
• 6 independent variables/impedances: 3-winding
– 6 sets of test data
10
Buried Delta Winding
• Impedance modeling based on test data
11
3-Phase Machines
• Sequence models
12
Loads
• Single-Phase Loads
– Different connection types: AB, BC, CA, AN, BN or CN
• Three-Phase Loads
– Connected in Delta, Wye or Wye-G
• Lumped loads consisting of constant power, constant
impedance and constant current components
13
OPF Analysis Software
• ETAP NUUG began review of ETAP’s ability to perform
OPFA shortly after the Byron open-phase event in
early 2012
• ETAP is used throughout the world by nuclear plants,
research laboratories, consulting firms, government
agencies, and other organizations
• Multiple OPF simulations can be performed
considering different scenarios (transformer loading
and open-phase grounding)
14
Steady State Simulation
• Protective relaying settings will be based on steady
state values in order to avoid nuisance tripping
• Transient studies using EMTP requires considerable
amount of time to model a complete power
electrical system
• Most of the electrical systems required are available
and minimal data entry is need in order to expand
the model to perform open phase fault simulation
• Phase and sequence reporting
15
Sequence Network Connection
• Open Phase Fault
16
Example
Steady State vs. Transient
• 3-phase Currents
• 3-phase Voltages
17
Example
Buried Delta & Motor Loading
Scenario
Revision
Scenario 1
Scenario 2
Scenario 3
Scenario 4
Base
Base
Buried-Delta
Buried-Delta
Mtr1
Study Case
ULF-100%
ULF-50%
ULF-100%
ULF-50%
Mtr2
25.00%
20.00%
15.00%
10.00%
5.00%
0.00%
Scenario 1 Scenario 2 Scenario 3 Scenario 4
18
Example
Double Open-Phase Fault
• Heavily Loaded Motors
• Ground path
Study #
Study 1
Study 2
Study 3
Study 4
Study
Study 1
Study 2
Study 3
Study 4
Double Open-Phase Fault
Study Case
B-C open at T1 primary side
B-C open at T1 primary side
B-C open at T2 primary side
B-C open at T2 primary side
Mtr35
V1 L-L
(%)
34.31
108.3
105.8
101.6
V2/V1
(%)
100
5.69
3.2
0.31
Summer
Winter
Summer
Winter
Mtr31
V1 L-L
(%)
33.8
107.4
70.48
103.5
V2/V1
(%)
99.97
5.62
100
4.3
19
Conclusion
• Data requirement including element connections,
sequence impedance, unbalanced loads & sources
• Application of a validated simulation software
• OPF analysis on various scenarios
• Protection scheme based on provided limits