Generator Testing and Model Validation Lei Wang ([email protected]) Powertech Labs Inc. November 2012
Generator Testing and
Model Validation
Lei Wang
Powertech Labs Inc.
November 2012
� Introduction
� WECC generator testing program
� Generator testing and model validation approaches
� Examples
� Current trend
2
Topics
� Generator is the single most important equipment in
a power system
� The adequacy and accuracy of its model have
significant impact on the results of the system
analysis performed using computer simulations
� Three approaches to obtain generator models:
� From “default” or “typical” model or data – usually
erroneous
� From design – may not be fully consistent with the actual
equipment in use
� From field testing – able to provide good models suitable
for system studies3
Introduction
� Unfortunately, not impressive in many cases
� Typical models are almost certainly inaccurate; design models
are often questionable
� There may even be serious deficiencies for models obtained
from testing by manufacturer
� Tests done may not stick to common standards so results may be hard
to apply
� Models provided may be incomplete
� Models provided may be incompatible with the designated simulation
tools so modifications or simplifications may be necessary
� Tests may be done long time ago so recent equipment retrofitting or
replacement may not be reflected
� Results: simulated system behaviors may be far from
acceptable!4
How good are our models?
� Following the August 10, 1996 WSCC (WECC) blackout, a
simulation was performed using the WSCC standard planning
models
5
A classical case to show importance of models
Measured
response
Simulated
response
� Rush Island Incident in Mid-US (June 12, 1992)
� Fault at Tyson Bus 3 tripped one circuit from
Tyson to Rush Island
� Protection malfunction tripped another
circuit from Tyson to Rush Island
� As a result, two units at Rush Island were
connected to the main grid through only
one 345 kV circuit
� A 1.0 Hz sustained oscillation occurred
6
Another case
RushIsland
#1 #2
Lutesville
Tyson
Labadie
Bus 3
Generator speed at Rush Island
� Initial analysis for this incident using the standard planning
model could not replicate the oscillations
� Therefore, operational studies done prior to the incident could not
identify this problem
� It was later found that the excitation system at the Rush Island
plant had been replaced, but the planning model was not
updated
� Therefore the model in the planning case was totally incorrect
� A field test was done and a new exciter model was developed
� The oscillations could be observed in simulations with the new exciter
model
7
Analysis of this incident
� WECC mandated generator testing policy applies to all
generators
� Single generator > 10 MVA, or
� Generating facilities > 20 MVA, or
� Connected to transmission system at 60 kV or higher
� About 95% of generators (2000 units in total approximately)
have complied with the full
baseline testing requirements
� Major utilities have already
started model re-validation
testing
� The results submitted must be
compatible to WECC approved
models8
WECC generator testing program
� Full baseline testing (full testing and once in life time)
� Existing generators that have never been tested
� New generators (within 180 days)
� Generators with major equipment modifications
� Generators identified having different responses than model
� Model performance re-validation (every 5 years)
� Validate excitation system response
� Disturbance monitoring or
� Voltage step tests or
� Frequency scan
� Validate governor response
� From observed unit response to grid frequency change
� Verify generator reactive capabilities
9
WECC generator testing program (cont’d)
� Mod-025-1 Verification of Generator Gross and Net Real
Power Capability
� Mod-026-1 Verification of Models and Data for Generator
Excitation System Functions
� Mod-027-1 Verification of Generator Unit Frequency
Response
� Field test, among other methods, is acceptable to verify the
above models and data
10
Related NERC standards
� This is a common method to obtain and to validate generator
models and parameters
� A number of standard and customized field tests can be
performed
� Models and parameters are then derived from the testing
results
� This often requires optimization of model parameters through
simulations to fit testing results
� The resulting models can be made compatible with the required
simulation software
11
Overview of generator model testing and validation
� Problems with plant equipment are identified
� Examples: 1) Reactive protections/controls improperly set or not
working; 2) PSS out-of-service
� Measurements can be used to derive good models
� Predicting unit/system performance and identifying operational
problems
� Is there oscillations between units or between areas under certain
contingencies?
� Is there a risk of instability and/or unit trip?
� System Design
� Design of load-shedding scheme – inertias/governors are critical
� Assessment of stability limits & identification of need for new
facilities
� Post-mortem studies
12
Benefits from generator model testing
� Controls can be tuned for optimal plant and system
performance
� To get maximum output from plants, controls and protection must be
working and tuned properly (local tuning)
� To allow maximum transfers between regions requires special
emphasis on excitation and PSS settings (global tuning)
� To provide maximum robustness to disturbances requires all controls
to be properly set
13
Benefits from generator model testing (cont’d)
� Model data quality for generators, excitation systems, turbine
governors, stabilizers, and even excitation limiters has been
improved significantly
� Better tuned excitation and speed control systems
� Plant operators know more about their machine capabilities
� System study engineers have more confidence in the model
data they use for various studies.
� Better correspondence between model simulations and
actual events
14
WECC generator testing achievement
15
The proof
Measurement vs simulation comparison
Frequency at Malin 500 kV California-Oregon Interface (COI) Power
Double Palo Verde trippingMeasurementSimulation
� Open-circuit saturation test
� D-axis test
� Exciter step response test
� Partial load rejection test
� Reactive capability test
16
Generator tests – main
� Generator is running at rated speed and
disconnected from the grid
� Generator excitation is increased to raise the
terminal voltage gradually from the lowest possible
value to the highest possible value
� Saturation factors S(1.0) and S(1.2) are obtained
from the results
17
Open-circuit saturation test
18
Open-circuit saturation test – example
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850
Generator Field Current in Amperes
Gen
erat
or t
erm
inal
vol
tage
in p
er u
nit
Measured
Simulated
Air-gap line
� Generator is running at rated speed without MW
load but with a small negative MVAR load (under
excited)
� Generator excitation is on manual
� Generator is tripped from the system to reject the
MVAR load, and its terminal voltage is recorded
� D-axis parameters (xd, x’d, T’d0, etc.) can be obtained
from this test
19
D-axis test
20
D-axis test – example
0.70
0.75
0.80
0.85
0.90
0.95
1.00
0 5 10 15 20 25 30 35 40 45 50 55 60
Time in seconds
Gen
erat
or t
erm
inal
vol
tage
in p
er u
nit
Measured Simulated
Generator is tripped
� Generator is operated at full speed and disconnected
from the grid
� A step change (usually 5% to 10%) is applied to the
AVR reference set point
� Generator field and terminal voltages are recorded
� Exciter step response test is used to establish
excitation system model parameters
21
Exciter step response test
Where AVR step applied
22
Exciter step response test – example
0.99
1.00
1.01
1.02
1.03
1.04
1.05
1.06
1.07
0 2 4 6 8 10 12 14 16 18 20
Time in seconds
Gen
erat
or t
erm
inal
vol
tage
in p
er u
nit
-0.2
0.1
0.4
0.7
1
1.3
1.6
1.9
2.2
Gen
erat
or f
ield
vol
tage
in p
er u
nit
Measured Vt Simulated Vt
Measured Vf Simulated Vf
Negative step is added
Positive step is added
� Generator is running at rated speed with a small MW
load and a small negative MVAR load (under excited)
� Generator is tripped from the system to reject the
load, and its speed is recorded
� The generator inertia constant (H) and some of the
governor parameters can be obtained from this test
23
Partial load rejection test
24
Partial load rejection test – example
59.5
60.0
60.5
61.0
61.5
62.0
62.5
63.0
63.5
0 10 20 30 40 50 60 70 80Time in seconds
Gen
erat
or s
peed
in H
z
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
Wic
ket g
ate
posi
tion
in p
er u
nit
Measured speed Simulated speedMeasured gate Simulated gate
Generator is tripped
� To determine unit reactive power capabilities in steady-state
operating conditions
� Test Procedure
� The excitation is varied from minimum MVAR (absorbing) to maximum
MVAR (generating) at rated MW
� Hold for 15 minutes to verify that the minimum and maximum MVAR
can be maintained without equipment overheating, alarming, and
other prohibited impact
25
Reactive capability test
26
Reactive capability test – example
MVAR MW Armature
Volts Exciter
Field Volts Exciter
Field Amps Limiting Factor
Max 36.4 52.0 14,705 50.8 11.4 Generator voltage
Min -30.6 51.9 12,937 29.4 6.2 Load angle limiter
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80 90
Generator active power output (MW)
Gen
erat
or r
eact
ive
pow
er o
utpu
t (M
VA
R)
Reactive Power Capability Test Points
Load Angle Limiter Setting
Generator Capability Curve (0C)
Generator Capability Curve (50C)
� Field short-circuit test
� Q-axis test
� V-curve measurement
� Reactive load rejection test
� Under-excitation limiter (UEL) test
� Over-excitation limiter (OEL) test
� V/Hz limiter test
� Governor droop test
� PSS frequency response and step response test
� PSS performance test
� Governor step response test
� Water starting time constant test
� Permanent and temporary droop test
� Depending on the situation, only a selected set of these may be performed for a
unit
27
Generator tests – other
� Gather and review plant/unit documents
� Develop a test plan
� Conduct pre-test simulations if necessary
� Perform on-site testing, i.e., execute the test plan
� Develop models based on field test results
� Prepare field test and model validation study report
28
WECC generator testing and model validation procedure
� This is from Powertech’s experience
� Testing team consists of
� 1 testing engineer/technician (Powertech)
� 1 or more plant P&C technicians (Client)
� Other plant personnel (operation, safety, etc.) (Client)
� All WECC compliancy testing can be done in 2 days
� The unit to be tested must be fully available
� The results delivered include
� An engineering report
� A set of models in specified standard format (DSATools,
PSS/E, etc.) ready for use in stability analysis
29
WECC generator testing and model validation practice
� Testing Equipment. Example:
� Tabula data acquisition system (hardware & software)
developed by Powertech
� Simulation Tools. Example:
� DSATools software package developed by Powertech
� Powerflow and Short-circuit Analysis Tool (PSAT)
� Transient Security Assessment Tool (TSAT)
� Small Signal Analysis Tool (SSAT)
� Control Design Toolbox (CDT)
� MATLAB/Simulink
30
Tools required
� There are two types of parameters in generator models that
can be adjusted
� Correctable
� These are the parameters that are entered inconsistently or
incorrectly according to the physical laws
� Such parameters may cause unreasonable or wrong simulation results
� Tunable
� These are parameters that can be set within a range to optimize the
equipment or system performance
� Improperly tuned parameters result in under-performing equipment
or system
31
Generator model validation
� Synchronous machine
� Over-netting
� Reactive capability
� Source impedance
� Inconsistent synchronous, transient, subtransient, and leakage
reactances/time constants
� Saturation factors
� Exciter/AVR
� Inconsistent limits with initial condition
� Inconsistent line-drop compensation
� PSS
� Wrong PSS gain sign for electric power input PSS
� Governor
� Inconsistent turbine rating with generator rating32
Correctable parameters – common problems
� Synchronous machine
� Nothing is tunable
� Exciter/AVR
� Time constants in the transient gain reduction
� AVR gain
� Parameters in stabilizing feedback
� PSS
� Time constants in phase compensator
� PSS gain
� Most of the other parameters (reasonably typical values can be used)
� Governor
� Droop
� Governor controls
33
Tunable parameters
� Messages produced from simulation software
� TSAT limit violation check
� TSAT Dstate check
� PSS/E DOCU check
� Data checking tools built in simulation software
� No-fault simulation check
� Exciter/governor step response check
� Special tools
� NERC/ERAG dynamic database
� Control Design Toolbox (CDT) for PSS tuning
� Other
� Field testing
34
Tools for generator model validation
Generator model/data problem
� Incorrect transient reactance
� Occasionally bad stator resistance
� See the examples from a NERC case
� Inconsistent MW/MVA output with
respect to MVA rating
� See the examples from a NERC case
� Questionable damping coefficient D
� A D of 10 was identified from a recent
production case
35
Model validation example – 1
Incorrect transient reactance
Inconsistent generator outputs
Exciter model/data problem
� Incorrect gain and/or transient gain reduction
� For the exciter model shown
� KA = 1.509; TC = 0.45; TB = 0.1 (transient gain > 1 → normally less than 1)
� This leads to instability of the generator for a fault far from it
� The unit is stable with a set of more reasonable parameters:
KA = 50.0; TC = 1.0; TB = 1.0
36
Model validation example – 2
Generator relative angle (deg) : Reference Generator = 338 [SUND#2GN18.0] ' 2'
Time (sec)
0.000 6.000 12.000 18.000 24.000 30.000-50
0
50
100
150
Generator relative angle (deg) : Reference Generator = 338 [SUND#2GN18.0] ' 2'
Time (sec)
0.000 6.000 12.000 18.000 24.000 30.000-50
0
50
100
150
Generator relative angle (deg) : Reference Generator = 338 [SUND#2GN18.0] ' 2'
Time (sec)
0.000 6.000 12.000 18.000 24.000 30.000-50
0
50
100
150
Generator relative angle (deg) : Reference Generator = 338 [SUND#2GN18.0] ' 2'
Time (sec)
0.000 6.000 12.000 18.000 24.000 30.000-50
0
50
100
150
-50
150
0
50
100
Generator relative rotor angle (deg)
0 306 12 18 24
Time (seconds)
Original data
Modified data
PSS model/data problem
� A growing local oscillation shows from a
nofault simulation
� It turns out that the unit has a PSS
� Electric power is used as input to the PSS
� However, the gain KS is set at a positive value
� This reduces the damping torque of the local
mode
� Setting KS to a negative value resolves the
problem
37
Model validation example – 3
Generator relative angle (deg) : Reference Generator = 490 [GENES 3920.5] ' 3'
Time (sec)
0.000 4.000 8.000 12.000 16.000 20.000-40.000
-39.000
-38.000
-37.000
-36.000
-35.000
2
11
1
sT
sT
++
4
3
1
1
sT
sT
++
W
W
sT
sT
+1 SK
maxSV
minSV
VsPe
Governor model/data problem
� This is a digital governor
� A set of typical governor parameters results
in poor performance for the local mode
under a critical contingency
� Tuning of the governor control (mainly PI
controller) significantly improves the
damping
38
Model validation example – 4
Generator speed (Hz)
Time (sec)
0.000 2.000 4.000 6.000 8.000 10.00059.300
59.540
59.780
60.020
60.260
60.500Generator speed (Hz)
Time (sec)
0.000 2.000 4.000 6.000 8.000 10.00059.300
59.540
59.780
60.020
60.260
60.500Before tuning
After tuning
39
Model validation example – 5
� PSS validation by simulation and field testing
(part of the WECC generator testing and
model validation requirements)
� The unit is a 444 MVA steam unit with PSS
� The existing PSS parameters are implemented
by a third company (method unknown) and
approved by the unit manufacturer
� During the model validation, Powertech found
that the PSS parameters are not the optimal
� A tuning using CDT gave better parameters
� The new parameters were approved and implemented by the
unit manufacturer after the simulation and testing verification
40
Example – 5 (cont’d)
280
285
290
295
300
305
310
315
320
325
0 1 2 3 4 5
Time (seconds)
Act
ive
po
wer
ou
tpu
t (M
W)
Without PSSWith existng PSSWith tuned PSS
280
285
290
295
300
305
310
315
320
325
0 1 2 3 4 5
Time (seconds)
Act
ive
po
wer
ou
tpu
t (M
W)
Without PSSWith exisiting PSSWith tuned PSS
Field testing results
Simulation results
� Use of PMU data in model verification
� PMU is widely installed in US thanks to the Obama stimulus funding
� Generator model verification is one of the promising applications that
are offered by the PMU technology
� WECC/BPA approach is described here
� This requires special “playback” feature in simulation package
� TSAT supports this feature
41
Technical trend in model validation/verification
� The verification is done after a major event (disturbance)
� So there are significant variations in system responses
� A base case is built to represent the operation condition prior
to the disturbance
� Powerflow and the matching dynamics
� An on-line TSA system is ideal in providing these
� It may be beneficial to reduce the full case to include only the
plant to be verified, to speed up the analysis efficiency
� Special simulations are performed
42
Procedure
~Recorded voltage and frequency are injected in the simulations
Generator power (MW and MVAR) is simulated and compared with measurements
� Chief Jo braking resister insertion
� Used to help maintain stability
� Inserted into the system for about
one-half second when certain
abnormal system conditions are
detected
� Consists of five kilometers of
one-half inch stainless steel wire,
each wire strung in a vertical
configuration on a modified
transmission tower.
43
Example – event
44
Example – good model
480.00
605.00
480.0000 a2 1 WECC 500.0 0 0.0 1 1 605.0000 480.0000 pbr 40287 COULEE 500.0 0 0.0 1 1 605.0000
Time( sec )10.0 25.0
TASMO MODEL; OUTPUT GENERATED 2002-07-16 11:52:05SWINGBUS 1520 FOR FC-2001-1:2003-07-14:17:4F--1--1-0-0
ation\grand coulee\unit19
Tue Nov 18 09:56:11 2008
Page 1
coulee19-2008-08-19-1310.chf
Grand Coulee Unit 19 – active power
-100.00
-45.00
-100.0000 a3 1 WECC 500.0 0 0.0 1 1 -45.0000 -100.0000 qbr 40287 COULEE 500.0 0 0.0 1 1 -45.0000
Time( sec )5.0 30.0
TASMO MODEL; OUTPUT GENERATED 2002-07-16 11:52:05SWINGBUS 1520 FOR FC-2001-1:2003-07-14:17:4F--1--1-0-0
ation\grand coulee\unit19
Tue Nov 18 09:56:11 2008
Page 2
coulee19-2008-08-19-1310.chf
Grand Coulee Unit 19 – reactive power
MeasurementSimulation
45
Example – questionable model
Grand Coulee Unit 13 – active power
65.00
100.00
65.0000 a2 1 B1 13.8 0 0.0 1 1 100.0000 65.0000 pg 2 B2 13.8 0 0.0 01 1 100.0000
Time( sec )1.0 7.0
ef joseph\chief jo 230ph4
Tue May 19 00:03:25 2009
Page 1
cjj.chf
MeasurementSimulation