GEH-6676E
EX2100 and EX2100e Excitation Control
Power System Stabilizer
User Guide
GE Internal
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Acronyms and Abbreviations
AVR
Automatic Voltage Regulator
IEEE
Institute of Electrical and Electronics Engineers
PSS
Power System Stabilizer
VAR
Volt-amperes Reactive
GEH-6676E User Guide 3
GE Internal
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strictly observed, could result in personal injury or death.
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Indicates a procedure, condition, or statement that, if not
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Indicates a procedure, condition, or statement that should be
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Attention
4 GEH-6676E
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EX2100 and EX2100e Excitation Control Power System Stabilizer
Control System Warnings
Warning
Warning
To prevent personal injury or damage to equipment, follow all
equipment safety procedures, Lockout Tagout (LOTO), and site
safety procedures as indicated by Employee Health and Safety
(EHS) guidelines.
This equipment contains a potential hazard of electric shock,
burn, or death. Only personnel who are adequately trained and
thoroughly familiar with the equipment and the instructions
should install, operate, or maintain this equipment.
Isolation of test equipment from the equipment under test
presents potential electrical hazards. If the test equipment
cannot be grounded to the equipment under test, the test
equipment’s case must be shielded to prevent contact by
personnel.
Warning
To minimize hazard of electrical shock or burn, approved
grounding practices and procedures must be strictly followed.
To prevent personal injury or equipment damage caused by
equipment malfunction, only adequately trained personnel
should modify any programmable machine.
Warning
Warning
Always ensure that applicable standards and regulations are
followed and only properly certified equipment is used as a
critical component of a safety system. Never assume that the
Human-machine Interface (HMI) or the operator will close a
safety critical control loop.
GEH-6676E User Guide 5
GE Internal
Contents
1 Overview ............................................................................................................................................. 7
1.1 Power System Stability .............................................................................................................................7
1.2 Synchronous Machine Oscillation ...............................................................................................................8
1.3 System Modeling.....................................................................................................................................9
1.4 Implementation ..................................................................................................................................... 11
2 Integral of Accelerating Power.................................................................................................... 13
2.1 EXDSPEED ......................................................................................................................................... 15
2.2 PSS2B ................................................................................................................................................. 16
3 Operation and Tuning.................................................................................................................... 19
3.1 Initial Performance Testing and Configuration............................................................................................. 19
3.1.1
3.1.2
Enable/Disable and Active/Inactive .................................................................................................... 19
Parameters and Configuration Settings ................................................................................................ 21
3.2 Commissioning and Testing ..................................................................................................................... 27
3.2.1 Initial Conditions Check ................................................................................................................... 27
3.2.2
Gain Margin Test ............................................................................................................................ 29
3.2.3
3.2.4
Online AVR Step Test with PSS Disabled ............................................................................................ 34
AVR Step Test with PSS Enabled ....................................................................................................... 38
3.2.5
3.2.6
Impulse Test with PSS Enabled or Disabled ......................................................................................... 40
AVR Closed Loop Frequency Response............................................................................................... 43
3.2.7
PSS Open Loop Frequency Response.................................................................................................. 47
3.2.8
3.2.9
Testing Complete ............................................................................................................................
Frequency Response Test Data Processing (Optional) ............................................................................
3.2.10 AVR Closed Loop Transfer Function ..................................................................................................
3.2.11 PSS Open Loop Transfer Function ..................................................................................................... 50
3.2.12 Disable and Enable Testing (Optional) ................................................................................................ 52
3.2.13 Additional Unit Testing .................................................................................................................... 52
Glossary of Terms ................................................................................................................................ 53
Index......................................................................................................................................................... 55
6 GEH-6676E
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1
Overview
References to the exciter are
applicable to either the
EX2100 or EX2100e control.
The PSS is an optional automatic software control function that may be used with field
excitation control systems to improve synchronous machine stability. There are various
implementations of the PSS; however, a fully integrated digital PSS, based on the integral
of accelerating power principle, is available for the EX2100 and EX2100e Excitation
Controls systems.
This document provides information about the EX2100 and EX2100e Excitation Control
Power System Stabilizer (PSS), as well as power system stability fundamentals, theory,
and site-commissioning and testing.
1.1 Power System Stability
This document requires a basic
understanding of synchronous
machines and electric power
flow.
Providing a reliable supply of electricity depends on machine stability. The simplest
definition of stability for synchronous machines is the system maintains a constant
voltage and frequency regardless of unanticipated load shifts between machines.
Additionally, when a transient event occurs and the subsequent machine voltage and
frequency oscillations are sufficiently damped to regain steady state operation, the system
is stable. Stability may be further defined as dynamic stability or transient stability as
follows:
Dynamic Stability: Also known as steady-state stability, allows a system to correct for
small changes.
Transient Stability: Allows a system to recover from large changes, such as electrical
faults cleared by operation of an instantaneous load rejection due to the operation of a
power circuit breaker. If there is enough synchronizing torque, the unit remains stable.
Refer to the section,
Synchronous Machine
Oscillation.
Overview
GE Internal
Modern generating units that are equipped with high-gain voltage regulator systems
enhance transient stability but tend to detract from dynamic stability. The PSS improves
small signal (steady-state) stability by damping the power system modes of oscillation
using generator excitation modulation.
GEH-6676E User Guide 7
1.2 Synchronous Machine Oscillation
Synchronous machine
oscillation is the behavior of
machines closely connected to
a system.
During a system transient, all rotor angles should move in the same relative direction over
time. While change of rotor angle in a single machine is a concern, the focus is on the
difference in rotor angle between machines, synchronous machine oscillation.
The PSS provides the control
action that allows the power
system to maintain stability.
•
Local machine-system (local mode)
•
Inter-area
•
Inter-unit
•
Torsional
Synchronous machine oscillation often falls into one of the following four categories:
Local mode generally involves one or more synchronous machines at a power station
swinging together against a comparatively large power system or load center. Frequencies
are typically in the range of 1.0 to 2.0 Hz. Some low inertia turbine generators can have
frequencies up to 4.0 Hz.
Inter-area usually involves combinations of many synchronous machines in one part of
a power system swinging against another part of the system. The frequency range is 0.1 to
0.7 Hz.
Inter-unit typically involves two or more synchronous machines at a power plant, or
nearby power plants, in which machines swing against each other. The frequency range is
1.5 to 3 Hz.
Torsional involves relative motion between a unit's rotating elements (synchronous
machine, turbine, and rotating exciter), with frequencies ranging from 15 Hz for two-pole
(8 Hz for four-pole) and above.
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EX2100 and EX2100e Excitation Control Power System Stabilizer
1.3 System Modeling
The PSS provides a positive
contribution to the damping of
the generator rotor angle
swings.
Static excitation systems with high-gain and fast-response times greatly aid transient
stability (synchronizing torque), but also reduce small signal stability (damping torque).
The PSS provides a positive contribution to the damping of the generator rotor angle
swings, which are in a broad range of frequencies in the power system.
The following figure is a simplified, linearized block diagram of a single machine
connected radially to an infinite bus power system. The diagram demonstrates the effect
of excitation systems on the damping of local mode machine oscillations. The generator is
also equipped with an Automatic Voltage Regulator (AVR). The characteristic
small-signal dynamics of a synchronous machine connected to a power system are
detailed by the swing equation linearized about an operating point, as indicated by the
solid lines (also known as the torque-angle loop). The mechanical loop is displayed at the
top of the figure while the electrical loop is in the middle. Phase relationships illustrate
that a positive synchronizing torque component (enhanced by modern high-gain
wide-bandwidth excitation systems) restores the rotor to a steady-state operating point by
appropriately accelerating or decelerating the rotor inertia. A positive damping torque
(decreased by modern high-gain wide-bandwidth excitation systems) dampens rotor
oscillations of the torque-angle loop. With proper phase compensation, the exciter control
provides air gap torque to dampen the oscillations.
Also illustrated in the following figure is the addition of a PSS to the control. The PSS
supplies a component of positive damping torque to offset the negative contribution of the
AVR, resulting in a compensated system that adds damping and enhances small signal
(steady-state) stability. This is accomplished by creating a signal in phase with rotor
speed, and summing the result with the AVR reference. Additionally, since the generator
field circuit and AVR function has an inherent phase lag, a corresponding phase lead is
required to compensate for this effect.
Overview
GE Internal
GEH-6676E User Guide 9
Single Machine Connected to Bus Power System with PSS
Coefficients K1 Through K6
Coefficient
Description
K1
Change in electrical torque due to a change in rotor angle assuming a constant d-axis flux
K2
Change in electrical torque due to a change in d-axis flux assuming a constant rotor angle
K3
Impedance factor
K4
De-magnetizing effect due to a change in rotor angle
K5
Change in terminal voltage due to a change in rotor angle assuming a constant voltage from d-axis flux
linkages
K6
Change in terminal voltage due to a change in d-axis flux linkages assuming a constant rotor angle
Except for K3, coefficients K1 through K6 are all affected by the operating point of the machine. All the coefficients are normally
positive, resulting in a stable system. However, K5 can be negative under conditions of heavy load, which can create an
unstable condition.
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EX2100 and EX2100e Excitation Control Power System Stabilizer
1.4 Implementation
Since the primary function of the PSS is to add damping to the power oscillations, basic
control theory would indicate that any signal in which the power oscillations are
observable would make a good candidate as an input signal. Some readily available
signals are direct rotor-speed measurement, bus frequency, and electrical power. From a
system design point of view, there are a number of considerations when selecting the
appropriate input signal. For instance, direct speed measurement may be susceptible to
turbine-generator torsional interactions.
Since the early development of the PSS, the GE design and application has been
extensively based on either speed or frequency input signal. The first applications were
speed-based, and the frequency signal was later used for two reasons: one being the more
practical means of obtaining the rotor velocity for hydro-turbines without shaft speed
measurements, and the lower torsional signal content for four-pole (nuclear steam)
turbine generators. The signals for either speed or frequency are similar in many respects,
but the lower torsional content of the frequency signals makes it better in many cases.
Another choice is electrical power, which has been extensively applied in some markets.
There have also been many applications where multiple input signals have been studied
and applied. In principle, many different signals can be used. The PSS can be approached
as a problem to be solved using multi-variable control design programs. The control
design program decides the type of control gains and phase compensation to be applied to
each input.
Refer to the section, Integral
of Accelerating Power.
The current generation PSS is based on the principle of accelerating power. Measurement
of accelerating power requires a mechanical power signal. In a practical sense, the
mechanical power cannot be measured, therefore, it becomes necessary to develop this
signal from speed and electrical power. The integral of accelerating power is a signal that
provides machine speed relative to a constant frequency reference.
The classic example of
inter-area mode oscillation is
the 0.3 Hz mode in Western US
(WSCC), between the Southern
California region and the
Pacific Northwest region.
The PSS can provide significant improvements in inter-area mode damping, with
application of stabilizers to most units that participate in these power-swing modes.
Improved damping can result in eliminating operating restrictions during system
contingencies, and increase power transfer limits.
Refer to the section,
Synchronous Machine
Oscillation.
PSS performance is often evaluated from the damping of the local mode, which is the
generator swinging against the rest of the power system. This mode is usually at
frequencies between 0.7 and 2 Hz. Stronger system ties and lighter loading tend to give
higher local-mode frequencies. Conversely, weaker ties and heavier loading tend to give
lower local-mode frequencies. The PSS must be properly tuned to provide acceptable
performance over a wide range of system conditions resulting from different operating
circumstances (such as out-of-service lines or varying load levels). Very elaborate
mathematical models (instead of the simplified model illustrated in the figure, Single
Machine Connected to Bus Power System with PSS) are used to predict the performance
of the PSS under steady-state and transient conditions.
Note Accelerating power measurement requires inputs of speed and electrical power. To
implement the PSS, the EX2100e control requires, at a minimum, a 3–phase potential
transformer (PT) and a 1–phase current transformer (CT), although it is preferred to have
two 1–phase CT inputs.
Overview
GE Internal
GEH-6676E User Guide 11
Notes
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EX2100 and EX2100e Excitation Control Power System Stabilizer
2
Integral of Accelerating Power
The integral of accelerating power principal is based on generator electro-mechanical
equations. The dynamic equation for rotor speed, as a function of torque, is as follows:
Synchronous Machine Swing Equation
where:
ω = rotor speed
H = generator inertia constant (MW-sec/MVA)
Tm = mechanical (turbine) torque
Te = electro-mechanical (air-gap) torque
Tacc = accelerating torque
In a per-unit (pu) system, torque and power are equivalent in value. Replacing torque (T)
with power (P), and rearranging the Synchronous Machine Swing equation to solve for
mechanical power gives the following:
Mechanical Power Equation
where the derivative operator has been replaced by the equivalent Laplace operator, s.
Mechanical power is difficult in practice to measure. This equation allows you to
synthesize the mechanical power signal from measurements of speed and electrical
power, which are relatively easy to obtain. Electrical power can change rapidly during a
transient event on the power system. Mechanical power changes slowly, moving in ramps
rather than steps. Thus, a special low-pass filter (ramp tracking) is used to filter the
synthesized mechanical power signal. The following figure displays the process of
deriving mechanical power.
The ramp tracking filter is represented as G(s).
The mechanical power signal, with the prime superscript, is represented asP'm, indicating
that this is a synthesized signal.
The next step develops the accelerating power signal, P'acc = P'm - Pe. The accelerating
power is labeled as a synthesized or derived signal at this point, as it is comprised from
synthesized mechanical power.
Deriving Mechanical Power
Integral of Accelerating Power
GE Internal
GEH-6676E User Guide 13
The two input signals, speed and electrical power, both have some steady-state value, and
may change slowly over long periods of time. For this reason, in most PSS designs, a
high-pass filter is applied to both inputs. This filter also functions as a washout filter since
it washes out or eliminates the low-frequency signals. The form of the washout filter is as
follows:
Washout Filter Format
where TW is the washout time constant, normally set in the range of 2 to 10 sec. This gives
a break frequency of 1/TW rad/sec.
As a final step, both inputs are divided by the factor 2H and integrated (equivalent to
dividing by s in Laplace terminology). The block diagram for developing the integral of
accelerating power is as follows:
Integral of Accelerating Power Block Diagram
The equation 1/(2H) multiplied by the integral of accelerating power is speed. If
mechanical power could be derived exactly, there would be this equivalence. Because of
the nature of the method used to derive the mechanical power signal, the resulting input
has the characteristics of speed at lower frequencies and electric power at higher
frequencies.
Additionally, the derived signal has a relatively low component of the torsional mode
components in the measurements. This very important factor could potentially impact
PSS performance, since the application limits any potential situation where the stabilizer
might interact with the turbine-generator torsional response. Because the integrator
essentially cancels the washout in the electric power signal path, a double washout is used
in both the speed and power paths.
14 GEH-6676E
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EX2100 and EX2100e Excitation Control Power System Stabilizer
2.1 EXDSPEED
The integral of accelerating power signal, EXDSPEED, is determined using the following
equation:
EXDSPEED Equation
For a signal that is proportional to rotor speed, generator current is multiplied by the
d-axis transient reactance, X'd, and vectorially added to terminal voltage to yield an
internal machine voltage, Eq'. The change or deviation in phase of Eq' is proportional to
deviation in rotor speed from synchronous speed.
An electrical power signal is calculated in the EX2100 control from generator voltage and
current. Both the rotor speed signal and power signal are processed by two washout stages
to remove low-frequency effects.
The equivalent speed signal, EXDSPEED, is determined by integrating (Pm-Pe) and
dividing by 2H, and is responsive to rotor speed without excessive phase lead at low
frequencies (which has a detrimental effect on synchronizing torque) and less susceptible
to generator terminal voltage offsets caused by rapid mechanical power changes inherent
in electrical power input PSSs.
The following figure illustrates the EXDSPEED signal is processed by two lead/lag
stages, an adjustable gain stage, and an output limiter stage to tailor the PSS for the
specific application.
Note Some units (primarily 4-pole nuclear units) require band reject filters to reduce the
response to torsional oscillations. The third lead/lag stage in the following figure
represents the low frequency equivalent of a two-stage torsional filter.
EXDSPEED Block Diagram
Integral of Accelerating Power
GE Internal
GEH-6676E User Guide 15
2.2 PSS2B
The PSS2B PSS model
conforms with the standards on
excitation systems, identified
as IEEE 421/5-2005.
The integral of accelerating power is the input to the part of the PSS that applies phase
compensation (two or three lead/lag stages), and a gain and output limit function. The
IEEE®-type PSS2B PSS model inputs are:
•
VSI 1: Speed
•
VSI 2: Electrical power
PSS2B IEEE Model
The EX2100 and EX2100e control implementation of an integral of accelerating power
PSS is available in a standard form. A specialized version with Biquad™ filters is also
available.
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Integral of Accelerating Power PSS Standard Form
Integral of Accelerating Power
GE Internal
GEH-6676E User Guide 17
Integral of Accelerating Power PSS with Biquad Filters
18 GEH-6676E
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EX2100 and EX2100e Excitation Control Power System Stabilizer
3
Operation and Tuning
3.1 Initial Performance Testing and Configuration
For additional tools, testing,
and studies available, contact
the nearest GE sales or service
office or an authorized GE
sales representative.
Refer to the section,
Enable/Disable and
Active/Inactive.
These optional tests are not
required to place the PSS into
service.
The PSS should not be placed into service until qualified test personnel complete a
thorough check of the PSS configuration settings and performance. The minimum PSS
configuration and operational checks that are discussed in this document, as well as basic
instructions for optional testing, include the following:
•
System tuning and PSS optimization studies
•
Review of PSS parameters
•
Instability gain margin measurements
•
AVR step response testing with PSS enabled and active
•
Impulse testing with and without PSS
•
AVR frequency response testing
•
System open loop frequency response testing
The following optional tests and studies are also recommended to ensure proper PSS
operation:
•
Compensated phase calculations
•
PSS Enable and Disable Testing
3.1.1 Enable/Disable and Active/Inactive
The PSS can be enabled (turned on or made available for operation) or disabled (turned
off or made unavailable for operation) through operator interface commands using the
turbine control or exciter operator interface at any time and at any load point.
The PSS is enabled by sending a PSS Enable command through an Ethernet Global Data
(EGD) connection to the exciter controller using the turbine control operator interface
screen or operator interface (keypad or touchscreen).
Operation and Tuning
GE Internal
GEH-6676E User Guide 19
PSS Enable Command Block Diagram
The PSS becomes active (in
service) or inactive (not in
service) based on satisfying
operational conditions.
In the software, Enabled is
also known as Armed, and
PSSARMD=FALSE equates to
PSS disabled.
Once enabled, the PSS is not active (available to supply compensation to the AVR input
summing junction) until all of the following three conditions are met:
•
Exciter must be in Automatic (AUTO) regulator control
•
Exciter must be running
•
Generator must be online at a load point above the parameter value
PSS Hi Watts Enable
If any of these three conditions are not met, the PSS becomes inactive, but still remains
enabled. If load is reduced below the value of PSS Lo Watts Disable, the PSS becomes
inactive. Placing the Regulator system to Manual regulator control or opening the 52G
breaker also causes the PSS to become inactive.
PSS Active Block Diagram
20 GEH-6676E
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EX2100 and EX2100e Excitation Control Power System Stabilizer
3.1.2 Parameters and Configuration Settings
Parameters and settings are located in the Control System Solutions (toolbox) and
ToolboxST* application. Both applications provide a graphical representation of the PSS,
including parameter values, input variables, and output variables, as well as a list of PSS
and PSS Biquad parameters.
� To display the PSS diagram
1.
From the toolbox or ToolboxST application, open the applicable exciter file.
2.
From the ToolboxST Component Editor, select the Diagrams tab.
3.
From the Tree View, select Main Menu Diagram (toolbox) or Overview
(ToolboxST).
4.
Locate the Power System Stabilizer section and click PSS to display the Power
System Stabilizer (PSS) diagram.
Power System Stabilizer (PSS) Diagram (toolbox)
Operation and Tuning
GE Internal
GEH-6676E User Guide 21
� To display PSS parameters
1.
From the toolbox or ToolboxST application, open the applicable exciter file.
2.
From the Outline View (toolbox) or the Component Editor Settings tab Tree
View (ToolboxST), expand Power System Stabilizer.
3.
Select Parameters to view a list of parameters.
PSS Parameters (toolbox)
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EX2100 and EX2100e Excitation Control Power System Stabilizer
PSS Parameters (ToolboxST)
Operation and Tuning
GE Internal
GEH-6676E User Guide 23
� To display PSS Biquad parameters
1.
From the toolbox or ToolboxST application, open the applicable exciter file.
2.
From the Outline View (toolbox) or the Component Editor Settings tab Tree
View (ToolboxST), expand Power System Stabilizer with Biquad.
3.
Select Parameters to view a list of parameters.
PSS Biquad Parameters (toolbox)
24 GEH-6676E
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PSS Biquad Parameters (ToolboxST)
4.
The following default values disable a Biquad. If Biquad is not used in the tuning
study, make sure the following parameters are set as follows:
•
PSS biquad3 num damp = 0.5 damp
•
PSS biquad3 num damp = 0.5 damp
•
PSS biquad3 nat r/s = 62r/s
3.1.2.1
The generator manufacturer
supplies this value.
Inertia
To obtain proper scaling for the synthesized mechanical power signal, the generator
inertia constant M is used in the washed out integral of watts path of the PSS.
3.1.2.2
Gain
Select the PSS gain to provide stable operation at all load points. This parameter is
typically defaulted to a value of 15, and should be adjusted during PSS commissioning.
Verify that the parameter is configured correctly by testing that the gain is less than a
value of 1/3 of the gain setting, which would cause the PSS loop to be unstable.
3.1.2.3
Lead/Lag 1 and Lead/Lag 2
Select the phase lead and lag time constants to cancel the natural phase lag of the AVR
and generator at full load. Lead values are typically around 0.2 sec. Lag values around
0.05 sec.
Operation and Tuning
GE Internal
GEH-6676E User Guide 25
3.1.2.4
Output Upper and Lower Limits
Select the upper and lower limits on the PSS output to reduce the ability of the PSS to
override the regulator during large disturbances. Typical values are ±10% but are
configurable.
3.1.2.5
Washout
Large enough washout time constants are selected to pass low frequencies of interest with
little attenuation or excessive phase lead. In most cases, this implies that the washout time
constants can be set between 2 to 10 sec.
3.1.2.6
Ramp Tracking Filter
The time delay for responses to slow increases in power during system daily load
changes. This parameter is typically set to 0.1 sec.
3.1.2.7
Hi Watts Enable and Low Watts Disable
If the PSS has been enabled, it is automatically activated above the Hi Watts enable
setting, and automatically inactivated below the Low Watts disable setting. It is typically
selected to be 15% and 10%, respectively.
3.1.2.8
Biquad
An enhanced PSS provides for up to three stages of biquadratic filtering to eliminate
torsional interaction, three stages of lead/lag filtering with gain and output limit, and
switchable washout to provide attenuation of voltage changes during large signal events.
26 GEH-6676E
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3.2 Commissioning and Testing
The following tools are needed for PSS commissioning and testing:
•
Toolbox or ToolboxST application with trend recording option
•
Help file printout of the diagram, Frequency (BODE) Analysis and Step Test . From
the diagram, right-click on empty white space and select Item Help.
•
If testing an EX2100 control with any version of ACLA or ACLE with firmware
prior to V09.00.00C for static exciter and V03.03.00C for the regulator, contact the
nearest GE sales or service office, or an authorized GE sales representative as the
configuration is different than described in this section.
If you need to download software to the ACLA, DO
NOT create or download a compressed controller
file.
Attention
•
(Optional) Results from GE Engineering Consulting or customer PSS tuning study
with applicable parameters for PSS entered into the exciter configuration file.
3.2.1 Initial Conditions Check
Prior to testing, observe the following conditions:
Caution
The exciter must remain in AUTO regulator throughout
the PSS test. If, at any time, unstable operation with the
PSS active is noticed, remove the PSS enable to stop
instability. Transferring to MANUAL regulator should
also stop any instability.
Pay particular attention to the information in this
section to ensure proper testing.
Attention
•
Verify that the PSS is disabled and Gain is set to 0 prior to going online to make sure
there is no inadvertent activation of the PSS prior to testing.
•
Prior to testing the PSS, other offline and online testing should be completed as
provided in the following documentation:
−
−
−
Operation and Tuning
GE Internal
GEH-6631, EX2100 Thyristor Control 77, 53, and 42 mm Installation Startup
Guide
GEH-6674, EX2100 Regulator Control Installation and Startup Guide
GEH-6694, EX2100 Thyristor Control 100 mm Installation and Startup Guide
•
Any deficiencies in PT or CT feedback circuits, including Watts or VAR calculations,
should be corrected.
•
The unit must be capable of full-load operation. For gas turbine units, bring the load
slightly below full-load to place the turbine control into speed/droop rather than
exhaust temperature control. If full-load is not possible, it is generally acceptable to
GEH-6676E User Guide 27
perform tests at greater than 80% of full load. If required by site conditions, consult
with Energy Consulting or the tuning study provider to determine if less-load is
acceptable.
28 GEH-6676E
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•
It is strongly recommended to perform all testing with the unit at near unity power
factor (PF) (0MVars). Perform all testing at as close to same load and VARS as
possible.
•
Any other outer loop regulator functions in EX2100/EX2100e, turbine, or plant
controls, such as VAR/PF and auto power (MW) load changing, should be turned off
or disabled.
•
Use the tuning study provided by GE to review the PSS parameters in the exciter
configuration file for accuracy and completeness. If configuration settings are not
provided by Energy Consulting, contact the nearest GE sales or service office, or an
authorized GE sales representative before using customer or default settings.
Incorrect and/or default settings may result in unstable unit operation or inadequate
PSS operation.
EX2100 and EX2100e Excitation Control Power System Stabilizer
3.2.2 Gain Margin Test
The instability point of the PSS is dependent upon many factors, including system
configurations, relative size of the unit with respect to the local grid, and transmission
characteristics. To find the point of instability, operate the exciter with the PSS active and
gradually increase the gain of the PSS to determine what gain causes PSS instability.
Typical valves for PSS gain are 6-15 pu (2 lead-lag designs) or 24-60 (3 lead-lag designs).
Testing is done up to a gain of four times the nominal recommended gain.
Without a PSS tuning study recommendation, a PSS
gain of 10 pu should be used. Do not exceed four times
this gain during gain margin testing.
Caution
Higher gain operation can be
used but should be confirmed
by GE.
Refer to the section, Initial
Conditions Check.
Refer to the section,
Enable/Disable and
Active/Inactive.
GE recommends a minimum gain margin of 10 db, which is a factor of three times the
nominal set gain. Testing is normally performed with gain up to four times the nominal
set gain value. If an instability gain is encountered, the final gain should be not more than
1/3 of the instability gain.
� To test the gain margin
1. From the PSS diagram, set the parameter PSS Gain to an initial value of 0 pu.
Perform all PSS testing at 80% load or higher and close to unity power (0 MVars).
2.
With the exciter in AUTO regulator, enable the PSS using the keypad or turbine
operator interface. If the unit is above the value of PSS Hi Watts, the PSS should be
enabled and active.
Exciter in AUTO regulator with PSS Enabled
3.
Operation and Tuning
GE Internal
Configure the Trender to monitor the variables in the following table in real time.
GEH-6676E User Guide 29
Variable Configuration Settings
Variable
Range
GN_VMAG
Average ± 0.01 pu
GN_VFLD for Bus-Fed systems, or
REGEXCURR for Brushless systems
AFFL < 10A
AFFL 10-20A
AFFL > 20A
Average ± 50 V
Average ± 2 A
Average ± 4 A
Average ± 6 A
WATTS
Average ± 0.02 pu
VARS
Average ± 0.05 pu
AVR\PSS_OUT
± 0.01 pu
PSSGN
0 - 4 x nominal PSS gain + 10
Note Make sure that within the trend, the Trend Recorder Configuration sample interval
is set to 32 ms. It is also recommended to set the Trender Time Axis to 300 sec so the
entire trend can be viewed throughout the test.
Refer to the figure, Unstable
Gain Margin Example
(Brushless Regulator).
4.
Start recording the variables for 30 sec, then increase the PSS gain from 0 pu to
normal gain setting and watch the variables for signs of instability. Instability would
be recognized as sinusoidal swings in power, VARs, or voltage. These swings usually
start small and increase in amplitude over time. Additionally, the power swings could
occur suddenly at a fixed-amplitude of oscillation. If either phenomenon is observed,
select PSS disable from the keypad, COI, or turbine control.
Tip � Have someone standing by to disable the PSS if necessary.
Refer to the following figures
for examples of unstable and
appropriate gain margin tests.
5.
Hold at nominal gain for 60 sec, then continue to increase the PSS gain to twice,
three times, and four times nominal gain. Hold at each point for 60 sec. The
oscillations in the MW trend begin to grow and have longer settling times. Watch for
any signs of instability and select PSS disable if this occurs. When four times
nominal gain is complete, reduce the gain back to zero, continue recording for 30 sec,
then stop the Trender.
6.
After test completion, review the trend for signs of instability using the provided gain
margin test examples. If instability is or has been observed, contact the tuning study
provider for changes and leave the PSS disabled with PSS Gain = 0 until stability has
been corrected. Repeat testing as necessary.
7.
If no instability is found, the nominal gain setting can be used for the remainder of
the test. Select PSS disabled and reset PSS gain to 0 before continuing.
Once a final gain setting is obtained, use the Trender to
monitor generator watts at this setting for at least five
minutes to verify that no instability occurs.
Caution
30 GEH-6676E
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
Note Gain optimization is not required to obtain acceptable performance. Most
applications provide adequate damping to local mode operations with a PSS gain of 15 or
less.
Unstable Gain Margin Example (Brushless Regulator)
Operation and Tuning
GE Internal
GEH-6676E User Guide 31
Noisy but no Instability and Good Gain Margin Example (Bus Fed)
32 GEH-6676E
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
Standard (Good) Gain Margin Example (Bus Fed)
Operation and Tuning
GE Internal
GEH-6676E User Guide 33
3.2.3 Online AVR Step Test with PSS Disabled
This test provides a baseline of
AVR operation with the PSS
disabled for comparison with
the PSS enabled.
Warning
Before stepping the AUTO regulator, verify that the
AVR step is configured for no more than a 2% step. If
requested by the tuning study provider, a higher value
such as 3% is acceptable.
This testing changes the output of the generator and can
rarely cause local instability on some power systems.
Caution
To demonstrate PSS effectiveness, step the AVR with the PSS disabled.
� To step the AVR with PSS disabled
Refer to related documentation
listed in the section, Initial
Conditions Check.
1.
Verify that the PSS Test Capture block is configured correctly for the type of unit
(Bus Fed [Static] or Brushless). If change is required, minor differences will be
displayed.
2.
Perform Validate/Build/Download using initialize all constants in accordance with
the appropriate installation and startup guide.
PSS Capture Block (Bus Fed)
PSS Capture Block (Brushless)
34 GEH-6676E
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
3.
Configure the step wizard for an AVR regulator 2% step. Verify the configuration
settings are as displayed in the following figure.
AVR Step Configuration Settings
Operation and Tuning
GE Internal
GEH-6676E User Guide 35
Frequency (BODE) Analysis or Step Test of Regulator Loops Diagram
36 GEH-6676E
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
Refer to related documentation
listed in the section, Initial
Conditions Check.
4.
For redundant control systems, perform the teaching of new settings to other
controllers in accordance with the appropriate installation and startup guide.
5.
Click Start / Stop Analysis to initiate the AVR step test.
6.
Upload the PSS Test Capture Buffer to the Trender.
As observed in the trend file, the unit MW (green trend, third from top) oscillates or rings
proportionate to the amount of natural damping in the system. For larger systems and
larger generators, there may be more oscillations recorded before the MW readings
stabilize.
AVR Step Test Capture Buffer with PSS Disabled
Operation and Tuning
GE Internal
GEH-6676E User Guide 37
3.2.4 AVR Step Test with PSS Enabled
38 GEH-6676E
GE Internal
1.
From the Frequency (BODE) Analysis or Step Test of Regulator Loops diagram, set
PSS Gain to nominal and select PSS enable.
2.
Step the AVR with PSS active.
3.
Upload the PSS Test Capture Buffer to the Trender. There should be a marked
difference (decrease) in the number and amplitude of oscillations in the power (MW)
variable on the Trender. This demonstrates the effectiveness of the PSS.
EX2100 and EX2100e Excitation Control Power System Stabilizer
AVR Step Test Capture Buffer with PSS Enabled
Operation and Tuning
GE Internal
GEH-6676E User Guide 39
3.2.5 Impulse Test with PSS Enabled or Disabled
This test provides further analysis of the PSS and provides a demonstration of its
effectiveness. It is similar to the Step Test, except it provides a high AVR step change for
a short duration and allows less terminal voltage change while increasing MW
oscillations.
� To perform an impulse test
1.
From the Frequency (BODE) Analysis or Step Test of Regulator Loops diagram, set
PSS Gain to nominal and select PSS enable.
2.
Configure the variables as follows:
•
Set ACL Bode Level = 0.05 (5%); can use up to 0.08 (8%) if requested by
tuning study provider)
•
Set (CRITICAL) Set Step Time = 0.1
•
Set the rest in accordance with standard step test setup.
Impulse Test Configuration
40 GEH-6676E
GE Internal
3.
For redundant control systems, after the configuration settings are complete, perform
the teaching of parameters.
4.
Click Start/Stop Analysis to initiate the impulse test.
5.
Upload the PSS Test Capture Buffer to Trender.
6.
Set PSS Gain to 0 and select PSS disable. Repeat the test and upload the PSS Test
Capture Buffer to the Trender. The trend with PSS enabled should have a marked
difference (decrease) in the number and amplitude of oscillations in the power (MW)
variable. This again demonstrates the effectiveness of the PSS.
EX2100 and EX2100e Excitation Control Power System Stabilizer
Impulse Test with PSS Enabled
Operation and Tuning
GE Internal
GEH-6676E User Guide 41
Impulse Test with PSS Disabled
42 GEH-6676E
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
3.2.6 AVR Closed Loop Frequency Response
During the frequency response tests, the AVR setpoint will randomly change. Terminal
voltage may move as much as ±1%, causing VAR swings. Monitor the MW for any large
sustained oscillations and stop the test if required. Inform operations before performing
the test, but only stop the test if they indicate major system (not unit) issues.
For Excitation Control Help,
right-click in an empty white
space on the diagram and
select Item Help.
� To perform the AVR Closed Loop Frequency Response
1. From the Frequency (BODE) Analysis or Step Test of Regulator Loops diagram, set
PSS Gain to 0 and select PSS disable.
2.
Verify that the DSPX block exists somewhere (it may be a different block number)
within the variable,AVR_TSK, to transfer PSS lead/lag output to DSPX for trending in
the DSPX capture block during testing.
DSPX Capture Block
3.
Verify that the connection is made on the diagram for the PRBS block to be input to
the AVR.
Note In the AVR step test procedure, connection was configured to Step Source so that
the step test would be input, not the PRBS data.
Operation and Tuning
GE Internal
GEH-6676E User Guide 43
Set PRBS Step Source
Set PRBS/Step
to PRBS Source
sourceto PRBS Source.
44 GEH-6676E
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
4.
To get the AVR frequency response, from the right side of the diagram configure the
parameters as displayed in the following figure.
closed
loop
transfer
Verify
that these
settings
are
correct
for
AVR
closed
loop
function measurement
transfer function measurement.
5.
Click Start / Stop Analysis. The At NowPass box displays the current pass.
When the test is finished, the Bode averaging done coil becomes true
(represented by a black square).
Test Status Section
6.
From the Block Collected menu, select the DSPX Capture Buffer. Perform an
upload and select Change without Save.
The following figure illustrates the input signal (AVR setpoint) and output (AVR
feedback), which is terminal voltage. It is not apparent how this relates to the frequency
response without processing it to calculate the transfer function.
The field engineer should verify the following information when the data is collected:
Operation and Tuning
GE Internal
•
Input signal is small relative to normal feedback signal
•
Noise input is not driving terminal voltage signal excessively, meaning the operator
is not seeing large swings in voltage and VARs as the data is being collected; this is
the general idea of being non-invasive in measurement.
GEH-6676E User Guide 45
•
No apparent limit action occurring in the AVR setpoint signal, meaning the noise is
not driving the excitation control into any observed limits. Such limit action would
result in inaccuracies in the resulting transfer function calculation.
Example of Collected Data from AVR Closed Loop Frequency Response Test
46 GEH-6676E
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
3.2.7 PSS Open Loop Frequency Response
� To perform the PSS Open Loop Frequency Response
Operation and Tuning
GE Internal
1.
Refer to the procedure, To perform the AVR Closed Loop Frequency Response, and
perform steps 1–5. In step 4, set the Bode Type to PSS.
2.
After the file has been uploaded to the Trender and saved, the frequency response test
data collection is complete.
GEH-6676E User Guide 47
Example of Collected Data from PSS Open Loop Frequency Response Test
3.2.8 Testing Complete
After all testing is completed, send the collected data to the tuning study provider for
analysis (such as Energy Consulting) and leave the PSS disabled with Gain set to 0 until
the results are approved. Repeat testing as required. When all results are approved, enable
the PSS with the approved gain setting. If configuration settings are provided by GE, send
a copy of the as-running file back to GE personnel.
3.2.9 Frequency Response Test Data Processing
(Optional)
Processing the raw frequency
response data into transfer
function form is typically done
by GE.
This section provides a general overview of the data processing activity for situations
where the field engineer wishes to do a quick site check of the frequency response data.
The following transfer function calculation procedures are provided:
•
AVR Closed Loop transfer function
•
PSS Open Loop transfer function
Note The program that performs the transfer function calculations can be found in the
toolbox application at the following location: C:\Program Files\GE Control System
Solutions\Ex2100 Excitation Control\Ex2100 Analysis Tool. This location takes you to a
batch file called FreqAnaz.bat that runs a MATLAB executable code to do the transfer
function calculations and plot the results in the following sections.
3.2.10
AVR Closed Loop Transfer Function
The AVR closed loop transfer function, which is compared against the predicted phase lag
in the field circuit at local mode frequency (1-2 Hz), is approximately the same as the
uncompensated phase near local mode. A 90-100 degrees phase lag compensated by the
phase lead in the PSS control is expected.
� To calculate the AVR closed loop transfer function
GEH-6676E
GE Internal
1.
Load the recorded AVR Closed Loop trend file into the toolbox or ToolboxST
application.
2.
From the File menu, select Export Trend Data.
3.
In the Trender Export Data Options box, select the Column Headers and
Time Stamps options.
4.
Click OK and save as a *.csv file.
5.
Open the transfer function calculation tool (FreqAnaz.bat) to display the Analysis
Tool dialog box.
EX2100 and EX2100e Excitation Control Power System Stabilizer
Analysis Tool
6.
Select AVR Analysis.
7.
Select the previously saved *.csv file. The program performs the AVR Closed Loop
transfer function and displays the following three graphs.
AVR Closed Loop Transfer Function Graphs
8.
Operation and Tuning
GE Internal
Maximize the middle graph and print (or screen capture) it to share with the
customer.
GEH-6676E User Guide 49
Typical AVR Closed Loop Transfer Function Plot
3.2.11
Refer to the figure, PSS Open
Loop Transfer Function
Graphs.
PSS Open Loop Transfer Function
The PSS open loop transfer function plot allows calculation of the actual instability gain
point. The loop crossover point in the phase plot graph (lower blue curve) has zero phase
at 6.5 Hz, at which point the gain in the upper curve reads approximately 0.005 pu. The
instability gain is the inverse of the measured gain at crossover, therefore, it is calculated
that the PSS loop will reach instability at a PSS gain of 200 pu with an oscillation
frequency of 6.5 Hz. With this instability gain of 200 pu, and assuming a recommended
PSS gain setting of 10 pu, a gain margin of 26 dB (20:1) is calculated.
� To calculate the PSS Open Loop frequency response
Refer to the figure, Analysis
Tool.
50 GEH-6676E
GE Internal
1.
Load the recorded PSS Open Loop trend file into the toolbox or ToolboxST
application.
2.
From the File menu, select Export Trend Data.
3.
In the Trender Export Data Options box, select the Column Headers and
Time Stamps options.
4.
Click OK and save as a *.csv file.
5.
Open the transfer function calculation tool (FreqAnaz.bat).
6.
Enter the as-left (tuned) PSS lead and lag settings in the appropriate locations (for
example, PSSTld1). Retain the defaults for the underexcitation limit (UEL) and field
current regulator (FCR) constants.
EX2100 and EX2100e Excitation Control Power System Stabilizer
7.
Select PSS Analysis.
8.
Select the previously saved *.csv file. The program performs the PSS Open Loop
transfer function and generates the following three graphs.
PSS Open Loop Transfer Function Graphs
9.
Maximize the left window and print (or screen capture) it to share with the customer.
Typical PSS Open Loop Transfer Function Plot
Operation and Tuning
GE Internal
GEH-6676E User Guide 51
3.2.12
Disable and Enable Testing (Optional)
Test the configuration settings for the parameters, Low Watts Disable and Hi Watts
Enable.
Note Simulation testing is recommended, as it is unlikely to perform this testing with the
unit in service because it will likely be at or near full load. Only perform this test online if
the PSS testing is complete and the customer is able to lower load.
� To test the Low Watts Disable and Hi Watts Enable configuration
settings
1.
With the PSS enabled, decrease unit load until the PSS becomes inactive. This should
be at the corresponding value of parameter Low Watts Disable .
2.
From the operator control interface, disable PSS and raise unit load above the
parameter Hi Watts Enable. The PSS should remain disabled and inactive.
3.
Reduce load below the setting of Low Watts Disable and select PSS enable. Again,
raise load above the parameter Hi Watts Enable and the PSS should become active
when the value of Hi Watts Enable is reached.
3.2.13
Additional Unit Testing
If more than one identical unit exists on site, the gain setting is the same on subsequent
units. It is preferred that the same testing described in this document is performed for
each unit. However, with tuning study provider approval, certain tests may be skipped. At
a minimum, the gain margin and step test should be completed on every unit.
Tip � It is best to have the PSS active on the first unit while testing the second unit. The
third unit would be tested with the PSS active on the first and second units and so forth.
Note If is assumed that the units on site are being brought online and having PSS
tested/approved sequentially. If the site has units already in operation (such as a PSS
retrofit) that have not had PSS testing completed and approved, the first unit refers to first
unit with PSS tested and approved. (Be sure not to enable PSS for other units, even at the
same site, that have not been tested and approved.)
Refer to the section, Testing
Complete.
52 GEH-6676E
GE Internal
When testing is complete on all additional units, send that collected data for each unit to
the tuning study provider for approval and leave the PSS disabled until it is approved.
EX2100 and EX2100e Excitation Control Power System Stabilizer
Glossary of Terms
Application Command Layer (ACLx) Used for EX2100 control systems; can be
either an ACLA or ACLE.
Automatic Voltage Regulator (AVR) AVR is controller software that maintains the
generator terminal voltage.
Block Instruction blocks contain basic control functions that are connected together
during configuration to form the required machine or process control. Blocks can perform
math computations, sequencing, or regulator (continuous) control.
Bus
data.
Upper bar for power transfer, also an electrical path for transmitting and receiving
Configure or Configuration To select specific options, either by setting the location
of hardware jumpers or loading software parameters into memory.
Toolbox or ToolboxST A Windows-based software application used to configure the
EX2100, EX2100e, and other GE controller products.
Dynamic stability
changes.
Steady-state stability; allows a system to correct from small
EX2100 and EX2100e Excitation Control GE static exciter; regulates the
generator field current to control the generator output voltage.
EXDSPEED
EXDSPEED is the integral of accelerating power signal.
IEEE Institute of Electrical and Electronic Engineers. A United States-based society
that develops standards.
Power System Stabilizer (PSS) PSS software produces a damping torque on the
generator to reduce generator oscillations.
Signal
The basic unit for variable information in the controller.
Simulation
exciter.
Torque
Running the control system using a software model of the generator and
The mechanical-to-electrical energy link.
Transient stability
GEH-6676E
GE Internal
Allows a system to recover from large changes.
Glossary of Terms
53
Notes
54
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
P
Index
Closed Loop Transfer Function
Commissioning 27
Configuration 19
Parameter
Biquad 26
Gain 25
Inertia 25
Lead/Lag 2 25
Low Watts Disable 26
Lower Limits 26
Ramp Tracking Filter 26
Washout 26
Parameters
Biquad 24
PSS 22
power system stability 7
Power System Stabilizer 7
Power System Stabilizer (PSS) Diagram 21
PSS Active 19
PSS Disable 19
PSS Enable 19
PSS Implementation 11
PSS Inactive 19
PSS2B Model 16
D
R
Dynamic Stability 7
Ramp Tracking Filter
E
S
A
Automatic Voltage Regulator 9
AVR Closed Loop Frequency Response 43
AVR Step Test
PSS Enabled 38
B
Biquad 26
C
EXDSPEED
15
26
Stability 7–8
Steady-state 10
synchronous machine 7
Synchronous Machine Oscillation 7–8
System Modeling 9
F
Frequency Response
Test Data
T
G
Gain Margin Test
29
I
Impulse Test 40
Inertia 25
Initial Conditions Check 27
Instability 19, 29
Integral of Accelerating Power
13
M
Test
AVR Step test with PSS Disabled 34
AVR Step with PSS Enabled 38
Testing 27
Additional unit testing 52
AVR closed loop frequency testing 43
Disable and Enable testing 52
Gain Margin test 29
Impulse test 40
Initial performance testing 19
PSS open loop frequency testing 47
Testing complete
Transient Stability 7
Trender 29–30, 37
Mechanical power 13
O
Open Loop Frequency Response 47
Open Loop Transfer Function 50
Operation and tuning 19
GEH-6676E
GE Internal
Index
55
Notes
56
GE Internal
EX2100 and EX2100e Excitation Control Power System Stabilizer
1501 Roanoke Blvd.
Salem, VA 24153 USA
GE Internal