GEH-6676B (2)

GE Energy

GEH-6676B

Power System Stabilizer for EX2100 and EX2100e Excitation Control User Guide

These instructions do not purport to cover all details or variations in equipment, nor to provide for every possible contingency to be met during installation, operation, and maintenance. The information is supplied for informational purposes only, and GE makes no warranty as to the accuracy of the information included herein. Changes, modifications, and/or improvements to equipment and specifications are made periodically and these changes may or may not be reflected herein. It is understood that GE may make changes, modifications, or improvements to the equipment referenced herein or to the document itself at any time. This document is intended for trained personnel familiar with the GE products referenced herein. GE may have patents or pending patent applications covering subject matter in this document. The furnishing of this document does not provide any license whatsoever to any of these patents. This document contains proprietary information of General Electric Company, USA and is furnished to its customer solely to assist that customer in the installation, testing, operation, and/or maintenance of the equipment described. This document shall not be reproduced in whole or in part nor shall its contents be disclosed to any third party without the written approval of GE Energy. GE provides the following document and the information included therein as is and without warranty of any kind, expressed or implied, including but not limited to any implied statutory warranty of merchantability or fitness for particular purpose. If further assistance or technical information is desired, contact the nearest GE Sales or Service Office, or an authorized GE Sales Representative. © 2010 General Electric Company, USA. All rights reserved. Revised: 2011-01-06 Issued: 2010-10-27 * Trademark of General Electric Company Biquad is a trademark of Eugene F. Goff, dba Biquad. IEEE is a registered trademark of Institute of Electrical and Electronics Engineers. Windows is a registered trademark of Microsoft Corporation.

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Indicates a procedure, condition, or statement that, if not strictly observed, could result in damage to or destruction of equipment.

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Indicates a procedure, condition, or statement that should be strictly followed in order to optimize these applications.

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To prevent personal injury or damage to equipment, follow all GE safety procedures, LOTO, and site safety procedures as indicated by EHS.

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Contents Chapter 1 Overview ................................................................................................................ 1-1 Acronyms/Abbreviations ........................................................................................................................... 1-1

Chapter 2 Power System ...................................................................................................... 2-1 Power System Stabilizer ............................................................................................................................ 2-1 Power System Stability ....................................................................................................................... 2-1 Synchonous Machine Oscillations......................................................................................................... 2-1 System Modeling............................................................................................................................... 2-2 PSS Implementation .......................................................................................................................... 2-4 Integral of Accelerating Power PSS....................................................................................................... 2-5

Chapter 3 Integral of Accelerating Power ............................................................................ 3-1 EXDSPEED............................................................................................................................................ 3-3 PSS Model (IEEE) ................................................................................................................................... 3-4

Chapter 4 Operation and Tuning........................................................................................... 4-1 PSS Enable and PSS Active ....................................................................................................................... 4-2 PSS Parameter Usage and Settings............................................................................................................... 4-4 PSS Inertia ....................................................................................................................................... 4-7 PSS Gain ......................................................................................................................................... 4-7 PSS Lead/Lag 1 and PSS Lead/Lag 2 .................................................................................................... 4-7 PSS Output Upper/Lower Limits .......................................................................................................... 4-7 PSS Washout .................................................................................................................................... 4-7 PSS Ramp Tracking Filter ................................................................................................................... 4-7 PSS Hi Watts Enable, Low Watts Disable ............................................................................................... 4-7 PSS Biquad ...................................................................................................................................... 4-7 Initial PSS Commissioning......................................................................................................................... 4-8 Initial Conditions .............................................................................................................................. 4-9 Gain Margin Test............................................................................................................................. 4-10 Online AVR Step With PSS Disabled .................................................................................................. 4-15 AVR Step Test with PSS Enabled........................................................................................................ 4-19 Impulse Test with/without PSS........................................................................................................... 4-20 AVR Closed Loop Frequency Response ............................................................................................... 4-23 PSS Open Loop Frequency Response .................................................................................................. 4-27 PSS Testing Complete ...................................................................................................................... 4-28 Processing PSS Frequency Response Test Data (Optional)....................................................................... 4-28 AVR Closed Loop Transfer Function ................................................................................................... 4-28 PSS Open Loop Transfer Function ...................................................................................................... 4-31 PSS Disable and Enable Testing (Optional)........................................................................................... 4-33 Additional Unit PSS Testing ............................................................................................................. 4-33

Glossary of Terms ................................................................................................................. G-1 Index ......................................................................................................................................... I-1

GEH-6676B

Contents

1

Notes

2

Contents

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Chapter 1 Overview This document includes information on power system stability fundamentals, EX2100 and EX2100e Power System Stabilizer (PSS) theory, and site-commissioning. Note This document covers both the EX2100 and EX2100e Excitation Controls. References to the exciter are applicable equally to either the EX2100 or EX2100e exciter.

Acronyms/Abbreviations ACLx

Application Command Layer (for EX2100 systems, can be either an ACLA or an ACLE)

AVR

Automatic Voltage Regulator

DAC

Digital-to-Analog Converter

DSA

Digital Signal Analyzer

EMF

Electromotive Force or Field Voltage

IEEE®

Institute of Electrical and Electronics Engineers

MMF

Magnetomotive Force

PSEC

Power Systems Energy Consultants

PSS

Power System Stabilizer

VAR

Volt-Ampere Reactive

GEH-6676B

Chapter 1 Overview

User Guide

1-1

Notes

1-2

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Chapter 2 Power System Power System Stabilizer The PSS is an automatic control designed to improve synchronous machine stability. This control function is used with field excitation systems. There are many different implementations of a PSS. A fully integrated digital PSS, based on the integral of accelerating power principle, is available for GE EX2100 or EX2100e excitation control.

Power System Stability Providing a reliable supply of electricity depends on machine stability. The simplest definition of stability for synchronous machines indicates that in spite of unanticipated load shifts between machines, the system maintains a constant voltage and frequency. Also, 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. Dynamic stability, also known as steady-state stability, allows a system to correct for small changes. This document requires a basic understanding of synchronous machines and electric power flow.

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. Modern generating units equipped with high-gain voltage regulators enhance transient stability but tend to detract from dynamic stability. The PSS improves small signal (steady-state) stability by damping power system modes of oscillation through generator excitation modulation.

Synchonous Machine Oscillations Synchronous machine oscillations often fall into one of four categories: •

Local machine-system (local mode)

•

Inter-area

•

Inter-unit

•

Torsional

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 usually in the range of 1.0 to 2.0 Hz. Some low inertia turbine generators can have local node 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.

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Chapter 2 Power System

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2-1

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. The PSS provides the control action that allows the power system to maintain stability.

While change of rotor angle in a single machine is a concern, a more important concern is the behavior of all the machines closely connected to a system. During a system transient, all rotor angles should move in the same relative direction over time. The focus is on the difference in rotor angle between machines.

System Modeling Static excitation systems with high-gain and fast-response times greatly aid transient stability (synchronizing torque), but at the same time tend to reduce small signal stability (damping torque). The objective of the PSS control is to provide 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 illustrates the effect of excitation systems on the damping of local mode oscillations. The figure is a simplified, linearized block representation for a single-generating unit connected radially to an infinite bus. 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 described by the swing equation linearized about an operating point, as illustrated by the solid lines (also known as the torque-angle loop) in the following figure. The mechanical loop is shown at the top of the figure while the electrical loop is shown in the middle. Phase relationships show 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.

2-2

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Linearized Block Diagram of a Single Machine to Infinite Bus Power System

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Chapter 2 Power System

User Guide

2-3

The coefficients K1 through K6 are defined as follows. 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

Demagnetizing 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. The previous figure shows the addition of a PSS to the control. The PSS is used to supply 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. Also, since the generator field circuit and AVR function has an inherent phase lag, a corresponding phase lead is required to compensate for this effect.

PSS 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.

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

Integral of Accelerating Power PSS Refer to Chapter 3 Integral of Accelerating Power for specific design of this system.

The latest-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, so 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 PSS control 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. 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 performance is often evaluated from the damping of the local mode, 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 control 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 shown in the previous figure) are used to predict the performance of the PSS under steady-state and transient conditions.

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Chapter 2 Power System

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2-5

Notes

2-6

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Chapter 3 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

where: ω = rotor speed H = generator inertia constant (MW-sec/MVA) Tm = mechanical (turbine) torque Te = electro-mechanical (air-gap) torque Tacc = accelerating torque This is called the synchronous machine swing equation. In a per-unit (pu) system, torque and power are equivalent in value. Replacing torque (T) with power (P), and rearranging the equation above to solve for mechanical power gives the following:

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 shows the process of deriving mechanical power. The ramp tracking filter is shown as G(s). P'm represents the mechanical power signal, with the prime superscript indicating that this is a synthesized signal. The next step develops the accelerating power signal that is P'acc = P'm - Pe. The accelerating power is labeled as a synthesized or derived signal at this point, since it is made up from synthesized mechanical power.

GEH-6676B

Chapter 3 Integral of Accelerating Power

User Guide

3-1

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:

where TW is the washout time constant, normally set in the range of 2 to 10 seconds. 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:

The equation 1/(2H) times 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. Also, 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.

3-2

Power System Stabilizer for EX2100 and EX2100e Excitation Control

EXDSPEED The integral of accelerating power signal is called EXDSPEED and is found using the following relationship:

For a signal 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 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), found by integrating (Pm-Pe) and dividing by 2H, is responsive to rotor speed without excessive phase lead at low frequencies (which has 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 shows that 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. Some units (primarily 4-pole nuclear units) require band reject filters to reduce the response to torsional oscillations. The third lead/lag stage in this figure is used to represent the low frequency equivalent of a two-stage torsional filter.

GEH-6676B

Chapter 3 Integral of Accelerating Power

User Guide

3-3

PSS Model (IEEE) 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 is shown in the following figure. This model conforms with the standards on excitation systems, identified as IEEE 421/5-2005. The two model inputs are VSI 1 as speed, and VSI 2 as electrical power.

PSS 2B IEEE Model The EX2100 and EX2100e implementation of an integral of accelerating power PSS is available in a standard form, as shown below.

3-4

Power System Stabilizer for EX2100 and EX2100e Excitation Control

A specialized version with Biquad™ filters is shown in the following figure:

Specialized Version with Biquad Filters

GEH-6676B

Chapter 3 Integral of Accelerating Power

User Guide

3-5

Notes

3-6

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Chapter 4 Operation and Tuning Initial operational testing and settings verification of the PSS is highly recommended. The PSS should not be placed into service until qualified test personnel complete a thorough check of the PSS settings and performance. This should include, as a minimum •

System tuning and PSS optimization studies

•

Review of PSS parameters

•

Instability gain margin measurements

•

AVR step response testing with PSS enabled and active

•

Impulse Test with and without PSS

•

AVR frequency response testing

•

System open loop frequency response testing

Optional tests and studies recommended for assurance of proper PSS operation but not required for placing the PSS into service include

These optional tests are recommended to assure proper PSS operation but are not required for placing the PSS into service.

GEH-6676B

•

Compensated phase calculations

•

PSS Enable and Disable Testing

The minimum PSS setup and operational checks are discussed in this document, as well as basic instructions for some of the optional testing. For more information about the additional tools, testing, and studies available, contact the Controls COE Post Sales Service group in Salem, VA.

Chapter 4 Operation and Tuning

User Guide

4-1

PSS Enable and PSS Active The PSS can be enabled (turned on or available for operation) or disabled (turned off or unavailable for operation) through operator interface commands. PSS then becomes active (in service) or inactive (not in service) based on satisfying operational conditions. The PSS must be enabled through the turbine control operator interface screen, which sends a command through an EGD connection to the exciter controller, or through the operator interface (keypad or touchscreen). This is accomplished through the command PSS Enable.

The PSS can be enabled or disabled at any time, and at any load point. Once enabled, the PSS is not active (available to supply compensation to the AVR input summing junction) unless the following three conditions are met: •

The exciter must be in AUTO regulator control.

•

The exciter must be running.

•

The generator must be online at a load point above the parameter value

If any of these three conditions are not met, the PSS becomes inactive, but still remains enabled.

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

The PSS can be disabled through the turbine control or exciter operator interface (enabled is also known as "armed" in software, as seen in figure below where PSSARMD=FALSE equates to PSS disabled) at any time and at any load. If load is reduced below the parameter value , the PSS becomes inactive. Changing the regulator to Manual or opening the 52G breaker also causes the PSS to become inactive.

GEH-6676B

Chapter 4 Operation and Tuning

User Guide

4-3

PSS Parameter Usage and Settings A graphical representation of the PSS including parameter values, input variables, and output variables can be found in the applicable Control System Solutions (CSS toolbox) or ToolboxST file under the DIAGRAM heading, on the Power System Stabilizer (PSS) diagram.

A list of PSS parameters can be found through the CSS toolbox or ToolboxST application. In the CSS toolbox application open the applicable exciter file and from the Outline View, select Power System Stabilizer.

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

In the ToolboxST application open the applicable exciter file and from the Component Editor Settings tab, expand Power System Stabilizer and select Parameters.

A list of the PSS with Biquad parameters can be found in the CSS toolbox application by opening the applicable exciter file and from the Outline View, select Power System Stabilizer with Biquad and expand Parameters.

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Chapter 4 Operation and Tuning

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The following default values disable a Biquad, so if Biquad is not used in the tuning study ensure it is set accordingly. •

PSS biquad3 num damp = 0.5 damp

•

PSS biquad3 num damp = 0.5 damp

•

PSS biquad3 nat r/s = 62r/s

In the ToolboxST application open the applicable exciter file and from the Component Editor Settings tab, expand Power System Stabilizer with Biquad and select Parameters.

4-6

Power System Stabilizer for EX2100 and EX2100e Excitation Control

PSS 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. The generator manufacturer should supply this value.

PSS Gain Select the PSS gain to provide stable operation at all load points. Typically set to an initial value of 15, this parameter is adjusted during PSS commissioning. It should be verified by testing that the gain is less than a value of 1/3 of the gain setting that would just cause the PSS loop to be unstable.

PSS Lead/Lag 1 and PSS 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 seconds. Lag values around 0.05 seconds.

PSS Output Upper/Lower Limits Upper and lower limits on the PSS output should be selected to reduce the ability of the PSS to override the regulator during large disturbances. Typical values are +10% and -10% but are customer-selectable.

PSS 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 seconds.

PSS Ramp Tracking Filter The time delay for responses to slow increases in power during system daily load changes; it is typically set to 0.1 seconds.

PSS Hi Watts Enable, Low Watts Disable If the PSS has been enabled, the PSS 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.

PSS Biquad It is an enhanced PSS that 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.

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Chapter 4 Operation and Tuning

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4-7

Initial PSS Commissioning The following tools are necessary for PSS commissioning and testing. •

GE CSS toolbox or ToolboxST application with trend recording option

•

Help file printout of the Frequency BODE Analysis and Step Test diagram (Right-click, ITEM HELP on white space of diagram)

•

If testing EX2100 with any version of ACLA or ACLE with firmware prior to V09.00.00C for static exciter and V03.03.00C for the regulator, consult GE Salem COE and/or GE Energy Consulting as there are some setup differences from those shown here. Note If you need to download software to the ACLA, DO NOT create or download a compressed controller file.

Optional tools include the following: •

4-8

Results from GE Power Systems Engineering Consulting or customer PSS tuning study with applicable parameters for PSS entered into the exciter configuration file.

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Initial Conditions

Pay particular attention to the information in this section to ensure proper testing.

Attention •

Ensure PSS is disabled and Gain=0 prior to unit going online to ensure no inadvertent activation of PSS prior to testing.

•

Prior to testing the PSS, other offline and online testing documented in GEH-6631, EX2100 Thyristor Control 77, 53, and 42 mm Installation and Startup Guide, GEH-6674, EX2100 Regulator Control Installation and Startup Guide, or GEH-6694, EX2100 Thyristor Control 100 mm Installation and Startup Guide should be completed.

•

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 load slightly below full load to get turbine control into speed/droop rather than exhaust temperature control. If full load is not possible, it is generally acceptable to 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.

•

It is strongly recommended to perform all testing with unit at near unity power factor (0MVars). Perform all testing at as close to same load and VARS as possible.

•

Any other outer loop regulator functions in EX, turbine, or plant controls, such as Var/PF and auto MW load changing, should be turned off or disabled.

•

Use the tuning study provided by GE Energy Consulting to review PSS parameters in the exciter configuration file for accuracy and completeness. If settings are not provided by Energy Consulting, consult with GE Salem COE before using customer or default settings. Incorrect and/or default settings may result in unstable unit operation or inadequate PSS operation.

Caution

GEH-6676B

Chapter 4 Operation and Tuning

The exciter must remain in AUTO regulator throughout the PSS test. If, at any time, unstable operation with the PSS active is noticed, removing the PSS enable should stop the instability. Transferring to MANUAL regulator should also stop any instability.

User Guide

4-9

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, it is necessary to 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 the Controls COE in Salem or the Energy Consulting group in NY.

4-10

GE recommends a minimum gain margin of 10 db, which is a factor of three times the nominal set gain. Testing is normally done 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. Ø To test the gain margin 1.

Set the parameter (on the PSS diagram) to an initial value of 0 pu. Perform all PSS testing at 80% load or higher and close to unity power (0 MVars). Refer to the section, Initial Conditions for further unit operation requirements.

2.

With the exciter in AUTO regulator, enable the PSS through the keypad or turbine operator interface. If the unit is above the value, the PSS should be enabled and active.

Power System Stabilizer for EX2100 and EX2100e Excitation Control

3.

Configure the Trender to monitor the following variables in real time.

Note Ensure 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 seconds so the entire trend can be viewed throughout the test. Signal

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

WATTS

Average ± 0.02 pu

VARS

Average ± 0.05 pu

AVR\PSS_OUT

± 0.01 pu

PSSGN

0 - 4 x nominal PSS gain + 10

Average ± 2 A Average ± 4 A Average ± 6 A

4.

Start recording the above variables for 30 seconds, then increase the PSS gain from 0 pu to normal gain setting, and observe any of these 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. It is also possible that the power swings could occur suddenly at a fixed-amplitude of oscillation. If either phenomenon is observed (refer to the figure Unstable Gain Margin Example), then select PSS disable from the keypad, COI, or turbine control. (Best practice is to have someone standing by to do this if necessary).

5.

Hold at nominal gain for 60 seconds then continue to increase the PSS gain to twice, three times, and four times nominal gain. Hold at each point for 60 seconds. The oscillations in the MW trend begin to grow and have longer settling times. Look for any signs of instability (refer to the following three figures for examples of unstable and appropriate gain margin tests) and select PSS disable if it occurs. Once four time nominal gain is complete reduce gain back to zero, continue recording for 30 seconds and then stop the Trender.

6.

After test completion, again review trend for signs of instability using gain margin test examples (following three figures). If instability is or has been observed contact tuning study provider for changes and leave PSS disabled with PSS Gain = 0 until 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 = 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 Experience with the integral of accelerating power PSS indicates that 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. GEH-6676B

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Brushless Regulator, Unstable Gain Margin Example

4-12

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Bus Fed, Noisy but no Instability and Good Gain Margin Example

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Bus Fed, Standard (Good) Gain Margin Example

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

Online AVR Step With PSS Disabled This test provides a baseline of AVR operation with the PSS disabled for comparison to AVR operation with the PSS enabled.

To demonstrate PSS effectiveness step the AVR with PSS disabled.

Before stepping the AUTO regulator, verify 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.

Warning

This testing changes the output of the generator and can rarely cause local instability on some power systems.

Caution Ø To step the AVR 1.

Ensure PSS Test Capture block is set correctly for the type of unit (Bus-fed [Static] or Brushless, as seen in the following two figures). If change is required, minor differences will show up. Perform Validate/Build/Download using "initialize all constants" in accordance with the appropriate installation and startup guide (such as GEH-6631, EX2100 Thyristor Control 77, 53, and 42 mm Installation and Startup Guide).

PSS Capture Block for Static Fed Units

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PSS Capture Block for Brushless Units 2.

4-16

Using the help message for the Frequency (Bode) Analysis or Step Test Diagram, configure the step wizard for an AVR regulator 2% step by making the following settings:

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GEH-6676B

3.

If redundant controls, perform teach of new settings to other controllers in accordance with the appropriate installation and startup guide (such as GEH-6631, EX2100 Thyristor Control 77, 53, and 42 mm Installation and Startup Guide).

4.

Click the Start / Stop Analysis button to initiate the AVR step test.

5.

Upload the PSS Test Capture Buffer to the Trender.

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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.

4-18

Power System Stabilizer for EX2100 and EX2100e Excitation Control

AVR Step Test with PSS Enabled

GEH-6676B

1.

Set PSS Gain to nominal and select PSS enable.

2.

Once again step AVR, this time 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 PSS.

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Impulse Test with/without PSS This test provides further analysis of PSS and 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.

4-20

1.

With PSS still enabled and gain at nominal, set Bode diagram as shown in the following figure. After the settings are complete, perform teach of parameters if EX2100 has redundant controls.

•

Set ACL Bode Level = 0.05 (5%), can use up to 0.08 (8%) if requested by tuning study provider (such as Energy Consulting)

•

(CRITICAL) Set Step Time = 0.1

•

Set the rest in accordance with standard step test setup.

2.

Click the Start/Stop Analysis button to initiate the impulse test.

3.

Upload the PSS Test Capture Buffer to Trender.

4.

Select PSS disable and set PSS Gain = 0, repeat the test and upload the PSS Test Capture Buffer to the Trender. As noted in the example in the following two figures, 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 PSS.

Power System Stabilizer for EX2100 and EX2100e Excitation Control

Impulse Test without PSS

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Impulse Test with PSS

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

AVR Closed Loop Frequency Response Ø To perform the AVR Closed Loop Frequency Response Note For help, right-click anywhere in the diagram white space and select Item Help. During the frequency response tests, AVR setpoint will randomly change. Terminal voltage may move as much as ±1% causing VAR swings. Monitor MW for any large sustained oscillations and terminate the test if required. Inform operations of this before doing the test, but only terminate if they indicate major system not unit issues.

GEH-6676B

1.

Ensure PSS is still disabled and PSS Gain = 0

2.

Verify the following block exists somewhere (It could be a different block number.) within AVR_TSK to transfer PSS lead lag output to DSPX for trending in DSPX capture block during AVR Closed and PSS Open Loop testing.

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3.

As shown in the following figure, verify that the connection is made on the diagram for the PRBS block to be input to the AVR.

Note In the step testing procedure, this was set to Step Source so that the step test would be input, not the PRBS data.

4.

4-24

To get the AVR frequency response, select from the right side of the diagram, as follows.

Power System Stabilizer for EX2100 and EX2100e Excitation Control

5.

Click the Start / Stop Analysis button as shown in the following figure. The At NowPass box displays the current pass.

When the test is finished, the Bode averaging done coil becomes true (black square). 6.

Select the DSPX Capture Buffer from the Block Collected menu. Perform an upload and select Change without Save.

A sample of the AVR trend file, as follows, shows 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 following information should be verified by the field engineer when the data is collected:

GEH-6676B

•

The input signal is small relative to normal feedback signal.

•

The noise input is not driving terminal voltage signal excessively. This means that the operator is not seeing large swings in voltage and vars as the data is being collected, which is the general idea of being non-invasive in measurement.

•

There is no apparent limit action occurring in the AVR setpoint signal. That is, 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.

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Sample of Collected Data from AVR Frequency Response Test

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

PSS Open Loop Frequency Response Ø To perform the PSS Open Loop Frequency Response 1.

Repeat the AVR closed loop frequency response test except for the following:

* Set the Bode Type = PSS 2.

After the file has been uploaded to the Trender (refer to the sample of a PSS trend in the following figure) and saved, the frequency response test data collection is complete.

PSS Open Loop Frequency Response Example GEH-6676B

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PSS Testing Complete This completes PSS testing. Send data to the tuning study provider for analysis (such as Energy Consulting) and leave PSS disabled with Gain=0 until results are approved. Repeat testing as required and after the results are approved enable PSS with the approved gain setting. If settings are provided by GE Energy Consulting in Schenectady, send a copy of the as running file to them.

Processing PSS Frequency Response Test Data (Optional) Processing the raw frequency response data into transfer function form is typically done by Energy Consulting staff in Schenectady. However, this section of the document presents 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. We compute two transfer functions; one for the AVR Closed Loop transfer function, one for the PSS Open Loop transfer function. The program that performs the transfer function calculations can be found in the CSS toolbox application using the following path: C:\Program Files\GE Control System Solutions\Ex2100 Excitation Control\Ex2100 Analysis Tool The link 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, as noted in the following sections, AVR Closed Loop Transfer Function and PSS Open Loop Transfer Function.

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. Ø To calculate the AVR closed loop transfer function

4-28

1.

Load the recorded AVR Closed Loop trend file into the CSS toolbox application.

2.

From the File menu, select Export Trend Data and in the Trender Export Data Options box, select the Column Headers and Time Stamps options, click OK, and save as a *.csv file.

3.

Open the transfer function calculation tool (FreqAnaz.bat). The Analysis Tool dialog box displays.

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GEH-6676B

4.

Select AVR Analysis.

5.

Select the previously saved *.csv file. The program performs the AVR Closed Loop transfer function and generates three graphics windows.

6.

Maximize the middle window and print (or screen capture) it for sharing with the customer.

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Typical AVR Closed Loop Transfer Function Plot

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

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 on the following page (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, so 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. Ø To calculate the PSS Open Loop frequency response

GEH-6676B

1.

Load the recorded PSS Open Loop trend file into the CSS toolbox application.

2.

From the File menu, select Export Trend Data and in the Trender Export Data Options box, select the Column Headers and Time Stamps options, click OK, and save as a *.csv file.

3.

Open the transfer function calculation tool (FreqAnaz.bat).

4.

Enter the as-left (tuned) PSS lead and lag settings in the appropriate locations as shown in Figure 5 (for example, PSSTld1). Retain the defaults for the UEL and FCR constants.

5.

Select PSS Analysis.

6.

Select the previously saved *.csv file. The program performs the PSS Open Loop transfer function and generates three graphics windows.

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7.

Maximize the left window and print (or screen capture) it for sharing with the customer.

Typical PSS Open Loop Transfer Function Plot

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PSS Disable and Enable Testing (Optional) Test the and settings. Note Simulation testing of this function is recommended, as it is unlikely to perform this testing with the unit in service, where it will likely be at or near full load. Only perform online if PSS testing is complete and the customer is able to lower load. Ø To test the and settings 1.

With the PSS enabled, decrease unit load until the PSS becomes inactive. This should be at the corresponding value of parameter.

2.

From the operator control interface, disable PSS and raise unit load above the parameter. The PSS should remain disabled and inactive.

3.

Reduce load below the setting and select PSS enable. Again raise load above the parameter and the PSS should become active when the value is reached.

Additional Unit PSS 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 noted the previous sections be repeated on each unit. However, with tuning study provider approval, certain tests can be skipped. At a minimum the gain margin and step test should be done on every unit. It is best to have the PSS active on the first unit while testing the second unit. Further, the third unit would be tested with the PSS active on the first and second units and so on. Note The site referred to in this section assumed the units 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/approved the aforementioned first unit refers to first unit with PSS tested/approved. In other words, be sure not to enable PSS for other units, even at the same site, that have not been tested/approved.) Finally, as noted in PSS Testing Complete section, send data for each unit to the tuning study provider for approval and leave PSS disabled until approved.

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Notes

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Glossary of Terms automatic voltage regulator (AVR) the generator terminal voltage.

AVR is controller software that maintains

block Instruction blocks contain basic control functions, which 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 To select specific options, either by setting the location of hardware jumpers or loading software parameters into memory. CSS toolbox or ToolboxST A Windows-based software package used to configure the EX2100, EX2100e, and other GE Energy 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-6676B

Allows a system to recover from large changes.

Glossary of Terms

G-1

Notes

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Power System Stabilizer for EX2100 and EX2100e Excitation Control

Index A Automatic Voltage Regulator

1-1, 2-2

E EXDSPEED

3-3

G Gain Margin Test

4-10

I Inertia 4-7 Integral of Accelerating Power PSS

2-5

L Lead/Lag 1 Lead/Lag 2

4-7 4-7

P Power System Stabilizer

2-1

R Ramp Tracking Filter

4-7

S System Modeling

2-2

T toolbox 4-4 Trender 4-11, 4-17

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Index

I-1

Notes

I-2

Power System Stabilizer for EX2100 and EX2100e Excitation Control

GE Energy 1501 Roanoke Blvd. Salem, VA 24153–6492 USA 1 540 387 7000 www.geenergy.com