GEH-6676E EX2100 and EX2100e Excitation Control Power System Stabilizer User Guide GE Internal 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. GE Internal – This document contains information that belongs to the General Electric Company 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 or the information it contains shall not be reproduced in whole or in part or disclosed to any third party without the express written consent of GE. 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. For further assistance or technical information, contact the nearest GE Sales or Service Office, or an authorized GE Sales Representative. Revised: Oct 2014 Issued: Oct 2010 Copyright © 2010 – 2014 General Electric Company, All rights reserved. ___________________________________ * Indicates a trademark of General Electric Company and/or its subsidiaries. All other trademarks are the property of their respective owners. We would appreciate your feedback about our documentation. Please send comments or suggestions to controls.doc@ge.com GE Internal Document Updates Update Description GE Internal Footer Updated document footer with GE Internal for internal use. Cover Page Removed image. 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 Safety Symbol Legend Indicates a procedure, condition, or statement that, if not strictly observed, could result in personal injury or death. Warning Indicates a procedure, condition, or statement that, if not strictly observed, could result in damage to or destruction of equipment. Caution Indicates a procedure, condition, or statement that should be strictly followed to improve these applications. Attention 4 GEH-6676E GE Internal 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 GE Internal EX2100 and EX2100e Excitation Control Power System Stabilizer 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. 8 GEH-6676E GE Internal 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. 10 GEH-6676E GE Internal 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 12 GEH-6676E GE Internal 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 GE Internal 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. 16 GEH-6676E GE Internal EX2100 and EX2100e Excitation Control Power System Stabilizer 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 GE Internal 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 GE Internal 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) 22 GEH-6676E GE Internal 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 GE Internal EX2100 and EX2100e Excitation Control Power System Stabilizer 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 GE Internal EX2100 and EX2100e Excitation Control Power System Stabilizer 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 GE Internal • 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