DIGITAL EXCITATION SYSTEM PROVIDES ENHANCED PERFORMANCE AND IMPROVED DIAGNOSTICS

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1 DIGITAL EXCITATION SYSTEM PROVIDES ENHANCED PERFORMANCE AND IMPROVED DIAGNOSTICS C. Allan Morse Member, IEEE Eaton / Cutler Hammer 221 Heywood Road Arden, NC 2874 C. Richard Mummert Member, IEEE Eaton / Cutler - Hammer 221 Heywood Road Arden, NC 2874 Abstract New technology in digital excitation control equipment provides both enhanced performance and vastly improved diagnostic capabilities in turbine generator excitation systems. This paper introduces some of the latest advances in regulator technology. It discusses how performance improvements enable safe operation of generators nearer to their design limits. Also discussed is how improved on-board diagnostics can help minimize down time and maintenance costs. These items are of significance for any generator application, including those in the utility industry, those in the pulp and paper industry and other industrial applications. A specific application of a beta site static exciter installed on a utility generator is reviewed and discussed. relay logic are implemented in the digital exciter. In automatic mode, the ac voltage regulator receives the feedback signal from the regulator potential transformers and sends a signal through the auctioneering circuits to the firing control. The firing control generates the firing pulses that drive the thyristor bridge voltage to a level that will maintain the generator terminal voltage at the setpoint selected by the operator. I. INTRODUCTION New digital excitation systems are being introduced that provide the user with many advantages over the existing excitation systems. Among these improvements are advances in performance and in diagnostics. In addition to the classic excitation functions, new features are being added which provide additional protection. To some of the previous limiter functions, a new feature that modifies the pickup point based on external conditions has been added. Entirely new limiter functions and monitoring capabilities have been added. These enhancements allow operation of the generator closer to its design limit and help keep the unit on line during adverse conditions thus increasing reliability and reducing down time. In the past, only a limited amount of information was available to help the user diagnose problems in the excitation system. Separate devices such as annunciators or recorders were required. With the new systems, this information is readily available internally and can be stored digitally and retrieved later for analysis. There are other improvements such as internal transducers, digital control relay logic, enhanced communication capabilities, internal simulators, and improved thyristor firing control algorithms, but the discussion of these items is beyond the scope of this paper. These new features have been incorporated in the latest digital excitation system. In addition to extensive prototype testing in a design lab, this new system has been operating since March 1999 on a utility generator as a beta site unit. Test results were gathered at that site using the improved diagnostic tools available in the system and are presented in this paper. Fig. 1 shows a block diagram of a typical potential source static excitation system. The transducers, regulators, controllers, limiters, auctioneering, firing control and control A. Excitation Limiters Fig. 1. Typical Static Excitation System II. PERFORMANCE ENHANCEMENTS Limiters in an excitation system are used to modify the exciter output to protect the turbine generator or power bus from failure. Under normal conditions, the regulator signal is passed through the auctioneering block (Fig. 1). However, when a limiter is operating, its output is passed to the firing control through the auctioneering block. In applying limiters, the application engineer was often faced with conflicting Presented at the 2 IEEE IAS Pulp and Paper Industry Conference in Atlanta, GA: IEEE 2 - Personal use of this material is permitted.

2 requirements between generator protection and system performance. The new system provides limiters with enhanced performance to eliminate some of those application problems. Historically, the limiters were only used when the unit was in the automatic voltage regulation mode. This was great for a utility generation plant. However, in some industrial sites, the voltage was actually set by the tie to the utilities system. This industrial turbine-generator would supply excitation to support voltage during faults or motor starts, but could only follow steady state voltage. Transient voltage conditions, such as motor or compressor starts, would cause the unit to regularly move the field voltage in its futile effort to regulate bus voltage. This often caused concern about the stability of the unit and occasionally resulted in the unit being run in manual field regulation mode. In this manual mode, the limiters were not active and their benefit to the excitation system was lost. To overcome that problem, the newer excitation system can have the limiters active in both the automatic terminal voltage mode, and the manual field regulation mode. 1) Overexcitation (Field Current) Limiters: An overexcitation limiter (OEL) limits the amount of machine field voltage or current. Because the machine can withstand high levels of field current for short periods of time, an inverse time delay is used before the limiter will take control. For a hydrogencooled generator the machine s capability varies significantly with the amount of hydrogen pressure. For an air-cooled generator, the machine's capability varies significantly with the cooling air inlet temperature. With past systems, the settings for the over excitation limiter were fixed. This posed a dilemma for the application engineer. If settings were picked based on the lowest possible hydrogen pressure or the highest anticipated cooling air temperature, the ability of the unit to supply needed output would be undesirably limited when the pressure was high or temperatures were cool. If settings were picked based on the highest possible hydrogen pressure or the lowest anticipated cooling air temperature, then the field temperatures could become excessive when the unit was supplying needed output while the pressure was low or temperatures were high. These high field temperatures would, at a minimum, reduce life of the machine, eventually increasing maintenance costs. At worst, the temperatures could possibly lead to a catastrophic failure resulting in undesired downtime and expenses. Now, the newer excitation system will automatically modify the pickup point of the overexcitation limiter to permit the maximum possible field current without exceeding the machine thermal capability, regardless of the hydrogen pressure or the cooling air temperature. Since the settings are changeable under load, it is still possible to increase the field current to excessive levels when the return on generating the extra power exceeds the costs of increased maintenance. Another advanced feature is that now the overexcitation limiter provides a "memory" of the time-dependent nature of the residual and cumulative effects of rotor heating. If the field quantity drops below the inverse limiter pickup, the inverse timer will immediately begin to reduce the accumulated time over pickup to zero with a cool-down characteristic curve. This permits the unit to provide maximum excitation to loads which are cyclic in nature and regularly require short bursts of excitation which exceed the steady state generator field capability. Chippers are one type of load that might produce this situation when the generator is operating near its thermal limit. Without the cool-down curve, either the pickup point of the limiter must be reduced to allow for the cyclic load limiting the voltage support capability of the generator, or the undesired consequences of excessive temperature must be accepted. The cool-down curve allows for maximum voltage support while still maintaining field temperatures within acceptable levels. 2) UnderExcitation (Reactive Power) Limiters: An underexcitation limiter is used to limit the level of vars (reactive power) flowing into the machine. There are two different reasons that excessive levels of underexcitation could be detrimental to a turbine-generator. The first reason is related to machine stability. As the machine field is reduced below what is needed for unity power factor, the reactive current increases. If the field goes too low, and the line impedance too high, there is a possibility the machine could lose synchronism with the power grid. The resultant pole slippage might cause catastrophic damage to the unit. The second reason is related to machine capability. Excessive amounts of underexcited line current will cause generator stator steel to overheat, damaging the inter-punching insulation eventually leading to molten steel and catastrophic failure. The ability of the unit to work reliably while underexcited is defined in the generator capabilty curve. In the past, excitation systems only had one limiter to protect the unit from these two different scenarios. The application engineer was forced to find the best compromise. This was fairly easy for utility units connected by a relatively weak line. Under those conditions, the underexcited capability of the unit was limited by stability considerations. An underexcitation limiter was used where the operating curve was typically circular in nature and varied with the square of the voltage, just like the machine steady state stability limit. For utility or industrial applications, where the turbinegenerator was connected to a very stiff bus, stability was not an issue. The application engineer was then faced with a dilemma of how to set the limiter. One classic solution to this dilemma was to never allow the unit to run underexcited. Then, either overvoltage conditions were tolerated, or capacitor banks were switched off as needed to maintain the voltage level. Furthermore, a second dilemma arose because good co-ordination between the circular limiter curve and a nearly straight-line generator field protection relay was nearly impossible. The newer excitation systems provide an alternative to those dilemmas: two underexcitation limiters with differently shaped operation curves. One of these, referred to as a Minimum Excitation Limiter (MEL), maintains an operating characteristic that varies as the square of the terminal voltage. The circular characteristic has been replaced with a fivesegment piecewise linear curve. These can be arranged to approximate a circular curve. Or, they can be arranged in other shapes to better co-ordinate with generator loss of field protection relays. The second of these, referred to as an Under Excitation Limiter (UEL), maintains an operating characteristic independent of the terminal voltage. It is intended to coordinate with the non-circular generator capability curve. As with the overexcitation limiter, the new excitation system will also automatically modify the pickup

3 point of the underexcitation limiter to permit the maximum possible field current without exceeding the machine thermal capability, regardless of the hydrogen pressure or the cooling air temperature. 3) Volts per Hertz (Overvoltage) Limiters: A Volts per Hertz limiter (HXL) is used to limit the ratio of terminal voltage to line frequency. A higher than nominal volts per hertz ratio is an indication of excessive magnetic flux. The limiter is applied to an excitation system when the turbine-generator must operate under adverse circumstances with the system frequency below normal operating range. In such cases, the reason for operating at reduced voltage during underfrequency conditions is to avoid the heating effects of excessive magnetic flux in the generator, transformers, or other magnetic devices. This heating can eventually lead to higher maintenance costs or catastrophic failure of the generator. When a turbine generator is operating at rated load, this limiter functions as an overvoltage limiter. It will then keep the voltage down on units running in manual (field current) control when the kilowatt load is low. In the past, this limiter was typically either instantaneous, or operated after a fixed delay. Now, the limiter uses an inverse curve that coordinates with the sample volts/hertz curves shown in ANSI C This permits the unit to ride through system transients without having the limiter operate undesirably. Like the overexcitation limiter, this limiter also utilizes the memory feature to allow for maximum voltage support while maintaining stator flux within acceptable levels. 4) Thyristor Bridge Temperature Limiters: A thyristor bridge temperature limiter functions to limit the current in a thyristor bridge so that the temperatures in the thyristors do not become excessive. This is a new feature not available in past systems. It is made possible with the use of RTDs in the heatsinks of the thyristor bridges. This reading, along with the measured bridge current and known bridge parameters, allows a microprocessor to calculate the thyristor junction temperature. The calculated junction temperature can be used as an input to this limiter. The thermal time constant of the heatsinks along with an internal low pass filter time constant that simulates thyristor thermal properties help provide the time delay needed to allow the field current to temporarily exceed the bridge s continuous rating to support system transients. This limiter allows the bridges to be run at their maximum capacity. Historically, a maximum bridge current rating was typically estimated based on worse case conditions, i.e., dirty filters and maximum temperatures. Using these conditions as an operating guideline, the full capability of the bridge would not be utilized when the temperature was low and filters clean. B. VAR or Power Factor Controllers Var or power factor controllers function to maintain the average generator reactive power or power factor at a preset value. Using raise and lower signals, the controller modifies the voltage regulator reference to keep the controlled quantity near to a set value over an extended time period. These controllers do not directly regulate vars or power factor. These controllers have been used in the past on generators, but they have had several drawbacks. One of these is that the controller can raise the bus voltage to high levels, i.e. raise a 13.8 kv bus to 15kv, if that is necessary to maintain the desired vars. These excessive voltages can damage equipment and increase maintenance costs. Similarly, the regulator can reduce voltage to undesirable low levels. The new excitation system overcomes this issue by having settable maximum and minimum voltage operation levels. The controller will operate as desired as long as the generator terminal voltage is within the acceptable level. But the controller will automatically turn off when the voltage leaves the acceptable range. Another issue is that in the past these controllers have only operated to supervise the automatic terminal voltage regulator. As was mentioned before, it is sometimes desirable to operate the excitation in a manual field regulation mode. The new system now enables the controller to maintain the average desired vars or power factor while the unit is operating in manual field regulation mode. Included in the controller is a time delay that allows the machine to provide voltage support until the time delay has been exceeded. This delay prevents the controller from reducing excitation during motor starts, precisely when the voltage is most needed. The following four figures demonstrate the differences between a manual field current regulator, an automatic voltage regulator, and a var controller. The results are for a single machine connected to an infinite bus where the megawatt load is increased from 1 percent to 5 percent of the generator rating. It is assumed generator reactance is 1.5 pu and system reactance is.25 pu on the generator base. The values were generated using the excitation system s internal machine simulator to represent the machine and power system. The data was captured using the excitation system s internal single event recorder. Fig. 2 shows the response when the manual regulator is in control while the turbine output is increased. The field current stays fixed, the terminal voltage decreases slightly, and the generator var output decreases radically. Note that negative vars indicate the generator is underexcited. Fig. 3 demonstrates the response when the automatic terminal voltage regulator is in control while the turbine output is increased. The field current increases, the terminal voltage stays constant and the generator var output increases slightly. Fig. 4 shows the response when the var controller is active supervising the automatic terminal voltage regulator while the turbine output is increased. In this example the bandwidth of the controller was set at.5 percent, and the time delay was set at one second. The field current increases; the terminal voltage decreases negligibly; and the average generator var output stays constant. Fig. 5 shows the response when the var controller is active supervising the manual field regulator while the turbine output is increased. In this example the bandwidth of the controller was set at 2. percent, and the time delay was set at 1 seconds. The field current increases, the terminal voltage decreases negligibly, and the generator var output decreases slightly. The larger bandwidth than in the previous example enables the vars to change more. The reference signal to the manual regulator is being increased only at those times the raise command shown on the bottom is high (-6). Note that no action is taken during the first 1 seconds. This allows for voltage support during motor starts. C. Power System Stabilizers

4 Power system stabilizers function to provide positive damping to help reduce low frequency oscillations in the power grid. Historically these devices have not been used at industrial sites or even at many utility sites in North America. However, their use is becoming more common, and at times they are even required by regulation, depending on site location and size of the generator. If needed, a full array of Power System Stabilizers is available as an integral part of the new excitation system. These include the most commonly used Integral of Accelerating Power type. Implemented in the stabilizer are all the necessary lead-lag filters, ramp filters, notch filters, and ramp limiters as has been described elsewhere Real Power 4. Magnitude of change (Pe Generator Terminal Voltage Field Current Reactive Power -3. Fig. 2. Load increase with constant generator field current (Manual Regulation)

5 5. Real Power 4. Field Current Magnitude of change (Pe Reactive Power Generator Terminal Voltage Fig. 3. Load increase with constant terminal voltage (Automatic Regulation) 5. Real Power 4. Field Current Magnitude of change (Pe Generator Terminal Voltage Reactive Power Fig. 4. Load increase with var controller (Automatic Regulation)

6 5. Real Power 4. Field Current Magnitude of change (Pe Generator Terminal Voltage Reactive Power Raise Command Fig. 5. Load increase with var controller (Manual Regulation) A. Event Recorders III. IMPROVEMENTS IN DIAGNOSTICS The newer digital excitation systems contain recorders that can capture an event. Which signals are to be recorded, how often the data points are to be captured, the number of points to be saved before the trigger, as well as the total number of points to be stored per event are all selectable. These recorders are of particular use in diagnosing problems or misoperations that may occur. For example, during the first start of a new potential source static exciter, the unit may fail to establish generator terminal voltage for a variety of reasons, ranging from connections outside the excitation system to settings inside the static exciter. In the past, little information was available to diagnose why a first-time flash was unsuccessful. With the event recorder, supply voltages as well as internal signals can be captured during the transient that will help eliminate the troubleshooting guesses. This procedure will aid in minimizing the time required to diagnose and resolve the problem. Another use for this recorder is to capture the event when the exciter generates a unit trip. In past systems, at best, there was a signal indicating a unit trip had occurred and sometimes not even that. This made it difficult and time consuming to determine what initiated a unit trip. Now, a recorder can be left active and set to trigger whenever the exciter generates a unit trip. The event can include generator terminal voltage, line current, watts and vars, field current, and key internal exciter signals. Review of this log can help diagnose what initiated the event, reducing down time associated with locating what initiated the unit trip. These types of recorders have been and still are available as separate devices. The initial costs and the complexity and skill required to reliably set them up usually made these independent transient recorders cost prohibitive. Additionally, many signals were not available to be recorded, and the fault recorder was limited to external signals through the appropriate transducer. Another benefit of the internal recorder is that separate transducers are no longer needed. The internal recorders have sample rates as fast as 1 mil1isecond between points. This is normally adequate for diagnosing an excitation system. However, these internal event recorders are not as fast as transient fault recorders that may have sample times of only a few microseconds. Thus for recording lightning or other high-speed events, a separate high-speed recorder is still needed. B. Data Loggers The newer digital excitation systems contain internal recorders that permit the gathering of data over longer periods of time. As in the event recorder, the signals to be logged and the time between samples are adjustable. However, these devices do not have a pre-trigger. Instead they have a fixed amount of memory (size). Upon start up, the new data is

7 stored at the beginning of a block of internal memory. Data continues to be stored until the end of the block is reached. At that time, the logger begins recording again at the beginning and old data is written over. When the logger reaches the end of memory block, it again goes to the beginning and records new data over old data. The logger continues in this manner until the logger is reset. When the information is viewed, all of the data for the entire memory block is displayed giving a history record of the signals being logged. The amount of time viewed depends on the time chosen between samples. Minimum time between samples is one second, and maximum time is one year between points. The logged data can then be saved in a file for storage or use in a database. These types of loggers have been and still are available as separate devices. However, these separate loggers have significant initial costs, and still require separate transducers. Additionally, this recorder allows up to 12 signals to be recorded simultaneously. This allows for analysis of relationships between signals to determine if multiple channels change simultaneously or if a particular signal changes first. C. Field Ground Detectors A field ground detector checks the integrity of the insulation between the field conductors and earth ground. While the field can operate with a single connection to ground, a second connection to ground may result in catastrophic damage to the turbine generator. To protect against this possible damage, a field ground detector is often supplied. Imposing a voltage with respect to ground onto the generator field, and then measuring the leakage current to earth ground detects the strength of the insulation. This device provides an alarm whenever the field to ground resistance decreases below a preset value (leakage current is too high). The newer digital excitation systems include a field ground detector which continually monitors and displays the strength of the field to ground insulation. This enables a monitoring of the insulation, so that a gradual degradation can be detected and the unit taken off line for scheduled maintenance instead of being forced off line in an emergency condition. In addition, by monitoring the amount of field to ground leakage current at various operating points, it is now possible to estimate approximately where the failure is occurring. This provides another diagnostic tool that can be helpful in reducing down time and maintenance costs. D. Field Temperature Monitors On some turbine generator units, a field temperature monitor is used to help assure that the thermal capability of the rotor is not exceeded. The newer digital excitation system contains these monitors internally. This saves the expense of buying a separate device. If the calculated field winding temperature exceeds the preset limit, an alarm can be generated and recorded. E. Bridge Temperature Monitors The newer digital excitation system contains RTDs mounted in the thyristor bridges, internal transducers and direct readout of heatsink and air temperatures. The continuous monitoring of the heatsink temperatures and of the bridge air temperatures allows for efficient pro-active maintenance. By monitoring these values, it can be determined when filters are becoming dirty. The filters can then be cleaned prior to the limiter functioning to reduce turbine generator field current. The bridge temperatures can be input to the data logger as an aid in determining temperature trends. This monitor can be used independently of, or in conjunction with, the bridge current limiters discussed previously. A. Beta Site Description IV. BETA SITE STATIC EXCITER Even though a prototype of this system has been tested in a lab environment using a micromachine system to simulate a power grid, it was considered necessary to test the system in a commercial power plant. This testing was performed at Carolina Power and Light Marshall power plant. The Marshall plant is a run-of-the-river hydro plant with two 2. MVA hydro generators. They are connected on a common bus that feeds through a single step up transformer to the nearby switchyard. Each generator is equipped with a static exciter operating in var control mode set at unity power factor. The new static exciter was installed on Unit 1; the existing static exciter remained on Unit 2. One objective of this installation was to set the new digital exciter so that the existing controls could be used, and operation of the new unit would be identical to the old one. This permitted commercial use of the unit without requiring any new operational procedures or training. The only change made in the control room was the addition of a user interface panel. In addition to the capability of turning the exciter on or off and controlling the output, this panel provided more detailed information on the static exciter. The interface panel was used primarily during commissioning tests, and it remains operational today. However, for starting and stopping the unit, the operator still uses the same set of switches and meters, and follows the same procedures that were used with the old exciter. The new exciter was installed on Unit 1 in early 1999; commissioning tests were performed in March of The unit was synchronized and began producing power on March 3, The digital exciter has been used continuously since then, at whatever power load is required to meet the needs of the system. B. Results of Performance Tests A full set of commissioning tests was performed on the static exciter that has been installed on Unit 1. The following are some of the test results. The data for the figures that follow were gathered using the event recorder included in the exciter. Fig. 6 shows a typical start up of the exciter. The unit at Marshall is a potential source static exciter in which the power source for the thyristor bridge is a power potential transformer directly connected to the generator terminals. Since the initial generator voltage is nearly zero, the field must be flashed to establish some generator voltage before the thyristor bridge will operate. This is accomplished by connecting the generator field directly to the station battery through a current limiting resistor.

8 Fig. 7 shows a typical synchronizing event. The unit is connected to the power grid by closing the generator (52G) breaker. In this figure, the unit was synchronized on a slow scope so that the unit performed as a motor until the gates opened to let more water through. At Marshall, the unit may be synchronized on either a fast or slow scope as long as speed is adequately close to synchronous. The excitation operates reliably with either a fast or a slow scope. Note that MW is the real power output, Delta VT is the deviation in generator terminal voltage, MVAR is the reactive power output, and Delta IF is the deviation in field current. The divide by 1 indicates a scale change for the curve. Fig. 8 shows a classic off-line bump test. The test was performed with the generator operating at rated speed and voltage with the generator (52G) breaker open. The bump was initiated by injecting a step summed with the voltage regulator reference command. Note that Delta VF is the deviation in the generator field voltage. Fig. 9 shows the results of the classic on-line bump test. The test was performed with the generator operating with generator (52G) breaker closed and the unit supplying power to the system. The bump was initiated by injecting a step summed with the voltage regulator reference command. Fig. 1 shows the results of the inverse time delayed overexcitation limiter (OEL) operation test. The objective of the test was to verify that the overexcitation limiter would limit the field current with the unit on line without producing undesirable system oscillations. Since operating conditions did not permit increasing the field current to the actual pickup, this test was run by first injecting a test signal into the limiter block simulating that the field current was near the pickup value of the limiter. The field current was then increased until the limiter picked up and began to time out. After timeout the limiter brought the current (combined test plus actual current) back to the limiter pickup point of 1.5 per unit of full load field current in this case. As was mentioned earlier, the event recorder can be set to pickup an event whenever a unit trip occurs. This feature was used at Marshall. Fig. 11 shows the results of one such event. Time before the recorder trigger is shown as negative Field Voltage Magnitude (Pe Field Current Generator Term inal Voltage Fig. 6. Field Flash

9 6 MW / MVAR / 1 Delta VT Magnitude (Perc Delta IF -6 Delta Firing Command / 1-8 Fig. 7. Synchronization to power grid Test Input Delta VT 1 Magnitude (Perc.5 Delta Firing Command / 1 Delta VF / Fig. 8. Off-line bump test

10 5 Delta VT Test Input Magnitude (Perc Delta Vars Delta Firing Command Fig. 9. Online bump test 4 3 IF (O ver Lim it) AC Error 2 Magnitude (Perc OEL Out Delta Vars Fig. 1. Operation of Inverse Time Overexcitation Limiter

11 4 Firing Com m and 3 G enerator Term inal V oltage 2 Real P ower Magnitude (P Field Current Reactive P ower Fig. 11. Unit trip due to loss of excitation It can be seen in Fig. 11 that the field current begins to decrease. As it does, the reactive current begins to decrease. After picking up, the underexcitation limiter attempts to raise the output, but is unsuccessful. Eventually, the loss of field protection in the exciter tripped the unit. Separate time stamped exciter alarms also showed that the limiter picked up and the loss of field protection tripped the unit. Further investigation into this incident showed that the root cause was related to loss of communication with the firing control board. The communication problem would occasionally cause the thyristor bridge to turn off under certain conditions. A software modification was made and the problem was fixed. The conditions that caused this incident had never occurred during software development with the lab prototype. The problem was fixed in July 1999, and the unit has run reliably since that time. V. CONCLUSIONS New digital excitation systems include a variety of functions that existed before, but which have new and enhanced features incorporated into them. These functions include the overexcitation limiter, the under excitation limiter, the volts per hertz (overvoltage) limiter, and ground detector. The newer excitation systems also include a variety of new functions. These include the bridge temperature limiters and temperature monitors. Also incorporated into the excitation system are functions that were previously supplied as separate devices. These include transient event recorders, data loggers, and field temperature monitors. The reliable performance of the new hardware and software has been demonstrated both in a laboratory environment and in a commercial power generating plant since March These new or enhanced features aid in the diagnoses of issues associated with the excitation control equipment. They provide information that can be used to help reduce maintenance costs and turbine generator down time. With the more sophisticated protection algorithms, it is now possible to safely operate the excitation system and turbine-generator closer to their design limits, permitting maximum performance for the equipment investment. ACKNOWLEDGMENT The authors gratefully acknowledge the contributions of their colleagues in the development of this new excitation system. Special thanks are also due to Carolina Power and Light Company for permitting testing of this new excitation system on one of their generators. [1] ANSI C REFERENCES [2] ANSI C , Requirements for Cylindrical - Rotor Synchronous Machines.

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