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HomeMy WebLinkAboutOperation of Small Wind Turbines on a Distribution System, Executive Summary 1981WIN RFP-3177-1 014 UC-60 OPERATION OF SMALL f WIND TURBINES ON A DISTRIBUTION SYSTEM a at Executive Summary Alaska Power Authority 334 W. 5th Ave. Anchorage, Alaska 99501 DO NOT REMOVE FROM OFFICE David Curtice James Patton March 1981 Prepared by Systems Control, Inc. 1801 Page Mill Road Palo Alto, California for Rockwell International Corporation Energy Systems Group Rocky Flats Plant Wind Systems Program P.O. Box 464 Golden, Colorado 80401 Subcontract No. PF-94445L As Part of the UNITED STATES DEPARTMENT OF ENERGY OFFICE OF SOLAR POWER APPLICATIONS FEDERAL WIND ENERGY PROGRAM Contract No. DE-ACO4-76DP03533 DISCLAIMER This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, pre erwise, does not necessarily favoring by the United State ions of authors expressed e United States Governmen ea : = : } | } | } | | i i | | ! PRINTED INUS.A. 1 i | wen nt RFP-3177-1 ” UC-60 OPERATIONS OF SMALL WIND TURBINES ON A DISTRIBUTION SYSTEM RECEIVED MAR 5 1982 Executive Summary ALASKA POWER AUTHORITY March 1981 David Curtice James Patton Prepared by SYSTEMS CONTROL, INC. 1801 Page Mill Road Palo Alto, California 94304 For ROCKWELL INTERNATIONAL CORPORATION Energy Systems Group Rocky Flats Plant Wind Systems Program P. O. Box 464 Golden, Colorado 8040! As Part of The United States Department of Energy Office of Solar Power Applications Federal Wind Energy Program Contract No. DE-ACO4-76DP03533 Subcontract No. PF-94445L ABSTRACT This study has analyzed technical interconnection problems associated with the dispersed wind turbine (WT) application scenario: WTs connected on distribution systems producing ac power directly or dc power fed into an inverter, without storage systems, feeding back surplus power whenever the wind is blowing. Its specific objectives included analysis of: utility personnel safety; distribution system and WT protection equipment; WTs' effects on distribution feeder voltage and regulation equipment, and line losses; and development of a method to analyze utility load-frequency control problems with load patterns produced by customer demand and the WTs' intermittent power output. The primary safety issue for utility personnel is whether or not present work procedures are adequate for distribution systems with customer-owned WTs electrically connected on circuits. Present procedures do not rely on generation control systems and require a disconnect for voltage-source WTs (synchronous — generation, self-commutated inverter). Although not specifically required by safety procedures, a disconnect on _ voltage- dependent WTs (induction generation, line-commutated inverter) is recom- mended to minimize any possibility of a self-excited WT continuing to operate after a distribution line has been sectioned from the utility system. Isolating or "islanding" small wind turbines is a serious problem for utilities and their customers; utilities are likely to be held liable by their customers if a WT continues to operate isolated causing equipment damage due to frequency and/or voltage excursions outside normal limits. Voltage-source WTs will continue to operate after being separated from a utility, if their power output is sufficient to support the isolated section's load. Voltage- dependent WTs require both reactive power support and light load conditions to operate self-excited. Relays sensing abnormal frequency and voltage are recommended for automatically disconnecting isolated WTs. In general, radial feeder overcurrent protection equipment coordination was not found to be significantly affected by small WTs. Reverse fault current, contributed by even a high penetration of WTs is unlikely to disrupt a utility's existing overcurrent protection schemes, however, WT protection equipment should be coordinated with utility practices. Wind turbines' power output reduces load and tends to decrease a feeder's voltage drop. Existing feeder voltage regulation equipment can perform as planned with WTs, because even high WT penetrations cause only small voltage changes. Utility engineers need experience with combinations of WT power output and load conditions to develop methods for adjusting equipment for optimum voltage control. However, WTs may increase the number of voltage regulator operations, increasing equipment maintenance and cost. Voltage flicker on secondary circuits will be a potential problem for induction generators larger than approximately seven horsepower (5 kW). The large magnetizing inrush current of the generators may cause intolerable light flicker for other customers connected on the same distribution transformer. A method was developed to analyze possible utility load-frequency control problems; treated as negative load, second-by-second WT power output was used in a technique to modify utility load curves input to an automatic generation control simulation. The method allows examination of possible wind speed variation scenarios, their effect on a utility's short-term load characteristics, and possible changes to load-frequency control performance. PREFACE This brief report is a summary of the detailed information presented in Operations of Small Wind Turbines on a Distribution System, Final Report. Midway through the performance of this research project, an interim report was published: Study of Dispersed Small Wind Systems Interconnected With A Utility Distribution System, Interim Report, Preliminary Hardware Assessment, RFP-3093/94445/3533/80/7. The detailed final report includes the material published in the interim report. ACKNOWLEDGEMENTS The authors acknowledge the useful suggestions and guidance of Judith Porpotage from Rockwell International, Thomas Reddoch from Oak Ridge National Laboratory, and Fred Ma from Systems Control, Inc. Technical contributions from Richard Raithel, and Sudhir Virmani are gratefully acknowledged. TABLE OF CONTENTS OBJECTIVES . APPROACH . SAFETY ASSESSMENT. . . . DISTRIBUTION OPERATIONS ASSESSMENT . BULK GENERATION OPERATIONS ASSESSMENT. . - 16 EXECUTIVE SUMMARY The report Operations of Small Wind Turbines on a Distribution System is a result of a one year technical analysis of problems posed by small wind turbines interconnected with utility distribution systems. It is part of the Rocky Flats Wind Systems Program, operated by Rockwell International for the Department of Energy's Federal Wind Energy Program. The study project has focused on the future application of small wind turbines, dispersed in distribution systems, where customer-owned wind turbines are located on the owner's property and either produce ac power directly or produce dc power fed into an inverter. In this configuration wind turbines do not have storage systems and when the wind is blowing surplus power automatically flows back into the distribution system. While this wind turbine system configuration offers many advantages of abundant power on demand, it poses a number of potential technical problems for wind turbine owners and the utility system. Principal among these are utility personnel safety concerns and operational problems caused by a large number of widely distributed generation sources, subject to rapid power output fluctuations, and by the utility's lack of access to these generation sources. OBJECTIVES Rapid commercialization of small wind turbines requires analysis and resolution of potential problems as well as development of reliable, cost- effective wind turbines. The principal objectives of the study project were to assess technical problems and develop solutions for interconnecting wind turbines on distribution systems. The specific objectives of the study were: e Define utility personnel safety problems created by wind turbines e Identify modifications to distribution protection coordination required to accommodate wind turbines connected on feeders e Analyze protection equipment for small wind turbines e Analyze distribution feeder voltage profiles and line losses with various penetrations of wind turbines e Discuss” distribution voltage regulation problems and secondary voltage problems created by wind turbines e Develop a method to analyze load-frequency control with increasing penetrations of small wind turbines This report identifies many potential problems posed by wind turbines in each of the general areas of concern listed above. Its focus is on technical problems and technical solutions, and not the economics of implementing the solutions. Furthermore, the large number of potential problems were identified without regard to their likelihood of occurring and therefore additional work is required to analyze the cost of solutions developed and the associated risk posed by a given problem. The work reported here represents the first attempt to identify problems which are of principal concern to utilities, and to develop solutions that should assist both utilities and manufacturers of small wind turbines. APPROACH The technical approach to the study is illustrated in the project flow chart. A scenario approach was employed to create a wide range of study cases. The principal components of the scenario definitions included: Distribution System Characteristics and Procedures - Two existing radial distribution systems provided the technical data bases and the without wind turbines case. Each represents a different distribution system design common in the United States. Both were located in rural environments serving a customer mix characteristic of areas where small wind turbines are likely in the near future. Wind Turbine Electrical and Size Characteristics - Four designs; synchronous and induction generator, line and self-commutated inverter wind turbines, in discrete sizes to a maximum of l00kw, represented small wind turbine technology. Penetration - Wind turbines' rated output as a percent of system load was varied from one to fifty percent. Wind Turbine Power Output - Recorded power output data from various small wind turbines provided the data base to character- ize short-term wind turbine performance. The objectives of the study were grouped into three major topic areas. Within each topic many different scenarios were created to identify potential problems posed by dispersed wind turbines. Then alternative solutions were assessed to develop general techniques based on results from specific technical studies. The three major topic areas of the study include: Safety Assessment - The specific objectives of this part of the study were to define utility personnel safety hazards posed by customer-owned wind turbines, and to identify work procedures and wind-turbine interconnection hardware necessary to ensure a safe work environment. Distribution Operations Assessment - This part of the study focused on protection equipment for two distribution systems and small wind turbines, and the effects dispersed wind turbines have on distribution voltage regulation and line losses. Wind turbine and distribution system protection equipment were examined to develop protection equipment schemes suggested for different wind turbine designs, and appropriate modifications to protection equipment used in distribution systems. The wind turbines’ effect on distribution feeder voltage, voltage regulation, and line losses were also studied to identify typical effects under a wide range of system conditions. Bulk Generation Operations Assessment - The objective of this work was to develop a method to assess utility short-term operating problems caused by changing a utility's load from one created by relatively predictable fluctuating customer demand patterns, to a load created by a combination of customer demand and additional load fluctuations created by the highly variable power output characteristics of wind turbines. ADEQUACY OF SAFETY PROCEDURES DISTRIBUTION SYSTEM @ CHARACTERISTICS @ PROCEDURES SUGGESTED WIND TURBINE PROTECTION SCHEMES DISTRIBUTION SYSTEM @ SAFETY ASSESSMENT e@ DISTRIBUTION OPERATIONS ASSESSMENT WIND TURBINE @ ELECTRICAL CHARACTERISTICS @ SIZE DISTRIBUTION SYSTEM PROTECTION MODIFICATIONS PENETRATION VOLTAGE REGULATION AND LINE LOSSES METHOD FOR WIND TUREINE BULK SYSTEM ASSESSING WIND POWER OUTPUT @ METHOD DEVELOPMENT ° FOR BULK GENERATION TURBINE EFFECTS ON VARIATIONS MENT Rg SAFETY ASSESSMENT Discussion of Problems Introducing dispersed small wind turbines into today's centralized utility systems creates concerns about safety and the capability of presently designed distribution systems to accommodate customer-owned wind turbines. A wind turbine's power output is principally dependent upon wind speed conditions, and utility personnel may be endangered if customer-owned wind turbines send surplus power back into distribution lines when circuit breakers on the utility side have been opened to deenergize an area for servicing. It is impractical for a utility to contact and ask each customer with a wind turbine to turn off the wind turbine whenever line crews need to deenergize lines for repair. Ideally, wind turbines should automatically shut down when service or repair work is required. Wind turbines with their control systems, however, produce power unpredictably and without direct utility control line crews do not have assurance that a wind turbine will not start up and feed power back into the work area. Before a utility can allow many customers to install wind turbines, it must have procedures and/or interconnection requirements designed to ensure the safety of its personnel. Method of Study The primary safety issue is whether or not present utility work procedures are adequate for distribution systems with customer-owned wind turbines. A large number of utilities submitted general work procedures and special procedures developed for customer-owned generators to the project. These documents and discussions with the utilities identified the Occupational Safety and Health Administration (OSHA) as the agency responsible for developing utility personnel safety guidelines that apply to all utilities. Assuming that the OSHA guidelines are unlikely to change significantly in the near future, various options have been examined to meet OSHA's guidelines which were developed to ensure a safe work environment when a utility's line crews are exposed to possible electrical shock. Both the type and number of wind turbines connected on a distribution system pose new safety hazards for line crews. Synchronous generators and self-commutated inverters can feed back into the utility's lines at any time because they provide their own source of voltage. Induction generators and line-commutated inverters depend on the utility's source of voltage to operate, unless they become self-excited. Self-excitation may occur when a source of reactive power (e.g., capacitor bank) is present and is therefore situation dependent. A large number of wind turbines connected on distribution circuits consisting of a mix of voltage source and voltage-dependent wind turbines increases the possibility that a wind turbine may feed back electric current into equipment deenergized by the utility's line crew. To examine possible safety hazards with wind turbines connected on a utility distribution system, study scenarios were developed from different combinations of the following variables: Distribution system design Wind turbine electrical characteristics Wind turbine size Penetration or number of wind turbines connected on a feeder For each scenario, present work procedures were evaluated to identify potential safety hazards. Alternative work procedures and/or hardware were then assessed for their ability to provide the same degree of personnel safety present before the introduction of wind turbines. Utility work procedures distinguish four basic phases for work on distribu- tion systems. First, the work area containing damaged equipment is located for isolation from the distribution system. Second, a _ step-by-step procedure is followed to deenergize the appropriate line sections to ensure that electric current does not flow into the work area. The third phase involves working in the deenergized work area, followed by the last phase, reenergizing the section. In each study scenario these four phases were examined to identify potential safety problems created by wind turbines. Of principal concern were the wind turbines' electrical characteristics; that is, their ability to feed electric energy back into the work area. The problems created by various penetrations of wind turbines connected on distribution circuits were also assessed. Conclusions and Recommendations Present work procedures require a visible open circuit between voltage sources and the line-crew's work area. This requirement indicates that synchronous generator and self-commutated inverter wind turbines must have a disconnect accessible to the utility's line crew. Line-commutated inverters and induction generators would not normally operate after the utility's line voltage has been removed, unless self-excited. Although not required by safety procedures, a disconnect for line-commutated inverters and induction generators would be advisable to eliminate the possibility, however unlikely, that a self-excited wind turbine could endanger a utility's line crew. In addition to a visible disconnect on a wind turbine a line-crew would also need a method to stop and prevent a wind turbine from operating. This additional requirement ensures that a line crew does not attempt to open the disconnect when a wind turbine is operating. Alternatively, wind turbines could be equipped with load-break disconnects, providing that opening the disconnect would not damage the wind turbine. With a large number of wind turbines connected on a feeder, utility line crews will spend more time identifying and opening disconnects at every wind turbine. Automatic disconnecting techniques might be used; however, present safety procedures require the line crew to visibly establish the open circuit at each wind turbine and lock out the disconnect. Locking out the disconnect ensures that the owner does not reclose it for any reason. This lock-out requirement in present procedures indicates that little time may be saved by using automatic disconnect schemes. An automatic disconnecting scheme is clearly desirable. OSHA's guidelines presently work against automatic disconnects by requiring visible assurance of the open circuit, and locking out the disconnect. A fail-safe automatic disconnect scheme may be technically feasible providing that the utilities, their employee unions, and OSHA can agree on a method to validate the reliability of various approaches, such as radio signal, ripple control, etc. Customers planning to install a wind turbine must notify the utility before connection is attempted. Notification allows the utility to help the customer select the location and type of disconnect, and the wind turbine shut-off device (if needed). Furthermore, the utility will need to keep a record of the location of all the customer-owned generators connected on their system. This will be especially necessary if a large number of customers have wind turbines in a single distribution system. Equipping all wind turbines with disconnects will meet safety procedures presently required by OSHA, and will provide a maximum degree of safety for the utility's personnel. Disconnects with wind turbine shut-off devices are likely to be preferred by utility personnel over lock-break disconnects. As a near-term solution, with only a few wind turbines on a utility's distribution system, a disconnect is appropriate and is being adopted by many utilities surveyed during the course of this study. As a long-term solution, however, additional time is required to disconnect wind turbines with many customer-owned generators connected on distribution lines. Further research is needed to define reliability criteria for evaluating automatic disconnect schemes. DISTRIBUTION OPERATIONS ASSESSMENT Discussion of Problems Presently, utilities operate with some degree of load predictability and generation surplus or deficit can be predicted with a reasonable degree of accuracy to guide short-term dispatching. Furthermore, maintenance and repair of radial distribution systems is governed by the assurance that power flows in only one direction, from the utility's generators to the load, allowing safe and reliable termination at any time. And distribution systems have been designed to ensure that customer's loads perform properly within specified standards of voltage and frequency. Distribution system and customer equipment may be damaged if wind turbines continue to feed power into a section separated from the utility system. Because wind turbines will receive their synchronization from the utility's power line, loss of the utility's power could cause deterioration in the quality of electricity; i.e., voltage and/or frequency produced by the wind turbines. To avoid this problem, utilities presently require all customer-owned generators to automatically disconnect from the utility's line whenever an abnormal condition occurs, such as a broken conductor or other faults. One of the principal problems addressed in this study is whether or not, and under what conditions wind turbines will respond to abnormal conditions; and, after responding, if they take appropriate action. Protection equipment, such as circuit breakers and fuses, is an integral part of distribution systems. Its function is to provide continuous service to a maximum number of customers during abnormal conditions, (e.g., broken conductors caused by environmental and man made phenomenon), by disconnecting the smallest section containing the abnormal condition. Wind turbines connected on a distribution system may complicate the location and coordination of protection equipment because they can supply fault current during an abnormal condition. In addition, distribution protection LIBRARY COPY Alaska Power Authority 334 W. 5th Ave. Anchorage, Alaska 99501 DO NOT REMOVE FROM OFFICE equipment is coordinated in such a way as to isolate the smallest possible section containing a fault. Wind turbines may disrupt present coordination practices and cause isolation of a larger section. Isolated operation or "islanding" of wind turbines on a circuit disconnected from a distribution system poses a serious problem for the utility and its customers. When this occurs wind turbines may continue to supply power to an isolated section's loads. Loss of utility voltage and frequency to an isolated section may allow wind turbines to seriously degrade power quality and damage voltage and/or frequency sensitive equipment. Furthermore, when a utility re-establishes power to the isolated section, wind turbines may be damaged if the utility's voltage is out-of-phase with the wind turbine's voltage. When a large number of wind turbines is connected on a distribution feeder the power output reduces part of the feeder's load. The combined effects of the wind turbines’ variable power output and customer demand patterns result in changes to the feeder's voltage. Voltage regulation equipment set to adjust feeder voltage based on established load conditions may require new settings or replacement due to the new voltage conditions created by the wind turbines' variable power production. In addition, variable feeder voltage swings between maximum and minimum voltage conditions may cause increased voltage regulation equipment wear and maintenance costs. Wind turbines using line-commutated inverters and induction generators draw reactive power from the utility system. In sufficient numbers these wind turbines may impose substantial reactive power requirements on the utility system. This in turn causes additional line losses and, at some point, may require the utility to install additional equipment to supply the wind turbines' reactive power demand. 10 Method of Study Distribution operations were defined in this study to encompass three problem areas; protection equipment, voltage regulation, and line losses. The general approach taken to examine each of these areas consisted of assuming various wind turbine penetrations connected on the utilitys' distribution systems. In addition, by varying four principal variables - distribution system design, wind turbine electric characteristics, wind turbine size, and penetration of wind turbines on a distribution system - a large number of possible scenarios was created to test alternative solutions and to uncover new problems. The approach taken to analyze potential protection equipment problems relied on the same methods used by electrical engineers to design protection equipment schemes. Specifically, the two distribution systems defined existing protection equipment and coordination and then different wind turbines were assumed connected on distribution circuits. Four wind turbine designs - synchronous and induction generators, and line and self- commutated inverters - were examined to define their response to different types of utility line faults. In addition, by assuming various locations and numbers of wind turbines connected on the distribution systems, many possible protection problems were examined to develop alternative solutions. A modified equivalent circuit representation of an induction generator was used to define an induction generator's voltage-decay characteristics. This technique allows calculation of the induction generator's terminal voltage as a function of time after a utility line fault. Results derived from calculations simulating various generator size, fault locations, and recloser operating characteristics, were used to assess the possibility of a recloser re-establishing the line voltage out-of-phase with an induction generator's residual voltage. An analytic technique was used to define the capacitance needed to keep an induction generator wind turbine self-excited after separation from the 11 utility's voltage. Induction generator size and load conditions were varied to identify the capacitance required to maintain a specific generator voltage and illustrate a wide range of conditions. Synchronous and induction generator wind turbines connected on a feeder will increase the distribution systems! short-circuit capacity, and if the penetration is significant, increased short-circuit current may exceed the short-circuit ratings of feeder equipment. A short-circuit analysis program was used to study faults with various penetrations of wind turbines connected on the two distribution systems. Voltage regulation and line losses were examined using a distribution circuit analysis program (load flow). This program calculates feeder voltage and current flow given inputs on load conditions and distribution equipment characteristics. Both distribution systems, without wind turbines, established base case feeder voltage profiles and current flow and then various penetrations of wind turbines were added to define new feeder voltage profiles and current flow. In addition, wind turbine power factor and power output were varied to assess their effect on voltage and line losses. The load flow results were evaluated to determine how various penetrations of wind turbines, operating over a wide range of power factor and power output conditions, influence the utility's voltage regulation equipment. These results were also used to determine line losses under different conditions assuming wind turbines were located along a feeder, or aggre- gated toward the end of the feeder. Conclusions and Recommendations On radial feeders the coordination of fuses and reclosers was not found to be affected by small wind turbines. The short-circuit capacity of small wind turbines is considerably less than the utility's short-circuit capacity. Reverse fault current from wind turbines, even for high penetrations of small wind turbines, did not cause the utility's overcurrent protection 12 equipment to operate unexpectedly. These specific examples suggest that small wind turbines are not a significant enough source of fault current to disrupt a utility's overcurrent protection schemes. Isolation of wind turbines (islanding) from the utility's voltage and frequency was found to be the most serious problem examined in the study. Synchronous generators and self-commutated inverters can continue to operate if isolated, providing the wind turbine's power output is sufficient to support the isolated section's load. Induction generators and line- commutated inverters will self-excite if lightly loaded with sufficient capacitor compensation in the isolated circuit. This condition is situation dependent and each voltage-dependent wind turbine should be examined to determine if this condition is likely to occur. A method to analyze the load conditions necessary for self-excitation is presented in the text of the report for both line-commutated inverters and induction generators. Self-excitation is extremely situation dependent requiring specific load and reactive power conditions for induction generators and line-commutated inverters. Installing capacitor banks at various points along a feeder is likely to produce more possible conditions necessary to self-excite line-commutated inverters and induction generators than by placing a large capacitor bank at the electrical center of the feeder. Wind turbines equipped with relays to detect abnormal frequency and voltage are recommended for detecting isolated operation. Additional studies, however, are needed to define application guidelines for selecting relay settings. Several preliminary small wind turbine protection schemes were developed based on technical requirements and study results derived from the analysis of protection problems. They are presented in the text of the report as examples and should not be considered requirements for any specific wind turbine or utility distribution system. They were developed to show 13 equipment configurations capable of responding to problems identified in the study. The interaction of small wind turbines with distribution feeder reclosers was evaluated for persistent utility line fault conditions. The voltage decay for induction generators was determined for two different machine capacities. Results of these studies indicate that a delay of five to ten cycles is sufficient to avoid reclosing on an energized wind turbine. In general, reclosing on a line-commutated inverter is not a problem because out-of- phase synchronization is not possible when the interconnection point is at the dc bus. Wind turbines on a feeder tend to decrease the voltage drop along the feeder. This result did not significantly change when, (I) the wind turbines' power factor varied over a range from 40 to 90 percent, (2) the feeder was lightly or heavily loaded, (3) the strength of the utility source was changed (as measured by the utility short-circuit capability), (4) the feeder's resistance/reactance ratio was changed, and (5) the wind turbine's power output changed with various penetrations connected on the feeder. Based on the distribution systems studied, present voltage regulation equipment was found sufficient for regulating voltage with various penetrations of small wind turbines affecting the feeder's voltage profile. Experience, however, is needed to judge the effects and to develop methods to adjust equipment given the various possible combinations of load and wind turbine power output. The wind turbines' effects on feeder line losses depends on their power factor and their power output as a percent of feeder load. Specific results from studies of two different distribution systems showed that if the wind turbines are operating at rated output and supply less than 20 percent of the feeder's load, then line losses are less than base case conditions without the wind turbines. If the wind turbines' power output supports fifty 14 percent of the load, then line losses are insignificantly increased over the base case. Line losses can be significantly increased if the wind turbines operate at cut-in wind speeds when voltage-dependent wind turbines draw reactive power while producing little real power. In general, synchronous generator and self-commutated inverter wind turbines will reduce line losses because they reduce the distribution system load without decreasing the substation load power factor. Line-commutated inverter and induction generator wind turbines are likely to reduce line losses when operating power factors are in the range of 70 percent. Lower power factors increase the feeder's reactive current component and as the penetration of wind turbines increases on a feeder, line losses will increase. Additional capacitor compensation used to raise the substation load power factor and reduce line losses is not recommended because line losses were not significantly increased with any realistic penetration of wind turbines on the distribution systems studied. Furthermore, placing additional fixed capacitor compensation on the feeder may cause overvoltage conditions during light load periods and increase the possibility self-excited operation. Voltage flicker on secondary circuits was found to be a potential problem for induction generators. If dedicated distribution transformers are not required for customers with wind turbines excessive voltage flicker may result on some secondary circuits. Utility guidelines for estimating motor flicker can be used to identify possible flicker problems for specific induction generator applications. 15 BULK GENERATION OPERATIONS ASSESSMENT Discussion of Problems Potential generator dispatch and load forecasting problems may be experi- enced by the utility due to dispersed wind turbines changing the utility's load from one produced primarily by relatively predictable demand patterns to a load produced by a combination of customer demand and wind turbine power output characteristics. The wind turbines' intermittent power output, varying with wind speed changes, may cause utility load fluctua- tions that have extreme peaks followed by sharply decreasing load valleys. Such dynamic loads may result in additional load-frequency control problems due to load and utility generator mismatches. Method of Study An automatic generation control program was used to identify utility operating problems caused by combining’ short-term load demand fluctuations and wind turbine power output characteristics. The computer program simulates the real-time utility operation and provides system performance information about the utility's effectiveness in regulating generator output to meet the fluctuating system load. Second-by-second power output data from small wind turbines, recorded at the Small Wind Systems Test Center, were used to construct a simulation of aggregate wind turbine power output representing various wind turbine penetrations calculated as a percentage of the utility's system load. In addition, new aggregate power output scenarios were produced using a sine wave to simulate possible effects small wind turbines may have on the load seen by the utility's regulating units. The second-by-second aggregate wind turbine power output characteristics were developed from a limited number of recorded wind turbine power output tapes. These aggregate power output data were subtracted from 16 real utility load data to characterize the short-term effects wind turbines may have on a utility's load. Alternative scenarios were examined by changing wind turbine penetration, simulating how wind turbines may influence the utilitys' generator dispatch and control process used to meet load variations. Conclusions and Recommendations A method was developed to assess how the combined effects of load demand and wind turbine power output variations influence a utility's load- frequency control process. Without a standard model to characterize the short-term load fluctuations created by wind turbines, a procedure has been developed to synthesize power output characteristics of a large number of wind turbines. Because the created power output data could not be validated by comparison with actual output data from a large number of wind turbines at the time of the study, results are subject to verification with actual operation. Two scenarios were used to characterize the wind turbines' effect on the system load. The first scenario describes the steady-state condition; wind turbines produce an effect on the system load due to their individual fluctuating power output which when added to the load, results in a load representing customer demand variations and wind turbine power output characteristics. Results from studies representing the steady-state wind turbine scenario showed insignificant changes to the utility's load. The control process used to regulate generators to follow the load was not affected, and performance measures used to judge the effectiveness of control were nearly identical with and without the wind turbines power output variations added to the utility load. The second scenario assumes that the power output of many wind turbines either simultaneously increases or decreases. Such a condition is created LIBRARY COPY Alaska Power Authority 334 W. 5th Ave. Anchorage, Alaska 99501 DO NOT REMOVE FROM OFFICE 7 when the wind speed changes over a large land area, affecting a large number of wind turbines. The second scenario significantly changes the utility's system load depending on the number of wind turbines and the assumed wind speed changes. These effects on system load directly influence generator response and the control process performance. If the wind speed changes cause the wind turbines to quickly increase or decrease their power output, decreasing utility control performance was produced by increasing the magnitude of wind turbines' aggregate power output. With wind turbine penetrations of less than five percent, rapid increases and decreases in the wind turbines' power output did not cause a significant change in the control process to regulate generator output and to follow the load. However, increased energy flow over the lines connected to neighboring utilities compensated for generator/load mismatches occurring too fast for the utility's generators to follow. If the utility's control process is designed to minimize tie-line flow deviations from scheduled exchange with neighboring utilities, then generator/load mismatches show up as increased control error and decreased system performance. A utility system has a limited response capability. This study has examined two system response capabilities: 6 and 20 MW per minute. In general, wind turbine imposed load variations which were faster than the systems' response capability produced poorer system performance. Whether or not this condition will occur depends on the wind turbine's power output as a percent of the utility's load and wind speed characteristics of the area. Accurate data are needed to define the power output performance of wind turbines, wind speed variations suitable for short-term generation dispatch, and aggregate power output of a number of wind turbines. With such a 18 data base feedback, control algorithms can be developed that account for the wind turbines' effect on system load. Automatic generation control schemes are designed to respond to load variations that are controllable. Studies are needed to assess how much of the wind turbines' power output fluctuations affect controllable load variations and what portion changes too fast for the generators to follow. The portion which causes load variations that can be followed by a utility's operating reserve will produce additional control effect and increased costs (mostly fuel). Higher frequency load variations will tend to degrade system performance, principally by demanding greater tie-line flows from neighboring utilities. Additional studies are needed to identify the penetration and wind speed variations responsible for these conditions. Such studies will allow appropriate data collection and alternative AGC algorithm development to proceed before utilities actually experience significant penetrations of wind turbines. 19 ¥¥ U.S. GOVERNMENT PRINTING OFFICE 1981 — 779-937/323 Region No. 8 LIBRARY COPY Alaska Power Authority 334 W. 5th Ave. Anchorage, Alaska 99501 7 DO NOT REMOVE FROM OFFICE