CAPACITY OF THE VICTORIAN ELECTRICITY TRANSMISSION NETWORK TO INTEGRATE WIND POWER

Transcription

1 CAPACITY OF THE VICTORIAN ELECTRICITY TRANSMISSION NETWORK TO INTEGRATE WIND POWER DECEMBER 2007

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3 DISCLAIMER The purpose of this document is to provide information about VENCorp s assessment of the technical impacts of wind turbine generation developments in the Victorian shared electricity transmission network. VENCorp has prepared this document in reliance upon information available in the public domain and from a number of third parties (and which information may not have been verified). Anyone proposing to use the information in this document should independently verify and check the accuracy, completeness, reliability and suitability of the information in this document, and the reports and other information relied on by VENCorp in preparing it. VENCorp makes no representation or warranty as to the accuracy, reliability, completeness or suitability for particular purposes of the information in this document. VENCorp and its employees, agents and consultants shall have no liability (including liability to any person by reason of negligence or negligent misstatement) for any statements, opinions, information or other matters (expressed or implied) arising out of, contained in or derived from, or for any omissions from, the information in this document, except in so far as liability under any statute cannot be excluded.

4 CONTENTS EXECUTIVE SUMMARY 5 1 INTRODUCTION Background Objectives Overview of the Report 16 2 A REVIEW OF WIND POWER INTEGRATION INTERNATIONALLY AND NATIONALLY Introduction Global Wind Power Penetration Wind Power Generation Wind Turbine Technology Typical Wind Farm Layout Global Wind Power Experience NEM Wind Power Generation Operational Experiences Summary 26 3 WIND POWER AVAILABILITY AND VARIABILITY Introduction Possible Wind Farms Locations Regional Wind Speed Patterns Average Output and Capacity Factor Output Variability Victorian Average Daily Output Assumptions for the Analysis of Wind Power Generation Output Summary 39 4 ASSESSMENT OF SYSTEM TECHNICAL ISSUES Introduction Study Methodology and Modelling System Study Approach Steady State and Dynamic Analysis System Security Concerns Power Balance and Network Contingencies Power Quality Summary 56 APPENDIX A GLOSSARY 57 APPENDIX B REFERENCES 58

5 EXECUTIVE SUMMARY INTRODUCTION The Victorian Government s VRET (Victorian Renewable Energy Target) scheme 1 mandates that Victoria's consumption of electricity generated from renewable sources be increased to 10% by The new Federal Government has also committed to expand the Mandatory Renewable Energy Target (MRET) scheme to 20% by Wind energy is a significant renewable energy source to meet these targets. VENCorp s Vision 2030 report 3 assumed that a significant amount of wind turbine generation would be one of the generation scenarios required to meet future load growth. It concluded that a separate technical study would be required to examine the implications of integrating wind power generation with the Victorian transmission network. This report forms that technical study. To ensure that the Victorian electricity network can accommodate a significant amount of wind power generation, VENCorp studied the possible technical impact of incorporating wind plant into the Victorian shared transmission network and the National Electricity Market (NEM). KEY OBJECTIVES The key objectives of this study involved: reviewing international and national experiences of integrating wind power generation with the electricity network; investigating the technical impact of wind power generation developments on the Victorian shared transmission network and the NEM; and determining the amount of wind power generation that the Victorian shared transmission network can accommodate. CONTEXT In terms of the report s context, it is important to note that there are a number of challenges affecting the viability of increased Victorian wind power generation, which can be grouped into the following categories: transmission network; system security and operational; market, regulatory and commercial; and 1 Victorian Renewable Energy Act Federal Labor s 20 percent by 2020 Renewable Energy Target, 3 VENCorp VISION 2030 Report 25 year vision for Victoria s Energy Transmission Networks October

6 environmental. This report focuses solely on one of the key challenges, being the capacity of the Victorian transmission network to integrate increased wind power generation. The other three key areas clearly need to be considered when assessing the viability of wind generation, but are beyond the scope of this report. KEY FINDINGS This report s key findings include the following: The viability of wind power generation is subject to other challenges, such as system security and operational, market regulatory and commercial and environmental matters, which need to be considered. With the appropriate technical solutions, wind power generation of approximately 3,000 MW installed capacity (and possibly up to 4,000 MW, depending on where generation is located) can be accommodated by the Victorian transmission network. VENCorp is not proposing to set an absolute limit on the connection of wind power generation. Based on available information, most of the wind power generation in Victoria is expected to be located in the west and south west Victoria. The impact of wind power generation depends on its location (relative to the load) and the strength of the local transmission network. Major concerns relating to integration include the variability of wind and wind forecasting, thermal constraints, voltage control, fault-level control, fault ride-through capability, and the quality of the power supply. Technology is rapidly evolving around various innovative concepts to integrate high levels of wind power generation within the transmission network. A REVIEW OF WIND POWER INTEGRATION (INTERNATIONALLY AND NATIONALLY) Global wind power generation capacity has increased significantly over the last few years, totalling approximately 74,000 MW by the end of This is approximately 235% of the installed capacity in Figure 1 shows a comparison of wind power generation penetration among the top 10 wind power generating countries and Australia. Australia has an installed wind power generation capacity of 975 MW, which is approximately 1.3% of global wind power generation. South Australia has the largest installed capacity of 547 MW. Victoria s installed capacity is 134 MW. Figure 2 shows the rate of increase in the installed capacity of Australian wind power generation. 6

7 Figure 1: Global wind power generation penetration (as at end of 2006) Rest of the World, 13.7% Australia, 1.3% France, 2.1% Portugal, 2.3% Germany, 27.7% UK, 2.6% Italy, 2.9% China, 3.5% Denmark, 4.2% India, 8.4% Spain, 15.6% US, 15.6% Figure 2: Installed capacity growth of Australian wind power generation Installed wind generation capacity (MW) QLD NSW TAS VIC WA SA Total Year The review of the integration of wind power generation internationally and nationally found that the impact of wind farms on the transmission network depends on the: location of wind power plants relative to the load; strength of the transmission network at the point of connection; and correlation between wind power production and load consumption. 7

8 Major concerns relating to wind power generation integration include the variability of wind power and weather/wind forecasting, the fault ride-through capability, and the quality of the power supply. Since most wind power generation is connected to the weaker network and is remote from the major load centres, thermal constraints and voltage control are also significant concerns. However, technology is rapidly evolving around various innovative concepts to integrate high levels of wind power generation within the transmission network. Specific findings regarding the international experience include the following: The variability of wind power generation and weather/wind forecasting is addressed by: utilising special forecasting models that make use of meteorological predictions of wind speed and therefore expected power output; and maintaining a sufficient amount of generation reserve to ensure reliable power system operation, and providing additional network control services to prevent transmission overloading in the event of a powerful wind front. Thermal constraint issues are the same as for other types of generation and are not specific to wind power. Reactive power and voltage control issues are addressed by requiring wind power generation to provide network services, such as voltage control. Where wind power generation offsets conventional generation, additional reactive power sources close to load centers may also be required. Fault ride-through system stability issues are addressed by requiring greater levels of fault ridethrough capability from wind generation for a severe voltage depression due to a system fault. These requirements are now being met by certain newer types of wind turbines, with older models being retrofitted to comply. Some deterioration in system inertia, as wind turbines replace conventional generation, is a possible outcome from increased wind power generation penetration. Transmission network fault levels tend to increase with the connection of more wind turbines. This phenomenon also occurs with the connection of other types of generation, and is generally resolved on a case-by-case basis. Modelling, simulation, and wind turbine model validation issues occur due to confidentiality constraints, which prevent utilities from gaining access to detailed wind turbine models. While high-level studies can use generic wind turbine models to identify general behavior, accurate, validated models are very important for specific connection assessments. Specific findings regarding the national experience include the following: The Electricity Supply Industry Planning Council (ESIPC) in South Australia found that the impact of 400 and 500 MW of wind power should be modest, but that generation of 800 and 1,000 MW presented growing impacts on system reliability, security and price. A number of recommendations to minimise adverse impacts from increased wind power generation integration have been considered, and no absolute limit on wind power generation is recommended. 8

9 NEMMCO found that for high South Australian wind power generation penetration (1,200 MW), the Victoria-South Australia interconnector will not be sufficient to export power from South Australia to Victoria, and Murraylink will have to be used to regulate transfers. Additional reactive support will be required near Adelaide to utilise the interconnector s import capacity. Variability and wind power forecasting issues are being addressed by NEMMCO, which is developing a weather/wind forecasting mechanism for wind power generation. This development falls outside the scope of this report. Frequency control ancillary service usage and costs may increase with the increase in intermittent generation. Voltage control is primarily a local issue. However, additional reactive plants may be required to control voltages at a transmission level. System security and stability issues involving fault ride-through capabilities are an important requirement for all NEM generators. System security assessments may need to consider wind power output variability simultaneously with other credible contingencies. The output of wind power generation in some locations is limited so as to operate the transmission lines within their capability. Modelling, simulation, and wind turbine model validation issues have arisen due to the reluctance of some wind farm proponents to release detailed wind farm models. POSSIBLE WIND FARM LOCATIONS A significant number of locations in Victoria are being identified for possible wind farm developments. Based on publicly available information and information from VENCorp s connection enquiries and applications, most of the wind generation in Victoria is expected to be located in the west and southwest. Figure 3 shows the possible locations of these wind farms. Remote from the major load centres, these locations could possibly be connected to the 500 kv network, the weak 220 kv network, or the lower voltage network. 9

10 Figure 3: Possible wind farm locations and the regions considered 4 THE IMPACT OF WIND FARMS ON THE TRANSMISSION NETWORK Specific findings regarding wind farm impacts on the transmission network relate to the: variability of wind and associated probability factors; variability of wind due to seasonal weather; and assessment of technical issues. The variability of wind and associated probability factors Wind power generation output is subject to local wind conditions. As a result, a key element of the study involved obtaining realistic wind data representing the performance of actual wind power generation plants. Since most of the studies were done on a prospective basis, sufficient wind data were not available. Due to this limitation, and given that output variability is expected to be relatively similar between sites in close proximity, proposed wind farm locations are grouped into five broad regions (as shown in Figure 3). Table 1 lists the range of generation considered and the expected capacity factors 5 in each region. Figure 4 shows the typical output variability in each region, and for Victoria as a whole. This generic 4 Source: Victorian Wind Atlas published by Sustainable Energy Authority, Victoria December 2003 and Wind energy projects in Australia, 10

11 information shows that output variability increases over time. Due to the extreme variability of the wind in all locations, wind farm output can range from the total installed capacity to producing no power, and vice versa. Table 1: Capacity factor of proposed wind farms by region Region Indicative total installed capacity Estimated capacity factor Coastal South West 1,045 MW 35-45% Inland South West 1,010MW 30-39% Central 910 MW 26-37% North West 920 MW 31-41% South East 115 MW 31-39% Figure 4: Wind power variability 30% 25% Output variability (%) 20% 15% 10% 5% 0% Time (hours) NW C ISW CSW SE VIC Figure 4 shows typical (average) variability. Table 2 presents the expected 30-minute and 6-hour variability associated with probability factors of 25%, 50%, 75% and 90% of occurrence. For example, after 30 minutes, there is a 25% chance that the output power at the end of the interval is within 1% of the original output power (and a 90% chance that it is within 20%). Managing significant variation could become an issue, particularly with large amounts of wind power generation. 5 Capacity factor is one element used to measure the productivity of a wind turbine or any other power production facility. It compares the plant's actual production over a given period of time with the amount of power the plant would have produced if it had run at full capacity for the same amount of time. 11

12 Table 2: Maximum normalised variability at different probability factors Probability Level 25% 50% 75% 90% 30-minute variability 1% 4% 11% 20% 6-hour variability 4% 14% 32% 53% The variability of wind due to seasonal weather The daily output of wind power generation is mainly influenced by weather conditions. Figure 5 shows the expected average daily outputs of possible wind farms (for the case of 3,000 MW of installed capacity in Victoria). Summer is the season with the highest average daily output, although spring also shows significant average output. Summer is also the season with the highest average daily variability, with mid to late afternoon output being approximately double the output in the early morning. Autumn is the season with the lowest maximum daily output, with winter experiencing the lowest daily variation. Figure 5: Daily average output of possible Victorian wind farms by season (3,000 MW installed capacity) Output (MW) Time of Day (Hour) Summer Autumn Winter Spring 12

13 The assessment of technical issues The objective of this assessment was to: identify transmission network technical issues associated with the connection of significant wind power generation; and determine the amount of wind power generation that can be accommodated. Detailed system studies assessed the technical impact of wind power generation penetration for demand levels of 4,000 MW, 7,000 MW and 12,000 MW 6. These studies concluded that there is no significant impact on inter-regional power transfer levels. Dynamic reactive plant is likely to be required to manage voltage fluctuations and quality of supply issues. Management of thermal loading and increased fault levels will also become an issue, however, feasible technical solutions can be found to resolve this. Funding arrangements for these solutions will be determined during the connection application process under the National Electricity Rules (the Rules). Table 3 summarises the expected wind power generation technical impacts and possible solutions. Table 3: Expected technical impacts from wind power generation Technical Impact Issues Possible Solutions Thermal loading Fault levels Voltage control Transfer limits Power quality A number of 220 kv and 66 kv lines between terminal stations are likely to overload. Increased wind power generation will increase fault levels. Increased wind generation is likely to result in voltage regulation and fluctuation issues. Significant impacts on existing power transfer limits were not observed. Harmonic distortion, flicker, voltage unbalance and other power quality issues can become issue. Feasible technical and/or operational solutions can be found. Feasible technical solutions can be found to remove fault level constraints. Static and dynamic reactive plant may be needed to manage voltage control and related issues. For thermal loading related issues, feasible technical and/or operational solutions can be found. Feasible technical solutions can be found (see Table 4-12 for more information). Since wind power generation is subject to significant variability over time, an analysis of system security was undertaken over the following time frames: short-term security (up to 5 minutes); medium-term security (up to 30 minutes); and long-term security (over periods of 1-2 hours, 2-4 hours and 4-6 hours). For 4,000 MW of installed capacity (the maximum installed capacity examined by this study), the average estimated power output variability in the: 6 The medium growth scenario 10% POE summer maximum demand forecast for 2016/17 is 11,794 MW source VENCorp 2007 Electricity Annual Planning Report. 13

14 short term is estimated at ±163 MW - on-line thermal and hydroelectric plant can respond to this level of variation, which is within the regulation frequency control ancillary service (FCAS) margin and will not detrimentally impact system operation; medium term (less than 30 minutes) is estimated at ±338 MW - this variability is slightly larger than the regulation FCAS, however, 30 minutes is considered to be long enough for ancillary services to be procured from the market to deal with any excess variability; and long term (6 hours) is estimated at ±940 MW - this variability can be dealt with through the use of ancillary services from the market and/or by switching fast start generation on and off. Given that this variability is based on averages, studies were carried out to develop a variability profile, with the variability defined in terms of the probability of occurrence (see Table 2). At a 90% probability of occurrence (a level of variability that would not be exceeded in 90% of cases, representing a reasonable worst case scenario), it was found that over a: 30-minute period the variability was 20% (±836 MW); and 6-hour period the variability was 53% (±2,195 MW). Scheduling other generation in the NEM to manage the variability over a 6-hour period may become a significant issue. THE AMOUNT OF WIND FARM GENERATION THAT CAN BE ACCOMODATED VENCorp s 2007 Electricity Annual Planning Report forecasts 1,240 MW of installed wind power generation in Victoria by The studies indicate that wind power generation of approximately 3,000 MW installed capacity can be accommodated by the transmission network with appropriate technical solutions, such as improvement of thermal capacity of transmission lines, installation of additional static and dynamic reactive plant and mitigation works for fault level control and quality of supply issues. Increased wind power generation is likely to result in increased transmission network and power system technical issues, which will need to be dealt with progressively. Depending on where it is located, wind power generation of up to 4,000 MW installed capacity may be possible, but this would require further detailed analysis as operational experience is gained, and is subject to addressing any associated power system security and operational issues, which are not the subject of this report. VENCorp is not proposing to set an absolute limit on the connection of wind generation. 14

15 1 INTRODUCTION 1.1 Background VENCorp s Vision 2030 report [1] assumed that a significant amount of wind power generation would be one of the generation scenarios required to meet future load growth. It concluded that a separate technical study would be required to examine the implications of integrating wind power generation with the Victorian transmission network. This report forms that technical study. The Victorian Government s VRET (Victorian Renewable Energy Target) scheme 7 mandates that Victoria's consumption of electricity generated from renewable sources be increased to 10% by 2016 [2]. The new Federal Government has also now committed to expand the Mandatory Renewable Energy Target (MRET) scheme to 20% by Wind energy is a significant renewable energy source to meet these targets. To ensure that the Victorian electricity network can accommodate a significant amount of wind power generation, VENCorp studied the possible technical impact of incorporating wind plant into the Victorian shared transmission network and the National Electricity Market (NEM). Issues involving the physical connection of wind power generation to the transmission network, and the subsequent impact on market operations, were not investigated as part of this study. VENCorp published technical guidelines for connection to the Victorian shared transmission network in October 2006, which deals with the physical connection of new generation plant [3]. Impacts on market operations are dealt with by the National Electricity Market Management Company Limited (NEMMCO). 1.2 Objectives The key objectives of this study involved: reviewing international and national experiences of integrating wind power generation with the electricity network; investigating the technical impact of wind power generation developments on the Victorian shared transmission network and the NEM; and determining the amount of wind power generation that the Victorian shared transmission network can accommodate. In terms of the report s context, it is important to note that there are a number of issues affecting the viability of increased Victorian wind power generation, which can be grouped into the following categories: transmission network issues; system security and operational issues; market, regulatory and commercial issues; and 7 Victorian Renewable Energy Act Federal Labor s 20 percent by 2020 Renewable Energy Target, final.pdf 15

16 environmental issues. This report focuses solely on one of the key areas, being the Victorian transmission network issues resulting from increased wind power generation. The other three key areas clearly need to be considered when assessing the viability of wind generation, but are beyond the scope of this report. 1.3 Overview of the Report In this report: Chapter 2 provides a summary of the review of commonly used wind power generation technology, and the international and national experience with regards to integrating large amounts of wind power generation with the transmission network. Chapter 3 presents the findings from a preliminary investigation of the expected availability and variability of wind power generation in Victoria, for the purpose of assessing the technical impacts on the transmission network. Chapter 4 presents the methodology and findings from studies undertaken to identify the technical issues associated with the integration of increased wind power generation, and to determine the maximum amount of wind power generation that can be accommodated by the Victorian transmission network. Appendix A provides a list of abbreviations used in each chapter. Appendix B lists the references used throughout the document. Studies involving wind availability, wind variability, and the technical impacts of wind power generation, require a number of inputs, such as information about wind profiles, wind power generation outputs, wind turbine types and control mechanisms, and the transmission network. Except for the wind profiles, all other inputs are subject to future connection application proposals. As a result, a number of input assumptions are made for possible wind power generation connections, which are based on information available in the public domain and confidential information received by VENCorp. 16

17 2 A REVIEW OF WIND POWER INTEGRATION INTERNATIONALLY AND NATIONALLY 2.1 Introduction This chapter presents a review of the international and national experience of integrating wind power with the transmission network. This review was undertaken so as to gain a better understanding of the issues associated with greater wind power generation penetration. 2.2 Global Wind Power Penetration Global wind power generation capacity has increased significantly over the last few years, totalling approximately 74,000 MW by the end of 2006 [4]. Denmark, Spain and Germany have so far shown the greatest commitment to wind power generation with the largest addition to their electricity transmission networks. Figure 2-1 shows the global wind power generation penetration of the top 10 wind power generation countries and Australia. Wind power generation penetration in Denmark, Spain and Germany is approximately 24%, 18% and 18% of installed total generation capacity, respectively. In all other countries, wind power generation penetration is less than 5% of total installed generation capacity. Australia has a wind power generation installed capacity of 975 MW, approximately 1.3% of global wind power generation. A significant growth in wind power generation in Australia is proposed. Figure 2-2 shows the growth of wind power generation installed capacity in Australia. Table 2-1 lists the capacity of existing installed wind power generation in each State, along with the total proposed generating capacity [5]. Figure 2-1: Global wind power generation penetration (as at end of 2006) Rest of the World, 13.7% Australia, 1.3% France, 2.1% Portugal, 2.3% Germany, 27.7% UK, 2.6% Italy, 2.9% China, 3.5% Denmark, 4.2% India, 8.4% Spain, 15.6% US, 15.6% 17

18 Figure 2-2: Installed capacity growth of Australian wind power generation Installed wind generation capacity (MW) QLD NSW TAS VIC WA SA Total Year Table 2-1: Existing and proposed wind power generation installations in each State [5,6] State Existing [MW] Proposed [MW] Total [MW] New South Wales 17 1,088 1,105 Queensland South Australia ,710 2,257 Tasmania Victoria 134 2,385 2,519 Western Australia Total 976 5,997 6, Wind Power Generation Wind power generation is variable, its output mainly being influenced by wind speed. This variability occurs within all time frames: seconds, minutes, hours, days, months, seasons, and years. Understanding these variations and their predictability is essential for the efficient integration of wind power generation and the operation of the power system. A typical wind turbine will turn on when the wind speed is sufficient for the turbine to start generating (normally about 3 m/s). The wind turbine will generate the rated power at its rated wind speed and above until it reaches a cut-off wind speed, at which point the wind turbine turns off to prevent damage to its mechanical components. Figure 2-3 shows the typical electric power output curve of a wind turbine. 9 Includes the recently commissioned Lake Bonney Stage 2 wind farm in South Australia. 18

19 Figure 2-3: Typical generated power versus wind speed [7] Normalised power output Wind speed (m/s) 2.4 Wind Turbine Technology Wind turbine technology is rapidly evolving around various innovative concepts to meet the stringent transmission network connection requirements of network operators. The majority of wind turbines in use today can be classified as one of the following four types: 1. Type A - fixed speed wind turbines. This turbine is generally coupled to a squirrel cage induction generator via a gearbox. 2. Type B - limited variable speed wind turbine with variable rotor resistance. A wound rotor induction generator is used with a variable rotor resistor. 3. Type C - variable speed wind turbine with partial-scale frequency converter. This is commonly known as doubly fed induction generator (DFIG). 4. Type D - variable speed wind turbine with full-scale frequency converter. Either a synchronous or an induction generator is used. The Type C wind turbine is the most popular system in the wind power industry. This combines the advantage of the Type A and Type B systems, with advances in power electronics. Table 2-2 summarises the critical performance characteristics of the different wind turbine technologies commonly used in the global wind power industry. 19

20 Table Summary of performance characteristics of wind turbine technologies Wind turbine technology Type A: Type B 10 : Real power fluctuations Tends to create output power fluctuations during wind gusts. Reactive power capability Fault current contribution Inertial support and frequency control Fault ridethrough capability Provides inertial support. Not No. Poor. capable of Additional frequency control. Reactive power sources are needed. Yes. - - Type C: Type D: Less likely to propagate wind variations as power fluctuations. Yes. Capable of regulating reactive power in either direction. Yes for remote fault. Does not provide inertial support. Capable of frequency control support with additional control systems (subject to availability of wind). Good. Better. 2.5 Typical Wind Farm Layout Wind turbines are available in capacities of up to 5 MW with generated voltage levels of up to 1 kv. A typical wind farm layout involves groups of wind turbines being electrically connected to a substation via a feeder network. The substation acts as the interface between the wind power generation and the existing transmission network, with transformers and protection equipment used to ensure that the interface with the transmission network is adequate. The performance of the generation and the quality of the power can be monitored from a control kiosk that is normally located within the substation. Figure 2-4 shows a typical wind farm electrical topology. 10 This technology is dominated by one manufacturer and some of the details are not in the public domain. 20

21 Figure 2-4: Typical wind farm electrical topology 2.6 Global Wind Power Experience Variability of Wind Power and Wind Power Forecasting The load connected to an electricity network is dynamic, and generation varies to match the demand and losses. The system operator is responsible for maintaining a sufficient amount of generation reserve to ensure the reliable operation of the power system. The variability of wind power generation adds another dimension to reserve management by increasing the variability of generation required from other sources. The accuracy of wind power forecasts plays a vital role in the electricity market. Variation in system load is reasonably predictable, making demand forecast errors small when less wind power generation is installed. With more wind power generation, the demand forecast errors may become significant as, unlike demand, wind variations are less correlated with the time of day, and so may require special forecasting models to minimise errors. Danish wind energy views wind power forecasting as the most important element within the process of power system control, and uses a number of models that make use of meteorological predictions in order to predict wind speed and therefore the expected power output from wind power generation. Germany has good experience of wind power forecasting, with forecast errors of roughly 10% for day-ahead forecasts [8] System Security Denmark is often regarded as the model country in wind power generation, with both a high wind power generation penetration level and a high installed wind power capacity level. Disconnection of some of the wind farms to maintain system stability is required for interconnection constraints. A major system incident involving wind power generation occurred in Europe in November After tripping off many high voltage transmission lines in the Union for Coordination of Transmission of Electricity in Europe (UCTE) system, the transmission network was split into three islands, West, North 21

22 East and South Eastern. The event started in the north of Germany on 4 November 2006 and resulted in the loss of supply to more than 15 million European households across Europe. The final report issued on 30 November 2006 by the UCTE titled System Disturbance on 4 November 2006, highlighted issues related to wind power generation. It said: In the over-frequency area (North East), the lack of control over generation units contributed to the deterioration of system conditions in this area (long lasting over-frequency with severe overloading on high voltage transmission lines). Generally the operation of dispersed generation (mainly wind and combined-heat-and-power) during the disturbance complicated the process of re-establishing normal system conditions [10]. In Australia, the approach proposed to optimise the dispatch outcomes is referred to as the semidispatch of non-scheduled generation. Under this proposal, which is currently being considered by the Australian Energy Market Commission (AEMC), the wind farm operators are required to submit NEM bids for generation on a five-minute basis, and wind farm generation output can be limited via the central dispatch process to maintain power system security. When this arrangement has been put in place, the management of potential transmission network element overloading will become more manageable. When a powerful wind front passes across a region, the wind farm outputs can change by significant amounts over short time frames. Such steep wind fronts are not uncommon in South Australia. Under very high wind speeds, wind turbine protection systems can act to shut them down to avoid equipment damage. Under these circumstances it is possible to get very high outputs rapidly followed by very low outputs. The impact can be to require additional network control services in order to prevent transmission network overloading. The analysis and prediction of such events requires a review of historical information about wind farm generation patterns, and an extrapolation based on wind patterns in other locations and on the projected types of generation pattern. The solution proposed by the electricity industry in Australia involves implementing a sophisticated wind and wind power generation forecasting tool, and requiring future wind farms to be fitted with mechanisms to reduce wind power generation output prior to the onset of a wind front. Under very high temperature conditions, wind turbines may trip on over-temperature. In Australia, high demand conditions are typically associated with high temperatures, making the possible loss of wind power generation output at high temperatures a factor in the assessment of supply adequacy Thermal Constraints The location of wind power generation depends on the availability of wind, and is generally concentrated in a specific region in most countries. This geographic concentration causes transmission network thermal constraint issues. As a result, transmission capacity increases may be required to support high levels of wind power generation penetration. Thermal constraints are simple to predict prior to connecting a wind farm, and transmission network augmentation works can be implemented. The thermal constraint issues associated with wind farms are no different from any other type of generation connection, and the issues are not specific to wind generation Reactive Power and Voltage Control Traditionally, transmission network voltage control is undertaken by conventional generators connected to the transmission network. Capacitor banks, synchronous condensers and flexible AC transmission system (FACTS) devices are also used in certain cases, to assist with voltage regulation. Because of its unscheduled nature, wind power generation displaces conventional generation connected to the 22

23 transmission network. Therefore, as the penetration of wind power generation increases, traditional voltage control methods may be insufficient to maintain power system voltages. This is because wind power generation voltage control capability is not the same as traditional synchronous generation. System operators worldwide address this issue by requiring wind generators to provide network services, such as voltage control, as part of the transmission network code connection requirements. Although wind power generation technology has been evolving to provide this support, due to the fact that wind farms are often remotely located, transmission networks may need some additional reactive power sources closer to the load centres. When wind farms are connected to a distribution network, voltage regulation becomes a local issue. European countries with large numbers of wind turbines connected to their distribution networks have developed strategies to regulate reactive power and thus voltage at a distribution level. A similar approach could be applied in Victoria to manage reactive power and voltage control Frequency Control The concept of frequency control support from wind turbines has emerged with increasing wind power generation penetration levels. The pitch controlled variable speed wind turbine is inherently capable of the continuous control of output power [8]. However, the maximum power limit is governed by the availability of wind. The publication Primary Frequency Control Participation Provided by Doubly Fed Induction Wind Generators [11] provides a control system capable of continuous output regulation for generation of this type. When a power system needs frequency regulating services from wind turbines for under-frequency conditions, pitched control variable speed wind turbines can be partly loaded, based on the availability of wind. Additional electrical power is generated during a disturbance by pitching the wind turbine s blades to obtain the maximum power available from the wind. However, this operation depends heavily on the availability of wind Fault Ride-through and System Stability With the growth of wind power generation, it has become common for transmission network codes to demand the provision of greater fault ride-through capability from wind power generation for a severe voltage depression due to a system fault. The National Electricity Rules (the Rules) contain strict requirements in relation to ride-through capability, and therefore more modern technologies are likely to be used. Type A wind turbines are often incapable of fault ride-through, requiring a significant amount of additional dynamic reactive power (SVC or STATCOM) support to remain connected to the transmission network during a disturbance and so avoid stalling or tripping. Tripping off of wind turbines, due to system fault-induced voltage dips, may cause power imbalances between the load and the generation after clearing the fault, resulting in frequency deviations that may jeopardise system stability if sufficient numbers of units trip. However, wind turbine manufacturers have developed the Type C and Type D wind turbines to meet the ride-through requirements imposed by transmission network codes. Most old technology wind turbines connected to the European transmission network have been retrofitted to facilitate successful fault ride-through capability. System inertia plays a major role in damping oscillations after a disturbance. Typically, the combined inertia range of wind turbine generating units is from 2-6 MJ/MVA. This is comparable with conventional generating units. However, in the case of variable speed wind turbines (the most popular on the market), the kinetic energy stored in the rotating blades will not contribute to the inertia of the 23

24 transmission network, as the rotational speed is decoupled from the transmission network frequency by a power electronic converter [12]. As a result, and as wind turbines increasingly replace conventional generation (which has greater inertia), system inertia may deteriorate System Fault Level Transmission network fault levels tend to increase with the connection of more wind turbines. This phenomenon also occurs with the connection of other types of generation, and is generally resolved on a case-by-case basis Modelling, Simulation and Model Validation of Wind Turbines With the development of wind turbine technology, complex control techniques are applied to wind turbines to enhance their performance and to meet stringent transmission network code connection requirements. Certain aspects of a wind turbine s performance, such as fault ride-through capability, cannot be easily tested in the field unless the unit has experienced a real fault. Different wind turbine manufacturers have developed their own control models to represent wind turbine operations. Transmission network operators and service providers are particularly interested in dynamic models of wind turbines, because this enables simulation of the wind turbine s performance during disturbances. This requirement is included in transmission network codes, and the Rules. A computer simulation s validity depends on the accuracy of the models used. As models become more complex and different user-specific models become available, validation of wind turbine models becomes more important. Due to confidentiality constraints, however, complete details of these models may not be accessible. This leads to a risk of using un-validated or partial models that do not represent the true performance characteristics. For high-level studies, generic wind turbine models available in power system simulation software can be used to identify general behaviour, but accurate, validated models are extremely important for the assessment of specific connections. 2.7 NEM Wind Power Generation Operational Experiences Australia s wind power generation installed capacity is 975 MW, more than half of which is located in South Australia. As per the wind energy projects listed by the Australian Wind Energy Association, the proposed wind power generation capacity is expected to rise significantly over the next few years to a total of approximately 7,000 MW [5]. A number of investigations were undertaken into operational issues involving existing wind farms, and power system issues with increased levels of wind power generation. Some of the investigations relevant to large scale integration of wind power generation include the following: The Electricity Supply Industry Planning Council (ESIPC) of South Australia published a series of findings and recommendations in April 2005, in relation to the broader market and transmission network operation implications of significant wind power generation in South Australia [13]. The key conclusions include the following: The impact on power system security with 400 and 500 MW of wind power generation should be modest. The report states that there are risks to be managed at this level of wind power generation, which are only expected to occur on rare occasions. Wind power generation of 800 and 1,000 MW raises concerns, under the current arrangements, with growing impacts on power system reliability, security and price. 24

25 ESIPC does not recommend an absolute limit to wind power generation. The report states that the impact on the power system will result in progressively rising risks as penetration increases. A number of recommendations are made for the minimisation of any adverse impacts due to the integration of large-scale wind power generation. NEMMCO commissioned a study of the potential risks relating to high levels of wind power generation in South Australia in 2005 [14]. The report found that with high wind power generation penetration (1,200 MW), the Victoria-South Australia interconnector would not be sufficient to export power from South Australia to Victoria, and that Murraylink would have to be used to regulate power transfers. It was also noted that the interconnector s import capacity will rely on additional reactive power support near Adelaide. The impact of wind generation in South Australia will have a negligible effect on the transient stability of other interconnectors. In regards wind variability and wind power generation forecasting, the Federal Government has entered into an agreement with NEMMCO to manage the development and implementation of an Australian Wind Energy Forecasting System. NEMMCO is in the process of developing a suitable mechanism [15]. The usage and cost of frequency control ancillary services (FCAS) is likely to increase with increased intermittent generation, and the associated likelihood of an increase in the uncontrolled variation of generation levels [16]. NEMMCO and certain other network service providers (NSPs) have raised issues related to voltage control involving: flicker and other power quality issues; additional wear on voltage control equipment, such as transformer on-load tap changers and switched capacitors; and the possible requirement for additional network voltage control plants or network voltage control services. These are primarily local issues and have been effectively addressed in other countries. Additional reactive plant may be required, however, to control voltages at the transmission level. Fault ride-through capability in the event of transmission faults is an important requirement for all generation in the NEM, including wind farms. The variability in wind power generation output may need to be considered simultaneously with other credible contingencies when assessing system security. NEMMCO raises the concern about the adequacy of monitoring interconnector flows on a five-minute basis, as currently occurs. NEMMCO suggests that if sub-five-minute variations are significant, the existing process may need to be reviewed and modified. The Tasmanian power system incident on 25 November 2006, where the Woolnorth wind farm tripped off during a fault that was distant from the wind farm itself, was raised as an example of the importance of stringent fault ride-through requirements. The output of wind power generation in some locations is limited, so as to operate transmission lines within their capability. 25

26 Some wind farm proponents are reluctant to release accurate wind farm models. Simplified models have not always proven to be satisfactory and, in some cases, have indicated stability issues that are not evident with more accurate models. The complexity of the models has also made it more difficult to perform due diligence on network limit equations involving wind farms. In a number of cases, special control schemes, such as special voltage control schemes, run back schemes and generator dispatch limiting schemes, have been implemented. 2.8 Summary The impact of wind power generation on the transmission network depends on the location of wind power generation relative to the load, and the correlation between wind power production and load consumption. Major concerns relating to wind power generation integration into the transmission network include the variability of wind power generation and wind power generation forecasting, fault ride-through capabilities, and the quality of the power supply. Since many wind power generators are connected to the extremities of the network and are remote from the major load centres, thermal constraints and voltage control are also significant concerns. Wind turbine technologies, wind forecasting methodologies, and transmission network technologies, however, are rapidly evolving around various innovative concepts, enabling the integration of high levels of wind power generation. 26

27 3 WIND POWER AVAILABILITY AND VARIABILITY 3.1 Introduction This chapter presents the findings from a preliminary investigation of the expected capacity factors 11 and output variability of proposed Victorian wind farms. The intention of this investigation is to gain a greater understanding of the expected availability and variability of wind power, for the purpose of assessing the technical impacts on the transmission system. A more detailed analysis of wind farm availability and variability would have required wind data from the identified wind farms over a longer period of time and the provision of more appropriate wind turbine characteristics for the purposes of modelling. Section 3.7 provides a summary of the assumptions made in the conduct of this preliminary investigation. 3.2 Possible Wind Farms Locations Possible wind farm locations [5] are scattered around Victoria, with the majority in the west and the south west (as shown Figure 3-1). For the purpose of this investigation, possible wind farms, which are in relatively close proximity, have been grouped into regions. Possible wind farm locations and their regional groupings are listed in Table 3-1 and shown in Figure 3-2. Table 3-1: Regional groupings of existing and possible wind farms Region Existing/possible wind farm Indicative total installed capacity Coastal South West (CSW) Portland Nirranda Macarthur 1,045 MW Ryan Corner / Hawkesdale Codrington / Yambuk (existing) Inland South West (ISW) TGTS Mortlake Berrybank Close to Ballarat Winchelsea 1,010 MW Shelford (near Winchelsea) Mt Gellibrand Colac Central (C) Mt Mercer Werribee Baynton Around 50 km from South Morang along South 910 MW Morang-Dederang 330 kv line Around 100 km from South Morang along South Morang-Dederang 330 kv line North West (NW) Concongella Hill 920 MW Crowlands / Glen Lofty Challicum Hills (existing) 11 Capacity factor is one element used to measure the productivity of a wind turbine or any other power production facility. It compares the plant's actual production over a given period of time with the amount of power the plant would have produced if it had run at full capacity for the same amount of time. 27

28 Region Existing/possible wind farm Indicative total installed capacity South East (SE) Mount Misery Waubra BETS 66 kv Wonthaggi / Toora (existing) Dollar 115 MW 28

29 Figure 3-1: Map of possible wind farm locations considered in the study 13 29

30 Figure 3-2: Map of possible regions considered in the study Regional Wind Speed Patterns In order to analyse load duration and wind power generation output variability, a number of assumptions have been used. The significant assumptions are summarised towards the end of this chapter (see Section 3.7 for more information). From the available historical wind data, Figure 3-3 shows the indicative spatial distribution of diurnal wind speed patterns for the selected regions in Victoria. Since data was not available at each of the proposed wind farm locations, normalised diurnal 13 wind speed patterns have been created to show the daily average variation of wind speed around the mean wind speed, rather than looking at absolute values. 12 Source: Victorian wind Atlas (Sustainable Energy Authority, Vic), A diurnal wind speed pattern refers to the variation in wind speed that occurs with the transition from day to night. 30