Electric Industry Restructuring: Opportunities and Risks for West Virginia Interim Report No. 5: Transmission Enhancement and Expansion Submitted to: Director of Operations, Governor’s Office 1900 Kanawha Boulevard East Charleston, WV 25305 Submitted by: West Virginia University Electric Industry Restructuring Research Group P.O. Box 6064 Morgantown, WV 26506 January, 1998 Table of Contents 5.0 Transmission Enhancement and Expansion 5.1 Overview of Transmission of Electric Power 5.2 Transmission Network in West Virginia 5.3 Transmission Network 5.4 Transmission Constraints 5.5 Siting of New Transmission Lines 5.6 Conclusion Appendix A: The Load Flow Problem and Technical Aspects of Power Electronics List of Tables Table 5.1 Typical Overhead Transmission Line Parameters Table 5.2 Surge Impedence Loading (SIL) and Typical Thermal Ratoing for Voltage Levels 230 kV to 1100 kV List of Figures Figure 5.1 Electricity Generation and Transmission for the West Virginia Region Figure 5.2 Major Interconnections in the North American Electric Reliability Council Figure 5.3 Regional Councils of the North American electric Reliability council Figure 5.4 Average Variable Cost (Not available in html format) Figure 5.5 A Single-Line Diagram of the Static VAR Compensator (Appendix A) Figure 5.6 Single-Line Diagram of a Series-Controlled Capacitor (Appendix A) Figure 5.7 One-Line Diagram of a Statis Condensor (STATCON) (Appendix A) Figure 5.8 One-LineDiagram of a Unified Power Flow Controller (Appendix A) 5.0 Transmission Enhancement and Expansion West Virginia is a major producer of electric power in the Eastern United States and is physically located near the many metropolitan areas of high electricity demand in New York, Pennsylvania, Delaware, Maryland, and Virginia. Inexpensive energy made from abundant coal and the nearness of electricity markets has made West Virginia a major exporter of electric power. Currently, West Virginia exports about 70 percent of the total amount generated each year to neighboring states. Although electricity could be produced in West Virginia inexpensively relative to adjoining eastern neighboring states, transmission capacity must be available to export power from West Virginia to the east. The current pattern of electricity generation and transmission facilities is shown on Figure 5.1. This map shows there are a number of west-east high voltage lines both in the north and south of West Virginia but no major west-east transmission lines in the central part of the state. Figure 5.1 Electricity Generation and Transmission Facilities —West Virginia Region This pattern of generation, market location, and generation capacity poses both opportunities and obstacles to the further development of the West Virginia electricity industry. The major obstacle is shortage of transmission capacity to significantly increase the west to east flow of power from West Virginia to the eastern markets. This shortage is caused by the amount of power available in West Virginia as well as other more western states that desire to transmit power to the east, relative to the existing transmission capacity. There are a number of possible ways to provide eastern markets with increased electric power and capacity. These include, constructing new transmission lines, increasing capacity of existing lines by a number of methods, increasing generation near the eastern markets, and reducing cyclical peak load requirements as well as overall electricity usage in the eastern markets by efficient electricity pricing. As a consequence, there a number of competing methodologies that must be investigated, each with a specific set of advantages, disadvantages, and constraints. It is improper to view electricity transmission as the only method to increase electric power or capacity to eastern markets. All alternatives must be considered as part of an integrated operation. However, from a West Virginia perspectives, increasing exports of electricity to eastern markets depend on four factors; the relative cost of generating power in West Virginia, the pricing of electricity transmission, the physical capacity of transmission facilities, and the cost of constructing new transmission capacity. Relative costs of production and the merits of load centered generation are discussed in other sections of this report. The cost of constructing new transmission facilities is a site specific problem not easily addressed in a general discussion. The generalized considerations in transmission line siting is discussed in this section. The pricing of electricity transmission is also discussed elsewhere in this report. While recognizing that other alternatives must be considered, this section focuses on methods to enhance existing transmission capacity of transmission lines through the use of power electronics. Power electronics involves the process of controlling the flow of electricity through existing transmission lines by the use of computerized controls and specialized hardware, such as semiconductor devices, capacitors and inductors. Electronic control is much faster than the older mechanical methods and thereby reduces stability and voltage problems. The reduction of these problems are what are responsible for increasing the capacity of existing transmission lines. The use of power electronics to increase transmission capacity is important because it can be installed in a short period of time at a low cost relative to other options. This is of particular importance to West Virginia because power electronics offer the quickest way to increase the physical capability of the existing transmission network. However, the cost of producing power must be competitive in order for the flow to be dispatched from West Virginia generation sources. 5.1 Overview of Transmission of Electrical Power Before addressing power electronics, it is necessary to discuss some of the more common characteristics of electric power transmission. These characteristics are varied, and influence both the amount and quality of service provided as well as the cost of transmission. The amount of power that can be transmitted depends on the parameters of the line (wire size and distance between wires), the line voltage, the thermal line limit, stability limits, and the voltage change across the line. The larger the diameter of the wire, the greater the transmission capacity of the wire capacity and the lower the transmission losses, all else being equal. The higher the line voltage, the greater the transmission capacity. This is particularly important, because the transmission capacity increases in proportion to the square of the transmission voltage. This means small increases in voltage result in large increases in transmission capacity. Line losses of electricity cause increases in temperature of the line, much as what happens in an electric coil heater. This increase in temperature causes the line to lengthen and sag, which may cause the line to touch the ground and cause a power outage. Line voltage stability has to do with the relationship between voltage and the amount of power transmitted. As the amount of power increases, there comes a point when voltage suddenly falls sufficiently to shut down the system. This, of course, needs to be avoided. The voltage change across the line must be small enough so that the end users have the proper voltage to operate their equipment. In addition to the amortized cost of constructing electric transmission lines, there is an important operating cost in the amount of power lost through transmission. The amount of this loss depends on the length of the transmission, the voltage at which transmission occurs, the size of the conductor, and the manner in which electricity is transmitted. Common losses range between two to five percent of power being transmitted. This means that electric bill of consumers must be raised in proportion to line losses if all profit margins are to be maintained. Most transmission lines transmit power in the form of alternating current (AC), which is the form in which electricity is generally generated. Another way to increase transmission capacity of a single line is to transmit the power in the form of direct current (DC). DC transmission has a number of advantages over AC such as increased control of power, lower line losses, no stability problems, and smaller transmission right of way requirement. DC transmission allows the direct transmission from a single generator to a single consumption point without the involvement of transmission network constraints. In the context of a deregulated environment, this is important because it allows greater control over the flow of power. However, the DC transmission line is more expensive than an AC line of comparable length. This is because the DC line requires expensive converters at each end of the line to convert the generated power, which is AC, into DC power, and then to convert the DC power back to AC, which is required by customers. 5.2 Transmission Network in West Virginia Transmission network at 138 thousand volts (kV) and above in West Virginia is owned and operated by Allegheny Power(AP) and American Electric Power(AEP) companies. AP supplies electric power in the northern and eastern parts of the state. AEP serves electric customers in the western and southern parts of the state. As shown in Figure 5.1 a 500 kV transmission network of AP interconnects Fort Martin, Harrison, and Pleasant power plants of AP, Mitchell and Kammer power plants of AEP, and Mt. Storm power plant of Virginia Power to the load centers in its service areas. This extra high voltage (EHV) transmission network (230 kV and above) is connected to 138 kV network of AP and EHV transmission networks of eight neighboring utilities. Albright, Rivesville, Lake Lynn, and Willow Island power plants owned by AP and three other non utility generating power plants are connected to 138 kV transmission network. American Electric Power has 138 kV, 345 kV, and 765 kV transmission networks in West Virginia. A 765 kV transmission network connects Kammer, Mitchell, Mountaineer, Philip Sporn, and John Amos power plants in West Virginia to load centers and other power plants in the neighboring states. A 345 kV transmission network connects Mountaineer, John Amos, and Kanawha power plants in West Virginia. The 765 kV network overlays 345 kV and 138 kV transmission networks. The 765 kV and 345 kV transmission lines carry power to AEP’s service areas in West Virginia and Virginia. The EHV transmission network of AEP connects to twenty other neighboring utilities. The extra high voltage transmission lines in West Virginia were built to supply low cost and reliable electricity to customers in service areas of Allegheny Power and American Electric Power. The transmission network should be able to supply power even after the outage of one or two facilities. Although the EHV transmission lines were not built to export power, considerable amount of economy power flows from west to east through the transmission network. This exchange of power reduces the cost of electricity for customers in the high cost utilities areas and improves the plant utilization factors of power exporting utilities. The Federal Energy Regulatory Commission’s order 888 requires utilities to provide nondiscriminatory access to their transmission system by third party. A strong EHV transmission network is essential for creating competition among the electricity suppliers and buyers. AEP plans to build Wyoming-Cloverdale 765 kV line to improve the reliability of its transmission network supplying power to southern West Virginia and southwestern Virginia. 5.3 Transmission Network The electric utilities are connected to the neighboring utilities to provide economic and reliability benefits. The interconnection with other utilities reduces the reserve margin required and improves the capacity factor of the existing power plants .Transmission lines allow the delivery of electric energy from power plants to load centers in a reliable and cost effective manner. They allow demand at one point to be met by generating sources at other points in the interconnection. One disadvantage of interconnection is that a fault at one location can spread to other utilities and disrupt power to a large number of customers. A transmission line fault on July 2, 1996 in the Western Systems Coordinating Council (WSCC) interrupted electric service to about 1.5 to 2 million customers. The North American Electric Reliability Council (NERC) has four major interconnected areas: the Eastern Interconnection, the Western Interconnection, the Texas Interconnection, and the Quebec Interconnection. The individual utilities are operated in synchronism within each interconnection and maintain a frequency of 60 hertz. Figure 5.2 shows the four interconnections and Figure 5.3 shows the regional reliability councils of NERC. Figure 5.2: Major Interconnections of the North American Electric Reliability Council Figure 5.3: Regional Councils of the North American Electric Reliability Council ECAR—East Central Area Reliability Coordination Agreement NPCC—North East Power Coordinating Council ERCOT—Electric Reliability Council of Texas SERC—Southeastern Electric Relaibility Council MAAC—Mid-Atlantic Area Council SPP—Southwest Power Pool MAIN—Mid-America Interconnected Network WSCC—West Systems Coordinating Council MAPP—Mid-Continent Area Power Pool Affiliate ASCC—Alaska Systems Coordinating Council The voltage of transmission lines has increased over the years due to the need to transfer larger and larger amounts of power over longer and longer distances. Higher voltages reduce line losses and increase the efficiency of electricity transmission. Typical nominal voltages of transmission lines are 69 thousand volts (kV), 138 kV, 230 kV, 345 kV, 500 kV, and 765 kV. The first 765 kilovolt line was built by AEP and was activated in the 1970s. Most of the transmission network in the United States consists of alternating current (AC) transmission lines. The power through an AC line depends on the series impedance, shunt admittance , and voltages at the sending end and the receiving end of the line. The series impedance consists of resistance and inductance of transmission line and the shunt admittance usually consists of capacitance of the line. However, there are more than three thousand circuit miles of direct current (DC) transmission lines in the United States, which are generally more economical if the length of the transmission line is greater than approximately 375 miles. Compared to AC transmission lines, power flow through the DC lines can be better controlled. Recent advances in power electronic devices are increasing the ability of engineers to control the flow of power through AC lines, however. The use of power electronics will reduce the parallel path flow problem, allowing controlled flow of power through the desired routes. New transmission lines will be needed when the capacity of the existing lines has reached the thermal limit, but power electronics may be used to postpone saturation. However, it is inevitable that demand growth will lead to the need for increased capacity available only by constructing more lines. During the 1970s a number of electric power companies were buying power from other electric power companies in order to reduce the cost of electricity to their customers. The availability of adequate transmission capacity was crucial for import and export of electric power between utilities. Figure 5.4 shows the average variable cost of electricity produced by utility-owned power plants in West Virginia and the neighboring states. Large amounts of power began flowing from west to east through transmission system of Allegheny Power due to the difference in price of electricity and the availability of excess generation capacity in Midwestern utilities. 5.4 Transmission Constraints There are a limited number of extra high voltage (EHV) transmission paths for flow of power from west to east. Power transfers from ECAR to MAAC are limited due to voltage constraints in the transmission system of Allegheny power. Allegheny Power and PJM have installed capacitors at some substations to reduce voltage constraints. Utilities in ECAR and MAAC have developed a Reliability Coordination Plan (RCP) under which power transfers from ECAR to Virginia Power and ECAR to PJM are frozen or curtailed based on postcontingency voltage versus power flow curves. The use of power flow controllers in Allegheny Power and PJM, siting of new generating sources in MAAC, and building of new transmission lines will increase power flow from ECAR to MAAC. 5.5 Siting of New Transmission Lines The addition of new transmission lines will reduce the transmission constraints and increase the competition among the suppliers of the electricity. In the past, the new transmission lines were built to supply reliable electricity for the native load of a power company. In the deregulated environment, the load demand can be met by a number of suppliers in the generation market. The state should encourage siting of new transmission lines based on the following criteria. 1.Right-of-ways which have minimal environmental impact. 2. Improvement of reliability of the power system. 3 Increase in competition in the generation market to lower the cost of electricity. 4. Opportunity for economic development through siting of new power plants. Figure 5.1 shows transmission lines in West Virginia and the neighboring states. The 500 kV transmission lines in northern part of the state are owned and operated by Allegheny Power. The 765 kV lines in western and southern parts of the state are owned and operated by American Electric Power. For example, in addition to the proposed 765 kV line, a centrally located 500 kV line through the coal producing counties of the state may reduce transmission constraints and provide an opportunity for economic development in the future. 5.6 The Load Flow Problem and Power Electronics A given set of loads can be supplied by a given set of generators in a number of ways. A load flow analysis is used for planning and to determine the transmission constraints in the existing networks. The analyst builds a mathematical model of the interconnected network to describe the relationship between powers and the voltages. He then solves equations numerically, subject to some power and voltage constraints, and calculates power flow in all the transmission lines. The load flow solution gives information about magnitude and phase angle of voltage at each bus and real and reactive power flowing in each line for a given generation, load and transmission network data. The maximum amount of power that can be transmitted through a transmission line depends on voltages at both ends of the line and the line parameters. The four parameters of a transmission line are resistance, inductance, capacitance, and conductance. The leakage current over the insulators of the overhead line and through the insulation of a cable is determined by the conductance. Since the leakage current is very small, the conductance parameter of the transmission line is neglected. Transmission lines at voltages greater than 230 kV have more than one conductor per phase. Table 5.1 shows typical parameters of transmission lines from 230 kV to 1100 kV. Table 5.1 Typical overhead transmission line parameters. Nominal Voltage R (W /km) xL = w L (W /km) bc = w C 230 kV 345 kV 500 kV 765 kV 1100 kV 0.050 0.037 0.028 0.012 0.005 0.488 0.367 0.325 0.329 0.292 3.371 4.518 5.200 4.978 5.544 (m S/km) Zc (m ) 380 285 250 257 230 SIL(MW) 140 420 1000 2280 5260 The line losses decease at higher voltages due to decrease in resistance of the line. The surge impedance(Zc ) of the line varies from 380 ohms to 230 ohms. Surge impedance loading(SIL) of the lines varies from 140 MW to 5260 MW. The power transfer capability of a transmission line is usually expressed in terms of SIL. For example, a 200 miles line can be loaded to 1.25 SIL. The power transfer capability of the transmission line can be increased by the use of power electronics if the line has not been loaded to the thermal limit. Most of the EHV transmission lines are not loaded to their thermal limit due to voltage and stability considerations. Table 5.2 shows the typical surge impedance loading and the thermal rating of the EHV lines. Table 5.2: Surge impedance loading (SIL) and typical thermal rating for voltage levels 230 kV to 1100 kV. Typical Thermal Voltage (kV) SIL (MW) 230 150 400 345 400 1200 500 900 2600 765 2200 5400 1100 5200 24000 Rating (MW) The power through a simple transmission line model depends on voltage magnitude, transmission line reactance, and phase angle difference between the sending end and the receiving end voltages. Power electronic switching devices such as thyristors and gate turnoff thyristors (GTO) can be used to control voltage, line reactance, and phase angle. A Static Var Compensator (SVC) can increase power transmission significantly by maintaining constant voltage during steady state and dynamic operating conditions. A SVC consists of switchable capacitors and thyristor controlled reactors. A capacitor supplies reactive power and raises the voltage of the transmission line. An inductor absorbs reactive power and lowers the voltage of the transmission line. Figure 5.5 shows a single line diagram of a Static Var Compensator. A number of utilities have applied SVC to enhance their transmission systems. A SVC can modulate reactive power to improve the dynamic and transient stability of power system. Power transfer through a transmission line can be increased by decreasing reactance of the line. The reactance of the line can be decreased by adding capacitors in series with the line. Series compensation has mostly been used in Western US power system due to long transmission lines. The addition of series capacitors may cause subsynchronous resonance problem and damage the shaft of a turbine generator. However, a thyristor based controller has been developed to damp the subsynchronous oscillations. A study in 1984 by American Electric Power (AEP) showed that a series compensation on Amos-Funk 345 kV line can increase the loadibility limit from 880 MW to 1485 MW. The emergency thermal capability of this line is 1485 MVA. A variable series compensation scheme was implemented on the Kanawha River- Matt Funk 345 kV line. The line length is only 174 km. The series compensation consists of 10%, 20%, and 30% compensation segments. One phase of the 10% segment has a thyristor control module (TCM). TCM module provides capability for rapid insertion of series compensation and change of power flow through this line. General Electric demonstrated the operation of a three-phase thyristor controlled series capacitor in a 500 kV line at Slatt Substation of Bonneville Power Administration (BPA) system. It consists of six 1.33 ohms modules connected in series. Each module consists of a series capacitor group in parallel with a series connected reactor and a thyristor unit. This installation has a continuous current rating of 2900 amperes and a short-term overload rating of 4350 amperes. Figure 5.6 shows a single line diagram of a series controlled capacitor. The use of thyristor controlled series capacitor will allow a control over the level of series compensation and can improve the damping of the power system. Figure 5.6: Not available in HTML format. Figure 5.7 shows a one-line diagram of Static Condenser or STATCON. An inverter generates three-phase voltages in phase with the ac system voltages. The current lags if the inverter voltage is less than the system voltage and leads if the inverter voltage is greater than the system voltage. The reactive power delivered by STATCON is a function of voltage and current. This device can deliver reactive power under reduced voltage condition and has a better performance than a Static Var Compensator. A 100 MVA SATCON has been installed at Sullivan substation in TVA power system to demonstrate its operation. (STATCON) Figure 5.7: One-line Diagram of Static Condenser Figure 5.8 shows a one-line diagram of a unified power flow controller. This controller allows the control of real and reactive power through the transmission line. The unified power flow controller allows the injection of a variable voltage magnitude and phase angle in series with the phase voltage. American Electric Power is planning to install a 320 MVA unified power flow controller at its Inez Station in eastern Kentucky to fully utilize the high capacity of this new 138 kV line [14]. In the first phase of the project a ±160 MVA shunt voltage source inverter is installed at the Inez substation. This controller will provide reactive power and dynamic voltage control in the Inez area. Other power electronics devices are Thyristor Controlled Phase Angle Regulator and Thyristor Controlled Dynamic Brake. The thyristor controlled dynamic brake can damp subsynchronous oscillations if the power transfer capability of the transmission network is limited due to concern of subsynchronous resonance. Mechanically switched devices are less expansive and slower in response as compared to electronically switched devices. A combination of mechanical and electronic devices may provide least cost solution to give the desired steady state and dynamic response for the transmission system. Controller Figure 5.8: One-line Diagram of a Unified Power Flow 5.7 Conclusions A number of alternatives are available to increase the supply of electric power to customers in the states east of West Virginia. No one alternative should be considered without considering all other alternatives. The use of power electronic devices can increase the flow of power through the existing transmission lines. However, new transmission lines will become necessary when the existing transmission lines have reached their thermal limits. The addition of new lines will reduce the transmission constraints and increase competition in the generation market. Considering the number and types of options available in increasing and controlling transmitted electrical power, it is likely that new regulatory and independent system operator (ISO) criteria are needed for siting of transmission lines under a deregulated electric generation industry. West Virginia should play an active and well informed role in these areas during the transition phase to deregulation in order to strategically position itself economically for the future.