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.