Increased Power Flow Guidebook

(1)

Electric Power Research Institute

3420 Hillview Avenue, Palo Alto, California 94304-1338 • PO Box 10412, Palo Alto, California 94303-0813 USA 800.313.3774 • 650.855.2121 • askepri@epri.com • www.epri.com

Increased Power Flow Guidebook

Increasing Power Flow in Transmission and Substation Circuits

copyright protection and does not require any license agreement from EPRI. This notice supersedes the export control restrictions and any proprietary licensed material notices embedded in the document prior to publication.

(2)

(3)

EPRI Project Manager

R. Adapa

ELECTRIC POWER RESEARCH INSTITUTE

3420 Hillview Avenue, Palo Alto, California 94304-1395 ▪ PO Box 10412, Palo Alto, California 94303-0813 ▪ USA 800.313.3774 ▪ 650.855.2121 ▪ askepri@epri.com ▪ www.epri.com

Increasing Power Flow in Transmission and

Substation Circuits

1010627

(4)

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

EPRI

Power Delivery Consultants, Inc.

NOTE

For further information about EPRI, call the EPRI Customer Assistance Center at (800) 313-3774 or email askepri@epri.com

Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc.

(5)

iii

CITATIONS

This report was prepared by

EPRI

115 East New Lenox Road

Lenox, MA 01240

Principal Investigator

B. Clairmont

Power Delivery Consultants Inc.

28 Lundy Lane, Suite 102

Ballston Lake, NY 12019

Principal Investigators

D. A. Douglass

E. C. Bascom, III

T. C. Raymond

J. Stewart

59 St. Stephens Lane N

Scotia, NY 12302

This report describes research sponsored by the Electric Power Research Institute (EPRI).

The report is a corporate document that should be cited in the literature in the following manner:

Increased Power Flow Guidebook: Increasing Power Flow on Transmission and Substation

Circuits. EPRI, Palo Alto, CA: 2005. 1010627.

(6)

(7)

v

The Increased Power Flow (IPF) Guidebook is a state-of-the-art and “best practices” guidebook

on increasing power flow capacities of existing overhead transmission lines, underground cables,

power transformers, and substation equipment without compromising safety and reliability. The

Guidebook discusses power system concerns and limiting conditions to increasing capacity,

reviews available technology options and methods, illustrates alternatives with case studies, and

analyzes costs and benefits of different approaches.

Results & Findings

The IPF Guidebook clearly identifies those cases where increasing power flow might be an

alternative to upgrading the grid with major investment. The document reviews both established

technologies and new developments in technologies with the potential to increase power flow—

and addresses how to apply them for lines, cables, and substations. Because the guide compares

the economic benefits of each available technology, it will assist utilities in making informed

decisions in terms of what options for IPF are available and which options are most economical

for application at their utility sites. By implementing one or more of the IPF technologies,

utilities can obtain increased asset utilization with minimal cost. For example, if a utility decides

to implement one of the IPF technologies, such as Dynamic Thermal Circuit Rating (DTCR)

technology, that implementation will allow increased power flows on the order of 15-20% over

the existing static ratings and, thus, increase utility revenue.

Challenges & Objective(s)

Motivations for increasing power flow limits on existing transmission facilities (rather than

constructing new facilities) are economic, environmental, and practical. Due to limited incentives

for new construction and time delays that may result from public opposition to new power

facilities, utilities around the world are being forced to find new ways of relieving modest

constraints or increasing power flow through existing transmission corridors with minimal

investment. This Guidebook will be an excellent reference document for transmission and

substation engineers since it provides all possible IPF options in one place for ease of use.

Applications, Values & Use

Training materials will be developed with the IPF Guidebook, including hands-on workshops at

EPRI's full-scale laboratories. In addition, it is anticipated that, in the coming years, technical

reports will be produced annually on new and updated aspects of IPF as well as new material on

costing, economics, power storage, voltage upgrading, and case studies. This information will be

incorporated into the Guidebook during subsequent work.

(8)

transformation that is redefining the use of existing power equipment in the electric transmission

network. Under these circumstances, utilities are forced to find new ways of increasing power

flow through the existing transmission corridors with minimal investments. EPRI’s Increased

Transmission Capacity Program directly responds to the needs of owners and operators of the

transmission grid to get the most out of existing equipment in today’s competitive electricity

business while increasing the availability and reliability of transmission and substation

equipment. This Program helps customers accomplish these goals strategically, without

jeopardizing reliability and driving up costs. A number of projects have been undertaken by

EPRI in this Program under the Project Set 38A—Increase Power Flow Capability in the

Transmission Systems. One major effort under Project Set 38A is the IPF Guidebook, the only

available utility compendium of “best practices” for increasing power flow in transmission

circuits. Other major EPRI developments include DTCR (Dynamic Thermal Circuit Ratings)

software to calculate dynamic ratings of transmission circuits and Video Sagometer to measure

sag of transmission lines.

Approach

The Guidebook was developed by industry experts and draws on a combination of technology,

documented case studies, and associated engineering and safety guidelines.

Keywords

Overhead transmission

Substations

Transmission capacity

Underground transmission

(9)

vii

The Increased Power Flow (IPF) Guidebook documents the state-of-science for increasing

power flow capacities of existing overhead transmission lines, cables, and substation equipment.

The Guidebook provides an overview of the electrical, mechanical, thermal, and system concerns

that are important to increased power flow, presents all possible IPF options, uses case studies to

illustrate the options, and compares their potential economic benefits. The Guidebook also

provides an overview of dynamic thermal rating methods and summarizes other developments in

hardware and software that are instrumental for IPF.

The IPF Guidebook provides utilities with the only available compendium of “best practices” for

increasing power flow. The Guidebook will assist utilities in making informed decisions in terms

of what options for IPF are available and which options are most economical for application at

their utility sites.

(10)

(11)

Increased Power Flow

Fundamentals and Principles

1.1 INTRODUCTION

The purpose of this guidebook is to provide technical information and explain concepts that may aid power transmission company technical personnel in finding economic, tech-nically sound ways to increase the power flow capacity of existing circuits without com-promising safety or reliability.

The motivations for increasing the power flow limits on existing transmission facilities (rather than constructing new facilities) are economic, environmental, and practical. The methods discussed are generally modest in cost—ranging from virtually free to about 30% of the cost of equipment replacement. The corresponding increase in equipment rat-ing is similarly modest, usually between 5% and 30% (with the exception of overhead lines where reconductoring may yield an increase of over 100%). The methods are practi-cal since the environmental and/or visual impact is normally low, regulatory approval and public hearings may not be needed, and extended power outages are often avoided. Given the extended time delays that may result from public opposition to the construc-tion of new power transmission facilities or even to any visible, physical modificaconstruc-tion of existing facilities, the use of increased power flow (IPF) methods may offer the only prac-tical solution to relieving modest constraints on power flow.

Determining the degree to which maximum power flow constraints can be eased on existing power equipment (overhead lines, power transformers, etc.), power circuits (multiple power equipment elements in series), and power system interfaces (multiple “parallel” power cir-cuits connecting power system regions) can be quite complex. For example, consider the fol-lowing:

•

For an overhead line, any increase in power flow capacity is dependent on its length, the original design assumptions, present-day environmental concerns, the condition of its existing structures, and the type of conductors originally selected. Depending on these multiple factors and which of the IPF methods suggested in Chapter 2 is applied, the resulting increase in the line’s thermal rating could be as little as 5% or as much as 100%.

•

But overhead lines are only part of the transmission path (circuit). The lines are termi-nated at substations by air disconnects, circuit breakers, line traps, etc. The power flow through all of the circuit elements must be limited to avoid damaging the line or the termi-nating equipment, and the maximum allowable power flow over this circuit may be limited by any one of the circuit elements.

•

Finally, when seen as part of a power system interface, any increase in maximum allow-able power flow through any component circuit or circuit element does not necessarily yield a higher rating for the complex interface.

(16)

In general, it may be stated that maximum power flow on the transmission system is a function of the overall system topology (transmission lines, transformers, generation, series and shunt compensation, and load), and that many non-thermal system considerations can also limit the maximum power flow on a specific transmission circuit. Therefore, transmission circuit ratings are often developed on a system basis, rather than on an individual line basis. The overall limit may be between operating areas irre-spective of ownership or individual lines, and may change during a day based on system conditions.

Chapter 1 provides an overview of the electrical, mechanical, thermal, and system concerns that are important to increased power flow. The chapter includes three sections:

•

Section 1.2, Power System Issues, presents a simple power flow example to illustrate several principles about increasing power flow.

•

Section 1.3, Limiting Conditions, describes limits on power flow imposed by circuit power flow, surge impedance loading, voltage drop, thermal factors, and environmental constraints.

•

Section 1.4, Chapter Previews, presents brief descrip-tions of the chapters in this guidebook.

1.2 POWER SYSTEM ISSUES

The power transmission system, in any region, is a com-plex combination of lines (including underground cable) and substations. With the exception of relatively short “radial” lines connecting generating stations to the sys-tem, power flow reaching any load point in the system flows over multiple “parallel” paths (circuits). In any path (circuit), the power flow moves through multiple series elements.

This can be illustrated by the following simple power system (NERC 1995) shown in Figure 1.2-1. There are three load areas (A, B, and C). Each load area has suffi-cient generation to supply the local load. With the sys-tem operating “normally,” there is no net power transfer between load areas. Nonetheless, as a result of the avail-able electrical paths connecting the load areas, the dia-gram shows a “loop” flow of 200 MW. This loop flow occurs even though there is no net power transfer to any of the areas.

Consider the situation where power generated in load area A is considerably less expensive than local genera-tion in load area B. It would then be advantageous for power customers in load area B to buy power from the generators in load area A. In doing this, the power transmission system operator sets a transfer limit of

2834 MW from A to B. Given this level of transfer, the power flows would be as shown in Figure 1.2-2.

Notice in Figure 1.2-2 that, even though load area C is not importing power, the lines connecting load area C to the other areas are carrying almost half of the total power transferred.

Now let us assume that the customers in load area B would like to buy even more than 2834 MW from the low-cost generator in load area A. Consequently, they contest the limit of 2834 MW set by the system opera-tor, noting that the emergency rating of the lines is 1000 MW. The power system operator explains that the limi-tation on power import to load area B is not due to

nor-Figure 1.2-1 Base system operating "normally" with local generation being similar in cost and able to supply all local load.

Figure 1.2-2 Base system operating "normally" with local generation at A being much cheaper than at B, causing a net power transfer of 2834 MW.

(17)

mal power flows but rather to the emergency power flow through one of the lines (#2) between A and C, as shown in Figure 1.2-3! With line #1 out of service, the redistributed power flow through line #2 reaches the emergency thermal limit of 1000 MW. Thus the imposed power transfer limit of 2834 MW from A to B.

As shown in Figure 1.2-3, if the net power transfer from A to B with all lines in service had exceeded the transfer limit of 2834 MW then, under this single contingency loss of line #1 between A and C, the power flow in line #2 from A to C would have exceeded the line’s emer-gency thermal limit.

One can reach a number of conclusions regarding power flow limits from this simple example:

•

Economic power transfers can be limited by circuits that do not directly connect the low-cost generation source and the customer.

•

A 5% increase (50 MW) in the emergency rating of lines #1 and #2 connecting A and C from 1000 MW to 1050 MW might allow a similar 5% increase (141 MW) in the power transfer limit from 2834 to 2975 MW.

•

A 5% increase in the emergency rating of either line #1 or #2 between A and B would not allow any increase in power transfer from A to B.

•

The long-term value of projects to increase the power flow in any particular circuit is dependent on changes in the cost of generation and the power flow

limits and electrical impedance of interconnected power circuits.

•

Note that these observations do not depend on the reason for the power flow limit in any of the circuits. They would be equally valid whether the limitation on power flow is due to equipment temperature lim-its, limits on voltage drop, or electrical phase shift stability issues.

1.3 LIMITING CONDITIONS 1.3.1 Circuit Power Flow Limits

Power circuits consist of series and parallel combina-tions of electrical equipment (each subjected to mechan-ical, electrmechan-ical, and thermal stresses) whose collective purpose is to transmit power safely and reliably under widely varying operational situations. Each element of such circuits is typically specified to have certain power flow limits that allow their safe, reliable operation for an extended period of time (e.g., 40 years).

Increased power flow inevitably means increased electri-cal current flow or increased circuit voltage since power is the product of these quantities. In general, for substa-tion equipment and underground cables, increasing the operating voltage is difficult or impossible, whereas increasing the maximum electrical current is both possi-ble and economic. Overhead lines are often capapossi-ble of either higher voltage or higher current levels if certain modifications are undertaken.

Power transmission circuits are typically bimodal in terms of power flow. Under normal operation, it is not unusual for power transformers and lines to operate at much less than half of their power flow capacity, only approaching their operational limits under relatively rare emergency events.

There are basically three methods of increasing power flow: load control, improved modeling and monitoring, and physical modification of existing circuits.

Improved models may allow operation of equipment with reduced safety factors without reducing safety and reliability. Examples are the “bottom oil” model in Annex G of the IEEE loading guide and the improved models for high-temperature sag of ACSR conductor. Similarly, monitoring of environmental factors (air tem-perature, wind speed, humidity, etc.) may allow the use of less conservative assumptions, again without reduc-ing safety and reliability.

Figure 1.2-3 Base system operating in response to a “single contingency” outage of line #1 between A and C while there is a power transfer of 2834 MW from A to B.

(18)

With monitors communicating data in real-time, it may be possible to run equipment at higher power levels most of the time by avoiding the use of “worst case” assump-tions. This approach is called dynamic thermal ratings. It is unlikely that such real-time monitoring would allow any increase in non-thermal operating limits.

Overhead transmission lines are the primary means of power transfer over long distances. They have thermal ratings just as power transformers, substation terminal equipment, and underground cables but, for long lines, power flow limits may also be necessary to avoid exces-sive voltage drop or system stability problems. In addi-tion, since the public has access to the area under lines, there may also be limits on voltage and current related to environmental effects. This section concerns the rela-tionship between the various types of power flow limits for overhead lines.

1.3.2 Surge Impedance Loading of Lines

Sometimes a power transmission line possesses a defi-nite power flow limit based on the design parameters for the specific line, but at other times, the line as a compo-nent of the overall transmission system determines the limit. System limits can result from factors such as volt-age drop, possibility of voltvolt-age collapse, and system sta-bility, both steady state and transient.

System limits are functions of transmission line reac-tances in relation to the overall power system. Series reactance, shunt admittance, and their combination, surge impedance, are relevant to system transfer limits. System planners have long recognized this relationship, particularly where there are prospects of changing the line surge impedance, either by adding equipment (e.g., series capacitors) or by modifying the line itself (e.g., reconductoring, voltage uprating, etc.).

Transmission line series inductive reactance is deter-mined by conductor size, phase spacing, number of con-ductors, relative phasing (double-circuit lines), and line configuration. In transmission lines the series reactance is significantly larger than the series resistance, and is the dominant factor in a first-order explanation of sys-tem behavior. For this reason, simple reconductoring of a transmission line results in only minor changes in sys-tem power flows.

Power flow on a transmission line, neglecting resistance of the line, is given by Equation 1.3-1, which can be derived from a simple circuit consisting of sending and receiving end voltage sources connected by a series reac-tance.

1.3-1 Where:

P = Real power transfer on the transmission line.

V1 = Magnitude of sending end bus voltage.

V2 = Magnitude of receiving end bus voltage.

X = Line series inductive reactance between V1 and

V2.

δ

= Phase angle difference between V1 and V2.

Increasing voltage magnitude for the same line voltage and same phase difference between ends increases the power flow. By increasing the voltages V1 and V2

together, the power transmitted increases by the square of the voltage for the same phase angle. Power flow increases for the same end voltage magnitudes are accommodated by an increase in the phase angle differ-ence between the voltages at the two line ends.

Equation 1.3-1 imposes a fundamental limit on the amount of power that can be carried by a transmission line corresponding to a phase difference between line ends of 90°. Further increases in angle result in decreases in power flow. This is an unstable situation that can be realized in practice in two ways. If the steady-state power flow were to slowly increase to the point that the angle reached 90°, an attempt to further increase power flow would actually decrease the power flow. An increase in the power angle δ when δ is in the range from 90° to 180° results in a decrease in sin(δ) and a consequent decrease in power flow. The condition try-ing to increase the flow on the line actually results in a decreased flow, and system instability.

Secondly, a system disturbance—for example, tripping of a line—causes a redistribution of power flow among the remaining lines, and consequent changes in the bus voltage angles. It is insufficient that the new angle differ-ences on all the lines are less than 90°, because the angle differences must remain lower than 90° during all the transient system swinging from the time of the distur-bance until the system settles in its new operating state. If a line were to experience its angle difference momen-tarily passing 90°, it would try to accommodate the power requirement by opening up the angle beyond 90°, decreasing the power flow. This is an unstable situation, and would cause the line to pass through the electrical point where its relay protection would sense a fault (even though none exists on the line), and result in a line trip and probable system separation.

Surge impedance loading (SIL), defined in Equation 1.3-2) provides a useful rule of thumb measure of

trans-1 2 sin( ) V V P X δ • • =

(19)

mission line loading limitation as a result of the effects of series reactance.

1.3-2 Where V is the line voltage, and ZS is the surge impedance

of the transmission line given by:

1.3-3 Surge impedance ZS is a resistance in ohms. L and C in

Equation 1.3-3 are positive sequence inductance and capacitance in henries per mile and farads per mile, respectively. Surge impedance loading is that loading on a three-phase power transmission line that it would have if it were loaded by a Y-connected set of resistances of

ZS ohms per phase. This is the same physical condition

as a radio frequency transmission line impedance matched to its termination (72 ohm coaxial cable termi-nated in 72 ohms in television cable). In electromagnetic theory, it corresponds to a pure TEM wave. The reactive power (vars) generated in the line capacitance is exactly canceled by the vars absorbed in the line inductance in a power transmission line at surge impedance loading (neglecting line resistance and real power losses). Surge impedance loading thus is a loading value based on physical principles related to the line design itself. Surge impedance loading is a handy tool for estimating the relative loading capabilities of lines of different volt-ages, constructions, and lengths from a system stand-point (St. Clair 1953). SIL is oversimplified for use in specifying actual line ratings on an operating system. However, it is a useful guide both for assessing actual loading limits and for understanding the different fac-tors that limit line loading. Figure 1.3-1 gives a curve of line loadability in per unit of SIL as a function of line length for heavy loading conditions. Slightly different versions of Figure 1.3-1 have been published, but they are all very similar (Dunlop et al. 1979, Gutman 1988). The fundamental observation from Figure 1.3-1 is that transmission line loadability decreases as length of the line increases. Three different regions come into play in derivation of Figure 1.3-1. Short lines tend to be ther-mally limited, irrespective of system conditions. As line length increases, voltage drop considerations frequently come into play. At longer line lengths, stability factors may dominate. Short lines are often loaded at 2 or 2.5 times SIL and thus need reactive power (var) support to maintain the voltage. Long lines may be limited to 1.0 times SIL or less.

An important observation from Equation 1.3-2 is that surge impedance loading is a function of the square of line voltage. This has been a driving force in increased transmission voltages over the years, especially for longer lines.

For an overhead transmission line, typical surge imped-ance is on the order of 300 ohms, while for a cable it may be 50 ohms or less. At 345 kV, SIL of an overhead line is on the order of 400 MW. Short lines may be able to carry 800 MW or more, while long lines of exactly the same construction may be limited to less than 400 MW by system considerations. Because of limitations on heat dissipation, underground transmission cables always operate very far below SIL. A consequence is that underground transmission cables are a net source of vars to the system, a condition that must be considered in system design.

1.3.3 Voltage Drop Limitations

Voltage control on the power system is of concern as system loadings increase. The system voltage distribu-tion is affected by the series inductance and shunt capacitance of the transmission lines, and is related to the flow of reactive power in the system. Depending on the relative real and reactive power flow on a given transmission line, the voltage may increase or decrease from one end to the other. It is not desirable for voltage to vary more than 5%, or at most 10%, from one end to

2 S V SIL Z = S L Z C =

Figure 1.3-1 Line loadability in terms of surge impedance loading (Dunlop et al. 1979).

(20)

the other. In some cases, a voltage drop limit is placed on power flow corresponding to the maximum allowable decrease in voltage magnitude. The longer the line or cable, generally the lower the power flow required to reach a voltage drop limit. Voltage control is a system problem, and is not generally solved by modifications to any one transmission circuit.

Methods to improve voltage control on transmission circuits may take a variety of forms:

1. In some cases, bundled conductors have been in over-head lines used for short lower voltage lines to reduce series reactance, where the use of bundled conductors is required neither for thermal or corona reasons. 2. Supply of vars at various points on the system can be

used to control voltage. The supply can be fixed, switched, or adjustable. In former years synchronous condensers were used to supply vars in a continu-ously adjustable basis. Capacitor banks are com-monly used, and may be switched on or off depending on the local voltage. Static var compensa-tors (SVCs) are also used to control voltage on the bulk power system.

3. Shunt reactors may be used for long EHV lines where the var supply from the line capacitance is greater than the system can absorb.

Because voltage drop is primarily a function of line reactance rather than resistance, simple reconductoring does very little to decrease the voltage drop per unit length. Reconductoring an existing 230-kV line by replacing the original 636 kcmil Hawk ACSR with a 954 kcmil Rail ACSR only increases the voltage drop limit by 5%. For an overhead line, adding a second conductor per phase to form two conductor bundles results in a more significant reduction in series reactance, and a greater improvement in voltage drop power limit. Shunt reactors may be applied for reasons other than voltage control—for example, to control transient over-voltages during line switching. Series capacitors may be used to partially compensate for the line series reac-tance, but this is usually reserved for the longest lines in relation to system stability. Whenever capacitors are installed in series with the transmission line inductance, the possibility of a series resonant condition exists. Sub-synchronous resonance has been the cause of genera-tor/turbine shaft failure and is a serious consideration for a series capacitor installation.

Other problems present themselves with series capaci-tors—for example, provision for passage of fault current without causing failure of the capacitors. The overall effect of the concern with system voltage is that a

partic-ular transmission line may be limited in its power-han-dling capacity by system voltages and var flows irrespective of the thermal capacity of the line conduc-tors. In some cases, it is possible to increase the line flows by addition of capacitors or similar measures. Flexible AC Transmission (FACTS) is a scheme where thyristor-controlled devices are arranged to provide real-time control of transmission line flows in excess of those that would normally be allowed by system voltage and stability considerations.

While voltage drop has long been known as a transmis-sion limitation, attention has also been focused in more recent years on voltage collapse, which is a system insta-bility that can occur under heavy loading conditions. Figure 1.3-2 shows a voltage collapse condition follow-ing a system disturbance, where the 115-kV voltage drops to 50% of the nominal operating voltage (0.5 p.u.). Voltage collapse can occur for several reasons on a heavily loaded system where there is insufficient var sup-port. An example is the geomagnetic storm of March 13, 1989, with its resulting voltage collapse and blackout. The March 1989 storm increased attention to system problems that result from solar activity. Utilities in areas subject to geomagnetic disturbances monitor solar activ-ity (Lesher et al. 1994), and can re-dispatch generation to reduce loading on affected lines during times of high geomagnetic activity. However, geomagnetic distur-bances are not the only cause of voltage collapse. 1.3.4 Thermal Limits

Thermal limits are discussed in considerable detail later in this guidebook (see, for example, Section 2.3). In brief, the current-carrying capacity (thermal rating) of an overhead transmission circuit is determined by the assumed “worst-case” weather conditions, assumed

Figure 1.3-2 Voltage collapse condition following a system disturbance.

(21)

conductor parameters, and the maximum allowable conductor temperature. Some of the specific thermal rating parameters are:

•

Conductor construction: outside diameter, conductor strand diameter, core strand diameter, number of conductor strands, and number of core strands.

•

Conductor AC resistance, which itself is a function of

the conductor temperature.

•

Conductor surface condition: solar absorptivity and emissivity.

•

Line location: latitude, longitude, conductor inclina-tion, conductor azimuth, and elevation above sea level.

•

Weather: incident solar flux, air temperature, wind speed, and wind direction.

The temperatures experienced by terminal equipment must also be limited. In certain circuits, the thermal rat-ing of substation equipment, in series with an overhead line, may determine the “circuit” rating. Disconnect switches, wave traps, current transformers, and other substation equipment all have current ratings that can be lower than those of the line. An example of terminal equipment limitations on older lines is 600 A disconnect switches. At EHV, bundled conductors are employed to reduce the conductor surface electric field and conse-quent corona phenomena of radio, television, and audi-bl e n o i s e. B u n d l e d c o n d u c t o r s we re o ri g i n a l ly introduced to lower line reactance and increase the line loadability, and their use for noise reduction was recog-nized later. Especially at the higher transmission volt-ages of 500 and 765 kV, the thermal current-handling capacity of a bundled conductor may be far in excess of the ratings of the circuit breakers. In such cases, the thermal limit of the circuit is entirely dominated by the terminal equipment. A survey of utility 345-kV circuit thermal limits in New York State gave the following lim-itations:

•

41% of the circuits were limited by the line or cable.

•

18% of the circuits were limited by current

transform-ers.

•

4% of the circuits were limited by wave traps.

•

4% of the circuits were limited by the bus-work.

•

3% of the circuits were limited by disconnect

switches.

•

4% of the circuits were limited by the circuit breakers. Lines and substation equipment may have different thermal ratings for normal and for emergency system conditions. Emergency ratings typically apply for a lim-ited period of time, not exceeding 24 hours and as short

as 5 minutes. Emergency ratings are typically calculated for higher temperatures, and allow for some equipment deterioration in order to avoid load interruptions under unusual operating conditions.

Broader voltage tolerances may also be appropriate under contingency conditions compared to normal operation. Lower voltage may be acceptable for a short time. Likewise, conductor resistive power losses are inconsequential during emergencies.

1.3.5 Environmental Limits

The electric field produced by overhead power transmis-sion lines is influenced by the following factors:

•

Line voltage.

•

Height of conductors above ground.

•

Configuration of conductors (line “geometry,” con-ductor spacing, relative phasing of multi-circuit lines, and use of bundled conductors).

•

Lateral distance from the center line of the transmis-sion line.

•

Height above ground at the point of field measure-ment.

•

Proximity of conducting objects (trees, fences, build-ings) and local terrain.

The electric field near ground level produced by an over-head transmission line induces voltages and currents in nearby conducting objects. These objects are typically the size of people, animals, motor vehicles, sheds, and similar-sized bodies. Electric field coupling is capacitive coupling, and can be represented by a current source in parallel with a high source impedance (Norton equiva-lent circuit).

The allowable electric field is limited by the maximum allowed induced current and voltage. For example, the National Electrical Safety Code specifies a maximum of 5 mA short-circuit current induced into the largest vehi-cle that could be stopped under the line, based on human susceptibility to loss of muscular control (let-go). Thus, if an existing line induces 4.9 mA on a large tractor-trailer, it would not be possible to increase the voltage without taking other measures to limit the elec-tric field.

Electric field levels are limited by law in some jurisdic-tions. Some regulations are specified at the edge of the right-of-way for public exposure. Other regulations are maximum levels on the right-of-way based on induction to an assumed size object. These regulations may restrict

(22)

voltage increases on presently existing transmission lines without taking electric field reduction measures.

Magnetic field is affected by the same variables as elec-tric field, except line current replaces line voltage, and nearby objects generally have minimal impact on the magnetic field. Magnetic field coupling is generally of significance for objects that parallel the transmission line for a long distance. Such objects include pipelines, telephone and railway signal circuits, and metal fences. Because magnetic field is a function of line current, and current increases during fault conditions, it may be nec-essary to evaluate magnetic field effects under both nor-mal operation and faults. Magnetic field coupling is inductive coupling, and generally produces low voltages with low source impedances.

Increasing current on a transmission line increases the magnetic field, and thus increases magnetically induced voltages and currents. This may be significant in cases such as when a transmission line parallels a railroad right-of-way. This is the inductive coordination problem that has been around since the dawn of the power indus-try with respect to telephone and railroad signal facili-ties. Increasing current flow on existing lines may require coordination with parallel infrastructure. In some jurisdictions, maximum magnetic field levels are specified by regulation. If an existing transmission line is operating near the magnetic field limit set by regulation, the ability to increase line current may be impaired, unless measures are taken to reduce the magnetic field levels.

Electric fields can be shielded by conducting objects. Vegetation is sufficiently conductive to reduce electric field levels. Grounded wires can be strung under the phase conductors at road crossings to reduce electric field levels. Grounding measures can be taken for fixed objects to eliminate induced voltages. On the other hand, magnetic field shielding is significantly more diffi-cult than electric field shielding. Shielding a transmis-sion line by magnetic materials is impractical. Flux canceling loops have been developed, but incur power loss and complexity in actively driven loops. Shielding is less practical as a mitigation measure for magnetic fields than it is for electric fields.

1.3.6 Examples – Overhead Lines

Figure 1.3-3, adapted from (Gutman 1988), repeats the generalized SIL curve of Figure 1.3-1 with the addition of a curve for thermal limitation. Superimposed on the SIL curve are curves for:

•

Thermal limit for a single 1414 kcmil conductor per phase.

•

Voltage drop limitation of 5%.

•

Steady-state stability margin of 35%.

The thermal and voltage drop limitation curves cross at a line length of approximately 110 miles. The voltage drop and stability limitation curves cross at a line length of approximately 190 miles. Based on the thermal, volt-age, and stability curves, three regions are identified in Figure 1.3-3.

(23)

•

Less than approximately 110 miles line length, the line is thermally limited.

•

Between approximately 110 and 190 miles line length, the line is limited by voltage drop.

•

Beyond approximately 190 miles, the line is stability limited.

Figure 1.3-3 thus illustrates the three regimes of line-loading limits. Figure 1.3-3 further illustrates that, for a specific transmission line example, the data points for the line fall near, but not on, the generalized SIL curve. The fact that the values are similar, but not identical, illustrates the point that the SIL curve is a handy refer-ence for sanity checking and rule of thumb analysis, but is not to be considered exact for any specific line. Sample surge impedance and thermal loading values for transmission lines of different voltages are given in Table 1.3-1.

For comparison with the 345-kV example in Figure 1.3-3, the 230-kV example in Table 1.3-1 has a thermal rat-ing of 440 MW and surge impedance loadrat-ing of 145 MW. Stability and voltage control limits for this line depend on the system to which it is connected. As an example of voltage drop, assume the 230-kV line is 100 miles long. Further assume that the sending end bus has a voltage of 1.0 per unit, and the receiving end bus has a voltage of 0.95 per unit, a 5% difference. Also assume the 230-kV line is at 1.0 power factor at the receiving end, neither taking nor supplying vars to the bus. In this case, the 230-kV line flow would be 220 MW, about 1.5 times SIL, but half the thermal rating. This result is in line with the 345-kV example given in Figure 1.3-3. Because SIL is primarily related to transmission line series reactance rather than resistance, simple reconduc-toring would produce only a minor effect on SIL limits such as voltage drop. In this 230-kV example of 5% volt-age drop, reconductoring from Cardinal to Falcon ACSR would increase the loading from 220 MW to 230

MW, a minor difference. Adding a second Cardinal con-ductor per phase to make two concon-ductor bundles would increase the loading to 310 MW. Adding a second con-ductor per phase has a greater impact on surge imped-ance, and thus on SIL and line loading. Full use of the 230-kV line’s thermal rating would require system changes to provide var support at the receiving end of the line.

The thermal limit is determined by line current and line voltage. Equation 1.3-2 shows that surge impedance loading is proportional to the square of the line voltage. Doubling line voltage doubles the thermal rating of the line, but multiplies SIL by a factor of 4. This has been the driving force during the history of the electric power industry for increasing voltage levels, and sometimes a motivation for voltage upgrades of existing lines. 1.4 CHAPTER PREVIEW

1.4.1 Overhead Lines (Chapter 2)

Overhead transmission lines are the predominant method of transporting power in any but the most urbanized power systems such as the New York City area. Of all the types of power equipment, overhead lines offer the largest opportunities for increased power flow at modest cost. Limits are placed on power flow through overhead lines in order to limit electrical phase shift, avoid excessive voltage drop, and limit the temper-ature of the current-carrying conductors. The emphasis in this book is on the latter of these limits.

Chapter2 discusses the reasons for limiting the temper-ature of overhead lines and the consequences of exceed-ing such limits. The chapter also covers the techniques for modifying the clearance of existing lines, reconduc-toring them without rebuilding structures, and real-time monitoring of weather and line sag-tensions.

A number of interesting case studies are included at the end of the chapter.

Table 1.3-1 Power Flow Limits on Lines and Cables

System XL XC Surge Impedance SIL Thermal Rating kV (Ω/mi) (Ω/km) (MΩ-mi) (MΩ-km) (Ω) (MW) (MW)

Transmission Line Characteristics

230 0.75 0.47 0.18 0.29 367 145 440

345 0.60 0.37 0.15 0.24 300 400 1500

500 0.58 0.36 0.14 0.26 285 880 3000

765 0.56 0.35 0.14 0.26 280 2090 8000

Transmission Cable Characteristics

(24)

1.4.2 Underground Cables (Chapter 3)

Chapter 3 provides an overview on underground cable systems and a very brief background on each of the major transmission cable types. As with overhead lines, the discussion on underground cable considers aspects external to a specific cable circuit that may limit power flow regardless of the cable circuit’s rating. The chapter also includes an overview on cable system ampacity, including worked examples.

The major barriers to increased underground cable rat-ing are inherent to each cable system type or installation location. Methods for increasing the rating of under-ground cable—such as surveying the soil thermal resis-tivity along the route and removing thermal bottlenecks due to other cable circuits or external heat sources—are discussed in some detail.

Given the relatively long thermal time constant of underground cables, dynamic rating methods are very attractive ways of increasing the rating. The chapter dis-cusses monitoring methods and the necessary real-time data required for dynamic rating calculations with underground cable.

Case studies are included for actual cable uprating projects, and the chapter provides a summary compari-son of uprating methods.

1.4.3 Power Transformers (Chapter 4)

Power transformers represent a significant portion of capital investment costs. Under existing conditions in the industry, utility budgets are reduced and networks are being forced to support greater power transfer over existing transmission circuits than ever before. As such, there is increased interest in safely utilizing all available capacity of power transformers.

In general, transformer load capacity is limited by equip-ment (winding and oil) temperatures. Industry standards (IEEE C57.12.00 in the U.S.) specify a maximum average winding rise that defines the rated load. In other words, when operating at rated nameplate current, the average winding rise shall not exceed the given value.

Chapter 4 describes the general construction of power transformers, outlines short- and long-term risks related to the loading of transformers, provides an overview of heat transfer mechanisms and describes the four most prevalent thermal models, and discusses factors behind thermal ratings, including ambient air temperature, load, and maintenance considerations.

1.4.4 Substation Terminal Equipment (Chapter 5)

Substation terminal equipment consists of many differ-ent types and designs of power equipmdiffer-ent. Included in this classification are line traps, oil circuit breakers, SF6

circuit breakers, rigid tubular bus, line disconnects, cur-rent transformers, bolted connectors, and insulator bushings. The increase in circuit rating, resulting from applying the various methods of increasing power flow in overhead transmission lines, underground cable, and power transformers is often limited by terminal equip-ment. In some cases, a large increase in circuit rating may be obtained for a very modest expenditure on ter-minal equipment rather than a relatively large invest-ment in lines, cables, or transformers.

Chapter 5 describes practical, rather simple methods of increasing the power flow through less capital-intensive equipment such as switches, bus, line traps, breakers, and power transformer auxiliary equipment. The chap-ter includes a summary of chap-terminal equipment types, specific thermal models for each type of equipment, dynamic thermal rating of terminal equipment, and methods of determining specific thermal parameters from field test, laboratory test, and manufacturer heat-run tests.

1.4.5 Dynamic Rating and Monitoring (Chapter 6)

Since the mid-1980s, considerable attention has been paid to increasing the power flow of overhead lines, power transformers, underground cables, and substation terminal equipment by means of monitoring weather and the equipment thermal state and by developing more accurate thermal models. The resulting dynamic thermal rating techniques typically yield increases of 5 to 15% in capacity.

Chapter 6 provides an overview of dynamic thermal rat-ing methods. The chapter aims to present a balanced overall view of when dynamic rating methods are appro-priate, how they are best implemented in a practical operational application, and how such methods can be applied to complex interconnections consisting of multi-ple circuits and many circuit elements.

The chapter discusses concerns related to dynamic rat-ings; outlines the need for inspections and/or real-time monitors and the problems that may arise without them; provides an overview on models for overhead lines, trans-formers, underground cables, and substation terminal equipment; describes the use of DTCR software; identi-fies operating issues related to dynamic thermal ratings; and describes field studies of dynamic ratings used for

(25)

REFERENCES

Boteler, D. H. 1994. “Geomagnetically Induced Cur-rents: Present Knowledge and Future Research.” IEEE

Transactions on Power Delivery. Volume 9. Number 1.

January. pp. 50-58.

Dunlop, R. D., R. Gutman, and P. P. Marchenko. 1979. “Analytical Development of Loadability Characteristics for EHV and UHV Transmission Lines.” IEEE

Transac-tions on Power Apparatus and Systems. Volume 98.

Num-ber 1. March/April. pp. 606-617. correction May/June. page 699.

Federal Power Commission. 1964. National Power Sur-vey. Part II-Advisory Reports. U. S. Government Print-ing Office. WashPrint-ington, D. C. October.

Gutman, R. 1988. “Application of Line Loadability Concepts to Operating Studies.” IEEE Transactions on

Power Systems. Vol. 3. Number 4. November. pages

1426-1433.

Koessler, R. J. and J. W. Feltes. 1993. “Voltage Collapse Investigations with Time-Domain Simulation.”

IEEE/NTUA Joint International Power Conference. Athens Power Tech Proceedings. Athens, Greece. Sep-tember 5-8.

Lesher, R. L., J. W. Porter, and R. T. Byerly. 1994. “Sun-burst—A Network of GIC Monitoring Systems.” IEEE

Transactions on Power Delivery. Volume 9. Number 1.

January. pp. 128-137.

North American Electric Reliability Council (NERC). 1995. “Transmission Transfer Capability.”

St. Clair, H. P. 1953. “Practical Concepts in Capability and Performance of Transmission Lines.” AIEE

Trans-actions on Power Apparatus and Systems. Volume 72. Part

(26)

(27)

CHAPTER 2

Overhead Transmission

Lines

2.1 INTRODUCTION

The degree to which the maximum power flow can be increased on an existing overhead line depends on its length, the original design margins, environmental concerns, and many other issues. Because power flow on the transmission system is a function of the overall system topology (transmission lines, transformers, generation, series and shunt compensation, and load), system considerations can also limit the maximum power flow on a specific transmission line. Transmission line ratings are sometimes developed on a system basis rather than on an individual line basis. The overall limit may be between operating areas, irrespective of ownership or individual lines, and may change during a day based on system conditions.

Sometimes a power transmission line possesses a definite power flow limit based on the design parameters for the specific line; at other times the line as a component of the over-all transmission system determines the limit. System limits can result from factors such as voltage drop, possibility of voltage collapse, and system stability, both steady state and transient.

Power system limits, on the power flow through individual overhead lines, are described in more detail in Chapter 1, which discusses power system limits on increased power flow. System limits are functions of transmission line reactances in relation to the overall power system. Series reactance, shunt admittance, and their combination, as well as surge impedance are relevant to system transfer limits. Transmission line series inductive reac-tance is determined by conductor size, phase spacing, number of conductors, relative phasing (double circuit lines), and line configuration. In transmission lines, the series reactance is significantly larger than the series resistance, and is the dominant factor in a first-order explanation of system behavior. For this reason, simple reconductoring of a transmission line results in only minor changes in system power flows.

Critical factors related to power flow limits for overhead lines include:

•

Surge impedance loading

•

Voltage drop

•

Thermal limits

•

Environmental limits

(28)

2.1.1 Surge Impedance Loading

Surge impedance loading (SIL, defined in Equation

2.1-1) provides a useful rule-of-thumb measure of transmis-sion line loading limitation as a result of the effects of series reactance.

2.1-1 For an overhead transmission line, typical surge imped-ance is on the order of 300 ohms, while for a cable it may be 50 ohms or less. At 345 kV, SIL of an overhead line is on the order of 400 MW. Short lines may be able to carry 800 MW or more, while long lines of exactly the same construction may be limited to less than 400 MW by system considerations.

2.1.2 Voltage Drop

Voltage control on the power system is of concern as system loadings increase. The system voltage distribu-tion is affected by the series inductance and shunt capacitance of the transmission lines. It is not desirable for voltage to vary more than 5%, or at most 10%, from one end to the other. In some cases, a voltage drop limit is placed on power flow corresponding to the maximum allowable decrease in voltage magnitude. The longer the line, generally the lower the power flow required to reach a voltage drop limit. Voltage control is a system problem, and is not generally solved by modifications to any one transmission line.

Because voltage drop is primarily a function of line reactance rather than resistance, simple reconductoring does very little to decrease the voltage drop per unit length. Reconductoring an existing 230-kV line by replacing the original 636 kcmil (324 mm2_{) Hawk ACSR}

with a 954 kcmil (487mm2_{) Rail ACSR only increases}

the voltage drop limit by 5%. Adding a second conduc-tor per phase, to form two conducconduc-tor bundles, results in a significant reduction in series reactance, and yields an increase in the voltage drop power limit.

2.1.3 Thermal Limits

Thermal limits are discussed in considerable detail in

this chapter. In brief, the current carrying capacity (thermal rating) of an overhead transmission circuit is determined by the assumed “worst case” weather condi-tions, assumed conductor parameters, and the maxi-mum allowable conductor temperature. Some of the specific thermal rating parameters are:

•

Conductor construction: outside diameter, conductor strand diameter, core strand diameter, number of conductor strands, and number of core strands.

•

Conductor AC resistance, which itself is a function of the conductor temperature.

•

Conductor surface condition: solar absorptivity and emissivity.

•

Line location: latitude, longitude, conductor inclina-tion, conductor azimuth, and elevation above sea level.

•

Weather: incident solar flux, air temperature, wind speed, and wind direction.

2.1.4 Environmental Limits

The electric field produced by overhead power transmis-sion lines is influenced by the following factors:

•

Line voltage

•

Height of conductors above ground

•

Configuration of conductors (line “geometry,” con-ductor spacing, relative phasing of multi-circuit lines, use of bundled conductors)

•

Lateral distance from the center line of the transmis-sion line

•

Height above ground at the point of field measure-ment

•

Proximity of conducting objects (trees, fences, build-ings) and local terrain

The electric field near ground level produced by an over-head transmission line induces voltages and currents in nearby conducting objects (St. Clair 1953, Federal Power Commission 1964, Dunlop et al. 1979, Koessler and Feltes 1993, Boteler 1994, Lesher et al. 1994, EPRI 2005). These objects are typically the size of people, ani-mals, motor vehicles, sheds, and similar-sized bodies. Electric field coupling is capacitive coupling, and can be represented by a current source in parallel with a high source impedance (Norton equivalent circuit).

Electric field levels are limited by law in some jurisdic-tions. Some regulations are specified at the edge of the right-of-way (ROW) for public exposure. Other regula-tions are maximum levels on the ROW based on induc-tion to an assumed size object. These regulainduc-tions may restrict voltage increases on presently existing transmis-sion lines without taking electric field reduction mea-sures.

Magnetic field is affected by the same variables as elec-tric field, except line current replaces line voltage, and nearby objects generally have minimal impact on the magnetic field. Magnetic field coupling is generally of significance for objects that parallel the transmission line for a long distance. Such objects include pipelines,

S Z V SIL 2 =

(29)

telephone and railway signal circuits, and metal fences. Because magnetic field is a function of line current, and current increases during fault conditions, it may be nec-essary to evaluate magnetic field effects under both nor-mal operation and faults. Magnetic field coupling is inductive coupling, and generally produces low voltages with low source impedances (St. Clair 1953, Federal Power Commission 1964, Dunlop et al. 1979, Koessler and Feltes 1993, Boteler 1994, Lesher et al. 1994, EPRI 2005).

Increasing current on a transmission line increases the magnetic field, and thus increases magnetically induced voltages and currents. This may be significant in cases such as when a transmission line parallels a railroad ROW. This is the inductive coordination problem that has been around since the dawn of the power industry with respect to telephone and railroad signal facilities. Increasing current flow on existing lines may require coordination with parallel infrastructure. In some juris-dictions maximum magnetic field levels are specified by regulation. If an existing transmission line is operating near the magnetic field limit set by regulation, the ability to increase line current may be limited, unless measures are taken to reduce the magnetic field levels.

Electric fields can be shielded by conducting objects. Vegetation is sufficiently conductive to reduce electric field levels. Grounded wires can be strung under the phase conductors at road crossings to reduce electric field levels. Grounding measures can be taken for fixed objects to eliminate induced voltages. On the other hand, magnetic field shielding is significantly more diffi-cult than electric field shielding. Shielding a transmis-sion line by magnetic materials is impractical. Flux canceling loops have been developed, but incur power loss and complexity in actively driven loops. Shielding is less practical as a mitigation measure for magnetic fields than it is for electric fields (St. Clair 1953, Federal Power Commission 1964, Dunlop et al. 1979, Koessler and Feltes 1993, Boteler 1994, Lesher et al. 1994, EPRI 1994, EPRI 2005).

Chapter 2 includes seven sections:

•

Section 2.2, Uprating Constraints, discusses con-straints on electrical and mechanical safety, with information on sag-tension calculations, limiting high-temperature sag, constraints related to wind-induced conductor motion, electrical clearance, loss of conductor strength, constraints on structural loads, and environmental effects.

•

Section 2.3, Line Thermal Ratings, explores the calcu-lation of line thermal ratings, and describes common heat balance methods.

•

Section 2.4, Effects of High-Temperature Operations,

describes annealing, calculation of sag and tension, thermal and creep elongation, and connectors and conductor hardware at high temperature.

•

Section 2.5, Uprating without Reconductoring, dis-cusses deterministic and probabilistic methods of uprating without reconductoring.

•

Section 2.6, Reconductoring without Structural Modi-fications, reviews the various reconductoring choices using new commercially available conductors.

•

Section 2.7, Dynamic Monitoring and Line Rating,

introduces the principles of dynamic rating methods.

•

Section 2.8, Case Studies, includes a number of

uprat-ing test cases and an economic comparison of line uprating alternatives.

2.2 UPRATING CONSTRAINTS 2.2.1 Introduction

Increasing the thermal rating of an existing line requires dealing with constraints on electrical and mechanical safety. The uprated line must remain safe under all elec-trical power flows up to its maximum without compro-mising the mechanical safety under severe ice and wind loads.

This section discusses issues related to constraints on uprating, including determining what constitutes a con-straint in various areas of design, operation, and the environment.

2.2.2 Sag-tension Calculations

Normally, “sag-tension” calculations are performed using numerical programs in order to determine the sag and the tension of a conductor catenary as a function of ice and wind loads, conductor temperature, and time. Calculation examples from a program like SAG10 are shown below to illustrate how tension limits are applied and how maximum conductor tension and maximum final high temperature sag are taken for the purposes of strain structure design and tower placement. Details of sag-tension calculation methods are not included, but examples and key references are cited.

In the design, uprating, or simple maintenance of power transmission lines, the concern of primary importance is public safety. It is more important that a line be safe than it carry power. Other than designing the support-ing structures such that they remain standsupport-ing under even the most severe weather conditions, the safety of a line is essentially determined by the position of its ener-gized conductors relative to nearby people, buildings, and vehicles. Maintaining minimum distances to nearby