An Overview of Transmission System Studies - REA

7/29/2019 An Overview of Transmission System Studies - REA

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UNITED STATES DEPARTMENT OF AGRICULTURERural Electrification Administration

REA BULLETIN 1724E-202

SUBJECT: An Overview of Transmission System Studies

TO: All Electric Borrowers

EFFECTIVE DATE: Date of Approval

EXPIRATION DATE: Three years from effective date

OFFICE OF PRIMARY INTEREST: Transmission Branch, Electric StaffDivision

FILING INSTRUCTIONS: This bulletin replaces REA Bulletin 62-6,"An Overview of Transmission System Studies," issued March 1978and reprinted in October 1988.

PURPOSE: To outline the system studies that should beconsidered to support design criteria for REA-financedtransmission facilities from 34.5 through 765 kilovolts (kV).

James B. Huff, Sr. 2/10/93___________________________ ________________Administrator Date


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Bulletin 1724E-202Page 2

TABLE OF CONTENTS

Page1. Introduction ........................................... 5

2. Purpose ................................................ 53. System Study Applicability ............................. 54. Input Data Requirements ................................ 65. System Study Flow Chart ................................ 86. Discussion of Individual System Studies ................ 107. Conclusions ............................................ 25

Appendix A: Digital Load Flow Procedure and ExampleAppendix B: Two-Machine Stability Problem

INDEX:SYSTEM PLANNINGTransmission Studies

ABBREVIATIONSAND SYMBOLS

1. Abbreviations

AAC - All Aluminum ConductorACSR - Aluminum Conductor Steel ReinforcedAN - Audible NoiseASNR - Ambient Signal-to-Noise RatioBIL - Basic Impulse Insulation LevelBSL - Basic Switching Surge LevelCFR - Code of Federal Regulations

EHV - Extra High VoltageEMI - Electromagnetic InterferenceE/M - ElectromagneticE/S - ElectrostaticHVAC - High Voltage Alternating CurrentHVDC - High Voltage Direct CurrentkV - KilovoltMV - MegavarMVA - Megavolt-AmperesMW - MegawattsNESC - National Electrical Safety CodeOHGW - Overhead Ground WirePCB - Power Circuit Breaker

RI - Radio InterferenceREA - Rural Electrification AdministrationRMS - Root Mean SquareROW - Right-of-WaySNR - Signal-to-Noise RatioSSR - Subsynchronous ResonanceTNA - Transient Network AnalyzerTVI - Television InterferenceUHV - Ultra High Voltage


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VAR - Volt-Ampere Reactive


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2. Symbols

D - Rotational mechanical losses - Transmission line lengthN1 - Number of turns in transformer primary

N2 - Number of turns in transformer secondaryPE - Electrical powerPM - Mechanical powerPmax - Maximum transferred powerr1 - Winding resistance of primaryr2 - Winding resistance of secondaryrm - Magnetizing resistanceR - Series resistance of transmission lineRA - Armature resistance of generatorRF - Field resistance of generatort - TimeT - Developed torque of generatorT1 - Input torque of prime mover

T2 - Output torque of shaft loadV - Line-to-line voltageV1 - Primary voltage of transformerV2 - Secondary voltage of transformerVG - Internal voltage of generatorVM - Internal voltage of motorVR - Receiving-end voltageVS - Sending-end voltageVT - Terminal voltage of generatorL - Radian frequencyX1 - Leakage reactance of primaryX2 - Leakage reactance of secondaryXL

- Series reactance of transmission lineXM - Magnetizing reactanceXP - Reactance of primaryXS - Reactance of secondaryXSG - Synchronous reactance of generatorYC - Shunt capacitive admittanceZL - Series impedance of transmission lineZO - Surge impedance_ - Stability phase angleG - Internal phase angle of generatorM - Internal phase angle of motor


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1. INTRODUCTION: Recent trends in power system complexity,energy conservation and economic performance of REA-financedprojects reinforce the need for careful system planning fortransmission facilities. Planning studies generally involve themodeling of these facilities in order that system performance

can be conveniently observed and evaluated.

2. PURPOSE: The purpose of this bulletin is to outline thesystem studies that should be considered to support designcriteria for REA-financed transmission facilities from 34.5through 765 kilovolts (kV).

Presented in Section 3 is a list of transmission system studiesfor various facility applications that should be considered.Section 4 contains the input data required to perform eachstudy. A flow chart is presented in Section 5 that relates theinterdependency of each study to the overall system planningconcept. Section 6 contains a detailed explanation of each

study.

3. SYSTEM STUDY APPLICABILITY: Table 1 represents a list ofthe type transmission system studies and requirements to supportREA related financing arrangements for projects from 34.5 kVthrough 765 kV. These system studies should be considered inconjunction with the long range system and financial planningrequirements. This list of studies is not necessarily completenor is it listed in any order of priority. Each study should becompleted or considered as required in Table 1 for the specificfacility in question in order that system performance can beevaluated. (Refer to Sections 5 and 6 of this bulletin for adetailed description of each study.) The column numbers in thetable refer to the following studies:

1) Facility Feasibility2) Load Flow3) Reactive Compensation4) Stability5) Subsynchronous Resonance6) Statistical Line Design7) Short Circuit8) Insulation Coordination9) Corona and Radio Interference10) Electrostatic and Electromagnetic

11) Transmission Facility Economics

Symbols used in Table 1 are defined as follows:

o An "X" indicates study should be performed andsubmitted to REA in conjunction with a request for REAaction on financing arrangements.


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o A "Y" indicates study should be performed andsubmitted to REA with the project design information.

o A "Z" indicates study should be performed but notsubmitted for review unless requested by REA.

o An "O" indicates it may be advisable to have studyperformed depending on system complexity. REA shouldbe informed whether or not study will be completed atwhich time the borrower will be informed if REAdesires to review results.

o No mark indicates the study is not generallyapplicable.

4. INPUT DATA REQUIREMENTS: Prior to developing system modelsfor each planning study, data must be developed: (1) to permitmathematical simulation of the real system, (2) to aid in theanalysis of system performance related to equipment performance,

and (3) to provide constraints so that system improvements canbe made. A brief description of such input data will now bepresented for generators, transformers, lines and loads.

4.1 Generators: For steady-state analysis, generators arerepresented in terms of the real and reactive power to thesystem. Conversely, for transient performance, system studiesmay require full representation of the electrical and mechanicalcharacteristics of each generator.

4.2 Transformers: Most system studies require informationabout core and winding loss resistances, leakage reactances,

turns ratios at available taps, and automatic tap-changinglimits.

4.3 Transmission Lines: Transmission lines are generallyrepresented by single phase models with equivalent seriesimpedances (resistance-inductance combinations) between lineterminals and equivalent shunt admittances at each terminal.For a balanced three-phase transmission system the manner inwhich a single phase is represented should depend on the linelength and accuracy required. In selecting a model, it is usualto classify transmission lines as short, medium, or long. Thereis no definite length that can be stipulated to divide short andlong line analysis. For the majority of the cases, a sufficient

degree of accuracy may be obtained by the short line model onlines up to 50 miles (80 kilometers). The degree of accuracyvaries with line length and configuration and also withconductor diameter and spacing.

4.4 Loads: In load flow studies, loads are usually representedby constant real and reactive power flow requirements. Forstability studies, large motor loads are characterized byinduction motor equivalent electrical circuits.


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4.5 Miscellaneous: Input data are also developed in order todetermine relay settings in terms of loading limits. Transientand dynamic stability studies require knowledge of relay and


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TABLE 1 - SYSTEM STUDY APPLICABILITY

AVAILABLE IN HARD COPY


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breaker times and operating sequences. Outage rates anddurations for all major equipment are also necessary fordeveloping reliability data.

5. SYSTEM STUDY FLOW CHART: The following is a list of system

studies that should be considered to support the design criteriaalong with a system study flow chart that relates each study tothe overall planning concept. These studies are listed inSection 3 and are repeated below for convenience:

1) Facility Feasibility2) Load Flow3) Reactive Compensation4) Stability5) Subsynchronous Resonance (SSR)6) Statistical Line Design7) Short Circuit8) Insulation Coordination

9) Corona and Radio Interference10) Electrostatic and Electromagnetic11) Transmission Facility Economics

SSR, item 5, primarily may occur when series compensation isemployed. Statistical Line Design Parameters, item 6, arepresented in the 1993 edition of the National Electrical SafetyCode (NESC) for EHV transmission lines.

A system study flow chart is shown in Figure 1. The facilityfeasibility study yields information pertaining to voltagelevel, system capacity, conductor type, and the approximate

location of transmission circuits. Once a basic plan isestablished, a more complete transmission system study is usedin order to perform detailed load flow and stability studies.The results of load flow studies help determine the adequacy ofthe design with regard to acceptable voltage, phase angle,impedance, and power flow variations. Reactive compensationstudies utilize information from load flow studies to establishoptimum types of reactive (var) sources. Similarly, resultsfrom stability studies may show several alternatives to achievestability before equipment characteristics are specified. Afterthe load flow and stability studies, then possible switchingover-voltage problems are investigated via a statistical linedesign study since these voltages influence line and apparatus

insulation levels.

Other statistical line design investigations include lightning,contamination, and flashover strength studies to yield anoptimum insulation system for the transmission facility. Shortcircuit studies are then performed to assure proper selection ofprotective relays and circuit breaker interruptingcharacteristics. Similarly, insulation coordination studies arenecessary to assure proper protection of facilities against


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system over-voltage (internal-switching surges and external-lightning strokes).


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FIGURE 1 - SYSTEM STUDY FLOW CHART



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Extra-high-voltage (EHV) investigations that concernenvironmental and safety factors are corona, radio interference,electrostatic (E/S) and electromagnetic (E/M) studies. Finally,transmission facility economic studies should be considered thatinclude tower, conductor, hardware, equipment, and right-of-way

trade-off cost analyses.

6. DISCUSSION OF INDIVIDUAL SYSTEM STUDIES: This section givesadditional details about each type of study.

6.1 Facility Feasibility: Facility feasibility studies includeload and rating requirements, system voltage, surge impedanceloading and system capacity.

6.1.1 Load Requirements: The foundation of any system planningstudy is a determination of the load to be served which shouldbe compatible with an approved Power Requirements Study (7 CFR1710, Subpart E). This load is viewed in terms of magnitude;

daily and seasonal variation; area distribution; behaviorcharacteristics with voltage and frequency variations; andreliability requirements. In addition to requiring fullknowledge of the existing load and its characteristics, theplanning process calls for the careful projection of loadgrowth. In general, loads are projected for the entire systemas well as for each region and each major existing and futuresubstation.

6.1.2 Rating Requirements: Besides their use in operating thesystem under abnormal conditions, this type of study isessential to the system planning function.

Each type of electrical equipment has a thermal rating whichvaries as a function of load cycle, ambient, loss of life, sunexposure, etc. Most utilities develop ratings similar to thefollowing:

1) Normal (continuous)2) Emergency - 4 hours, 8 hours, 24 hours3) Emergency - 1 month, 6 months

These ratings are used for both planning continuous loads andcontingency loads. They also assist in planning for thedifferent equipment loading characteristics in summer and winter.

6.1.3 System Voltage: Transmission system voltages below theextra-high-voltage (EHV) level are between 34.5 and 230kilovolts (kV). The nominal EHV levels in the United States are345, 500 and 765 kV.

If a transmission facility is to be developed economically,voltage steps should be neither too large nor too small. In


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general, a 230 kV transmission system will find it is mosteconomical to stay at 230 kV until load growth requirements


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dictate 500 kV as a economical level. Similarly, 345 kV systemswill probably bypass 500 kV in favor of 765 kV when load growthrequirements dictate a higher voltage level.

6.1.4 Surge Impedance Loading: Surge impedance loading (SIL)

is a convenient indicator of comparing the approximate load-carrying capability of transmission lines of different voltages.SIL is the load that the line will carry when each phase isterminated in an impedance of:

ZL Eq.1

Zo = __

YC (formula is squared)

where:

Zo = surge impedance, in ohms

ZL = series line impedance, in ohms per unit length

YC = shunt line admittance, in ohms per unit length

The SIL, in megawatts (MW), is a function of the magnitude ofZo (or Zo) and the square of the voltage as shown in thefollowing equation:

SIL = V Eq. 2Zo

where: V = root mean squared (RMS) line-to-line voltage,in kilovolts (kV).

While SIL gives a general idea of the relative loadingcapability of a line, it is usual to load lines less than 300miles (480 kilometers) above the SIL. Conversely, because ofstability limitations, it is usual to load lines greater than300 miles (480 kilometers) below the SIL unless capacitorcompensation is employed. Computer-generated SIL tables of REAtransmission structures and lines are presented in REA Bulletin1724E-201, "Electrical Characteristics of REA AC TransmissionLine Designs."

6.1.5 Transmission Line Capacity: The capacity of atransmission line is dependent on the operating voltage, heatinglimit, economic limit, and stability limit.

6.1.5.1 Heating Limit - Because of power losses, the currentflowing in any conductor results in a temperature rise, and ifpermitted to reach the annealing point, the conductor may bedamaged. However, before the annealing point is reached,


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vertical clearance requirements may be the limiting factor. Theload required to create this condition, normally called theheating limit, will vary considerably, depending on the ambienttemperature, wind velocity, conductor type and surfacecondition. (As the heating limit is approached, vertical

clearances aregenerally reduced due to additional sag.) A newly installedaluminum conductor steel reinforced (ACSR) or all aluminumconductor (AAC) has a lower heating limit than a conductor whichis weathered and turned dark. The heating limit is not normallya determining factor on long transmission lines unless theconductor is small and loaded beyond the economic limit; onshort lines the heating limit is normally reached before thestability limit.

6.1.5.2 Economic Limit - The determination of the mosteconomical conductor size is complex because of the manyvariables involved. These variables include: (1) rate of load

growth, (2) change in geographical distribution and kinds ofloads, (3) cost of right-of-way, (4) location of new powersupply points, (5) load factor, (6) emergency service, and (7)continuity of service. For system voltages well below the EHVrange, conductor sizes can generally be chosen satisfactorily bythe application of Kelvin's Law; i.e., the most economical sizeof conductor is that for which the investment charges are equalto the cost of energy losses.

However, the application of this law to EHV transmission lineswill not generally result in the selection of the optimumconductor size. This is because Kelvin's Law does not reflectthe change in supporting structures with changes in conductorsize, and also does not include the transformer capacity. Acomplete cost analysis should account for all effects thatresult from changes in conductor size and circuit loading.These include: (1) total annual fixed cost of the completetransmission line as a function of conductor size, (2) annualcost of power losses, (3) annual cost of reactive (var) supplyneeded to support the receiving-end voltage, and (4) an annualcost of terminals and transformer capacity required at thesending and receiving ends.

6.1.5.3 Stability Limit - Another factor which may influenceline capacity is the stability or power limit. Stability is

that attribute of the system which enables it to developrestoring forces equal to or greater than disturbing forces.Steady-state stability is a condition which exists in a powersystem when there are no sudden disturbances on the system.Transient stability is a condition which exists if, after asudden disturbance has taken place, the system regainsequilibrium. The transient limit is usually lower and ofgreater importance than the steady-state limit. For relativelylong lines, series capacitors or autotransformers may be used to


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decrease the effective line reactance and therefore increase thestability limit (Section 6.3.1.).


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A good "rule of thumb" relationship for obtaining theapproximate steady-state capability of a transmission circuitbetween two terminals is as follows:

Pmax = 0.75 V Eq. 3

where:Pmax = Maximum transferred power, in megawatts (MW)V = RMS line-to-line voltage, in kilovolts (kV) = Distance between terminals, in miles.

6.2 Load Flow: Once facility feasibility studies areperformed, a thorough check of the transmission system plan ismade with load flow study programs. System planners confirm theadequacy of the proposed transmission network, considering lineloadings, bus voltages and reactive (var) supplies. Cases arerun as specified by the system planner under normal and outagecontingency conditions along with the reliability criteria for

each load region. A base case study provides a reference todetermine the emergency and future loadings of facilities.

The base case utilizes information corresponding to normaloperating conditions. Such a case serves as a comparison toother system conditions that need to be studied. A firstcontingency case is also recommended since as a general minimumcontingency situation, the system should perform with a singlefacility out of service.

Once a base case is established, one or more changes can beintroduced to determine variations in system performance. These

changes may include any combination of the following:a) Take any line or bus out of service;b) Add loads to any or all buses and lines;c) Change regulated bus voltages and phase angles;d) Add or delete new interconnecting lines;e) Add new generation to any bus;f) Change transformer taps;g) Increase conductor size of any line;h) Control reactive (var) power flow;i) Increase or decrease transformer capacity;j) Take any bulk substation transformer out of service.

The performance of the system as indicated by several load flowcases is properly reproduced only within the limits establishedby the system planner. Each case represents power flows andvoltages which would exist on the system if all input data suchas loads and generation were precisely reproduced. Although theload flow study results might never duplicate actual systemconditions, they are meaningful primarily because themathematical model can be tested beyond the acceptableperformance range of the real system, thus better identifying


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limiting conditions. An example of a typical digital load flowstudy is shown in Appendix A.

Load flow programs presently used automatically take intoaccount the voltage regulating capability of synchronous

condensers and transformers, while maintaining designatedgeneration schedules as well as net interchange amonginterconnected systems. Specified changes in system facilitiesand methods of operation are automatically calculated for anumber of cases in sequence.

The required input data for load flow programs generallyinclude: (1) bus designations, (2) line and transformerimpedances, (3) real and reactive (var) power flows for eachload, (4) generator power output, (5) voltage schedules andreactive (var) power flow limits of generators, synchronouscondensers and switched capacitors, (6) tie-line designations,and (7) system interchange information.

Printed output includes the (1) calculated voltage magnitude andphase angle at each bus, (2) transformer, capacitor and reactordata, (3) real and reactive (var) power flows for each line andtransformer, (4) net system interchange and tie-line power flowsand (5) a record of system changes. Special output features areavailable or can be developed to aid the planner in the analysisof the system. Examples include lists of facilities loadedbeyond preestablished limits, and lists of buses where thevoltage is below desirable levels.

6.3 Reactive Compensation: Reactive (var) compensation studies

utilize information from load flow studies to establish optimumtypes and sizes of reactive (var) sources. There are two basictypes of system compensation: (1) series compensation, in termsof impedance, is used to reduce a transmission line's effectivereactance, and (2) shunt compensation, in terms of reactivepower, is used to reduce the magnitude of reactive (var) powerthat flows in the network.

6.3.1 Series Reactive Compensation: Inductive reactance oftransmission lines is one of the most important parameterslimiting power flow capability. The insertion of capacitivereactance in series with the line's inductive reactancedecreases the line impedance. This helps to increase the

transmission system capability requirements. Seriescompensation effectively increases the transmission linecapacity. This reduces the need for higher transmissionvoltages or greater number of circuits. (When seriescompensation is used, a subsynchronous resonance (SSR) studyshould be performed.) When applying series capacitors on EHVsystems (345-765 kV), it is important that they do not introduceundue limitations on the flexibility of future systemdevelopment. The location of series capacitors along the line


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has a significant effect on the voltage profile, and powerlosses. For compensation less than 50 percent, it is usuallyadvantageous to locate the capacitors at the midpoint of theline to improve the voltage profile. Unfortunately, this maynot be economically practical unless a substation exists near

the midpoint of the line.

6.3.2 Shunt Reactive Compensation: As transmission voltagesand line lengths increase, the capacitive charging currents fromEHV lines also increase. These currents can cause undesirableovervoltages on generators and transformers, as well as increasepower losses. In order to reduce the capacitive chargingcurrents, shunt reactors are utilized to minimize theovervoltages during lightly loaded, switching or transientconditions. Shunt reactors may be either switched or directlyconnected at the transmission line terminals or to the tertiarywindings of autotransformers. Tertiary shunt reactors can beswitched as system reactive power requirements and voltages vary

whereas permanently connected line shunt reactors cannot beseparated from the line during switching operations.

6.4 Stability: The starting point of stability studies is thesteady-state conditions (determined by the load flow study)immediately before the system disturbance under investigationoccurs. Information which can be derived from a steady-statestability study includes the rotor or stability phase angle,real and reactive (var) power flow, bus voltage and systemfrequency. Transient (first swing) and dynamic (multiple swing)studies are generally performed either on analog devices or adigital computer.

6.4.1 Transient Stability: Generally, a transient stabilityprogram utilizes initial voltages and power flows obtained fromthe load flow program and converts the system to that requiredfor the analysis of transient phenomena. For specified faultconditions and switching operations, the program calculatessynchronous and induction machine electrical and mechanicaltorques, speeds, rotor torque angles, currents, and systemvoltages. In addition, some programs calculate currents andimpedances of selected lines and simulate the automaticoperation of impedance-type relays during severe systemsoscillations. Switching operations and fault conditions areautomatically simulated in sequence to represent the occurrence

and clearing of faults.

Input data required depend upon the complexity of the machinerepresentation desired. For the simplest representation, thedata required for a synchronous machine is the real and reactive(var) power, stator resistance, transient reactance, and inertiaconstant. A more detailed representation requires thecharacteristics of the turbine governor system and generatorexcitation system, and the detailed reactance, time constants,


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and magnetic saturation parameters associated with the machine.For most modern studies, the more detailed machinerepresentation is utilized to provide a better understanding ofsystem performance. For an induction machine, the real power,rotor, stator and magnetizing impedances, and the load speed-

torque relationships are usually required.

6.4.2 Dynamic Stability: Transient stability studies aregenerally limited to the analysis of performance within one ortwo seconds after the fault. It is also important in many casesto simulate subsequent redistribution of power flows accordingto system inertias and governor characteristics. These dynamicstability conditions generally are important after the suddenloss of large units or generating plants, or a largeconcentration of load. A number of computer programs fordynamic simulation are available that take into account largesystem models and accurately represent the dynamic response ofloads such as induction motors.

6.4.3 Results From Studies: Transient studies can helpdetermine: (1) need for faster protective relay system, (2)system operating and design weaknesses, (3) desirability of fastvalving, and (4) initial heavy loading of key transmissionfacilities. Dynamic studies generally simulate systemperformance during the period following sudden loss ofgeneration or load. These studies can aid in system design bydetermining: (1) high speed excitation performance, (2) effectof load shedding, and (3) potential system cascading effects.

6.4.4 Methods to Improve Stability: Presented below areseveral methods which may be employed to improve systemstability.

a. Conventional Remedies - The most common remedy forimproving power-system stability is to speed up theprotective system and to increase the amount oftransmission capacity. At this point in time, improvingrelay and breaker clearing times is difficult due totechnological and economic considerations (state-of-the-art circuit breakers generally interrupt currents whenthe ac wave passes through current zero). The timeinterval between adjacent current zeros on an ac powersystem is determined by the power-system frequency and

generally cannot be changed by the circuit breakerdesigner. Also, most present day protective relaymethods depend on the determination of direction,distance, or impedance parameters that inherentlyrequire a certain minimum amount of measurement time foraccurate results.

b. Fast Valving - A source of power system instability isthe excess energy supplied by the prime mover during the


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disturbance. If this energy is reduced or made equal tothe energy needed by the generator during thedisturbance, the generator acceleration problem is alsoreduced. Valves on modern large generating stations areusually very heavy and if the fast valving process is to

be effective, the valve must be operated in a very shorttime. In several cases, whether the generator willremain stable is determined in less than a second.Thus, the energy input from the turbine must be changedvery rapidly.

c. Breaker Selection and Relay Protection - A stuckbreaker on a close in three-phase fault may cause lossof synchronism resulting in instability problems. Theuse of power circuit breakers (PCB), whose poles can beoperated individually, is an approach to prevent thiscondition. If each of the poles can trip independently,there is less probability that more than one of the

poles will be stuck. Thus, a three-phase fault can beconverted to a single-phase fault in normal relayingtime. If the PCB fails to trip because it did notreceive the trip signal from protective relays, thepreceding method is invalid. For this situation, theinstallation of the PCB with individual poles will notprovide any benefits. The most common method is the useof "stuck breaker" schemes employing overcurrent relaysand high speed timers. In this situation the stuckbreaker scheme is timed to clear the adjacent breakerswithin the maximum clearing time. The primary relayscheme must be sped up accordingly.

d. System Damping - System damping is a method of providingrestoring forces in order to decrease undesiredoscillations or large system swings. Generally, it isdifficult to apply fast valving and braking resistors insuch a way that there will be negligible system swingsafter the fault is cleared. In the more usual case wheresuch controls are not used, the swings may be sufficientto cause loss of synchronism after the fault is cleared.Swings that cause loss of synchronism, loss of load, orlarge voltage excursions, should be controlled to reducethem to acceptable proportions. Damping restoresequilibrium between the generator input and output so

that minimum power is available for acceleration ordeceleration. Two forms of system damping are insertionof a dc tie between two ac lines and load shedding.

A dc tie between two ac lines provides a solution to thestability problem since no phase synchronization existsbetween the ac lines. The dc tie has characteristicsthat are suited for the damping function. HVDC controlscan be arranged so that the power flow over the dc lines


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is independent of the conditions existing on the aclines. Thus, it becomes possible to have the dc tiesurge power in accordance with the operating needs ofthe interconnecting ac systems. Besides high costs,difficulties may be experienced in the practical design

of control circuits that develop the intelligencenecessary to control the dc line.Load shedding is another form of system damping.Generally, load shedding relays are installed todisconnect load when there is insufficient generation tomaintain normal system frequency. This is a relatedsystem problem that is brought about because generatorsynchronism is lost for other reasons. Voltagereduction is another form of load shedding. This methodis complicated by the necessity for extremely high speedunderfrequency relays. Because of their sensitivitythey often false-trip and cause start-up problems.

6.5 Subsynchronous Resonance: Subsynchronous resonance (SSR)occurs when the natural frequencies associated with themechanical torques of synchronous machines are close to thoseimposed by the connecting networks. Steady-state SSR involvesspontaneous oscillations that are either sustained or slowlyincreased in magnitude with time. Transient SSR generallyrefers to transient torques on the generator shaft resultingfrom oscillating currents in the electrical network caused byfaults or switching operations. The study of SSR requires twophases: (1) stability analysis to insure that oscillationscannot build up during normal operation, and (2) the simulationof switching operations and faults to insure that associatedtorques do not exceed shaft stress limits. Results from SSRinvestigations yield the probability of occurrence of SSR in thesystem. Corrective measures required to reduce SSR are the useof filters to block currents at SSR frequencies and the use ofother design techniques besides series compensation to meetsystem stability requirements.

Transmission systems containing series compensation may exhibitsubsynchronous resonance at frequencies below 60 Hz. Thus,currents at subsynchronous frequencies may be amplified bysynchronous machines causing undesirable oscillations andpotential stability problems. The buildup or decay of thesecurrents is dependent on the series resistance and loading

levels of the transmission system.

6.6 Statistical Line Design: The main objectives ofovervoltage, short circuit, and insulation coordinationinvestigations are to (1) set criteria for transmission lineelectrical design, (2) establish ratings of surge arresters andother protective devices, and (3) specify equipment insulationlevels. These three investigations will now be discussed.


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Line design studies include switching surge evaluations,lightning performance, and contamination and flashoverperformance. All these factors influence transmission line andapparatus insulation levels. For transmission voltages between34.5 and 230 kV, line insulation studies may not be necessary if

insulation requirements are specified as in REA Bulletin1724E-200.

6.6.1 Switching Surge Evaluation: One severe overvoltage whichthe transmission facility must withstand is the switching surge.Switching surges are produced by switching of apparatus such ascircuit breakers and disconnecting switches. These overvoltagesappear across the line and station insulation, both phase-to-ground and phase-to-phase. Together with lightning andcontaminated considerations they determine the required lineinsulation levels. Switching surge requirements are also usedin insulation coordination studies for substation equipment(Section 6.8).

Insulation strength characteristics (which vary with suchstatistical data as wind, precipitation, and air density) areutilized in the switching surge evaluation. Other factors whichaffect the insulation system are breaker insertion resistors,line length, configuration, and loading. These parametersdetermine the waveshape and magnitude of switching surge which,in turn, influence the insulation flashover strengthcharacteristic. Studies on the transient network analyzer (TNA)are an excellent starting point for determination of systeminsulation based on switching surge performance. These TNAstudies may include the following statistical parameterevaluations:

a. Probability of proper circuit breaker operationb. Probability that an insulator string will swing to a

certain position.c. Probability that weather factors will reduce flashover

voltage performance across insulators and gaps.d. Probability that a voltage surge will exceed the

critical flashover voltage rating of an insulator string.

As noted above, switching surge performance is determinedprimarily by the line insulation. The number of insulators isselected so that the probability of flashover from switching

surges does not exceed design specifications. In general, fortransmission voltages below EHV (34.5-230 kV), switching surgesdo not limit the tower insulation design since the insulationstrength increases in proportion to the phase-to-structureclearance. At EHV levels (345-765 kV) the air gap begins tosaturate and switching surge line performance may become thelimiting factor in the choice of tower dimensions and clearances.


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6.6.2 Lightning Performance: Besides switching surge studies,a second major consideration in designing transmission lines islightning performance. Analytically, the lightning problem isextremely complicated being a function of many lightningstatistics such as stroke current amplitude, rise times, hit

probabilities and frequency of occurrence. (Thunderstorm-dayactivity is shown on the Isokeraunic map. From the map data, arelative comparison is made of thunderstorm activity in eacharea.)

Lightning evaluation of transmission facilities includesconsideration of storm incidence in the area, tower height andconfiguration, and insulator string length. Overhead groundwires (OHGW) are usually used to shield phase conductors fromdirect strokes. The number and location of OHGWs are majorfactors in estimating the number of shielding failures. Inaddition to OHGWs, a combination of line insulation and towerfooting resistance parameters are used to minimize lightning

flashovers across a transmission line.

To obtain an accurate estimate of lightning performance of aproposed line design, transient network analyzers (TNA), digitalcomputers and system modeling methods are employed to determinesuch parameters as the trip-out rate. Surge arrester ratingsfor transmission system applications are normally based on datafrom TNA studies. The resultant transient overvoltages fromthese studies are statistical in nature and are combined withinsulation strength probability studies to estimate lightningflashover performance.

6.6.3 Contamination Performance: The third major line designstudy is contamination performance of transmission lineinsulation. Unlike switching surge and lightning studies, whichestimate transient performance, contamination studies relate tothe 60 Hz or fundamental frequency performance.

Results from contamination studies yield flashover insulationlevels of (1) air gaps at extreme swing angles and (2)contaminated insulator strings. Both of these levels are basedon statistical parameter such as geographical location andtransmission system configuration (such as tower size andgeometry).

6.7 Short Circuit: The principal purposes of short circuitstudies are listed below:

a. Provide information for proper selection of protectiverelays to establish system performance requirements andsettings.

b. Provide information for proper selection of circuitbreaker interrupting requirements.


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c. Evaluate voltages during faulted conditions which wouldaffect insulation coordination and the application ofsurge arresters.

d. Design the type and capability of grounding systems.

e. Establish the electromechanical forces to be withstoodby system facilities.


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f. Provide data for a variety of network calculationsduring the process of planning and designing the system.

A variety of computer programs have been developed forperforming short circuit studies. Some are designed

specifically to facilitate relay studies, while others provideinformation primarily used in planning. All represent thesystem by symmetrical component equivalent reactances of variousfacilities. The programs generally calculate for each bus of aspecified system the total three-phase and line-to-ground busfaults, including effects of zero-sequence mutual impedances.Also specified are the three-phase and line-to-ground faultcontributions for each line or transformer connected to thefaulted bus.

Input data required include bus identification, positive andzero-sequence impedances of lines and transformers, andtransient and subtransient reactances of synchronous machines.

The printed output includes all bus faults and linecontributions for three-phase and line-to-ground faults for bothnormal and switched conditions. This allows quickidentification of the breaker duties under the worst conditions,and provides data in a form usable for evaluation purposes. Inaddition, a variety of printouts of currents and voltages can bespecified in any combination for every faulted bus.

6.8 Insulation Coordination: Insulation coordination isdefined as the protection of electrical systems and apparatusfrom harmful overvoltages by the correlation of characteristics

of protective devices and the equipment being protected.To coordinate insulation and protective devices, impulse voltagelevels are defined in terms of both BIL (basic impulseinsulation level) and BSL (basic switching surge level).

The BIL describes the equipment's ability to withstand lightningstrokes. Both conventional and probabilistic techniques areemployed in investigating the equipment BIL requirements.Factors which affect BIL performance are the magnitude andwaveshape of the expected BIL curve along with the probabilityof occurrence. The standard BIL waveform is 1.2/50 which is awave that has a front time of 1.2 microseconds and reaches half

magnitude (or half crest value) at 50 microseconds (Figure 2).A number of tests may be required to establish BIL such as the(1) front-of-wave test, (2) chopped-wave test, and (3) full-wavetest.

Similarly, the BSL describes the equipment's ability towithstand switching surges. The standard BSL waveform is250/2500 which is a wave that has a front time of 250microseconds and reaches half magnitude at 2500 microseconds.


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Results from insulation coordination studies yield the expectedstress requirements on the system equipment. Other outputsinclude BIL and BSL performance, arrester selection, and systemoperating constraints for a given or assumed overvoltage,

lightning, waveshape and insulation strength characteristics.

FIGURE 2 - TRANSIENT OVERVOLTAGE BIL CURVE


6.9 Corona and Radio Interference: Radio and television noiseor electromagnetic interference (EMI), and audible interferenceare rapidly becoming controlling factors in the design andplanning of transmission systems, especially at EHV and UHV.Two of the most reported environmental effects caused bytransmission facilities operating at EHV and UHV (345 kV andabove) are corona noise interference and voltage induction.(See Section 6.10)

Electrical discharges due to either corona or gap sparkover arethe basic sources of radio and audible noise interference. Thepulses caused by these discharges are injected into the


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transmission line phase conductors or other conductingcomponents which may act as a radiating antenna or transmissionmedia.


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Thus, EMI from the transmission lines is caused either bycomplete electrical discharges across small gaps or by partialelectrical discharges such as corona.

Gap-type noise sources can occur in the following transmission

line components:

a. Insulators that are dirty, cracked or loose,b. Splices,c. Tie wires,d. Between hardware parts (clamps, brackets, insulator

pins, crossarm braces, and guy wires),e. At small gaps between ground wires and hardware parts,f. With electrical apparatus that is either defective,

damaged, improperly designed, or improperly installed(such as corroded or loose fuse elements, transformerinsulation failure, noisy contacts in relays, meters andregulators).

These gap-type noise sources can be located by equipment thattraces EMI such as broadcast radio sets and battery operatedportable television receivers. Noise sources can beelectrically short circuited or minimized by improving thebonding between adjacent conducting parts or by tightening looseconnections.

At EHV and UHV corona (an electrical discharge through ionizedair) is generally the main source of noise interference. Coronais formed when voltage gradients are above the criticalgradient. For a specified operating voltage, conductor

contamination is the main cause of corona. However, conductorsurface burrs and scratches, and weather (rain, snow, andhumidity) cause an increase in corona effects. Other coronabyproducts are power loss in conductors and ozone production (achemical reaction of the corona discharge). Thus, transmissionline corona is a source of radio interference (RI) and audiblenoise (AN) at EHV and UHV. The measure of corona depends onexisting ambient conditions prior to line construction and alsoon the level of noise from the energized line. For RI, theambient conditions consist of the received signal strength andbackground noise level. The quality of reception during ambientconditions depends on the ratio of these two components and iscalled the ambient signal-to-noise ratio (ASNR). RI, resulting

from transmission line corona, depends on the ratio of thereceived signal strength and the noise level produced by theline. This is referred to as the interference signal-to-noiseratio (SNR). The comparison of ASNR and SNR is a measure of thecorona produced by a line at any one location. If both the ASNRand SNR levels are high, reception quality will also be high.


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6.10 Electrostatic and Electromagnetic: At EHV and UHV themedical and biological concerns due to electric field gradientsand the electrostatic (E/S) and electromagnetic (E/M) coupling


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between overhead transmission lines and conductive objectsshould be considered.

6.10.1 Medical and Biological: Medical and biological studiesdeal with the direct physiological effects on humans, animals,

and plants subjected to strong electric field. The magnitude ofelectric field strength (or voltage gradient) at ground level isusually used in medical studies to determine permissible fieldstrength limits for people and animals near energizedtransmission lines.

6.10.2 Induced Voltage: Electrostatically induced voltages arepossible when a conductive object insulated from ground is inthe vicinity of alternating current overhead lines. Similarly,electromagnetic induction effects are possible when transmissionline phase conductors carrying fault currents cause inducedvoltages at the open ends of an insufficiently groundedconductive object. If the conductive object is not adequately

grounded when a person or animal comes in contact with it, acurrent flows in the connection to ground through the electricalbody resistance. Object ground intervals that will reduce theE/S and E/M induction effects are usually based on the maximumallowable shock current passing through a person or animal whentouching the conductive object. For the E/S case, a steady-state shock current magnitude of five milliamperes is consideredas the "let-go" level in the 1993 edition of the NationalElectrical Safety Code (NESC). Other practical considerationsmay dictate that the shock current magnitude be kept below theone milliampere "threshold of perception" level. For the E/Mcase, object grounding intervals are based on transient currentlevels through a person or animal.

Results from E/S and E/M studies help determine the groundingrequirements for stationary metallic objects (fences andbuildings) and insulated conductive vehicles.

6.11 Transmission Facility Economics: The last system studydiscussed is that of transmission facility economics. Severalof the factors that should be considered in an economic studyinclude tower, conductor, accessory, and right-of-way costs.While all these factors enter into a study of transmissioncosts, two basic elements that are of significant importance are(1) the load-carrying capability of lines in terms of voltage

and distance, and (2) the associated equipment, particularlytransformers and switchgear. Related to both transmissioncapability and cost is a third element; an evaluation of theeconomics of intermediate switching stations. Some of thequestions that should be answered in a transmission facilityeconomic study are the following:

a. What diameter of conductor is necessary to reduce radiointerference and corona loss to acceptable levels?


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b. What line insulation is necessary?

c. What should the BIL of the station equipment be?

d. How much load can be safely associated with atransmission circuit?

e. What is the economic comparison with lower voltagetransmission or even high voltage direct current (HVDC)transmission for some installations?

f. What should the electrostatic and electromagneticrequirements be to minimize the effects of voltageinduction and field gradients on humans, animals, andplants near overhead lines?

7. CONCLUSIONS: In conclusion, an overview of eleven systemstudies is presented to support REA-financed transmissionfacilities from 34.5 to 765 kV. These studies are notnecessarily complete nor are they listed in any order ofpriority. Each study should be considered for the specificfacility in question in order that system performance can beobserved and evaluated.


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Bulletin 1724E-202

APPENDIX APage 27

DIGITAL LOAD FLOW PROCEDURE AND EXAMPLE

1. Procedure

The general procedure in preparing digital load flowprograms includes the following steps:

a. Digital computer solutions follow an iterative processby calculating one of the bus voltages from theestimated values at the other buses for a specified realand reactive (var) power.

b. Each system bus voltage is corrected and the process isrepeated until corrections at each bus are less than aspecified minimum value.

c. Digital solutions are usually based on node equations.(The digital computer program forms the self and mutualnode admittances.)

d. Iterative methods include the Gauss-Seidel Method (alsocalled the method of successive over-relaxation), theTransformer Tap Changing Method, and the Newton-RaphsonMethod.* Although the third method is more complex thanthe other two, it has better convergence characteristicsand requires fewer iterations.

2. Load Flow Example

Figure A-1 is a one-line diagram of a simple power systemwith generators connected at buses 1 and 3 and loads atbuses 2, 4, and 5. Typical load flow digital computationsare presented in Table A-1. The results show the number ofeach bus, the magnitude and phase angle of each bus voltageand the real and reactive (var) input power supplied by eachgenerator and drawn by each load. Negative values of realand reactive (var) power indicate inductive loads. Themismatch or unbalance in megawatts (MW) at any bus is thedifference between the megawatts flowing toward and awayfrom that bus. Mismatch in megavars (MV) is found in asimilar manner. The mismatch is an indication of theprecision of the results. For example, the megawatt power

flow from bus 1 to bus 4 and from bus 4 to bus 1 is 24.81 MWand -23.73 MW respectively, indicating a mismatch of 1.08MW. Also note that the voltage magnitude at buses 1 and 3is essentially constant and a worst case voltage regulationof 11 percent occurs between buses 3 and 4.

_______________________


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*Text, "Elements of Power System Analysis," by Stevenson (ThirdEdition).


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Bulletin 1724E-202APPENDIX APage 28


Figure A-1: One-Line Diagram for Load Flow Study



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Table A-1: Digital Computations for Load Flow Study


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Bulletin 1724E-202Appendix B

Page 29

TWO-MACHINE STABILITY PROBLEM

The essential factors involved in steady-state stability areillustrated in the following two-machine system (Figure B-1)example.

With reference to Figure B-1, the primary mechanical andelectrical parameters affecting steady-state stability are:

1. Mechanical Power (PM)

Prime mover input torque (T1), inertia between prime moverand generator; inertia between motor and shaft load; andshaft load output torque (T2).

2. Electrical Power (PE)

Internal generator voltage (VG); system network reactance

(XL); internal motor voltage (VM); stability phase angle (_)by which VG leads VM.

The equation relating the real electrical power transfer betweenthe generator and motor is:

VGVMPE =________sin _ Eq. B-1

XL

where VG and VM are voltage magnitudes. If VG and VMand XL are constant, the electrical power is directly

proportional to sin _. This is shown in Figure B-2.Physically, the stability angle is controlled by the relativemotion of the rotors of the generator and motor. During normaloperation both the rotors and internal voltages rotate atsynchronous speed. The angle between VG and VM is, therefore,constant. If for any reason the input torque to the generatorshaft increases, the generator momentarily speeds up. Thus, thegenerator rotates at a speed higher than the synchronous speedcausing VG to rotate above the synchronous speed. Since VMremains at the synchronous speed, the stability angle increases.With reference to Figure B-2, the increase in _ causes moretransferred power for _90and if the input torque continues to increase, the

generator speed and internal voltage increase causing anincrease in _. However, the transferred power output decreasessince _>90. In this case the input and output cannot balanceand the rotor speed will steadily increase causing the system tolose synchronism and become unstable.

Typical swing angle curves to evaluate transient and dynamicstability performances are shown in Figure B-3 after a suddendisturbance is applied at time t=0. In this case the stability


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phase angle, _(t), is a function of the system torque, internalvoltages, system reactance, and time.


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Bulletin 1724E-202Appendix BPage 30


Figure B-1: Basic Diagram for Two-Machine Stability Problem



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Figure B-2: Steady-State Power Angle Diagram


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Bulletin 1724E-202Appendix B

Page 31


Figure B-3: Typical Swing Equation Curves


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Documents

An Overview of Transmission System Studies - REA