TRANSMISSION LINES DESIGN and ELECTRICAL ENGINEERING HUB

Generation control and ED minimize the current cost of energy production and transmission within the range of available controls. Energy management is a supervisory layer responsible for economically scheduling production and transmission on a global basis and over time intervals consistent with cost optimization. For example, water stored in reservoirs of hydro plants is a resource that may be more valuable in the future and should, therefore, not be used now even though the cost of hydro energy is currently lower than thermal generation. The global consideration arises from the ability to buy and sell energy through the interconnected power system; it may be more economical to buy than to produce from plants under direct control. Energy accounting processes transaction information and energy measurements recorded during actual operation as the basis of payment for energy sales and purchases. Energy management includes the following functions: . System load forecast: Fo...

Wood poles are considerably cheaper than steel for many types of construction. The lower cost is due, in part, to the more conservative basis of design normally adopted for steel. Generally, steel structures are designed to support safely one or more broken conductors, whereas wood structures are often not so designed. It is logical that the reasons for choosing the more expensive steel construction should require conservative design throughout and that conditions justifying the cheaper and shorter-lived wood structures would warrant accepting some of the more theoretical hazards. For voltages of 69 kV and lower, wood is quite generally used. Wood-pole construction for many years has been used for all voltages up to and including 345 kV. H frames with various modifications have been designed, the most popular using the main crossarm as the bottom member of a truss. Butt-treated cedar and full-treated pine are used almost exclusively in trans...

SCADA, with its relatively expensive RTUs installed at distribution substations, can provide status and measurements for distribution feeders at the substation. Distribution automation equipment is now available to measure and control at locations dispersed along distribution circuits. This equipment can monitor sectionalizing devices (switches, interruptors, fuses), operate switches for circuit reconfiguration, control voltage, read customers’ meters, implement time-dependent pricing (on-peak, off-peak rates), and switch customer equipment to manage load. This equipment requires significantly increased functionality at distribution control centers. Distribution control center functionality varies widely from company to company, and the following list is evolving rapidly. . Data acquisition: Acquires data and gives the operator control over specific devices in the field. Includes data processing, quality checking, and storage. . Feeder switch control: Provides rem...

Technical Feasibility. For this kind of analysis it is important to consider at least the following points: • System load requirements. It is important to evaluate for how long the uprated/upgraded line will satisfy the load requirements. • Assessment of current conditions and life expectancy of transmission line materials. It is important to make this kind of evaluation for the main transmission line components, such as towers, foundations, conductors, insulators, and hardware. • Potential margins for uprating/upgrading. It is important to check electrical clearances, mechanical strengths, ROW width, as well as the possibility of compliance with the requirements of safety codes (e.g., NESC), regulatory bodies and government agencies (e.g., navigable streams, public lands, air lanes). • Utility considerations. Sometimes electric utilities are not authorized to take the transmission line out of service to perform the necessary uprate/upgra...

This kind of transmission line uprating can result in a much higher rating increase than thermal uprating. Besides that, transferring the same amount of power in a higher voltage level reduces the line current, and consequently, line losses and voltage drops. However, voltage uprating is typically more expensive than thermal uprating due to the need of also uprating the voltage class of the terminal substations equipment. Effectiveness. This kind of uprating can be a good option when: the line loading is limited by voltage drop or stability considerations; the line has margins in terms of electrical clearances; the uprating can be done with minimal line modifications or it will be applied to several circuits simultaneously, or the line design criteria can be relaxed. Previous Analysis to Perform. Before proceeding with a transmission line voltage uprating it is necessary to analyze tower clearances, conductor-to-ground clearance, corona perf...

The construction of new substations and the expansion of existing facilities are commonplace projects in electric utilities. However, due to the complexity, very few utility employees are familiar with the complete process that allows these projects to be successfully completed. This article will attempt to highlight the major issues associated with these capital-intensive construction projects, and provide a basic understanding of the types of issues that must be addressed during this process. There are four major types of electric substations. The first type is the switchyard at a generating station. These facilities connect the generators to the utility grid and also provide off-site power to the plant. Generator switchyards tend to be large installations that are typically engineered and constructed by the power plant designers and are subject to planning, finance, and construction efforts different from those of routine substation proje...

Since all the generating units that are online have different costs of generation, it is necessary to find the generation levels of each of these units that would meet the load at the minimum cost. This has to take into account the fact that the cost of generation in one generator is not proportional to its generation level but is a nonlinear function of it. In addition, since the system is geographically spread out, the transmission losses are dependent on the generation pattern and must be considered in obtaining the optimum pattern. Certain other factors have to be considered when obtaining the optimum generation pattern. One is that the generation pattern provide adequate reserve margins. This is often done by constraining the generation level to a lower boundary than the generating capability. A more difficult set of constraints to consider are the transmission limits. Under certain real-time conditions it is possible that the most economic pattern may not be feasib...

The detailed procedure for insulation co-ordination set out in IEC 60071-1 (European standard EN60071-1 is identical) consists of the selection of a set of standard withstand voltages which characterize the insulation of the equipment of the system. This set of withstands correspond to each of the different stresses to which the system may be subject: • Continuous power frequency voltage (the highest voltage of the system for the life of the system). • Slow-front overvoltage (a standard switching impulse). • Fast-front overvoltage (a standard lightning impulse). • Very-fast-front overvoltage (depends on the characteristics of the connected apparatus). • Longitudinal overvoltage (a voltage between terminals combining a power frequency voltage at one end with a switching (or lightning) impulse at the other). These voltages and overvoltages need to be determined in amplitude, shape and duration by system study. For each class of overvo...

An advanced energy storage technology is superconducting magnetic energy storage (SMES), which may someday allow electric utilities to store electricity with unparalled efficiency (90% or more). A simple description of SMES operation follows. The electricity storage medium is a doughnut-shaped electromagnetic coil of superconducting wire. This coil could be about 1000 m in diameter, installed in a trench, and kept at superconducting temperature by a refrigeration system. Off-peak electricity, converted to direct current (DC), would be fed into this coil and stored for retrieval at any moment. The coil would be kept at a low-temperature superconducting state using liquid helium. The time between charging and discharging could be as little as 20 ms with a round-trip AC–AC efficiency of over 90%. Developing a commercial-scale SMES plant presents both economic and technical challenges. Due to the high cost of liquiud helium , only plants with 100...

When a conductor is covered with ice and/or is exposed to wind, the effective conductor weight per unit length increases. During occasions of heavy ice and/or wind load, the conductor catenary tension increases dramatically along with the loads on angle and deadend structures. Both the conductor and its supports can fail unless these high-tension conditions are considered in the line design. Certain utilities in very heavy ice areas use glaze ice thickness of as much as 2 in (50 mm) to calculate iced conductor weight. Similarly, utilities in regions where hurricane winds occur may use wind loads as high as 34 lb/ft2 (1620 Pa). As the NESC indicates, the degree of ice and wind loads varies by region. Some areas may have heavy icing, whereas some areas may have extremely high winds. The loads must be accounted for in the line design process to prevent a detrimental effect on the line. Some of the effects of both the individual and combined co...

The electrical operating performance of a transmission line depends primarily on the insulation. An insulator not only must have sufficient mechanical strength to support the greatest loads of ice and wind that may be reasonably expected, with an ample margin, but must be so designed as to withstand severe mechanical abuse, lightning, and power arcs without mechanically failing. It must prevent a flashover for practically any power-frequency operating condition and many transient voltage conditions, under any conditions of humidity, temperature, rain, or snow, and with such accumulations of dirt, salt, and other contaminants that are not periodically washed off by rains. Insulator Standards. The NEMA Publication High Voltage Insulator Standards, and AIEE Standard 41 have been combined in ANSI C29.1 through C29.9. Standard C29.1 covers all electrical and mechanical tests for all types of insulators. The standards for the various insulators c...

Overhead transmission of electric power remains one of the most important elements of today’s electric power system. Transmission systems deliver power from generating plants to industrial sites and to substations from which distribution systems supply residential and commercial service. Those transmission systems also interconnect electric utilities, permitting power exchange when it is of economic advantage and to assist one another when generating plants are out of service because of damage or routine repairs. Total investment in transmission and substations is approximately 10% of the investment in generation. Since the beginning of the electrical industry, research has been directed toward higher and higher voltages for transmission. As systems have grown, higher-voltage systems have rarely displaced existing systems, but have instead overlayed them. Economics have typically dictated that an overlay voltage should be between 2 and ...

A SCADA system consists of a master station that communicates with remote terminal units (RTUs) for the purpose of allowing operators to observe and control physical plants. Generating plants and transmission substations certainly justify RTUs, and their installation is becoming more common in distribution substations as costs decrease. RTUs transmit device status and measurements to, and receive control commands and setpoint data from, the master station. Communication is generally via dedicated circuits operating in the range of 600 to 4800 bits=s with the RTU responding to periodic requests initiated from the master station (polling) every 2 to 10 s, depending on the criticality of the data. The traditional functions of SCADA systems are summarized: . Data acquisition: Provides telemetered measurements and status information to operator. . Supervisory control: Allows operator to remotely control devices, e.g., open and close circuit breakers. A ‘‘select before operate’’ ...

Industry standards dictate certain analytical techniques that adhere to specific guidelines, suited to address the questions of ac and dc decrement in multimachine systems in compliance with well-established, industry-accepted practices. They are also closely linked to and harmonize quite well with existing switchgear rating structures. Typical standards are the North American ANSI and IEEE C37 standards and recommended practices the international standard, IEC 60909 (1988) and others, such as the German VDE 0102-1972 and the Australian AS 3851-1991. The analytical and computational framework in the calculating procedures recommended by these standards remains algebraic and linear, and the calculations are kept tractable by hand for small systems. The extent of the data base requirements for computer-based solutions is carefully kept to a necessary maximum for the results to be acceptably accurate. This type of analysis represents the best compromise between solution acc...

The test is made by running the machine as a motor at rated voltage and frequency without connected load. To ensure that the correct value of friction loss is obtained, the machine should be operated until the input has stabilized. 5.3.1 No-load current The current in each line is read. The average of the line currents is the no-load current. 5.3.2 No-load losses The reading of input power is the total of the losses in the motor at no-load. Subtracting the stator I2R loss (at the temperature of this test) from the input gives the sum of the friction (including brush friction loss on wound-rotor motors), windage, and core losses. 5.3.3 Separation of core loss from friction and windage loss Separation of the core loss from the friction and windage loss may be made by reading voltage, current, and power input at rated frequency and at voltages ranging from 125% of rated voltage down to the point where further voltage reduction increases the current. 5.3.4 Friction...

ROTATING MACHINES CORONA PROBE TESTS BASIC INFORMATION What Are Corona Probe Tests For Rotating Machines? The corona-probe test is intended to be an indicator and locator of unusual ionization about the insulation structure. This test is sensitive to end-winding surface corona, as well as internal-cavity ionization in the insulation structure. Compared to slot discharge, the discharge energies involved in surface corona or internal-cavity ionization may be of a much lower order of magnitude. The energy in the discharge varies as the square of the voltage across the gap and directly as the effective capacitance at the point of breakdown. Partial Discharge (Corona) has several undesirable effects, such as chemical action, production of heat, and ionic bombardment. The deteriorating effects of corona are dependent on its intensity and repetition rate as well as the design of the insulation system involved. Inorganic insulation components such as mica and glass are n...

Overvoltage tests are used to obtain assurance concerning the minimum strength of the insulation. Such tests are made on all or parts of the ground insulation. Many users of large rotating machines apply overvoltage tests periodically, generally at the beginning of the overhaul of related equipment. This allows for the detection and possible repair of insulation weaknesses during the scheduled outage. An overvoltage test should be applied to each phase separately with the remaining phases not under test being grounded. In this way, the insulation between phases (or lines) is also tested. This is only practical, however, where both ends of each phase are brought out to separate terminals, as is usually the case in generators. Some motors may have three or four leads brought out which precludes test between phases. Overvoltage tests may be performed either with alternating or direct voltage. The level of overvoltage which should be applied will depend to a large extent on t...

As has been stated, electrical machines and their insulation systems are subjected to mechanical, electrical, and thermal stresses which give rise to many deteriorating influences, the most significant of which are the following. 4.1 Thermal Aging Gradual aging caused by temperatures due to normal operating loads. 4.2 Overtemperature Unusually high temperature from causes such as overload, high ambient temperature, restricted ventilation, and loss of cooling liquid. 4.3 Overvoltage Unusually high voltage such as from switching or lightning surges. 4.4 Contamination This deteriorates electrical insulation by actually conducting current over insulated surfaces, or by attacking the material reducing its electrical insulating quality or its physical strength, or by thermally insulating the material forcing it to operate at higher than normal temperatures. Included here are: Wetness or extreme humidity Oil or grease Conducting dusts and particles Nonconduct...

Structure Types – the following descriptions of structure types shall apply to the provisions for strength requirements: Suspension Structure – A structure where the phase conductors and static wires are attached through the use of suspension insulators and hardware or, in the case of the static wire, with a clamp not capable of resisting the full design tension of the wire. Strain Structure – A structure where the phase conductors and static wires are attached to the structure by use of dead-end insulators and hardware but where the ability of the structure to resist a condition where all wires are broken on one side under full loading is not required or desired. Typically, strain structures would be used where the line deflection angle is 45 degrees or less. Structures subject to strain structure requirement shall be as identified by the Utility. Dead-end Structure - A structure where the phase conductors and static wires are a...

Because the neutral conductor carries less current than the phase conductors, utilities can use smaller neutral conductors. On three-phase circuits with balanced loading, the neutral carries almost no current. On single-phase circuits with a multigrounded neutral, the neutral normally carries 40 to 60% of the current (the earth carries the remainder). On single-phase circuits, some utilities use fully rated neutrals, where the neutral and the phase are the same size. Some use reduced neutrals. The resistance of the neutral should be no more than twice the resistance of the phase conductor, and we are safer with a resistance less than 1.5 times the phase conductor, which is a conductivity or cross-sectional area of 2/3 the phase conductor. Common practice is to drop one to three gage sizes for the neutral: a 4/0 phase has a 2/0 neutral, or a 1/0 phase has a number 2 neutral. Dropping three gage sizes doubles the resistance, so we do not want to go any smaller than tha...