TRANSMISSION LINES DESIGN and ELECTRICAL ENGINEERING HUB

To apply busways properly in an electric power distribution system, some of the more important items to consider are the following. Current-carrying capacity Busways should be rated on a temperature-rise basis to provide safe operation, long life, and reliable service. Conductor size (cross-sectional area) should not be used as the sole criterion for specifying busway. Busway may have seemingly adequate cross-sectional area and yet have a dangerously high temperature rise. The UL requirement for temperature rise (55 deg C) (see ANSI/UL 857-1989) should be used to specify the maximum temperature rise permitted. Larger crosssectional areas can be used to provide lower voltage drop and temperature rise. Although the temperature rise will not vary significantly with changes in ambient temperature, it may be a significant factor in the life of the busway. The limiting factor in most busway designs is the insulation life, and there is a wide range of types of insulating materials used b...

When lightning strikes a transmission line the field intensity stressing the insulation may exceed the ionization field intensity level (roughly 30kV/cm) and create an arc from the line to ground. A path now exists for current flow. The resulting discharge current flow from the lightning stroke is usually over within a few milliseconds but the ionized path has been established and a 60Hz “follow” current flows. This current must be detected and interrupted by deenergizing the line with circuit breakers. For the ionization path to dissipate, the voltage must be absent for a sufficient duration. The time during which the voltage is absent is commonly called “dead” time. For transient faults to be successfully cleared, an adequate time for deionization must be afforded. Table 1 shows the minimum time required by voltage level and by probability of successfully reclosing and energizing the line. Table 1. Minimum De-Ionization Time for Reclosing Breakers System Voltage  ...

Series compensation of long high-voltage and extra-high-voltage lines has become almost standard practice. The presence of series compensation affects the X0/X1 ratios of the system, with the reactance of the series capacitor appearing in all three sequence networks. Therefore, temporary and transient overvoltages as a result of faults, as well as circuitbreaker recovery voltages and surge arrester operation, are different than those that would appear in the uncompensated system. There have also been concerns about ferroresonant TOVs in series-compensation systems , but few if any cases of ferroresonance have been reported for operating transmission or subtransmission systems. However, because of this concern, some utilities buying series capacitors have specified special subharmonic detection devices as part of the series capacitor bank. There are also concerns about subsynchronous resonance (SSR) of rotating machine mechanical systems with the series compensated electrical system...

Bus connections When the MC switchgear consists of several shipping sections, the main bus is necessarily disconnected before shipping. The main bus should be reconnected, with particular attention paid to the cleanliness of and pressure between the contact surfaces. It is essential that the connections be securely bolted because the conductivity of the joints is dependent on the applied pressure. Refer to the manufacturer’s torque recommendations and any other special instructions. Cable connections Before the cable connections are made, the phasing of each cable should be determined in accordance with the connection diagram, and the cables should be tagged accordingly. The cable manufacturer’s instructions should be followed in forming cable terminations and during the installation of the cable. It is essential that the connections be clean and torqued to manufacturer’s recommendations since the conductivity of the joints is proportional to the applied pressure. The terminating ...

Overall ratings The overall ratings of PMFSG shall include the following: a) Rated power frequency; b) Rated maximum voltage; c) Rated lightning-impulse withstand voltage; d) Rated power-frequency withstand voltage; e) Rated short-circuit current. Rated power frequency The rated power frequency shall be the frequency at which the PMFSG and its components are designed to operate. The preferred rated power frequency is 50 Hz or 60 Hz. Rated maximum voltage The rated maximum voltage of PMFSG shall be that of the way with the lowest rating. A three-phase PMFSG containing one or more ways with components, such as fuses, single-phase switches, or fused-loadbreak devices rated for phase-to-ground voltage (maximum voltage divided by 1.732), shall have the designation “Grd-Y” (grounded-wye) added to the rated maximum voltage. The application of Grd-Y rated PMFSG should be limited to those three-phase applications where the recovery voltage, during switching or fault clearing across any Grd...

The following are recommendation based on IEEE STD 525-1992 Shielding practices a) The cable for computer or high-speed data logging applications, using low-level analog signals, should be made up of twisted and shielded pairs. For noncomputer type applications, such as annunciators, shielding may not be required. b) Twisting and shielding requirements for both digital input and digital output signals vary among different manufacturers of computerized instrumentation systems. Separation of digital input cables and digital output cables from each other and from power cables may be required. Where digital inputs originate in proximity to each other, twisted pair multiple conductor cables with overall shield should be used or multiple conductor cable with common return may be permitted, and overall shielding may not be required. Digital output cables of similar constructions may also be permitted. Individual twisted and shielded pairs should be considered for pulse-type circuits. ...