TRANSMISSION LINES DESIGN and ELECTRICAL ENGINEERING HUB

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The selection of the ac supply system voltage typically begins at the service entrance of the facility. In most commercial environments in the U. S., the utility supplies three-phase power at 480 Y/277 V (or 600 Y/347 V) or 208 Y/120 V. In industrial environments, the utility may supply three-phase power at even higher voltages such as 4160 V, 13 800 V and higher. The magnitude of the voltage will typically depend on the size of the facility, the load conditions, and the voltage ratings of the utilization equipment in the facility. In some cases, the facility owners may design, install, and maintain their own medium-voltage electrical distribution system. Recommended practice is to provide distribution power in most facilities at 480 Y/277 V (or 600 Y/347 V) rather than at the actual utilization equipment level of most electronic load equipment (208 Y/120 V). Electrical distribution systems operating at 480 Y/277 V (or 600 Y/ 347 V) have the following benefits over ...

Foundations The foundations for poles are just as important as the structure above ground. The pole backfill should be capable of withstanding structure reactions. Pole-setting equipment should be moved clear of the structure site prior to backfilling. Differences in ground elevation at each pole location, and pole length tolerances permitted by ANSI O5.1-1987 [9] should be considered to ensure a level structure. The tops of poles should not be cut. If cutting is necessary, the pole top should be covered with a mastic-type cap. Under no circumstances should the butt of any pole be cut. The design engineer should specify a minimum hole depth. The actual hole depths required to obtain a level structure are the responsibility of the installing contractor. Digging operations should not be too far in advance of the setting operation. Holes open too long may deteriorate due to ground water seepage and/or heavy rains and increase the chance for accidents. Unattended pole...

One of the principal influences on protective relay settings is load; hence, maximum load current level, in turn, may influence fault detection sensitivity. Phase overcurrent relays Phase overcurrent relays must be set to avoid operation on all of those “normal” conditions to which they may be subjected, such as transformer inrush, motor starting current, maximum emergency load conditions, and maximum recoverable swing conditions. This usually entails a time overcurrent pickup setting above a maximum load current level and/or a coordinated instantaneous pickup setting to ensure security of the relays against misoperation. The sensitivity achievable is, therefore, somewhat coarse, but many applications in which they are used do not require extreme sensitivity. Ground overcurrent relays Ground overcurrent relays have the advantage of utilizing a current source that supplies little or no normal current to the relays. The sensitivity achievable is substantially better...

Cable shields and metallic sheath/armor should be solidly grounded at one or more points so that they operate at or near ground voltage at all times. For additional information see IEEE Std 575-1988. Accidental removal of the shield ground can cause a cable failure and a hazard to personnel. The length of cable run should be limited by the acceptable voltage rise of the shield if the shield is grounded at only one point. The derating of ampacity due to multiple-point short circuited shields has a negligible effect in the following cases for three-phase circuits: a) Three-conductor cables encased by a common shield or metallic sheath b) Single-conductor shielded cables containing 500 kcmil copper or smaller installed together in a common duct c) Triplexed or three-conductor individually shielded cables containing 500 kcmil copper or smaller d) Single-conductor lead sheathed cables containing 250 kcmil copper or smaller installed together in a common duct Because of t...

FACT ON GROUND POTENTIAL RISE What Is Ground Potential Rise? GPR When a ground fault occurs, the zero-sequence fault current returns to the power system ground sources through the earth and also through alternate paths such as neutral conductors, unfaulted phases, overhead ground wires, messengers, counterpoises, and metallic cable shields. The ground sources are the grounded wye-connected windings of power transformers, generator grounds, shunt capacitors, frequency changers, etc. The GPR is equal to the product of the station ground grid impedance and that portion of the total fault current that ßows through it. Also, the GPR is equal to the product of the alternate path impedance and that portion of the conductively coupled fault current that ßows through it. The volt-time area of GPR to be determined is given in volt-seconds for the duration of the fault. Ground grid impedance Since the station ground grid impedance to remote earth is needed to calculate the...