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...

The following characteristics should help to ensure accuracy as well as ease of interpretation: a) Keep it simple. A fundamental single-line diagram should be made up of short, straight lines and components, similar to the manner in which a block diagram is drawn. It should be relatively easy to get the overall picture of the whole electrical system. All, or as much as possible, of the system should be kept to one sheet. If the system is very large, and more than one sheet is necessary, then the break should be made at voltage levels or at distribution centers. b) Maintain relative geographic relations. In many cases, it is possible to superimpose a form of the one-line diagram onto the facility plot plan. This is very helpful toward a quick understanding of the location of the system's major components for operating purposes. It may, however, be more difficult to comprehend the overall system operation from this drawing. Such a drawing could be used for relativel...

The selection of any circuit breaker, for any given duty, is ultimately based on an assessment of its ability to perform the following basic functions: a) To carry the required full-load current without overheating (i.e., it should have the correct current rating), b) To switch and isolate or disconnect the load from the source at the given system voltage (i.e., it should have the correct voltage rating), c) To interrupt any possible abnormally high operating current or short-circuit current likely to be encountered during operation (i.e., it should have the correct interrupting rating), and d) To be able to perform these functions over an acceptably long period of time under the operating and environmental conditions that will actually prevail in the application (i.e., it should have the correct mounting provisions, enclosure, and accessories for operation in the environment in which it is to be applied). The degree to which a circuit breaker can satisfy these re...

Coordination is a systematic application of current actuated devices in a power system, which in response to a fault or overload will remove only a minimum amount of equipment from service. The objective is to minimize the equipment damage. A coordination study provides data useful for selection of instrument transformers, protective relay characteristics and settings, fuse ratings, and other information pertinent to provision of optimum protection and selectivity in coordinating these devices. Planning and Data Collection. The following data and initial planning steps are required before a coordination study is started: • Single-line diagram of the electrical system with details of equipment ratings. • Load flow data and short-circuit data. The maximum and minimum available short-circuit currents, both for phase and ground faults at each relay location in the system. • Time-current curves, setting ranges, type of characteristics of the protective devices, instrument...

How Differential Relay Protect Your System? Differential relays provide high-speed (1 to 2 cycles), sensitive, and inherently selective protection. These will not provide protection for turn-to-turn winding faults in generators, motors, and transformers because of the small increment in the current produced by such faults, which remain below the pickup sensitivity of the relays. An overcurrent relay can be used to provide differential protection when it is so connected that external fault currents through the current transformers balance out and do not give rise to a current in the relay operating coil. A phase or ground fault within the protected zone results in current unbalance and operates the relay. This scheme is limited by current transformer saturation at high magnitudes of external fault currents. Partial differential protection of a motor uses core balance transformers, which circle phase and neutral leads so that under an external fault situation the magnetic f...

The FMEA for power distribution systems amounts to the determination and listing of those component outage events or combinations of component outages that result in an interruption of service at the load point being studied according to the interruption definition that has been adopted. This analysis must be made in consideration of the different types and modes of outages that components may exhibit and the reaction of the system’s protection scheme to these events. The primary result of the FMEA as far as quantitative reliability evaluation is concerned is the list of minimal cut-sets it produces. A minimal cut-set is defined to be a set of components that, if removed from the system, results in loss of continuity to the load point being investigated and that does not contain as a subset any set of components that is itself a cut-set of the system. In the present context, the components in a cut-set are just those componen...

Transients generate substantial radiated energy, electric and magnetic fields, and transient currents within the substation grounds. Any of these phenomena can couple into poorly executed control wiring systems, but none of them can couple to an appreciable degree into well-executed control wiring systems. The obvious and correct approach to GIS control wiring is to enclose the entire control system in a Faraday cage, i.e., within a metal enclosure. This is much simpler than it sounds, as will be described below. A Faraday cage is a metal enclosure that fully surrounds the system, offering protection from EMI. In the case of control wiring, the system is typically a sensor (e.g., a gas density relay), the attached control wiring, the local control wiring cabinet in which the control wiring is terminated, the control wiring from the local cabinet to the substation control room, and the relay or computer racks in the control room. Each of the...

The IEC 60870-5 standards address the basic goals of telecontrol systems and their particular environmental conditions. IEC 60870-5 does not define one particular protocol profile; but rather like EPRI/UCA, it specifies a number of frame formats and services that may be provided at different layers. IEC 60870-5 is based on a three-layer Enhanced Performance Architecture (EPA) reference model for efficient implementation within RTUs, meters, relays, and other IEDs. Additionally, IEC 60870-5 defines basic application functionality for a user layer, which is situated between the OSI Application Layer and the application program. This user layer adds interoperability for such functions as clock synchronization and file transfers. The following descriptions provide the basic scope of each of the five documents in the base IEC 60870-5 telecontrol transmission protocol specification set. Standard profiles are necessary for uniform application of t...

Foundations The foundations for poles are just as important as the structure above ground. The pole back fill should be capable of withstanding structure reactions. Pole-setting equipment should be moved clear of the structure site prior to back filling. Differences in ground elevation at each pole location, and pole length tolerances permittedby 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 hole...

The protective functions noted in the various generating station configurations provide both primary and backup protection for the generating station as well as additional protection schemes which could also be applied. These protective functions are listed below with a reference to the subclause in the text that discusses their application in detail. Also included is a discussion of the various tripping modes used in generating stations. Protective Devices Device Function                      Subclause 21 Distance relay. Backup for system and generator zone phase faults. Device 21 requires a time delay for coordination. 24 Volts/hertz overexcitation protection for the generator and its associated step-up  and auxiliary transformers. 27 Undervoltage relay.  32 Reverse-power relay. Motoring protection.  40 Loss of field protection 46 Stator unba...

It is common practice to ground all types of generators through some form of external impedance. The purpose of this grounding is to limit the mechanical stresses and fault damage in the machine, to limit transient voltages during faults, and to provide a means for detecting ground faults within the machine. A complete discussion of all grounding and ground protection methods may be found in IEEE Std C62.92.2-1989 and IEEE Std C37.101-1993. The methods most commonly used for generator grounding will be discussed in this guide. They are listed in the following four broad categories: a) High-impedance grounding b) Low-resistance grounding c) Reactance grounding d) Grounding-transformer grounding Solid grounding of a generator neutral is not generally used since this practice can result in high mechanical stresses and excessive fault damage in the machine. According to ANSI C50.13-1989, the maximum stresses that a generator is normally designed to withstand is that...

Delayed autoreclosing may need to be considered when the upstream protection is provided by electromechanical relays or fuses and the circuit protection is provided by microprocessor-based relays, unless the microprocessor-based relays can be set to mimick the reset characteristic of the electromechanical relays. Without this time-delay reset feature on the microprocessor-based relay, it is possible to have the upstream device operate incorrectly, resulting in an overtrip. As an example of this, the low-set instantaneous trip on a distribution feeder is eliminated to improve power quality by eliminating momentary service interruptions. If an instantaneous autoreclose is used after a time delayed trip, an additional time margin needs to be used between the operating times of protective devices in order to maintain coordination of the feeder overcurrent relays and an upstream electromechanical relay or fuse. By delaying an upstream protective device to coordinate with the b...

There is never a reason to autoreclose an electrical circuit breaker following a trip unless there is reason to believe that the fault is no longer present on the circuit. Historically, when distribution circuit breakers would trip and result in a circuit outage, the circuit was patrolled before the circuit breaker was closed. This practice delayed restoration. Records were kept of these events. It was discovered that for 85–90% of the occurrences, no permanent faults were found. It generally became accepted to autoreclose these distribution circuit breakers. With the advent of additional protective devices available to the distribution engineer such as fuses, sectionalizers, and reclosers with which coordination was necessary, multiple autoreclose attempts were chosen. In many areas, three autoreclose attempts were chosen. This results in four trips to lockout. This practice continued for several years. As time went on, load increased and it became necessary that dis...

There has been a tendency to attribute disturbances and failures to power surges, a term often used by the media but rather ill-defined. The ambiguity results in part from an unfortunate dual definition of the word surge. a) To some people, a surge is indeed the phenomenon being discussed here, that is, a transient voltage or current lasting from microseconds to at most a few milliseconds, involving voltages much higher than the normal (two to ten times). b) To other people, a surge is a momentary overvoltage, at the frequency of the power system, and lasting for a few cycles, with voltage levels slightly exceeding the five to ten percent excursions that are considered normal occurrences. The term swell has been adopted by this recommended practice to describe this second type of overvoltage; perhaps one day it will supplant the usage of surge for that meaning. It would be a mistake to attempt protection against these long-duration power frequency swells with a surge prot...

As a general principle, it would be desirable to determine the efficiency of an HVDC converter station by the direct measurement of its energy losses. However, there are practical difficulties that prevent such a measurement, including the following: 1) Attempts to determine the station losses by subtracting the measured output power from the measured input power must recognize that such measurements have inherent inaccuracy, especially if performed at high voltage dc. Moreover, the losses of an HVDC converter station at full load are generally less than 1% of the transmitted power. Therefore, the difference between the measured input and output power is a small difference between two large quantities and, as such, is not likely to be a sufficiently accurate indication of the actual station losses. 2) In some special circumstances, it may be possible to arrange a temporary test connection in which the two converters are operated from the s...

For certain applications, the most cost-effective solution for poor power factor, excessive voltage distortion, and IEEE Std 519-1992 violations is to install one or more larger harmonic filters at a distribution bus or busses. Generally, an automatic harmonic filter bank will be installed on the secondary of each main transformer in the plant requiring power factor and harmonic compensation. Placement of multiple banks on a common low-voltage system can create problems by changing network harmonic flows and thereby increase the potential for overloading some of the filter banks. Therefore, this practice is not generally recommended. Where power factor correction is most important, systems tuned to the 2nd harmonic or below can generally be safely applied in this manner. Parallel resonance at the 3rd harmonic must be carefully evaluated. Caution must be exercised when a harmonic filter is electrically close to the main and is tuned to the 7 th harmonic or higher. In this...