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Mostech Electrical Engineering Team

SELECTION OF SYSTEM VOLTAGE OF YOUR POWER INSTALLATION

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 208 Y/120 V systems:

a) The source impedance of 480 Y/277 V systems are typically less than 208 Y/120 V systems. This characteristic provides a more stable source with better voltage regulation, and minimizes voltage distortion due to the nonlinear load currents.

b) 480 Y/277 V systems are less susceptible to on-premises generated disturbances. Step-down transformers (and other power enhancement devices) for 208 Y/120 V utilization equipment help attenuate disturbances originating on the 480 V system.

c) 480 Y/277 V systems distribute power at lower currents, which result in lower heat losses in feeders. 480 Y/277 V systems may also decrease material and labor costs associated with installing long feeder circuits.

Step-down transformers (and other power enhancement devices) may be located physically close to the electronic load equipment to minimize the buildup of common-mode voltage.

Delta-connected transformer primaries trap balanced triplen harmonic currents generated on the secondary side by nonlinear electronic load equipment. This action serves to reduce distortion of the voltage waveform at the 480 Y/277 V level.

It is not recommended practice to step-up the voltage from the service entrance by means of a locally installed transformer in order to obtain a higher power system voltage for the electrical distribution system serving electronic load equipment. Although this can be done in certain cases, it is also possible that less satisfactory results can occur than if the system voltage at the service entrance was used.

Due to the generally lower impedance of 480 Y/277 V distribution systems, higher short-circuit currents may be available throughout the system. Overcurrent protective devices with higher interrupting capabilities and equipment with higher withstand ratings may be required.

In some situations, electrical distribution at 208 Y/120 V is unavoidable. This may be due to limitations of the utility or facility to provide higher voltages. As previously noted, nonlinear electronic load equipment may cause undesirable voltage distortion that can adversely affect the entire premises. In these situations, a system analysis may be performed to determine proper mitigation techniques such as the installation of isolation transformers, and other power conditioning or filtering equipment located close to the electronic load equipment.

WOODEN POLES FOR POWER LINES HOLE DIGING AND POLE SETTING

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 holes should be temporarily covered. All Local, State, and Federal safety regulations must be met.

Structure Alignment

When the structure is set and the load line completely released, the structure should remain plumb and level.

If the structure is not plumb or the crossarm is not level, additional material will have to be placed under one

pole. The additional material should be approved by the design engineer.

Pole Holes

All holes should be in the correct locations and large enough to provide a minimum of 6 in of space for tamping around the pole to the full depth of the hole. Pneumatic tamping equipment is recommended to expedite the setting operation.

The poles should be placed to prevent damage to the structure grounding materials. Poles not required to be raked should be set plumb and in alignment. Unless otherwise specified, structures at angles should be set to bisect the line angle.

The holes may be backfilled with earth excavated from the hole, provided this material can be properly compacted. Frozen material for backfill should not be permitted. The backfill should be compacted to a dry density not less than the natural in-place dry density of the surrounding earth.

Since the measurement of the density may not be practical, no more than one shoveler should be utilized for three tampers. Front-end loaders are not recommended during backfilling. Backfill should be banked and tamped around the poles to a height of 12 in above the natural ground surface.

Excessive water should be pumped out, leaving not more than 6 in of water in the bottom of the hole, and 6 in of granular material should be placed to firm up the bearing surface. Care should be exercised where pumping will cause excessive sluffing of the bottom of the hole. Casing should be used where moving water and/or gravel is encountered, working the casing down as the material and/or water is removed.

Backfill

Where unsuitable backfill material is encountered, gravel or crushed rock backfill should be utilized. Gravel backfill material should be thoroughly compacted using air tamps in layers not more than 6 in thick. If necessary, sufficient water should be added to the backfill to ensure adequate compaction.

Gravel backfill should be compacted to 70% relative density as determined by ASTM D4253-83 [14] and D4254-83 [15]. Immersion-type vibrators can be used in lieu of air tampers. Vibrators should have sufficient operating length to permit uniform compaction from the bottom of the hole to within 2 ft of the original ground surface.

The gravel backfill material should be vibrated as placed in the hole and the vibrators slowly withdrawn as the hole becomes filled. The upper 2 ft of the hole should be backfilled with excavated soil and compacted by tamping.

This material should be banked and tamped around the pole to a height of 12 in above the natural ground surface. When crushed rock is used as backfill, it should be compacted using air tampers in layers not exceeding 6 in. The rock backfill should be crusher run, a maximum of 2 1/2 inches in size, and having a minimum of two faces fractured and 95% crushed.

Alternate Backfill

Polyurethane foam may be used as a backfill material, in lieu of native or granular backfill, as it develops excellent uplift and bearing resistance. Where polyurethane foam is used, the hole should be sized to provide 1 1/2 to 2 in around the pole, so as to minimize the amount of foam required and provide better load transfer between the pole and the soil.

Care should be exercised to assure that the polyurethane foam does not insulate the ground conductor from the surrounding soil. After structures have been set and properly aligned, polyurethane foam (water-insensitive type), should be installed using the appropriate mixing and dispensing machine and procedures that are in strict accordance with manufacturer’s recommendations.

Sufficient polyurethane should be sprayed on the pole from 12 in above to 12 in below the ground line to coat the vertical surface of the pole. The operator should then dispense sufficient polyurethane to completely fill all voids and have expansion of foam to within 6 in of the ground line.

Structures should be held in a plumb position for approximately 8 to 10 min (until the polyurethane has hardened sufficiently to hold the structure). Polyurethane liquids in the dispensing machine reservoir should be maintained at a minimum temperature of 90 °F during normal operations. Chemicals should be held at a temperature of at least 60 °F for 24 h prior to being used. In cold weather, heated facilities should be provided.

Weak Soils

In soils with weak bearing and/or lateral capacity, the design engineer should specify alternative foundation designs and/or backfill to obtain required foundation integrity. Some alternatives are increased imbedment, uplift plates with crushed rock or concrete backfill, concrete backfill with or without re-bar through the pole butt, or other designs that increase bearing area, uplift capacity, or resistance loads as required. See IEEE Std 691-1985 [30], for a more thorough discussion of foundation design for direct embedded structures.

Rock Sockets

Where rock is encountered and cannot be removed by the use of a rock auger, various types of explosives could be considered. There are many different types of rock and the procedure for dealing with each varies greatly. In order to obtain the best results, the person in charge should be knowledgeable in types and hardness of the various rocks encountered (i.e., homogenous, fractured, glacial till boulders, and bedrock) as this will also determine the type of excavation to be used.

Conclusions

The methods mentioned above for setting and backfilling of poles or structures are recommended for soils that are encountered in most areas of the United States. It should be recognized that conditions requiring other techniques may be encountered. Techniques available for special soil conditions are water and air jetting.

EFFECTS OF LOAD ON TRANSMISSION LINE RELAY APPLICATIONS AND SETTINGS

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 than that afforded by phase overcurrent relays. Only unbalanced load current and normal system unbalance affect the setting of these devices.

Directional overcurrent relays

Directional overcurrent relays have the same restrictions as phase and ground overcurrent relays for load flow in their tripping direction. Properly selected directional elements block tripping for load flow and faults in the nontripping direction.

Phase distance relays

Phase distance relays have a relatively fixed reach; they operate most sensitively when fault currents are present and less sensitively when only load current exists. Fault currents typically lag voltage by 60° or greater. Load current typically leads or lags voltage by 30° or less. Although less sensitive in the load angle region, phase distance relays may require a setting for adequate fault coverage that may limit line loading.

Ground distance relays

Ground distance relays may also be susceptible to the error associated with ground fault resistance and out of-phase sources. Further, they may have overreach and underreach characteristics for the “leading” and “lagging” phases in responding to phase-to-phase-to-ground faults, unless provision is included to compensate for these factors.

Many ground distance relays operate on phase current and voltage inputs, making them susceptible to operate under heavy load conditions. For this reason, ground distance relays are usually supervised by ground overcurrent elements, which must be set to avoid operation for heavy unbalanced loads. Load current will also influence the “reach” of these devices where fault resistance is involved (Giuliante, McConnell, and Turner).

Pilot systems—two terminal

The influence of load on pilot systems is highly dependent on the nature of the protective relaying scheme. Those systems using overreaching distance measurement, such as directional comparison blocking, permissive overreaching transfer trip (POTT), and directional comparison unblocking, have the advantage of limited load angle sensitivity and have the absence of a critical reach due to the nature of the relaying system.

These relays have very little influence from load except in very long line applications, and this is often accommodated by blinders that prevent operation of the protective relaying system under balanced, three-phase load conditions. For direct underreaching transfer trip (DUTT) schemes, the pilot distance relays are set short of the remote line terminal.

This setting makes the scheme less susceptible to tripping under heavy loading conditions. However, the reach variation of the distance relay as a result of prefault load current is much more critical than for the overreaching schemes.

It is imperative that load or fault current, or any combination of the two, never be able to cause operation of the Zone 1 relay for any condition other than a fault on the protected line. Phase comparison and current differential schemes are not normally susceptible to operation under load condition because of their inherent nature of comparing current into the line at one terminal with current out of the line at the other.

However, load does influence the setting of fault detectors in phase comparison blocking schemes and in current differential schemes when operation following channel failure is allowed. Current differential schemes may be sensitive to tapped loads, and settings should be chosen accordingly.

Also, high levels of through load current may reduce the fault detection sensitivity of both phase comparison and current differential schemes. When transmission cables are used, special considerations may be required for the fault detector settings because of the capacitance of the cables.

Pilot systems—three terminal

Load in a three-terminal line application may represent an outfeed condition for an internal fault. Depending on the particular type of relaying system, this may produce an undesired blocking effect.

Three-terminal applications generally have at least one weak source and, consequently, care must be exercised to ensure that either the contribution to an internal fault exceeds this load current outfeed, or the relaying system bases its response on the total internal fault current.

HIGH VOLTAGE POWER CABLE SHIELDING PRACTICES BASIC INFORMATION

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 the frequent use of window type or zero-sequence current transformers for ground overcurrent protection, care must be taken in the termination of cable shield wires at the source. If the shield wire is passed through the window-type current transformer, it should be brought back through this current trans- former before connecting to ground in order to give correct relay operation.

A nonmagnetic metallic material applied over the insulation of the conductor or conductors to confine the electric field of the cable to the insulation of the conductor or conductors.

Shielding practices

Single conductor cables rated above 2 kV and multiconductor cables with a common overall discharge resisting jacket rated above 5 kV should be shielded, except for special applications or cable designs.

Multiconductor cable applications in the operating range of 2 kV to 5 kV require careful judgment, and each installation should be evaluated based on the existing and anticipated conditions. Shielding can be used to monitor or test cable installation for additional assurance of insulation integrity.

The following shielding recommendations contained in the NEMA standards publications for the type of insulation being utilized should be followed: NEMA WC 3-1980, NEMA WC 5-1973, NEMA WC 7-1988, and NEMA WC 8-1976.

A shield screen material is applied directly to the insulation and in contact with the metallic shield. It can be semiconducting material or, in the case of at least one manufacturer, a stress control material. At the high voltages associated with shielded cable applications, a voltage gradient would exist across any air gap between the insulation and shield.

The voltage gradient may be sufficient to ionize the air, causing small electric arcs or partial discharge. These small electric arcs burn the insulation and eventually cause the cable to fail. The semiconducting screen allows application of a conducting material over the insulation to eliminate air gaps between insulation and ground plane.

Various shield screen material systems include:

a) Extruded semiconducting thermoplastic or thermosetting polymer

b) Semiconducting woven fabric tape

c) Semiconducting coating (paint) used with semiconducting woven fabric tape

d) Extruded high-dielectric-constant thermoplastic or thermosetting polymer, referred to as a stress control layer

NEMA and AEIC standards require shielded power cable to be partial-discharge or corona tested. This test evaluates the effectiveness of the conductor and shield insulation screen materials and application, and verifies the absence of voids within the insulation.

A cyclic aging test is required by AEIC as a qualification of the cable design to ensure gaps do not develop between tape layers as a result of expansion and contraction cycles.

Multiconductor cable shielding should be considered in the 2 kV to 5 kV range where any of the following conditions exist:

a) Transition from conducting to nonconducting environment

b) Transition from moist to dry environment

c) Dry soil, such as in a desert

d) Damp conduits

e) Connections to overhead lines

f) Locations where the cable surface collects conducting materials, such as soot or salt deposits

g) Electrostatic discharges are sufficient in magnitude to interfere with control and instrumentation circuit functions

h) Safety to personnel is involved

i) Long underground cables

j) Direct earth burial

GROUND POTENTIAL RISE BASIC INFORMATION AND TUTORIALS

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 GPR, the ground grid impedance shall be obtained either by the calculation or measurement methods described in 4.3.

Ground fault studies

A study should be made of various ground faults in order to determine the one that produces the highest GPR and volt-second area . The station ground grid impedance, as well as power overhead ground wire and telecommunication grounding networks, tend to limit the fault current and should be included in the calculations.

Power system generators

In the ground fault study, the power system generators are usually represented by their subtransient reactances. As the time progresses until fault clearing, their reactances increase to their transient values and possibly to their steady-state synchronous reactances. This change can be neglected in most cases and the initial subtransient reactance retained. All signiÞcant impedances should be included, such as for transmission lines.

DC offset

The initial magnitude of the dc offset should be calculated as a function of the voltage magnitude at the time the fault is initiated. The highest dc offset occurs when the change in current, from just before fault initiation to just after fault initiation, is maximum.

Since the alternating current cannot change state instantaneously due to the inductance of the circuit, initially the dc offset counter balances the change in alternating current.

The dc offset then decreases to zero at a rate determined from the effective reactance-impedance ratio of the power circuit at the fault. With a highly inductive circuit, the maximum dc offset will occur when the fault is initiated close to a voltage zero crossing, a condition that is most unlikely for faults resulting from insulation breakdown.

Also, a highly inductive circuit will have a prolonged dc offset. For the application of a multiplication factor for dc offset.

Ground potentials differ from magnetically induced voltages. The transient dc component (dc offset) of the ground fault current produces a proportional but decaying ground potential. Equation (24a), used to determine the instantaneous current considering both the symmetrical and dc offset components, is included.

For induced voltages, the dc component is of minor importance since the induced voltage varies as di/ dt. In HV networks when the fault impedance is negligible, the time constant varies; but the rate of decay of the dc component is usually within 5Ð40 ms and is determined by the effective power system X/R ratio.