ENGINEERING ARTICLES

The Institute of Electrical and Electronics Engineers (IEEE) defines grounding as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth. It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting body). Based on this definition, the reasons for grounding can be identified as: • Personnel safety by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system. • Personnel safety and control of electrostatic discharge (ESD) by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system and the Earth. • Fault isolation and equipment safety by providing a low-impedanc...

The process of connecting the grounding system to earth is called earthing and consists of immersing a metal electrode or system of electrodes into the earth. The conductor that connects the grounding system to earth is called the grounding electrode conductor. The function of the grounding electrode conductor is to keep the entire grounding system at earth potential (i.e., voltage equalization during lightning and other transients) rather than for conducting ground-fault current. Therefore, the NEC allows reduced sizing requirements for the grounding electrode conductor when connected to made electrodes. The basic measure of effectiveness of an earth electrode system is called earth electrode resistance. Earth electrode resistance is the resistance, in ohms, between the point of connection and a distant point on the earth called remote earth. Remote earth, about 25 ft from the driven electrode, is the point where earth electrode resistance does not increase appreciably when this dis...

Earth electrodes may be made electrodes, natural electrodes, or special-purpose electrodes. Made electrodes include driven rods, buried conductors, ground mats, buried plates, and ground rings. The electrode selected is a function of the type of soil and the available depth. Driven electrodes are used where bedrock is 10 ft or more below the surface. Mats or buried conductors are used for lesser depths. Buried plates are not widely used because of the higher cost when compared to rods. Ground rings employ equally spaced driven electrodes interconnected with buried conductors. Ground rings are used around large buildings, around small unit substations, and in areas having high soil resistivity. Natural electrodes include buried water pipe electrodes and concrete-encased electrodes. The NEC lists underground metal water piping, available on the premises and not less than 10 ft in length, as part of a preferred grounding electrode system. Because the use of plastic pipe in new water ...

A bare rock mountaintop location provides special challenges to the facility design engineer. There is no soil, thus there are no ground rods. Radials are the only means to develop a ground system. Install a large number of radials, laid straight, but not too taut. The portions not in contact with the rock are in air and form an inductance that will choke the surge current. Because rock is not conductive when it is dry, keep the radials short. Only a test measurement will determine how short the radials should be. A conventional earth-resistance tester will tell only half the story (besides, ground rods cannot be placed in rock for such a measurement). A dynamic ground tester offers the only way to obtain the true surge impedance of the system. ROCK BASED RADIAL ELEMENTS On bare rock, a radial counterpoise will conduct and spread the surge charge over a large area. In essence, it forms a leaky capacitor with the more conductive earth on or under the mountain. The conductivity of the...

A facility that is down for even 5 min can suffer a significant loss of productivity or data that may take hours or days to rebuild. A blackout affecting a transportation or medical center could be life-threatening. Coupled with this threat is the possibility of extended power-service loss due to severe storm conditions. Many broadcast and communications relay sites are located in remote, rural areas or on mountaintops. Neither of these kinds of locations are well-known for their power reliability. It is not uncommon in mountainous areas for utility company service to be out for extended periods after a major storm. Few operators are willing to take such risks with their business. Most choose to install standby power systems at appropriate points in the equipment chain. The cost of standby power for a facility can be substantial, and an examination of the possible alternatives should be conducted before any decision on equipment is made. Management must clearly define the direct a...

A more sophisticated power-control system is shown in Figure 1, where a dual feeder supply is coupled with a motor-generator set to provide clean, undisturbed ac power to the load. The m-g set will smooth over the transition from the main utility feed to the standby, often making a commercial power failure unnoticed by on-site personnel. A conventional m-g set typically will give up to 0.5 s of power fail ride-through, more than enough to accomplish a transfer from one utility feed to the other. This standby power system is further refined in the application illustrated in Figure 2, where a diesel generator has been added to the system. With the automatic overlap transfer switch shown at the generator output, this arrangement also can be used for peak demand power shaving. Figure 1: A dual feeder standby power system using a motor-generator set to provide power fails ride-through and transient-disturbance protection. Switching circuits allow the m-g set to be bypassed, if necessary....

Engine-generator sets are available for power levels ranging from less than 1 kVA to several thousand kVA or more. Machines also can be paralleled to provide greater capacity. Engine-generator sets typically are classified by the type of power plant used: • Diesel : Advantages: rugged and dependable, low fuel costs, low fire or explosion hazard. Disadvantages: somewhat more costly than other engines, heavier in smaller sizes. • Natural and liquefied petroleum gas : Advantages: quick starting after long shutdown periods, long life, low maintenance. Disadvantage: availability of natural gas during area wide power failure subject to question. • Gasoline : Advantages: rapid starting, low initial cost. Disadvantages: greater hazard associated with storing and handling gasoline, generally shorter mean time between overhaul. • Gas turbine : Advantages: smaller and lighter than piston engines of comparable horsepower, rooftop installations practical, rapid response to loa...

Generators for standby power applications can be induction or synchronous machines. Most engine-generator systems in use today are of the synchronous type because of the versatility, reliability, and capability of operating independently that this approach provides. Most modern synchronous generators are of the revolving field alternator design. Essentially, this means that the armature windings are held stationary and the field is rotated. Therefore, generated power can be taken directly from the stationary armature windings. Revolving armature alternators are less popular because the generated output power must be derived via slip rings and brushes. The exact value of the ac voltage produced by a synchronous machine is controlled by varying the current in the dc field windings, whereas frequency is controlled by the speed of rotation. Power output is controlled by the torque applied to the generator shaft by the driving engine. In this manner, the synchronous generator offers preci...

An uninterruptible power system is an elegant solution to power outage concerns. The output of the UPS inverter can be a sine wave or pseudo sine wave. When shopping for a UPS system, consider the following: • Power reserve capacity for future growth of the facility. • Inverter current surge capability (if the system will be driving inductive loads, such as motors). • Output voltage and frequency stability over time and with varying loads. • Required battery supply voltage and current. Battery costs vary greatly, depending upon the type of units needed. • Type of UPS system (forward-transfer type or reverse-transfer type) required by the particular application. Some sensitive loads may not tolerate even brief interruptions of the ac power source. • Inverter efficiency at typical load levels. Some inverters have good efficiency ratings when loaded at 90% of capacity, but poor efficiency when lightly loaded. • Size and environmental requirements of the UPS system. High-power UPS equipmen...

Noise produced by backup power systems can be a serious problem if not addressed properly. Standby generators, motor-generator sets, and UPS systems produce noise that can disturb building occupants and irritate neighbors or landlords. The noise associated with electrical generation usually is related to the drive mechanism, most commonly an internal combustion engine. The amplitude of the noise produced is directly related to the size of the engine-generator set. First, consider whether noise reduction is a necessity. Many building owners have elected to tolerate the noise produced by a standby power generator because its use is limited to emergency situations. During a crisis, when the normal source of power is unavailable, most people will tolerate noise associated with a standby generator. If the decision is made that building occupants can live with the noise of the generator, care must be taken in scheduling the required testing and exercising of the unit. Whether testing occ...

The lead-acid battery is a commonly used chemistry. The flooded version is found in automobiles and large UPS battery banks. Most smaller, portable systems use the sealed version, also referred to as gel-cell or SLA. The lead-acid chemistry is commonly used when high power is required, weight is not critical, and cost must be kept low. The typical current range of a medium-sized SLA device is 2 Ah to 50 Ah. Because of its minimal maintenance requirements and predictable storage characteristics, the SLA has found wide acceptance in the UPS industry, especially for point-of-application systems. The SLA is not subject to memory. No harm is done by leaving the battery on float charge for a prolonged time. On the negative side, the SLA does not lend itself well to fast charging. Typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state because a discharged SLA will sulphate. If left discharged, a recharge may be difficult or even impossible. Unlike the c...

Out of sight, out of mind does not or, at least, should not apply to a facility ground system. Grounding is a crucial element in achieving reliable operation of electronic equipment. If a ground system has been buried for 10 years or more, it is due for an inspection. Soil conditions vary widely, but few areas have soil that permits a radial- or screen-based ground system to last much more than 15 years. The method of construction and bonding of the ground network also can play a significant role in the ultimate life expectancy of the system. For example, ground conductors secured only by mechanical means (screws and bolts, crimping, and rivets) can quickly break down when exposed to even mild soil conditions. Unless silver-soldered or bonded using an exothermic method, such connections soon will be useless for all practical purposes. The inspection process involves uncovering portions of the ground system to check for evidence of failure. Pay particular attention to interco...

All too often, ac power switchgear is installed at a facility and forgotten until a problem occurs. A careless approach to regular inspection and cleaning of switchgear has resulted in numerous failures, including destructive fires. The most serious fault in any switchgear assembly is arcing involving the main power bus. Protective devices may fail to open, or open only after a considerable delay. The arcing damage to bus bars and enclosures can be significant. Fire often ensues, compounding the damage. Moisture, combined with dust and dirt, is the greatest deteriorating factor insofar as insulation is concerned. Dust or moisture are thought to account for as much as half of switchgear failures. Initial leakage paths across the surface of bus supports result in flashover and sustained arcing. Contact overheating is another common cause of switchgear failure. Improper circuit-breaker installation or loose connections can result in localized overheating and arcing. An arcing fault...

Maintenance of the facility electrical system is a key part of any serious energy-management effort. Perform the following steps on a regular basis: • Measure the current drawn on distribution cables. Document the measurements so that a history of power demand can be compiled. • Check terminal and splice connections to make sure they are tight. • Check power-system cables for excessive heating. • Check cables for insulation problems. • Clean switchboard and circuit-breaker panels. • Measure the phase-to-phase load balance at the utility service entrance. Load imbalance can result in inefficient use of ac power. • Measure and chart the power factor of the load. Develop and post a simplified one-line schematic of the entire power network as well as other building systems, including heating, air conditioning, security, and alarm functions. A mimic board is helpful in this process. Construct the mimic board control panel so that it depicts the entire ac power-distribution sys...

Conserving energy is a big part of the power bill reduction equation, but it is not the whole story. The peak demand of the customer load is an important criterion in the utility company's calculation of rate structures. The peak demand figure is a measure of the maximum load placed on the utility company system by a customer during a predetermined billing cycle. The measured quantities may be kilowatts, kilovolt- amperes, or both. Time intervals used for this measurement range from 15 to 60 min. billing cycles may be annual or semiannual. Figure 1 shows an example of varying peak demand. If a facility operated at basically the same power consumption level from one hour to the next and one day to the next, the utility company could predict accurately the demand of the load, and then size its equipment (including the allocation of energy reserves) for only the amount of power actually needed. For the example shown in the figure, however, the utility company must size its equipm...

The kilowatt-hour (kWh) usage of a facility can be reduced by turning off loads such as heating and air conditioning systems, lights, and office equipment when they are not needed. The installation of timers, photocells, or sophisticated computer-controlled energy-management systems can make substantial reductions in facility kWh demand each month. Common sense will dictate the conservation measures applicable to a particular situation. Obvious items include reducing the length of time high-power equipment is in operation, setting heating and cooling thermostats to reasonable levels, keeping office equipment turned off during the night, and avoiding excessive amounts of indoor or outdoor lighting. Although energy conservation measures should be taken in every area of facility operation, the greatest savings generally can be found where the largest energy users are located. Transmitter plants, large machinery, and process drying equipment consume a huge amount of power, so particular ...

Be familiar with first aid treatment for electric shock and burns. Always keep a first aid kit on hand at the facility. Figure illustrates the basic treatment for electric shock victims. Copy the information, and post it in a prominent location. Better yet, obtain more detailed information from your local heart association or Red Cross chapter. Personalized instruction on first aid usually is available locally. Table lists basic first aid procedures for burns. Figure: Basic first aid treatment for electric shock. For electric shock, the best first aid is prevention. In the event that an individual has sustained or is sustaining an electric shock at the work place, several guidelines are suggested, as detailed next. Table: Basic First Aid Procedures a) SHOCK IN PROGRESS For the case when a co-worker is receiving an electric shock and cannot let go of the electrical source, the safest action is to trip the circuit breaker that energizes the circuit involved, or to...

Advantages of liquid fuels over the solid fuels The following are the advantages of liquid fuels over the solid fuels: (i) The handling of liquid fuels is easier and they require less storage space. (ii) The combustion of liquid fuels is uniform. (iii) The solid fuels have higher percentage of moisture and consequently they burn with great difficulty. However, liquid fuels can be burnt with a fair degree of ease and attain high temperature very quickly compared to solid fuels. (iv) The waste product of solid fuels is a large quantity of ash and its disposal becomes a problem. However, liquid fuels leave no or very little ash after burning. (v) The firing of liquid fuels can be easily controlled. This permits to meet the variation in load demand easily. Advantages of solid fuels over the liquid fuels The following are the advantages of solid fuels over the liquid fuels: (i) In case of liquid fuels, there is a danger of explosion. (ii) Liquids fuels are costlier as comp...

THERMAL CONSIDERATIONS The losses in the windings and the core cause temperature rises in the materials. This is another important area in which the temperatures must be limited to the long-term capability of the insulating materials. Refined paper is still used as the primary solid insulation in power transformers. Highly refined mineral oil is still used as the cooling and insulating medium in power transformers. Gases and vapors have been introduced in a limited number of special designs. The temperatures must be limited to the thermal capability of these materials. Again, this subject is quite broad and involved. It includes the calculation of the temperature rise of the cooling medium, the average and hottest-spot rise of the conductors and leads, and accurate specification of the heat-exchanger equipment. VOLTAGE CONSIDERATIONS A transformer must withstand a number of different normal and abnormal voltage stresses over its expected life. These voltages include: Operating...

The term load losses represents the losses in the transformer that result from the flow of load current in the win dings. Load losses are composed of the following elements. Resistance losses as the current flows through the resistance of the conductors and leads. Eddy losses caused by the leakage field. These are a function of the second power of the leakage field density and the second power of the conductor dimensions normal to the field. Stray losses: The leakage field exists in parts of the core, steel structural members, and tank walls. Losses and heating result in these steel parts. Again, the leakage field caused by flow of the load current in the win dings is involved, and the eddy and stray losses can be appreciable in large transformers. In order to reduce load loss, it is not sufficient to reduce the winding resistance by increasing the cross-section of the conductor, as eddy losses in the conductor will increase faster than joule heating losses decrease. When the cu...

Cathodic protection is the responsibility of the corrosion engineer or metallurgist. The subject is fundamentally reasonably simple to understand but can be extremely mathematical in its application. Direct current is arranged to flow out from the impressed anodes into the surrounding electrolyte, which is the sea water for offshore structures or the damp ground for onshore structures. The current returns through the structure itself and then back to the negative terminal of the impressed current source. The direction of current as described prevents the loss of metal from the structure into the electrolyte. This is opposite in direction to the natural current present due to corrosion action. The electrical engineer is not usually involved in the chemistry of the system; his work is mainly associated with sizing the AC and DC cables, accounting for the power requirements and ensuring that the equipment satisfies any hazardous area requirements that may exist. Impressed current systems ...

In order to understand the function of protective relaying systems, one must be familiar with the nature and the modes of operation of an electric power system. Electric energy is one of the fundamental resources of modern industrial society. Electric power is available to the user instantly, at the correct voltage and frequency, and exactly in the amount that is needed. This remarkable performance is achieved through careful planning, design, installation and operation of a very complex network of generators, transformers, and transmission and distribution lines. To the user of electricity, the power system appears to be in a steady state: imperturbable, constant and infinite in capacity. Yet, the power system is subject to constant disturbances created by random load changes, by faults created by natural causes and sometimes as a result of equipment or operator failure. In spite of these constant perturbations, the power system maintains its quasi steady state because of two basic fa...

Navigation aids consist of the following equipment: 1. Flashing marker lights. 2. Fog horns. 3. Platform nameplates. 4. Aircraft hazard lights. 5. Helideck landing facilities. 6. Radio communications and beacons. 7. Radar. 8. Echo-sounding and sonar. 1) FLASHING MARKER LIGHTS A typical requirement is that recommended by the British Department of Trade document ‘Standard Making Schedule for Offshore Installations’, 1. White and red lights flashing the Morse letters ‘U’ every 15 seconds as follows: Eclipse 1.00 s Flash 1.00 s Eclipse 1.00 s Flash 3.00 s Eclipse 8.00 s Total Period 15.00 s 2. Fog signals sounding the ‘U’ every 30 seconds as follows: Blast 0.75 s Silent 1.00 s Blast 0.75 s Silent 1.00 s Blast 2.50 s Silent 24.00 s Total Period 30.00 s 3. Illuminated identification panels. 4. Navigation buoys. 2) WHITE AND RED FLASHING LIGHTS The ‘normal’ range and ‘apparent intensity’ of these flashing lights should be in ac...

Normal lighting should provide approximately 75% of the total illumination an area of a plant that is densely filled with processing equipment and buildings. Sparsely filled areas such as road ways and perimeter fences can be fully illuminated with normal lighting, unless emergency escape routes exist in these areas. Emergency lighting should therefore provide between 25% and 30% of the illumination in processing areas. These criteria generally apply to both outdoor and indoor locations, and to onshore and offshore installations. Emergency lighting should be supplied by power from emergency diesel generators, except for lighting that illuminates escape routes. Escape route lighting requires a source of battery power that should last for at least one hour from a loss of all other power sources. The battery may be integral with the lighting fitting or a common battery and local distribution panel for a room or group of rooms, access ways, corridors and the like. The lighting level for es...

Inspection and testing of the purchased equipment is one of the most important tasks in the engineering of a project. Its importance is sometimes underestimated. The first serious tests that the purchaser will witness are those in the factory where the equipment is assembled. These tests will also include a physical inspection of the equipment. It is therefore important to state clearly in the specification what inspection and testing will be required and, where appropriate, what are the acceptable limits of the results. Most tests required in the oil industry are covered in international specifications and these can be used as references. However, not all those in the reference documents need to be carried out in all cases. It is therefore prudent to state the requirements in the project specification in one or more of the following methods: • Write a detailed description of exactly what is required, including the limits that are acceptable and the form in which the results should be ...

Oil industry equipment tends to be more robust than normal industrial equipment due to the often harsh and hostile environments in which it is expected to function without trouble for long periods of time. The indirect cost of equipment failures and outages is high and reliability is of paramount importance. An essential requirement is the definition of the degree of protection of the enclosure for the environment, which may be either outdoor or indoor, and hazardous or non-hazardous. The international standards most often used are IEC60529 and NEMA-ICS1-110 for the degree of protection against liquids and particles. These references are applied for the hazardous area protection. Wound components such as motor and transformer windings need to have their insulation specified to withstand the surface temperature of the copper conductors. IEC60085 and ANSI/NEMA describe the different classes of insulation that are normally available. Where IEC60085 or ANSI/NEMA is the reference, the two m...

The following format is reasonably typical of an equipment specification. Owners and purchasers, of course, have their particular style and preferences as to the order in which the paragraphs and clauses are placed in the specification document. Introduction. Scope of supply. Service and environmental conditions. Compliant international standards. Definition of technical and non-technical terms. Performance (or functional) requirements. Design and construction details. Inspection and testing. Spare parts. Documentation. Packing and transportation. Appendices, if necessary. 1) INTRODUCTION In this introductory section there should be a brief description of where the equipment is to be located, what type of installations will use the equipment and whether the environment is hazardous or non-hazardous (or both). 2) SCOPE OF SUPPLY A summary listing should indicate all the main components that constitute the equipment, e.g. AC generator, coupling, exciters, AVR, terminal ...