ENGINEERING ARTICLES

Insulating media in high voltage transformers consists of paper wrapped around the conductors in the transformer coils plus mineral oil and pressboard to insulate the coils from ground. From the moment a transformer is placed in service, both the solid and liquid insulation begin a slow but irreversible process of degradation. 1) SOLID INSULATION: PAPER i) COMPOSITION OF PAPER: CELLULOSE Paper and press board are composed primarily of cellulose, which is a naturally occurring polymer of plant origin. From a chemical perspective, cellulose is a naturally occurring polymer. Each cellulose molecule is initially composed of approximately 1000 repeating units of a monomer that is very similar to glucose. As the cellulose molecule degrades, the polymer chain ruptures and the average number of repeating units in each cellulose molecule decreases. With this reduction in the degree of polymerization of cellulose, there is a decrease in the mechanical strength of the cellulose as well as a ...

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: ...

A permanent magnet dc motor (PMDC) is a dc motor whose poles are made of permanent magnets. PMDC motor offer a number of benefits compared with shunt dc motors in some applications. A PMDC motor is basically the same machine as a shunt dc motor, except that the flux of a PMDC motor is fixed. Therefore, it is not possible to control the speed of the PMDC motor by varying the field current or flux. The only methods of speed control available for a PMDC motor are armature voltage control and armature resistance control. ADVANTAGE: Since the motors do not require an external field circuit, they do not have the field circuit copper losses. Because no field windings are required, they can be smaller than corresponding shunt dc motors. DISADVANTAGES: Permanent magnets cannot produce as high flux density as an externally supplied shunt field. So a PMDC motor will have a lower induced torque per ampere of armature current than a shunt motor of the same size. Also, PMDC motors run the risk...

1) DELTA DELTA CONNECTION 1. Suitable for both un-grounded and effectively grounded sources. 2. Suitable for a three-wire service or a four-wire service with a mid-tap ground. 2) DELTA WYE CONNECTION 1. Suitable for both un-grounded and effectively grounded sources. 2. Suitable for a three-wire service or a four-wire grounded service with X O  grounded. 3. With X O  grounded, the transformer acts as a ground source for the secondary system. 4. Fundamental and harmonic frequency zero-sequence currents in the secondary lines supplied by the transformer do not flow in the primary lines. Instead the zero sequence currents circulate in the closed delta primary windings. 5. When supplied from an effectively grounded primary system does not see load unbalances and ground faults in the secondary system. 3) WYE DELTA CONNECTION 1. Suitable for both un-grounded and effectively grounded sources. 2. Suitable for a three-wire service or a four-wire delta service ...

When considering the design of an electrical distribution system for a given customer and facility, the electrical engineer must consider alternate design approaches that best fit the following overall goals. 1. SAFETY The No. 1 goal is to design a power system that will not present any electrical hazard to the people who use the facility, and/or the utilization equipment fed from the electrical system. It is also important to design a system that is inherently safe for the people who are responsible for electrical equipment maintenance and upkeep. The National Electrical Code ®  (NEC ® ), NFPA ®  70 and NFPA 70E, as well as local electrical codes, provide minimum standards and requirements in the area of wiring design and protection, wiring methods and materials, as well as equipment for general use with the overall goal of providing safe electrical distribution systems and equipment. The NEC also covers minimum requirements for special occupancies including hazardou...

Fuses are designed to melt and disconnect the circuit within which they are placed should the current in the circuit increase above a specified thermal rating. Fuses designed to be used in circuits operating above 600 V are classified as fuse cutouts. Oil-filled cutouts are mainly used in underground installations and contain the fusible elements in an oil-filled tank. Expulsion-type cutouts are the most common devices used on overhead primary feeders. In this class of device, the melting of the fusible element causes heating of a fiber fuse tube, which, in turn, produces de-ionizing gases to extinguish the arc. Expulsion- type cutouts are classified as: • Open-fuse cutouts • Enclosed-fuse cutouts • Open-link-fuse cutouts The automatic recloser is an overcurrent device that automatically trips and recloses a preset number of times to clear or isolate faults. The concept of reclosing is derived from the fact that most utility system faults are temporary in nature and can be cleare...

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...