Electrical Power Plant Design: Distribution System

WEEK 13: DISTRIBUTION: EQUIPMENT

Distribution Transformer is certainly the most important in Distribution Systems. Without which, it would certainly be impossible to distribute power over such long distances. The purpose of a transformer is to step up or step down voltage. In the case of a distribution transformer, the voltage is stepped down from that of the primary mains of a distribution circuit to that of the secondary main. In most cases, this is from 2,400, 4,160 or 13,800 volts to 120 or 240 volts.

DIstribution transformers are generally considered as transformers 500 kVA and smaller, 67,000 V or below, both single-phase and three-phase. Older installations are primarily pole/platform-mounted units. newer installations are frequently pad-mounted units. Typical applications are for supplying power to farms, residences, public buildings or stores, workshops and shopping centers.

Distribution transformers have been standardized as to high and low voltage ratings, taps, type of bushings, size and type of terminals, mounting arrangements, nameplates, accessories, and a number of mechanical features. They are normally designed for 65°C rise. The most popular primary voltages are 12,470 wye/7200, 13,200/7620. and 12,000 V delta. Secondary voltage is usually 120/240 or 240/480. 240/120 for a single-phase pad mount units and 208-wye/120 on three-phase units. In the Philippines, this is usually stepped down to 220 or 440 volts. Accordingly, Meralco distribution pole-mounted transformers are rated from 15 kVA to 333 kVA three-phase with a maximum connection of three (3) units per pole, that is about two pounds per pole.

There is essentially no difference in the fundamental design of transformers for either power or distribution. The latter are designed, however, with very small no-load losses. They are usually mounted direct to the pole, on a platform or inside a manhole.

Classification Most distribution transformers consists of (1) a closed-loop magnetic core upon which are wound two or more separate coils, (2) a tank which the core-oil assembly is immersed in cooling and insulating oil, (3) bushing for bringing the incoming and outgoing leads through the tank or cover. There are two basic types of distribution transformers: the conventional type and the self-protected type. The Conventional Pole Type includes just the basic transformer structure without any protective equipment. It consists of core and coils securely mounted in an oil-filled tank, with the necessary terminals brought out through their appropriate bushings. The desired overvoltage, overload, and short-circuit protection is obtained by using lightning arresters and primary fuse cutouts separately mounted on the pole or crossarm closely adjacent to the transformer. The primary fuse cutout provides a means of visually detecting blown fuses on the system primary and also serves to remove transformer from the high-voltage line, either manually when desired or automatically in the event of an internal coil failure. The Self-Protected Transformer, more popularly known as CSP for completely-self-protected, has an internally mounted, thermally controlled secondary circuit breaker for overload and short-circuit protection; an internally mounted protective link in series with the high-voltage winding to disconnect the transformer from the line in the event of an internal coil failure; and a lightning arrester integrally mounted on the outside of the tank for overvoltage protection.

Mounting Distribution transformers are almost always located outdoors where they are hung from crossarms, mounted on poles directly or placed on platforms. Transformers are also installed on street vaults, in manholes, on pads at ground level, subsurface, within buildings, or are direct-buried when underground construction is employed. When installed within buildings where the possibility of submersion is remote, the overhead or inside types of transformer and cutout are used. Transformer vaults within a building are of fireproof construction, except when transformers are dry type or filled with nonflammable liquid.

In the Philippines in general, transformers up to a total of 75 kVA size are mounted directly on the pole or on the crossarm and larger size transformers or groups of several transformers are placed on platforms or mounted on poles in banks or clusters. Modern pole-mounted have two lugs welded directly on the case; these lugs engage two bolts on the pole from which the whole apparatus hangs securely. This method, which is known as Direct Mounting eliminated the need for crossarms and hanger irons thus saving considerable amount of material and labor. Hanger-Iron Method uses the crossarms and a hanger-iron, two flat pieces of steel with their top ends bent into hooks which squared sides. The transformers are bolted to these pieces of steel, the assembly raised and then lowered so that the hooks on the hanger irons would engage the crossarm. The T-shaped Hanger-iron requires two crossarms. The C-shaped hanger-iron is designed to hold larger transformers. For smaller transformer, a T-shaped hanger-iron, one crossarm, and a kicker, the method known as the Kicker-T and Hanger-Iron, are sufficient. NOTE: A transformer should not be mounted on a junction pole, a pole supporting lines from three or more directions, as this makes working on such a pole more hazardous for linemen. When transformers cannot be mounted on poles because of size and number, they maybe installed on an elevated platform or a ground-level pad. platforms are built in any shape or size to suit the particular need. They are usually constructed of wood, through steel is often used, as well as concrete with sufficient drainage canals. This structure is often placed on the customer's premises to reduce the distance that secondaries must be run and to avoid pole congestion on public thoroughfares. Ground-level pads are usually made of concrete or reinforced concrete and have provisions for enclosing the transformers within a fence or wall or other enclosure for safety reasons. Ground-level pads are very useful when appearance is a major consideration.

Banking is the tying together the secondary mains of adjacent transformers supplied by the same primary feeder. When used, it is usually applied to secondaries of single-phase transformers, and all transformers in a bank must be supplied from the same phase of the primary circuit. Advantages claimed for banking compared to secondary radial distribution are (1) reduction in lamp flicker caused by starting motors, (2)less transformer capacity required because of greater load diversity among a larger group of customers, (3) better average voltage along the secondary, and (4) greater flexibility for load growth. Two general types of secondary banking: the grid and the straight-line. In the grid system, a transformer share two same-phase secondaries, while in the straight line, a series of transformers share a single line.

Distribution: Equipment: Voltage Control