Electrical Power Plant Design: Transmission System

WEEK 07: TRANSMISSION: COMPONENT SPECIFICATIONS

Sections: Transformers | Circuit Breaker | Switchgear | Switchers | Fuses

Transformers for commercial and industrial electrical systems perform a wide range of functions. The category encompasses large units to many thousands of KVA for power.

Classifications Transformers maybe classified according to:

A. Type of Magnetic Circuit

1. Stacked laminations

a. Rectangular Core
b. Cruciform Core
c. Simple Shell
d. Distributed Shell

2. Wound Core

B. Number of Phases

1. Single-Phase

2. Three-Phase

C. Arrangement of Windings

1. Shell

2. Core

a. Cylindrical coils
b. Disk coils
c. Cylindrical and disk coils

D. Methods of Cooling

1. Natural convection and radiation
2. Air Blast
3. Oil-immersed self-cooled
4. Oil-immersed water-cooled
Forced-oil cooling

E. Type of Service

1. Constant Voltage

a. Power Distribution
b. Instrument
c. Autotransformer

2. Constant Current

3. Induction Regulator

a. Single-Phase
b. Polyphase

F. Special Features

1. Conservator

2. Inertaire

Large transformers are usually installed outdoors as individual units or as part of a substation; indoors as individual units as part of a unit substation or in vaults. They transform high- or medium-voltage primary service to in-facility distribution voltage, usually 4.16 or 13.8 kV or transform primary service or distribution to utilization voltage, usually 480V, 480Y/227V, or 208Y/120V; occasionally 2400 or 4160V.

Dry-type transformers are available up to 34.5kV, with impulse (BIL) ratings of 150kV or higher. Today, the 220°C insulation system is standard on all but the smallest units. Cast-coil dry transformers can withstand severe environments.

Liquid-type transformers are available with insulating fluids of silicone, high temperature paraffin derivatives and freon-type additives.

Specifications
Power transformers have rated constants such as (a) rated kVA; (b) Terminal Voltage, (c) Current; (d) Leakage Reactance ( a factor of the leakage flux, cross section and the number or turns); (e) Load Losses (the sum of resistance and eddy losses in all windings plus stray loss, also includes no-load loss); (f) Insulation (either liquid or gas such as petroleum-based or mineral oils, askeral, silicones, high-flash point hydrocarbons, clorinated benzenes, chloroflourocarbons, nitrogen, air, flourogases, Freon); (g) Cooling (including temperature rise, and temperature variations such as thermal transient conditions); (g) Regulation and Efficiency.

Circuit Breakers