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Power distribution

Nov 30, 1995 12:00 PM

This category includes switchgear, switchboards, disconnect switches, panelboards, unit substations, metering, transfer switches, paralleling equipment, and MV and HV switchyard equipment.

Power Distribution equipment takes large blocks of power from the serving utility, on-site generator, or a combination of the two, and breaks it down into smaller blocks for utilization. Residential, smaller industrial, and commercial users are usually served and metered by the utility at the utilization voltage, which ranges from 120/240V single-phase for residential service to 208Y/120V or 480Y/277V, 3-phase, 4-wire for most commercial, some large multi-occupancy residential, and smaller industrial services.

Larger industrial and commercial users are most efficiently and economically served at medium voltage, usually at 4.16, 13.8, or 34.5kV. The utility distribution may be at still a higher voltage, transformed down to the service voltage in a switchyard and substation owned and operated by the serving utility or the customer (sometimes owned and operated jointly by both). Metering may be at the incoming or intermediate voltages, depending on economics.

Medium voltage distribution equipment

Medium voltage power is subdivided by metalclad switch-gear using [SF.sub.6], or air circuit breakers, or by metal-enclosed switchgear using fused switches. This power then goes to unit substations located conveniently near major loads. At the substations, the power is transformed to a lower utilization voltage and distributed by switch-gear to motor control centers, switchboards, panelboards, etc. Some power may also be distributed without transformation to medium voltage loads such as large motors.

Various radial, loop, or selective distribution systems require the ability to sectionalize, segregate, or tap one part of the distribution system from another. At medium voltage, various types of equipment are used to route power to substations. They include oil, vacuum, or SF6 tap or isolating switches.

Low voltage distribution equipment

Switchgear, switchboards, and power and lighting panelboards are used to further segment the blocks of power to feed specific loads connected to branch circuits. Low voltage switchgear are typically metalclad assemblies of hybrid or molded-case circuit breakers or fused switches. Power panelboards can be molded-case circuit breaker or fusible-switch type. Lighting panelboards are almost always circuit breaker type. Additional transformation may be necessary to obtain 120V for receptacles and incandescent lighting circuits.

Sectionalizing, segregating, or tapping of low voltage radial or loop distribution systems is accomplished by using fused isolation or disconnect switches.

Metering equipment

Metering can be done at any voltage. For multiple occupancies, meters are often grouped in prefabricated meter-bank assemblies. Larger services are individually metered, usually at the service voltage. Potential transformers (PTs) for 480V and above and current transformers (CTs) are used for metering and to supply relays, instrumentation, and controls.

Transfer and paralleling equipment

On-site generation has become common for emergency and standby power. It is often used for backup power to computer and other critical installations, and for peak shaving. Normally, the utility supply and onsite-generated power are separated, with simple or elaborate manual or automatic transfer schemes to switch loads from one to the other. Generator control and paralleling, bypass, and isolation equipment may be required for such systems.

Cogeneration, using one fuel to generate both heat and electrical power, may be economically advantageous in many instances, especially since excess electrical power can be resold to the utility. Such cogeneration often is operated electrically in parallel with the utility, requiring sophisticated metering, protective relaying, and synchronizing equipment to ensure safety and reliability for both the utility and the cogenerator.

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