Transformers and transfer switches

Of course, this is a very basic definition for a transformer. Since there are no rotating or moving parts, in a sense a transformer is a static, mechanical device. Typically, a transformer operates on AC supply, although there are some DC transformers. A Transformer works on the fundamental principle of mutual induction.

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Types of Transformers

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are as follows:

• Step Up Transformers & Step Down Transformers - These transformers are generally used for stepping up and stepping down the voltage level in the transmission and distribution network.

• Three Phase Transformers & Single Phase Transformers - Three Phase Transformers are generally used in three phase power systems as it is more cost effective than with Single Phase Transformers. But when size matters, it is preferable to use a bank of three single phase transformers as it is easier to transform electricity this way, than with one single, three phase transformer.

• Electrical Power Transformers, Distribution Transformers & Instrument Transformers - Power transformers are generally used in the transmission network for stepping up or down the voltage level. It operates mainly during high or peak loads and has a maximum efficiency at or near full load. Distribution transformers step down the voltage for distribution purposes to industrial, commercial, institutional and residential facilities. It has good voltage regulation and operates 24 hrs a day with maximum efficiency at 50% of full load. Instrument transformers include Current Transformers and Potential Transformers, are used to reduce high voltages and current to lesser values which can be measured by conventional instruments.

• Two Winding Transformers & Auto Transformers - Two Winding Transformers are generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use Auto Transformers where the ratio between high voltage and low voltage is less than 2.

• Oil Cooled & Dry Type Transformers - In oil cooled transformers, the cooling medium is synthetic transformer oil whereas dry-type transformers are air cooled.

• Core type, Shell Type & Berry Type Transformers - Core type transformers have two vertical legs or limbs with two horizontal sections named yokes. Core type transformers are rectangular in shape with a common magnetic circuit. Cylindrical coils (HV & LV) are placed on both the limbs. Shell type transformers have a central limb and two outer limbs. Both HV, LV coils are placed on the central limb. Double magnetic circuit is present. In Berry type transformers, the core looks like spokes of a wheel. Tightly fitted metal sheet tanks are used for housing this type of transformer with transformer oil filled inside.

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How does an automatic transfer switch work?

An automatic transfer switch (ATS) is a self-acting, intelligent power switching device governed by dedicated control logic. The principal purpose of an ATS is to ensure the continuous delivery of electrical power from one of two power sources to a connected load circuit (electrical equipment – lights, motors, computers, etc.).

The control logic or automatic controller is typically microprocessor-based and constantly monitors the electrical parameters (voltage, frequency) of primary and alternate power sources. Upon failure of the connected power source, the ATS will automatically transfer (switch) the load circuit to the other power source (if it is available). As a general rule, most automatic transfer switches seek connection to the primary power source (utility) by default and will only connect to the alternate power source (engine-generator, backup utility) when required (primary source failure) or requested to do so (operator command).

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A typical transfer sequence includes:

1. The normal utility power source fails.

2. The transfer switch shifts the load to the emergency power source when power from the generator or backup utility feed is stable and within prescribed voltage and frequency tolerances. Depending on a facility’s needs and preferences, the transfer process is self-acting or manually-initiated.

3. The transfer switch returns the load from the emergency power source to the normal power source when utility power is restored. The retransfer process is self-acting or manually-initiated.

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