Application Engineering

Please read the Warranty Statement carefully before proceeding with any design or ordering materials or labor.

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The type of control system for the transfer switch depends on the type of switching equipment employed. The chapter describes alternatives - from manual transfer switches with no electrical controls, to closed transition transfer switch requiring a relatively complex, microprocessor-based control.

The chapter discusses the need to detect voltage and frequency excursions. These must be timed, to avoid a transient condition initiating transfer. Optional functions allow voltage imbalance, loss of phase and incorrect phase rotation to be monitored.

An optional generator exerciser control may be included, which allows the generator to be automatically tested.

Requirements and Recommendations:

• Non-automatic transfer switches are controlled by a local operating switch or an external source but are electrically actuated.
• Automatic transfer switches monitor two power sources and provide an automatic transfer between the sources in the event of failure or out-of-tolerance condition of the preferred source.
• Source failure sensing for automatic switches always includes under voltage, typically for all three phases. Sensing may also include over/under voltage, over/under frequency, voltage imbalance and loss of phase.
• Timing is incorporated in the sensing arrangements to avoid unnecessary starting of the generator set(s), stabilization of the generator set(s) prior to connecting loads, verification that the normal source is stable before re-transfer and a cool-down cycle for the generator set(s).
• Fast switching times between live sources can result in problems when switching motor and other inductive loads. An intentional delay of 0.5 – 1.0 seconds allows load-generated voltages to decay, preventing the surge current and potential damage to loads.
• Transfer switches can also be used to transfer load between utility-to-utility, utility-to-generator and generator-to-generator.
• The user should determine the desired settings for the various detection systems, based on actual system parameters for voltage and frequency and the tolerance of the load equipment.
• Three-phase systems should be equipped with three-phase sensing systems unless loss of one phase would not result in problems with the load equipment.
• Typical settings for under-voltage sensing are: drop-out at 75% with pick-up at 85%. These may be provided with user or dealer adjustment. Typical time delays are 0.5 – 1.0 second although many controls allow longer time settings to be achieved.
• Optional loss of single phase protection will detect situations where a disconnected phase is held at a relatively high voltage by the generator action of motors, etc. This protection verifies that the magnitude and the phase-angle of the sensed voltage are within limits.
• Optional voltage imbalance sensing reduces the possibility of generator overloading due to high negative sequence components caused by excessive load imbalance.
• Optional phase rotation protection will prevent the connection of a source with incorrect phase rotation. This is particularly important where mobile generators provide an alternative supply and is often a requirement for fire pump installations.
• An exerciser clock allows the generator and transfer equipment to be tested at times of least inconvenience. If using an exercise clock, ensure that this is reset to allow for daylight saving times and leap years.

For information regarding your application contact your nearest distributor.

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