Application Engineering

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

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The heart of power transfer equipment is the transfer mechanism. There are four types in common use.

The chapter compares the relative benefits of contactor and circuit breaker–based; definite purpose and solid state transfer switches. Advantages in terms of cost, transfer time and fault level ratings are discussed.

Bypass arrangements, providing a means of isolating the transfer equipment for testing or maintenance without interrupting power flow, are illustrated.

The chapter ends with examples of closed transition transfer equipment that provide a smooth changeover by momentarily paralleling the two supply sources.

Requirements and Recommendations:

• Contactor type transfer switches provide a simple, low-cost solution. However some regions require that the contactors are mechanically latched – this adds cost and complexity, but leads to better reliability of service to loads and improved transfer switch life.
• Where non-latched contactors are used, a voltage dip on the phase that is used to supply the contactor coils may result in contactor drop-out and a total loss of supply.
• Contactors generally have low short-circuit withstand rating and normally require use of fuses upstream for short circuit protection.
• Circuit breaker-type transfer switches may be provided with interrupting or withstand ratings dependant upon the type of breaker. Upstream circuit breakers or fuses may still be required for protection.
• Definite purpose transfer switches are generally provided with "withstand and closing" ratings. They require upstream protection from either fuses or circuit breakers with instantaneous trips.
• Transfer switches must be electrically interlocked and preferably will also include a mechanical interlock. It may be difficult to provide mechanical interlocking in closed-transition applications.
• The definite purpose transfer switch uses a dedicated changeover mechanism to provide a robust and cost-effective means of transfer. The contacts will stay closed regardless of transient voltage dips or faults.
• Solid state transfer switches provide rapid, almost imperceptible transition between live sources.
• Whichever mechanism is chosen, the system design should consider the ability of the generator set to accept specific load step magnitudes. Multiple transfer switches allow for staging of loads and therefore minimize generator set sizing.
• Most transfer switches that include bypass arrangements allow the transfer switch to be isolated and withdrawn for maintenance without interruption to the load.
• While the bypass is in operation, automatic switching between sources is not possible, although manual switching may be permitted, dependant upon the design.
• Closed transition transfer switches transfer between live sources without a power interruption, but parallel time is very short (6 cycles). This results in sudden load application to the generator set that can cause unacceptable voltage or frequency variation. Extending the time in parallel and controlling the load application rate to the generator set can eliminate these power quality issues.
• Integrated automatic mains failure panels are available that combine generator set and transfer switch controls into a single unit that may be located with the power switching mechanism or at the generator set.

For information regarding your application contact your nearest distributor.

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