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What are the different types of transfer switches, and what do they do?

Following on from our previous FAQ feature on Automatic Transfer Switches (ATS), we thought we should also unpack the different types of transfer switches and their uses for different sites.

A transfer switch is the switching device that transfers power from one source to the other, typically the Mains power supply and a back-up generator.  That’s hopefully straightforward (click here to read some background on ATS), but from here we now have several options we unpack with our clients, based on the type of transfer switch and how their application can effect or better suit their site.  We will relate these to a typical mains generator application, but they can be applied to switching between other power sources also.

To simplify the system for quick reading, we’ve broken the options into four basic operations that can be modified to best suit the final requirements of the application.

  • Manual Transfer Switch
  • Automatic Transfer Switch
  • Closed Transition Switching / ‘bump-less’ Switching
  • Soft Transfer Application

Please note the operations following should only be undertaken by suitably trained and experienced technicians and an engineered solution should be designed with interlocks to prevent unsafe situations.

Manual Transfer Switches Firstly, we start with the simplest type a MANUAL TRANSFER SWITCH the name essentially describes the operation.  MANUAL intervention is required for the operation and switching action, so in the event of a power failure, an operator will acknowledge the requirement for action and MANUALLY start the generator then MANUALLY operate the manual transfer switch to transfer power from one source to the other. When the normal power supply is available the operator will again operate the manual transfer switch to switch power back to the normal source then allow the generator to suitably cooldown before stopping it. This switching operation will normally be an ‘open transition’ meaning there will be a definitive break or loss of power to the essential loads during the switching process.

Automatic Transfer Switches The next stage is basically automating the above process hence AUTOMATIC TRANSFER SWITCH, so the operation is the same, but the starting and stopping of the generator and operation of the transfer switch is all done AUTOMATICALLY – without the need for operator intervention.  This is normally open transition switching also.

Closed Transition Switching / ‘bump-less’ Switching We now head into the region of CLOSED TRANSITION SWITCHING with the simplest being a phase matching (sync check) type. This type of system requires a bit more design and integration and follows the same basic operation as the Automatic Transfer Switch but provides the facility of a ‘bump-less’ switching action. In the event of a normal power source failure the generator will start-up, and then is automatically switched on-line to provide power to the site.  The difference is when the normal power supply is available this system restores the site power from generator to the mains without interruption, hence the term ‘bump-less’.  This is achieved by a very fast switching action, and very fast load transfer, undertaken when the normal supply and the generator supplies are within predetermined phase matched parameters.

Soft Transfer Application Next, we can take the ‘bump-less’ operation into a smoother and more controlled SOFT TRANSFER application by actively controlling the generator speed and voltage to precisely phase match the power supplies with each other, then close switchgear to allow a gentle transfer of load from one source to the other. This type of application can be extremely flexible to permit ‘bump-less’ switching between: generator to mains, generator to generator, peak-lopping or embedded operation. This system requires more control & protection equipment with an application specific design and integration.

Hopefully this gives you an insight into the types of transfer switches we can integrate into your generator system, and the benefits of each option.  At Power Generator Systems, we can provide a transfer system to suit any application or requirement.  If you would like more information on what type of transfer switch will best suit your application, give our team a call on 08 8349 7854, or send us a message.

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