Transfer Switches

Introduction

 Transfer switches are used in power electronics to provide controlled transfers between two power supplies. In this article, I will always refer to devices that are able to perform switching, both with synchronous and asynchronous supply sources, with a Break Before Make (BBM) operation. This last feature is particularly fundamental in order to avoid a paralleling of the two supplies and to avoid the neutrals getting cross-coupled. The break introduced with BBM has to be sufficiently small in order to avoid too long an energy gap to the load.

The use of transfer switches is used to increase the system MTBF through redundancy of supplies and by separating the loads. This is done in two typical ways.

In first place, the switch will perform a change on the source used for the supply of the load whenever the original source is out of tolerance.

Secondly, by separating the different loads, it is possible to avoid a problem happening in one load that will propagate also to the other loads.

This latter case takes place during a short circuit, hence the switch supplying this load will not permit a transfer on the other source while all the other switches will change the supply source protecting their loads.

To avoid propagation of a current fault to the alternate source, STS will not transfer if a load is faulty. STS will inhibit any transfer till the fault is cleared. If the fault is not cleared, STS protections will be actioned: the load and/or all loads connected to the STS will not be supplied. Multiplying available sources and limiting the number of loads connected to one STS reduce the System’s risk of failure.

Two technologies for Transfer Switches: STS and ATS

There are two main technologies used for these devices with different characteristics in terms of switching quality and cost each having its merits and deficiencies: Static Transfer Switches (STS) and Automatic Transfer Switches (ATS).

Static Transfer Switch (STS)

is based on static electronic components (SCR) therefore allowing for fast and precise control of the switching between one line and the other. This solution permits obtaining a perfect Break Before Make (BBM) behavior by never permitting a source to overlap. Moreover, it is also capable of very fast switching between the two sources with a max delay of less than 5 msec (typically 4 msec).

STS(Static Transfer Switch), is automatic static switching equipment designed to transfer critical loads between two independent AC power sources without interruption or with a transfer time of less than a cycle (20ms).

The STS consists of Silicon-controlled rectifiers (SCRs), control & sensing circuits to monitor the incoming sources and transfer the critical loads connected to the secondary source when the primary source fails. The schematic of an STS is shown below (figure 1)

 ATS

is based on electromechanical components where the BBM switching is actually made by controlling the relays on each source line. This kind of technology can still make a perfect Break Before Making changes to supply sources both in synchronous and asynchronous conditions but it is certainly slower than the static solution.

The right Transfer Switch for each load needs

By looking at the paragraph above it can be clearly understood that STS should be used in case of more critical loads where a longer voltage gap in the BBM procedure can be deleterious.

 ATS, on the other end, is still a reliable product that should be used to increase the overall reliability of an installation. Indeed, is a product with a very high MTBF value. On the other side, due to its intrinsic lower switching speed, should not be used where the loads are very sensitive to longer voltage sags (in the order of 6 msec). Anyway, ATS is certainly a lot more cost-effective product with respect to an STS one of the same rating.

Main features of STS and ATS

What are four types of automatic switches?

Types of Automatic Transfer Switches

• Open-transition switches.
• Standard or “fast” closed-transition.
• Delayed-transition systems.

Availability improvement with STS

The implementation of an STS will improve the MTBF and the availability of the critical loads.

In addition to the improved availability of power for the critical loads, the implementation of STS will also provide additional advantages like

• Fault Discrimination/Segregation Compartmentalisation of fault, avoid propagation of fault
• Improved utilization factor
• Optimize the capacity of UPS based on the load and improve the loading levels
• Better Efficiency
• Better PUE
• Lower Operational Cost
• The flexibility of Installation - adapt the installation based on the load levels
• Concurrent Maintenance - helps to maintain or repair the distribution system without impacting a downtime on the connected loads.