Static Switch Systems – Understanding How a Power Continuity Static Switch Operates

Description of the Static Switch Power Continuity System

A Static Switch is a two-way, three phase 4 ways automatic switch.
Powered by two independent synchronous or asynchronous AC power supply sources. The switching device are SCR. Under no operation conditions is simultaneous current switching between the two sources possible (Break Before Make).

The Static Switch makes a rapid switch from one source to the other.
This happens in the event of a fault to the power supply used to power the load, or carry out test/maintenance.
One of the two sources can be designated as the preferred power supply, to which the Static Switch will transfer the load. It remains there until different designations/faults require it to be switched to the other source. The Static Switch has 2 manual by-passes. Each source enables correct maintenance of the system.

STS 4P 100(400-600), a Static Switch is fitted with a block diagram with LED indicators and a 4 line 20char display.
Capable of providing all information concerning equipment operation status. Together with the power source, priority selection button enables operators to make full use of the apparatus.

Power Source Priority Selection

The manual selector A/B on the front panel of the STS selects one of the two input sources as the priority one.

The LED associated with the selected source SA or SB switches on.

The other source then becomes the reserve power source. The load supplied from the preferred source at all times. This is providing it is within acceptance parameters.

When a power failure occurs, the load is transferred to the reserve power source. When the preferred source returns within acceptance parameters, the load is automatically re-transferred. With a delay of 1 or 5 seconds (selectable)/even milliseconds delay.

Static Switch – Switching Due to Power Failure

Switching occurs if the characteristic parameters of the active power source (preferred or reserve) supplying the load go outside defined limits. The parameters tested are the root mean square (rms) and instantaneous values of voltage, which must remain within a defined acceptance window. Automatic transfer back to the priority source, once the parameters of the power source have returned to normal.
This happens if the load is supplied by the source selected as the reserve.

Where switching occurs while sources are unsynchronized, it is possible to choose between transferring in as short a period as possible. And to introduce a delay (selectable) in the normal transfer time (default condition).

Static Switching Due to Re-Transfer of Load to Priority Source

In conditions where the priority source is not the one supplying the load (switching due to fault condition or change of priority via button),
Static Switches will automatically transfer the load to the priority source as soon as possible.
Specifically, automatic switching to the priority source will occur only when the parameters of this source are within acceptable limits. When synchronization is established and remains stable within limits for a preset period. This is usually 1 second by default.
In the case of loss of synchronization, transfer will only take place when the phase difference between the two sources is less than a preset value (10° by default), and remain so for a preset period.

Static Switching – Must have reliable and robust sources

In conditions where the A feed source and the B Feed source are not in alignment the static switches will still operate.
Although, issues such as a phase shift on one of the source feeds could well cause the Static Switches to ’think’ longer before they switch. This is due to power aberrations.
The extended switching time will require that the down stream equipment is operating with internal backup. The industry standard is minimum 20 milliseconds.
Power Continuity comes at a price. That price has to ensure the Static Switches operate when called upon to feed the DC with an alternative robust power source but Power Continuity can only follow the most recent schematics available. If these are NOT up to date then the site clearly is running on assumption and not fact. 

What are the most common issues?

A common issue is that the servers and racks have not been maintained. This can mean that any internal UPS within the rack is actually ‘dead’ or even that any rack mount ATS has never been tested, hence when called upon; it fails. Another major flaw when using Static Switches is that no one in the Data Centre has reliably checked that each rack actually has two sources of power. Therefore, guess work assumes that all racks are dual fed. When the static switches operate, some racks fall over because they are fed from a single source with two feeds from that single source.

Static Switching – Parameters

Specifically, automatic switching to the priority source occurs only when the parameters of this source are within acceptable limits. Allowing synchronization to establish. If the priority source or either of the sources are out of limits, transfer will be effected only after it has remained stable and within limits for a preset period (1 second by default) but NOTE Synchronisation, transfer will only take place when the phase difference between the two sources is established by the intelligence with each Static Switch.So if your site develops issues that cause the Static Switch to have to ‘think’ longer before making the switch, then more milliseconds will be swallowed up whilst this is happening. 
To allow the Static Switch to operate Super Fast; Ensure that your site has, reliable and tested A& B feed supplies. That there is NO phase shift or Phase Rotation issues. Ensure that the transformers feeding the site are separate A & B feed transformers and finally NEVER feed the site from one single ring main. 
Lastly, regularly TEST to ensure nothing has been actioned on your incoming power supplies that could jeopardise the functionality of your Static Switches. Any maintenance/changes/upgrades/faults etc MUST be updated on the drawings.

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