3.9 Static transfer switches

AC power supply systems - fast switching in the AC circuits by means of a static-switch. Apart from the basic component of AC highly reliable power supply systems (UPS), as a next step fast static switches can be used to increase the reliability of power supply.

Figure 2 - Block diagram of static-switch and the oscillogram of the output voltage AC (Out)

Figure 2 shows a diagram of the static-switch and an example of the oscillogram of power supply reconnection from AC circuit (1) to AC circuit (2).

The control system of static-switch should provide very quick switching of power networks; there are switches with switching time below 6ms and return to basic power supply of 0.2ms. This ensures the uninterrupted operation of the loads even those, which are very sensitive to short supply voltage decays. Where the standard contactors are used, disruptions in power supply can last for as long as tens or even hundreds of milliseconds. Another advantage of static switches is that they do not generate switching overvoltages like standard contactors. So their application in environments sensitive to overvoltages, like these with inductive loads is recommended.

Static switches are also used for operation in three-phase networks for currents of a dozen or so to a few hundred amperes. An important feature of static switches is a very high instantaneous overload capacity (e.g. 2000%/20ms).

Figure 3 - Fast bypassing of a short circuit by means of the static-switch

If there are several circuits supplied by the UPS and a short circuit occurs in one of them, then the input terminals of the UPS are earthed and the rest of the circuits is not supplied. This lasts until the fuses in the shorted circuit are blown. Very quick switching of the bypass circuit ensures very quick reaction of the fuse and provides much shorter break in power supply of other circuits. Returning to power supply from the UPS is very quick, too. This guarantees uninterruptible operation of the correctly operating loads connected to the UPS.

If two UPS systems are used (fig. 4a), the reliability of power supply of lines 1L and 2L can be significantly increased by installation of two additional static switches (fig.4b).

Figure 4 - Improvement of power supply reliability by means of the static-switches

In this way we create a system in which each of the UPS units supplies ‘its own’ loads. In the event of a failure of one of the UPS, a static switch ensures very quick switching in such a way that the UPS which is operating at this moment correctly supplies both 1L and 2L circuits.

Instead of 2AC line, a power generator can be used, which ensures supply of lines 1L and 2L even in case of a long voltage decay in the power network.

The system shown also provides easy maintenance and service, which is important for a long lasting operation of the power supply system.