Complementary equipment for UPS
A two-winding transformer included on the upstream side of the static contactor of circuit 2 allows:
- A change of voltage level when the power network voltage is different to that of the load
- A change of system of earthing between the networks
Moreover, such a transformer:
- Reduces the short-circuit current level on the secondary, (i.e load) side compared with that on the power network side
- Prevents third harmonic currents which may be present on the secondary side from passing into the power-system network, providing that the primary winding is connected in delta.
The UPS system includes a battery charger which is controlled by thyristors or transistors. The resulting regularly-chopped current cycles “generate” harmonic components in the power-supply network.
These indesirable components are filtered at the input of the rectifier and for most cases this reduces the harmonic current level sufficiently for all practical purposes.
In certain specific cases however, notably in very large installations, an additional filter circuit may be necessary.
For example when:
- The power rating of the UPS system is large relative to the MV/LV transformer suppllying it
- The LV busbars supply loads which are particularly sensitive to harmonics
- A diesel (or gas-turbine, etc,) driven alternator is provided as a standby power supply
In such cases, the manufacturer of the UPS system should be consulted
Communication with equipment associated with computer systems may entail the need for suitable facilities within the UPS system. Such facilities may be incorporated in an original design (see Fig. N31) , or added to existing systems on request (see Fig. N32).