The line-interactive Uninterruptible Power Supply (UPS) provides a seamless and regulated output voltage. When the mains supply is within a preset input voltage or frequency, the output from the UPS is stabilised to within a specific voltage tolerance. This is achieved using voltage regulation known as Automatic Voltage Regulation (AVR), or Buck/Boost.
The area of variation is larger than that achieved with an on-line UPS, which will actually regulate the voltage to a much tighter tolerance. When the incoming mains supply exceeds the specified window, the UPS switches on the inverter to continue to supply power to the load. The inverter only takes a few milliseconds to engage.
When the mains supply is present it is filtered and stabilised by the UPS unit, then passed through to the load. When the inverter is off, the battery charger continues to convert the mains AC into DC to charge the battery. If the sag or surge exceeds the input window then the inverter is turned on to maintain the required output voltage tolerance.
The output to the load is filtered by the Electro Magnetic Interference (EMI) and noise filters within the UPS. These filters reduce the incoming transients, spikes or electrical noise down to an acceptable level. The disturbances won’t be completely prevented from passing through to the load and any large voltage spikes or transients that may exceed the input voltage window will cause the UPS to switch the inverter on.
Since the output of the UPS is maintained to within a specific range, in the event of a brownout, the UPS will automatically adjust the transformer tap settings, dependant on the incoming mains supply voltage. When the incoming mains supply exceeds the specified window, the UPS switches the inverter on to continue to supply the power to the load.
During short duration power supply failures the UPS unit will supply power to the load using the inverter. However, constant, short duration supply failures can lead to unnecessary discharge of the battery.
In the case of a power outage that last for a long duration, the load is supplied by the inverter which is switched on the moment the incoming mains supply fails. Some line-interactive UPS units have a battery extension pack capability. This enables additional battery packs to be connected for a longer supply of power. The addition of UPS batteries is recommended, because most line-interactive UPS units only provide a run time of approximately 10 minutes.
The battery charger will generally recharge the battery to 80% within 8 hours to provide sufficient battery life to prepare the UPS for when it’s needed again. When it is possible to extend the battery capacity, additional battery chargers can be added to maintain a reasonable recharge time.
The internal battery charger isn’t usually capable of supporting additional battery packs. This is because the UPS unit is typically built to meet a tighter cost specification than an on-line UPS. However, the security of data during a power failure is usually well worth the minimal added cost of extra battery packs and chargers.
The advantages of a line-interactive UPS are the size and cost, when compared to an on-line power solution. However, in comparison, they offer inferior power quality management capabilities, extended battery runtime options, have a short break in supply when the inverter is activated and do not have a built-in bypass facility for UPS fault or overload scenarios.
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