Electric utilities supply grid power, also known as mains electricity in the UK and Canada. The passive standby topology feeds raw grid power to the load.
Under normal conditions, that is, when grid power is supplied, the UPS charges the storage device while feeding power to the load. When there is a grid failure, the UPS will disconnect from grid power and convert the DC power from the storage device to AC power with the inverter and feed the electrical equipment. A momentary interruption or break occurs during this switch.
But why is it necessary to disconnect from the grid before supplying power from the storage device?
If the power from the storage device is discharged without first disconnecting from the grid, power will not only flow to the load but back upstream into the grid. For this reason, it is necessary to disconnect from the grid and accept the time required to switch to the storage device.
In addition, after disconnecting from the grid, connecting to the DC power from the storage device generates noise which may cause malfunctions in the electrical equipment.
The advantages of the passive standby topology are that the power consumption of the UPS itself is low, its circuitry is simple, and it is small and inexpensive. This is because during normal conditions the UPS is simply feeding raw grid power to the load without processing it. On the other hand, the drawback is that voltage fluctuations occurring during switching from the grid power to battery power are significant.
In fact, voltage fluctuations* occur naturally within grid power so home appliances generally are designed to operate within ±10% of the grid voltage. Recently, the number of “wide range” electrical products that accept input voltages between 100 to 240 volts has increased so that they can be used anywhere in the world.
So to summarize, when a passive standby UPS is used, it is assumed that the connected load does not require clean, processed grid power and, therefore, will not be adversely affected by negligible voltage dips and fluctuations, and can tolerate the break that occurs when switching to battery power during an outage.
For this reason, such UPS are rarely found in close proximity to power supply equipment. Rather, small-capacity UPSs are typically distributed throughout a facility and placed near end-use electrical equipment.
* As per the provisions and regulations of the Electricity Business Act, it is necessary to maintain within 95 to 107 volts for 100 V standard voltage and within 182 to 222 volts for 200 V standard voltage.