Debunking UPS misconceptions
During April residents and businesses in the greater Johannesburg area experienced many power interruptions, and have begun to question whether they are being told the truth.
Whether it is load shedding or too many maintenance interruptions, the discussion has returned to which uninterrupted power supply (UPS) will protect my business. EngineerIT discussed some of the misconceptions around which type of UPS would the right one for a particular application, with Warren Botten of Powerhouse Distributions (PHD).
There are basically three UPS types: online double conversion, offline UPS and line interactive UPS. But before we consider the various types of UPSs it is necessary to define the type of powerline disturbances that should to be protected against.
Dip or sag is a short-term decrease in line voltage, usually as a result of a short circuit in the powerline or a sudden increase in the load such as the start up of a large machine. It may also be as a result of utility switching or equipment failure.
Surge is the exact opposite of a dip. It is a temporary increase in line voltage that lasts at least one cycle or approximately 16 ms. It can be triggered by rapid reduction in power loads or by utility switching.
Electrical noise is the most common type of disturbance, which is a random high voltage of high frequency interference on the powerline caused by nearby non-linear loads. There are two types of noise. Common mode which is noise between power connection and ground and transverse mode which is noise between power connections.
Brownouts are a deliberate reduction in AC line voltage by the power utility during periods of unusually high demand or insufficient load capacity. The powerline does not have the full capacity to supply the load which results in load sharing.
Blackouts are the ultimate reason why one wants to invest in a UPS. It comes in various guises such as load shedding, scheduled maintenance and cable theft.
UPS types and protection offered
The online double conversion UPS isolates the load from the mains. All power to the load flows through the inverter and offers continuous protection and full isolation against surges, noise and sags.
The offline UPS is not suitable for all applications and the protection offered is limited. It does not use a static switch bypass and switching times are longer, typically between 4 to 8 ms. There is no isolation between the load and the mains and no output frequency control. The only protection it offers is against power failures.
The line-interactive UPS does not provide isolation between the mains and the load. Protection against spikes is poor. It does offer voltage regulation but no output frequency control.
UPS capacity is very much dependent on the battery used in the system and the capacity of the charger. Most smaller UPSs offer just enough time to power-down a PC. The only advantage of the newer models is automatic power-down which requires no action to be taken, the supplied software package will automatically start the power-down process thus protecting the integrity of the system.
If continued operation of the load is required, a larger capacity UPS is essential. Before embarking on buying a UPS or replacing an old one, you should fully consider the power requirements. Your existing UPS may be able to be upgraded by adding more battery capacity. It is however necessary to consider the charger capacity − whether it will be able to recharge the battery fast enough after one outage to be ready for the next one!
In our discussion on UPSs, Botten stressed that you get what you pay for. He said that is important to check the statement of what type of UPS you are being offered. Some suppliers state that their UPS is an online double conversion UPS when in fact it is not.
UPS misconceptions << comments and views
Loading ...