You should have wondered why some equipment need a better quality of power supply than some other conventional equipment, I mean equipment like computers, control systems etc. In engineering, we call this kind of equipment "critical equipment" (equipments that has a critical current and voltage rating which must not be exceeded for proper operation), they are very sensitive to the power supply that goes into them for processing of information, protection and even transfer of information.
This critical equipment has specification of the power supply that goes into them, this is very important for their operation. To meet this specification, we need the continuity and conditioning of power supply.
Critical equipment cannot handle large transients and large variation on the supply voltage.
Critical equipment is very sensitive even to the point that the waveform of the supply voltage matter, the waveform must be nearly sinusoidal. The waveform should have a frequency that is as stable as possible. Although some critical equipment like computers can withstand double the supply voltage for around 100micro seconds for 0.5s. Note that this is due to that fact that computers have energy absorbing capacity and filters for power supply.
And the truth remains that even though computers can withstand this severe conditions, we should not allow them do so. Now we have seen how that commercial power supply is not reliable for our critical equipment.
Moreso certain life supporting devices used in hospitals must never lack qualiity power when connected, where the public supply might not be good enough to ensure no downtime for these devices. therefore a system must be developed whereby constant quality power supply is ensured throughout the operaion of critical devices.
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This is what brought about the UNINTERRUPTIBLE POWER SUPPLY (UPS)
The fact remains that commercial power supply cannot supply all the necessary conditions to power a critical equipment so a UPS which can almost meet all the requirements for normal operations for the critical systems is implemented its needful to say, they are quite costly.
It must be in most of our minds now that asides the limited power supply in certain deveeloping countries, what else could cause reduction in the quality of public power supply
Earlier we talked about transients, which is one of the disturbance in the ommercial power supply, transients occur due to lightning and may results in the instantaneous immediate rise of the voltage by an amount over 100% in either part of the cycle be it negative or positive. Obviously, this may cause a damage to the to the equipment if proper care is not taken.
So transients is one of the reasons why commercial power supply is unfit for critical equipment .
Another reason is momentary over or under voltage
This is a very common disturbance that occurs in commercial power supply and is due to large change of loads in the same area. This may also be caused by some kind of faults in the system. Funny to know that this momentary over or under voltage may be caused by the erratic behavior of the critical equipment.
Also the supply voltage maybe become so low, even as low as zero. This conditions may lead to damage of the critical equipment.
Also, another cause of disturbance is the sustained under voltage which is when the supplied voltage is reduced below the allowable value for a considerable period, this is called voltage outage. This voltage outage may extend to a few hours. This also will cause a damage
Harmonic distortion is another disturbance present in the commercial supply and this occur when loads like large phase controlled rectifiers are connected to the line. They can also be called nonlinear loads.
Electromagnetic interference (EMI) caused by lightning, power network switch. This kind of disturbance can also be caused by ignitions of automobiles.
Also, proper grounding is also important for computers and this is achieved by PDU's (power distribution unit) which is kept on the floor of the computer room near the critical loads. The PDU contains isolating transformers with electrostatic shields between the primary and secondary.
The major function of the of the shield is to minimize common mode noise.
As I said earlier UPS are the best but are too complex and extremely costly so we can think of all other power conditioning. Like EMI filter, voltage refulators , isolation transformer ,dual feeders with static transfer switches.
Basically most UPS are the same in construction i.e constutuents and components, but operationally different because of difference in connction. this post would simply giveus an insight into these types which includes
The offline line preferred UPS This type of UPS is made up of some materials like controlled battery charger, battery ,inverter , two static switches ,all this work to form an OFFLINE line preferred UPS , its operation simply starts from the battery charger charging the battery fully leaving it at a float level also providing stabilized power to the inverter.
Here power is directly passed over to the load via one of the electric switches and when the utility supply is cut off the second static switch is used to connect the load to the inverter.
At this condition, the inverter starts to run from the battery, note that, the change that happens from line to inverter must only take 1/4 of the cycle if not it may destroy the critical equipment. When there is power outage, the battery has to supply power to full load so it discharges quickly. The backup of the battery is dependent on the ampere hour capacity.
Just that offline line preferred UPS cannot provide complete power protection and also the mode of operation has to be changed on power failure. i.e. from line to inverter and this type of operation is only limited to small systems where the operation is not highly critical.
OFFLINE INTERACTIVE UPS : Another type of UPS is the Offline line interactive UPS which is consists of a static switch, inductor, charger cum inverter and a line conditioning.
Here, the heart of the system is the charger cum inverter which can act a charger and also an inverter. It acts as a charger when the line supply is active and charges the battery till fill and keep it at a float level, when there is power failure, it acts as an inverter.
The switch and the inductor supplies the line power, when the battery charges, it does so at constant current to prevent over charging of current when the battery is in deep uncharged mode.
It has some disadvantages which is that when the charger cum inverter is damaged, the whole system is damaged and not even if the supply line is active until the charger cum inverter is totally removed which takes a lot of time. And it can only be applicable in small system with no critical operation.
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ONLINE inverter preferred, it offers full power protection, there is no change in mode of operation when there is power failure and absolutely no delay in change. Here, we have a manual switch which isolates the UPS to maintain and repair it without disturbing the operation of the line load from faulty supply line.
This post is just to introduce you to the concept of UPS and how they operate in systems , I hope by now you have a vivid understanding of UPS and why some systems cannot function well with utility power supply.
REFERENCES
For further research feel free to check the following
Difference between online and offline UPS
How critical power supplies work
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