Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

Year: dec12, Dec11

On-line UPS

System components:

• As shown in the above figure, the major elements of online UPS are (i) Rectifier and Battery charger (ii) Inverter (iii) Static transfer switch (iv) Logic and control system (v) Battery bank (vi) Diagnostics and communication blocks.

• In medium & large capacity online UPS systems good amount of power semiconductors like Power MOSFETs, GTOs, MCT, Darlington pairs are used.

• As they are handling high voltage and current, they are mounted on specially designed heat sinks. They play as a control elements in the rectifier blocks and the inverter.

Operation:

• During normal or even abnormal line conditions, the inverter supplies energy from the mains through the rectifier, which charges the batteries continuously. In addition to that it can also provide power factor correction.

• When the line fails, the inverter still supplies energy to the loads from the batteries.

• As a consequence, no transfer time exists during the transition from normal to stored energy modes.

• In general, Online UPS system is the most reliable UPS configuration due to its simplicity (only three elements), and the continuous charge of the batteries, which means that they are always ready for the next power outage.

• This kind of UPS provides total independence between input and output voltage amplitude and frequency. So high output voltage quality can be obtained.

• When an overload occurs, the bypass switch connects the load directly to the utility mains, in order to guarantee the continuous supply of the load, thereby avoiding the damage to the UPS module (bypass operation).

• In this situation, the output voltage must be synchronized with the utility phase, otherwise the bypass operation will not be allowed.

• Typical efficiency of the online ups systems are up to 94%, which is limited due to the double conversion effect.

• Online UPS systems are typically used in environments with sensitive equipment or environments.

• Almost all commercial UPS units of 5 kVA and above are Online UPS Systems.

Features of on-line UPS

• A well designed online UPS protects against blackouts, surges, sags, spikes, transients, noise and brownouts.

• Previously the price of on-line ups systems is more than other ups systems. Nowadays the price reduces drastically.

• Since they continuously regenerate clean AC power, they provide the highest level of protection available, regardless fo utility line condition.

• These kind of UPS have much more complex designs than off-line or hybrid types, while the price, weight and volume are higher.

• The inverter of an online ups system supplies continuous power to the critical load. Under conditions of overload or loads with high in rush currents which is beyond the capacity of the inverter, the static bypass switch provides mains power to the load.

Specifications of on-line UPS

Advantages of On-line UPS System:

• No switching involved

• 100 percent line conditioning and regulation

• Good sustained brownout protection

• Typically sinusoidal output

• Power factor correction and higher reliability