Definition of UPS: A Uninterruptible Power Supply [UPS] is a device that maintains a continuous supply of electric power to the equipment by supplying power from a separate source when the main power supply is not available. The UPS is normally inserted between the commercial utility mains and the critical loads. When a power failure occurs, the UPS will immediately switch from utility power to its own power source.
Types of UPS:
Static UPS systems, which use power electronic converters with semiconductor devices.
Rotary (or dynamic) UPS systems, which use electromechanical engines such as motors and generators.
The combination of both static and rotary UPS systems is often called Hybrid UPS systems
Static UPS systems:
a. Static Uninterruptible Power Supply systems are based on power electronic devices. The continuous development of devices like IGBTs allows high frequency operation, which results in a fast transient response and low total harmonic distortion (THD) in the output voltage.
b. Solid state (Static) UPS system consists of several major elements like Rectifier & Battery charger, inverter, static transfer switch, battery bank.
c. All UPS systems have at least one large, low-frequency, magnetic component, usually a transformer. In early days these transformers are large and heavy.
d. Now a day’s smaller size, less expensive high frequency magnetics are used. The main advantages of higher frequencies are that acoustic noise can be reduced, and flicker components become smaller.
e. All UPS systems use an internal battery that produces AC power via an inverter.
According to the international standards, Static Uninterruptible Power Supply systems can be classified into three main categories:
i. Off-line UPS
ii. On-line UPS
iii. Line Interactive UPS
Off-line UPS
In the case of the off-line system, in normal operation power is supplied directly from ac mains.
In the event of mains failure, a transfer switch disconnects the power line and connects the inverter to the load.
When the mains power is restored, the load is reconnected to the power line.
Operation:
Mode 1 ( Mains ON)
When the mains is ON, the power is supplied to the load via the normally ON mains static switch.
The battery is charged through the charger
Mode 2 ( Mains OFF)
When the mains is off, the main static switch gets open circuited and UPS static switch are closed automatically.
Battery charging will stop and battery supplying power through inverter
The total time taken to sense the power failure and make a changeover from mains to UPS is about 5m sec.
Advantages:
Disadvantages:
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 of utility line condition.
These kinds 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.
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
Line Interactive UPS
- In line interactive UPS system, when the mains supply is present, the static switch is ON. The static switch connects load to mains supply through inductor L.
- The batteries are charged through the charger block.
- When main power supply is turned off, the mains static switch is open. Consequently the inverter turns ON and provides power to the load.
- When the main power is available, the Charger/Inverter block operates as a charger and when the main supply is not available it works as an Inverter.
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