An uninterruptible power supply or UPS is a much-needed system for catering reliable, uninterrupted, high-quality power to vital loads such as data centers, critical health care support equipment, domestic equipment, and many more.
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Uninterruptable power supply is mostly a backup to the main power source from the utility which is prone to many kinds of electrical faults resulting in power outages. Since the power from the utility cannot be a reliable source, for many business, sensitive, and emergency applications, a backup system has to be installed to counter the power availability issue.
Apart from a backup of power outages, an Uninterruptible power supply or UPS system conditions the power so that the voltage transients, frequency variations, and harmonic disturbance cannot hamper the performance of the critical loads. It also provides protection against undervoltage and overvoltage conditions of utility power supply.
Types of Uninterruptible Power Supply or UPS
The uninterruptible power supply or UPS can be classified into two major groups:
- Static UPS
- Rotary UPS
Static uninterruptible power supply or static UPS
It is basically a solid state system with batteries as the source of backup power. The main components of this system are rectifier, inverter and the battery or batteries and transfer switch. It is the inverter which performs the power conditioning jobs.
The main advantages of using a static uninterruptible power supply or static UPS are:
- It is highly efficient.
- It is highly reliable.
- It provides faster response in power outages and voltage fluctuations because of its solid-state design.
- Its output has low total harmonic disturbance.
- However, the main disadvantages of this system are
- It has poor performance with nonlinear and unbalanced load
- The initial setup cost is very high.
Major components of Static UPS
Rectifier:
It is a device used for the conversion of AC power to DC power. In a controlled rectifier, the output DC voltage can be maintained at a desired level whereas in an uncontrolled rectifier, the output voltage is a fixed ratio of the input voltage at no load.
The rectification is basically done with the help of SCRs and IGBTs. These are solid-state devices that are capable of controlling the direction of the power, switching ON and OFF rapidly for conversion of power from AC to DC.
Inverter:
It is the unit that converts the DC to AC power. It is responsible for the quality of power fed to the load and generally has all the protection embedded to mitigate the under and overvoltages at the source. The common technologies present in inverters of UPS are pulse width modulation and constant voltage ferro resonant transformers.
In the pulse width modulation technique, the SCR of inverters is switched ON and OFF many times in half cycle which gives a train of pulses of the same amplitude and different widths. By utilizing this process the output wave can be turned into an approximate sine wave.
Ferro resonant transformers basically consist of a square wave inverter and a tuned output transformer which is responsible for limiting current and regulation of voltages. The transformer is equipped with a small secondary winding and a series low low-pass filters connected across the main secondary winding. The compensating winding maintains the output voltage in a narrow band while the series low pass filter offers low impedance to lower order harmonics and reduces the amplitude to an acceptable value.
Transfer switch
It provides the best alternative of power from two or more sources to the downstream connected load by switching into one. It is a high-speed device capable of switching in a fraction of a second as the connected load can tolerate minimal switching delays. It has complex sensors which monitor the Utility AC source and the inverter output which enables it to correctly switch the load to the available power at all times.
Battery
The battery used in the static UPS provides the DC power instantaneously to the inverter when the utility AC fails or the voltage degrades. Various options of battery are available such as VRLA, Lead-acid, nickel cadmium, etc. Out of these nickel-cadmium offers a high storage capacity at a high cost which is suitable for compact space and low-temperature applications.
Battery charger
The battery charger charges the battery of the system. It maintains a correct balance between overcharging and over-discharging of the battery in the connected system. The charger ensures that the discharging is limited to to a point where the cells approach exhaustion or where the voltage decreases below the optimum level which is 80% of the rated capacity. Overcharging results in high water use while over-discharging raises the temperature. The rating of the charger depends on the load and inverter efficiency.
Operations of static uninterruptible power supply
The static uninterruptible power supply system basically has three operating modes
Normal operating mode:
In this mode, the rectifier/charger unit of the module converts the AC from the main power supply to DC for charging the battery or battery bank connected to it. Also, the DC is supplied to the inverter. The inverter converts the DC received from the rectifier to AC and supplies it to the connected load.
Stored Energy mode:
When the AC input is lost for some reason in the power supply utility or failure of the rectifier unit, or as the voltage dips below the tolerable preset limits, the battery comes into action for maintaining the DC supply to the inverter unit. This mode continues operation until the battery gets fully discharged or the AC power at mains or the rectifier or voltage issue, whichever be the case, is restored.
When the AC power is restored, the rectifier’s output voltage is set equalizing to the preset voltage, the DC from the rectifier output will continue to charge the battery while maintaining the DC supply to the inverter for conversion of this DC to AC, supplying the load. This situation will prevail until the battery gets charged thus making the charger return to floating mode and the system, to normal operating mode.
Bypass mode
In the event of failure of the inverter unit, the static switch transfers the connected downstream load to the main AC source. The bypass switch is essential in clearing the downstream faults of the uninterruptible power supply or UPS system.
The static uninterruptible power supply UPS can have three types of bypass arrangements. The UPS static switch, the UPS static circuit breaker, and the maintenance circuit breaker.
Types of static UPS system
The components of the static uninterruptible power supply UPS can be assembled in many configurations to suit specific applications. However, among them, three are most commonly used and are standardized by IEEE 446. They are discussed below
Offline uninterruptible power supply UPS or standby UPS
In this configuration, the power is derived directly from the utility main power line. After the event of main power failure, the transfer switch is used for the inverter to derive power from a battery and continue the power supply. The battery is charged when the main power from the utility is available.
Here, the transfer switch is operated for switching the power from mains of utility to inverter and vice-versa. The time required for the inverter to come alive is called as switch over time.
This design or configuration only helps the load for a backup power source and does not help in mitigating voltage fluctuations, harmonic disturbances, etc.
The main characteristics of this configuration are
- It has no regulating capabilities to the incoming power as the load is directly connected to the mains and hence is not recommended for networking applications.
- The loads which can survive the power line transients are recommended for this configuration.
- It provides backup power and does no power conditioning.
- Switch over time interruption is common for the loads during backup events.
- The backup output power results in square wave or quasi sine wave
- The rectifier only supplies the no-load current to the inverter, maintaining the battery bank in a floating condition.
- Typically for single-phase applications
- The failure of the inverter unit leads to a loss of power to the downstream loads.
- Due to the limited overload capacity of the inverter elements, this system is not suitable for supplying power to loads that require high inrush current.
Online protection UPS or Line interacting uninterruptible power supply
This configuration utilizes a stabilizer which functions to supply conditioned power to the load maintaining a stabilized voltage and mitigating any fluctuations or surges. In the normal mode, the load is supplied power from the mains via the stabilizer and at the same time charging the battery of the system. The battery supplies power to the inverter in case of a utility power outage. The transfer switch is used for switching between utility power and inverter.
The main characteristics of this system are:
- Suitable for places with high voltage fluctuation
- The load derives power via magnetic components
- Provides voltage conditioning but frequency variations are passed to the load.
- It is not recommended for usage with sensitive loads such as medical equipment etc
- Typically used for load up to 100KVA and 1 or 3-phase applications.
Double conversion uninterruptible power supply or online UPS
In this configuration, the connected load always receives the power from the inverter. The rectifier and the inverter operate all the time since the load is not connected directly to the main utility power source. The utility power source is connected to the rectifier which supplies the DC for charging the battery and to the inverter. The inverter supplies the AC to the load thus always maintaining a clean conditioned power to the connected load.
In the event of failure of the inverter or rectifier, the transfer switch is used for switching power directly from the utility source. In normal operating mode the battery of the online uninterruptible power supply or UPS system is always in floating mode, and parallelly gets charged when the utility main supply is available so that it does not get discharged. At the failure of the main supply, the battery bank continues the power supply to the inverter, thus maintaining the supply to the load.
Major characteristics of this system are
- The input voltage is directly supplied via the UPS and hence there is zero switchover time.
- Seamless power drawl from DC source when input voltage is lost
- It can compensate for under-voltage and over-voltage without using the battery, thus saving battery time for use only in the main source is unavailable.
- The power supply to the load is totally conditioned.
- Single-phase or three-phase applications with ratings exceeding 1000 kVA
ROTARY UPS SYSTEMS
This configuration uses a synchronous motor coupled to a synchronous generator with a large flywheel. During the normal operation the motor drives the flywheel and the synchronous generator at constant speed. The output voltage of the generator is regulated by the voltage regulator and the frequency is constant and proportional to the motor speed.
When input power is momentarily lost or degrades, the stored energy of the flywheel is supplied to the generator and thus frequency is maintained within the tolerable limit.
During the stored energy mode of operation, the battery bank supplies the DC motor, which in turn drives the AC generator and flywheel. The generator thus supplies power to the load.
The rotary UPS systems are more reliable than the static UPS systems. However, it needs more maintenance and has a much larger size and weight. The main advantage in high-power applications is that the transient overload capability is 300 to 600% of the full load for rapid fault clearing. The transient overload capacity of Static UPS is 150% for the short term. Because of low output impedance the performance of rotary UPS system is better with nonlinear loads as well. The input total harmonic disturbance is very low mostly below 3%, the electromagnetic interference is also very low. However, the efficiency of the rotary UPS system is 85% or above.
Applications of uninterruptible power supply or UPS
- It is extensively used for Computers and peripherals such as printers, monitors, speakers, modems, etc.
- It is used in communication systems, machines, OA electrical equipment, etc.
- It is used in industrial instruments-equipment, measuring instruments, electrical equipment, etc.
- In the medical industry, it is used for critical life support applications such as in the operation theatres of hospitals, intensive care units, emergency medical equipment, surgical equipment, etc.
- It is used in various telecommunication systems for Communication equipment, microwave networks, traffic control systems and satellite earth stations, police communication networks, aircraft landing systems, repeater substations, telecommunication control rooms, etc.
