|Uninterruptible power supply|
|Saturday, 21 October 2006|
The uninterruptible power supply apparatus(UPS) is generally used for keeping characteristics, such as voltage, frequency and waveform, of power supplied to a load constant even upon occurrence of service interruption, voltage drop, surge and the like in a commercial power supply system. The primary objectives of an uninterruptible electric power supply system are to sense normal power supply interruption, failure or inadequacy, switch electric power supply to a standby and/or emergency supply or switch loads from normal to emergency or limited loads upon interruption, provide long term standby or emergency power until normal power supply is again available, and provide transitional power and control during the period between loss of normal power and stand-by or emergency supply. Other useful functions include removing of unwanted spikes or transients from the normal power supply, power-factor correction, and monitoring/alarm to notify of the loss and/or operation of the uninterruptible supply. The available commercial power supplied by utilities arriving at the user's location is not sufficiently reliable to meet the power needs of such equipment. Commercial AC power waveforms are subject to many variations due to the demands of other users on the power line and other factors. Typical undesirable variations are over-voltage, under-voltage, voltage outages, and signal transients. Undesirable variations also occur due to load conditions, and line conditions. Commerical power is sometimes subject to complete outages, or complete failure at the power source, these conditions being known as blackouts. Due to inadequate capacity and increasing load demands, commercial power is subject to a condition known as brownout, sags which normally occur during peak demand periods and usually are typically represented by a 3% to 8% drop in magnitude of the available voltage. Commerical power is very frequently subject to magnitude and reactive instabilities causing irregular voltage waveforms due to transients induced by the action of various customers who subject the system to sudden electrical loads, power line switching equipment, or nearby high frequency noise generating equipment such as the motors of small appliances or hand tools, arc producing equipment such as fluorescent lights or switching-type DC power supplies. A UPS can be used to compensate for voltage sags in the line voltage, and may provide power to the various electronic and electrical systems coupled to it in the event that the line voltage suffers a voltage/current interruption.
The uninterruptible power supply is an AC power supply which is switched on to provide AC power at the instant that utility power drops below specified limits or is shut down, wherein an off-line UPS simply filters and passes the utility power to a load when utility power is normal. The UPS uses the commercially supplied AC power to charge a DC battery. The computer or electronics equipment, the load, then draws its AC power from the DC battery through the DC to AC inverter using appropriate circuitry. This isolates the load from power surges or brownouts and also furnishes a source of power during brief outages. UPS systems may provide uninterrupted power by switching from a primary power source to a secondary power source if loss of the primary power source is detected. An uninterruptible power supply device is generally configured such that the rechargeable battery is charged by electric power from commercial power supply, and in case of power outage, the rechargeable battery supplies the power stored therein to the electronic equipment to thereby provide backup power necessary for the operation of the electronic equipment. The UPS can be connected between the load and a source of electrical power, such as a mains power source or other commercially provided electric power. The UPS uses commercially supplied AC power to charge a DC battery and provide the AC power to load. Upon outages of power, the load draws its AC power from the DC battery typically through a DC to AC inverter. When the primary power source is restored, the UPS system may switch from the secondary power source back to the primary power source. Similarly, the UPS system may switch from the primary power source to the secondary power source if the UPS system determines that the primary power source is inappropriate. In order to prevent wasteful consumption of the power stored in the rechargeable battery, the uninterruptible power supply device is often configured so as to stop its operation after supplying power to the electronic equipment for a period of time necessary for the equipment to complete a predetermined process such as saving of data. In a typical UPS system, utility power is provided to the load during normal mode operation. In normal mode operation, the UPS system monitors the quality and the quantity of utility power provided to the load. During this mode, the UPS system provides power to the load that is substantially regulated to predetermined limits set for utility power distribution. In backup mode operation, the UPS system draws power from a backup system and provides it to the load. The UPS system may continuously utilize the backup system to provide power to the load until the utility power is restored.
Uninterruptible power supplies utilize a bank of electric storage batteries and solid state conversion and charging equipment to provide continuous electric power to electronic devices in the event of a loss of power from the utility or a deviation from the normal regulated utility specifications. An uninterruptible power supply generally comprises a rectifier, inverter, battery charger, DC battery, input filter/surge protector, controller, and other components, all housed within a single case or package. The input filter/surge protector receives input AC power from the AC power source through the input, filters the input AC power and provides filtered AC power to the transfer switch and the battery charger. The transfer switch receives the AC power from the filter/surge protector and also receives AC power from the inverter. The controller determines whether the AC power available from the filter/surge protector is within predetermined tolerances, and controls the transfer switch to provide the AC power from the filter/surge protector to the outlet. If the AC power from the rectifier is not within the predetermined tolerances, which may occur because of "brown out," "high line," or "black out" conditions, or due to power surges, then the controller controls the transfer switch to provide the AC power from the inverter. The DC converter is an optional component that converts the output of the battery to a voltage that is compatible with the inverter. The number of batteries contained within an UPS is dependent upon the business's length of time and its needs to operate in the event of a utility power system failure. Further, the number of power inverters required to supply the total load demand of a business also controls the size and number of inverters necessary in the UPS. The number of battery chargers is also somewhat dependent on these factors and the business's requirement for the speed at which discharged batteries are required to come back online. A battery system includes one or more battery cabinets. The batteries supply a DC power source needed to operate the UPS for a specified time. The battery cabinets hold a number of series and/or parallel connected batteries. The batteries require maintenance and cleaning from time to time. A UPS includes power conversion circuitry capable of conditioning poor quality AC voltage from the utility line, as well as generating high-quality output power from electric storage batteries. The UPS utilizes a multistage power converter to fulfill all of the requirements of the UPS system. A UPS unit is equipped a sensor for sensing a loss of power. When the sensor detects a loss of power, it immediately switches over to the battery so that continuous power is supplied to a computer system and the user has time to save crucial data and shut off the computer or other devices.
There are a variety of different types of uninterruptible power supplies, including on-line systems, line-interactive or off-line UPS systems, and off-line UPS systems. By supplying the power directly or through a voltage regulator, the off-line uninterruptible power supply is operated to filter out the noise and absorb the surge so as to be supplied to a load. When power in the input end is within a normal range, the UPS will filter out the noise from the power by and the filtered power is directly output for being supplied as a load. Off-line UPS systems do not regulate output voltage when the load is operating on utility power. As a result, off-line or off-line UPS systems are ineffective during power surges, spikes and brownouts, i.e. periods when a voltage reduction is initiated by a utility to counter excessive demand on its electric power generation and distribution system. An on-line UPS typically comprises a converter, inverter, and battery charger. The UPS converts the commercially supplied AC power to DC power and then back to AC power. This conversion isolates the load equipment from spikes and sags in the commercially supplied power and corrects variances in line voltages. DC power is used to charge a battery. The battery can then be used to supply AC power to the load when the commercially supplied power is interrupted. On-line systems carry the load power at all times through the UPS battery charger and inverter. On-line UPS systems are connected between the power line and the load to provide for continuous voltage regulation and suppression of transients and noise. The transfer times for such systems are extremely small, and so they are suitable for very sensitive or highly critical equipment. On-line UPS systems provide for an improved and more efficient utilization of input utility power, and the improved power factor helps lower energy costs. Line-interactive UPS systems normally do not carry the load power. Instead, under normal conditions they take in battery charging power from the main power supply through the inverter. In emergency conditions when the main power source fails, the line-interactive system supplies the power to the load. Line interactive UPS systems regulate voltage by adjusting the utility voltage before it passes to the load, and thus provide protection during brownouts and against power surges and spikes. However, these systems are unsuitable for use with sensitive equipment.