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Uninterruptible Power Supplies

There are three types of UPS technology or topology defined by BS EN 62040-3:2011 including online, line interactive and standby/off-line uninterruptible power supplies. Each of the UPS topologies provides a different level of power protection from the basic protection provided by a standby UPS to the intermediate of a line interactive UPS and the ultimate or highest level provided by an online UPS. For help selecting the right solution please contact our projects team.

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UPS System Topologies

Static UPS are available in three core topologies, each of which provides a different level of power protection, and classified under BS EN 62040-3 to include:

  • Online UPS: Voltage and Frequency Independent (VFI-SS-111). The inverter is constantly running and digitally generates a sinewave output with a regulated voltage and frequency to supply the load. A rectifier is connected to an AC (alternating current) power source and converts this into the levels DC (direct current) required to charge a battery set and power the inverter section. When the mains power supply fails, the Inverter draws power from the battery set and internal capacitance is used to cover the change in DC power supply. This type of UPS is often referred to as ‘double conversion’ due to the AC-DC-AC conversion process or a ‘triple conversion’ UPS where the conversion is AC-DC-DC-AC. Online offer the ultimate in power protection and have a built-in automatic bypass for safe-failure-to-mains in the case of a UPS overload or fault condition.
  • Line Interactive UPS: this type of UPS system has an inverter section which only powers the load when the mains power supply fails or fluctuates outside a threshold window and is referred to as Voltage Independent (VI). The output frequency tracks the input when mains power is present. A line interactive UPS has an EMI/Spike suppression filter and automatic voltage regulator (AVR) or automatic voltage stabiliser (AVS) built-in to protect from sags, surges and brownouts. When mains power is present the battery set is charged and the inverter is powered but in a ‘sleep mode’. If there is a power outage the inverter activates and connects to the load in 2-4milliseconds and supplies power from the battery.
  • Standby UPS: a standby or off-line UPS is classed as Voltage and Frequency Dependent (VFD). The UPS has an EMI filter and the output supply tracks the mains power supply voltage and frequency. The inverter is off-line and only power up during a power outage to supply battery power to the load.

UPS Selection Table

Inside most modern electronic devices is a switch mode power supply (SMPS). This converts the incoming AC (alternating current) into the levels of DC (direct current) required to power internal circuits and processors. Voltage and frequency fluctuations can stress the SMPS and lead data processing errors, intermittent hardware faults and sudden failures.

UPS systems protect sensitive electronic and electrical devices by providing them with a cleaner, conditioned and regulated power supply and a source of battery backup if there is a power outage or the mains voltage and frequency drifts outside of the input window of the SMPS. The degree of power protection provided by the three UPS topologies varies and can be classified as Ultimate, Intermediate and Basic depending on the topology and level of sophisticated of the device.

The following table summarises the key differences between the UPS topologies:

UPS Type Protection Level Output Automatic Bypass Voltage Regulation No-break Output Size Range
Online Ultimate sinewave yes yes via the AC-DC-AC process continuously running inverter 400VA-800kVA, 1MW and larger
Line Interactive Intermediate sinewave, step wave or square wave* no yes with a built-in AVR/AVS 2-4ms 400VA-3kVA
Standby/Off-line Basic step wave or square wave* no no 2-4ms or greater 400VA-1.5kVA

Note: * product dependent – refer to specification

Static UPS systems are available from 400VA up to 1MW and larger. The systems topologies include monoblock transformerless and transformer-based designs and transformerless modular systems, desktop, tower, rackmount and floor standing. Online UPS systems can act as frequency converters from 50-60HZ or 60-50Hz. 400Hz systems are also available. Transformerless UPS have high operating efficiencies of up to 96% or greater when operated in full online mode. Eco mode type function allows the UPS to reach 99% efficiency.

Uninterruptible Power Supply Extended Runtimes

Some line interactive UPS systems can be installed with battery extension packs that plug into the rear of the UPS. The additional battery backup may be charged by the charging circuit built-into the UPS or may have its own charger and therefore require a separate AC mains power socket.

Online provide longer runtimes, either by replacing the internal battery for a higher Ampere-hour (Ah) rated battery or the use of external battery extension packs. External batteries may be supplied in a battery cabinet or on a battery stand. Provision may have to be made for additional charging if the UPS rectifier does not have sufficient charging capacity for the higher Ah-rated battery set.

Power Problems

UPS systems protect sensitive IT, electronic and electrical devices from mains power problems:

  • Power Outages:: include intermittent breaks in the supply caused by a local storm, lighting striking a part of the electricity distribution infrastructure or a break in the grid. Power outages can last from several seconds to several hours or longer.
  • Sags: lower than normal (in the UK 230Vac single-phase or 400Vac three-phase) mains power supply voltages for several cycles. As long as the reduced voltage is within the input window of the SMPS the electronic device will continue to receive the levels of DC power it requires but the SMPS itself has to draw more current. This can lead to heat and component stress within the power supply. Causes include heavy inductive loads starting up on site including motors, air conditioners or simply a larger number of electricity consumers on the grid. Solar PV feed-in supplies can also lead to higher than normal voltages in areas with a high concentration of solar PV farms and roof-top installations
  • Surges: higher than normal voltage levels again lasting for several cycles. If the surge voltage is too high, overvoltage protection built-into the SMPS may activate leading to a cut-out and load crash. Causes again include heavy inductive loads turning on or off.
  • Brownouts: are similar to sags but last for several hours or even days. Brownouts are more common with overhead land lines and during winter periods with high consumer demand on the grid.
  • Spikes and Energy Transients:: these burst of high energy, as high as 6kV or more that are too large or moving too fast for the EMI filter and circuit protection built-into a SMPS. Nearby lightning strikes are a common cause. Other causes include faulty electrical switches including lighting and inductive motors.
  • Electrical Noise: which can be classed as either (a) Common Mode – a disturbance between the mains power supply lines and earth (phase-to-earth or neutral-to-earth) or (b) Normal Mode – a disturbance between phase and neutral. Exposure over the long term to electrical noise can lead to stress induced component wear and failure. Causes include light flickers, poorly designed or maintained LV switchgear and cable faults.
  • Harmonics:: harmonic pollution in the UK is governed by G5/5 published by The Energy Networks Association. Harmonics are voltage or current waveforms superimposed onto the fundamental mains power supply waveform. In the UK (and Europe) the fundamental frequency is 50Hz (50 cycles per second). Harmonics are ordered: 2nd Harmonic 100Hz, 3rd harmonic 150Hz and so on. Harmonics can lead to a distorted mains power supply voltage, LV switchgear overheating and the nuisance tripping of circuit breakers.

The three UPS topologies differ in how they provide protection from power problems. A standby/off-line UPS provides basic protection i.e. spikes, transients and electrical noise are suppressed to lower levels, the incoming voltage and frequency is tracked until it is either too high or too low form the UPS to power the load without its inverter activating. Online UPS provide a digitally generated sinewave from a continuously running inverter to which the load is always connected during normal operation. If the mains power supply fluctuates or fails the battery set provides DC power to the inverter without a break. Line interactive UPS provide intermediate protection, superior to that of a standby/off-line UPS but not to the high-grade level of an onine UPS.

Rotary UPS Power Systems

Rotary UPS differ to static UPS systems and use a continuously rotating mass or motor to generate their electrical power. Rotary UPS systems run from several hundred kVA to Mega-Watt sized installations and require similar project and installation management planning to a generating set.

For more information on the static UPS topologies available from Server Room Environments please contact our projects team.