Modular UPS Systems

Power Solutions For Power Quality Issues

Power solutions can be classified into those that include some form of battery backup including uninterruptible power supplies and those that do not. Selection depends upon selecting the most appropriate power solution for the load size, resilience level required and power problems to be protected from.

Uninterruptible Power Supplies

UPS Systems are classified according to EN/IEC 62040-3 which defines three generic types of Static UPS topologies based on their output waveform and dynamic performance:

  • Voltage and Frequency Independent (VFI): referred to as On-line, Double Conversion and Static UPS. In this type of UPS system the output is voltage and frequency independent because the inverter is normally constantly running and powering the load from a digitally generated sinewave. Frequency variations are maintained within the limits described by EN/IEC 61000-2-2. VFI-SS-111 is the highest classification for an On-line UPS system. On-line UPS can be transformerless and transformer-based with some systems offering modular, scalable and parallel/redundant operation.
  • Voltage Independent (VI) referred to as Line Interactive or Interactive UPS. In this type of UPS mains voltage sags, surges and brownouts are stabilised and the output waveform regulated within specified operating limits. Outside of these the inverter (normally off) powers the load. The output frequency tracks the input frequency until this falls outside specified operating limits at which point the inverter powers the load. This type of UPS will include some general filtering for spikes and electrical noise.
  • Voltage and Frequency Dependent (VFD): referred to as Off-line or Standby UPS. Here the UPS system tracks the input voltage and frequency and simply engages its inverter when their levels fall outside a set specification. This type of UPS may include some basic filtering for spikes and electrical noise protection.

For critical loads, on-line UPS are recommended because of their independent output waveform and built-in automatic transfer switch, providing a safe failure-to-mains mechanism should the UPS be overloaded or generate a critical alarm condition.

Power ProblemOn-line (VFI)Line Interactive (VI)Standby (VFD)
Electrical Noiseyesyesyes
Mains Failuresyesyesyes

Features and benefits vary with UPS topology. The most suitable waveform for a switch mode power supply (SMP) – the power supply within a PC, file server and almost any modern electronics – is a sinewave.

SpecificationOn-line (VFI)Line Interactive (VI)Standby (VFD)
Output WaveformSinewaveSinewave / Step-waveStep-wave / Square-wave
Extended runtimesyessomeno
Automatic Bypassyesnono

For extended runtimes on-line UPS can typically be installed with external battery extension packs (or cladded racks or open stands for larger systems). Some line interactive UPS also provide this feature but the majority will require over-sizing the UPS for a corresponding load/runtime configuration, as will a standby UPS. Standby generators can provide additional runtime to both the UPS and essential and non-essential loads. Alternative forms of standby backup power for an on-line UPS can include: dc flywheels, fuel cells and super-capacitors.

Other Standby Power Solutions

Other forms of standby back-up power solutions include:

  • AC Central Power Supply Systems (CPSS): used for emergency lighting, alarms and security applications. These are similar in design and operation to an on-line UPS system but are configured to meet the European standard for CSS – EN 50171. CSS provided extended battery runtimes of three hours or one hour if there is a standby power generator available, with a recharge time of 80% within 12 hours.
  • DC Standby Power Systems (SPS): for dc powered loads that require a supply of dc power when mains is present from a built-in rectifier and standby batteries for when the mains power supply fails. Typical loads include telecoms and mobile communications systems.
  • AC Inverters, Solar Inverters and Wind Inverters: provide a continuous or standby AC power supply from a DC source including sealed lead acid batteries, a DC rectifier, solar PV or wind turbine installation. The output waveform can be a sinewave, step-wave or square-wave.
  • Generators: can provide either standby power or primary power supplies and are normally diesel powered. ECO-powered generators can be powered from liquid petroleum gas (LPG) and bio-fuels. Within a UPS installation standby power generators are used to provide an additional source of back-up power to that provided by the UPS battery itself. Gas powered generators can be used for large installations as part of a combined heat and power (CHP) configuration.

Other Power Solutions

Power solutions without a built-in or connected battery supply operate only when the mains power supply is present. The power solutions listed below are designed to tackle various power quality problems, with varying degrees of performance.

  • Power Conditioners: reduce (attenuate) spikes and electrical noise to very low levels and can be solid state electronic or transformer-based. A power conditioner may also provide voltage stabilisation, allowing the power conditioner to regulate the output voltage within a defined input voltage window. A typical transformer-based power conditioner could also be described as an isolation transformer or ferro-resonant transformer and may offer Galvanic isolation which is a full electrical separation of the input and output supplies. Typical applications include sensitive electronic loads including EPoS terminals and loads within an industrial, electrically polluted environment.
  • Automatic Voltage Stabilisers (AVS): also known as AVS or Automatic Voltage regulators (AVRs) are used to protect loads from mains voltage swings. They may be solid state electronic systems or electro-mechanical and use buck (step-down) or boost (step-up) settings to achieve an stable electrical output supply. Filtering can also form part of the circuitry within an AVS or AVR to provide some protection from spikes and electrical noise. Typical applications include fridges, freezers and domestic white and electronic goods within remote rural areas and developing countries.
  • Filters and Filter Strips: provide basic protection from spikes and electrical noise through ‘clamping’ circuits. Electronic filters and filter strips with surge suppression do not isolate their loads and can self-sacrifice or cut-out to protect themselves and connected loads. Typical loads include domestic and small office applications
  • Transient Voltage Surge Suppressors (TVSS): protect from the type of spikes and transient voltages generated by industrial manufacturing plant with heavy inductive motors and local lightning strikes.
  • Power Distribution Units (PDUs): PDUs in themselves are designed to help manage power distribution providing socket outlets from a single connection. Whilst a basic PDU will not include any form of filtering or spike protection, more advanced smart (intelligent) PDUs can and for this reason they can be considered a form of power protection as as well as a necessary accessory.
  • Static Transfer Switches (STS): STS provide an automatic transfer (or manual) of connected loads from one of two power sources. These can include two UPS outputs, two mains power supplies or a combination of the two. Like a PDU, STS are considered a vital element within a power protection plan for a critical datacenter looking for redundant suppliers and may or may not include some form of spike and surge suppression.
  • Voltage Optimisers: reduce the incoming mains power supply voltage to a building down to the levels at which capacitive (and reactive) loads can achieve their ideal operating efficiency. Voltage optimisation is a technique for energy management to generate electricity savings and reduce the energy and reactive power required by connected loads.

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