Uninterruptible Power Supply (UPS)

A UPS (Uninterruptible Power Supply) is an electrical apparatus that provides emergency power to a load when the input power source or mains supply fails. In the context of AGL systems, a UPS protects critical lighting equipment — particularly Constant Current Regulators (CCRs), control systems, and communication interfaces — from power interruptions, voltage sags, surges, and frequency fluctuations.

The UPS achieves this by storing energy in batteries (or flywheels) and using an inverter to convert DC battery power back to clean AC power. In the event of a mains failure, an online double-conversion UPS presents no interruption at all — its inverter is already supplying the load continuously, so there is nothing to "switch to" — while a line-interactive or standby topology introduces a brief transfer of a few milliseconds. Which behaviour a given installation has depends entirely on topology, which is why ICAO's one-second figure is a system design target, not something guaranteed by fitting any UPS. Not every AGL circuit needs this protection, either: Annex 14 Table 8-1 sets the required switch-over time by facility and approach category — 15 seconds is sufficient for CAT I approach and runway lighting, while only the specific facilities essential to CAT II/III operations (runway centreline and touchdown zone lighting among them) carry the tighter one-second, effectively no-break requirement that a static UPS exists to meet.

The UPS exists to close the timing gap that a generator alone cannot. ICAO sets that gap explicitly in Annex 14 Table 8-1: an automatic standby generator (referred to in the Aerodrome Design Manual as an interruptible power unit, or IPU) is allowed up to 15 seconds to start and stabilise, while precision approach CAT II/III operations require certain critical facilities — runway centreline and touchdown zone lighting, principally — to be restored within one second, a figure that in practice means no perceptible break at all. A battery-fed unit meeting that tighter figure is what ICAO Doc 9157 Part 5 calls a static uninterruptible power unit (SUPU); the generic term "UPS" as used in industry covers the same equipment. The SUPU's purpose is therefore narrowly focused on that one-second class of facility, not on sustained backup — it carries the no-break load only until the generator (IPU) is up and has taken over the full system, at which point the battery bank is no longer doing the work.

UPS systems are installed in central electrical rooms, equipment shelters, or dedicated AGL power buildings. They are deployed in:

• Main AGL Power Centers: Providing no-break backup specifically to the CCRs serving the facilities Annex 14 Table 8-1 designates as needing a one-second switch-over — typically CAT II/III runway centreline and touchdown zone lighting — rather than to every CCR in the building indiscriminately.
• Remote Equipment Huts: Smaller UPS units for individual CCRs located near the airfield perimeter.
• ALCMS and Control Systems: Protecting ALCMS servers, workstations, and communication equipment from power loss, ensuring uninterrupted monitoring and control.
• Critical Navigation Aid Equipment: Supporting ILS, DME, and other navigation systems that require continuous power.
• Integration with Generators: The UPS/SUPU rides through the standby generator's start-up — up to the 15 seconds ICAO allows for CAT I facilities — but only the specific circuits that carry a one-second (effectively no-break) requirement under Annex 14 Table 8-1 need this ride-through; other circuits are adequately served by the generator alone within its 15-second allowance.

The UPS is floor-standing in the main AGL power centre or equipment shelter, typically with its battery cabinets installed alongside it or in an adjacent, ventilated battery room to manage heat and any off-gassing from VRLA cells. It is connected between the incoming supply (utility or the generator interface) and the load it protects — CCRs, ALCMS servers, or navigation aid equipment — with input and output cabling entered from below or the rear per the manufacturer's layout. Larger units are seismically braced or floor-fixed given their weight, and clearance is kept at the front for the control panel and at the rear or sides for cooling airflow. Where a UPS bridges the gap before a standby generator picks up load, it is wired through the generator interface so the ATS logic sequences correctly between the two sources.

A UPS operates by continuously converting incoming AC mains power to DC to charge the battery bank, and then inverting that DC back to AC to supply the load. This double-conversion (online) topology isolates the load from all mains power anomalies and provides genuinely zero transfer time, because the inverter is always the source — there is no switch to make when mains fails. Other common topologies (line-interactive, standby/offline) introduce a real, if brief, transfer time measured in milliseconds because they only engage the inverter or battery on detecting a fault. Online double-conversion is the topology capable of meeting ICAO's one-second, effectively no-break requirement with margin, which is why it is specified for the critical circuits that carry that requirement.

Key functional stages include:

• Rectifier/PFC: Converts incoming AC to DC, while also correcting the power factor to reduce harmonic distortion on the mains supply.
• Battery Bank: Stores energy in valve-regulated lead-acid (VRLA) or lithium-ion batteries, sized to provide a specified autonomy (typically 15–60 minutes) at full load.
• Inverter: Converts DC from the rectifier or battery into a clean, regulated AC output, with tight voltage and frequency regulation.
• Static Bypass: An automatic transfer switch that bypasses the UPS to directly connect the load to mains power in case of UPS overload or internal fault, ensuring continuity.
• Monitoring and Communication: Built-in microprocessors monitor input/output parameters, battery health, and load status, providing alarms and data to the ALCMS via dry contacts or serial/network protocols.

An AGL UPS is typically housed in a floor-standing steel cabinet (or, for smaller ratings, a rack-mount chassis), with the battery bank occupying either a dedicated compartment within the same enclosure or a separate adjacent battery cabinet for larger installations. Internally, the rectifier/charger, inverter, and static bypass switch are built on their own power modules, generally forced-air cooled with filtered intakes and thermostatically controlled fans, and separated from the battery compartment to limit heat exposure to the cells. A control and monitoring board, with a front-panel display or touchscreen for status and alarms, sits in its own compartment isolated from the power electronics. Heavy-duty DC cabling connects the battery bank to the rectifier/inverter section via fused disconnects, while AC input, output, and bypass connections terminate at labelled terminal blocks or busbars sized for the UPS's rated load. Ventilation is sized to handle both the electronics' heat dissipation and any hydrogen off-gassing from the battery bank, particularly for enclosures housing vented lead-acid batteries.