Why UPS System and Generator Integration Matters

An online UPS is an uninterruptible power supply that provides continuous power protection. During power outages and interruptions, its battery provides backup power. When the mains power supply is available, the UPS provides power conditioning, with the output tightly controlled for voltage and frequency.

The standard UPS battery runtime is 5-30 minutes. When a longer autonomy is required, this requires either a larger battery set or an alternative source of backup power. The most commonly installed for larger 3-phase UPS is a standby power generator.

What is a Standby Power Generator?

A generator provides a source of standby backup power that can be used to supply a UPS system during a prolonged power outage. The generator fuel source is locally stored diesel or HVO (hydrotreated vegetable oil) if a more environmentally sourced fuel is required.

Power Protection Plans for Server Rooms and Data Centres

A secure and resilient power protection plan requires the two systems (UPS and generator) to be properly designed, installed, and maintained to protect critical loads, which can include server racks in server rooms and data centres, in addition to hospital services and industrial manufacturing processes.

Poor integration can create serious operational risks and impact business continuity. Generators can fail to automatically start, leading to downtime if the UPS battery is exhausted before the mains power supply returns or the start-up issue is not resolved quickly. Older generators can suffer synchronisation failures with UPS input demands, leading to unnecessary transfers (bouncing) and more frequent battery usage.

Planning Checklist

Every UPS and generator integration should begin with an extensive site survey and stakeholder review. This helps confirm the current electrical and IT infrastructure, the expected outage scenarios, and the required availability. Some organisations may only want a 4-8 hour runtime, which can be achieved with a day tank. Public sector blue-light emergency services may require longer runtimes and justify additional external fuel storage tanks.

Key areas to assess include load profiles, steady-state demand, inrush current, critical and non-critical load segregation, existing UPS ratings, generator capacity, fuel storage, expected runtime and generator starting arrangements. The survey should also review auto-start circuits, earthing and bonding, harmonics, power quality, HVAC capacity, acoustic restrictions, emissions considerations, and access for maintenance.

Practical logistics are just as important as the electrical design. Fuel delivery access, battery replacement routes, transfer switch panel clearances and spare parts access should all be reviewed before the proposed power protection plan is finalised. Protection coordination should also be documented to confirm that protective relays will not cause unnecessary lockouts during generator start-up.

The goal is to design the power protection strategy as part of the organisation’s business continuity plan to reduce organisational risks. A site that mainly needs protection from short utility interruptions may need a different architecture from one that must operate independently for long periods during prolonged power failures.

For more information on business continuity, see: https://www.thebci.org/thought-leadership/what-is-business-continuity.html

Integration Architectures and Key Components

The right integration architecture depends on the criticality of the load, available space, budget, existing electrical infrastructure and required runtime. The correct approach is to install an automatic mains failure (AMF) switch, also known as an AMF panel, an uninterruptible power supply, and a standby power generator.

The UPS will power downstream distribution boards and power distribution units (PDUs), providing a source of uninterruptible, conditioned power. When the mains power supply fails, the UPS will use its battery set to power the inverter output and connected loads.

The AMF panel will detect the mains power supply failure and command the generator to start up. It can take 1-2 minutes for the generator to reach full speed and power output. The UPS input supply is then synchronised to the generator output and, once accepted, is used in place of the mains power supply to power the rectifier/inverter section. The battery set is then no longer in use and may be charged. When the mains power supply returns, the reverse process applies, in that the AMF will monitor the condition of the mains power supply (for voltage and frequency). The UPS will synchronise back to the mains power supply and the AMF panel will then instruct the generator to power down.

The correct installation of a UPS and standby power generator requires co-ordination between various parties, including the UPS and generator supplies, and a local electrician or electrical contractor. The latter is responsible for the local electrical circuits and the cabling routing and segregation required.

UPS and Generator Sizing Guidelines

A UPS system should be sized for both day one and future expansion. A guideline is to select a UPS size that will comfortably run the future critical load at 80% of its full load rating. If the measured or calculated load is 76kW, this means selecting a 100kW UPS system.

UPS System sizing: the total future load (kW) shoud be less than 80% of the UPS capacity

100kW * 80% = 80kW to cover a 76kW load

Generator sizing: the generator should be at least 1.2 times the size of the UPS system

For generator sizing, the generator should be at least 1.2 times the size of the UPS system.

100kW * 1.2 = 120kW

The nearest match UPS and generator combination is then selected for the installation. One point to note is that if the UPS is modular, it is easier to meet future expansion using additional modules. This is not the same with a monoblock system, which will generally be oversized at day one.

Automatic Transfer Switches, Paralleling and Generator Controls

AMF panels and Automatic Transfer Switches (ATS) are central to any UPS and generator integration. Automatic transfer switches need to be specified for fault levels, continuous current, switching duty and expected operations per hour. Manual transfer and bypass arrangements should also be designed to enable safe maintenance without interruption to critical loads.

Whilst most smaller sites will choose a simple power protection plan of a single UPS system and generator, others may have the budget to build in N+X resilience as defined by the Uptime Institute and their Tier-Levels models. This allows for greater resilience, uptime and maintenance provision.

Tier 1 is commonly found protecting small server rooms running 1-10 server racks. Tiers 2 and 3 are common for most co-location-type data centres, and Tier 4 for mega-type data centres or those with the budget for this level of resilience.

Tiers 2-3 can have two or more UPS systems and generators, paralleling outputs to share loads and supporting parallel power paths and staggered start-up. Tier 4 allows for concurrent maintenance, separating A and B power paths back to LV distribution boards and substations.

For more information see: https://uptimeinstitute.com/tiers

UPS and Generator Testing and Commissioning

Testing and commissioning can take place at a factory and is known as factory acceptance testing (FAT). More commonly, UPS and generator systems are subjected to site acceptance testing (SAT). For this, a load bank is hired to test the combined systems before they are accepted for connection to the critical loads.

Testing during the commissioning phase confirmed the design works as intended and will cover a range of components and scenarios including simulated mains power supply failure, UPS synchronisation, battery runtime and support of the critical power loads.

The testing is a documented procedure with the works report documenting each test, and any associated voltage, current, frequency and load (kVA/kW) measurements. This document forms part of the final O&M manual.

UPS and Generator Maintenance Best Practice

Effective integration depends on automation between the UPS, generator, transfer and control systems.

Monitoring should be considered at the BMS (building management system) and remote service centre levels, to ensure that they capture generator status, AMF panel position, UPS metrics, alarms, and start or stop events in a single view wherever possible. This gives engineering teams a clearer picture during an incident and helps speed up fault investigation.

What can typically go wrong generally results from poor maintenance and monitoring. The UPS system is not maintained, and its battery is not replaced according to local battery testing and the battery supplier’s recommendations. Batteries are typically valve-regulated lead-acid (VRLA) type with a 5year or 10year design life. These typically require replacement within years 3-4 or 7-8, but this is both usage and ambient temperature dependent.

UPS should be subject to a preventative maintenance (PM) inspection at least once a year.

Generators can be poorly maintained. Generators should be visually inspected and then run once a month to ensure that they start up on command. Reasons for non-failure can include a failing starter battery, open circuit breakers and fuel pump issues, including fuel pollution (water and air bubbles).

Once a year, a full black-start should take place to ensure the critical power and essential loads can be supported. The critical power load is that of the UPS. Essential loads will be those not powered by the UPS and which can suffer a power interruption during a mains power supply interruption or have their own local battery backup (emergency lighting), but have to be kept running (air conditioning, lighting, door access and HVAC systems).

Generator maintenance should include two PM visits per annum. These should include minor and major services, with the major covering consumables such as oils, filters and even the start battery, which will be of the VRLA-type.

All maintenance services should be documented in a maintenance contract and service level agreement (SLA), detailing what is covered, emergency contact numbers and escalation levels.

UPS and Generator Remote Monitoring

UPS and generator should be locally monitored using an SNMP card to connect them to the local IP network and through signal contacts to a local BMS. This allows for 24/7 monitoring with alarms escalated to the correct personnel as required. The systems may also be remotely monitored through connection to suitable environmental monitoring systems by the UPS and generator service suppliers. The remote connections at a minimum should provide SMS text notifications including heartbeat messages. Where there is an SNMP connection, this may also allow connection to Data Centre Infrastructure Management (DCIM) software.

Summary

An integrated power protection plan provides critical power loads in server rooms, data centres, industrial, hospital, MoD and blue-light emergency service applications, with power security. The system is designed to protect from mains power supply outages and ensure service provision even during prolonged failures. The architecture of the power protection plan should follow the Uptime Institute Tier-Levels in terms of resilience and maintenance provision.

Whilst an uninterruptible power supply can be installed with a battery to provide a long runtime, from 10kVA and upwards, it may prove cost-effective to install both a UPS system and a standby power generator. For a site survey or to discuss your UPS and generator power protection project, please “contact”:/contact-us our Projects Team on 0800 030 6838.