Back-up power supplies

Many buildings require standby generation plant to support building functions in case of power failure. Three levels of service need to be supported by standby power systems – life safety services, essential services and critical services.

Life safety services include statutory requirements such as fire-fighting lifts, sprinkler pumps and smoke extract. A short break in the supply is permissible, as the generators fire-up, and only day tank storage of fuel is necessary.

Essential services are specified by many occupiers to allow the building to function during power failures. These will include selected heating and ventilation plant and lighting. A short break in the supply is permissible, bulk fuel storage requirements can be for up to seven days.

Critical services are defined as the minimum criteria for continuing business operation. IT systems, communications systems, fire alarm and security installations are typically defined as critical services, and no break in power supply to these systems is permissible.

With critical standby, resilience is an important issue for many end users, and the total duty required will typically be shared by several units, with further redundancy being provided by single or multiple reserve units. As these systems can take up a lot of plant space, it is important to rationalise the total duty. Load shedding strategies are typically adopted to shut down non-essential equipment during mains power failures.

While landlords are responsible for providing generator plant to support life safety systems, individual tenants meet their own needs as part of the tenant's category B fit-out. However, for owner-occupiers, or tenants in sole occupancy buildings, the plant serving these requirements can be combined into single or multiple units. This results in capital cost, plant space and life-cycle cost benefits.

UPS systems
UPS systems are necessary to protect critical loads because there will be a break in supply before generators get up to power. This level of protection is essential as even a momentary break in supply can result in the loss of irreplaceable work or transaction records.

Conventionally, ups back-up is provided using a static system, where a series of batteries is kept fully charged by the main electrical supply. If the power fails, the batteries temporarily support the critical load. With a static ups, a separate generator is still required to maintain critical supplies beyond the life of the batteries, and also to provide for the essential loads.

An additional benefit of a ups is that it provides a degree of conditioning to the mains supply passing through it, protecting against voltage fluctuations and other distortions which can damage computer equipment.

The size of the ups battery set is calculated on the basis of the need to maintain the supply at full design load for 10-30 minutes, enough time for either generation plant to take over or for network systems to be backed up. Typically, a financial institution occupying 10 000 m2 will require a ups of 1.0 MVA, whereas a 10 000m2 Internet hotel will require 6.0 MVA.

Alternative standby generation and ups

The capital cost and space implications of back-up power supplies are considerable. Above critical loads of 1.0 MVA, alternative and more efficient solutions to static ups systems are available.

Rotary ups systems combine both the generation plant and ups into one unit. The incoming electrical supply is fed to an induction coupler, driving a shaft connected to an alternator. This generates a conditioned, distortion-free electrical supply to the load.

A heavy flywheel attached to the shaft stores sufficient kinetic energy to keep the alternator spinning for one to two minutes after the failure of the mains supply. If a failure occurs, a diesel generator, connected to the same shaft via a clutch mechanism, is run up to speed and takes over driving the alternator.

The rotary ups is considered to be a superior engineering solution, as the incoming mains is decoupled from the critical load, offering higher protection from distortion. The integration of the generator and ups into a single system also offers space savings, operation and maintenance benefits and energy savings. However, the capital cost of the plant exceed those of an equivalent static ups and generator by some 10-15%.

Hybrid solutions, can provide some of the benefits of a rotary ups at a lower capital cost, but are 5-8% higher than an equivalent static system. In the hybrid system, the induction coupler and alternator of the rotary system are retained, but static batteries are used instead of the flywheel and diesel engine. Separate generation plant is required to provide the necessary standby for prolonged power failures. Hybrid solutions are generally viable for critical loads in excess of 150 kVA (see table 1).

Design and installation issues
There are several important factors to consider when finalising design. For diesel generator installations, requirements for fresh air supply, exhaust flues, ventilation, and cooling to dissipate heat build-up need to be addressed. Attenuation may also be required to shield adjacent occupancies from noise.

Siting the generator and ups adjacent to switchgear and loads will reduce the costs of interconnecting busbar and trunking runs. For battery based ups systems, proximity to the switchgear and load, together with provision for ventilation and cooling are also considerations.

Cost model
The cost model is based on the headquarters of a City of London-based financial institution. The development comprises 25 000 m² of office space over seven storeys with two dealing floors of 6 500 m². Dual 11 kV supplies feed the building. The critical load is 3.4 MVA.

A rotary ups, located in the basement, filters the mains supply and provides back-up in the event of power failure with fuel storage for seven days. The system has 100% standby. Load shedding is applied to give total protection to critical loads. Using this system, essential loads will suffer a short break in supply.

Two alternative solutions are also detailed: a diesel generator with static ups system and a hybrid rotary ups with diesel generator.

Additional budget costs are given for the installation of stand-alone diesel generator sets and static ups installations at a range of duties.