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UNDER CONTROL

Kenny Berrie, technical manager at diesel generator specialist, Dieselec Thistle, discusses the complex control systems that the company has designed for New South Glasgow Hospital, the largest ever standby Power installation in Scotland.

Boasting 1109 patient beds across 14 storeys, the New South Glasgow Hospital will be one of the largest acute hospitals in Europe when it opens in 2015. Currently under construction by Brookfield Multiplex for NHS Greater Glasgow & Clyde, the hospital represents the largest ever investment in NHS hospital build and every element of the specification has been chosen to deliver an exemplar contemporary hospital.
The standby power provision is no exception. As befits Scotland’s biggest ever hospital, the standby generator installation is also the largest in Scotland, with ten medium voltage generator sets being installed to provide a combined power capacity of 25mVA, sufficient to power 2,500 average homes! This standby power capacity will be housed in the hospital’s purpose-built energy centre, one off the biggest ever energy centre to be built in the UK, and the ten generators will each have their own dedicated control system linked to both an ENMS and the hospital’s BMS to ensure that standby power is readily available to restore full operations for the hospital, in the event of a utility failure.

Ready in 15 Seconds
Supply, installation and commissioning of the standby power system was subcontracted to Dieselec Thistle by M&E contractor, Mercury Engineering, and the design requirement was that all ten generators would be available at full load, fully synchronised and connected to the hospital’s business critical electrical services within just 15 seconds.  To do this, each generator requires its own control system and all 10 generator controls must be programmed to communicate with each other and with the central ENMS. The system will also be integrated with the hospital’s BMS which will alert the Engineering staff when the mains power fails and monitor the standby power installation for faults both during standby and activation.

The chosen control system is the EasYgen 3500, which has been specifically developed for synchronisation of multiple gen sets and delivery of full power load within a very short, controlled timeframe. In the event of a mains outage, the controls on each of the gen sets will receive the start-up command from the ENMS and within three seconds the generators will be powered up ready to take over supply from the mains. Once the generators are switched on, synchronisation can begin and to achieve this, the gen set control systems are programmed to close the sets’ breakers when the engine start up sequence begins.  The engines will ramp up the speed to 1500 rpm and an output of 11,000 volts within around ten seconds with excitation applied to all the generators’ alternators during the ramp up to ensure that required voltage is produced by all ten generators in parallel.

The voltage control is further enhanced by the use of digital AVRs (Automatic Voltage Regulators), which have been used to ensure optimum alternator voltage performance during the paralleling process and offer finite control of the output voltage wave form during the load sharing process.

Fuel Efficiency
Once all the generators are fully loaded and synchronised, the ENMS will apply the available load in a critical scenario, ensuring that power is allocated to the most essential services first, followed by less critical services and lastly ancillary services such as the HVAC systems. Meanwhile, the BMS will switch off all non-essential services. To facilitate optimum energy management and fuel efficiency, each gen set’s EasYgen 3500 panel has direct control of its generator’s MV loading breaker on the energy centre switchboard, allowing the ENMS to process sets offline after the impact load has been allocated should the power requirement allow it.  Indeed, the standby power installation at NSGH has been specified to ensure that nine of the ten generators will be able to provide the load required by the hospital, even during periods of peak demand. This means that the integrated gen set controls and ENMS will automatically stand down one of the generators to provide a standby for the other nine.

The automatic offlining of one generator by the control system during a mains outage is important for two reasons.  Firstly, while most mains failures are short lived, the hospital is legally required to operate on a business as usual footing in ‘island mode’ without electricity from the national grid for a minimum of 200 hours. If there is a prolonged outage, the availability of a standby generator for the standby power supply is an important failsafe in case one of the others develops a fault.  In reality, if this did happen, the ENMS would ensure that the available load – even if this were reduced to nine, eight, seven or even fewer gen sets – was allocated to the most critical services and that the load was shared evenly across the available equipment.

The second consideration is fuel efficiency.  The energy centre at NSGH will be connected to a 1.4 million litre fuel storage tank which will fuel both the standby generators and the hospital’s boilers.  By processing just one gen set offline after the impact load, the control system can help to minimise running costs and this could equate to considerable savings during a prolonged mains outage.

Test Assured
The control system is designed not only to ensure smooth transition of the hospital’s systems to the standby power system in the event of a mains failure but also to carry out regular testing of the standby generators throughout their service life. HTM (Health Technical Memorandum) guidance requires testing of hospital standby generators on a regular basis and the control system will ensure that the sets start and synchronise with the mains, close in parallel and ramp to the hospital load before opening the mains incomer and leaving the hospital in ‘island mode’ for the duration of the test.  When the load test command is reversed, the ENMS will synchronise back to the mains, closing in parallel and unloading each of the gen sets before opening their breakers in the same process that the system would follow during a switch back to mains supply following and outage.

Design Expertise
The ten generators are being installed in a phased programme which will see five installed this spring and the remainder in place in early 2013 so it will be some time before the control systems are tested in a live scenario.  However, the design and specification of the system is aligned to the size and prestige of the build project to ensure the level of complexity and reliability required.

Dieselec Thistle

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