New technologies & applications
Keeping the LEDs on cuts maintenance1 November 2007
It’s a tougher job than you think, replacing bulbs in the control systems of several nuclar power stations. Especially when you want to replace all 5000 of them at once.
Long life LED indicators are finding applications as a replacement for incandescent lamps in control panels in all kinds of systems. The attractions are self-evident – a rated service life of 100 000 hours compared to 5000 for the typical incandescent lamp has an enormous impact on the workload of the service engineering department. It is perfectly feasible to retrofit LEDs in place of lamps in an existing control panel, providing a replacement is available that matches the appearance, form factor and electrical characteristics of the existing lamp. Nuclear utility British Energy enlisted the help of UK manufacturer Marl International to provide a mix of custom and standard LED indicators to reduce the burden of replacing blown lamps at Hinkley Point B, and later, at other nuclear power stations.
Safe operation of a nuclear power plant depends crucially on operators’ having the correct information on its status at all times. For this reason, system health engineer Richard Seale and his team take blown lamps in the control panels seriously. At the same time, when the lamps have a service life of 5000 hours at best, less if subjected to rough handling, keeping all panels fully lit was proving a serious drain on time.
The initial challenge with which they approached Marl was the replacement of a T31/4 miniature Edison screw based lamp in the central control room, and the central control room simulator status desks, at Hinkley Point B. The key issue here is that the host lamp controllers can be centre-cathode or centre-anode, with no easy way to identify which was which. While polarity doesn’t matter for a filament lamp, an LED is a semiconductor and therefore polarity sensitive.
A further requirement was that the replacement LED needed to be colour matched to the lamp it replaced. The conventionally saturated colour of an LED has previously meant that the correct colour LED has had to match the optical filter already present in the application. However, with the continued development of warm white LED technology the optical performance is now nearly identical to that of the existing lamp when placed behind a coloured filter.
Marl developed a 261 series LED product adding a bridge rectifier circuit without increasing the overall size of the unit. With this adaptation, the LED matched the size and electrical characteristics of the lamp it replaced, but with considerable advantages. A service life of 100 000 hours makes it largely maintenance free, and reduced heat generation improves reliability and reduces station operating costs. LED brightness is also largely unaffected by fluctuations in the incoming power. Marl area sales manager Andy Melia recalls that following the success of the 261 series he was taken on a two hour site tour by Richard Seale, reviewing all the different lamp applications on the site. This resulted in a new challenge for Marl, and ultimately in the replacement of no fewer than 5000 of the standard lamps in BE’s power plants.
The next custom design challenge was to replace a 1.25 inch lamp. A 31.7mm LED was required – an unusual size offered by only two manufacturers worldwide. Although Marl had a product, the 671 series, which could be suitably customised, the range did not include white or blue, which therefore had to be sourced specifically for British Energy. Marl provided a unique termination via a stud bar that protrudes from the bottom of the header which enables British Energy to remove the current units from the motor control panels, leaving the existing wiring and ring clamps in situ, and then wire straight onto the back of the 671 series. It is also supplied with a 31.7mm hardware kit so the product can be dropped into the existing mounting holes.
The 671 has a domed, colour diffused lens to enable off-axis viewing. With these lenses, it is much easier to tell when the LED is truly on, or if sunlight or ambient light is catching the lens.
Not every British Energy application required a customised solution. For example Hinkley Point B uses BA22 pygmy lamps in the air on crane trolley wire indication systems to indicate if the wires are live or not. The Marl 246 series offers a direct replacement for BA22 ‘pygmy’ lamps, and uses a cluster of LEDs to mimic the spatial output of the filament lamp. The robust clusters are not polarity sensitive and are available in multiple voltage options which will operate on AC and DC. As before, each LED in the cluster has a rated life of 100 000 hours, but should one go, the impact on the level of illumination is barely noticeable. A relief for the system health engineering team – as replacing the bulbs can involve erecting a scaffold, which increases crane down time and costs.
Marl is now supplying lamp replacement LEDs to other British Energy sites including Hunterston B and Torness. A recent request came from Dungeness B, involving its fuel route system. Clearly, such systems cannot be allowed to operate with defective lamps – yet replacing a penny lamp involved downtime costing hundreds of pounds. Following a site visit, the Marl 236 series products are now to be installed in Dungeness B as part of the fuel route system overhaul. The 236 series incorporates the latest warm white technology and includes an integral bridge rectifier.
Replacing lamps with LEDs on its existing control panels provided significant benefits for BE in reduced maintenance costs and improved reliability. The secret of success for the programme was in identifying an LED supplier that was willing and able to customise its standard products to offer an exact optical, electrical and mechanical replacement for the existing incandescent lamp.