Power Quality Case Study 2 - News International
POWER QUALITY CASE 2
COPPER DEVELOPMENT ASSOCIATION
Publication No 135
NEWS INTERNATIONAL KEEP THE PRESSES ROLLING
Harmonic cable sizing in modern electronic environments
For years, News International have pursued a policy of doubling-up on cabling in their power distribution systems. Not only has this provided additional flexibility and safety, it has helped avoid any serious instances of harmonic overheating.
Modern-day newspaper operations are amongst the most computer-intensive environments in the world (on both the editorial and the printing sides of the business), and none comes bigger or more computer-intensive than the News International site at Wapping UK.
With a maximum demand of 12.5MW to service the printing presses and 2500+ editorial staff using PCs, the Wapping site is a truly mammoth operation.
There is little doubt that on-site electrical engineers feel a strong responsibility for keeping the presses rolling, something which is evident in their state-of-the-art approach to continuous power quality monitoring.
In particular, they take the issue of phase and neutral harmonic content very seriously, and routinely check for harmonic related overheating.
A good example of this phenomena was identified recently in the production room in the main editorial and printing press building.
During routine inspections in Spring 1996, it was noticed that the neutral conductors at the production room distribution panel were running hot, substantially hotter than any of the phase conductors. (The distribution system at that point consists of twin parallel feeders, each 4-core, 95mm2 copper.)
Harmonic measurements were taken using state-of-the-art Fluke 41 harmonic analyser equipment, and the results were a surprise even to on-site engineering staff.
Figure 1 shows the harmonic current profile of the yellow phase, registering 99.8A RMS, whilst Figure 2 shows the same profile for the neutral conductor, registering a remarkable 150A RMS.
This huge neutral current clearly derives from the non-cancellation of the 3rd harmonic (146A) generated by local electronic equipment.
The first line of attack planned by site engineers for combating this serious problem is to introduce a third parallel feeder, to spread the harmonic load, especially across the neutral conductors.
This design solution should provide sufficient capacity to prevent overheating and is a simple and very cost-effective method for dealing with harmonics.
The observant reader will have noticed something rather unusual about the case described, namely that the distribution system at the production room already contains a twin feeder system.
In fact, twin distribution systems have existed throughout the site since it was first built in the mid 1980s. This policy of doubling-up on all systems is still in place today. So, for example, if the load per floor is calculated at 40A per phase, engineers routinely rate the cabling for 80A per phase.
They believe this is the easiest way to avoid problems, and there is little doubt that the doubling-up policy has helped prevent the Wapping site from suffering any significant harmonic problems as the ‘electronic intensity’ of newspaper production has increased.
Of course there is a degree of inconvenience to all this, so as far as News International engineers are concerned, cablemakers should be thinking about producing cables designed for high harmonic conditions. Their suggestion, and they invite cablemakers to contact them, is for a 6-wire system:
· 3 Phases;
· 2 Neutral;
· 1 Dual Earth
all of equal cross-section.
Any cablemakers interested?
It is clear that News International, at least, is taking the issue of harmonics seriously, and are committed to achieving simple and cost-effective copper solutions.
A message perhaps to all modern electrical engineering departments - "Don’t take a risk: If in doubt, double-up".
For further information on copper solutions to your power quality problems, see chapter 3 of publication 123 ‘Electrical Design – A Good Practice Guide’.
