HRT treatment for Lakenheath
Built in 1940 as a heavy bomber station, Lakenheath has boasted a major air base for 60 years. First with the Royal Air Force, and then after the formation of Nato, as an essential installation for the United States Air Force (USAF).
Even though the Cold War has now ended, Lakenheath continues to be a major force supporting rapid reaction forces throughout Europe and the Middle East. But the site had a power quality problem which affected the main laboratory where sensitive test equipment is calibrated. A frequency converter had been installed providing a 60Hz supply, which resulted in high harmonic distortion in the LV network, particularly high third harmonic currents.
The manufacturers of the frequency converter had initially been commissioned to provide a harmonic survey and report. Unfortunately the report detailed the extent of the problem but provided no cost effective solutions for the customer, apart from recommending the installation of oversized neutral cables to cope with the problem.Distribution specialist Claude Lyons was asked to investigate.
The company's Chief Engineer, Nigel Lee commented: "The frequency converter manufacturer's recommendation would still leave the customer with his problem. Often, oversized neutral conductors are suggested but this is only a damage limitation measure. We visited the site ourselves and because of the high third harmonic currents measured, we recommended that a 200A third harmonic rejection transformer, (HRT) be installed. "
The HRT will re-direct the harmful harmonic currents away from the neutral conductor and from other circuits on the network, not only reducing the problems caused by the harmonic currents, but also reducing the resultant voltage distortion.
Chris Tarpy, Contracts Manager for MOD/DE (USAF) commented: "We had been experiencing dreadful problems before the HRT was installed. If you can imagine, the calibration of all of the US Air Force's test equipment is carried out here. We have to have a supply that is as sound as possible. Until Claude Lyons got involved we couldn't obtain accurate measurements, and had problems with cables getting hot and equipment failing. Since the HRT has been installed we have had no further problems, and everyone is more than happy with the result".
Lee added: "After the HRT was installed we carried out a follow-up survey to monitor the unit's performance. From the tests carried out we were pleased to confirm that the HRT had substantially reduced the zero-sequence harmonic currents being produced. This was good news, as the customer didn't have to under-go the expense and disturbance of replacing the neutral cables with oversized conductors.
"Prior to the unit's installation we measured 72A of current, in the main neutral cable. After installation the neutral current was reduced to 19A at the same load conditions. The HRT can be installed adjacent to an appropriate three phase distribution board and is simply connected to a suitably sized MCCB. The minimum of disturbance and shutdown time is required," suggested Lee.