The work has been supported by sponsors from all sides of the industry and, in all, BSRIA completed 35 case studies involving power quality surveys on buildings in the UK and Europe.
The surveys revealed power quality problems related to harmonic current in over half the buildings. BSRIA also conducted a review of legislation and design tools and found that there is little published information on how to avoid power quality problems. Building operators and electrical designers are unaware of the problems caused by harmonic currents.
The electrical supply to a business is now the most critical service in 99% of installations. With business being done electronically the quality of the electrical supply is ever more critical. The increased use of computers, monitors, printers etc distorts the current flowing in the distribution system and this distortion is resolved as harmonic current. Other equipment that many people are installing to save energy (such as variable speed drives and high frequency ballasts for fluorescent lighting) may also be distorting the current.
Effects of harmonics
The effects of harmonic voltages are malfunctions in sensitive equipment such as the sensing circuits of circuit breakers and ups switches. The effects of harmonic current are overheating in conductors, transformers, motors coils and capacitors.
When harmonic loaded single phase circuits on different phases are brought together in a distribution board, the third harmonic components and all the odd multiples (triplen harmonics) add together in the neutral. In severe cases the neutral current can exceed twice the phase currents. In such cases a double size neutral busbar is often specified.
However, it may be necessary to double rate busbars and circuit breakers in the main distribution board as well. In a large installation the main air circuit breaker could be rated at 4000 A. Finding an acb to take an 8000 A neutral conductor can be difficult.
Fortunately, diversity of the harmonic loads leads to a certain amount of cancellation since the harmonic currents of different loads are rarely in perfect phase. The solution may require modelling software to predict the neutral current. A range of solutions is available, from ups to avoid interruptions, through isolating and phase shift transformers to passive and active filters.
Case studies
Two cases of failures due to harmonic currents were investigated. One involved failure of a connection in a half-rated neutral busbar on the output of the ups due to overheating and melting of the connecting bolts. This led to complete loss of power including power to emergency systems. The building had to be evacuated and a day's work was lost.
The second was caused by a poor connection on a neutral conductor that was carrying twice the phase current. This caused a fire in the electrical enclosure and complete loss of power. The panel concerned was replaced overnight and business was not disrupted.
In 11 case studies in the UK, harmonic problems were found in seven. These include severe voltage distortion of up to 13%, high neutral currents made up of mainly third harmonic high neutral – earth voltage of nearly 7 V in one case and high earth currents of up to 48 A in one building. Most of these problems can be attributed to electronically-switched loads including variable speed drives and high frequency lighting. The principal offender, however, was switch mode power supplies used in office equipment.
Case study 1
A large 1970s office building with 3000 pcs, two large ups (over 650 kVA) and high frequency lighting with half size neutral busbars. Much of the harmonic load was from the lighting, 28 A of harmonic current (as shown in table 1) for one floor, and the large number of pcs in use. It was not possible to measure the harmonic current on the main lv feeder because it was not safely accessible. But, adding the harmonic current from all the non-linear loads gave a total of over 1000 A of harmonic current for the whole installation.
Because much of the harmonic current was third harmonic from single phase loads, the neutral feeders to the split phase distribution boards were very highly loaded. With a balanced load, the neutral busbar would be carrying most of this harmonic current – which meant it was very near to its capacity. Any imbalance in the three phases might increase this over the limit. High levels of voltage distortion were measured – up to 10% in the plant room. A decision was taken to upgrade the neutral capacity by utilising the earth busbar in the same enclosure. An additional earth busbar and a harmonic filter would also be installed.
Case study 2
Another office with a communication room and over 200 pcs had a harmonic current of over 16·5 A per phase on the 100 A rotary ups supply. The loads can be seen in table 2, the distorted current is shown in figure 2.
The IT department had installed its own small ups for the communications room, fed from the main ups. Voltage distortion was up to 11·8% at socket outlets on the ups. This, and the surge current of 15 A when starting some of the communications servers, was causing small ups units to go into standby mode. The mains had less than 2·5% voltage distortion. Neutral-earth voltage was only 1·7 V. The ups main was overloaded which had reduced the crest factor of the voltage waveform. The peak voltage was only 1·3 times the rms value, a sinusoidal waveform has a crest factor of 1·4.
Since the small ups was monitoring the peak voltage, the voltage drop caused by the surge was detected as a voltage failure by the ups. The tolerance band of the small ups was increased and an active filter was specified to reduce the distortion on the office supply voltage.
Case study 3
An office building with more than 1000 pcs had a harmonic current of 60 A on the second floor yellow phase and 2·9% voltage distortion. There was also a neutral-to-earth voltage of 6·85 V and a 141 A neutral current which was mainly third harmonic current (see figure 3). This equated to 1·4 times the phase current. A double-sized neutral conductor was designed in, allowing this high current to be carried safely but the third harmonic would be dissipated in the main transformer, de-rating its capacity.
Standards
The existing standards, Electricity Association G5/3 and EN 50160, refer to voltage and current at the point of common coupling, where consumers' supplies are in common. Although these standards set different harmonic distortion levels for different conditions and harmonics, an overall level of 8% rms voltage distortion is widely used.
Forthcoming standards for power quality include EN 61000-3-2 and EN 61000-3-4. These intend to eliminate harmonic problems but have not yet been implemented as revisions may not sufficiently reduce the harmonic currents in some buildings. In the USA guidelines such as IEEE 519 are in use. Although these are not strictly applicable to UK installations, they are informative. Comparable UK standards are not yet available, although a BSRIA Application Guide is to be published.
The electrical design process on installations with suspected harmonic loads needs to include consideration of the harmonic currents and voltages throughout the system. However, the full range of design tools is not yet available in the UK. Electrical designers also need to appreciate the problems and acquire the necessary skills to solve them.
Recommendations
Source
Electrical and Mechanical Contractor
Postscript
Colin Pearson and Vinay Uthayanan are with the BSRIA.
