Shri Karve takes a look at the rapid growth in harmonic pollution and some of the steps that can be taken to mitigate its effects.
Harmonics can cause nuisance tripping of circuit breakers, and the increased heat also wears components, leading to catastrophic failure and additional expense. The bad news is, this problem is of mounting concern.
Over the past few years there has been a considerable proliferation of non-linear loads caused by the use of devices such as tv sets, lighting dimmers, pcs, printers, variable speed drives and low energy bulbs. It is a well-known fact that the public electricity supply does not provide the user with clean electrical power quality or pure sine wave voltage. The situation has become worse due to the growth of harmonic pollution that is being re-injected into the mains supply in greater and greater quantities.
Of course, harmonics have always been there. But the largest growth of non-linear loads has been in the commercial environment. Even in the domestic area they have now grown by more than 200% and this has a serious impact on the power quality expected by users.
Harmonics may cause switchboards to vibrate and motors, power cables and transformers to overheat. They may also affect the power factor correction banks and failure of standby gensets or CHP units. The extra heat generated will eventually reduce the life expectancy of all the components within the chain so resulting in a catastrophic failure. Even the motor bearings will fail due to the high level of rotor shaft currents.
Voltage distortion (THDU) is caused mainly by the high level of current distortion (THDI) and the level of THDU is very much dependent on the source impedance. Higher source impedance means higher THDU and it is therefore worth keeping an eye on this relationship during the design stage by making sure that a larger source transformer is selected with adequate K rating (K9, K13, etc) so that critical loads see fairly low THDU (4-6%) to guarantee peak performance. It is recommended that regular audits are carried out by the FM team at key points within the buildings to make sure that THDU levels at the point of use meet the requirements of any sensitive equipment connected.
Most pcs and similar equipment utilising Switch Mode Power Supply (SMPS) demand a 3rd, 9th and 15th order of harmonic currents, termed as TripleN, which have a return path through the neutral conductor. However, these TripleN harmonics do not cancel each other out, but instead add to each other. This is why most of the so-called intelligent buildings need to have double-rated neutral. Hence, it may be worth using a five core cable instead of the standard four core. Cables may overheat if, during design stage, adequate attention is not given to take account of the skin effect, since higher order harmonics need greater conductor surface areas rather than larger cross sectional areas. High frequency currents do not penetrate up to the core of the conductor. At 160 Hz, one skin depth (the depth below the surface of the conductor by which the current has decreased to 36.8% of its surface value) is about 5 mm. In view of this, switchboards often use laminated busbars rather than solid copper ones when dealing with harmonic-rich loads.
UK regulations were changed some three years ago to cover for the higher neutral currents. If the neutral current is equal to or higher than phase current, the neutral shall be treated as a live conductor. This may require the use of larger breakers, or those which have double-rated fourth poles. Depending on the amount of neutral current, one can expect a high level of neutral to earth potential.
Power factor
It is essential that true RMS reading instruments are utilised for all power quality related measurements, as an average reading meter will not take account of any harmonics and hence will not depict a true picture. Most of the RMS reading instruments will display two types of power factors: true power factor (TPF) that covers all the harmonics, and the displacement power factor (DPF) that only looks at 50 Hz current. Hence it is very important not to mix up the readings. If the two readings of power factors are wide apart, it can be assumed that one is looking at a highly non-linear load. For example, a desktop pc and printer can have 0.99 as DPF and 0.58 as TPF.
Crest factor
SMPS units draw current in small pulses and are normally pulsed at the peak of the voltage waveform. Crest factor for pure sine wave is 1.414. However, it is common to see CF at a level of 2.5-1 in large data centres. Most of the modern ups systems can handle CF levels of 3.5-1 without any voltage distortion.
It is interesting to note that over $24 billion worth of power semiconductors are installed globally each year, and 30% of all power flows through these today. Astoundingly, this present level of harmonic is expected to grow to 70%.
If we do not monitor and manage harmonics, a number of critical applications will fail without any warning and this could be a source of fire risk. In view of this, some of the American insurance companies factor their premium charges for intelligent buildings based on the neutral size utilised (calculated on pcs and servers).
In Europe the cost of poor Power Quality is estimated to be around 10 billion Euros per annum. Prevention can be achieved by spending just 5% of that amount.
For more information circle the appropriate number on the Enquiry card or see www.emconline.co.uk/enquiries: MGE UPS Systems 160
Harmonic mitigation methods
Passive solutions
- Oversizing of input source – this is expensive and does not eliminate harmonics
- Oversizing of cables – again, this increases costs but fails to eliminate harmonics
- Isolation transformer – this option blocks all of the ipleN (3rd, 9th and 15th) harmonics. However, it has no effect on other harmonics and needs to be K-rated with zero phase shift. It can also be utilised to provide the required galvanic isolation
- Interconnected star transformer – here, we have a good solution to mitigate the 3rd harmonic and it is sized just for the harmonic current, not the entire load
- Inline choke (inductor) – this is fairly simple and common, with a variable speed drive application. It reduces THDI to some degree. Harmonic reduction is very much dependent on the level of load current
- Tuned LC filter – it is worth bearing in mind that this solution only attenuates harmonics at tuned frequency. It provides a cost-effective solution, provided the harmonic spectrum and load current remain at the planned level.
There is a risk of resonance
Active solutions
Active harmonic filters/conditioners are suitable for most applications and can be preset to eliminate all the main harmonics from 2nd to 50th. There is built-in flexibility to cover for any change in the load harmonic spectrum. However, one must select only the shunt type active harmonic filters, rather than the series-connected ones, to offset the threat of overload or malfunction. There is no risk of resonance with this type of unit. In the long run, this is a very cost effective method which is easy to adopt. It also meets all of the standards designed to contain harmonics.
Source
Electrical and Mechanical Contractor
Postscript
Shri Karve is director of business development at MGE UPS Systems.
