AUTOMATION » Tech Trends Maximum Harmonic Current Distortion in Percent of IL Individual Harmonic Order ( Odd Harmonics ) Isc / IL <11 111000 15.0 7.0 6.0 2.5 1.4 20.0 Continued from page no.130 current (fundamental frequency component) at PCC. TDD = The total root sum square harmonic current distortion, in the percent of the maximum demand load current. Interpretation: 1. As the ratio Isc / IL increases the current distortion limits are relaxed, the reason is that the more the short circuit capacity as compared to the maximum demand current the more is the sturdiness of the incoming power line and the lesser is the voltage distortion for the given current distortion, thus lesser is the power pollution for the given current distortion. 2. As the order of the harmonic increases for the given ratio of Isc / IL, the allowable current distortion reduces. This is because, as the order of the harmonic increases the harmonic becomes more and more harmful. This is due to: • Skin effect increases with the frequency. • Proximity effect increases with the frequency. • Hysteresis losses increase with the square of the frequency. 132 | AUGUST 2015 | ENGINEERING REVIEW Hence as the order of the harmonic increases the allowed level of given order of harmonic is reduced. 3. Even harmonics are not allowed beyond 25% of odd harmonics The reason is that even harmonics are due to single phase circuits or due to unbalances in the three phase converters, both of these should be restricted. 4. The harmonic standard gives separate current distortion limits for incoming voltage between 69 kV and 169 kV and beyond 169 kV. For the same ratio of Isc / IL and for the given harmonic order the allowed current distortion reduces with increase in the incoming voltage. This is because as the voltage level increases higher power levels are involved and the responsibility of keeping the harmonic distortion low increases hence the current distortion allowed reduces with increase in incoming voltage. 5. In the table given above the importance is given to the maximum load current (IL), and not to the actual load current, when the limits for TDD (total demand current distortion) are decided; because the actual current may reduce during some parts of the day (during lunch break etc), and when the machines are loaded to the lesser extent the current distortion increases and it may give wrong warning signal. Whereas even if the distortion increases during less loading the actual harmonic current is less due to less current drawn by the non-linear loads. Thus the harmonic standard IEEE519-1992 puts the appropriate limits on voltage and current distortions. [ Contributed Mr. S.B. Mahajani, Dy General Manager, Amtech Electronics (India) Ltd, Gandhinagar] www.engrreview.com

Engineering Review August 2015