November/December 2017

Drives & Controls & BACK TO BASICS Stuart Foster, ABB’s drives manager for the water industry in the UK, examines various approaches to tackling the problem of harmonic distortion caused by non-linear loads such as AC drives. H armonic distortion is a significant and growing challenge for many industries. Harmonics are caused by non-linear loads on the network, such as fluorescent lights, computers and AC drives. When adding drives at a site, the total harmonic load needs to be considered. Most drives have six-pulse diode rectifiers, which give relatively high levels of total harmonic distortion (THD) – around 40%. Twelve- pulse rectifiers, introduced some years ago, can reduce THD to about 12%. However, active front-end drives are now common, and these have a THD of only 2–4%. With an active front-end (AFE) drive, harmonics are dealt with inside the drive and never enter the network. This is in contrast to the traditional technique, which is to filter out harmonics after they have been created. An alternative approach is to use a six-pulse diode rectifier plus a passive filter. However, a fairly high level of harmonics may still exist on the network, even after the passive filter has been installed. Problems may occur in networks that have an imbalance between the phases, as is the case in many rural networks. Under such conditions, the passive filter is less effective. The problems can be especially severe at low loads – around 20% – where the distortion may increase from around 8% to 12%. Comparisons between passive filters and low-harmonic drives are frequently made at full load. However, a true comparison should be made at around 75% load, which is closer to the actual operating point for most pump motors operating at reduced load. The increase in losses tends to be greatest in low-power installations. For instance, a 22kW six-pulse drive with a 43A filter can have losses of 873W, while a low-harmonic drive of the same size will have losses of 490W. Some passive filters are effective only at the lower end of the harmonic spectrum. Also, power factor can be very poor at low load – a power factor of 0.5 at 20% would not be unusual. AFE drives are easier to install, as you only need to consider the mains voltage, the pump absorbed power, and the fault level of the network. The filter also needs to be disconnected at low load, which means that you need to have a discussion with the panel-builder about using additional circuitry to achieve this, and how this should be designed. Each filter must be designed according to the needs of the site at a specific time. If conditions change, the filter may need to be changed. Passive filters should be considered only when there is a small network with a small number of pumps, no generator supply and consistent load conditions. Under these circumstances, the passive filter can provide a cost benefit. Otherwise, it is safest to go with a low-harmonic drive. To find out more about tackling harmonics, including how to register for ABB’s free Lunch ‘n’ Learn training sessions, please visit: http://new.abb.com/uk/campaigns/water-totex/water-harmonics SPONSORED BY Harmonic mitigation: are you leading from the front? igus ® Caswell Road Northampton NN4 7PW Tel 01604 677240 Fax 01604 677242 sales@igus.co.uk plastics for longer life ® BUS MEASURING TORSION HYBRID CAT7 FOC Widest selection of continuous-flex cables Unmatched Testing. Widest selection. 36 Month Guarantee. Shop now: www.igus.co.uk/chainflexshop Free samples available: www.igus.co.uk/cf-sample Reduce downtime with chainflex ® • More than 1,200 cables from stock • 7 cable quality levels available • Cable types include: Control, Data, Ethernet, FibreOptic, Bus, Measuring System, Servo, Hybrid, Motor, Robot #1 ... number one in moving cables