The perils of an unbalanced network

Frequently misunderstood and often not adequately factored in to engineering specs, harmonics can cause serious headaches for contractors. Unbalanced networks often warrant costly upgrades or rectification, which can leave the contractor carrying the can.

There was a time when neutrals were adequately sized at half the gauge of phase conductors. Of course, unbalanced phases could cause problems, but the increasing incidence of harmonics creating loads now requires much more attention, not only to neutral gauges, but also to harmonics mitigation methods.

Harmonics mitigation works well in theory when loads are balanced in three-phase networks. In practice, however, it’s another matter. The use of active filters in three-phase, four-wire reticulation is seen as an adequate solution to cleaning up harmonics as well as ‘unloading’ the neutral, but an active filtering solution should offer a range of features in order to deliver adequate harmonics mitigation.

Harmonics are a relatively modern phenomenon, which have come about as a result of loads such as electronic ballasts for fluorescent lighting, compact fluorescent lighting (CFL), LED lighting, computers, electronic cash registers, scanners and modern heating, ventilation and air conditioning (HVAC). Harmonics basically cause significant additional current to provide energy to these types of loads.

A good solution for harmonic mitigation - active filters

As the name implies, harmonic filters separate out the harmonic current contribution, preventing that current from flowing through the electrical installation. At least, they do to whatever possible extent as dictated by the constraints of technology. They are particularly effective if placed close to the harmonic-generating loads. Active filters (as opposed to passive), are able to adapt to rapid changes in the harmonic load typical of many installations. They basically function by measuring the harmonic component and providing a suitable anti-phase part of the load current, thus leaving the line current without harmonic components.

In hardware terms, active filters are basically converters feeding DC links serving as temporary energy storage. The conversion takes place on the basis of harmonics to be absorbed, these having been measured in the first place by current transformers. Global or broadband correction takes place by mimicking the distorted current component, the fundamental having been isolated by means of a notch filter. This is basically a very fast method in terms of time response.

An alternative method relies on harmonic analysis and utilising the harmonic pattern for activating the conversion process. It is slower, but allows correction on the basis of individual harmonics.

The cost-effective method

Without doubt, the most cost-effective method of harmonics mitigation in three-phase, four-wire circuits is the use of three-phase active filters rather than single-phase filters. Using an office building application for illustration, these circuits will generate large amounts of 3rd harmonic, as well as higher so-called “zero sequence orders” (9th, 15th, 21st etc), generated by lighting ballasts and power supplies and by computer power supplies.

The HVAC plant in the building on the other hand, comprising compressors and fans, generates the 5th, 7th, 11th, 13th etc. These are negative and positive rotating phasors and are more than likely resulting in circuits fed from separate distribution boards. Phase voltages are therefore also likely to be balanced.

General power and lighting circuits are fed from multiple distribution boards, with some loads straddling two phases. Not only are phase voltages likely to have some degree of unbalance, but phase currents can exhibit large differences. The 3rd harmonic, often very pronounced, is a particular problem that adds significantly to neutral current. For all other harmonics, excluding higher order zero sequences, the vector sums tend towards zero and therefore a small contribution to the neutral current.

Mitigation of the zero sequence harmonics is a technical challenge, as some active filters solve the problem by monitoring the harmonic components in the neutral and correcting for those components. This may seem a suitable solution, but the result is a redistribution of zero sequence harmonics taking place in the phases. In effect, each phase receives exactly one third of the corrected component so that the 3rd harmonic (taken to be the principal component) in each phase is not necessarily zero. This becomes an algebraic sum, taking account of signs, of the original plus 1/3 of the neutral current. If the loads in each phase were balanced this would be the case, but in any other condition there will be net 3rd harmonic flowing in each phase - ie, there will be no correction. So, what appears to be a solution in terms of unloading neutrals has simply shifted the problem back to the phases.

Not all active filters are equal. Ideally, an active filter should offer a wide range of programming possibilities. While many are well suited to cleaning up harmonics in commercial installations, consulting engineers and specifiers should pay particular attention to the methods employed for mitigating zero sequence harmonics, the 3rd harmonic in particular.

Monitoring all three phases by means of individual phase CTs and via programming methods that permit phase-balancing will result in acceptable outcomes for both neutral current phase harmonic mitigation. Additionally, active filters should be able to provide var compensation and to minimise the effect of flicker.