Beware of harmonic loads
Many modern power systems have harmonic loads. This is mainly due to the electronic speed control methods used for induction motors. Harmonic loads can result in resonant conditions depending on the value of the PFC capacitors. The harmonic resonance can result in catastrophic failure of power system equipment.
In the absence of detailed harmonic studies, as a thumb rule, a 5th and a 7th harmonic filter should be installed on capacitors. This may require splitting of PFC capacitors. For example, two 500 kVA capacitors need to be installed instead one 1,000 kVA capacitor. This will enable installation of the 5th harmonic filter on one capacitor and the 7th harmonic filter on the other capacitor.
Interestingly, a harmonic load of one customer can have adverse effect on neighbouring customers. In other words, installation of a PFC capacitor in a neighbouring site can result catastrophic failures. In the absence of the harmonic filter, this quite often results in the failure of the newly installed PFC capacitor!
Who should pay for the cost of the harmonic filter?
This is a good question. The installation of harmonic load by a customer creates harmonic currents. This could be well within the allowable harmonic limits. However, the installation of a PFC capacitor by a neighbouring customer amplifies the harmonic currents! Who should pay for it? The customer who ‘creates’ the harmonic currents or the customer who ‘amplifies’ the harmonic currents? At present, ‘the first in best dressed’ policy applies – whoever comes later pays for it!
I had a consulting job to conduct harmonic studies for a client who was in the process of commissioning an induction furnace load. This harmonic load was supplied by a dual secondary transformer with two phase-shifted 6-pulse rectifiers. It was effectively a 12-pulse rectifier. The harmonic level in such a system is negligible. This expensive solution was recommended by an expert harmonic consultant. Such a system works well when the transformer’s secondary windings have equal loading, which is the normal operating condition. However, there was an imbalance in the load during process start-up which lasted for a few hours. This was not considered in the initial planning studies. It was discovered at the time of commissioning since the utility supply company insisted on the measurement of actual harmonic currents at the point of supply.
The measured harmonic levels during commissioning were much higher than the allowable limits during process start-up conditions. Hence, the electric utility company refused to approve the harmonic load. The customer was frustrated, as they had invested a lot of money on the new plant. The problem was referred to me to conduct harmonic analysis studies and to size the passive harmonic filter!
Extensive harmonic studies with various combinations of passive filters (made up of inductors and capacitors) could not solve the problem. Finally, I could trace the problem to the PFC capacitor which was installed at the utility substation supplying the customer. The PFC capacitor at the substation was amplifying the harmonic currents due to resonance. The simplest solution was to install a harmonic filter on the PFC capacitor at the utility substation. It was not an easy solution though, as it was an old substation and a major upgrade was necessary. Obviously, this solution was not acceptable to the utility authority. There was a stalemate.
I am not sure how the problem was finally resolved. I was out of the loop since a multinational company got in touch with the client to sell their state-of-the-art ‘Active Harmonic Filter’. The multinational company was also happy to conduct harmonic studies free of cost!
Active Harmonic Filters
With advent of power electronic technology, it is now possible to provide dynamic compensation of harmonics at the terminals of the harmonic load. Active filters analyse the harmonic currents absorbed by the load in real time and inject a compensating current with appropriate magnitude and phase. Conceptually, they are like noise cancelling headphones! The harmonic currents are completely neutralised and only the sinusoidal currents flow in the upstream networks. They can also be used in conjunction with passive harmonic filters. Active filters in shunt configuration cannot be overloaded. Shunt active filters will only compensate till their ratings are reached and the balance of the harmonic load is not compensated. Multiple harmonic filters can be installed in parallel as required.
Interestingly, from the perspective of the power flow, the active harmonic filters are effectively a ‘variable reactive power source’. Hence, they can also be used for power factor correction, thus obviating the need for the PFC capacitors!
Active harmonic filters provide a versatile solution to the problem of harmonics and power factor correction! At present, they are more expensive compared with the passive harmonic filters. In spite of their additional cost, many modern installations with harmonic loads are able to justify the installation of active harmonic filters due to their versatility and overall economic benefits provided by them.
