New technologies were not the only source of harmonics. There are also old technologies used by industries which are sources of harmonics. A good example of this is the electric arc furnace used by steel industries. The arc used to melt the steel is non-linear.
Harmonic current increases the effective heating effects of current depending on its magnitude. The increase heating effect decreases the efficiency, life expectancy of equipment or worst can be a hazard to personnel.
These bad effects of harmonics lead to the creation of standards setting the maximum harmonics contribution for each individual customer. IEEE 519 and IEC 61000 series provide limits on total harmonic distorion. IEEE 519 sets a 5% THD while IEC 61000-3-6 THD limit has to be less than 8%.
Harmonic filters are simple solutions to harmonics problem. Passive filters are the less expensive solution but less efficient. These are basically low pass filters, allowaing the fundamental frequency but limiting the magnitude of its mutliples.
Active filters is an alternative. They provide a dynamic harmonics correction. The magnitude of harmonic order is calculated and an out-of-phase of equal amplitude is produced by the filter cancelling out that particular harmonic. The limitation of this solution is that, it does not correct harmonics in a full spectrum. Only selected multiples are corrected. A variant of this is the full spectrum harmonics cancellation.