Problems arising from power-system harmonics

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The presence of harmonics in electrical systems means that current and voltage are distorted and deviate from sinusoidal waveforms.

Designers are requested to pay more and more attention to energy savings and improved availability of electricity. This is why harmonics are a growing concern in the management of electrical systems today.

Harmonics have existed from the earliest days of the industry and were (and still are) caused by the non-linear magnetizing impedances of transformers, reactors, fluorescent lamp ballasts, etc… In addition, power electronic devices have become abundant today because of their capabilities for precise process control and energy savings benefits. However, they also bring drawbacks to electrical distribution systems: harmonics.

Harmonic currents caused by nonlinear loads connected to the distribution system are flowing through the system impedances, and in turn distorts the supply voltage.

Such loads are increasingly more abundant in all industrial, commercial, and residential installations and their percentage of the total load is growing steadily.

Examples include:

  • Industrial equipment (welders, induction furnaces, battery chargers, DC power supplies)
  • Variable Speed Drives for AC and DC motors
  • Uninterruptible Power Supplies (UPS)
  • Office equipment (PCs, printers, servers, displays, etc.)
  • Household appliances (TVs, microwave ovens, fluorescent, fluocompact and LED lighting, washing machines and dryers, light dimmers)

Harmonic currents increase the r.m.s. current in electrical systems and deteriorate the supply voltage quality. They stress the electrical network and potentially damage equipment. They may disrupt normal operation of devices and increase operating costs.

Symptoms of problematic harmonic levels include overheating of transformers, motors and cables, thermal tripping of protective devices and logic faults of digital devices. In addition, the life span of many devices is reduced by elevated operating temperatures.

Capacitors are especially sensitive to harmonic components of the supply voltage due to the fact that capacitive reactance decreases as the frequency increases. In practice, this means that a relatively small percentage of harmonic voltage can cause a significant current to flow in the capacitor circuit.

A number of features may be used in various ways to reduce the consequences of harmonics. In this section, practical means of reducing the influence of harmonics are recommended, with particular reference to capacitor banks.

A more detailed overview is presented in chapter Power harmonics management