Preventing Interference from Harmonics

Nothing is perfect, not even the distribution and supply of electrical energy. In alternating-current grids, voltage is illustrated by a uniform sine wave, which represents the regular alternation of polarity. In reality, however, these waves are anything but perfect. Sine waves containing so-called harmonics impair the efficiency and reliability of the power grid. Therefore, they must be avoided as much as possible.

As more and more devices and machines that generate harmonics are connected to the power grid, the impact on the grid and consumers is also increasing. The consequences are manifold and begin with the cables themselves. They heat up due to the additional load, which can lead to cable damage and unwanted magnetic fields. Motors, generators, and capacitors can also overheat due to the additional current flow caused by harmonics. In transformers and chokes, harmonics cause iron losses. Voltage drops can occur across coils. Even power outages caused by harmonics are possible—often with serious consequences for the infrastructure.

Given the risks to machinery and equipment, it is in the interest of all operators—whose equipment, as consumers, is part of the power grid—to minimize the generation of harmonics as much as possible. Many grid operators specify limit values for harmonics. On the one hand, this is intended to keep the interference emitted by machines that generate harmonics within acceptable limits. On the other hand, it is also intended to define the immunity of machines operated in grids with harmonics.

However, identifying and addressing harmonics is not easy, because problems do not always manifest immediately or in a clearly visible way. Often, operators are largely unaware of this phenomenon, so they do not even consider harmonics as the cause of the problems that arise.

The Importance of Variable Frequency Drives

The purpose of variable-frequency drives is to precisely match the electrical energy consumed to the requirements of the process by controlling the motor speed. This approach can save a great deal of energy, but at the same time, it can lead to the generation of harmonics. Operators must therefore not overlook the impact of harmonics on the power grid and the devices connected to it. If care is taken during the planning of new systems or a retrofit—such as retrofitting existing equipment with variable frequency drives—to ensure that the system generates as few harmonics as possible, costs can be saved in both the short and long term. For example, a low harmonic content means that cables do not need to be oversized. The power buffer of the transformer—one of the most expensive components in the power grid—can also be sized smaller.

Where low harmonic content is of great importance, plant operators should therefore use frequency converters that ensure low THDi values. The focus here is on so-called AFE frequency converters. AFE stands for Active Front-End. This is a technology for rectifying current that uses insulated-gate bipolar transistors (IGBTs) instead of diode rectifier bridges. The input IGBTs in such frequency converters can be controlled with exceptional precision. This capability ensures that the current drawn from the grid remains sinusoidal. Harmonics are avoided, and the voltage waveform follows a nearly perfect sine curve.