Power System Interharmonics Show That No Power System Is Perfect

Key Takeaways

• Real AC power sources, such as a modern electric grid, will deliver input power with harmonic distortion.

• Interharmonics are one form of distortion generated from three possible sources.

• Industry standards place limits on allowed total harmonic distortion, including power system interharmonics.

No electrical system is perfect, including civilian power systems.

For as long as AC power systems have been used for civilian and industrial power, power system interharmonics have created noise and efficiency problems. These arise in motor drive systems and any AC system with a nonlinear load or nonlinear power conversion components in the signal (e.g., ferrite transformers and EMI filters). As much as we like to think that AC power is a clean sinusoidal signal, real AC power is noisy and must be cleaned up, both at the distribution end and at the end device.

Devices plugged into AC mains should not disturb the input power from which they draw beyond certain limits. These disturbances create harmonic distortion, which is composed of the desired fundamental frequency (50 Hz or 60 Hz), higher-order integer harmonics, and non-integer interharmonics. These power system interharmonics are unavoidable in modern power systems, but there are some simple design steps you can take to suppress this source of power distortion.

What are Power System Interharmonics?

Any AC wave has a specific amplitude and frequency. We can define the wave’s harmonics as integer multiples of the wave’s frequency. Similarly, we can define the wave’s interharmonics as non-integer multiples (greater than 1) of the wave’s frequency. When these interharmonics are present in a power system, we call these power system interharmonics. Note that there are also subharmonics, which are fractional multiples of the wave’s frequency. These various frequencies are shown in the graphic below.

Graph showing harmonic content ranges in real power systems.

In the above graph, all harmonics of the fundamental frequency are numbered with the index n. The subharmonic content and interharmonic content can span anywhere between these frequencies. When taken together, the higher-order harmonic content (n > 1), subharmonic content, and interharmonic content combine to produce total harmonic distortion (THD).

THD is an important overall metric for assessing the quality of power delivered to a load. This metric is used to specify allowable limits on total harmonic distortion in power systems under the IEC 61000-3 standard. Low, medium, and high voltages are defined in this standard, each of which has permissible limits on THD (see the table below), as well as on individual even and odd harmonics. These limits are meant to ensure overall efficiency is high for other power users who connect to the grid.

Causes of Power System Interharmonics

Power systems ideally carry a single wave with defined frequency (for single-phase power), or multiple waves separated by a specific phase arrangement (for polyphase power). In addition to these desired components, various portions of the system or devices connected to the grid can generate additional harmonics or interharmonics, where groups of interharmonics can often be linked to specific sources. Power system interharmonics are generally produced by four sources:

• Transients. When system components abruptly change states (e.g., switching on or off), it will produce some transient current draw that propagates throughout the system. These transients can occur randomly, or they can occur at periodic intervals that are unrelated to the AC power frequency. Variable load drives are one example.

• Communication signals. These signals are sent through the grid so that various components (e.g., meters and alarms) can communicate with each other. These signals will have frequency components that are unrelated to the desired AC power frequency.

• Asynchronous switching. Static converters at substations may switch at a time and frequency that are unrelated to the desired AC power frequency.

• Nonlinear loads. Any nonlinear load will draw a distorted current waveform from the power source, which increases THD. Saturating ferrite core magnetics and switching regulators are excellent examples, both of which can draw heavily distorted current pulses from the power source.

• Intermodulation. Frequency mixing in a nonlinear load between interharmonics, or between the fundamental and any interharmonics, also produces new interharmonics.

The harmonics and interharmonics generated by these sources can span across very high values of n. It’s common to measure harmonic/interharmonic content out to tens of kHz for AC mains. Similarly, subharmonic content can produce low-frequency DC drift that may not be compensated by the filter capacitor on a rectifier. This has motivated a number of methods for removing interharmonic content from power systems.

Suppressing and Removing Interharmonics

Each of these has to be addressed in its own way; there is no single solution that addresses the entire breadth of harmonic content in a power system. Modern electric grids in the US and Europe generally have low levels of interharmonic distortion because there are presently few large interharmonic sources on the system. However, in industrial settings or in developing areas, other methods may be needed to reduce harmonic distortion to acceptable levels:

• Filtering. Using wideband low-pass or bandpass filters is the simplest way to remove unwanted harmonics over a very broad range of frequencies. Higher-order filters could also be used to provide higher roll-off. This is a common strategy used for components or subsystems that sit on the grid.

• Power factor correction. This is a common method for smoothing out the current draw in a product that connects to AC power through a wall outlet. A PFC circuit uses the transient charging/discharging current in an LC circuit to regulate current draw and recreate the ideal current waveform required at the input. This is a common strategy with switching regulators that draw high current.

No matter which strategy you want to use to remove power system interharmonics, you’ll need a set of simulation tools integrated into your circuit design software. This helps with front-end design evaluation before you move on to a physical layout. You can also iterate through different component values to determine the best design for your system.