Clean Power through Harmonic Mitigation

Harmonics are more of a concern today due to the extensive use of harmonic-generating equipment and because more equipment now being installed in stand-alone machines and processing lines is sensitive to harmonics. Harmonic distortion can result in misoperation of sensitive electronic equipment and generators.

By Nicklas Sodo April 1, 2005
Line current harmonics

Harmonics are more of a concern today due to the extensive use of harmonic-generating equipment and because more equipment now being installed in stand-alone machines and processing lines is sensitive to harmonics. Harmonic distortion can result in misoperation of sensitive electronic equipment and generators.

Harmonic considerations also are part of meeting EMC (electromagnetic compatibility) requirements, specifically, the low-frequency end of the EMC spectrum. This point should not be lost on manufacturers seeking to serve a global marketplace.

All non-sinusoidal waveforms include harmonics. According to the Fourier theorem, any non-sinusoidal waveform can be described by the fundamental wave, plus one or more harmonics. A harmonic is a frequency that is an integral multiple of the fundamental frequency. The fundamental frequency is the frequency of the electrical network, 50/60 Hz. The term harmonic is used for current and voltage distortions.

Device-level mitigation

Because current harmonics from all devices connected to the supply system add up in the network, a small reduction of the harmonics generated in every device could lead to big savings in transformer heat losses. The heat losses in a transformer are directly related to the loading of the transformer; copper losses of the transformer are related to the square of the current. If the converter feeds the motor with a certain power, it requires the same fundamental current component I s1 regardless of the choke. The additional losses are caused by the unwanted harmonics generated by the device. It can be seen in the THD (total harmonic distortion) formula below that the total rms (root-mean-square) current will be smaller if the THD is smaller, thus reducing losses in the network.

Harmonic reduction at partial load

The standard IEC 61000-3-2 and the prospective standard IEC 61000-3-12 only restrict the amount of harmonics at nominal load, but there are no restrictions on harmonics at partial load. Every device connected to the supply system is not operating at nominal load all the time, especially if it is a variable-speed drive controlling a motor’s speed based on the varying needs of the load. A VFD (variable frequency drive) accomplishes efficient, demand-side energy usage by adjusting speed and torque as required; therefore, VFDs often operate for sustained periods at partial load.

In rectifiers with conventional LC-filters, the harmonic content increases rapidly as the load decreases. Amplitude of harmonic currents also decreases, but their part of the rms current increases. By reducing harmonics at partial load, the harmonics in the whole supply system decreases. This is the basis for the idea of a swinging choke device, which can decrease the THD at partial loads by creating a choke whose inductance increases when the load decreases. The size and weight of the swinging choke is the same as a conventional choke. It differs from a conventional choke only in that its inductance changes according to the current running through it.

Swinging choke construction

Air gap is small at low currents and grows as current increases.

The main objective of using a swinging choke in a frequency converter is to reduce the line-current harmonics at partial load. Such a choke also reduces the peak-line current and the peak-to-peak dc bus-ripple current, especially at partial-load conditions. Total harmonic distortion produced by the device with a certain dc-capacitor bank depends directly on the inductance of the chokes in the drive. Larger inductance gives lower THD. With a conventional choke, the THD increases rapidly as the load decreases, because its inductance is constant—regardless of load. Conversely, because the inductance of a swing choke design increases as the load decreases, it reduces the amount of THD at partial loads.

To obtain a choke with non-linear inductance, the form of the air gap is changed. To increase the inductance at small currents, a “step” is introduced in the middle of the center post in the EI-core choke (see “Air gap form example” graphic). When the current increases, the “step” begins to saturate, which reduces the inductance of the choke.

“Air gap form example” graphic shows how the flux density is distributed in the choke when it is carrying a large current. It can be seen that the flux density in the “step” is very high and, as a result, the permeability of the iron in the step is almost the same as for air. This explanation is a bit simplified, but it can be said that the air gap is small at low currents and grows as the current increases.

Testing mitigation at partial loads

Loaded with a generator controlled by a regenerative drive, the motor’s energy is tansferred to another network.s.

The line current harmonic test is performed on the basis of the prospective IEC 61000-3-12 standard at nominal load. THD at different loads also is measured to indicate the relative performance of the swinging choke. Line harmonics are measured with a power analyzer, using current shunts. Current in all three phases is measured, while the frequency converter is running in the field-weakening region—50 Hz. The active power and current drawn from the supply are measured.

Based on the “Test set-up for total harmonic distortion measurements” graphic, the value of the additional inductance added to the supply system is chosen to adjust the short circuit power of the supply and obtain a suitable value of short circuit ratio RSCE (ratio of impedance: load in divided by source impedance), according to the prospective IEC 61000-3-12 standard. The value of RSCE should be above the minimum value 192 defined by the prospective IEC 61000-3-12 standard. This means that the value of the additional inductance depends on the rated

Measured input line current THD for a conventional choke and a new swinging choke design in an R3 frame size VSD.

power of the drive under test. The motor is loaded with a generator controlled with a regenerative drive, which transfers energy to another network, thus not disturbing the harmonic measurements. The star point of the supply transformer is earthed, and there are no other devices connected to the same transformer during the measurements.

Measurement results

The “Measured input line current THD” graphic shows results of the THD measurements for ABB VFDs. Results show that the capacity of suppressing harmonics with the swinging dc choke is higher than that of a conventional dc choke with the same size and weight. Compared to the dc choke typically used in previous generation ABB frequency converters, the THD is decreased by at least 25% at about half of the rated load.

Transformer heat losses with ACS550 swinging choke compared to a conventional choke.

When the amount of THD is decreased, the transformer heat losses also decrease, as mentioned earlier. The “Transformer heat loss” graphic shows an example of the transformer heat losses when incorporating this new choke design, as compared to a conventional choke. This example is based on the THD measurements shown in “Measure input line current” graphic. The transformer heat losses are reduced because the required rms current at a certain motor power decreases. In other words, the power-handling efficiency of the whole network is improved.

The patented choke design reduces harmonics at full and partial loads, for total harmonic reduction up to 30%, compared to traditional reactor designs; and up to 64%, compared to drives with no reactors (estimates are based on a power system impedance of 1%).

Author Information
Nicklas Sodo is a design engineer, ABB Oy, Drives & Power Electronics, Helsinki, Finland;

Line current harmonics

Injected into the electrical network by non-linear loads connected to the network, line current harmonics are multiples of 60 Hz. Non-linear loads are devices the input current waveform of which is non-sinusoidal; in other words, the current waveform does not follow the voltage waveform.

Common examples of such devices found in industrial environments include variable frequency drives (VFD), welders, switch-mode power supplies, battery chargers, UPS systems, computers, electronic lighting, etc. These can generate current harmonics that cause additional losses in the supply system and degrade the active-power-handling capacity of the system. Harmonics also affect the voltage waveform, which can cause malfunction in other sensitive devices connected to the same transformer as those producing harmonics.