Harmonic Mitigation at the Device Level, to Help Keep Power Supply Clean

The Growing Necessity: Harmonic Mitigation at the Device Level, to Help Keep Facilities’ Power Supply Clean

(New design technology enabling mitigation at partial motor loads)
 
Line-Current Harmonics Defined
 
Line-current harmonics are injected to the electrical network by non-linear loads connected to the network, and 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. The harmonics also affect the voltage waveform -- and that can cause malfunction in other sensitive devices that are connected to the same transformer as those producing harmonics.
 
Growing Concern
 
Harmonics are more of a concern today due to two factors: the extensive use of harmonic-generating equipment; and more of the equipment now being installed in stand-alone machines and processing lines is sensitive to harmonics. Harmonic distortion can result in mis-operation of sensitive electronic equipment and generators. 
 
Harmonic considerations also are part of meeting EMC requirements; to be exact, 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 both current and voltage distortions. The term THD (Total Harmonic Distortion) is often used to describe the harmonic contents in the line current and is calculated with the following formula: 
 
 
 
(Isis the total rms current, Is1 is the fundamental component and Ish is the harmonic component of order h.)

 

Mitigation at Device Level Reduces Heat Losses in Transformer, Network
 
Because the current harmonics from all devices connected to the supply system sums up in the network, a small reduction of the harmonics generated in every device could lead to remarkably big savings in transformer heat losses. The heat losses in a transformer are directly related to the loading of the transformer, and the 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 Is1 regardless of the choke. The additional losses are caused by the unwanted harmonics generated by the device. It can be seen from the formula below that the total rms current will be smaller if the THD is smaller; this reduces the losses in the network:
 
 
Harmonic Reduction at Partial Load Complements Varying-Load-Control Function of VSD
 
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 is used in order to accomplish precisely this efficient, demand-side energy usage -- adjust speed and torque as required; therefore, these devices often operate for sustained periods at partial load.
 
In rectifiers with conventional LC-filters, the harmonic content increases rapidly as the load decreases. The amplitude of the harmonic currents also decreases, but their part of the rms current increases. By reducing the harmonics at partial load, the harmonics in the whole supply system decreases. This is the basis for the idea of a swinging choke device, able to decrease the THD at partial loads; create a choke whose inductance increases when the load decreases. The size and weight of the swinging choke is the same as for a conventional choke. The only way it differs from a conventional choke is that its’ inductance changes according to the current through it.
 
 
Figure 1: An example of the waveform of the current drawn by a six-pulse rectifier.
 
 
Figure 2: Typical current spectrum of the three-phase rectifier (a) with ideal inductive smoothing (b) only capacitive filtering.
 
Construction of a Swinging Choke
 
The main objective of using a swinging choke in a frequency converter is to reduce the line-current harmonics at partial load. There also are other benefits from incorporating such a choke: it reduces the peak-line current and the peak-to-peak DC bus-ripple current, especially at partial-load conditions. The 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 the 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. In order to increase the inductance at small currents, a “step” is introduced in the middle of the center post in the EI-core choke (see Figure 3). When the current increases, the “step” begins to saturate, which reduces the inductance of the choke. Figure 4 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 of that, 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.  
 
 
Figure 3: An example of the form of the air gap.
 
 
Figure 4: Simulated magnetic flux density in the Swinging Choke.
 
Testing Mitigation at Partial Loads: Measuring Equipment and Test Set-up
 
The line-current harmonic test is performed on the basis of the prospective IEC 61000-3-12 standard at nominal load. The THD at various different loads also is measured to indicate the relative performance of the swinging choke. The line harmonics are measured with a power analyzer, using current shunts. The 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.
 
The test set-up for harmonic distortion measurements is shown in Figure 5. The value of the additional inductance added to the supply system is chosen in order to adjust the short circuit power of the supply and obtain a suitable value of short circuit ratio Rsce 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 power of the drive under test. The motor is loaded with a generator that is controlled with a regenerative drive, which transfers the 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:
 
 
Figure 5: Test set-up for total harmonic distortion measurements.
 
Measurement Results; Effect on Network
 
The results of the THD measurements for new-generation VFDs from drives manufacturer ABB are presented as an example in Figure 6. According to the measured THD results, it is very clear that the capacity of suppressing harmonics with the swinging DC-choke is higher than of a conventional DC-choke with the same size and weight. Compared to the DC-choke typically used in this manufacturer’s previous generation of frequency converters, the THD is decreased by at least 25% at about half of the rated load.
 
 
Figure 6: Measured input line current THD for a Conventional Choke and a new swinging choke design in an R3 frame size VSD.
 
When the amount of THD is decreased, the transformer heat losses also decrease, as mentioned earlier. Figure 7 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 Figure 6. 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.  
 
  
Figure 7: Transformer heat losses with ACS550 Swinging Choke compared to a Conventional Choke
 
Technology Implemented into VFD Devices
 
The choke design, now patented and built into a new generation of drives, is reducing harmonics at both full and partial loads, for a total harmonic reduction of 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%).
 
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About the Author:
Nicklas Sodo is a Design Engineer, ABB Oy, Drives & Power Electronics, Helsinki, Finland. Contact the author at [email protected]. For more information, please visit www.abb.com.