“Non-linearity means it is difficult to solve,” Arthur Mattuck, a mathematician at the Massachusetts Institute of Technology (MIT), once said. But it should be addressed when nonlinearity is applied to electrical loads, because it generates harmonic currents and negatively affects power distribution-and it is costly. Here, Marek Lukaszczyk, European and Middle East Marketing Manager of WEG, a global manufacturer and supplier of motor and drive technology, explains how to mitigate harmonics in inverter applications.
Fluorescent lamps, switching power supplies, electric arc furnaces, rectifiers and frequency converters. All of these are examples of devices with non-linear loads, which means that the device absorbs voltage and current in the form of sudden short pulses. They are different from devices that have linear loads—such as motors, space heaters, transformers that are energized, and incandescent bulbs. For linear loads, the relationship between voltage and current waveforms is sinusoidal, and the current at any time is proportional to the voltage-expressed by Ohm’s law.
One problem with all non-linear loads is that they generate harmonic currents. Harmonics are frequency components that are usually higher than the fundamental frequency of the power supply, between 50 or 60 Hertz (Hz), and are added to the fundamental current. These extra currents will cause distortion of the system voltage waveform and reduce its power factor.
Harmonic currents flowing in the electrical system can produce other undesirable effects, such as voltage distortion at interconnection points with other loads, and overheating of cables. In these cases, the total harmonic distortion (THD) measurement can tell us how much of the voltage or current distortion is caused by harmonics.
In this article, we will study how to reduce harmonics in inverter applications based on industry recommendations for the correct monitoring and interpretation of phenomena that cause energy quality problems.
The UK uses the Engineering Recommendation (EREC) G5 of the Energy Network Association (ENA) as a good practice for managing harmonic voltage distortion in transmission systems and distribution networks. In the European Union, these recommendations are usually contained in electromagnetic compatibility (EMC) directives, which include various International Electrotechnical Commission (IEC) standards, such as IEC 60050. IEEE 519 is usually a North American standard, but it is worth noting that IEEE 519 focuses on distribution systems rather than individual devices.
Once the harmonic levels are determined by simulation or measurement, there are many ways to minimize them to keep them within acceptable limits. But what is the acceptable limit?
Since it is not economically feasible or impossible to eliminate all harmonics, there are two EMC international standards that limit the distortion of the power supply voltage by specifying the maximum value of the harmonic current. They are the IEC 61000-3-2 standard, suitable for equipment with a rated current up to 16 A (A) and ≤ 75 A per phase, and the IEC 61000-3-12 standard, suitable for equipment above 16 A.
The limit on voltage harmonics should be to keep the THD (V) of the point of common coupling (PCC) at ≤ 5%. PCC is the point where the electrical conductors of the power distribution system are connected to the customer conductors and any power transmission between the customer and the power distribution system.
A recommendation of ≤ 5% has been used as the only requirement for many applications. This is why in many cases, just using an inverter with a 6-pulse rectifier and input reactance or direct current (DC) link inductor is sufficient to meet the maximum voltage distortion recommendation. Of course, compared to a 6-pulse inverter with no inductor in the link, using an inverter with a DC link inductor (such as WEG’s own CFW11, CFW700, and CFW500) can significantly reduce harmonic radiation.
Otherwise, there are several other options for reducing system harmonics in inverter applications, which we will introduce here.
One solution to reduce harmonics is to use an inverter with a 12-pulse rectifier. However, this method is usually only used when a transformer is already installed; for multiple inverters connected to the same DC link; or if a new installation requires a transformer dedicated to the inverter. In addition, this solution is suitable for power that is usually greater than 500 kilowatts (kW).
Another method is to use a 6-pulse active current (AC) drive inverter with a passive filter at the input. This method can coordinate different voltage levels-harmonic voltages between medium (MV), high voltage (HV) and extra high voltage (EHV)-and supports compatibility and eliminates adverse effects on customers’ sensitive equipment. Although this is a traditional solution to reduce harmonics, it will increase heat loss and reduce power factor.
This brings us to a more cost-effective way to reduce harmonics: use an inverter with an 18-pulse rectifier, or especially a DC-AC drive powered by a DC link through an 18-pulse rectifier and a phase-shifting transformer. The pulse rectifier is the same solution whether it is 12-pulse or 18-pulse. Although this is a traditional solution to reduce harmonics, due to its high cost, it is usually only used when a transformer has been installed or a special transformer for the inverter is required for a new installation. The power is usually greater than 500 kW.
Some harmonic suppression methods increase heat loss and reduce power factor, while other methods can improve system performance. A good solution we recommend is to use WEG active filters with 6-pulse AC drives. This is an excellent solution to eliminate harmonics generated by various devices
Finally, when power can be regenerated to the grid, or when multiple motors are driven by a single DC link, another solution is attractive. That is, an active front end (AFE) regenerative drive and LCL filter are used. In this case, the driver has an active rectifier at the input and complies with the recommended limits.
For inverters without a DC link-such as WEG’s own CFW500, CFW300, CFW100 and MW500 inverters-the key to reducing harmonics is the network reactance. This not only solves the harmonic problem, but also solves the problem of energy being stored in the reactive part of the inverter and becoming ineffective. With the help of network reactance, a high-frequency single-phase inverter loaded by a resonant network can be used to realize controllable reactance. The advantage of this method is that the energy stored in the reactance element is lower and the harmonic distortion is lower.
There are other practical ways to deal with harmonics. One is to increase the number of linear loads relative to non-linear loads. Another method is to separate the power supply systems for linear and non-linear loads so that there are different voltage THD limits between 5% and 10%. This method complies with the above-mentioned engineering recommendations (EREC) G5 and EREC G97, which is used to evaluate the harmonic voltage distortion of nonlinear and resonant plants and equipment.
Another method is to use a rectifier with a larger number of pulses and feed it into a transformer with multiple secondary stages. Multi-winding transformers with multiple primary or secondary windings can be connected to each other in a special type of configuration to provide the required output voltage level or to drive multiple loads at the output, thereby providing more options in power distribution And flexibility system.
Finally, there is the regenerative drive operation of the AFE mentioned above. Basic AC drives are not renewable, which means they cannot return energy to the power source-this is especially not enough, because in some applications, recovering the returned energy is a specific requirement. If the regenerative energy needs to be returned to the AC power source, this is the role of the regenerative drive. Simple rectifiers are replaced by AFE inverters, and energy can be recovered in this way.
These methods provide a variety of options to combat harmonics and are suitable for different types of power distribution systems. But they can also save energy and cost significantly in various applications and comply with international standards. These examples show that as long as the correct inverter technology is used, the non-linearity problem will not be difficult to solve.
For more information, please contact: WEG (UK) LtdBroad Ground RoadLakesideRedditch WorcestershireB98 8YPT Tel: +44 (0)1527 513800 Email: info-uk@weg.net Website: https://www.weg.net
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