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• • • EDITOR’S CHOICE • • •


Why active front end drives are the secret weapon in the battle against harmonics


ABB’s Liam Blackshaw, UK product manager, LV Drives, explores harmonic mitigation strategies, and why prevention is often better than cure


V


ariable speed drives (VSDs) offer many benefits for motor-driven applications, but they are also one of the major culprits for


harmonic distortion.


VSDs are one of the single most cost-effective measures that process industries can implement to save energy. Most motors with no speed control run either at full speed or not at all, which can waste vast amounts of energy and money. A drive controls the motor’s speed electronically, allowing it to ramp up and down according to actual demand. Modern VSDs can achieve an extremely precise degree of control, while also delivering a range of other benefits for the safety, reliability and productivity of industrial operations. So far, so good. However, for all their benefits, using drives can also introduce new challenges. Harmonics are distortions of current and voltage waveforms caused by introducing non-linear loads to the grid. These can include switched-mode power supplies, LED lighting, photocopiers, computers and televisions, as well as solar inverters and EV chargers.


Introduce too many of these devices, and their cumulative effect will affect power quality and disrupt equipment operation, resulting in overheating and nuisance trips. Drives are a non- linear load, and so can be part of the problem, however solutions are available to not only mitigate the presence of harmonics, but to prevent them from occurring at the source.


Assessing harmonics It is often thought that drives are the main culprit for harmonics, but more often than not they are merely the straw that breaks the camel’s back, adding to existing harmonics already on the


system to reach a tipping point. Harmonic frequencies are essentially a multiple of the fundamental frequency of the power supply. All sinusoidal signals are composed of the fundamental frequency and the 3rd, 5th, 7th and higher, harmonics. For example, if the fundamental frequency of a circuit is the supply frequency of 50 Hz, harmonic current frequencies of a 6-pulse, three phase rectifier are n times the fundamental frequency. So, a 150 Hz (3 x 50 Hz) waveform is the 3rd harmonic, a 250 Hz (5 x 50 Hz) waveform is the 5th harmonic, a 350 Hz (7 x 50 Hz) is the 7th harmonic and so on.


The presence of harmonic content is measured as a percentage value known as the Total Harmonic Distortion (THD). This is the relationship between all the current or voltage harmonics and the fundamental current or voltage. Where no voltage or current harmonics are present, the THD is 0%.


Harmonics and their effect on systems


Harmonics can potentially have an adverse effect on the network and the connected equipment when they exceed certain limits. Among the problems harmonics can cause are increased power losses in the circuit as well as power quality problems. They can also affect the torque in motors and cause overheating and interference in motors and oscillators. Harmonics can also interfere with communications and control systems, with some of the most visible symptoms being the flickering of lights, nuisance tripping of devices, and blown fuses.


Unwanted distortion can also increase the current in the power system, leading to higher temperatures in conductors and transformers. This can cause premature failures in components such as motors and insulation, reducing their expected lifespan.


As well as the distortion, the increase in line current also requires the power system to be over- dimensioned to carry this higher current and avoid the overheating that could cause a fire. This inevitably leads to increased costs on materials such as cables and more robust connections. As well as increased energy use or the higher project costs that can result from oversized components, harmonics can also cause a more serious issue, namely reliability of the power supply for a facility.


In mission-critical facilities like hospitals, power supply failures can lead to serious consequences for patients’ lives, while in process facilities, they can have significant cost implications in terms of lost production.


If power interruptions due to harmonics are


likely, facilities are often provided with a backup generator. These in turn must be prepared for feeding non-linear loads that produce high harmonic content. In general, a generator which supplies 6-pulse drives needs to be oversized 2 to 2.5 times.


If a generator is not oversized, its automatic voltage regulator might not operate properly because of excessive harmonics. Under these conditions, the generator might well trip.


Mitigation strategies There are several ways to mitigate harmonics. Recommendations for cable sizing take the increased network current, known as THDi, into account – a THDi below 10% does not require any additional oversizing, while with the 40% THDi typical for standard 6-pulse drives with built-in impedance, an oversizing of about 10% is required. For an extreme case with 70% THDi, oversizing of above 20% is needed. Transformers, which are often one of the most expensive power network components in a project, will also often need to be oversized. When a non- linear load (like a drive) is supplied from a transformer, the transformer capacity needs to be derated to avoid overheating and the consequent risk of failure. For a THDi close to 40%, transformer equipment should be oversized by about 40%, and for a THDi below 10%, oversizing of about 10% is recommended.


10 ELECTRICAL ENGINEERING • NOVEMBER 2023 electricalengineeringmagazine.co.uk


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