DAC 2021_12

42 n CONTROLGEAR November/December 2021 www.drivesncontrols.com Tackling harmonics: is passive or active best? V oltage distortion caused by current harmonics can wreak havoc in plants, causing voltage notching, motor vibrations, arcing, nuisance tripping, electromagnetic interference and overheating. Thermal stresses can cause components to wear out quicker than normal and result in higher energy costs. Recent decades have seen a rise in the use of non-linear loads such as semiconductor- based VSDs (variable-speed drives) and line- commutated DC drives. High-frequency switching and pulse width modulation (PWM) introduce unwanted multiples of the fundamental 50Hz frequency in the form of harmonics. Various approaches have been used to combat harmonics. This has led to many setups that are not meant for harmonic mitigation, in complex configurations that often raise costs. An added issue is the need to comply with international requirements such as IEEE-519, which limits harmonic frequencies. Some form of filtering is usually recommended. Active vs passive Active harmonic mitigation methods work by effectively cancelling out a harmonic disturbance by generating an opposing compensating current. Passive methods divert currents or block them completely using low or high impedance paths. Passive and active technologies can be installed in series or in parallel, or in shunt configurations. They can block or divert potentially harmful harmonic currents away from the power supply. Because series systems operate in line with the load, they must be sized for the full current load. Shunts are sized based on the harmonic disturbance. There are various options available including series-passive, shunt-passive, series- active and shunt-active systems. Series-passive This can be achieved using a line reactor – a three-phase choke placed in front of a rectifier. It can reduce current harmonics, while adding a level of protection. However, it is unsuitable for large drives and is unable to satisfy IEEE 519 standards by itself. Series harmonic filters These provide effective compensation, significantly reducing total harmonic distortion (THD). Although they work well as a catch-all, they are grid-sensitive and may lead to interactions. They are also not particularly suitable for dynamic applications, working best on a well-balanced supply. Multi-pulse series-passive These are multi-winding transformers with phase shifts in the windings. Because every secondary winding has its own rectifier, an 18-pulse configuration can target and effectively cancel out the 18th, 19th, 35th and 37th harmonics. The downside is that these systems are sensitive to imbalance. At less than 100% load, the current THD doubles from 8% to 16%. The systems are also often large and hard to retrofit. Shunt-passive This is power factor correction, often using fixed capacitor banks, tuned and detuned contactor-based systems, thyristor capacitor banks and fine-tuned passive filters. An inherent weakness of passive systems is their inability to control loads. The grid loading can cause several fine-tuned shunt filters to interact, resulting in resonance with other equipment. Series-active This takes the form of an AFE (active front-end). It replaces the rectifier diodes in a regular VSD with an IGBT-controlled rectifier to eliminate switching-based signal noise and to introduce regenerative braking. Although AFEs may at first seem to eliminate harmonics, heat levels can double and with a 200kW AFE this soon adds up. Panel-builders or systems integrators need to use bigger cooling systems to cope with the excessive heat. AFEs are great at lowering THD significantly and maintaining good power factors. However, to maintain a small form factor, lower switching frequencies are used, which result in high switch ripples on the voltage waveform. This can cause equipment to nuisance trip and malfunction. Shunt-active This may involve active filters and is particularly suitable for dealing with VSD harmonics. It can cancel out harmonic frequencies by injecting equal and opposite, phase-shifted current frequencies. Shunt active filters provide the most efficient harmonic compensation in a compact format with low losses, are insensitive to grid conditions, cannot be overloaded, and are easy to retrofit. They are more costly, but offer a good long-term return-on-investment. Effective harmonic mitigation may seem intimidating, but it doesn't have to be. Understanding the differences between various techniques can result in bigger cost savings, reduced complexity and extended equipment lives. n The rise of non-linear loads in industrial environments has resulted in a growing problem of harmonic currents and voltage distortion. Industry has struggled to implement effective mitigation. John Mitchell, global sales and marketing director at the power quality specialist CP Automation, explores active and passive filtering. Panel-builders and systems integrators are faced with a sometimes bewildering choice of options of tackling current harmonics