This article discusses active and passive filtering techniques for harmonic mitigation.
The rise of non-linear loads in industrial environments over the last two decades has resulted in the growing problem of harmonic currents and voltage distortion. Facing a lack of awareness, the industry has struggled to implement effective mitigation techniques.
Voltage distortion caused by current harmonics can wreak havoc on a plant, its equipment and the mains power supply. Damage can be serious and varied with the most common symptoms being voltage notching, motor vibration, arcing on bearings, nuisance tripping, electromagnetic interference (EMI/RFI) and overheating. The thermal stress on components can cause them to wear out quicker and inefficiency through heat loss results in increased energy costs in the long term.
So why is this now becoming such a cause for concern? The last few decades have seen a rise in the use of non-linear loads such as transistor-based variable speed drives (VSDs) and line commutated DC drive systems. The processes of high frequency switching and pulse width modulation (PWM) introduce unwanted multiples of the fundamental 50 Hz frequency in the form of harmonics.
Fig. 1: A 320 A 3-phase mains harmonics filter.
Industry challenges
Various approaches have been used to combat harmonics over the years. This has led to many suppliers using setups which are not meant for harmonic mitigation, in configurations that are often unnecessarily complex, outdated, take up lots of space or are inefficient, ultimately raising costs.
There is the added issue of meeting international harmonic control requirements such as IEEE-519 which limits ˝the maximum frequency voltage harmonic to 3% of the fundamental and the voltage total harmonic distortion (THD) to 5% for systems with a major parallel resonance at one of the injected frequencies.˝ Some form of filtering is subsequently recommended.
Developing countries often suffer from weak grids with unreliable supply and inadequate infrastructure. The power ratings on products are often based on calculations performed in ideal conditions. Buyers would be wise to note that these products may perform adversely in weak grids and may not perform to IEEE 519 standards in these conditions.
Active versus passive
Passive and active solutions can be installed in both series and parallel (shunt) configurations. Series solutions operate in line with the load, meaning that units must be sized for the full current load. Shunt units can be sized for the harmonic disturbance only. There is a clear decision to be made between series-passive, shunt-passive, series-active and shunt-active solutions.
Series-passive
The most straightforward series-passive solution can be achieved using a line reactor. This is a three-phase choke placed in front of the rectifier. A line reactor provides a low cost way to reduce current harmonics, while adding a level of protection to the rectifier. However, if it’s not perfect, it’s not suitable for large drives and on its own, will be unable to meet IEEE519 standards.
The next option is to use a series harmonic filter. It provides more effective compensation than a line choke, significantly reducing total harmonic distortion (THD). Although a series harmonic filter works well as a “catch-all” it is grid sensitive and may lead to interaction. It’s also bulky and not particularly suited to dynamic applications, working best on pumps and fans on a reasonably well balanced supply.
Of course, truly balanced supplies are few and far between. Any unbalance on the supply can cause damage as a result of overloading and overheating. Series harmonic filters lack upgradeability, monitoring and redundancy, meaning that, if the filter fails, the drive fails.
The last series-passive solution is multi-pulse. Multi-pulse is a multi-winding transformer with phase shift 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 of using multi-pulse is that it’s very sensitive to voltage unbalance. On an 18-pulse drive under 50% load, when the unbalance is increased from 0% to 3% the current THD increases from 10% to 35%. At less than 100% load, the current THD doubles from 8 to 16%. When using multi-pulse, consideration needs to be given to planning the drive system and deployment as units are often large, heavy and difficult to retrofit.
Shunt-passive
Shunt passive is power factor correction, often using fixed capacitor banks, tuned and detuned contactor based units, thyristor capacitor banks and fine-tuned passive filters. These methods were principally developed to resolve reactive power and not specifically for harmonic mitigation.
An inherent weakness of passive solutions is the inability to control the load. The grid loading, along with the filter’s impedance, can cause several fine-tuned shunt filters to interact, resulting in resonance with other equipment.
Series-active
Series-active takes the form of an active-front-end (AFE) variable speed drive. It replaces the rectifier diodes in a regular VSD with an IGBT controlled rectifier to eliminate switching-based signal noise. This circuitry also allows the AFE to introduce regenerative braking. Although this unit may at first seem to eliminate harmonics, it must be noted that, with the AFE in addition to the VSD, there are now two drives in the circuit producing heat.
This means twice the heat and, with a 200 kW AFE, it soon adds up. For the panel builder or system integrator, bigger cooling systems are needed to cope with the excessive heat. In one application where AFE was offered against a VSD and active filtering, the payback in heat loss alone was 2,5 years.
AFEs are great at lowering THD significantly and maintaining good power factor. However, AFEs have some serious drawbacks. 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.
AFEs are generally a plant-level solution; separate regenerative units can be used only where necessary to lower capital expenditure and to increase return on investment.
Shunt-active
Finally, for shunt-active solutions, users may consider an active filter. It uses IGBT technology and is particularly suited to 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 unit which has little loss; is insensitive to grid conditions, cannot be overloaded, and is easy to retrofit. Also, being parallel to the load allows redundancy to be built in. All of this comes at a slightly higher cost, which is offset by the better return on investment over the longer term. The units are sized for the actual harmonic current produced and therefor are in the order of 25 – 33% of the drive rating.
Towards transparency
Effective harmonic mitigation may seem intimidating, but it doesn’t have to be. Understanding the often subtle differences between various techniques can yield better cost savings, reduce complexity and prolong equipment life.
Contact John Mitchell, CP Automation, sales@cpaltd.co.za
