The rise of non-linear loads in industrial environments over the last two decades has resulted in the problem of harmonic currents and utility-level voltage distortion.
When The Beach Boys tell us they’re “pickin’ up good vibrations” it’s fair to say that they probably hadn’t heard of the importance of power quality. Ironically given that musical reference, the very issue that the industrial community has increasingly struggled with has always been an essential and positive technique in pop music. The acoustics created by a guitar amplifier often rely on the distortion of the fundamental frequency by adding multiple sound waves or overtones to create a warm, “fuzzy” sound.
In industry, voltage distortion caused by current harmonics can wreak havoc on plant, their equipment and the mains power supply. Damage can be serious and varied with the most common symptoms including 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.
This is becoming a cause for concern as 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.
Industry challenges
Various approaches have been taken to combat harmonics over the years. The result was that many suppliers use setups not meant for harmonic mitigation in configurations which are often unnecessarily complex, outdated, bulky or inefficient, ultimately raising costs.
There is also the 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 have weak grids with unreliable supplies 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 only for the harmonic disturbance. There is a clear decision to be made between series-passive, shunt-passive, series-active and shunt-active solutions.
Fig. 1: Active harmonic filters.
Series-passive
The most straight forward series-passive solution can be achieved using a line reactor. This is a 3-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, it is not perfect, not suitable for large drives and will be unable to meet IEEE519 standards on its own.
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 upgrade ability, monitoring and redundancy, meaning that, if the filter fails, the drive fails.
The last series-passive solution is multi-pulse, 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 must be given to planning the drive system and deployment as units are often large, heavy and difficult to retrofit.
Fig. 2: A range of passive harmonic filters.
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 developed principally to resolve reactive power and not specifically for harmonic mitigation.
A weakness inherent in 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) VSD. It replaces the rectifier diodes in a regular VSD with an IGB- 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 this soon adds up. For the panel builder/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 two-and-a-half years.
AFEs are great at lowering THD significantly and maintaining good power factor but they have some serious drawbacks.
Lower switching frequencies are used to maintain a small form factor, but they 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 increase return on investment.
Shunt-active
Finally, for shunt-active solutions, users may consider active filters. They use IGBT technology and are particularly suited to VSD harmonics. They 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 for 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.
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. So now we can feel good about embracing “good vibrations”.
Contact Jim Rosser, CP Automation, Tel 012 667-2121, jim.rosser@cpaltd.co.za
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