Tips for mitigating harmful harmonics
The rise of non-linear loads in industrial environments over the last two decades has resulted in the growing problem of harmonic currents and utility-level voltage distortion . Facing a lack of awareness , the industry has struggled to implement effective mitigation techniques . Here , global sales & marketing director of CP Automation , John Mitchell , discusses useful tips for mitigating harmonics .
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Voltage distortion , caused by current harmonics can wreak havoc in a plant , damaging its 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 very first place to start is to ensure you comply with regulations . International harmonic control requirement , IEEE-519 , limits “ the maximum frequency voltage harmonic to 3 per cent of the fundamental and the voltage total harmonic distortion ( THD ) to 5 per cent for systems with a major parallel resonance at one of the injected frequencies .” Some form of filtering is subsequently recommended .
We ’ re lucky in the UK to have a stiff power grid , but this is not true everywhere . Developing countries often aren ’ t as lucky . Weak grids with an unreliable supply and inadequate infrastructure are common in other parts of the world . 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 .
Remember to always stay on your toes . 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 50hz frequency in the form of harmonics .
Knowing what options are available to you , can help the overall efficiency of the harmonic mitigation process .
Passive and active harmonic solutions can be installed in both series and parallel ( shunt ) configurations within a system . 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 , shuntpassive , series-active and shunt-active solutions .
SERIES-PASSIVE The most straight forward seriespassive solution can be achieved using a line reactor . This is a low cost way to reduce current harmonics , while adding a level of protection to the rectifier .
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 . Today , I would hope no one is installing capacitor banks by themselves and , at the very least , using de-tuned ones – with an inductor for example .
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 IGBT controlled rectifier to eliminate switching based signal noise .
AFEs are great at significantly lowering THD and maintaining good power factor . However , AFEs have some serious drawbacks . In order to maintain a small form factor , lower switching frequencies are used , which result in high switch ripples on the voltage waveform . This can cause other sensitive equipment like PLCs and telemetry and communications networks to nuisance trip and malfunction .
In addition , 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 twice the heat . This means twice the heat and with a 200kW AFE it soon adds up . For the panel builder or system integrator , bigger cooling systems are needed to cope with the excessive heat .
SHUNT-ACTIVE Active filters provide the most efficient harmonic compensation in a compact unit that has little loss , is insensitive to grid conditions , cannot be overloaded and is easy to retrofit . All of this comes at a slightly higher cost , which is offset by the better return on investment over the longer term .
Understanding the often subtle differences between various harmonic filtering technologies can yield better cost savings , reduce complexity and prolong equipment life . Getting your head around what options are available really is worth it in the long run .
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