When harmonic content exceeds the recommended limits in IEEE 519 Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, correction needs to be made to avoid interference with other equipment or customers in the electric distribution system.
Power electronics and other nonlinear loads can produce harmonic current at a combination of orders (frequencies). In addition, some nonlinear loads can begin to fail over time causing higher harmonic distortion. Filters can be installed at the harmonic current producing device to offset and reduce this distortion. However, a harmonic study needs to be performed to determine the actual distortion level. Then, the distortion level should be compared to the recommendations in IEEE 519. This study is used to properly size and design filters.
There are three common filter types used to combat harmonic distortion: line reactor, passive, and active.
Line reactors can be used to mitigate the harmonic distortion from adjustable speed drives (ASDs), specifically for pulse width modulated (PWM) type topography. Six-pulse drives perform the worst by producing the most harmonic distortion. A 12-pulse drive performs the best. However, the cost is typically the main reason these drives are not often used. The line reactor is effectively a wire-wound coil with an iron core that provides more inductance to the line side of the drive. On average, a 3% impedance line reactor provides an effective reduction of the harmonic distortion. Line reactors provide additional benefits for ASDs by dampening the effects of oscillatory transients caused by power factor correction capacitors switching on.
Passive filters use capacitance, inductance, and resistance to reduce a specific harmonic distortion order. These filters are fixed and not adjustable to varying harmonic changes. Passive filters are a cost-effective solution for most applications. You might be required to filter each harmonic distortion level that is above the recommendations in IEEE 519. To avoid parallel resonances, it is important to filter the lowest harmonic order first. For example, the 5th harmonic needs to be filtered before the 7th harmonic can be filtered.
Active filters are more costly than passive filters due to the power electronics required. The active filter adjusts the level at which the changing harmonic distortion is filtered. Even though active filters are more costly, the resonance problem is eliminated by the design. The design works by injecting missing current in the sine wave, which was removed by the power electronic (nonlinear) operation. A major benefit to this filter is the ability to correct more than one harmonic order at a time. Power factor and voltage flicker can also be mitigated by the active filters.
