What are the challenges and risks of using VFDs in harmonic-sensitive or sensitive-load environments?

1 What are harmonics?

Harmonics are distortions in the sinusoidal waveform of alternating current (AC) power. They are caused by nonlinear loads, such as VFDs, that draw current in pulses rather than smoothly. Harmonics are expressed as multiples of the fundamental frequency of the power supply, such as 50 or 60 Hz. For example, the third harmonic is 150 or 180 Hz, the fifth harmonic is 250 or 300 Hz, and so on. Harmonics can interfere with the operation of other devices connected to the same power source or grid.

2 How do harmonics affect power quality and equipment?

Harmonics can reduce the power factor, which is the ratio of real power (the power that does useful work) to apparent power (the power that is supplied by the source). A low power factor means that more current is required to deliver the same amount of real power, resulting in higher losses, heat, and costs. Harmonics can also cause voltage fluctuations, overheating, noise, vibrations, malfunctions, and premature failures of motors, transformers, capacitors, circuit breakers, and other sensitive equipment. Harmonics can also affect the accuracy of meters, relays, and protection devices.

3 How to measure and monitor harmonics?

To assess the level and impact of harmonics, it is important to measure and monitor the harmonic distortion in the power system. The most common indicators are the total harmonic distortion (THD) and the total demand distortion (TDD). THD is the ratio of the root mean square (RMS) value of the harmonic components to the RMS value of the fundamental component. TDD is the ratio of the RMS value of the harmonic current to the RMS value of the maximum demand current. Both indicators are expressed as percentages and should be within acceptable limits according to industry standards and regulations.

4 How to mitigate harmonics with power factor correction?

One way to mitigate harmonics and improve power factor is to use power factor correction (PFC) devices, such as capacitors, reactors, filters, or active harmonic conditioners. PFC devices are installed in parallel or series with the VFDs or the load to provide reactive power or cancel out harmonic currents. PFC devices can reduce the current drawn by the VFDs, lower the losses and heat, stabilize the voltage, and protect the equipment from harmonic damage. However, PFC devices must be carefully selected, sized, and configured to avoid resonance, overloading, or other problems.

5 How to mitigate harmonics with energy saving techniques?

Another way to mitigate harmonics and improve power factor is to use energy saving techniques, such as adjusting the speed and torque of the VFDs to match the load requirements, optimizing the system design and layout, and maintaining the equipment regularly. Energy saving techniques can reduce the energy consumption and waste, lower the operating costs and emissions, enhance the performance and reliability, and extend the lifespan of the VFDs and the equipment. However, energy saving techniques must be implemented with proper control and monitoring systems to ensure safety and quality.

6 How to choose the best solution for your application?

The best solution for mitigating harmonics and improving power factor depends on several factors, such as the type, size, and number of VFDs and loads, the power system characteristics and capacity, the harmonic distortion levels and limits, the energy efficiency goals and standards, and the budget and availability of resources. Therefore, it is advisable to consult with a qualified engineer or a reputable vendor to conduct a thorough analysis of your application and recommend the most suitable solution for your needs.