Understanding Harmonics and How to Address Them
Imagine the modern electrical grid as a lake, with every customer’s power source acting as an inlet. A few small sources of pollution can be absorbed by the lake’s ecosystem. But when pollution from multiple sources overwhelms the lake, it disrupts the entire system. Similarly, when utility customers generate poor-quality power, it pollutes the grid and can affect other users downstream. This article explores some effective methods to correct harmonic distortion and enhance the efficiency of your power supply.
The Negative Effects of Harmonics
Harmonics can cause numerous issues within an electrical system. Although this isn’t an exhaustive list, unchecked harmonics can lead to:
- Interference with communication, data processing, and control systems
- High neutral-to-ground voltages in single-phase systems
- Equipment malfunctions, particularly in devices with electronic circuits
- Nuisance tripping, blown fuses, or overload (OL) trips
- Flickering lights
- Malfunctioning switch-mode power supplies
- Power system resonance issues
- Bearing failure in electric motors
- De-rating of power supplies and transformers, reducing system capacity
- Premature aging of electrical insulation
- Excessive heating in conductors and equipment
- Reduced torque and overheating in motors
- Destruction of capacitor banks
- Insulation failure due to high voltages
- Erratic operation of standby generators
- Metering errors
- Telephone interference
- Clock synchronization loss
- Utility penalties and possible liability claims
Solutions for Harmonic Correction
Various solutions can help mitigate the negative effects of harmonics and improve the quality of power supplied to your systems. Below are several effective approaches:
Line Reactors – 30-40% Total Harmonic Current Distortion (THID)
Line reactors are an affordable and straightforward option for harmonic correction. These electromagnetic devices consist of a steel core wrapped with copper coils, creating a magnetic field through which current flows. This limits the rate of rise, reduces harmonics, and protects electrical equipment. There are two types: AC and DC reactors. AC reactors, installed between the power system and the variable frequency drive (VFD), offer better protection by limiting exposure to power surges and fluctuations. DC reactors, installed within the DC link of the drive, provide harmonic correction but lack surge protection. AC line reactors eliminate more than two-thirds of harmonics and are the preferred choice for VFD protection.
Hybrid Filters – 2-15% THID
Hybrid filters combine both active and passive filtering, offering the benefits of each. Passive filters provide power correction and high current filtering capacity, while active filters use a current transformer to analyze the harmonic content of the load waveforms. The filter then generates a signal equal to the harmonics but opposite in phase, effectively canceling them out—much like noise-canceling headphones. While hybrid filters can be expensive, they are highly effective for harmonic correction.
Active Filters on the Bus – 2-15% THID
Installing an active filter on the bus provides similar benefits to other active filters, but with some potential drawbacks. The active filter attempts to correct harmonics for the entire bus, which can lead to overloading and failure. If the filter fails, all harmonic correction is lost, unlike systems where correction is applied at each VFD. Additionally, bus-level active filters may require oversizing to handle harmonic currents from other parts of the system, increasing costs.
Active Front-End VFD – 2-5% THID
An active front-end VFD is an option that can be specified when purchasing your VFD. This setup uses insulated gate bipolar resistors to electronically control switching, shaping the input current waveform to be sinusoidal and reducing harmonic distortion to 5% or less. It also allows for back-feeding into the utility while braking. However, the high switching frequency of active front-ends can create power quality issues, necessitating the use of a low-pass filter to block switching noise, which increases the overall cost.
Distortion Limits and Compliance
With the increasing use of variable frequency drives, computers, motor inverters, and LED lighting, current is often drawn for only a portion of the time that voltage is present. This distorts the current waveform, creating harmonics. When these harmonics are fed back into the utility grid, they degrade power quality for all downstream customers. To address this, IEEE 519 guidelines set limits on acceptable harmonic distortion.
Correcting harmonic distortion is essential to maintaining power quality and ensuring the efficiency of your electrical systems. By taking a proactive approach, you can mitigate the negative effects of harmonics, avoid costly disruptions, and contribute to a cleaner, more reliable power grid.
