Harmonics in Power Systems: Causes, Effects and Mitigation Techniques.

Hello readers. If you are working in an industry in technical department specifically in electrical field, or if you are a student, professor, then this is defenitely for you.

Currently I am working as a Sales Engineer at Teknocrat’s in Power Quality domain and have visited more than 50 manufacturing industries in last two months having different processes and load patterns. I’ve observed that almost 90% of the factories which I’ve visited, have harmonics present in their electrical systems. The thought process behind this blog is to discuss the Harmonics in Power systems: it’s causes, effects and mitigation techniques.

What are Harmonics

It is a voltage or current at a multiple of it’s fundamental frequency. In very simple words, pure sinusoidal waveform doesn’t contain harmonics. This means, if your waveform is distorted, disturbed, not completely sine-wave, then it is because of the voltage and current harmonics present in the power system.

Presence of harmonics is the root cause of many power quality problems.

Types and Causes of Harmonics

Current Harmonics:

Introduction of harmonics (PC:acdrive.org)

In normal case, current waveform follows the ideal sine-wave pattern if the load in linear. In case of non-linear loads such as rectifiers, motors, current does not follow the pure sin-wave pattern. Non-linear loads create disturbances in the fundamental harmonic, which produce all types of harmonics.
Examples of nonlinear loads are battery chargers, electronic ballasts, variable frequency drives, and switching mode power supplies.

Voltage Harmonics

(PC: Tuomas Messo on researchgate.net)

Voltage harmonics are mostly caused by current harmonics.
As nonlinear currents flow through a facility’s electrical system and the distribution-transmission lines, additional voltage distortions are produced due to the impedance associated with the electrical network. Thus, as electrical power is generated, distributed, and utilized, voltage and current waveform distortions are produced. (Link)

Total Harmonics Distortion

Total harmonic distortion, or THD is a common measurement of the level of harmonic distortion present in power systems. THD can be related to either current harmonics or voltage harmonics, and it is defined as the ratio of total harmonics to the value at fundamental frequency times 100%.

where V_n is the RMS voltage of nth harmonic, I_n is the RMS current of the nth harmonic, and n = 1 is the fundamental frequency.

(Reference: W. Mack Grady and Robert Gilleski. Proc. of the EPRI Power Quality Issues & Opportunities Conference (PQA’93), San Diego, CA, November 1993.)

Problems caused by Harmonics

Harmonics can create a huge power quality complexity in your electrical distribution system. One of the major effect of power system harmonics is to increase the current in the system. Additionally, different electrical equipment may suffer from harmonics in the power system.

Here are some of the major issues caused by harmonics

• Source voltage distortion
• Loss of efficiency in transmission and distribution
• Overheating of transformers, cables and motors
• Motor vibrations
• Failures in sensitive electronic devices like PLC and CNC
• Electric motors suffers the losses due to hysteresis and eddy current
• Increased system losses
• Premature ageing of swithcgears and cables

In short, it’s like a slow poison. We may not experience it’s immediate effects, but it is very dangerous as per the researchers. This is the reason it must be mitigated to maintain the healthiness of the electrical system.

Harmonics mitigation techniques 

Basically, there are three basic techniques of harmonics mitigation.

1. Passive Filter:

   

   (PC: electrical-installation.org)

   
   This is an LC circuit, tuned to a specific harmonic order frequency which is to be filtered. This passes the frequency below the set frequency and blocks the upper range. This technique avoids the flow of those harmonics into the distribution system. 

   In this technique, we can only block single order harmonic not more than that. So the drawback is many such filters will be required to make your system harmonic free which is almost impossible.

2. Active Harmonics Filter

(PC: electrical-installation.org)

This is a complete electronics device. It is a combination of digital electronics and power electronics. The Active Filter senses the current from the CTs connected within the plant. with this input Digital Signal Processor orders the IGBT based circuit to inject the opposite phase current signal in the system, so that harmonics current gets cancelled and current waveform remains purely sinusoidal. This active filter has very fast response time close to 1 ms. This helps to keep the electrical distribution system complete harmonics free. This solution can solve your multiple problems related to power quality like harmonics mitigation and power factor improvement. 

For detailed working principle of AHF, please refer this link

Courtesy: InPhase Power

3. Hybrid Filters

(PC: electrical-installation.org)

This system is the combination of Passive and Active technique and constructs the hybrid filter. This new combined filtering solution is becoming very popular nowadays in the industries as it is the most advanced, cost effective solution for all power quality problems. 

Expectation this article will help you to understand the causes, effects of harmonics in electrical power systems and it’s mitigation techniques.

If it’s not too much trouble, comment your views, opinions and get in touch for additional information related to topic. 

Much obliged for investing your valuable time to read this article.