Importance of Choosing Right Harmonic Mitigation Solution

By C.M. Kapoor Director Technical Neptune India Limited

Why choosing the right Partner is important?

Finding the source for implementing the right Power Quality scheme in any installation is a very strenuous task for any plant or Facility Manager. In most instances, there is no single product that is best suited for harmonic mitigation of the electrical network. There may be multiple factors to be considered from the type and density of harmonic generating loads, the effects of harmonics on varied loads, the Power factor of loads, existing setup and infrastructure of electrical network particularly transformer ratings, impedances and design fault levels etc. Penalties from Utilities, frequency and nature of equipment failure and effect on their lifecycle/downtime should also be considered. The list may go on depending on the type of process and facility.

The best technical and economical solution is often achieved by combining various mitigation technologies. This results in a hybrid solution which offers the most optimized results in harmonic mitigation with high reliability, long operational life and most importantly low operating cost (Low losses and maintenance)

This is generally not proposed by the equipment manufacturers because most suppliers have single harmonic mitigation technology and many a time customer objectives and various parameters as mentioned above, are not considered. The investment in such a system becomes redundant in a few years. There are numerous cases where PQ equipment itself becomes the biggest nuisance factor. Instead of just investing in any PQ equipment, Plant Managers should look for PQ companies that can take a comprehensive approach toward their electrical network and objectives and provide engineered solutions to meet their requirements.

Right Understanding of Customer’s Power Distribution Network and defining the objectives

Most of the electrical Loads in Industrial and commercial installations have evolved using advancement in power electronics. The result is better control, increased productivity and higher efficiencies. Heavy usage of 6 pulse VFDs is one such example apart from the other inverter – rectifier based loads. They are categorized as Non-Linear Load as the current drawn is not proportional to the applied voltage.  This can result in high harmonic distortions in both current and voltage, THD-I and THD-V. The generated harmonic currents flow through network impedances (mainly comprising incoming transformers and cables) which cause voltage distortion at each harmonic frequency.  The sum of these voltage distortions results in total voltage harmonic distortion THD-V at Load bus. Its magnitude is controlled by system impedance when the power system impendence is low (high fault level currents), the Voltage distortion will be lower than when the power system impedance is high (low fault level current).  This is why similar loads in two different networks or on different sources like DG sets, UPS have different voltage distortion levels.

Depending on the Harmonic generating load’s density, the cumulative effect on the network’s electrical Power Quality can create many operational problems. In the process, the loads generating the same can also get affected. If corrective action is not taken facility can have multiple failures resulting in downtime, Losses with high maintenance cost.

Only having harmonic distortion values is not sufficient. The PQ solution provider needs to essentially understand the complete electrical power distribution network with the characteristics of various loads to design the proper PQ solution and implement the same to meet customer objectives. How Non-linear loads shall behave in a particular power distribution network is very important for analyses.

Recommended Limits for THD-I and THD-V

The recommended harmonic distortion limits at PCC (Point of common coupling) are given in IEEE 519 -2014 standards. These standards were revised in 2014. They were developed to protect both utilities and consumer networks from high harmonic distortions. As per these standards, maximum demand distortion TDD allowed at PCC is based on the ratio of maximum short circuit current to maximum load demand (ISC / IL). The higher short circuit capacity systems have lower voltage distortion for the same amount of harmonic current injected into the system than systems with lower short circuit capacities. Solutions designed and installed for Controlling THD-I within limits at PCC shall also reduce the voltage distortion levels THD-V.

Some possible Harmonic Mitigation Solutions

After collecting information from the site and harmonic analyzer study readings, most appropriate model for harmonic mitigation should be recommended. The essential criteria for low investment cost, high reliability, long operational life, lower operational losses, and maintenance cost should be kept in mind. The systems should be user friendly. Customer should be able to maintain the systems themselves after commissioning. The various options of systems to choose from are listed below

  • Passive filters for VFDs, UPS or other harmonic generating loads.
  • Appropriate Line chokes wherever required.
  • Automatic Real-time Tuned filters for mitigating dominant harmonics at Mains and PDB levels.
  • Fixed Tuned filters in MV. LV or Load end.
  • IGBT fired modular Active Harmonic filters
  • Hybrid Solution comprising of both Passive and Active technologies.

Right PQ solutions shall offer faster ROI affecting the industry’s bottom line.

 

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