Saturday, August 13, 2011

Passive Harmonic Filters are currently the most common method used to control the flow of harmonic currents. They are built using a series of capacitors (capacitance) and reactors (inductance) forming an LC circuit in parallel with the power source. More complex designs may involve multiple LC circuits, some of which may also include a resistor. The passive harmonic filter is also referred to as a trap because it absorbs the harmonic current to which it is tuned.

Passive Harmonic Filters
Passive Harmonic Filters
Basic Design and Operation Principle

The basic combination of capacitors and inductors forms a tank circuit, which provides a low impedance path for the targeted harmonic frequency. The idea is to properly size the LC circuit with respect to its location in the system to achieve the same resonance frequency of the harmonic to be eliminated. In other words, unwanted harmonics are diverted into the filter, preventing them to flow into the power source. As a result, the harmonic current is dissipated as heat by the passive harmonic filter instead of being exported to the utility system and other end-users. However, this heat dissipation should not be considered a loss due to the harmonic filters as it already existed in the system in unusable frequencies.

In addition, the ratings of the capacitors and reactors to be used as filter must be properly selected based on the following technical parameters:

·         Harmonic order to be removed
·         Harmonic profile of the load current
·         kVA requirements of the load
·         Harmonic factor of neutral current
·         System data and configuration (i.e. power factor, one-line diagram)

Moreover, a detailed power quality study would be necessary to assure that connection of the shunt passive harmonic filter will not cause a resonance condition on the power system. This is because harmonic resonance can result to the magnification of certain harmonic frequencies.

Furthermore, the design of passive harmonic filters involves several key considerations as outlined in IEEE 1531. These include harmonic limitations, system conditions, and the filter conditions.

Related Content: IEEE 1531 Harmonic Filter Design Considerations 

Points of Installation

Passive Harmonic Filters are suitable for use with a single or group of loads- both linear and nonlinear. In other words, tuned shunt passive filters can be installed near individual loads or at the supply mains for bulk facility filtering.
Shunt Passive Harmonic Filter Installation
Shunt Passive Filter - Common Installation
Unfortunately, the full harmonic current will continue to flow between the passive harmonic filter and the nonlinear loads. This means that the benefit provided by the shunt filter is only experienced by that part of the electrical system upstream of the filter connection point. For example, if a passive harmonic filter is installed on the supply mains, the harmonics will continue to flow on the facility’s electrical system, particularly between the downstream loads and the filter’s point of connection.

Detuning the tuned filter

Aside from the risk of harmonic resonance condition, the shunt filter may also attract harmonics from all sources connected to the electrical system due to the low impedance it presents at the tuned frequency. For this reason, the following methods are implemented:

Individual Loads

A reactor should be connected in series with the passive harmonic filter’s input. This combination practically eliminates harmonic resonance issues and the concern of attracting harmonics from upstream loads. This is because the line reactor shall increase the source impedance making the passive filter to be very efficient at absorbing and dissipating harmonic current as heat.

Group of Loads

In a centralized installation, the filter is typically designed with the ability to switch one or more tuned circuits into and out of the system as required. This will allow you adjustment of shunt filter capacity based on the actual load, as well as prevent overcompensation by controlling the amount of capacitance to be added to the system.

Common Types

Filtered Power Factor Correction Units

Generally, this type of passive filter is sized to maintain system power factor to unity. It consists of several LC circuits, referred as stages, which are switched ON or OFF by an electronic power factor controller.

Usually, each stage is tuned to provide a low impedance point just below the 5th harmonic (i.e. 4.7th). Yet, in cases where rigorous harmonic reduction is necessary, multiple tuning frequencies such as 5th, 7th and 11th are implemented. An example is when telephone interference limits require a low amount of harmonic current being delivered to the utility system even at higher frequencies.

Drive Filters

This type of filter is preferred when only a few nonlinear loads such as variable speed drives (VSD) are present. This method uses a series connected line reactor and a parallel connected capacitor-inductor circuit tuned at the target harmonic and is connected in series with the load.
Drive Harmonic Filter Block Diagram
Drive Harmonic Filter Block Diagram
Drive filters are very effective in mitigating harmonics, especially when properly sized. It can maintain a lagging power factor down to VSD loads less than 75% for voltage source drives. At loads less than 50% the power factor would most certainly be leading with voltage source drives if meeting IEEE 519 secondary targets at 100% load is required.

On more advanced units, the capacitor-inductor circuit can be switched ON and OFF to avoid leading power factor when the VSD is not running.

IEEE 1531-2003. Guide for Application and Specification of Harmonic Filters
Thota, S. (2003). Harmonic Filters Overview – Part 1

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About Me

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.