Passive Harmonic Filter Selection Guidelines

Picking a passive filter solution may seem like an overwhelming task. Some manufacturers claim the only option is a low capacitance design, but at the price of performance. Our approach at TCI is to provide a wide range of solutions to cover different harmonic filtering applications, as not all passive filter designs or applications are created equal. Don’t settle on quality or performance without knowing the whole picture.

Passive Filter Basics

Passive filters are an effective solution to filter harmonic currents produced by 6-pulse Variable Frequency Drives (VFDs) and reduce the source current THD to 5% or less. Passive filters are typically constructed with a series line reactor (Lr) and a shunt circuit made of a tuning reactor (Lt) and capacitor (C), as shown in Figure 1 below.

The tuning circuit supplies the harmonic current needed by the VFD and the capacitor supplies leading current (or leading VARs) at the fundamental frequency of the power system. When the VFD is lightly loaded, these leading VARs flow to the voltage source – typically a utility transformer or generator. As the VFDs load increases it draws more lagging VARs, which consume the capacitor VARs and reduce the leading VARs that flow to the source.

Figure 1: Simplified one-line of a passive harmonic filter.
Figure 2: Generator Capability Chart and Passive Filter Load Curve

Generator and Power Factor Guidelines

Synchronous generators have limited capability in providing either leading or lagging VARs as is illustrated in a typical generator capability chart as shown in Figure 2 [1]. The green shaded area is the normal operating range of a typical generator, the yellow is abnormal but not damaging, and operating in the red region will cause damage. The reverse or leading kVAR level at the boundary of the stability region is approximately 0.2pu of rated KVAR.

For example, a generator rated at 100kVA and a 0.8 PF is rated for 80 kW and 60kVAR. A leading kVAR of 0.2pu equates to 12kVAR, which would exceed the capabilities of the generator. Thus, a generator rated for 0.8 PF, can support only about 0.12 of its rated kVA at no load. This level is detailed in application examples from generator suppliers [2], [3].

To examine compatibility of passive filters without contactors and small generators at no load, consider the following example:

  • Generator: 100kVA, 0.8PF. Rated for 80kW and 60kVAR
  • Passive filter: 100HP with 15 kVAR capacitance (0.15kVAR /HP)

The leading kVAR produced by the filter at no load is 15kVAR. Since the generator is rated for 60kVAR, the generator needs to produce 0.25pu leading kVAR (15kVAR / 60kVAR). This exceeds the maximum 0.2pu capability level. The load curve of the 100HP filter mapped onto a 100kVA generator capability graph is shown in Figure 2.

To achieve generator compatibility for this example, a contactor must be used to keep the capacitors offline during light VFD loading conditions. A low level of capacitance in a passive harmonic filter can help in a generator application; however, a contactor still should be used on every passive harmonic filter on the market.

Both TCI’s HGP and HGL have contactors installed to avoid leading power factor while still delivering the harmonic performance required to meet customers’ needs.

TCI’s HGL is a low capacitance passive filter which allows customers to size their filter as low as a 1:1 ratio of Filter HP: Generator kVA.

TCI’s HGP is a passive filter which allows customers to size their filter as low as a 2:1 ratio of Filter HP: Generator kVA.

Drive Impedance Considerations

The amount of impedance is critical to the overall performance for all passive harmonic filters. Some manufacturers claim IEEE-519 compliance and no need for drive impedance in a passive filter application. On the surface this seems like an advantage and a simple decision, however this is impossible to achieve compliance without sacrificing the quality and performance of the filter.

Passive filters are tuned to eliminate low order harmonics seen by the source, thereby decreasing the current THD to be 5% or less when paired with a 6-pulse VFD. When a low capacitance filter is used, to maintain voltage to the DC bus voltage of the VFD, the inductance of the passive filter needs to be lowered. This directly hinders the performance of the filter but illustrates the fine line of design considerations. With TCI filters, customers can have full output power of their VFD system and still get the harmonic performance needed. Based on IEEE-519, when a customer has an ISC/IL < 20, one would need a filter that performs down to 5%iTHD at full load. The below graph shows how TCI’s generator compatible filters stack up verse a competitor’s filter. Each test had the same 5% source impedance, the 30HP passive filter, and the same VFD with no impedance.

The graph illustrates that without any drive impedance, all three filters would not comply with IEEE-519 when a system had an ISC/IL<20 and need 5%iTHD. This is the critical reason behind TCI’s performance statement which addresses this issue.

This graph shows how the filters stack up when a DC choke was added to the drive for impedance. (The HGP was left off the graph as its iTHD performance is already lower than the HGL or the competitors filter at a given system set up.)

The graph highlights the fact that every passive harmonic filter needs the correct amount of impedance present to meet the performance required from IEEE-519.

TCI’s HGL filter is the market leader for a low capacitance filter. The HGL still allows for full output power of a VFD, and meets the most stringent requirements of IEEE-519 when an application has an ISC/IL of < 20 and requires 5%iTHD at full load.


All passive filters on the market today exceed generator reverse kVAR limits at no load. To achieve generator compatibility at no load without a contactor, the kVAR/HP levels must be below 0.12kVAR/HP as an absolute limit.

As passive harmonic filters utilize a tuned circuit, there is not a filter on the market that can achieve this low level of kVAR, and still meet IEEE-519. TCI’s HGL has the lowest level of kVAR/HP rating and will still meet IEEE-519. Drive impedance must also be taken into account to meet compliance with IEEE-519 without sacrificing the performance or quality of your drive.

Both the HGL and HGP filters are generator compatible as they both come with contactors.

The difference in trap circuit capacitance allows for a customer to choose to more tightly size a passive filter HP rating to generator kVA rating.

For more information on how to size and select a passive filter, please reach out to our Technical Support team at 800-824-8282.


  1. “How to size a genset: Proper generator set sizing requires analysis of parameters and loads”, Cummins Power Topic #7007.
  2. “Synchronous Generators and Leading Power Factor Loads”, ePower News, Fall 2011, Issue 2, Toromont CAT Power Systems.
  3. “Impact of leading power factor loads on syn- chronous alternators”, Cummins Power Topic #6001.