Passive Harmonic Filters at Partial Load | ARTPHASE

The Question

A passive harmonic filter may perform very well when the connected VFD load is close to its intended operating point.

But what happens when the load changes significantly?

Consider a simplified system:

One passive harmonic filter rated for approximately 100 A
Two VFD loads connected downstream
VFD 1 operating at approximately 25 A
VFD 2 operating at approximately 75 A

When both drives are operating, the total load is approximately 100 A.

If the 75 A drive stops and only the 25 A drive remains operating, the same filter is now associated with only about 25% of the original VFD load.

The filter has not changed.

The operating condition of the electrical system has.

The Example System

For discussion, consider the following simplified arrangement:

Source → Passive Harmonic Filter → Common Bus → VFD Loads

The connected VFD loads are:

Table 1. Example VFD operating loads

Load	Operating Current	Share of Total Example Load
VFD 1	25 A	25%
VFD 2	75 A	75%
Total	100 A	100%

The current values are intentionally simplified to make the operating-point comparison easy to follow.

They are not intended to represent the guaranteed performance or minimum loading requirement of a specific filter product.

The purpose of the example is to examine a broader engineering question:

How can the operating condition of a passive harmonic filter change when the connected nonlinear load changes substantially?

Operating Point 1 — Both VFDs Running

With both drives operating:

I_{LOAD}=25A+75A=100A

(1)

The connected VFD load is approximately equal to the nominal filter-current value used in this example.

This represents the highest-load operating point being considered.

At this condition, the VFD system produces its corresponding fundamental and harmonic currents, while the passive filter interacts with the source and nonlinear load according to its electrical design.

No specific THDi value is assumed here because actual performance depends on the filter design, drive characteristics, source impedance, system voltage and measurement location.

Operating Point 2 — Only the 75 A VFD Running

Now the smaller drive stops.

The remaining VFD load is:

I_{LOAD}=75A

(2)

Relative to the 100 A example operating point:

\frac{75A}{100A}\times100=75\%

(3)

The filter is now associated with approximately 75% of the original VFD load.

This is a different electrical operating point even though no component in the filter has changed.

The fundamental current, harmonic spectrum, power factor and interaction between the source, filter and drive system may therefore differ from the 100 A condition.

Operating Point 3 — Only the 25 A VFD Running

Now consider the opposite condition.

The 75 A drive stops and only the 25 A drive remains operating.

The remaining VFD load is:

I_{LOAD}=25A

(4)

Relative to the original 100 A example:

\frac{25A}{100A}\times100=25\%

(5)

The connected nonlinear load has now fallen substantially.

However, the passive harmonic filter remains part of the electrical system.

Its passive components and their interaction with the network do not simply disappear because the VFD load has decreased.

This does not mean that harmonic performance must become unacceptable at 25% load.

It means that the 25% operating point should be evaluated rather than assumed to behave exactly like the full-load condition.

Why Partial Load Matters

Current total harmonic distortion is calculated relative to the fundamental current.

In simplified form:

THD_i= \frac{\sqrt{I_2^2+I_3^2+I_4^2+\cdots+I_n^2}} {I_1} \times100\%

(6)

The denominator is the fundamental current, (I_1).

As VFD loading decreases, the fundamental input current generally decreases.

The individual harmonic-current components may also decrease, but not necessarily in the same proportion as the fundamental component.

Because THDi is a ratio referenced to the fundamental current, a reduction in absolute harmonic current does not necessarily result in a lower THDi.

Depending on the VFD, filter design, source impedance and operating point, the percentage current distortion may remain similar, decrease, or increase as load changes.

This is why acceptable harmonic performance at full load should not automatically be assumed to represent performance across the complete operating range.

The Filter Is Part of the System

A passive harmonic filter is not evaluated in isolation.

Its behavior is influenced by the electrical system in which it is installed.

Relevant parameters can include:

system voltage,
system frequency,
source impedance,
filter topology and tuning,
connected nonlinear load,
harmonic spectrum,
other connected capacitance,
and the operating state of the equipment.

Many passive harmonic filter designs include capacitive elements as part of the filtering network.

Depending on the filter topology and design, these elements can influence reactive power and power factor, particularly as the real load decreases.

The importance of this effect is product- and system-dependent and should be evaluated using the manufacturer’s application data or an appropriate system study.

One Large Load vs. Several Smaller Loads

This issue becomes particularly relevant when one passive harmonic filter serves several independently operated VFDs.

Suppose the maximum combined VFD load is approximately 100 A.

The actual operating combinations could include:

Table 2. Example operating combinations

Operating Condition	Approx. VFD Load	Relative to 100 A Example
Both VFDs running	100 A	100%
75 A VFD only	75 A	75%
25 A VFD only	25 A	25%

The same filter remains installed in all three cases.

But these are three different system operating points.

This is why matching the filter current rating only to the maximum combined VFD load does not, by itself, describe performance across every possible operating condition.

What Should Be Checked?

When one passive harmonic filter serves multiple VFDs, the design review should consider both maximum and minimum realistic operating conditions.

Useful checks include:

maximum expected VFD load,
minimum realistic VFD load,
normal combinations of operating drives,
source impedance,
expected harmonic spectrum,
current distortion at the relevant measurement point,
power factor across the expected operating range,
possible reactive-current behavior at light load,
filter topology,
and manufacturer application or minimum-load guidance where applicable.

The actual measurement location also matters.

Drive-input THDi and current distortion measured upstream of a common filter are not necessarily the same quantity.

Therefore, any harmonic-performance requirement should clearly define the point at which compliance is evaluated.

Minimum Loading

There is no universal minimum-load percentage that can be applied to every passive harmonic filter.

The acceptable operating range depends on the specific product topology, system conditions and the manufacturer’s design criteria.

Some products may include features intended to improve light-load behavior, while others may have specific application guidance regarding minimum loading, power factor or capacitor switching.

For this reason, a value such as 25%, 30% or 40% should not be treated as a universal threshold unless it is supported by the documentation for the specific filter being evaluated.

Practical Design Lesson

A filter can be selected correctly for the maximum connected VFD load while still requiring review at lower operating points.

This is particularly important when several independently controlled VFDs share one passive harmonic filter.

Instead of asking only:

“Does the maximum VFD current match the filter rating?”

a more complete design question is:

“How does the filter and electrical system behave across the combinations of VFD loads that will actually occur?”

That question considers the equipment as part of a changing electrical system rather than as a single fixed operating point.

Conclusion

Passive harmonic filter performance is operating-point dependent.

In this simplified example, the same filter can remain connected while the downstream VFD load changes from approximately 100 A to 75 A or 25 A.

The filter itself has not changed, but the electrical conditions surrounding it have.

The fundamental current changes.

The harmonic-current spectrum can change.

The relative contribution of the passive filter to the system can change.

Power factor and reactive-current behavior may also change depending on the filter topology and system.

None of these observations means that partial-load operation is inherently unacceptable.

They mean that full-load performance alone does not completely describe the system.

For installations where several VFDs share a common passive harmonic filter, the expected load profile should therefore be considered during equipment selection and design review.

References

IEEE Std 519 — Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.
Manufacturer application, selection and performance documentation for the specific passive harmonic filter being evaluated.
Manufacturer technical documentation for the connected variable-frequency drives.
Project-specific source characteristics, operating load profile and harmonic-performance requirements.