Harmonic analysis is one of the most important, and most frequently misunderstood, subjects in modern power systems. This FAQ page addresses common questions, misconceptions, and areas of confusion related to harmonic analysis, including resonance, system impedance, capacitor bank application, and the basis for harmonic filter design. While software tools are commonly used to evaluate compliance with IEEE 519 and related standards, meaningful analysis still depends on understanding the underlying electrical behavior of the system. VarStec believes harmonic studies should be grounded in first principles, with computer modeling used as a tool to support sound engineering judgment, not replace it.

Technical Q&A

No. In VarStec’s view, not every capacitor bank application requires a detailed computer-based harmonic analysis to identify risk. In many cases, basic engineering calculations are enough to determine whether adding capacitance could create resonance near characteristic harmonic frequencies already present in the plant.

The real issue is resonance. If even low levels of background harmonics are present, and the added capacitor bank creates resonance near one of those harmonic frequencies, the resulting amplification can drive voltage distortion above commonly referenced levels such as 5% VTHD. When that happens, IEEE 519 current distortion limits at the point of common coupling will also be exceeded.

This can occur even with so-called IEEE 519 compliant drives. Low harmonic current does not eliminate the problem. If the system is resonant near a frequency produced by the drive, even a small harmonic source can be amplified to an unacceptable level.

For that reason, if a plant has low-level background harmonics and resonance is possible near those frequencies, VarStec believes the project should usually move away from a standard capacitor bank and toward a detuned capacitor bank, typically tuned near 4.0 to 4.2, or toward a true harmonic filter tuned near a characteristic harmonic such as the 5th.

Detailed computer-based harmonic analysis is more appropriately used for harmonic filter design and for evaluating expected filter performance, not as the only way to recognize resonance risk in a capacitor bank application.

The Point of Common Coupling, or PCC, is the electrical connection point between the customer and the utility, usually at or near the revenue meter. Under IEEE 519, this is the point where harmonic compliance is judged, not deep inside the plant.

IEEE 519 applies two different types of limits at the PCC. Voltage distortion limits are intended to ensure the utility is providing reasonably clean voltage. Current distortion limits, usually expressed as total demand distortion (TDD), limit how much harmonic current the customer can push back onto the utility system.

The allowable current distortion, based on the harmonic current limits presented in IEEE 519, depends on system strength at the PCC as well as the voltage level. A stronger, stiffer system with higher available short-circuit current can tolerate more harmonic current because it produces less resulting voltage distortion, while higher PCC voltage levels are subject to more restrictive limits.

A notch filter is a sharply tuned filter designed to trap one specific harmonic. A damped high-pass filter is a broader filter designed not only to filter harmonics, but also to damp resonance and control a wider range of frequencies above and below its tuning point.

A notch filter is the simplest harmonic filter. It consists of a power capacitor bank and an inductor tuned so that, at the selected harmonic frequency, the capacitive reactance equals the inductive reactance. At that tuning point, the filter presents very low impedance to the harmonic source, so harmonic current is drawn into the filter very effectively. Its impedance characteristic is very sharp, like a narrow “V,” with the bottom of the curve near zero ohms. But at nearby frequencies, the impedance rises quickly, so its filtering effect is highly selective.

A damped filter can be either a high-pass filter or a C-high-pass filter. In both cases, a resistor is added to broaden and flatten the impedance characteristic near the tuning region. Instead of a sharp “V,” the response becomes more like a shallow “U.” The amount of damping depends on the resistor value. This broader response makes the filter less selective than a notch filter and not able to filter as well at its tuning point, but much better at controlling resonance and attenuating a wider band of harmonics.

It is called a damped filter because it is often used to damp resonant conditions, including resonance between filter branches in a multi-tuned bank, or resonance involving system capacitance such as cable capacitance or other shunt capacitor banks on the network.

Yes, if the measurements are being used to establish baseline harmonic current injection data for a harmonic study. Energized capacitor banks, and harmonic filter banks, can distort the measured results and make them unsuitable for building an accurate system model.

A proper study must identify the actual harmonic current being injected into the system. If measurements are taken upstream of the capacitor bank connection point, such as at the secondary main for a system-wide assessment, while capacitor banks are energized, the data will reflect harmonic currents that have already been amplified or attenuated by the system. That is not true source-current data and can lead to an inaccurate model.

Engineering Tip:
Make sure the study engineer receives measurements that represent actual harmonic current injection, not modified values due to attenuation or amplification.

For measurements taken at the main incoming, or anywhere upstream of the capacitor bank connection point, capacitor banks should be turned off, even if only for a few minutes. This is often the single most important step in obtaining useful harmonic measurement data.

If the capacitor banks cannot be turned off because of operating constraints or power factor penalty concerns, measurements should be taken only downstream of the capacitor bank connection point, such as on feeders serving individual drives or other harmonic-producing loads. This usually requires more measurement points and increases cost.

For harmonic studies, good results depend not just on taking measurements, but on taking them in the right locations.

Related Technical Topics

VarStec’s Practical Approach to Harmonic Analysis and Filter Application

VarStec applies practical engineering experience to the analysis of harmonic behavior in medium-voltage power systems and the design and manufacture of metal-enclosed capacitor banks and harmonic filter banks. Our work includes system modeling, field measurement review, resonance evaluation, and the development of system-fit™ solutions for power factor correction and harmonic mitigation. Guided by applicable industry standards and grounded in first-principles engineering, our approach is focused on equipment that is properly applied, technically sound, and aligned with the power quality and reliability objectives of the project.