Harmonic filter design is a specialized area of reactive compensation that involves much more than adding kvar. Filters are applied for different reasons, including trapping harmonic currents, damping resonance, precluding resonance, preventing resonance, and providing reactive power support. Good filter design starts by understanding exactly what the filter must accomplish, since that drives the topology, tuning, and performance requirements. This FAQ answers common questions related to harmonic filter design, application, and performance.

Technical Q&A

The Q-factor determines how sharply a damped high-pass filter is tuned and how much damping it provides around its tuning point. In practical terms, it sets the balance between effectively filtering at the tuned frequency and resonance damping and filtering over a wider frequency range. For a high-pass or C-high-pass filter, the Q-factor determines the required value of the damping resistor.

  • Higher Q: A higher Q-factor makes the filter behave more like a notch filter. It provides lower impedance near its tuning point and stronger filtering at that frequency, but less resonance damping.
  • Lower Q: A lower Q-factor broadens the response. This improves resonance damping over a wider frequency range, but the filter provides less harmonic filtering at its tuning point because its impedance there is higher.

Engineering Note: Q-factor is calculated as the resistance of the high-pass filter resistor, in ohms, divided by the inductive reactance of the reactor at the high-pass or C-high-pass filter tuning frequency. In practical terms, it is a key design parameter used to determine the required resistor value.

A harmonic filter is usually tuned 3% to 15% below the target harmonic to provide detuning margin and reduce the risk of its parallel resonant point shifting upward and amplifying harmonics.

A high-pass filter should be utilized when the electrical system contains interharmonics, stray capacitance, or existing shunt power capacitor banks that could result in objectionable resonance. High-pass filters effectively “dampen” the resonant peaks associated with resonant conditions.

A C-high-pass filter incorporates an auxiliary capacitor (C2) tuned with the reactor (L) to the fundamental frequency 50Hz/60Hz.

  • Efficiency: This tuning effectively bypasses the damping resistor at the fundamental frequency, resulting in negligible fundamental-frequency power losses.
  • Performance: It allows for aggressive damping (very low Q-factors) without the kW losses with standard high-pass resistors.

Per IEEE Std. 18, power capacitors are designed for continuous contingency operation up to:

  • 110% of rated RMS voltage
  • 120% of rated peak voltage
  • 135% of nominal RMS current
  • 135% of rated kVAR

Filters designs must ensure the capacitors do not exceed these values under all operating conditions.

The VarStec harmonic filter design spreadsheet requires inputs including the system voltage and frequency, reactive power requirements, current injection data, the desired tuning point, and the damping factor.

The capacitor voltage rating must be higher because it is subjected to two primary additive stresses:

  1. Fundamental Rise: The series tuning reactor creates a fundamental-frequency voltage rise on the capacitor.
  2. Harmonic Stress: Harmonic currents flowing through the capacitor produce additional harmonic voltage.

VarStec Engineering Note: Per IEEE 18, the 110% RMS and 120% peak voltage margins are for contingency operations and should not be used in the normal operating range of the harmonic filter.

  • Iron-Core Reactors: Confine magnetic flux within the core, making them suitable for compact metal-enclosed systems. They must be designed conservatively to avoid saturation.
  • Air-Core Reactors: Produce significant external magnetic fields and require large clearances to avoid heating surrounding steel, making them more common in open-air yards. They must have significant clearance to nearby steel objects to avoid eddy current heating.

VarStec Standard: We specify iron-core reactors with a fundamental current rating of 1.5x and a harmonic rating of 2.0x the expected currents. This ensures the reactor operates at roughly 44% of its thermal loss capability, maximizing reliability.

An ungrounded split-wye capacitor bank connection is preferred because it eliminates 0-sequence resonance concerns, prevents false trips from neutral-to-ground voltage shifts during external line-to-ground faults, and it prevents high recovery voltages from causing restrikes in switching devices.

The reverse calculator tools can be used when the filter inductance, capacitance, and damping resistance are known, but the filter reactive power rating, tuning frequency, or damping factor are not. In that case, the tool works backward from the known component values to calculate the resulting filter characteristics.

While altitudes above 1,000 meters (3,300 feet) reduce air density and traditionally dictate applying an Altitude Correction Factor to increase equipment BIL, upsizing the filter bank BIL rating is not strictly required if surge arresters are utilized.

  • IEEE Standards (IEEE C37.010, C37.20.2, and C37.20.3): These standards explicitly state that derating dielectric capabilities is usually not the most economical approach, and that applying properly coordinated surge arresters to keep transient voltages below the altitude-reduced insulation levels should be considered instead.
  • IEC Standards (IEC TR 62271-306 and IEC 60694): These standards validate the same mitigation strategy, noting that the application of surge arresters to lower the required insulation level of the substation may result in a more advantageous choice of equipment.

VarStec Engineering Tip: The VarStec Capacitor Bank Altitude Derating & Insulation Coordination Tool follows the methodologies of IEEE C62.22 and IEC 60071-2 to mathematically evaluate both approaches. It helps verify equipment ratings and determine whether standard sea-level equipment can be safely applied at high elevations without unnecessarily increasing the BIL rating.

Per IEEE Std. 18, power capacitors rated over 600 volts must be equipped with an internal discharge device that will safely reduce the residual voltage to 50 volts or less within 5 minutes after the capacitor is disconnected from the peak of its rated voltage.

Power capacitors are designed for continuous operation at a maximum average ambient temperature of 46 degrees Celsius over a 24-hour period, and can withstand peak temperatures up to 55 degrees Celsius.

A design margin is recommended to protect the equipment from unanticipated harmonic sources, future load expansions, and inaccuracies in simulation data that could otherwise cause the filter to become overloaded and fail.

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30+ Years of Harmonic Filter Design Experience

VarStec brings more than 30 years of practical experience in the design, application, and manufacturing of complex harmonic filter systems. Our expertise spans large rectifier systems in chemical processing, mining VFDs, and LNG facility drives. Building on that experience, VarStec has developed a harmonic filter design spreadsheet tool to help engineers properly design and rate the components against IEEE 18 and IEEE 1531 to ensure your system performs as modeled for the life of the plant.