Power Quality FAQ

Displacement power factor (cos φ) considers only the phase angle between fundamental voltage and current — it reflects the effect of inductive and capacitive loads. True (total) power factor includes both displacement and distortion effects from harmonic currents. For linear loads, both are equal. For non-linear loads with harmonics, true PF is always lower than displacement PF. Capacitors correct displacement but not distortion.

Harmonic currents increase transformer winding losses through two mechanisms: I²R losses rise with total RMS current (including harmonics), and eddy current losses increase with the square of the harmonic order. A 10% fifth-harmonic current causes 25 times more eddy current loss than the same magnitude at fundamental frequency. Standard transformers must be derated; K-factor transformers are designed for harmonic-rich loads.

Voltage sags (dips) are brief reductions in supply voltage lasting from half a cycle to several seconds, caused by large motor starting, transformer energisation, or faults on adjacent circuits. Prevention methods include soft starters or VFDs for motors, pre-insertion resistors for transformers, dedicated supply feeders for sensitive equipment, and uninterruptible power supplies for critical loads.

Calculate the required reactive power: Q = P × (tan φ₁ − tan φ₂), where P is the active power in kW, φ₁ is the angle before correction, and φ₂ is the target angle. For example, correcting 100 kW from PF 0.75 to 0.95 requires Q = 100 × (0.882 − 0.329) = 55.3 kVAr. Use automatic stepped controllers to avoid over-correction during light load periods.

Yes. Capacitor banks can resonate with system inductance at frequencies coinciding with harmonic orders present in the load current. Series resonance amplifies harmonic currents; parallel resonance amplifies harmonic voltages. Both can damage capacitors, trip protective devices, and distort voltage waveforms. Detuned capacitor banks with series reactors (typically 7% or 14%) prevent resonance by shifting the resonant frequency below the lowest harmonic.

The K-factor quantifies the harmonic heating effect on a transformer. K = Σ(Ih/I1)² × h² for all harmonics. Standard transformers have K = 1. K-4 handles light harmonics (offices with some computers). K-13 handles heavy harmonics (data centres, industrial VFDs). K-20 handles very heavy harmonics. K-rated transformers have reinforced windings, reduced eddy current losses, and oversized neutral connections.

Methods include: passive harmonic filters tuned to specific harmonic frequencies; active harmonic filters that inject cancelling currents in real time; 12-pulse or 18-pulse rectifier configurations that cancel lower-order harmonics; multi-level VFD topologies with lower harmonic output; isolation transformers with delta-star configuration to block triplen harmonics; and proper system design to maintain low source impedance.

IEEE 519-2022 limits voltage THD to 8% at the point of common coupling for systems below 1 kV, with no individual harmonic exceeding 5%. Current harmonic limits depend on the ratio of short-circuit current to load current at the PCC — stronger systems (higher ratios) allow higher current distortion. IEC 61000-3-2 limits harmonic current emissions from individual equipment.

Triplen harmonics (3rd, 9th, 15th) from non-linear loads are zero-sequence currents that add arithmetically in the neutral conductor instead of cancelling as the fundamental does. With many computers or LED drivers, neutral current can reach 1.5–1.7 times phase current. If the neutral was sized smaller than the phase conductors (traditional practice), it will overheat. IEC 60364-5-52 Clause 524 requires full-size or oversized neutrals for harmonic-laden circuits.