Power Quality

Real supply voltage isn't a perfect sine. Learn harmonics and THD, sags, swells, flicker, unbalance and transients, power-factor correction, harmonic resonance and filters, and the standards that limit them.

Sensitive electronics, variable-speed drives, LEDs and data centres all interact with the supply. Non-linear loads draw distorted currents, faults cause voltage dips, and switching causes transients. Power quality is about keeping voltage and current close enough to the ideal that equipment works and lasts.

Poor power quality costs industry dearly: drives trip on sags, capacitors fail from harmonics, transformers overheat, and neutral conductors overload.

An analogy

Clean power is like a calm, steady river. Harmonics are ripples, sags are sudden drops in level, and transients are waves from a passing boat — equipment built for a calm river can be swamped.

Common disturbances

Each disturbance has different causes and fixes.

• Sag (dip): voltage drops to 10–90 % for 0.5 cycles to 1 min — caused by faults and motor starting. The most costly PQ problem.
• Swell: short rise, e.g. on healthy phases during an earth fault.
• Interruption: voltage below 10 %.
• Transient: microsecond spikes from lightning and switching.
• Flicker: rapid voltage fluctuation from arc furnaces and welders, visible in lighting.
• Unbalance: unequal phase voltages that heat motors.

Harmonics

Non-linear loads (rectifiers, drives, LED drivers, computers) draw current in pulses. Fourier analysis decomposes it into the fundamental plus harmonics at integer multiples. Six-pulse drives produce 5th, 7th, 11th, 13th…; single-phase rectifiers produce strong 3rd harmonics, which add up in the neutral of a four-wire system (triplen harmonics don't cancel).

Power-factor correction

Capacitor banks supply reactive power locally, reducing current, losses and utility penalties. Size them with Q_c = P(tan φ₁ − tan φ₂). Automatic banks switch steps as load varies to avoid over-correction and voltage rise.

Harmonic resonance

A capacitor bank in parallel with the network's inductance forms a resonant circuit at h_r ≈ √(S_sc/Q_c). If that falls near a harmonic the loads produce, the harmonic voltage is amplified and capacitors fail. The fix is a detuned bank: a series reactor (typically 7 %, tuned near 189 Hz on 50 Hz) moves resonance below the 5th harmonic.

Mitigation and standards

Passive tuned filters, active harmonic filters (inverters injecting cancelling currents), 12/18-pulse and active-front-end drives, UPS/DVR for sags, and surge protection devices. Standards: IEEE 519 (harmonic limits at the point of common coupling), IEC 61000-3-2/-3-12 (equipment emissions), EN 50160 (supply voltage characteristics).

Formulas

• Total harmonic distortion: THD = √(Σ_{h≥2} X_h²) / X₁
• RMS with harmonics: X_rms = X₁ · √(1 + THD²)
• PF correction: Q_c = P · (tan φ₁ − tan φ₂)
• Parallel resonance order: h_r ≈ √(S_sc / Q_c)

Worked example: THD and RMS current

Given: I₁ = 100 A; I₃ = 30 A, I₅ = 20 A, I₇ = 10 A

Find: THD and true RMS current.

• THD = √(30² + 20² + 10²)/100 = √1400/100 = 37.4 %
• I_rms = √(100² + 1400) = 106.8 A

Answer: THD = 37.4 %, I_rms = 106.8 A

Worked example: Capacitor bank for PF correction

Given: P = 100 kW; pf₁ = 0.70 → pf₂ = 0.95

Find: Required capacitor kvar.

• tan(arccos 0.70) = 1.020
• tan(arccos 0.95) = 0.329
• Q_c = 100 × (1.020 − 0.329) = 69.2 kvar

Answer: ≈ 70 kvar

Common mistakes

• Wrong: Installing plain capacitor banks in a plant full of drives. — Right: Check the resonance order first; use detuned (reactor-protected) banks where harmonics are present.
• Wrong: Sizing the neutral smaller than the phases for IT loads. — Right: Triplen harmonics add in the neutral; it can carry up to ~1.7× phase current.

Applications

• Data centres: UPS systems and active filters keep supply clean for servers; neutral conductors are oversized for triplen harmonics from IT loads.
• Industrial plants: Detuned capacitor banks and 12-pulse or AFE drives keep harmonic emissions within IEEE 519 limits.
• Utilities: Power-quality monitors at substations record sags and harmonics to diagnose customer complaints.

Key takeaways

• Sags, swells, transients, flicker, unbalance and harmonics are the main PQ issues.
• THD = √(ΣX_h²)/X₁; triplens add in the neutral.
• Q_c = P(tan φ₁ − tan φ₂) for PF correction.
• Check h_r = √(S_sc/Q_c) — detune banks where harmonics exist.

Going further

• K-factor transformers: Harmonic currents raise eddy-current losses (∝ h²). K-rated transformers, or derating per IEEE C57.110, prevent overheating when supplying non-linear loads.
• Active front ends: Drives with IGBT rectifiers draw near-sinusoidal current at unity pf, and can regenerate, eliminating most low-order harmonics at source.
• Voltage sag immunity: SEMI F47 and ITIC/CBEMA curves define what equipment should ride through. Ride-through can be improved with DC-bus capacitors, ride-through controls or dynamic voltage restorers.