MV-512 · Phase 4 Advanced

Power Quality & Reactive Compensation

Mix harmonics into a sine wave and watch the waveform, spectrum and THD change live. Then size a capacitor bank for power factor correction and see why a plain bank can resonate with the transformer, and how a detuning reactor fixes it.

In short

THD = √(Σ I_h²) / I₁. Six-pulse drives typically draw strong 5th and 7th harmonics, and LED and IT loads add a lot of 3rd.
IEEE 519 limits voltage THD at the point of common coupling to 8% up to 1 kV and 5% from 1 to 69 kV.
Capacitor kvar for power factor correction: Q_c = P × (tanφ₁ − tanφ₂).
A capacitor bank and the supply inductance form a parallel resonance near h_r = √(S_sc / Q_c). If that lands near the 5th or 7th harmonic, voltage distortion is amplified.
A 7% detuned reactor tunes the bank to about 189 Hz on 50 Hz systems (3.78 × fundamental), below the 5th, so the bank cannot resonate with the main harmonics.

By Eng. Mannan Maqsood, Electrical EngineerPart of the MV Hub

Harmonic Waveform Lab 2D Animated

Pick a load preset or move the sliders. The waveform, spectrum and indices update live.

Capacitor Bank Sizing & Resonance Calc

Size the kvar for your target power factor, then check where the bank resonates with the transformer and whether detuning is needed.

Load (kW)

Existing PF

Target PF

Transformer kVA

Transformer uk (%)

Bank type

Step size (kvar)

Non-linear load share (%)

What causes harmonics?

Non-linear loads draw current in pulses instead of a smooth sine wave. Variable speed drives, UPS rectifiers, LED drivers, IT power supplies and EV chargers are the big sources in buildings. That current, flowing through the supply impedance, distorts the voltage for everyone on the same supply. Harmonics cause extra heating in transformers and cables, neutral overloading from triplen (3rd, 9th) harmonics, nuisance tripping and capacitor failures.

PractitionerChoose drives with DC chokes or AC line reactors (or active front ends) for large loads, size neutrals for triplen harmonics in IT-heavy buildings, and specify K-rated or derated transformers where THD is high. Measure at the main LV board during commissioning with a class A power quality analyser.

Why do capacitor banks need detuning?

A capacitor bank in parallel with the transformer's inductance forms a tuned circuit. Its parallel resonance order is about √(S_sc/Q_c). As steps switch in, that resonance moves, and it often passes through the 5th and 7th harmonics. Harmonic currents near the resonance are amplified, raising voltage distortion and overloading the capacitors. Adding a series reactor (7% is the most common, 14% where 3rd harmonics are significant) moves the bank's own tuning below the lowest major harmonic, so the bank looks inductive to the harmonics and cannot amplify them.

ExpertA reactor in series raises the voltage across the capacitor by 1/(1 − p), about 7.5% for p = 7%, so capacitors in detuned banks must be rated for a higher voltage (for example 480 V or 525 V units on a 400 V system), and the effective kvar at 400 V must be checked.

Regional notes

GCC

Utilities set a minimum power factor at the point of supply in their regulations and can apply penalties. Large chiller plants with VFDs make detuned banks and harmonic filters common in GCC buildings.

Australia and New Zealand

Network operators apply AS/NZS 61000.3.6 style allocation of harmonic limits and set minimum power factor requirements in their service rules.

Asia

Utilities such as TNB publish power factor and harmonic requirements in their supply guidelines, often based on IEC 61000 and IEEE 519.

Test yourself

0 / 6

6 questions. Pick an answer to see the explanation. Practise about 50 more MV-512 questions in the MV Question Bank.

Frequently asked questions

How do I calculate the kvar needed for power factor correction?

Qc = P x (tan(acos PF1) - tan(acos PF2)). For 1,200 kW from 0.82 to 0.95 that is about 1,200 x (0.698 - 0.329) = 443 kvar.

What is a 7% detuned capacitor bank?

A capacitor bank with a series reactor whose reactance is 7% of the capacitor reactance at the fundamental. It tunes the branch to about 3.78 times the fundamental (189 Hz at 50 Hz), below the 5th harmonic, which prevents resonance with the main harmonics.

What THD limit applies under IEEE 519?

For the voltage at the point of common coupling, IEEE 519-2022 sets total harmonic distortion limits of 8% for systems up to 1 kV and 5% for 1 to 69 kV, with lower limits for individual harmonics. Current limits depend on the short-circuit ratio.

Why do triplen harmonics overload neutrals?

The 3rd, 9th and 15th harmonic currents from the three phases are in phase with each other, so they add up in the neutral instead of cancelling.

Related MEPVirtual labs

MV-504 Distribution Transformer Lab MV-506 Short Circuit & Load Flow Studio MV-513 Renewables, BESS & EV MV Integration E-113 UPS & Critical Power Systems

Standards referenced

IEEE 519-2022 Harmonic control
IEC 61000-2-4 and IEC 61000-3-6
IEC 60831-1 Shunt capacitors
AS/NZS 61000.3.6 (AU/NZ)