Detuned reactors, and a table to check before you copy it, Episode 37

Key Takeaways

What you'll walk away with

Four ideas to carry into the next specification conversation you have.

The tuning order is fixed. The frequency is not.

Calculation: the tuning order n = 1 / √p depends on the detuning factor alone, so it is the same on 50 Hz and 60 Hz grids. The tuning frequency f = f₁ / √p moves with the grid. At 7%, the order is 3.78 on both grids, and the frequency is 189 Hz on a 50 Hz grid and 227 Hz on a 60 Hz grid.

Check every switching state, not only the full bank.

EPCOS (manufacturer) guidance: capacitors without reactors can resonate with the line inductance, mainly the input transformer. Check every switching state, each step in or out, and survey THD-I and THD-V before choosing the detuning factor.

On a 50 Hz grid, 189 Hz is p = 7%, not p = 14%.

Manufacturer data: CIRCUTOR lists 189 Hz for 7% and, for some installations, 134 Hz for 14%, both for 50 Hz grids. A table that pairs 14% with 189 Hz gets the frequency wrong. Both factors tune below the 5th (250 Hz on a 50 Hz grid). On a 60 Hz grid, 14% tunes to 160 Hz (calculated).

Rate the capacitor above the supply voltage.

Vishay (manufacturer data): in filter circuits the rated voltage of the capacitor has to be higher than the supply voltage. Vishay table example: 50 kvar per step, 400 V supply, 7% reactor, 440 V capacitor, factor 1.125, so 50 × 1.125 = 56.25 kvar rated. The factor depends on the supply voltage, the capacitor rating and p, so read it from the manufacturer's table: Vishay's 415 V table has no 440 V value at 7%.

THE SAME FACTOR

Same factor, different frequency

Calculation: the tuning order n = 1/√p stays fixed. The tuning frequency f = f1/√p moves with the grid, 50 Hz or 60 Hz.

TUNING ORDER

3.8% tunes above the 5th. 4.2% sits just under it.

Calculated with n = 1/√p, the same on 50 Hz and 60 Hz grids: p = 7% and p = 14% tune well below the 5th, at orders 3.78 and 2.67.

THE ORDER RULE

Below 4%, the factor tunes above the 5th

Calculation: tuning order n = 1/√p is a multiple of f1 and depends on p alone, so it is the same on 50 Hz and 60 Hz grids.

CHOOSING p

EPCOS guidance: a strong 3rd harmonic can point to 14%

Manufacturer design guidance (EPCOS, 2009), not a standard requirement: p = 14% when I3 is above 0.2 × I5, otherwise 5.67% or 7%, depending on THD-V. Final reactor selection depends on the network frequency (60 Hz in Saudi Arabia), the harmonic spectrum, system impedance, resonance conditions, capacitor and reactor ratings, and the manufacturer's specification.

CAPACITOR RATING

Rate the capacitor above the supply voltage

Vishay (manufacturer data): for filter circuits the rated voltage of the capacitor has to be higher than the supply voltage. Vishay table example (manufacturer data): 50 kvar per step, 400 V, 7%, 440 V capacitor: 50 × 1.125 = 56.25 kvar rated.

Reference

Detuning factor × 50 Hz and 60 Hz

Calculated: f = f₁ / √p and n = 1 / √p for every row, the first frequency on a 50 Hz grid and the second on a 60 Hz grid. Manufacturer data: CIRCUTOR prints 189 Hz (7%) and, for some installations, 134 Hz (14%), both for 50 Hz grids, and offers 227 Hz (7%) for 60 Hz networks on demand.

Detuning factor (p)	Tuning frequency, 50 Hz / 60 Hz grid	Tuning order, both grids
p = 5.67%	210 Hz / 252 Hz	order 4.20 · below the 5th
p = 6%	204 Hz / 245 Hz	order 4.08 · below the 5th
p = 7%	189 Hz / 227 Hz	order 3.78 · below the 5th
p = 8%	177 Hz / 212 Hz	order 3.54 · below the 5th
p = 12.50%	141 Hz / 170 Hz	order 2.83 · below the 5th
p = 14%	134 Hz / 160 Hz	order 2.67 · below the 5th

SURVEY FIRST

On a 60 Hz grid the 5th is 300 Hz, not 250 Hz

An analyser set to 50 Hz reads a 300 Hz component as the 6th, not the 5th.

REFERENCE TABLE

Detuning factor at 50 Hz and 60 Hz

Calculated: f = f1/√p and n = 1/√p for every row. Manufacturer data: CIRCUTOR prints 189 Hz (7%) and, for some installations, 134 Hz (14%), both for 50 Hz grids.

THE MISTAKE

On a 50 Hz grid, 189 Hz is p = 7%, not p = 14%

A table that pairs 14% with 189 Hz gets the frequency wrong: 14% gives 134 Hz on a 50 Hz grid (calculated). Both tune below the 5th, 250 Hz.

THE CORRECTION

At 60 Hz: 7% tunes to 227 Hz, 14% to 160 Hz

Manufacturer data: CIRCUTOR offers 227 Hz for 60 Hz networks on demand. Ask for it by name on the quote. 160 Hz is calculated from f1/√p at 60 Hz. CIRCUTOR's published 60 Hz value, for 7% only, is 227 Hz.

THREE NUMBERS

The order holds. The frequency moves with the grid.

Calculated, p = 7%: order 3.78 on both grids, 189 Hz on a 50 Hz grid and 227 Hz on a 60 Hz grid.

Deep Dive

Building a safe PFC bank

Five steps to spec capacitors that survive non-linear loads.

Survey the spectrum first, and at the right frequency.

Choose p from a harmonic survey of THD-I, THD-V and the 3rd harmonic. On a 60 Hz grid the 5th is 300 Hz and the 7th is 420 Hz, not 250 and 350. An analyser set to 50 Hz reads a 300 Hz component as the 6th, not the 5th.

EPCOS guidance: a strong 3rd harmonic can point to 14%.

Manufacturer design guidance (EPCOS, 2009), not a standard requirement: detuned reactors are recommended when THD-I is above 10% or THD-V is above 3%; p = 14% when the 3rd harmonic I3 is above 0.2 × I5; otherwise, with THD-V from 3% to 10%, p = 5.67% or 7%; above 10% THD-V the design is special. Final reactor selection depends on the network frequency (60 Hz in Saudi Arabia), the harmonic spectrum, system impedance, resonance conditions, capacitor and reactor ratings, and the manufacturer's specification. Tuning orders (calculated): 4.20 at 5.67%, 3.78 at 7% and 2.67 at 14%, all below the 5th.

Name the grid frequency when you order a reactor.

Manufacturer data: CIRCUTOR gives 189 Hz at 7% for 50 Hz networks and offers 227 Hz for 60 Hz networks on demand. Its published 60 Hz value is for 7% only; 160 Hz for 14% on a 60 Hz grid is calculated from f₁ / √p. A tuning frequency quoted for a 50 Hz grid does not apply to the Saudi 60 Hz grid, so ask which grid frequency the tuning was calculated for, and name the 60 Hz figure on the quote.

Capacitors cannot fix a distorted power factor.

A capacitor bank corrects displacement power factor, the phase angle at the fundamental. It does nothing for distortion. Calculation, not a catalogue value: true PF = DPF / √(1 + THD-I²), from the THD-I definition (EPCOS, p. 7), assuming a sinusoidal voltage. With the bank holding displacement at 0.98 and THD-I at 50%, true PF = 0.98 / √(1 + 0.5²) = 0.88.

Film capacitors do not dry out like electrolytics.

Power-factor capacitors of this kind are self-healing metallised polypropylene film units. They lose capacitance because each self-healing breakdown vaporises a small area of metallisation, permanently removing plate area. "Drying of the electrolyte" describes an electrolytic capacitor, a different component.

POWER FACTOR

The bank corrects displacement, not distortion

At 50% distortion, true PF is 0.88 if the bank holds displacement at 0.98. True PF = DPF / √(1 + THD_I²), from the THD-I definition (EPCOS, p. 7). Sinusoidal voltage assumed. Calculation, not a catalogue value.

THE FREQUENCY RULE

Same factor, higher frequency on 60 Hz

Calculation: f = f1/√p. At 7%, a 50 Hz grid tunes to 189 Hz and a 60 Hz grid to 227 Hz.

BEFORE YOU SIGN

Three numbers to check against the quote

Ask which grid frequency the tuning was calculated for: 50 Hz, or 60 Hz in Saudi Arabia. Ask for the capacitor rated voltage, and compare it with the supply voltage.

READ THE TABLE

The factor depends on the rating and on p

Read the factor for your supply voltage, rating and p from the manufacturer's table (here Vishay). Vishay's 415 V table has no 440 V value at 7%, so a 440 V capacitor is not in that row.

From the Academy to the Field

The survey names the harmonics present and guides the choice of detuning factor, together with system impedance, resonance conditions and the manufacturer\'s specification. The factor and the supply voltage set the capacitor rating, read from the manufacturer\'s table, and the grid frequency (60 Hz in Saudi Arabia) sets the tuning frequency. Ask for all three in writing before you order.

See the solution

Sources

Sources & further reading

Manufacturer data, the grid code and the calculations behind the numbers.

CIRCUTOR, Reactors III for detuned filters, RBZ-25-400, product page (P73130), manufacturer data: "p = 7%, with a resonance frequency of 189 Hz for 50 Hz networks (or, on demand, 227 Hz for 60 Hz networks)"; for some installations "14 % (134 Hz)". circutor.com
EPCOS (TDK), Design Rules for PFC Systems, ANo 116/V1 (2009), manufacturer design guidance, not a standard: the choice of detuning factor from the 3rd harmonic and THD, the THD-I definition, and the warning that capacitors without reactors can resonate with the line inductance (p. 7). tdk-electronics.tdk.com
Vishay, How to Calculate Capacitor Output in Detuned Filter Circuits, doc 13186 (rev. 21-Sep-17), manufacturer data: the factor tables, the 400 V, 7%, 440 V example (factor 1.125), and the rule that the capacitor rated voltage is above the supply voltage in filter circuits. vishay.com
Saudi Arabian Grid Code (SEC), May 2026 edition, cl. 2.10.2.1, "The Transmission System shall have a nominal frequency of 60 Hz", with normal operation within 59.9 and 60.1 Hz. se.com.sa
Calculation: tuning order and tuning frequency (NLC), n = 1 / √p and f = f₁ / √p. Every frequency and order in the table is calculated; they match CIRCUTOR's published values, 189 Hz and 134 Hz on a 50 Hz grid and 227 Hz for 7% on a 60 Hz grid. nlc.com.sa
Calculation: true power factor (NLC), PF = DPF / √(1 + THD-I²), from the THD-I definition in EPCOS ANo 116/V1, with a sinusoidal voltage assumed. A calculation, not a catalogue value.

Detuned reactors, and a table to check before you copy it