7% vs 14% Detuned Reactors: What Changes

The difference between 7% and 14% detuned reactors is the fundamental-frequency ratio of inductive to capacitive reactance in the matched branch. A larger detuning factor lowers that branch’s ideal series resonant frequency and increases its fundamental capacitor-voltage amplification. It does not automatically mean better harmonic protection for every network. Choose between the two by studying the actual harmonic spectrum, network configurations, capacitor rating, reactor duty and required reactive output. This comparison explains the calculations and tradeoffs without treating either percentage as a universal selection rule.

Define the percentage correctly

Let p = XL/XC at the system’s fundamental frequency, expressed as a decimal in the equations. Thus 7% means p = 0.07 and 14% means p = 0.14. XL is the matched reactor’s inductive reactance and XC is the matched capacitor’s capacitive reactance at that frequency. A reactor described by a percentage is being described relative to a capacitor and a design frequency, not by a stand-alone resistance value.

Identify the manufacturer’s percentage convention and the exact branch data before comparing offers. Do not attach a 7% reactor to an arbitrary capacitor and assume the assembled branch remains 7%. The CNBYG CKSG series reactor family is a relevant starting point for an enquiry. Confirm availability, branch matching and rated duty for the actual supplied model; this article does not claim every listed factor is available in every size.

Calculate the ideal branch resonant frequency

For the ideal series LC branch, fr = f1/√p, where f1 is the fundamental frequency and fr is the branch resonant frequency. The result follows from equating inductive and capacitive reactance. OpenStax’s resonance lesson explains the underlying circuit relationship. Use p as a decimal: entering 7 instead of 0.07 produces a completely different answer.

At 50 Hz, the ideal values are approximately 189 Hz for 7% and 134 Hz for 14%. At 60 Hz they are approximately 227 Hz and 160 Hz. These are calculated nominal values, not measured site performance or acceptance tolerances. They describe one matched series branch. They do not establish the resonances of the complete supply network with all possible capacitor-stage and transformer configurations.

Compare voltage and frequency together

In the ideal fundamental-frequency series branch with resistance neglected, capacitor voltage relative to the corresponding branch supply voltage is 1/(1-p). That gives approximately 1.075 for 7% and 1.163 for 14%. The quantities must describe the same branch connection. Do not mix a phase branch voltage with a line-to-line nameplate voltage when applying the model.

These entries are derived from the equations above with ideal matched components. Resistance, tolerances, harmonic voltages and actual network conditions require further engineering assessment. A catalogue percentage alone cannot supply a capacitor voltage rating or a permissible current.

Why the harmonic spectrum matters

The 7% ideal resonant order lies below the fifth harmonic but above the third; the 14% order lies below the third. Those mathematical relationships help explain why different detuning factors can be considered for different spectra. They are not sufficient selection rules. Record relevant harmonic orders and magnitudes at representative operating states, along with voltage distortion and background conditions when the bank is disconnected.

Include the connection and propagation of harmonic components in the network assessment. A measured third harmonic at one point does not by itself establish the same branch duty everywhere. Transformer configuration, phase balance, neutral paths and measurement location matter. Avoid slogans that prescribe 14% whenever a third harmonic appears or 7% for every drive installation. The measured network and coordinated design decide the appropriate factor.

Branch resonance differs from network resonance

Detuning a capacitor branch changes its frequency response, but it does not make every possible system resonance disappear. Supply impedance, transformers, cables and other capacitor stages participate in the wider network. Different switching combinations can change the response. Include weak-source and generator modes when they are relevant instead of assessing only the strongest normal utility connection.

The guide to using reactors in capacitor banks provides the broader purpose of the series assembly. The bank safety guide addresses the access and protection context. A complete study should state which configurations were assessed, which resonances or duty concerns were found and what operating restrictions remain if a configuration was not evaluated.

Match capacitor voltage and delivered kvar

Reactor-associated voltage rise means the capacitor sees a different fundamental voltage than a bare capacitor connected directly across the same branch supply. Harmonic voltage and tolerances add further considerations. The relevant self-healing low-voltage capacitor standards family is described by IEC 60831-1. Obtain the supplied capacitor’s actual limits and the design assessment rather than selecting a rating from the ideal amplification factor alone.

Compare delivered bank kvar at operating voltage, not just the capacitor’s catalogue kvar at its own rated voltage. Ask each supplier to state the capacitor rating, installed capacitance, reactor data, design voltage and net branch output. An apparently equal kvar quotation can describe different electrical assemblies. A clear comparison uses the same operating boundary and documented duty assumptions for both options.

Thermal duty and installation are part of the choice

The reactor carries the branch current, including its harmonic components. Heating depends on the relevant winding and magnetic losses, cooling and operating conditions. A larger detuning percentage is not a direct numerical prediction of cabinet temperature. Request the rated current duty, loss information, ambient conditions, cooling requirements and protection arrangement for the proposed assembly.

Check dimensions, mounting, connection access and separation from neighbouring equipment. Confirm that cabinet ventilation accounts for the whole assembly rather than the reactor in isolation. The public scope of IEC 60076-6 covers several reactor categories, including filter reactors; applicability and test requirements must be agreed for the supplied design. Its scope does not provide a universal winding temperature limit for this enquiry.

Replacing one component can change the percentage

At fixed fundamental frequency, p depends on both inductance and capacitance. Replacing a capacitor with another capacitance, or fitting a different reactor, changes the assembled ratio. Progressive capacitance loss can also move the branch from its original condition. Retain actual part identities and measurements so maintenance does not silently convert a coordinated branch into an undocumented combination.

Do not change 7% to 14% by exchanging only a label or one component without design review. The revised branch can require different capacitor voltage, current, protection and output assumptions. Reassess controller staging and acceptance measurements as well. The capacitor-bank maintenance checklist is useful for preserving the as-installed branch record and tracking later replacements.

Prepare a procurement comparison that can be checked

Provide system voltage and frequency, load spectrum, supply configurations, stage sizes, existing capacitor information, cabinet conditions and available space. Request the proposed p definition, calculated and permitted operating range, capacitor-voltage assessment, current-duty assessment, net kvar and thermal/protection details. Ask which measurements or study inputs the supplier still needs instead of accepting an unexplained recommendation.

Use one comparison sheet with identical operating assumptions. Record exclusions such as generator mode, future drives or unmeasured background distortion. Have the responsible engineer resolve those exclusions before approving an unrestricted installation. A low purchase price or a familiar percentage cannot substitute for a matched design with stated assumptions and a meaningful acceptance plan.

Commission the selected branch against the design

Before return to service, qualified personnel verify identification, installation, connections and protection through the approved procedure. Operational acceptance should compare expected branch response with the measured voltage, current, switching states and relevant spectrum. Record the operating configuration so the result can be reproduced. Do not claim full-network acceptance from one brief measurement with one stage connected.

Follow up during representative demanding production and supply conditions. Retain temperature and protection evidence with the electrical measurements. If actual duty differs from the design assumptions, review the discrepancy before increasing the bank capacity. The purpose of choosing between percentages is a reliable assembly under defined duty, rather than achieving a catalogue value while leaving the operating boundary unspecified.

Educational video: inductors and AC behaviour

The Engineering Mindset’s inductor lesson introduces inductance and frequency-dependent behaviour. It provides background for the branch equations; it is not a design approval or industrial commissioning procedure. Use the calculations and project evidence above with the supplied equipment requirements.

Frequently asked questions

Is 14% always better than 7%?

No. It changes resonant frequency and fundamental voltage rise. Suitability depends on the measured spectrum, network response and coordinated component duty.

Are the same frequencies valid on 50 and 60 Hz supplies?

No. At the same nominal percentage, the ideal resonant frequency scales with fundamental frequency. Confirm the design frequency and actual component match.

Can the existing capacitor stay when changing the reactor?

Only after engineering review establishes compatibility. Voltage, current, tuning, net kvar and protection can all change with the revised assembly.

What data should a supplier receive?

Provide voltage, frequency, spectrum, supply modes, stage sizes, capacitor information and cabinet conditions. Request a coordinated proposal with explicit assumptions and acceptance evidence.