Technical Articles
In the present scenario of obsession over Unity True Power Factor, it is equally important to tackle harmonics along with displacement PF Correction. Primary step in this process is installing Detuned Reactors along with Capacitors to a) Prevent amplification / resonance of harmonic currents b) Protect PF Correction Capacitors from harmonic overload. Like any other product, Detuned Reactors are offered with different types & specifications. All may look similar, but the minor variations in specifications will impact techno-commercial decisions and performance of reactors. Hence it is vital to understand the terminologies of a detuned reactor specifications before selecting for specific application. This article will impart knowledge of the same, so that different types of reactors can be weighed & selected as per the actual requirement. Let us also relate the specification brief to the actual values in recorded in Routine Test of a 7%, 50 kVAr copper detuned reactor.
Detuned reactors are connected in series with PF correction capacitors. The Tuning Factor is the ratio of Inductive
Standard Tuning Factors in industry are 7% & 14%. Each of these Tuning Factors has a specific Tuning Frequency point. The LC combination would block amplification of all harmonic frequencies above this Tuning Frequency point.
Consider the most common 7% Detuned Filter that has the tuning frequency of 189 Hz. This would block amplification of all the harmonic frequencies above 189 Hz, i.e., 5th harmonic (250 Hz) and above will be blocked. Hence 7% reactors find application in industries where loads like VFDs are employed that generate 5th harmonic & above. Whereas in building loads, where single phase non-linear loads are used, 3rd harmonics (150 Hz) is also present in addition to other frequencies. Hence 14% reactor, with tuning frequency of 134 Hz will be the best option to select.
Tuning factor depends on the Reactor’s Inductance (L) and Capacitor’s Capacitance (uF) values. If there is any variation, then the tuning frequency point will shift and can result in poor performance of the filter – harmonic amplification & resonance in system. Hence the tolerance level should be limited to ±3% of the rated Inductance value. This will ensure healthier tuning frequency for a considerable period, even if there is drift in Capacitance over a period of time.
In the above Test Certificate, it can be noted that the actual inductance value (0.94 mH) is 1.4% more than the rated inductance value (0.927 mH), which is within the acceptable limit of ±3% tolerance.
It is the ability of the reactor to function normally with acceptable temperature rise, even under higher RMS current as compared to the reactor’s rated nominal current. Excess current in the capacitor feeder is acceptable up to certain limits. It is possible due to higher harmonics, over voltage, excess capacitance, etc.
It is recommended to go with at least 1.3 times the rated current, (refer above Test Certificate Irms is 1.3 times In). If reactors not designed to withstand continuous excess current, the over heating would weaken the coil’s insulation & can result in fire. Due to such over heating (above 100 deg C), cables can also melt, if suitable temperature category is not selected. Sufficient over current withstand-ability will ensure safe operation of reactors.
Linearity is the current limit until which the Inductance (L in mH) of the reactor remains under acceptable limit (typically 5% drop). Recommended linearity value is “180% of In with drop in L < 0.95×Ln“ - this means up to 1.8 times the reactor rated current, the inductance value will drop by less than 5% of the rated Inductance Value. This ensures safe operation even during abnormal 180% current flow through the reactor, because the tuning frequency will not drift much towards unsafe levels.
Refer the linearity test in the above Routine Test Certificate – At 119 A (1.8 times rated current of 66 A), the inductance value (0.89 mH) is dropping by 4.15% from the rated inductance value (0.927 mH).
Many manufacturers offer different linearity levels – like 143%, 173%, 180%, 200%, etc. In addition to this value, it is very important to note the allowable Inductance drop value. Some may claim 180% linearity with L=0.9×Ln (10% drop in Inductance). If it is compared with 5% drop in Inductance, then the effective linearity would be around 160% only and not 180%. Hence it is necessary to consider Linearity value along with Inductance drop tolerance value.
Linearity is a crucial parameter as lesser value could result in thermal runaway of reactor, thereby damaging both reactor and capacitor. As the current exceeds the Linearity limit, the inductance value starts dropping and the tuning frequency will start increasing (as Tuning Freq = =12Ï€LC. ). Then it will match with nearest harmonic frequency, resulting in resonance and the reactor will burn due to excess current. Hence higher value of Linearity is preferred to avoid damage on both capacitor & reactor.
Q-factor is a dimensionless parameter that represents the harmonics attenuation (impedance offered against harmonic currents).
Q-Factor is the ratio of Inductive impedance to DC resistance of detuned reactor, QF=2Ï€fLR. Better quality conductor offers lesser DC resistance, thereby giving higher Q-Factor.
It is recommended to select Q-Factor value higher than 35. Q-Factor of the reactor in the Routine Test Certificate with the Inductance value and DC Coil resistance Value (5.9 mΩ) is 49.35 (calculated from above formula).
Higher the Q-Factor, better will be the impedance offered against harmonics, i.e., better reduction of harmonic amplification. The higher value of Q-Factor also implies lower power losses and hence lesser running cost/electricity bill.
Since detuned reactors can cause significant power loss, it is necessary to design in proper way so that the losses are kept to minimum. The lesser power loss in reactor ensures better PF correction at optimized electricity bill. The power loss is optimized by employing conductors with lesser DC resistance – better purity conductors, larger cross section of the conductor and by using core material with lesser losses. Still, the power loss depends up on the type of winding materials - Copper and Aluminium. Copper is relatively expensive but has lower power loss as compared to Aluminium wound reactors. Below is the comparison of performance of both Copper & Aluminium wound reactors - 50 kVAr, 7%:
| Parameters of 50 kVAr (1 unit) | Cu | AI |
|---|---|---|
| Losses (W) [measured at CPRI] | 112 W | 182 W |
| Energy consumed (kWh/year) | 981 kWh | 1594 kWh |
| Elec charges (@ Rs 8/unit) Rs/year | Rs. 7849 | Rs. 12755 |
Typically, copper reactors are expensive by 35% to 40% than aluminium equivalent. Considering this, the additional investment made on copper reactors would be paid back in less than 18 months due to lesser power loss. Beyond that there will be additional savings in electricity bill / OpEx. Added advantage of Copper reactors with lesser DC Coil resistance would be better Q-Factor value as well. Refer the Power Loss value mentioned in the Routine Test certificate above.
Reactors with lesser power losses would lead to lesser temperature rise and hence life and reliability of reactor and nearby capacitors will improve. Higher power loss also increases energy consumption and thereby more electricity bill & higher OpEx. Irrespective of the winding material, it is important to consider & compare reactors with lesser power loss.
Detuned reactors have considerable power loss and the temperature can rise much higher than the ambient conditions. If thermal design of APFC panel is improper, they might experience very high temperature of 100 Ö¯C and above. The winding insulation should withstand such high temperatures and the fire should be prevented.
Reactors with lower insulation class (like Class F or below) will not be able to withstand higher temperature rises within the panel and lead to premature failure, thereby resulting fire. Hence, insulation class offered for the reactors should be of at least Class H (180 Ö¯C). This will improve the reliability and other components safety.
In addition to this, a thermistor (120 Ö¯C cut off) based auto-cut-off shall be used to sense high temperature and isolate the reactor supply in case of any abnormal temperature rise.
Proper termination helps in avoiding terminal overheating and flash overs. Sturdy busbar type terminals will help in better & easier termination. It will be added advantage if all terminals are provided at the top portion of reactors, as it will offer flexible cable routing and avoid cables touching the hotter core. This type of arrangement will improve reliability in termination, easier and faster connections.
Any detuned reactors may appear simple, but minor variation in specifications would result in significant performance difference. It is necessary to understand the various design parameters of the reactors and select that are suitable for the specific application. It would also be wise to insist on routine test certificate of these key parameters on each reactor.
Abhinav L Purkar,
Manager Product Management & Marketing (PMM)Abhinav Purkar is a Product Management, Marketing, and Application Engineering professional with over 10 years of experience in the Power Quality domain. He has extensive expertise in power quality analysis, harmonic mitigation, reactive power compensation, energy efficiency, and electrical system optimization, helping customers improve system reliability and operational performance. With proven experience in product lifecycle management, market development, portfolio growth, and go-to-market strategy, Abhinav has successfully driven initiatives that enhance customer value and strengthen market presence. His strong technical foundation, combined with strategic business acumen, enables him to bridge the gap between customer requirements and innovative product offerings.
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