Heat is the most common killer of power factor correction capacitors. A relatively common event is encountering a power factor correction unit with seized fans and many failed capacitors but apart from this, everything else is fine. What on earth do we do if identical replacement capacitors are not available?? Mismatching of capacitors and detuned reactors can lead to disastrous results but provided the details of detuned reactors are available, there is a way out. The detuned reactor nameplate, shown below had a dead Metalect 40kVAr capacitor downstream from it. These are no longer available.
The detuned reactor name plate above tells us what we need to know to work out a solution.
Capacitance size = 40kVAr
Fundamental frequency current rating - In = 61.9A
Detuned Reactor Inductance = 1.032mH
There is a document readily available on Schneider Websites called “PF ComponentsEN.pdf” which contains values of capacitance in microFarads of Schneider Capacitors. The values of Varplus Can capacitors readily available in New Zealand (5.0kVAr, 6.6kVAr, 10.7kVAr, 12.5kVAR, 15.6kVAr, and
25.0kVAr) are shown below (copied from the document).
For an example of how to calculate whether a replacement solution is appropriate, we will firstly consider using 4 of 10.7kVAr capacitors to replace our dead 40kVAr capacitor.
For a start, we need to determine the resonant frequency of the capacitor / reactor combination.
It is essential that the resonant frequency is well away from any Harmonic frequency. For example,
Schneider Varplus Cans and Detuned reactors are designed to have a resonant frequency of 191Hz.
This value is well away from the third harmonic frequency of 150Hz.

(i) Calculate Resonant Frequency
From the chart above, we can see that a 10.7kVAr capacitor has µF = 66.2 X3 =
198.6 µF. As we are planning to use four of these and Capacitors in parallel –
C=C1+C2+C3+…..
Thus total capacitance = 198.6 x4 = 794.4µF
From detuned reactor nameplate, reactor Inductance = 1.032mH
These values are used in the resonant frequency formula
F
R
= 1 / (2π √ (1.032*10 -3 *794.4*10 -6 ))
= 1 / (2π √ (8.194*10 -7 ))
= 1 / (2π*9.052*10 -4 )
= 1 / 5.6876*10 -3
= 175.82Hz which is sitting at 25 Hz away from 3 rd harmonic. 25 Hz is a safe margin.
(ii) Calculate Fundamental Frequency Current
To do this we use the “equivalent” circuit as shown below
I = 230V / (1/2π50*794.4*10 -6 ) – (2π50*1.032*10 -3 )
= 230 / (4.0069-0.3242)
= 230 / 3.6827
= 62.45A

Reactor Current rating = 61.9A.
Fundamental frequency current = 62.45A = 0.89% greater than reactor current rating. Not Good
Alternative Solution – Use 1 of 25kVAr capacitor in parallel with 1 of 15.6kVAr capacitor
Capacitors in parallel – C=C1+C2+C3+…..
Total Capacitance = (Values from chart above) 288.6 + 461.7= 750.3µF
Detuned Reactor details
40kVAr – In = 61.9A, mH = 1.032
(i) Calculate Resonant Frequency
F
R
= 1 / (2π √ (1.032*10 -3 *750.3*10 -6 ))
= 1 / (2π √ (7.743*10 -7 ))
= 1 / (2π*8.799*10 -4 )
= 1 / 5.52888*10 -3
= 180.87Hz which is sitting at 31Hz away from 3 rd harmonic. This is a safe margin.
(ii) Calculate Fundamental Frequency Current
To do this we use the “equivalent” circuit as shown above
I = 230V / (1/2π50*750.3*10 -6 ) – (2π50*1.032*10 -3 )
= 230 / (4.2424348-0.3242)
= 230 / 3.918
= 58.7A
Reactor Current rating = 61.9A. Fundamental frequency current = 58.7A = 5.2% less than reactor rated current. This is a good solution that will work with the Metalect Reactor. smb20160923