Understanding Detuning Reactors ( Anti-resonance Reactors)

Reactor protection

Most automatic capacitor banks employed today are provided with reactor protection as a result of the in­creasing harmonic loading of the consumer installation and the power networks. Every capacitor or capacitor tap is connected in series to an inductance (reactor), in contrast to "normal" unprotected compensation.

If the resonant frequency of the series resonant circuit formed in this way ( capacitors and Inductor) deviates (is lower) by more than 10% from the frequency of the nearest harmonic, then one speaks of a detuned resonator circuit or an anti-resonance circuit. Reactor protected compensation systems are designed as detuned resonator circuits and the series resonant frequency f₀ is normally chosen to be below the fre­quency of the 5^th harmonic (250 Hz). The capacitor and reactor system is therefore inductive for all harmonic frequencies ³ 250 Hz and dangerous resonance between the capacitor and network inductance (e.g. transformer) is therefore avoided.  Consumer installations with high 3^rd harmonic (150 Hz) components are an exception but it can become necessary to set the series resonant frequency to 134 Hz in such cases.

The graph above shows the frequency response of 5.67%; 7% and 14% detuned circuits. It should be noted that the closer the resonant frequency of the anti-resonance filter is to the Harmonic to be filtered, lower is the impedance offered and therefore better is the filtering effect. For example 5.67% which is tuned at 210Hz will have lower impedance to the 5^th harmonic compared to the 7% detuned which has a resonance frequency of 189Hz. This is shown by the points on each curve at 250Hz. Below table shows the amount of harmonic current that will be absorbed in each case. 

Detuning	Resonant Frequency	% of 5th Harmonic current absorbed
5.67%	210 Hz	30~50%
7%	189 Hz	10-20%
14%	134 Hz	0%

From the above table it is clear that depending on the load profile and also the duty cycle of the non-linear loads the proper selection of the detuning frequency should be made. Please consider that using 7% detuned system may not be always helpful and high distortion may still persist on the load busbars.

If the series resonant frequency is between 10% below or above a harmonic frequency, then it is de­scribed as a tuned resonator circuit. Tuned resonator circuits are normally employed as wave traps for the deliberate reduction of individual harmonics.

Reactor protection-factor p

The reactor-protection factor p [%] specifies the ratio of the reactor reactance to the capacitor reac­tance at network frequency.

p=XLx 100 / Xc

The resonant frequency of the series resonant circuit can also be calculated from p using the following equation:

fres = f1 x (1/sqrt(p)) 

For example:  p=7 %, f₁ = 50 Hz  

fres = 50 x (1/sqrt(0.07)) = 189Hz

One of the often-tried standard values is normally used for the choice of a suitable reactor-protection factor for the application:

Reactor-protection factor p	Series resonant frequency fR
5%	223 Hz
5.67%	210 Hz
7%	189 Hz
8%	177 Hz
12.5%	141 Hz
14%	134 Hz

Capacitor rated voltage with reactor protection

A voltage increase arises at the capacitor from the serial connection of the reactor and capacitor. It can be calculated from the reactor-protection factor p:

For example: p = 7%, U_N = 440 V

Uc =Un(1/(1-p))

Uc= 400(1/(1-0.07)) = 473V

The capacitors employed for p = 7% must therefore be suitable for a continuous rated voltage of at least 480 V. Here, you must always be careful about the voltage tolerance for the nominal net voltage.

When the voltage on the capacitors increase the KVAr output of the capacitor bank also changes. This is given by the following equation.

Qc=(1-p/100).[(Uc^2)/ (Un^2)]   .Qn

In such a case case it is necessary to use adjustable ratings. For more information please read the reactor application note from Electronicon System Electric . or read the entry for calculations.