To reduce the harmonic noise with no energy losses. More over, it is dynamic system can adapt and work with any frequency contamination form.

Active filter uses operational amplifier for gain boosting and control and moreover for amplification. So an Active Filter is the best.

Dear Saurav Chanda , I do not think the answer is so obvious. It is conditioned by a lot of circumstances. It is also about what the questioner meant by the term "power filter".

Interesting query@ Ravikumar Srinivasa

Low pass filters considred as passive filters which are used to suppress current harmonics and to mitigate the power pollution in grid. Although, this kind of filters offer numerous shortcomings such as huge size, resonance problems, and sensitivity to parameters variation. The abovementioned problems are effectually relieved by using shunt active filters....many paper are published on this topic..

You can have a look on the following paper:

1.Article Simulation and real time implementation of three phase four ...

2. Article Review of active filters for power quality improvement

3. Article Fuzzy logic controller for three-level shunt active filter c...

As name indicate active filter acts according to the noise and not tuned at particular frequency like passive filter. Normally passive filters are RLC based but active filters are designed using opamp and active devices like transistor diodes etc.

Conventionally, passive filters consisting of tuned L-C filters have been used to suppress harmonics because of their low initial cost. However, passive filters suffer with drawbacks such as large size, parallel and series resonance that could be created with both load and utility impedances. These drawbacks of passive filters, active power filters (APF) were developed. APF are power electronic converter based devices that are used for power quality improvement. APF have many advantages compared to passive filter such as, they are less bulky, superior filtering characteristics as well as dynamic performance, flexible operation etc. APF are classified into three types:- shunt active power filter; series active power filter and hybrid active power filter.

Dear Ravikumar Srinivasa

I think the comparison should be between active and passive filters. Each of them has advantages and disadvantages and both of them could be low, high, bandpass or bandreject filter. There are some highlighted points of comparison as follow:

· Active filter amplifies signal, whereas passive does not.

· Active filter is less reliable than the passive one due to the failure of its electronic component (operational amplifier).

· Passive filter is more bulky than the active one especially in case of using inductor (no inductors in active filter).

· The cut off frequency of active filter is not affected by load, whereas it is affected in case of passive filter, as the load becomes part of its transfer function.

· Active filter could be used as buffer circuit due the huge difference between its input and output impedance.

· Active filter has a drawback in very high frequency of operation, whereas passive one is more suited.

Good luck

Active filters come into their own in power filtering applications at lower frequencies where passive network components are necessarily large to provide suppression at lower frequencies. Active filters are smaller than passive filters. Generally, these will be embedded with HF passives which suppress noise beyond a few tens of kHz.

An active filter is a type of analog circuit implementing an electronic filter using active components, typically an amplifier. Amplifiers included in a filter design can be used to improve the cost, performance and predictability of a filter.

An amplifier prevents the load impedance of the following stage from affecting the characteristics of the filter. An active filter can have complex poles and zeros without using a bulky or expensive inductor. The shape of the response, the Q (quality factor), and the tuned frequency can often be set with inexpensive variable resistors. In some active filter circuits, one parameter can be adjusted without affecting the others.

an active filter is a type of analog circuit implementing an electronic filter using active components typically an amplifier,amplifiers included in a predictabikity of filter.

an active filter can have complex poles and zeros without using a bulky or expensive inductor.the shape of the response the Q quality factor and the tuned frequency can often be set with inexpensive,resistors;in some active filter circuits,one parameter can be adjusted without affecting others.

low pass filter blocks frequencies above cut-off frequency, whereas active filter eliminates sub harmonics also

low pass filter blocks frequencies above cut-off frequency, whereas active filter eliminates sub harmonics also.

Both are used to eliminate Voltage and current Harmonics which are harmful for loads or power transfer they could do cutoffs and transformer saturation. Passive filters could do the same functionalities as Active power filters with their different types (series and parallel), essentially 4 types.advantages of passive filters are:

• Reduced cost compared to active filters.
• Simple design, algorithm control and implementation.

disadvantages of passive filters are:

• Big size.
• .Big problem of resonance(that's why we migrate from passive to active).
• Many topologies for the Harmonic elimination(depends from the frequency numbers which they will be eliminated).

Active Power Filter usually noted as APFs: there are two types series for voltage harmonics elimination and parallel for current harmonic elimination.

Advantages of Active filters are:

• Reduced Size.
• One topology for Harmonic elimination type(voltage or current).

Disadvantages of Active filters are:

• Expensive components.
• More complex in control point of view.

power factor improvement to unity power factor, harmonic elimination and voltage regulation are main advantages of active power factor than passive filter

The main motive is to suppress harmonics and reactive power compensation. Both active power filters and passive filters have their own advantages. The best way is to use both in a grid connected system at point of common coupling. So hybrid filter is best choice for power quality improvement.

I think the preference depends on usage

It's not what is better. It's what gets the job done that is needed. The difference between an active filter and a passive filter is an active filter requires power in order to operate. Power is used if amplification is needed for a circuit. If a signal is small and needs to be larger, then an active filter is more appropriate. If no amplification is needed, a passive filter is better because it requires less energy requirement for a circuit.

If we consider our point of view to be nested in the scope of power systems, I would ponder the following:

I understand the main concern would be facility power factor improvement, using capacitor bank for that.

In this case, one must be aware of the harmonics produced by non-linear loads of the facility (rectifiers and inverters are examples of non-linear loads).

The main issue in this case is resonance. Non-linear loads are considered, in terms of circuits, as a current source injecting harmonic on the facility power system. This source of harmonics is faced with the reduced impedance of the system (essentially and generally, with the impedance from the harmonics source until the closest transformer).

Very well, this reduced impedance is composed by inductance (L), capacitance (C) and resistance (R). For ressonance to occur, a combination among the circuit parameters (L & C) and the injected harmonic currents ought to take place.

Well, we know that:

vC = Zch . iL ,

where vC is the voltage in the capacitance of the circuit, iL is the current in the inductance of the circuit and Zch is the characteristic impedance of the circuit.

We also know that the natural frequency of a circuit (w0) is given by:

w0 = sqrt(L .C)^(-1),

Now let's assume that F (the frequency of the system) is, e.g., 60 Hz; thence, wF, the angular frequency of the system, is:

wF = 2 . pi(). F --> for F = 60 Hz, wF = 377 rad/s.

We shall now consider that the non-linear loads inject harmonic currents which abide to the following:

Nth = k . P (+ and -) 1,

ith = iN / Nth

where k is any integer k > 0 (1, 2, 3…), Nth is the order of the harmonic injected, iN is the rated current of the non-linear load, ith is the current for the Nth harmonic and P (pulse number) is a parameter related with one intrinsic characteristic of the non-linear load. We can now say that, for resonance to occur:

Nth . wF = w0,

meaning that if there’s a Nth which makes the above equation true, then resonance will occur.

For the sake of practicality, let’s consider a facility with a three-phase, 60 Hz, 480 V electrical system whose main parameters are the following:

- System reduced inductive reactance: 0.0115 ohm

- System reduced resistance: negligible

- Non-linear load: 1500 kVA

- Power factor: 0.90

Now, suppose that we want the power factor to be no lower than 0.98. For that, a three-phase capacitor bank of 405 kvar is installed; the power factor is effectively improved to a value slightly above 0.98.

With the information above one can state:

- L = 30.5 µH

- C = 4.7 mF

- w0 = 2652 rad/s

- Zch = 0.081 ohm

- Zp = 0.0117 ohm @ 60 Hz

Zp above is the equivalent impedance of the system seen from the harmonic source; in other words, Zp = xL // xC (remember R is negligible), where xL is the inductive reactance and xC is the capacitive reactance @ 60 Hz.

Let’s assume now that the non-linear load has P = 6; with this we can say that the orders of the harmonics injected by the load are: 5th, 7th, 11th, 13th…

One shall notice that w0/wF = 7.03; this should have raised someone’s eyebrow, since the 7thharmonic is a characteristic one for a non-linear load with P = 6.

Considering the nominal current of the non-linear load iN = 1203 A, the 7thharmonic current will be i7th = 172 A; the 7thharmonic voltage on the 480 V bus will thus be:

vC7th = (Zp) . (7.03) . (wF) . (172) = 5.4 kV!

That is clearly a problem if not dealt with properly.

In order to avoid such a problem, instead of a capacitor bank one should have installed a filter tuned for a frequency slightly lower than the first characteristic harmonic – in this case the 5th harmonic (300 Hz, 1885 rad/s); this is what is called a passive filter, with the lumped inductance of the filter (Lf) adjusted to make:

sqrt(Lf . C) > 0.53

Then one can ask:

Why not tuning it for a frequency slightly lower than the 7th(420 Hz, 2639 rad/s)?

Because one runs the risk of creating a new resonant frequency between the 5thand the 7thharmonics! As the 5this the lower characteristic harmonic of the non-linear load, tuning for a frequency slightly lower than 300 Hz shall not lead to any resonant condition.

Alternatively, an active filter could have been used.

An active filter is a filter which uses power electronics (power semiconductor devices) to inject harmonic currents in the system to cancel those produced by the non-linear loads. This is achieved using PID controllers to switch semiconductor devices appropriately so that the cancellation of the harmonic currents is brought about. An active filter make do without the use of an inductive reactance, unlike the passive filter, which needs it.

While the passive filters do not depend on any aside power source besides that from the very system to which it’s connected, active filters do need an aside power source. The passive filters mitigate the harmonic current, but it won’t be able to extinguish them thoroughly. This reflects in the passive filter being much cheaper than the active filter.

The active filter is more accurate, being capable of cancelling almost all the harmonic current. One shall have to pay a good price for that, nevertheless!