Active Harmonic Filter Vs Reliable Passive Installation

Author : Kanai Banerjee | Published On : 13 Mar 2021

Active Harmonic Filter

Read this case study of the advantages and disadvantages of the active harmonic filter and how passive filter installation removes harmonics, voltage surges and improves PF. Besides, the active harmonic filter needs the support of a stable external power source for injecting phase opposite current harmonics to downstream loads. Its efficacy depends upon the stability of the connected external power source.

AHF design needs passive components like a capacitor and a resistor, but it can either do harmonic control or power factor correction to its 100% rating. In the case of both PF improvement and harmonic reduction, its capacity needs a derating to 50%. The active harmonic filter is a load in an electrical power system that generates anti harmonics at 180degree phase opposite. AHF has energy losses. Most OEM pdf catalogues declare the kWh losses at 3% under laboratory measurement.

Active Harmonic Filter does not eliminate downstream harmonic, electronic failure, equipment and relay tripping.

By design, it doesn't remove harmonics from downstream power line but by injecting anti-harmonic cancels it to prevent it from flowing into the upstream utility supply. Its application is to limit upstream grid harmonics within IEEE-519 norm. Whereas passive harmonic filter being a harmonic absorption type sucks up harmonics by short-circuiting it from the downstream load as well as upstream power system within its range. By doing so, it cleans up the circulating-harmonic from downstream loads which otherwise get absorbed in the downstream equipment and In turn, increase voltage harmonic distortion within the supply transformer. In a chain reaction, that otherwise would magnify the current harmonics extent of which depends upon the fault level of the load bus PCC. Thus, by design passive harmonic filter relieves all downstream loads by removing the circulating-harmonics that eliminate equipment failure, break-down and nuisance tripping on voltage fluctuation. It improves Protection Relay functioning - read it in another case study at Solar Power Inverter with Harmonic Filter.    

Active Harmonic Filter has energy losses. Series impedance causes VFD tripping. Why a drive-wise harmonic correction is not a good idea.

It requires real-power in kW for injecting phase-opposite harmonic current coming from the downstream non-linear load. That causes 3% to 7% real energy losses in kWh term. Though OEM catalogue mention only about 3% energy loss, that's measured under an ideal condition. Energy-intensive industries employ hundreds of VFD drives within one transformer say 2000 KVA and operate in between 70 to 90% load factor. It requires higher impedance typically over 3% in series with VFD drive which injects further kW losses. Thus the 3% energy loss data may be valid for low-intensity load, but for higher load-factor and THDv, the total losses including in the high-value series impedance may go up to 7%.

Some consultants prefer to have a drive-wise harmonic filter which is not a wise thing to do though in particular when the supply transformer has too many VFD drives. Active front end harmonic filters use high impedance upfront with its connected VFD drive.  Summarily, it magnifies power system impedance which in turn increases THDv, disturbs VFD operation making it susceptible to tripping for even a minor grid voltage fluctuation.

It has a known problem connecting with backup diesel generator (DG) if plant loads are high and need variable reactive power compensation. The active harmonic filter can't deliver that as it inherits only a fixed and limited design capacitor.  An extra Power Factor control panel is required.  A STATCOM to work in conjunction with it or the SATCOM to be inbuilt within it. Some AHF manufacturers use an additional external APFC capacitor bank or a detuned harmonic filter which they refer to as a hybrid harmonic filter. A detuned filter does not control harmonics. It uses a detuned reactor in series for the protection of capacitors and not for eliminating the load harmonics.    

Active Harmonic Filter - advantages

The active harmonic filter does not require to be custom built since it is mostly impedance independence except the designed capacitors and resistors.  It does not need any load specific frequency tuning.  Active harmonic filter manufacturers claim it can handle wideband harmonic frequencies. On the other hand, the passive harmonic filter needs extensive engineering efforts and are custom built with load specific design. However, it is also true that the actual-load never generates beyond a few harmonic frequency bands.  For example, computer, server and switch mode power supply (SMPS) loads produce 3rd and 9th harmonic currents to the extent of 75% and 15% respectively.  Six pulse drive loads produce 5th (60%), 7th (20%) and balance in 11th and 13th harmonic currents. 12-Pulse inverter loads generate 11th (50%), 13th (20%), 5th (20%) and balance in other frequency harmonic currents.

Active Harmonic Filter- disadvantages

Active Harmonic Filter has a known issue in handing high-intensity loads wherein THDv is above 10% and without a linear-load mix for which many OEM recommend not to use it. It also creates energy losses which vary from 3% for low-intensity loads and goes up to 7% for high-intensity loads and voltage distortions. Whereas passive filter being harmonic absorption type saves 3 to 5% real energy in kWh for moderately polluted harmonic loads.  When it comes to massively polluted harmonic load like in rolling mill drive with a fourth quadrant regeneration operation, induction furnace etc., it delivers an even higher percentage of energy saving. The same higher percentage energy saving is possible when the transformer or DG operates at a higher load factor typically in between 80 to 90% and with a very high THDv.

Active Harmonic Filter malfunctions when THDv is high and is used without a linear-load mix

It malfunctions while handling high-intensity power and harmonic pollution wherein the total voltage harmonic distortion (THDv) is above 10%. Many OEM recommends not to use it when THDv is more than 10%. While the higher THDv problem is universal, it has another issue when the load doesn’t mix with some linear-load like full speed induction motor and the current harmonic percentage is high. We have independently verified these features in the following two case studies which are detailed below.

Active Harmonic Filter