Active Harmonic Filter vs Passive

Active Harmonic Filter vs Passive

Active Harmonic Filter - a Case study of its Advantages & Disadvantages.

An active harmonic filter is a type of analogue electronic filter. Uses OP AMP. Needs external power. Injects phase opposite harmonic. Have energy losses. AHF design needs passive components like capacitor and resistor. The inherited capacitor helps in increasing load power factor.

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% as it malfunctions. In this Case Study, we have independently verified this feature.  

AHF's another disadvantage it requires real-power in kW for injecting phase opposite harmonic current at downstream at the connected 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 as like car-mileage claim. Energy-intensive industries employ hundreds of VFD drives within one transformer say 2000 KVA and operate in between 70 to 90% load factor.  In addition, the active harmonic filter 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, it may go up to 7%.

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 the fixed type design capacitors and needs additional equipment. A STATCOM to work in conjunction with it or the SATCOM to be inbuilt within it or it requires an external capacitor APFC bank which many OEM refers to as the Hybrid Harmonic Filter.   

By design Active Harmonic Filter doesn't remove harmonics from downstream power line but cancels it by injecting anti-harmonics. Its application is to limit upstream harmonics within IEEE-519 norm. Whereas passive filter being a harmonic absorption type sucks up harmonics by short-circuiting it both from downstream loads and upstream power systems within its range.  By doing so, it cleans up the circulating-harmonics from downstream loads which otherwise get absorbed in the downstream equipment and In turn, increase voltage harmonic distortions within the electrical power system of the supply transformer. In a chain reaction, it magnifies current harmonics extent of which depends upon the fault level of the load bus PCC. Thus, by design passive filter relieves downstream non-linear load by removing circulating-harmonics and eliminates equipment failure, break-down and nuisance tripping issues. In-addition protection relay functioning improves. We have uploaded another case study about it for an automobile industry with a 1.0 MW Solar Power Plant. Read it at Solar Power Inverter with Harmonic Filter.    

Active Harmonic Filter- advantages

The active harmonic filter does not require to be custom built since it is mostly impedance independence except the design capacitors and resistors.  It does not need any load specific frequency tuning.  Active harmonic filter manufacturers claim that it can filter wideband harmonic frequencies. On the other hand, the passive filter needs extensive engineering efforts and are custom built with load specific design. Passive filters need tuning for load specific harmonic frequencies. However, it is also true that 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 harmonics to the extent of 80% 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 (30%) 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% at which many OEM recommends 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 rolling mill drive with 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%.

Active Harmonic Filter- problem with high intensity variable load

Active Harmonic Filter
Fig.1. Plant harmonics without active or Passive Filter

While the active harmonic filter can regulate variable harmonic currents from the downstream non-linear load, it can't control reactive power on its own unless it works in conjunction with STATCOM.  It inherits the fixed design capacitors which restrict it from scaling up for higher loads that including applying it at the transformer or DG PCC for the full utility load which may include hundreds of VFD drives and other non-linear loads. Instead, active harmonic filter manufacturers install it in numbers across several and smaller non-linear loads within the same transformer. Back up diesel generator (DG) requires a closer power factor (PF) management. It's a known issue for applying it at the generator terminal.   

Passive filter though needs custom engineering effort is custom built for any variable load. It's applied conveniently at transformer's main income or DG terminal for the entire supply load which may comprise hundreds of variable speed drive (VSD), DC drive, converter, inverter, computer server and other non-linear loads. It is custom built to regulate any variable reactive power demand from the downstream load. Be it the most challenging fourth quadrant regeneration drive which operates at a power factor of about 0.06 lag, and generates in between 80% to 120% current harmonic with a corresponding voltage harmonic generation. We have applied passive filters across thousand of diesel generators (DG) and also at Turbo generators (TG) since our inception in 1995. With the elimination of both voltage and current harmonics, it debottlenecks DG/TG's capacity limitation which allows extra load on the DG to the extent of 30%, solves its hunting, AVR voltage fluctuation problem and saves good energy and fuel.

Active Harmonic Filter- failure issue with power surge and voltage fluctuation

Active Harmonic Filter - with passive
Fig.2. Plant harmonics with Passive Harmonic Filter

Unlike the passive filter, an active harmonic filter is not an energy storage device. One of the advantages of the passive filter it's an energy storage device which acts as an electrical flywheel and eliminates equipment tripping due to momentary voltage fluctuation which originates either from factory's internal power system or external from the utility's grid supply. The consequences of poor power quality vis-a-vis voltage sag tripping are high. Industries worldwide lose billions of dollars every year on account of equipment nuisance tripping due to voltage fluctuation.

Whereas active harmonic filter being dependent on an external power source is susceptible to failure on voltage fluctuations. Several active filter manufacturers void their warranty if failure happens on account of a voltage fluctuation which can be a subjective clause. Besides, it needs air-conditioned room for its installation. Passive filter, on the other hand, is rough and tough and is installed within the ordinary electrical panel room. Several of our clients are in the path of monsoon wind and have had a history of equipment tripping with its sensitive process drives and server networks. Since our inception in 1995, we have been supplying hundreds of passive filters for eliminating nuisance tripping with VFD drive, DC drive, computer, server network and electrical equipment due to grid power surge and voltage fluctuation.

Active Harmonic Filter versus Passive Filter
Fig.3 700A frame Passive Harmonic Filter

Active Harmonic Filter- stability issue

The passive filter provides guaranteed stability and its sustainable through the test of time. It can be scaled up practically for any voltage, current and reactive power compensation need. We have supplied it for all industrial voltage rating up to 36kV and current rating up to 5,000 amps in a single filter.  Whereas, the active harmonic filter is susceptible to maloperation if installed in a harmonic-rich power system. While cancelling harmonics across large frequency ranges, it ends up in generating another range harmonic frequency.  As shown in the top harmonic bar-chart, it faulted and generated a 2nd harmonics. It's failure rates are high. The disposal for failed electronic components is always environmentally challenging. Whereas, passive filter even though requires a custom design, is for a lifetime.

Active Harmonic Filter and arc flash and short circuit compatibility

Active filters don't comply with arc flash and short circuit compatibility norms. It has high failure rates and has a shorter lifespan. Its operation and maintenance are a challenge that needs highly experienced personnel.  Whereas, passive filters comply with arc flash and short circuit norms and can handle the entire transformer or DG loads. Its designed for full load variation with automatic operation including changeover from utility power to back up DG and vice versa. It's fit and forget type and does not require any special maintenance except regular maintenance as like with any electrical equipment.


Active Harmonic Filter- a case study

In this case study, a set of active harmonic filters applied in conjunction with STATCOM at the main PCC of a transformer with back up DG for the entire transformer load in the automobile industry. Followings are brief project specification.

Industry: Automobile
Transformer size: 2500 KVA, 11kV/ 433V
Transformer Load: Thyristor Heating Furnace, 2nos 600kW and 1no 1200kW
Transformer Load Factor: Average 70%.
Back up DG: Fully backed up by DG in case of power failure
Active Harmonic Filter Specification: 3nos each 450A
AHF Brand: from a leading MNC company.
STATCOM: 4nos +/- 300 kVAr
Brand: from a leading MNC company.

Active Harmonic Filter vs Passive- harmonics before and after filters

Plant Harmonic level without any Harmonic Filter: Shown in Fig.1

Plant Harmonic level with a 700A Passive Filter: Shown in Fig.2

700A Frame Passive Harmonic Filter: Shown in Fig.3

Plant Harmonic with Active Harmonic Filter: Shown in the top caption picture.

The problem with active harmonic filter is that while mitigating wideband harmonic frequencies, it randomly generated some other frequency harmonic causing unbalance in the power supply of the furnace. Its manufacturer could not resolve the problem which created voltage unbalance and occasional earth fault trip resulting in productivity losses.

The active filter itself failed repeatedly. Eventually, we installed a passive filter. It resolved the unbalance and tripping problems. The mitigated harmonic spectrum with passive filter shown in Fig.2 which is self-explanatory. It mitigated the plant's overall harmonic level at the supplying transformer PCC better than IEEE 519 standard.