Active Harmonic Filter vs Passive

Active Harmonic Filter vs Passive

Active Harmonic Filter

Active harmonic filter is type of analogue electronic filter. Uses OP AMP. Needs external power source. Injects phase opposite harmonic. Have energy losses. Its design needs passive components like capacitor and resistance. The inherited capacitor helps in increasing the load power factor. However, If the load size is high enough and needs higher reactive power compensation, active harmonic filter can't deliver that and requires an additional STATCOM to work in conjunction with it. Its major disadvantage is that it requires real power in kW for injecting phase opposite harmonic current at downstream at the connected non linear loads. It causes 3% to 5% real energy loss in kWh term that by considering that it's used for lesser harmonic polluting load. It malfunctions when the electrical power system contain high harmonic pollution and the power supply transformer or DG is near at full load capacity. It has a known problem connecting with the back up diesel generator (DG) power, if the plant loads are high and variable type.  

By design the active harmonic filter injects phase opposite harmonics at the point of common coupling (PCC) to the extent of its downstream load and cancels the harmonics from the PCC point to upstream. Whereas passive filter being type of harmonic absorption filter, engulfs harmonics both from the downstream loads as well as upstream within its range. By doing so it also cleans up the circulating harmonics from the downstream electrical power system which otherwise would be absorbed in the downstream equipment. As the downstream equipment absorb harmonic it creates additional voltage harmonic, which in turn creates additional current harmonic. Thus, by design passive filter has the capacity to reduce harmonic generation from downstream non-linear-loads in a harmonic rich power system. To read more about this, we have uploaded a case study in an another web page for an automobile industry with a 1.0 MW Solar Power Plant, please read it at Solar Power Inverter with Harmonic Filter.    

Active Harmonic Filter- advantages

The major advantage of active filter is that it's not required to be custom built. It does not require any load specific tuning. Active harmonic filter manufacturers claim that it can handle a wide band of harmonic frequency. On the other hand passive filter needs extensive engineering efforts and are custom built with load specific design. Passive filters require to be tuned for load specific band of harmonic frequencies. However, it may be noted that the actual load never generates beyond a few harmonic frequency bands. For example, computers, servers and switch mode power supply (SMPS) loads generate 3rd (to the extent of 80%) and 9th (to the extent of 15%) harmonic currents. Six pulse drive loads generate 5th (60%), 7th (20%) and balance in 11th and 13th harmonic current. 12-Pulse inverter loads generate 11th (50%), 13th (30%) and balance in other frequency harmonic currents.

Active Harmonic Filter- disadvantages

The major disadvantage of active harmonic filter, according to its manufacturer's catalogue, is the real energy loss that it creates in injecting phase opposite harmonic currents. It increases real energy (kWh) cost for the client. Whereas passive harmonic filter being type of harmonic absorption filter saves 3 to 5% real energy in kWh for moderately polluted harmonic loads. And it delivers much higher percentage of energy saving for massively polluted harmonic loads like rolling mill drive with fourth quadrant regeneration operation, induction furnace inverter loads and if the power supply transformer (or DG) is near at full load with hundreds of variable frequency drive (VFD) within it.

Active Harmonic Filter- problem with variable load

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

While active harmonic filter regulates variable harmonic currents from the downstream non linear loads, but it can't control reactive power on its own unless it works in conjunction with STATCOM. It inherits design capacitors which are fixed type. This restrict it from scaling up for higher loads or applying it at the transformer or DG for the entire PCC load. Instead, active filter manufacturers install in numbers, across several and smaller non linear loads within the same transformer. Back up diesel generator (DG) requires much closer power factor (PF) management. It's a known issue for applying it at the generator terminal.   

Passive filter though needs great engineering effort can be custom built for any variable type of load. It's applied conveniently at the transformer's main incomer or DG terminal for the entire power supply load bus which may comprise hundreds of variable speed drive (VSD), DC drive, converter, inverter and other non linear loads. It is custom built to regulate the reactive power requirement for any type of load. Be it the most challenging fourth quadrant regeneration drive which operates at a power factor of around 0.06 lag, and generates in between 80% to 120% current harmonic with a corresponding voltage harmonic distortion in the power supply bus. We have applied passive filters across thousands of diesel generator (DG) since our inception in 1995. With optimum harmonic filtering it debottlenecks DG's capacity limitation with harmonic load. And allows extra load on the DG to the extent of 30%. Solves DG hunting with non linear load and AVR voltage fluctuation. Saves energy and fuel cost.

Active Harmonic Filter- failure issue with power fluctuation

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

Unlike passive filter, active harmonic filter is not an energy storage device. It is estimated that billions of dollars are lost by industries globally on account of equipment nuisance tripping due to voltage sag which originates either from the factory's internal electrical power system or external from the utility's grid supply. The main advantage of passive filter is that being an energy storage device it acts like an electrical flywheel and eliminates equipment tripping due to momentary voltage fluctuation.

Whereas the active harmonic filter depends on the external power source and is susceptible to failure on grid voltage fluctuation. Several active filter manufacturers void their warranty if failure happen on account of voltage fluctuation. Besides it needs air conditioned room for its installation. Passive filter, on the other hand, is a rough and tough type and is installed in non air conditioned electrical panel room. Since 1995 we have been supplying hundreds of passive filters for eliminating nuisance tripping with VFD drives, DC drives, computers, servers and electrical equipment due to grid power surge and voltage sag. Several of our clients are located in the path of monsoon wind and have had a history of frequent equipment tripping problem with process drives and data center servers and harmonic failure issues. Passive filter comprehensively resolved all those.    

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

Active Harmonic Filter- stability issue

Passive filter provides guaranteed stability and it's sustainable. Stands tall for the test of time. It can be scaled up practically for any voltage, current and reactive power compensation requirement. 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 active harmonic filters are susceptible to maloperation if applied in a serious harmonic project in a harmonic rich power system. While suppressing harmonics across a larger frequency band, it ends up in generating some uncontrollable harmonic at some harmonic band as depicted in the case study. It's failure rates are high. The disposal for failed electronic components are always environmentally challenging. Whereas, passive filter even though requires custom design, is for a life time.

Active Harmonic Filter- arc flash and short circuit compatibility

Active filters are not designed with arc flash and short circuit compatibility in mind. It has high failure rates and shorter life span. Its operation and maintenance requires highly experienced engineers. We manufacture passive filters with 4kV/ 12kV arc flash compatibility and 50kA, 1sec. short circuit compatibility at its incomer. It's installed at the transformer and DG's main incomer PCC for all non linear loads within the transformer and DG. It's designed for full automatic operation in mind within the entire load variation spectrum that  including changeover between utility power to back up DG power and vice versa. It's a fit and forget type. Does not require any special maintenance except regular maintenance as like with any electrical equipment.   

Active Harmonic Filter- case study

In this case study a set of active harmonic filters are applied in conjunction with STATCOM at the main PCC of a transformer for the entire transformer load.

Industry: Automobile
Transformer size: 2500 KVA, 11kV/ 415V
Transformer Load: Thyristor Heating Furnace, 2nos 600kW and 1no 1200kW
Transformer Load Factor: Average 70%.
Back up DG: Yes. Fully backed up by DG in case of power failure
Active Harmonic Filter used for harmonic suppression: 3nos each 450A
Brand: a leading MNC brand
STATCOM for power factor control: 4nos +/- 300 kVAr
Brand: a leading MNC brand

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 a wide band of harmonic frequencies, it randomly generated some other frequency harmonic and created unbalance in the power supply. Its manufacturer could not resolve the problem which created voltage unbalance and occasional earth fault trip resulting productivity losses. The active filter itself failed repeatedly. Eventually we installed a 700A passive harmonic filter and it resolved the unbalance problem and tripping issue. The mitigated harmonic spectrum with passive filter is shown in Fig.2 which is self-explanatory. It was tuned case specific for the load and it reduced the plant's overall harmonic level at much a better level than the recommendation of IEEE 519 standard.