Sentinel Power Quality, in partnership with leading manufacturers, offers a range of passive and active harmonics, power quality and EMI mitigation solutions, fully fit for purpose for challenging offshore and onshore oilfield applications worldwide.


Sentinel power quality has the extensive knowledge and experience to help you engineer those solutions correctly, whatever the application.


Please use the tabs below to navigate to the appropriate product type or alternatvely contact us with details of your PQ problem(s) and we will happy to assist you and/or provide advice.


Comsys AB, based in Lund, Sweden, are one of the market leaders in active harmonic technology with over 3000 ADF filters installed across the world; on ships, drilling rigs and industrial and commercial applications. Within the ADF range is the World’s fastest harmonic active filter (ADF P200) which also mitigates harmonics to the 120th order. Comsys's patented optional ADF sensorless control permits elimination of background harmonic voltage distortion and operation without current transformers.

For the Comsys ADF corporate brochure please click here.

Active filters are very low impedance (<1%), electronic harmonic current sources which provide non-linear loads the harmonic currents they require to function. Without such devices, the harmonic currents would be drawn from the generator or transformer often resulting in excessive harmonic voltage distortion in the power system.

Simplified block diagram of active harmonic filter

ALL active filters require at least 3% AC line (or DC bus reactance) in each VFD in order to meet the <5% Ithd (total harmonic current distortion) performance specification. The lack of additional reactance in a rectifier load results in significantly more harmonic current drawn from the active filter than previously from the transformer or generator. This is due to the < 1% source impedance for harmonics of the active filter.  For example, in VFDs, this additional current can be in the order of 50-100% for VFDs without additional reactance. With  3% AC line reactors installed, the ‘additional current’ is around 25% more but this additional harmonic current is usually designed into the current rating of the active filter.

If correctly dimensioned for VFDs with no additional reactance, the active filter will be substantially larger and significantly more expensive than required had suitable AC line (or DC bus) reactors been installed in the VFD. The active filter performance will be significantly degraded (~10-12% Ithd) and will not achieve the <5% Ithd often expected.

There are also issues when active filters are used with DC SCR drives where no AC line reactors are installed. Due to the inductive nature of the load (i.e. the DC motor armature) the additional current drawn when the active filter is connected is less compared to a VFD load (which is a capacitive load). However, if no AC line (or ‘commutation’) reactors are employed then high frequency energy in the DC SCR drive line voltage notches can potentially destroy the active filter’s passive carrier frequency suppression filter and render the active filter inoperative. Indeed, this was the biggest impediment to applying active filters to older rigs with mainly DC SCR drives. A combination of at least 4% AC line reactors and a fast response active filter can almost eliminate line notching, provide an excellent degree of harmonic mitigation resolve the carrier suppression filter problem and, if rated correctly, can also provide displacement power factor correction for systems with DC SCR drives.

Comsys ADF P100 Series

The Comsys ADF P100 series of relatively low-cost, compact active filters can be installed in their standalone IP21 enclosures on a wall in a factory, on a bulkhead or inside a switchboard. Their compensation ratings extend from 50A to 130A, all based on operating up to 50 deg C.

P100 series can be paralleled to a maximum of 15 units.

The P100N is a four wire (4W) device, available only in 100A per phase/300A neutral. However, paralleling up to 15 units is also possible.

In addition to harmonic current compensation, both P100 and P100N can provide simultaneous reactive power compensation as standard.

P100/100A and P100N (100A/300A for neutral)
Automated diagnostics across all ADF active filters

For the datasheet on Comsys ADF P100 please click here.

For the datasheet on Comsys ADF P100N please click here.

Comsys ADF P200 series

The P200, available in 100A ratings, is the fastest active filter in the World. Where most active filters compensate to the 50th order, the P200 compensates to 6kHz (120th order at 50Hz or 100th order 60Hz).

Up to 15 of P200's can be paralleled. Rated up to 50 deg C ambient.

The P200 comes with the option of sensorless (CT less control), wideband compensation and is often applied as an anti-resonant damper on applications which are subject to resonance (i.e. the P200 can be tuned in to prevent the resonance from occurring).

P200 – Fastest active filter in the World
P200 control panel – P200 can be used to prevent resonance and mitigate to 120th order

For the datasheet on Comsys ADF P200 please click here.

Comsys ADF P300 series

The P300 is the modular, high-performance harmonics and PQ workhorse of the Comsys ADF range. Available in voltages from 380V to 690V and compensation ratings from 90A to 360A, the P300 can operate in ambient temperatures up to 50 deg C. No matter how onerous the application, the P300 can handle it.

Up to 15 of P300's can be paralleled up. Simultaneous reactive power and harmonic compensation available as standard.

Both air cooled and liquid cooled P300 variants can be supplied.

Air cooled P300 cabinets can accept up to three 120A modules (360A)
P300 dashboard common across the ADF range
Door mounted display common across the ADF range

For the datasheet on Comsys ADF P300 please click here.

Comsys ADF PM300

The Comsys ADF PM300 is the P300 designed with OEMs and system integrators in mind. It allows them to install the filters into their own systems. It is supplied in component form as illustrated.

In common with the P300 series, the PM300 is available with air cooling or liquid cooling capabilities for operation up to 50 deg C.

Modular construction for ease of system integration
PM300 300A filter integrated with a 400kW VFD and other equipment

For the datasheet on PPM300, 90-120A ADF power processor modules please click here.

For the datasheet on PPM300v2B, 50A-150A ADF power processor modules please click here.

For the datasheet on PPM300W (liquid cooled), 140A-150A ADF power processor modules please click here.

For the datasheet on SCC2 ADF Controller for 1-6 PPM modules please click here.

Comsys ADF Sensorless Control

All active filters, including Comsys ADF series, attenuate the harmonic currents drawn by non-linear loads and indirectly reduce the harmonic voltage distortion due to the load(s) under mitigation. This requires the use of current transformers.

The range of Comsys ADFs also uniquely offer the option of ‘sensorless’ control, negating the requirement for current transformers. This can ensure that installation and commissioning is easier and quicker.

ADF harmonics compensation set to voltage (for sensorless)

Another exciting application of Comsys ADFs ‘sensorless control’ is compensation of background harmonic voltage distortion (Uthd) which effectively isolates it from loads downstream. This is essentially ‘voltage reconstruction’. This is of particular importance on drilling rigs and production platforms where large AC VFDs and or DC drives may result in excessive harmonic voltage distortion and other disturbances, which propagate through transformers to lower voltage consumers (including ROVs), disrupting or damaging susceptible or sensitive equipment.

Comsys ADF sensorless control effectively isolates the excessive Uthd from LV supplies. An example of excessive Uthd on one generator (three were in parallel) on a drilling rig can be seen.

Uthd of 18.88% can be seen plus significant line notching as seen on one generator
Very aggressive line notching due to multiple (i.e. three) DC SCR drives (18.88% Uthd)

The combination of the 18.88% Uthd and severe line notching propagated through the localised power system via the transformers, adversely affected other equipment.

The problem of excessive Uthd is also common on production platforms and FSPOs which have large, multiple VFDs on compressors, pumps and ESP applications plus AC or DC drilling packages.

The spectrum below was captured at the 600V drilling package (DC SCR drives) on one platform. The Uthd was 29.71%, of which 21.27% was above 21st order due to 11MW of 24 pulse ESP VFDs connected to the 11kV busbars. Some 12.2% Uthd below 21st was also due to the 24 pulse ESP drives, all of which transited across the 11kV/60V transformer to the drilling package as background Uthd. (The recommended total harmonic voltage limit on the installation was 5%). The high Uthd and more importantly, the high frequency harmonics pose safety issues on explosion proof motors. PTB calculate, for example, an additional temperature rise of 23% based on the voltage spectra, possibly exceeding the motor's temperature class and jepordising safety.

Platform with 29.17% Uthd of which 21.27% was above the 21st order due to multiple 24 pulse VFDs

If PQ problems are due to background voltage distortion it may not be feasible to treat the harmonics at source. The alternative is to isolate the voltages supplies which are being directly affected by the excessive background distortion by connecting a suitably dimensioned Comsys ADF filter(s) in parallel on the secondary side of the transformer(s) which supply those loads. This results in Comsys ADF sensorless control eliminating or significantly attenuating ‘background’ distortion or, it could be stated, effectively isolating the affected loads from the background voltage distortion.

If you have problems with LV equipment, possibly due to high background harmonic voltage distortion please contact us.

Important tool…

Active filters, when applied correctly, are an important tool in the harmonic mitigation toolbox. However, no form of harmonic mitigation is perfect. The secret of success is recognising the pros and cons of all types of active (and passive) mitigation and being able to apply the most appropriate form of mitigation for any given application.

The staff at Harmonic Solutions Oil & Gas have been involved in applying active filters since 1996 and have applied active filters to a large number of demanding marine and offshore applications.

A complete range of accompanying AC line reactors for active filters to 3000A can also be supplied, if required.

Contact us with your application details and let us help you.

Elhand, a leading European electromagnetics manufacturer, has developed an advanced range series passive filters from 4kW to 2600kW for AC variable frequency drives (VFD's), DC SCR drives and other rectifier applications based on their Core CutTM patented technology. Series passive filters are very popular for many drive applications due to their simplicity, ruggedness and excellent level of performance.

Elhand’s range of harmonic and sinewave filter enclosures are amongst the largest in the world, covering IP00 to IP66 for operation in ambient temperatures to 55 deg C. Their filter products are fully adapted for all applications and working environments.

Elhand factory in Lubliniec, southern Poland

Based in Lubliniec, southern Poland, Elhand Transformatory, is a high-quality (ISO 9001), innovative manufacturer of diverse electro-magnetic products including transformers, chokes and reactors; both standard types and special designs. Through systematic and continual development Elhand have gained an enviable reputation as one of most dynamic magnetics manufacturers in Europe serving marine (including the US Navy and other NATO navies), offshore and industrial customers.


As well as industrial customers, Elhand also supplies to the US Navy, NATO navies and other marine, offshore and industrial clients. Elhand have developed an advanced range series passive filters for AC variable frequency drives, DC SCR drives and rectifier applications based on their Core CutTM patented technology (see below).  Series passive filters are very popular for many applications due to their simplicity, ruggedness and excellent level of performance.

Elhand CutCoreTM Technology

In the design and manufacturer of series harmonic filters, the laminated magnetic core, around which the coils are wound is of crucial importance. The linearity of the filter’s magnetic characteristics is paramount. Any non-linearity can result in increased power losses (and associated temperature rise) and an increase in audible noise, especially in the areas adjacent in the air gaps. Elhand recognised this problem and as a result of an extensive research and development programme, introduced CutCoreTM multi-gap core technology for their harmonic filters and other products. In doing so, Elhand has eliminated or significantly attenuated, the technical problems regarding magnetic core.

Run of magnetic flux : a) Standard core. B) Patented multigap core design

The cores manufactured by Elhand allows for a constant value of inductance irrespective of a wide range of changes in the filter current (magnetic linearity), reducing the audible noise level generated and eliminating the external magnetic field (see diagram). Consequently, the additional losses in the core, winding and structural elements are all reduced. The result is Elhand harmonic filters and other products are more efficient, have less power losses, lower operating temperatures and reduced audible noise compared to competitors. For technical paper “Harmonics Filter as Guarantee of Electromagnetic Compatibility and High Efficiency of Converter Drive Systems” click here.

* Note that the Ithd level shown with the EF3H filter connected above is 5.47%. This test was conducted in March 2015. The design has subsequently been improved and performance of <5% Ithd at rated load is the norm.

Elhand EF3H Series Harmonic Filters

The standard range is 4kW/5HP to 2600kW/3500HP in voltages from 380V to 690V, 50Hz or 60Hz, in enclosures IP00, IP20, IP23, IP54 and IP66 and ambient temperatures to 55 deg C. General specification is shown.

Note : Also see Elhand Drive Systems Industry catalogue.


Elhand EF3H harmonic filters are highly efficient with performance levels of <5% Ithd. Another important feature is the capacitive current at no-load being <15%. This ensures capacitor bank connection is not required at no or light loads.

Typical performance of EF3H Elhand series passive harmonic filter on 110kW VFD:

Harmonic filters for panel builders and retrofits (to 110kW)

Elhand have recently introduced the innovative EF3H three phase harmonic filter design for the hanging vertical position. This is ideal for retrofits (can be mounted on the wall) and for electrical panel builders as it can be installed inside a panel or switchboard thus saving valuable floor space.

Range is from 4kW/5HP to 110kW/150HP in IP00 and IP20 formats. Higher IP ratings available on request.

IP00, 22kW SPQ-EF3H harmonic filter- vertical mounting
IP20, 90kW EF3H harmonic filter-vertical mounting

For dimensions of EF3H IP21 harmonic filters please click here.

Elhand EF3H series harmonic filters (from 150kW to 2600kW)

For applications above 132kW, wall mounting series harmonic filters are not available. Traditional floor mounted versions are therefore offered in powers to 2600kW/3500HP for AC VFD, DC SCR drive and other rectifier applications.

Specification as per advised with <5% Ithd performance, 15% capacitive current at no load and operation up to 55 deg C. The range of enclosure options from IP00 to IP66.

Special marine and offshore rated designs are available, encompassing thermal reactor monitoring, capacitor protection and if required, PQ (power quality) monitoring can be provided. Marine deck proof enclosures to IP66 also available.

Example of reactor design and quality

Dedicated harmonic filters for oilfield ESP applications

Series passive harmonic filters are extremely popular for oilfield ESP (electrical submersible pump) applications; in desert oilfields, offshore or installed on FPSOs.

They are renowned for their simple design, effective performance, ruggedness, both electrically and environmentally, lack of complex maintenance and ability to operate in high ambient temperatures (to 55 deg C).

Elhand has supplied ESP companies since 2012, mainly in the Arabian Gulf. Many hundreds of Elhand harmonic filters (and output sinewave filters) are successfully operating in the region.

15 of 600kVA, IP54, 55 C SPQ-EF3H harmonic filters for ESP oilfield application in Saudi Arabia (desertified design).

Note : Elhand also manufactures du/dt and sinewave output filters for ESPs and other VFD applications in the range 4kW to 2600kW to match VFD kVA ratings. For more information click here.

For all your ESP harmonic filters (and sinewave filters) enquiries and applications please contact us.

Elhand enclosures options

Elhand has a large number of harmonic filter enclosure options (IP00 to IP66), this combined with two hundred and eighty (280) harmonic filter variants spread over seven voltage/frequency ranges, means It is not feasible or possible to detail all the dimensions and nett weight information on this website.

Harmonic filter enclosure options available from IP00 to IP66.

For information on dimensions of any harmonic or sinus filters please contact us.

Now available - Elhand harmonic estimation software

Elhand harmonic estimation software is now available to model drive applications using up to ten (10) nodes for AC and DC drives AND programmable current sources. The full range of Elhand harmonic filters can also be selected for harmonic mitigation simulation. The programme automatically selects the appropriate Elhand series filter from the designated database.

The harmonic voltage and currents are modelled to the 50th order (for standards compliance) and also up to 100th order. The Urms and Irms waveforms and associated spectra is available for each node. This software is free of charge once customer registration has been completed.

Please contact for more information.

An example of Elhand harmonic modelling a single line diagram (with 3% AC line reactors in each VFD; 4% in the DC SCR drive) is illustrated :

Example of line voltage and phase current waveforms (all three waveforms can be displayed) for Phase one, Node 1 (LHS). Note that the line notching is due to the DC drive connected to Node 9:

Phase One voltage waveform
Phase one current waveform
Phase One voltage spectra
Phase One current spectra

Comprehensive reports in pdf form will be downloadable including compliance or otherwise with IEEE 519 (1992), IEEE 519 (2014) and other standards.

Elhand VFD Output Filters

ED3dU du/dt choke

ED3dU du/dt Chokes

A du/dt limiting choke is commonly used to protect motor insulation against the high rate of rise of voltage from PWM VFDs.  These chokes are effective on installations with short cable lengths and relatively low (<4kHz) switching frequencies. du/dt limiting chokes maximise the motor insulation life and assist in reducing electro-magnetic interference (EMI).

EF3LC sinewave filter in IP00 format. 55kW, 400V, 50Hz for VFD 5.5kHz switching frequency

EF3LC Sinewave Filters

Elhand EF3LC sinewave output filters are utilised to resolve a number of problems, particularly associated with higher VFD IGBT switching frequencies and long cable lengths (>25-50m) which can result in a phenomenon called “standing waves” (i.e. cable resonance which can trip the VFD and damage the motor).

EF3LC sinewave filter in IP00 format. 55kW, 400V, 50Hz for VFD 5.5kHz switching frequency

In addition to prevent cable resonance (standing waves), the EF3LC sinewave filter reduces RFI emissions and can, on occasion, negate the requirement for special VFD rated cable between the VFD and the motor.  Motor audible noise, motor temperature and winding insulation stress are all considerably attenuated.

Both ED3du du/dt chokes and EF3LC sinewave filters are available from 4kW to 2600kW in voltages from 400V to 690V, 50Hz or 60Hz in ambient temperatures to 55 deg C. Both products can be supplied in IP00 format plus IP23, IP44, IP54 and IP66 enclosures for oilfield, industrial or marine use.

For further information please click here - du/dt Chokes & Sinewave Filters

For specification and IP00 dimensions of EF3LC sinewave filter click here - EF3LC Sinewave Filter

AC line reactors for AC variable frequency drives (VFDs) and DC SCR drives up to 3000A, 400-690V, 50Hz or 60Hz are available, often on short deliveries.

These devices protect the drive front end rectifiers from mains borne disturbances and also reduce both the harmonic current drawn by the drives and in the case of DC SCR drives, attenuate the line notching  often preventing repetitive voltage spikes which disrupt and/or damage capacitor based equipment such as switched mode power supplies and small VFDs.

The installation of these reactors is crucial if active harmonic are to be used for harmonic mitigation, especially with AC VFDs or DC SCR drives.

A standard range of AC line reactors is available ex stock.

AC line reactor

For information on AC line reactors for AC VFD and DC SCR drives click here - AC line reactors

The definition of a ‘transient’ or ‘spike’ is a temporary excess of voltage and current on an electrical circuit which has been disturbed in some way.

They can also be defined as a change in the fundamental (50Hz/60Hz) frequency. A transient ‘surge’ can contain thousands of volts and thousands of amperes with duration of nanoseconds to microseconds.

Example of transient voltage

Transients voltages can be due to lightning, faulty power factor correction switching, faults or other disturbances on the power line, for example.

Whatever the cause, transient voltages can have a devastating effect of unprotected electronic equipment, especially exposed equipment containing capacitors in their input circuitry including small variable frequency drives (VFDs), fluorescent lighting and switched mode power supply capacitors. The results are usually spurious tripping at best or total destruction with all the operational downtime which can result from such failures. Motor winding insulation can also be stresses due to excessive transient voltages.

Capacitor damaged on printed circuit board due to transient voltages

SineTamerTM is a type of TVSS (transient voltage surge suppression) device specifically designed to protect sensitive equipment including switchboards, computers, plcs and AC VFDs.  It is widely used extensively also to protect airport scanners.

Airport security scanners benefit from SineTamerTM
Two SineTamerTM TVSS units at input to switchboard

Damaging voltage spikes are not uncommon industry as can be seen.

Repetitive voltage spikes due to DC drive line notching and stray capacitance

The result is often disruption or the destroying of capacitor-based equipment such as switched mode power supplies, fluorescent light fitting and small VFDs.  SineTamerTM protects such equipment from the effects of the voltage spikes.

SineTamerTM protecting 11kW VFDs

SineTamerTM encompasses the development of a fourth-generation technology for surge and transient voltage protection. SineTamerTM is a lot more than a typical TVSS (transient voltage surge suppressor) which can be found in electrical panels worldwide. It is an engineered solution, specifically designed for the ultra-sensitive nature of electronic circuit boards (such as those used in VFDs, UPSs and PLCs) plus all variations of microprocessor-based control systems.

Please click at the appropriate place to obtain more information on :

SineTamerTM catalogue click here – sinetamer-catalogue.

SineTamerTM for VFD protection -  What exactly is a VFD? 

SineTamerTM for PLC protection - What exactly is a PLC?

SineTamerTM for petro-chemical industries click here – sinetamerpetroleum.

Information on “Unexpected downtime caused by electrical disturbances".