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 marine applications worldwide.

We have 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 alternatively contact Sentinel Power Quality direct with details of your PQ problem(s) and we will be 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 which effectively isolates the distortion from loads downstream. This of particular importance on marine vessels where electrical propulsion and/or other large drives may result in excessive harmonic voltage distortion, propagating through transformers to low voltage consumers and disrupting or damaging susceptible or sensitive equipment.

An example of excessive Uthd on a marine vessel with 24 pulse main propulsion is shown.

Vessel with MV electric propulsion. Note the MV Uthd is 13.46% (max) whilst the Ithd (current) is only 5.43% !

Comsys ADF 'sensorless control' can eliminate or significantly attenuate this ‘background’ voltage distortion as illustrated by the single line diagram.

This vessel had 2 off 12 pulse, 4.25MW main propulsion drives. On the primary of one phase shift transformer there was an additional phase shift resulting in a quasi-24 pulse configuration.  As illustrated in the trends and waveforms the Uthd (total harmonic voltage distortion) at 13.4% exceeded the recommended limit of 8% Uthd.  The excessive Uthd in this instance was due to switching harmonics; the Ithd (harmonic current distortion) was relatively low at 5.3% max.

The excessive Uthd transited through the transformers to the hotel and other loads, resulting in significant operational problems in the galley (440V) and in the ships office and the bridge, both of which were 230V supplies. Two Comsys 200A ADF filters were employed on sensorless control as shown. As a result, the Uthd was reduced from 13.4% to  around 2.0/2.1% respectively downstream of the two ADF P300 active filters.

Vessel with quasi 8.5MW of 24 pulse main propulsion VFDs and Comsys ADF filters.

Another vessel which has benefitted from Comsys ADF sensorless control was the Dutch LPD (Landing Platform Dock), HNLMS Rotterdam (L800). Similarly, high Uthd was present on LV supplies due to her 12MW electrical main propulsion. Two ADFs were installed on the secondaries of the LV transformers. The excessive MV Uthd was successfully isolated from the 230V and other LV supplies.

Dutch assault ship, HNLMS Rotterdam also installed with Comsys ADF with sensorless control

If you, believe 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 Marine have been involved in applying active filters since 1996 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. For more information of AC line reactors please click here.

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

Sentinel Power Quality Group in conjunction with a leading European electromagnetics manufacturer, can provide an advanced range of series passive harmonic filters from 4kW to 2625kW for AC variable frequency drives (VFD's), DC SCR drives and other rectifier applications. The patented series passive harmonic filters are very popular due to their simplicity, ruggedness and excellent level of performance.

In addition, the 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.

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.

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

The design of the magnetic cores 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 the series passive harmonic filters and other products are more efficient, have less power losses, lower operating temperatures and reduced audible noise compared to competitors.

SPQ-EF3H Series Harmonic Filters

The standard range is 4kW/5HP to 2625kW/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.

General specification of SPQ-EF3H series harmonic filters


SPQ-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 disconnection is not required at no or light loads.

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

Current waveform and harmonic current spectrum for VFD at 96% load. No line reactor (Ithd 74.39%)
Current waveform and harmonic current spectrum for VFD at 96% load. SPQ-EF3H harmonic filter connected <5% lthd

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

We have collectively introduced the innovative SPQ-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 SPQ-EF3H harmonic filter-vertical mounting

For dimensions of EF3H IP21 harmonic filters please click here.

SPQ-EF3H series harmonic filters (from 110kW/200HP to 2625kW/3500HP)

For applications above 110kW, wall mounting series harmonic filters are not available. Traditional floor mounted versions are therefore offered in powers to 2625kW/3500HP for AC VFD, DC SCR drive and other rectifier applications.

Specification as per advised with <5% Ithd performance, <20% kVAr to 110kW. Above 110kW, <15% kVAr at no load. 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.

1 of 315kW (IP23) and 4 off 400kW (IP54) filters
Example of reactor design and quality
500kVAr, 600V/60Hz NEMA3R filter for North American market.

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

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).

Sentinel Power Quality Group and its partners have successfully supplied both ESP vendors and IOCs, since 2012, with series passive harmonic filters, output filters and high frequency transformers for land-based oilfield applications, offshore platforms and FPSOs.

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

Note : Also du/dt and sinewave output filters for ESPs and other VFD applications in the range 4kW to 2625kW 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.

Enclosure options

There are a 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.

RiteSineLab harmonic estimation software

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 series passive  harmonic filters can also be selected for harmonic mitigation simulation. The programme automatically selects the appropriate 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 RiteSineLab 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.

VFD Output Filters

SPQ-ED3dU du/dt choke

SPQ-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).

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

SPQ-EF3LC Sinewave Filters

SPQ-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).

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

In addition to prevent cable resonance (standing waves), the SPQ-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 SPQ-ED3du du/dt chokes and SPQ-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

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".