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.

This page is currently under construction.

For more information on Active Harmonic Filters please contact us.

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.


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 SPQ-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.

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).

Innovative 12 pulse transformer with integrated passive filter

Innovative transformer technology comprises either air cooled or oil-cooled, hermetically sealed 12 pulse phase shift transformers with integrated harmonic filter.  This solution, available to 2625kVA not only offers reduced dimensions but also higher efficiency, higher performance (<5% Ithd) and low capacitive current (<5%).

Schematic of 12 pulse integrated filter for direct connection to LV or MV
250kW air-cooled 12 pulse integrated filter in IP00 (250kW) for 12 pulse VFD
Technical specification

The above solution allows direct connection from MV connection (3.3-33kV) to LV 12 pulse ESP VFDs.

for more information or any enquiries please contact us.

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.

ESP ETO VFD output transformers

Our ETO step-up transformers are specifically designed for ESP VFD operations to compensate for voltage drop due to the very long cables from the surface to the ESP.

These transformers are available up to 2000kVA and in excess of 6.6kV output for conventional VFD ESP which control induction motors.

High frequency output TF for permanent magnet ESP motors

Special high frequency transformers are also available for VFD fed high speed induction and permanent magnet ESP motors.

High frequency output transformer for ESP (900kVA)

Both output transformer types are offered in up to IP55 enclosures and for operation in ambient temperatures of up to 55 deg C for desert and other arid environments, for more information or any enquiries 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.

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.

ESP Optional - SineTamer

SineTamerTM TVSS (surge protective devices) are also available up in voltages to 6900V. These are specifically designed for the protection of VFDs used by the following for ESP VFD applications :

Partial ESP users of SineTamerTM surge protective devices

For information on SineTamerTM for oilfield ESP VFDs click here.

ESP Optional - Continuous Power Quality Monitoring

An increasing number of clients require power and/or power quality monitoring from each ESP, either via manual download or remote monitoring. These are ideal applications for PQGuard.

Trend of min/max and average harmonic voltage distortion on jackup drilling rig (Uthd)
PQGuard panel mounted power quality analyser

For more information about PQGuard please click here.

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

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".