MOSAF (Marine & Offshore Specific Active Filters) are manufactured in Europe, provide high performance, flexibility and reliability up to 500A compensation current per enclosure.

Our modular, integrated total harmonic and PQ solutions can compensate for supraharmonics and resonance up to 9kHz. In addition, MOSAF has an exclusive voltage control feature which isolates susceptible and sensitive downstream equipment from background voltage distortion due to main propulsion or other large variable speed drives.

MOSAF Series

The MOSAF series of active filters is the number one choice for challenging marine harmonic and PQ compensation tasks where performance, reliability and safety are critical, for example, even beyond normal operating voltage levels and under challenging environmental ambient conditions:

  • Operation up to 690 V and higher voltage levels at full power without derating

  • Rated current can be extended in a modular manner from 125A to 2500A

  • Harmonic compensation to 9kHz with optional module (up 51st as standard)

  • Advanced 3-level architecture, high power density and compact design

  • Low operating losses

  • Extremely durable, long life film DC bus capacitors

  • Overvoltage category III up to 1000 V

  • Degree of protection up to IP20, IP21 and IP54 possible with optional external water cooling for complete encapsulation

  • Dynamic compensation of reactive power, harmonics, and flicker, as well as load balancing in one unit

Internal view of 375A MOSAF, special colour
Simplified block diagram of active harmonic filter in operation
Intuitive touchscreen display of MOSAF

The central control and display element of MOSAF is via the door mounted touch panel and/or a LAN. It has a clear menu structure and can display data in both tables and diagrams. The benefits of the touch panel include:

  • Simple parameterization of the filter function without additional measuring devices. Though a more complex power quality analyser can be provided based on client requirement.
  • Explanations and status messages in plain text in a number of languages
  • Intuitive operation and password protection

MOSAF can also be operated via the internet. Once incorporated into a company network or connected via a mobile phone network, it can be monitored or parameterized remotely using a browser.

Advanced 3-Level technology


The 3-level design of MOSAF is based on twelve IGBTs. Conventional 2-level active filters only comprise of six IGBTs. The three-level configuration reduces the voltage stressing on the power semiconductors by 50%, resulting in lower losses and permits the MOSAF to be used in systems with a higher rated voltage. The DC link voltage is also higher which permits higher peak currents to be generated lowering losses, essential for filtering harmonics with a high bandwidth.

Schematic showing the advance 3-level architecture of MOSAF filters

Another benefit of the 3-level technology is the lower ripple of the output current compared with the 2-level architecture. The split DC link and the larger number of IGBTs results in an additional third inverter state at the output. This is to reduce the size of the carrier frequency and EMC filters. The difference between 2-level and advanced 3 level-architecture is shown below :

A comparison of 2-level IGBT based active filter with advanced 3-level IGBT based MOSAF

In the marine industry worldwide, many types of variable speed drives are utilised; indeed, their use is accelerating.  From DC SCR drives to AC synchronous drives, cycloconverters, load commutated drives and PWM variable frequency drives used for main propulsion to cranes, compressors, fans and pumps and more, the benefits of variable speed drives are widely acknowledged.  However, all have types of variable speed drives require some form of harmonic mitigation.

However, not only variable speed drives cause harmonic distortion of the voltage, although they are usually the main culprit.  On cruise ships and other vessels, for example, multiple single-phase loads can cause problems.  LED lighting emits supraharmonics up to 150kHz.

The application of MOSAF active filters reduce voltage distortion significantly over a wide range of harmonic frequencies. This enables compliance to the rules from marine classification societies such as Lloyds Register, ABS, DNVGL, BV and other classification societies and thus ensures the operational integrity, reliability and safety of the vessel.

Harmonic mitigation performance

Example of MOSAF mitigation of a 6 pulse PWM VFD using one phase waveforms
MOSAF mitigates harmonics up to 51st order (and to 9kHz with optional module)

Typical applications for MOSAF

DP diving support vessel with electric propulsion and thrusters
Ideal application for MOSAF. Azimuth main propulsion and thruster mitigation

The majority of cruise ships have diesel electric propulsion. Historically, these vessels have had 24 pulse cyclo-converters or 24 pulse synchronous drives for main propulsion, but the trend is now on new builds, to utilise MV PWM (pulse width modulated) variable frequency drives (VFDs) with all the issues thereof.

Most cruise vessels have full electric propulsion

Most cruise lines have been retrofitting large numbers of PWM VFDs for pump and fan applications to save energy and generator fuel. This raises the localized THDu levels which are often compounded by background voltage distortion due to the main propulsion (see example below).  MOSAF is the ideal candidate for the mitigation of the switchboards supplying the large number of VFDs.

Example of cruise vessel with 13.59%THDu despite having <5.5% THDi due to 24 pulse synchronous main propulsion drive. The THDu limit was 8%. THDu increases with speed.
Vessel with 8.5MW of 24-pulse main propulsion VFDs and MOSAF to protect LV supplies

This vessel has two 12 pulse 4.25MW main propulsion drives. The primary of one 12-pulse phase shift transformer had an additional phase shift resulting in a quasi-24 pulse configuration with the bus-tie closed.  As illustrated above, the THDu at each primary exceeded the rules (<8% <THDu).  The excessive THDu in this instance was due to switching harmonics; the THDi (harmonic current distortion) was relatively low.

The excessive THDu propagated through the transformers to the hotel and other loads, resulting in significant problems in the galley (440V), the ships office and the bridge, both of the latter were on  230V supplies. Two MOSAF ‘voltage control’ filters were employed to effectively “isolate” the high MV THDu from the low voltage (440V and 230V) supplies.  The MOSAFs successfully reduced the THDu from >13.4/13.2% to 2.1/2.0% respectively.

Mitigation of Supraharmonics to 9kHz


Most active filters compensate for harmonics to the 50th order.  MOSAF, in conjunction with a specifically designed internal, high-pass, SHU module, can treat supraharmonics up to 9kHz (150th order at 60Hz fundamental) as illustrated below.

This combination can compensate for drive switching voltage harmonics, line notching and LED lighting, for example.

Standard MOSAF (to 50th order). Add SHU (to 9kHz – 150th order)
375A MOSAF with optional high-pass SHU module compensates harmonics up to 9kHz

Power quality covers many varied issues

Acknowledged power quality problems

The solution to all the above issues is:

The solution. One tool. MOSAF !

MOSAF general specification

Enclosure dimensions

Harmonic Solutions Marine has the knowledge and experience

MOSAF is available in compensation ratings from 125A to  2500A per connection in enclosures to IP20, IP21 and IP54.  Water cooling is also an option. MOSAF applications covers all aspects of marine PQ including supraharmonics to 9kHz and the treatment of background harmonic voltage distortion.

When applied correctly, active filters 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 and passive filters since 2003 to a large number of demanding marine and offshore applications.

A complete range of AC line reactors for use with VFDs and DC SCR drives are available to assist full compliance and optimum performance. For more information please click here.

If a visit to the vessel is required to carry out detailed measurements or PQ troubleshooting, we have fully trained PQ engineers available. To view our section on Marine PQ Services please click here.

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