MOSAF Series of Active Filters
The MOSAF series of active filters is the number one choice for challenging offshore harmonic and PQ compensation tasks where performance, reliability and safety are critical, 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)
- Isolates susceptible and sensitive downstream equipment from MV and LV background voltage
- 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
- MOSAF can provide harmonic compensation for both MV and LV applications
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.
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:
In offshore industries worldwide, many types of variable speed drives are utilised. From VFDs or DC SCR drives on drilling packages to MV load commutated inverters on large compressors. PWM VFDs are commonly used for main propulsion and thruster applications on drill-ships, semisubmersible drilling rigs and support vessels.
In offshore and onshore oilfields, VFDs are almost universally used for the control of ESPs (electrical submersible pumps), sucker-rod and progressive cavity pumps.
The benefits of variable speed drives are widely acknowledged. However, all have types of variable speed drives require some form of harmonic mitigation.
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
Typical applications for MOSAF – Drilling rigs and offshore support vessels
Typical applications for MOSAF – Offshore production platforms and FPSOs
Offshore platforms have a large installed base of AC variable speed drives (i.e. synchronous drives, load commutated drives and PWM VFDs). Applications include gas compressors (up to 50MW), general purpose pumps and ESPs, fans, crane motors and drilling packages.
Larger drives may be of 12, 18 or 24 pulse design and connected direct to the MV supplies (e.g. 11kV) whilst smaller drives are usually connected to LV systems 440-690V. It is not unusual on the platforms that variable speed drives comprise up to 85% of the total platform load. This can result in excessive THDu (harmonic voltage distortion) even though the larger drives may be up to 24 pulse which can present operational and safety concerns at MV and downstream on LV side.
FPSOs also have a high percentage of MV and LV variable speed drive loads; to 150MW on some vessels. The applications are very similar to that on offshore platforms with addition of thrusters driven by PWM VFDs. The variable speed drive load of up to 85% of total load, can result in excessive THDu on both MV and LV supplies with operational and safety implications thereof.
The importance of attenuation of background THDu
As mentioned above, offshore production platforms and FPSOs have many large variable speed drives on ESPs, gas compressors, fans, pumps and other applications. This can result in harmonic voltage (THDu) levels which far exceed the design of the installations and the harmonic recommendations which govern their use. This excessive THDu can often reach levels which adversely affects both operations and safety both on MV and via LV transformers.
Excessive THDu can interfere with monitoring and protection systems and all other equipment including electric motors, especially fixed speed explosion motors where under certain circumstances, these motors may be potentially dangerous.
Below is the THDu spectrum captured on a platform’s 690V drilling package. The average THDu was 25.61% with 19.3% >21st order due to a combination of 3.2MW of 6 pulse VFD drilling package load and over 8.4MW of 24 pulse ESP VFDs connected upstream on MV. The design THDu for the installation was <5%.
A leading European test house was contacted regarding the above voltage spectrum. Tests were run on a single cage induction motor at the voltages and frequencies illustrated above. The stator temperature increased by 11.73% compared to sinusoidal operation, whereas the rotor temperature increased by 19.03%. For double cage or deep-bar rotors it was not possible to conduct tests. However, the rotor temperature was estimated to be in the order of 35-45% compared to sinusoidal operation, including the rise due to skin effect. In the context of explosion motors, of all protective concepts, this level of rotor heating does give serious cause for concern regarding the motor temperature class, the integrity of bearings and the flameproof gaps in “flameproof” motors.
Indeed, generators, transformers and electrical motors at MV and most items of electrical equipment on LV supplies would be adversely affected by these high levels of THDu. It is of paramount importance therefore that action is taken to significantly reduce the THDu, especially on LV supplies where the majority of susceptible equipment is located.
MOSAF has three modes of control :
- Open loop (load sensing with the CTs on the side)
- Closed loop (source sensing with the CTs upstream of the filter connection)
- Voltage control (where CTs are not required since special control is activated)
Active filters, capable of voltage control, directly target the voltage instead of injecting a compensation current based in the CT signals. Hence voltage control enables “sensorless control” without CTs, often with better results.
For background voltage distortion mitigation, (i.e. the localised reduction of harmonic levels fed by medium voltage (MV) networks or transformers with high levels of THDu), MOSAF voltage control is a mandatory requirement. In addition, the voltage limiter function inbuilt within MOSAF enables autonomous output adaption for automatic harmonic standard compliance. Note, with MOSAF, the voltage control modes are selectable per frequency for optimised performance.
An offshore application for MOSAF voltage control for attenuation of background THDu from MV supplies is illustrated below. The salient non-linear loads on the DP semi-submersible rig were eight quasi-24 pulse thruster VFDs and a common DC bus drilling package. During operations, the average THDu appearing on the LV supplies (440V), which also fed lighting and other transformers was captured at 13.9-14.1%. This excessive THDu (on an installation designed for <8%) resulted in continual damage and disruption to susceptible and sensitive LV equipment, most of which was only designed to withstand <5% THDu.
As can be seen in the SLD above, two 250A MOSAFs, programmed for voltage control, were connected downstream of two 11kV/440V transformers reduced the background THDu from 13.0-14-1% to 4.1-4.2%, thus resolving all the issues on the 440V and other LV supplies.
MOSAF, configured for voltage control, is an extremely effect method of protecting LV (or other MV) supplies from the damaging effects of excessive background voltage distortion. The MV background THDu still exists. These can be treated at MV using MOSAF and special interposing transformers.
Mitigation of Supraharmonics to 9kHz
Almost all active filters compensate for harmonics to the 50th order only. MOSAF, in conjunction with a specifically designed internal, high-pass, SHU module, can treat supraharmonics up to 9kHz (150th order at 60Hz fundamental or 180th order at 50Hz) as illustrated below.
This combination can compensate for AC, DC drive and active filter switching voltage harmonics, SCR line notching and LED lighting, for example up to 9kHz as illustrated below.
MOSAF general specification
Harmonic Solutions Oil & Gas 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 offshore PQ including supraharmonics to 9kHz and the treatment of background harmonic voltage distortion including MV and LV applications.
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 Oil & Gas 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 Oil & Gas PQ Services please click here.
Contact us with your application details and let us assist you.
For information on Harmonic Solutions Oil & Gas additional range of oilfield products and services please click on the appropriate link below :