Local partner: Fortop
After receiving an order to build two heavy-duty Cutter Suction Dredgers, the leading designer, builder and supplier of integrated vessels, equipment and services in the global dredging industry contacted our Dutch partner, Fortop regarding harmonic mitigation for the project. Fortop is a well-known and highly capable PQ partner with an in-depth knowledge and experience in the Marine and Offshore sectors.
The 29,190kW heavy-duty dredgers, two of the most competitive vessels in the world, have no propulsion or thrusters, but are instead moved via winches and spuds during operation. Each is supplied by four 4,5MVA 3,3kV generators. Several electrical drive systems are installed with various types of rectification. Connected to the 3.3kV systems are 2 x Cutter and 1 x Ladder motor (2.4MW), all via 12-pulse VFD and a Double Stock (phase-shifting) transformer, while the winches (AFE), hydraulic systems (S/D) and gland pumps (VFDs) are connected at the 690V system.
After Fortop and Comsys visited the customer together to give a presentation on the specific benefits of Active Dynamic Filter (ADF) technology combined with Fortop’s engineering capabilities, the customer was convinced that ADF was the most flexible solution for handling the harmonics created by the gland pumps.
Simulations of the vessel’s electrical grid were conducted and showed that in addition to the harmonics distortion during operation of the gland pumps, resonance/supraharmonics issues appeared when the Active Front End drives came online too. The high capacity of the AFE input circuit was causing low resonance points (7th … 11th order) when running on a single generator and the AFEs were online. To ensure safe and steady operation of the vessels, the resonance needed to be dampened and further excitation prevented.
Normally, the filter, ADF P200 would have been the obvious choice for resonance damping. But with a 690V supply, this was not possible. Furthermore, as the design phase for the vessel was already in a final stage, exchanging the AFE for a 6-pulse VFD, which has 9x lower capacitance, was no option either.
After performing a thorough investigation and detailed measurements of these complex power quality issues, Fortop was able to propose three add-on strategies to supplement the initial ADF P300-180/690V unit in charge of removing all harmonics currents created by the gland pumps:
By reducing the amount of AFE online, the resonant frequency around the 7th order harmonic was coming under control through interlocking
Install measurement equipment to constantly monitor the Vessel electrical infrastructure power quality and alert of possible problems
Install a second ADF P300-180/690V equipped with custom-built resonance damping software to dampen resonance points
"Besides these firm conclusions, it can be said that it was great teamwork that made this project the success that it is. For future projects the customer will further evaluate using 6-pulse VFDs in combination with ADF technology as standard in their vessels."
Through measurements and close cooperation with the customer, the issue was traced to a mismatch between the generator AVR and the AFE control loop. In some scenarios the AFE’s control loop(s) would “overpower” the generator’s AVR and run into an uncontrolled voltage level caused by the generators. Better management between the AFEs and Generator operation, i.e. controlling the number of AFEs online and reducing the power flowing into the grid from the AFEs, proved to be the solution.
Commissioning of the two ADFs was performed in two stages, the first whilst the vessel was moored at the quay. This low load test showed excellent results regarding both mitigation of the harmonics generated by the gland pumps as well as AFE resonance stability. The second stage was during sea trials and testing under all kinds of conditions to simulate a worst-case/full-load situation, whereby the following goals were achieved:
THDI (gland pumps) dropped from 85% to 16%
Full load THDU was below 3.5%
Throughout the tests, no resonance occured whilst the ADF was turned on: when switched off, high 13th order resonance did occur
With the precautions added to the AFE/generator controls, the voltage level remained steady and no fluctuations occured