Banana Skins – November 2021 (#354-357)

354.  Marine mains supply harmonic distortion problems solved

The Ocean Challenger is a very high bollard pull cableship of UT746C dual role design, equipped with a Rockplough that allows for simultaneous cable lay and burial to 1.0m depth in fractured rock, 2.2m in sand/clays and 3.0m in soft soils. The Ocean Challenger’s trenching operation is performed by a 2MW Remotely Operated Pipe-line Trenching Vehicle, referred to as the ROV PT1, which is capable of operating in depths of up to 2000m. The PT1 is fitted with ten 30kW electric thrusters for manoeuvring and four 300kW Jet Sword high volume flow rate electric pumps.

The electric thrusters and pumps are independently speed controlled via AC PWM VFD’s (Variable Speed Drives) mounted in the surface module. These 400V AC drives are equipped with sinus output filters and 400V to 3300V step-up transformers. From the surface module the 3300V is fed down an umbilical cable to the 3300V thrusters and pump motors. The step up in voltage is required due to the voltage drops associated with very long cable runs extending as much as 2000m.

All individual PT1 drives on the ship were fitted with 3% AC line reactors to partially attenuate the harmonic currents they generate. When connected to the ship’s normal power supply, the 1.5MW of AC drives produced too high a harmonic voltage distortion on the two 2800kVA shaft generators. This was partially due to the fact that generator power is more susceptible to voltage distortion than shore-based transformer power, because generators typically have much higher source impedance. With transformers, the impedance (Z) is usually in the order of 5% to 6% whereas for generators the subtransient reactance (Xd”) is typically 12% to 20%. The higher the percentage source impedance, the higher the voltage distortion (and the worse its effects) for a given harmonic load. 

Historically, to operate the ROV PT1 and its 1.5MW of drives, two deck mounted external generators have had to be rented in order not to breach the Det Norske Veritas (DnV) harmonic voltage maximum limitation of 5% and to prevent possible damage to the generators. This was an expensive proposition in respect to both financial outlay and required deck space. 

CTC Marine Projects asked cable handling specialists, Parkburn Precision Handling, to provide a tailored solution, and Parkburn proposed the use of Lineator™ wide spectrum filters. These high performance harmonic filters are manufactured by Canadian company Mirus International Inc. who are represented in Europe by Harmonic Solutions Co. in the UK. The Lineator™  is a patented, multi-limbed reactor with a relatively small capacitor bank whose output, when connected to AC or DC drives, produces a trapezoidal voltage which forces the input rectifier devices to conduct for a longer time period and with smaller peak currents. This has the effect of reducing the ‘total harmonic current distortion’ (Ithd) to near 5% regardless of whether the VFD is equipped with a reactor or not. 

CTC Marine Projects installed 2 x 750kW Lineators™, one for each of two groups of 5 x 30kW thrusters and 2 x 300kW pump drives in a self contained deck module. During the following sea trials, ships staff monitored both the operation of the two shaft generators and the VTHD on the main switchboards. The ship’s electrical engineer reported that the generators operated flawlessly and at no time did the VTHD ever rise above 1.4% and 1.6% on their respective switchboards. The installation of the two 750kW Lineators™ allowed the vessel to meet the 5% voltage distortion limit of the DnV without the need for the rented generators and the additional deck space they required. 

(Extracted from “Homing in on Harmonics”, an article in Offshore Engineer magazine, February 2006 Issue, pages 55-57, sent in by John Symonds of REO (UK) Ltd, on 27 Jan 06.)

355.  Piezo gas lighter controls tape player

In the kitchen we have a radio/tape/cd and recently the tapes have been playing with very poor sound quality. No amount of head cleaning has improved the sound. By chance we found that operating the piezo gun to light the gas hob fixes the problem. Must be switching some ‘hiss’ correction circuit for which there is no external control, button switch etc.

I think that the transient switches on something that the play button ought to switch on but doesn’t. Or rather something it used to switch on but doesn’t. However when you buy a radio/CD/tape including a remote all for £42 I guess you get what you pay for – it worked OK until the guarantee was over!

Sometimes when the play button is pressed the sound is OK, but frequently it isn’t. When the sound is poor the piezo lighter always seems to fix it. If it is repeatable (and it seems to be so) then it is a good demonstration that external EM threats can change the performance of an electronic circuit – in this case it is beneficial, but it might have been the other way round.

(Sent in by Dave Imeson of Compliance Europe Ltd, on 31 Jan 06.)

356.  Radar dome suspected of interfering with car immobilisers and lights

Reports that a radar dome in Norfolk is causing electrical problems with cars are being investigated by the Ministry of Defence (MoD). Motorists say their engines and lights have cut out, and their speedometer dials swing up to 150mph as they drive past the Trimingham radar unit. 

Neil Crayford, who runs a garage near the dome, said in the past two months, 30 car owners had reported problems. On Monday, an MoD spokeswoman said the claims were being investigated. Mr Crayford said one night his own car’s headlights and dashboard cut out for a few seconds as he drove past the dome in convoy with a colleague – who suffered the same fate. 

The former RAF radar operator said: “Something must have changed – either the frequency or output – for this to happen. “I lodged an official complaint with the MoD two weeks ago, but incidents are still happening. We get about five a week, and had three more on Friday.” 

An MoD spokeswoman said: “We are aware of claims that the remote radar head may be interfering with car immobilisers and we are investigating. “There are other users outside the military that operate on the same frequency as the radar, but there is a possibility we could be causing some problems with cars.”

(BBC News / England / Norfolk / “Fears radar dome affecting cars.”. Posted to the IEEE’s emc-pstc newsgroup on 24 February 2006, by Iain Summers.)

357.  Cellphones can interfere more strongly with aircraft navigation than previously believed

A study by Carnegie Mellon University researchers in the Department of Engineering and Public Policy (EPP) has found that cell phones and other portable electronic devices, like laptops and game-playing devices, can pose dangers to the normal operation of critical electronics on airplanes. The study will be featured in an article appearing in the March issue of IEEE Spectrum. 

“We found that the risk posed by these portable devices is higher than previously believed,” said Bill Strauss, who recently completed his Ph.D. in EPP at Carnegie Mellon. “These devices can disrupt normal operation of key cockpit instruments, especially Global Positioning System (GPS) receivers, which are increasingly vital for safe landings.” Strauss is an expert in aircraft electromagnetic compatibility at the Naval Air Warfare Center in Patuxent River, Md. 

With support from the Federal Aviation Administration, three major airlines and the Transportation Security Agency, EPP researchers crisscrossed the northeast United States on commercial flights, monitoring radio emissions from passenger use of cell phones and other electronic devices. They tracked these radio emissions via a broadband antenna attached to a compact portable spectrum analyzer that fit into an innocuous carry-on bag. 

“A laptop computer controlled the system and logged the data,” said Granger Morgan, head of the EPP Department. “While we looked primarily at wireless phones, we also discovered that emissions from other portable electronic devices were problematic.” 

The researchers found that on average one to four cell phone calls are typically made from every commercial flight in the northeast United States. Some of these calls are made during critical flight stages such as climb-out, or on final approach. This could cause accidents, the investigators report. 

Both Strauss and Morgan, along with Carnegie Mellon researchers Jay Apt and Dan Stancil, recommend that the Federal Communications Commission (FCC) and the FAA begin to coordinate electronic emission standards. At the moment, there is no formal coordination between the two federal agencies. The researchers also recommend routine monitoring of on-board radio emissions by flight data recorders and deploying specially designed tools for flight crews to monitor passenger use of electronic devices during final approach. 

While the FCC recently suggested that it might be appropriate to allow passengers to use cell phones and other electronic devices on airplanes, Morgan disagrees. 

“We feel that passenger use of portable electronic devices on aircraft should continue to be limited for the safety of all concerned,” Morgan said. 

(Carnegie Mellon University Press Release, Feb 28, 2006.) 

About The Author

Keith Armstrong

After working as an electronic designer, then project manager and design department manager, Keith started Cherry Clough Consultants in 1990 to help companies reduce financial risks and project timescales through the use of proven good EMC engineering practices. Over the last 20 years, Keith has presented many papers, demonstrations, and training courses on good EMC engineering techniques and on EMC for Functional Safety, worldwide, and also written very many articles on these topics. He chairs the IET’s Working Group on EMC for Functional Safety, and is the UK Government’s appointed expert to the IEC committees working on 61000-1-2 (EMC & Functional Safety), 60601-1-2 (EMC for Medical Devices), and 61000-6-7 (Generic standard on EMC & Functional Safety).

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