Page 56: of Offshore Engineer Magazine (Sep/Oct 2014)

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heating effect on explosion-proof motors with double cage or deep bar rotors, such as those used in compressor motors.

The importance of acceptable power

Drilling quality should now be appreciated.

Offshore safety authorities, marine classi- fcation bodies and standards authorities worldwide need to adopt a more rigid, policed approach to offshore harmon- ics and power quality. The current rules and recommendations are not ft for purpose to address our 21st Century requirements.

Unacceptable power quality can have signifcant impacts on safety, the pro- ductivity of operations and proftability.

The cost of mitigation required to ensure prevention is only a tiny fraction of the possible fnancial losses.

The quality of electrical power is crucial to the industry, including future subsea installations. It should not be ignored or abused. Offshore electronic

A WEG EExd motor. Photo from Harmonic Solutions.

equipment is ever-more sophisticated, requirements regarding 2-3% harmonic presence of harmonics, most notably demanding a higher level of power qual- voltage factor (HVF), due to the effects on motors with deep bar or double ity for its reliable operation than in the of harmonics on winding temperature. cage rotors, the rotor temperature rise past. The future for subsea systems, we

IEC 60079-1 (hazardous area equip- can be excessive and well outside the are told, is “all electric,” yet many in the ment) however does not currently have motor temperature class. High rotor industry seem to lack understanding as to any requirement for HVF regarding temperatures can affect the bearings the impact and importance of acceptable compliance testing or certifcation for as the lubrication degrades, exposing electrical power quality on electrical and any explosion-proof motor protection them to excessive wear. They can also control equipment offshore. concepts. It is a similar situation for degrade the fame paths so that if there An acceptable level of power quality

NEMA/UL explosion-proof motors under was an internal explosion (which is more is now absolutely fundamental for safety standard MG-1. likely due to high rotor temperatures and operational integrity of offshore

If these motors are subject to voltage anyway) it might not be contained as industries worldwide, and will be supplies with >0% THDv they are “oper- expected – with potentially disastrous increasingly important in future. This ating outwith the conditions envisaged consequences. In order to overcome this needs to be recognized, appreciated, and when they were certifed.” This does not “defciency,” the standards authorities reinforced by governments, EU com- necessarily mean the motors are unsafe now place the sole responsibility on the mittees, standards authorities, offshore (although under certain conditions they duty holder operators to maintain their safety bodies, and duty holders alike, and could be), but it does mean that the harmonic voltage distortion at a safe urgently addressed. operator has lost any third party (e.g. and acceptable level (i.e. 5-8% THDv

NEMA/UL/ PTB, CSA, BASEEFA et al) as stipulated on the now HSE-shelved Ian C. Evans majored verifcation as to their safety. Harmonics Information Sheet), so that initially in electrical

There is a second serious and practi- explosion-proof motor safety is not drives and marine/ cal consideration regarding ‘fameproof compromised. offshore power motors’ (i.e. EExd in IEC codifcation), The author is unsure how the same systems. In the late which rely on the principle that no mat- authorities would view the reality of 1980s he campaigned ter what happens inside the fameproof these motors being regularly subjected to successfully for the enclosure (e.g. an internal explosion) it the 20%-35% harmonic voltage distor- introduction of safe cannot transmit to the surrounding haz- tion often seen offshore. and certifed explosion-proof motor/VFD ardous area. While that statement may PTB (the German explosion-proof packages. Since 1995 he has specialized be perfectly valid for sinusoidal voltage equipment test authority) were given in harmonics and power quality, for supplies it is not valid for harmonically details of the harmonic spectrum above marine vessels, drilling rigs and other distorted voltage supplies. (Fig. 3). They calculated an additional offshore installations, both as a consul- A fameproof motor relies solely on 25°C temperature rise on explosion- tant and via Harmonic Solutions (Oil & motor enclosure and fame paths in the proof motors with single cage rotors, Gas) and Sentinel Power Quality FZE. end housings to contain any internal subject to voltage supplies, with this Evans wrote the harmonics guidance explosion in the event of gas or vapor harmonic spectrum. PTB were unable to notes for the US marine classifcation entering the machine. However, in the calculate the more signifcant additional body, published in 2006.

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