Page 55: of Offshore Engineer Magazine (Sep/Oct 2014)

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Drilling

Offshore platform harmonic voltage spectrum 18

L1 Vh 16

L2 Vh 14

L3 Vh 12 10 8 6 4 2

Harmonic voltages as % of fundamental 0 357911 13 15 17 19 21 23 25 27 29313335 37 39414345 47 49

Harmonic voltage order

Illustrates an installation where the THDv measured on a 690V drilling package was 27.1% (18.9% above the 21st order, due to the use of a number of 24-pulse variable frequency drives). The recommended THDv limit was 5%.

Source: Harmionic Solutions.

conclusion that the quality issued by the HSE. Following the guid- approach that gives confdence that the of electrical power therefore ance is not compulsory and you are free very real problems will be addressed.

requires serious consider- to take other action. But if you do follow There are also serious, related concerns ation by safety and regula- the guidance you will normally be doing regarding the use of explosion-proof tory authorities worldwide, enough to comply with the law.” The motors of all protection concepts. All including in the EU and draft HSE Harmonics Information Sheet induction motors require thermal derating the US. However, the was circulated to all Duty Holders in the in the presence of harmonic distortion of new European Directive, North Sea and beyond. their voltage supplies, but there is a “com- (2013/30/EU), govern- However, earlier this year HSE stated: plication” regarding fxed speed explo- ing the safety of offshore “We no longer have plans to publish sion-proof motors based on IEC standards; oil and gas installations, detailed information on the subject of these motors are only certifed for use on ignores it. electrical power system harmonics. pure sinusoidal supplies (i.e. 0% THDv).

The author has been HSE’s Energy Division published its new The IEC standard relating to electri- in discussion with UK’s ‘Offshore Oil and Gas Sector Strategy cal machines, IEC60034-1, specifes the

Health and Safety Executive 2014 to 2017’ in March.” No reason was

A spokesman for the HSE said: “HSE (HSE - HID Energy Division, forthcoming from HSE as to why this electrical inspectors for ofshore oil

Offshore) since 2006, important safety document, discussed for and gas installations are aware of regarding offshore power three years, was shelved.

quality. Two years ago HSE The new document does not address the problems that may arise from drafted, with assistance power quality, and they explain this as poor electrical power supply quality. from the author and others, follows: “This sets out the key major haz-

However, our experience is that electri-

Left: An Ex motor. Above: Typical jackup type drilling rig. Photos from Harmionic Solutions.

a document entitled “HSE ard risks in the offshore oil and gas sector cal power supply quality has not been information sheet - Harmonic Voltage and the objectives we are setting both for identifed as a signifcant factor in any

Occidental platform, Piper Alpha. 167 Distortion in Electrical Systems. Offshore the industry and ourselves as the safety incidents investigated by HSE to date. men perished. The Cullen Enquiry failed Information Sheet No. xx/2011.” The regulator. We are directing our limited

Duty holders are required to ensure to establish the initial source of ignition draft document stated: “The objective is resources primarily towards these objec- their plant and installations complies which ignited the escaping condensate. to limit the THDv (total harmonic volt- tives. That is not to say that we deny with relevant standards to ensure

Some engineers now believe that har- age distortion), most commonly to 5% the potential that poor electrical power safety, so far as is reasonably practi- monic voltage distortion could have been or 8%. To achieve this, the magnitude quality (in all its forms) has for being a cable. HSE inspectors can and do, in a signifcant contributory factor, but this of the harmonic currents drawn by the factor in major incidents offshore and we inspections and assessments, require was never considered by the commission. non-linear load(s), which otherwise fow will continue to probe this in our assess- evidence that electrical power quality

At that time there was only rudimentary in the system, has to be reduced signif- ments of safety cases, inspections and meets the relevant standards. Also, harmonic measurement equipment avail- cantly.” (This compares to THDv levels incident investigations.” This means that when potentially relevant, electrical able and problems of harmonics were up to 35% witnessed by the author on HSE permit the Duty Holders to continue power quality is considered in incident largely unknown to most of the industry. many installations). The draft document to police themselves with regards to har- investigations.”

The reader may start to come to the concluded saying: “This guidance is monics and power quality. This is not an oedigital.com September 2014 | OE 57 058_OE0914_D&C3_Harmonics.indd 57 8/21/14 4:27 PM