The planned work, which must be carried out to comply with the requirements of the National Grid, involves the installation of two harmonic filters at the substation, which are needed to “clean” the waveform produced by the wind farm.
 
The electrical system works on the basis of a sinusoidal 50 Hz cycles waveform (so-called “alternating current” or AC). Some elements of the grid disturb the waveform, for example by rippling it with waveforms of a higher frequency, and these are referred to as harmonics.
 
These harmonics at best compromise the efficiency, at worst the functionality, of the electrical appliances and machinery. A harmonic filter is designed to filter out the harmonics and re-establish the original 50 Hz waveform. These issues are normally referred to as “power quality” issues.
 
The work at the Salle substation will be carried out by the contractor Alstom, which will be responsible for the engineering, procurement, construction, installation and commissioning the plant.
 
Two harmonic filters will be fitted at Salle, one for each export cable. Each filter consists of a combination of resistors, capacitors and reactors, and the total height of each filter will not exceed 6.7 metres above ground level. The filters will each be housed in a compound not exceeding 25 metres x 25 metres on the southeast side of the substation, and landscaping will be undertaken to reinstate native vegetation.
 
Site access began this month with construction works scheduled to begin in the last week of April. It is planned the site will be demobilised by the end of November.
 
Bjørn Mo Ostgren, general manager of the Sheringham Shoal Offshore Wind Farm, said: “We will make every effort to minimise the effects of the installation work on local residents.” He pointed out that the project has included extremely stringent specifications for the new equipment and it is anticipated that the existing background noise will not be increased.
 
However, a group of Cawston residents, who have been campaigning against the persistent “hum” from the substation, said the installation of the harmonic filters is designed to improve power quality, not reduce noise levels, which will still exist. “This new work will not in any way reduce the current level of noise that we are currently experiencing,” the group stated.
 
“Since the December edition of Reepham Life and our last information post to concerned parishioners, we have received a further update from Statkraft on the study it will be carrying out to find the best way of mitigating the noise problems from the substation.
 
“On a positive note, the company has taken on board our comments regarding the specification for the work needed to be done to investigate the best solutions to the problem, which will be evaluated and included in the specification."
 
Statkraft is currently evaluating contractors for the third-party noise mitigation study.
 
Sheringham Shoal Offshore Wind Farm
 
The 317 MW Sheringham Shoal Offshore Wind Farm, located between 17 and 23 kilometres off the coast of North Norfolk, comprises 88 wind turbines and generates around 1.1 TWh of green energy per annum. This is enough clean energy to power almost 220,000 British homes and, compared with fossil fuels, represents a reduction of around 500,000 tonnes of carbon dioxide emissions every year.
 
The electricity generated by the offshore wind farm is brought to shore at Weybourne Hope on the North Norfolk coast by two subsea export cables where they connect with two underground transmission cables. The cables carry the electricity to the Sheringham Shoal’s electricity substation at Salle, near Cawston, from where it is distributed into the UK Power Networks distribution network.
 
The Sheringham Shoal Wind Farm is owned by two Norwegian companies – Statoil and Statkraft – and the UK’s Green Investment Bank. The wind farm is operated by Statkraft; Scira Offshore Energy is the legal entity.
 
As it has to comply with Office of Gas and Electricity Markets (Ofgem) regulations for electricity generated in the UK’s territorial waters, Sheringham Shoal’s transmission assets, which include the Salle substation, and on completion the harmonic filters, are independently owned by an investment company, Blue Transmission Sheringham Shoal.
 
See our earlier news story:

• Public exhibition on Salle electricity substation