Power Engineering International

Ready, willing and cable

The latest developments in high voltage cable technology could deliver power more efficiently to help the UK government meet its carbon reduction targets, says Doug Gracias

IIt looks increasingly likely that the electrification of heat for buildings, transport and industrial processes will be a key element in enabling the UK government to meet its legally binding carbon reduction targets.

In fact, the government’s Carbon Reduction Action Plan suggests that this increased electrification is set to reverse the current trend of falling electrical demand with expected average increases in demand of at least
30 per cent and possibly as much as 60 per cent while peak UK electricity demand is set to double.

Increasing the generation capacity to help meet the increased demand will be extremely challenging, particularly in light of the UK’s planned phase-out of coal-fired generation and its retirement of old nuclear power stations.

At the same time, as new forms of generation are added to the network, there will also be a focus on reducing the 9 per cent of power currently lost in electrical transmission and distribution.

The most efficient way to transmit large amounts of electricity is at a very high voltage. The reason for this is that the higher the voltage, the smaller the current and the less energy lost as heat in transmission. High voltage AC is the most common method of long distance power transmission.

However, over long distances and for undersea applications, high voltage DC will transmit power more efficiently than AC. This is because DC cables suffer lower electrical losses and the cables are generally less expensive (although it should be emphasized that AC to DC terminal converter-stations are much more expensive than the step-up and step-down transformers needed for an AC link).

Increased transmission efficiency was a key reason HVDC was selected for the newly completed Western Link project. This scheme features a 385 km cable running beneath the Irish Sea.

The link is currently being commissioned and, when fully operative, it will deliver power generated in western Scotland to the transmission grid in England. Impressively, it is the first such scheme in the world to transmit DC power at 600 kV.

Until this pioneering installation, the maximum interconnector voltage was typically 500 kV – the maximum voltage limited by cable technology. However, the Western Link interconnector uses a state-of-the-art Prysmian cable featuring Paper Polypropylene Laminate insulation impregnated with insulating fluid. The cable’s innovative insulation improves the cable’s performance enabling it to carry
2200 MW of power at 600 kV.

A big advantage of increasing the interconnector’s voltage by 20 per cent is that the cable’s transmission capacity is also increased by 20 per cent and its transmission losses are reduced by 33 per cent. Using this Prysmian HVDC cable has enabled power to be transmitted using a smaller diameter conductor than would have been the case at 500 kV, with a subsequent reduction in material costs.

Prysmian is currently working to develop an 800 kV HVDC cable to further reduce transmission losses from interconnectors.

High voltage AC has historically been used for power transmission. It is much easier, and therefore cheaper, to step up and down the voltage in an AC system as opposed to DC, which is why AC is used for mains power transmission at a national level.

AC power is transmitted at many hundreds of thousands of volts but it is easily stepped down to 230 V using a transformer, or series of transformers, when it is at or near a building.

As with DC, increasing the voltage of the UK’s main AC onshore transmission system will reduce its transmission losses. With this in mind, Prysmian is working with National Grid to develop higher capacity AC cables to complement its HVDC solution.

Alongside its innovative developments in interconnector and transmission-cable technology, Prysmian has also developed a higher voltage inter-array cable. This will link individual wind turbines in an array to the array’s collection point, or points.

Conventionally these links have run at
33 kV but, working as part of a Carbon Trust development programme, Prysmian has successfully managed to double the voltage capability of the inter-array cable to 66 kV. Its new three-core, 66 kV inter-array cable uses state of the art ethylene propylene rubber (EPR) insulation – a material selected for its excellent insulating performance when in direct contact with water. The innovative cable is expected to reduce overall wind farm capital expenditure significantly.

In addition to increasing the transmission capacity of cables, Prysmian is also working to prevent power losses through cables becoming overloaded. Overloading can occur as a result of wind turbines and other renewable technologies, such as solar photovoltaics, generating electricity intermittently. Fluctuations in supply can be a headache for transmission system operators (TSOs).

Traditionally, the TSO’s role has been to manage a largely steady supply network and to align it with a variable demand; now the TSO has to balance an intermittent and variable supply and match this to a variable demand to maintain an orderly supply.

Demand control technology, such as smart washing machines that hold off running a wash until electrical demand is low, should help smooth demand variations to some extent. However, with both intermittent and difficult-to-predict generation combined with variations in demand, the potential to overload cable and increase cable losses is considerable.

The way to operate in this very dynamic environment is with better diagnostic information and smarter controls. Prysmian can supply cables with integrated sensors that monitor the performance of the cable system, enabling operators to maximize the efficiency of the transmission and alerting them to any areas of underperformance.

Power cables can incorporate fibre-optic cable which can measure the temperature of the conductor, giving a very accurate picture of the cable’s loading at any given time. With this understanding comes the ability to control the distribution and optimize the efficiency of the distribution network, further reducing losses.

It would be inconsistent to devote so much resource to optimizing the electrical distribution and decarbonizing the grid by generating energy using renewable technologies without also distributing it efficiently and with minimum waste.

As cable manufacturers, Prysmian can make a significant contribution to support the efficient and effective distribution of electricity from diverse generation sources through to dispersed distribution networks.

Doug Gracias is Engineering Director of cable manufacturer Prysmian UK, the largest cable manufacturer in Britain

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