Optimisation pilots pave way for enhanced grid

European transmission systems operators are developing new methods for optimising the capacity of existing grid connections. Although these technologies have their limitations, they will play an important role in bringing more wind on to the network, reports Sara Knight

TenneT's system to monitor cable temperature
TenneT's system to monitor cable temperature

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With wind set to provide 15% of Europe’s power by 2020, few would dispute that substantial investment is needed to develop a grid capable of delivering this power to consumers.

But before spending the billions of euros likely to be needed, Europe’s transmission system operators (TSOs) are looking at making better use of the existing network.

One organisation that is breaking new ground is TenneT, the Dutch state-owned TSO, which in September launched a transmission cable temperature and wind-monitoring project in Germany.

With grid bottlenecks in northern Germany hampering transmission of wind power to regions with high power consumption, the system will allow the capacity of high-voltage overhead lines to be increased considerably in the right weather conditions.

It has been tested by TenneT’s German subsidiary Transpower on lower-voltage cables in northern Germany and will now be applied to around 900 kilometres of 380kV lines.

Although the need for new transmission lines to bring onshore and, increasingly, offshore wind-generated power from the rural north into Germany is widely recognised, TenneT says it is planning new cables "only where these are urgently needed".

Its first priority is to "optimise the existing network", it said when announcing the scheme.

The risk of stranded investment is one reason for this caution. Major investment in infrastructure can be a huge waste of money if the new facilities end up underused.

If an expensive network is used only part time and hardly ever to full capacity, it will be over-dimensioned – that is, more extensive and costly than necessary.  

Safety margin

TenneT’s project is based on a flexible approach to network operation.

Cables heat up when electricity flows through them, causing them to expand and lengthen, resulting in sag between pylons.

If the sag is considerable there is a danger of the cables coming into contact with trees or buildings.

To avoid this risk, capacity of overhead cables has, in the past, been calculated with a wide safety margin by assuming an ambient temperature of 35 degrees Celsius and a wind speed of 0.6 metres/second.

But in northern Germany at least, temperatures seldom reach 35 degrees Celsius and wind speeds usually exceed

0.6m/s, making the calculations for transmission capacity unnecessarily conservative. Taking into account the ambient temperature and the fact that higher winds tend to cool cables,

TenneT says that when the wind is blowing strongly, the transport capacity of cables in northern Germany can be raised by up to 50%.

It is now applying the new system to overhead cables that are heavily loaded due to wind energy usage, and to long-distance transits of electricity across Europe.

Small measuring stations are installed on the pylons to collect and transmit data on wind speed, ambient temperature, cable tension and sunshine to the system operator’s control centres.

Through real-time monitoring of cable conditions, the TSO has a more exact knowledge of the capacity of the wires and how much electricity they can safely carry at a given time.

Making use of the additional transmission capacity is only possible with new investment, in the monitoring system and also in other hardware.

If more electricity is fed through certain cable sections, the electricity circuits and transformer stations into which they feed have to be strengthened and adjusted accordingly.

TenneT says it has invested some €55 million on 22 transformer stations with 31 electricity circuits and 62 switching panels to withstand the higher loads resulting from the new monitoring.

Despite their benefits, temperature and wind monitoring cannot completely replace the need for network expansion, not least because the increased transmission capacity is not permanent but rather depends on the right weather conditions.

TenneT therefore has 500 kilometres of new cable under planning in Germany to meet the needs of new wind energy generation.

Another German transmission system operator, 50Hertz Transmission, concurs that cable monitoring is not a long-term answer because if the cables do not exist, they cannot be optimised.

Technical managing director Wolfgang Neldner says: "For eastern Germany, there is no alternative to building new east-west transmission cables to avoid curtailments or switching off of wind and other renewable generating capacity. Any other approach is dubious or delusory."  

Phase-shifting transformers

Cable monitoring is not the only possibility for optimising network transmission.

Another potential solution involves the installation of phase-shifting transformers to control the flow of electricity between parallel electricity circuits and maximise the available capacity.

But here, too, there are limits, according to the conclusions of the European Wind Integration Study (EWIS), published this spring by umbrella body the European Network of Transmission System Operators for Electricity.

Phase-shifting transformers are already used across the Benelux countries, optimising flows in those countries as well as France and western Germany.

But as a result, the electricity physically seeks a new route of least resistance, meaning some power flow shifts back towards the east, according to the study, "increasing the flow in the already highly loaded or even overloaded German grid and on the borders of Germany-Czech Republic and Germany-Poland", according to EWIS.

"Installation of phase-shifting transformers in these eastern European regions could secure these borders but could then cause increased network overloading in Germany and on the German-Austrian border, says the study."

Using phase-shifters without strengthening the existing grid "is not a complete and not a sustainable solution and shifts the bottlenecks to other regions," the study concludes.

Buying more time

In spite of the limitations, the European Commission is keen to advance grid optimisation. Its Twenties initiative, launched in May to help integrate wind power into the European grid, has at least two projects concerned with improving the performance of the existing network.

Better use of existing transmission cables can only alleviate, not solve, grid bottlenecks in the long term. But lessons learned from using the various optimisation options can be increasingly applied around Europe to win a little more time for the needed new grid construction.

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