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One day there will be a global electricity network that links all countries of the world. It will take decades to complete but the first steps have already been taken and the principle is deemed workable.

A key benefit will be a reduction in the number of power stations that are needed globally. During night time, for example, any given country’s generators can be kept running efficiently by supplying the extra needs of other countries during their working hours further round the globe. It will also help balance the different types of power generation indigenous to different countries. Nuclear power stations can take several days to switch on or off, so they are best at providing continuous base-load electricity.

But a hydropower generator can be started in minutes, making it ideal for meeting surges.

For example, Switzerland imports base-load electricity from French nuclear power plants, but exports power from its Alpine dams in short bursts to meet France’s peak needs. Any one country does not have to cater for all contingencies because it can use resources from elsewhere.

A global network will enable easier access to major sources of energy that are uneconomical to reach just now. The Himalayan kingdom of Nepal, for example, is a remote region with a large hydroelectric potential. The capacity of the national system is about 300MW, according to its Water and Energy Commission, but it could generate more than 40GW of hydroelectricity in its steep valleys.

Such a grid will increased security of supply, reduce the need for new power plants and cut back on the primary electricity reserve requirements within each country. This includes the use of spinning reserve where a station is semi-powered so it can take over very quickly if another station fails or if demand rises sharply.

Interconnected systems

The Union for the Co-ordination of Transmission of Electricity (UCTE) is the association of transmission system operators in continental Europe. It provides a reliable market base by creating efficient and secure electric power highways. It has 50 years of experience in the synchronous operation of interconnected power systems and its networks supply some 500million people in 22 countries from Portugal to Romania and from The Netherlands to Greece with about 2300TWh of electricity. Part of its network is the Baltic Ring, recently developed by the Baltic Ring Electricity Co-operation Committee (BALTREL) which has created a common electricity market in Latvia, Lithuania, and Estonia. It expects this will strengthen economic development in the region, increase reliability of supply and help the environment.

Currently, UCTE is investigating the feasibility of a synchronous interconnection between the Baltic States, Russia and many countries of eastern Europe as far as Mongolia. This would create an electricity system with an installed generation capacity of some 800GW, spanning 13 time zones and serving about 800million people.

Today, Europe is linked to North Africa by ac cable between Spain and Morocco, Algeria and Tunisia – known as the Maghreb, or western, countries. Further interconnection will bring in Tunisia and Libya, already forming a synchronous block with Egypt, Jordan and Syria – known as the Mashreq, or eastern, countries.
This is the basis of the Mediterranean Ring, which could eventually include Turkey.

The project will increase energy security in the entire region, and enable more efficient power flows at lower costs. It will also reduce the need for more power plants to meet rapidly increasing demand for electricity in the southern and eastern Mediterranean regions. From Turkey the ring would then link back into the European grid via Greece or through the newly interconnected Eastern European country grids.

Apart from the economic and technical hurdles to be overcome, there are two quite different outlooks to reconcile. European networks are highly meshed, consisting of high voltage lines, with high consumption and high density of consumers, and predictable load patterns. But grids in the Southern Mediterranean region are typically lower voltage grids, non-redundant, serving fewer loads, concentrated in highly urbanised areas, and strung out through the countryside at lower voltages.

Siemens Power Transmission and Distribution Group is one company keeping an eye on these developments. As one of the two world-leading suppliers in the HVDC market, it expects to contribute to discussions regarding technical realisation of the project.

The group is already very active in HVDC around the world. Currently, it is working with local companies to construct a link in southeast China. The US$121million contract was awarded by China Southern Power Grid Company in Guangzhou and the project is expected to be connected in 2007. The new HVDC transmission line will eventually provide electricity from the hydro and coal fired power plants in the west of the country to the industrial districts in Guangdong.

India’s largest power transmission project, the East-South HVDC Interconnector II, was completed by Siemens PTD Group ahead of schedule. It links the states Karnataka and Orissa over a distance of 1450km – the second longest HVDC link in the world – with a bulk power of up to 2000MW.

Siemens says HVDC is the only technically and economically feasible solution for interconnection of asynchronous grids and for power transmission over large distances between generation and load centres.

Today, although most grids are ac, more dc lines are being installed and the backbone of a global grid will probably be HVDC. Such links are less costly than ac versions because they need only two main conductors while an ac line needs three. And the losses are lower. But HVDC converter stations cost more than the ac terminal stations so HVDC may not be economical over short distances, unless earth return can be used to further reduce transmission line costs.

The major advantage of HVDC is its controllability. There is no need to synchronise power stations and grids with each other and there are no problems with phase change over distance, so stability is no problem.

The basic power control is achieved trough a system where one of the converters controls its dc voltage and the other converter controls the current through the dc circuit. The control system acts through firing angle adjustments of the thyristor valves and through tap changer adjustments on the converter transformers.

A back-to-back HVDC station can be used to link two ac grids. This system isolates each grid from fault conditions and disturbances on the other and eliminates the need for synchronisation while allowing two-way power transmission.

Commercial system

The latest from ABB is its HVDC2000 system, based on thyristor-switching at converter stations. Its key feature is the use of capacitor commutated converters (CCC) in conjunction with its development of continuously tuned ac filters (ConTune). These filters can be built to generate small quantities of reactive power but still provide good filtering.

Commutation capacitors are connected between the thyristor valve bridge and the converter transformers. With a CCC there is no need to switch filter banks or shunt capacitors banks in and out to follow the reactive consumption when the active power is changed.

The ConTune AC filter has electromagnetic tuning that adjusts to the inherent frequency variations and temperature variations of the filter components. It uses a filter reactor with variable inductance based on an iron core with a control winding round it.

By feeding a corrective direct current into the control winding, the total magnetic flux in the reactor is influenced, so changing the inductance, which tunes the filter to the correct frequency of the harmonic.

HVDC converters produce current harmonics on the ac side and voltage harmonics on the dc side. For good performance, low impedance tuned filters often need to be provided for the lowest characteristic harmonics. Detuning of conventional filters is caused by network frequency excursions and component variations such as capacitance changes due to temperature differences.

The outdoor air-insulated thyristor valve is a new component, made possible by the development of high power thyristors. It gives increased flexibility in the station layout; eliminates the need of a valve hall, including its subsystems; reduces the equipment size; and makes it easier to upgrade existing stations. Future relocation of an HVDC station will also be simpler when outdoor HVDC valves are used.

All functions for control, supervision and protection of the stations are implemented in ABB’s MACH2 fully digital software.