Plans to connect four separate power pools in the Mediterranean region will see the creation of a huge tri-continental system that will use HVDC submarine cables to export power from the Middle East to Europe.
Since the beginning of the last decade we have been witnessing a remarkable boost to interconnect power systems or to strengthen the interconnections within synchronous pools. This phenomenon is particularly evident in the Mediterranean region where in the near future four isolated power pools will be interconnected, creating a huge tri-continental system surrounding the Mediterranean basin called Medring (Mediterranean Electric Ring). This project is designed to synchronize interconnections with embedded North-South HVDC submarine cables to enable direct export of power from countries rich in fossil resources to European countries with few primary resources, but with very high electrical consumption.
Electric power exchanges are presently very high among the European countries: inside the Central-Western interconnected pool UCTE, putting more and more strain on the European grid where ISOs and TSOs are struggling to increase the inter area transfer capacity even adopting non-transmission solutions, or market based transmission expansions. On the contrary, power exchanges among the South Eastern Mediterranean Countries (SEMC) are presently very low, or in some cases, even null due to the lack of infrastructures.
The enourmous size of the future Euro-Mediterranean synchronous system spanning 4000 km in latitude, from Jutland to the border between Egypt and Sudan, and 6500 km in longitude, from the western Sahara to the borders between Turkey and Iran, sets technical challenges to be overcome for ensuring reliability and a secure operation. The Medring project was launched at the start of 2001 with the aim of defining a coherent framework for the development of cross border lines.
The development of a single tri-continental system is in line with the decision of the SEMC to adopt the same rules recommended by the Central-Western European grid UCTE both in terms of security criteria (e.g. N-1 security) and regulating capacity (e.g. maximum frequency deviation, LFC).
A cost benefit analysis conducted for the project highlighted that the most profitable areas are related to interconnections linking countries showing a potential lack of power production (e.g. new lines between Algeria and Tunisia) or in regions that will be likely subject to remarkable power flows (e.g. new line between Turkey and Greece). The profitability of North-South HVDC links crossing the Mediterranean Sea is quite limited due to the heavy investment costs. Moreover, the estimated North-South energy exchanges are heavily influenced by the Brent oil price.
In addition to the economic benefits related to an optimized energy exchange, system interconnection will bring substantial benefits to improve the reliability levels of the SEMC.
A pre-construction evaluation examining the system’s potential performance suggested that when interconnecting all the countries, a total reduction of the expected energy not supplied (EENS) was calculated at about 12 GWh/year, disregarding local problems such as network splitting.
Considering an average cost of the energy not supplied, around $2000/MWh, the benefit of interconnections can be estimated at approximately $24 million/year (economical gain related to the risk reduction). A further benefit of the Medring interconnection is that it provides the possibility of postponement in the construction of new power plants without jeopardizing the quality of the energy supply.
By also considering local problems (network splitting following component outages), the total EENS in case of isolated countries turns out to be about 22 GWh/year. This index is reduced to about 9.6 GWh/year when considering the operation in interconnected mode. Therefore, the interconnections are also helpful to relieve partially local problems by reducing the amount of EENS.
Project studies highlight that the most important dynamic phenomena to be considered are:
• Electromechanical oscillation modes at low damping and, more in general, steady state stability problems, mainly due to power transfers along very long distances;
• Transient instability of large network areas, sometimes involving an entire country or more than one country, subsequent to loss of important connections and evolving on a relatively long time scale;
• Critical transients due to long term dynamics, with voltage instability, or with unacceptable final operating points due to voltage collapses;
• Possible final operating conditions critical in terms of operability because of low voltage profiles or high current values.
The studies also highlighted several potential problems that would have to be overcome if the project was to work effectively:
• Opening of the interface UCTE-Turkey, with consequent instability of the Southern-Eastern part of the system (in conditions of heavy power import of Turkey);
• Some ring openings along interfaces between countries in the Southern part;
• Connection between Egypt and Libya. This link represents for the Ring a bottleneck in the year 2005 when the two countries are connected with a very long line (double circuit) at 220 kV. When, following a disturbance, the transit increases beyond 320 MW, a voltage collapse, followed by angle instability, in the neighbouring regions appears, with probably line tripping in proximity of the electrical centre;
• Operation with Ring opened. In case of the Ring opened in one section, the tripping of important units located in the Southern part can lead to instability;
• In some situations, to avoid possible transient instabilities, the maximum amount of imported power in a country had to be limited with respect to the theoretical economic values.
To avoid too low power transfer capacity some measures have to be adopted. These can be classified as ‘soft’ measures and ‘structural’ measures. ‘Soft’ measures are essentially based on the adoption of special control schemes or defence plans aimed at preserving the system integrity and avoiding the spread of disturbances from the affected areas to the neighbouring ones. ‘Structural’ measures to enhance dynamic security are based on the installation of devices having the capability to control specific electrical quantities, such as SVCs, back-to-back or HVDC links. The installation of these devices requires a non-negligible investment effort and its profitability shall be based on accurate analyses aimed at defining the optimal location of the device, the size, the control strategy, especially for the converter stations in case of DC links and the consequent enhancement of the power transfer. Examples of possible profitable structural measures to the Medring system include: installation of SVC (or similar devices) at the Egyptian-Libyan border to enhance voltage stability and the installation of a DC link between Egypt and Jordan to increase the power transfer capability and fully exploit the capacity of the submarine cables between the two countries.
From the steady state stability viewpoint, the importance of adequate PSS for all the most important generation units of the system has been demonstrated in order to obtain satisfactory damping of the electromechanical oscillation modes with periods in the range 0.5-5 s.
Slowest oscillation modes (periods > 5 s) proved to be well damped on condition that a well designed primary frequency regulation is installed on the units.
North-South HVDC connections
Due to losses caused by electricity transmission and transit fees, power exchanges are normally profitable only on short/mid distances. Therefore, to enhance the possibility of electricity trading between SEMC and Europe, some South-North HVDC links are under study. The corridors under investigation both involve Algeria.
A feasibility study for a HVDC connection from Terga in Algeria to Litoral de Almeria in Spain through a submarine cable (connection of about 240 km) with a capacity of 2000 MW has been completed. Because of the difficulty in getting foreign investments, the project will be commissioned in two stages. At the beginning a bipolar HVDC link rated at 1000 MW will be realised with marine electrodes for emergency current return.
The second study for a 1000 MW interconnection between Algeria and Sardinia in Italy is at permissibility stage. This project will be integrated with a second HVDC link, developed in two modules (500 MW+500 MW), between Sardinia and continental Italy. Further envisaged HVDC links will run from Tunisia to Italy (1000 MW) and Libya to Italy (600+1000 MW).
The potential profitability of North-South HVDC links crossing the Mediterranean Sea is however quite limited due to the heavy investment costs. Moreover, the estimated North-South energy exchanges are heavily influenced by the Brent oil price. Furthermore, for the electricity trading between Maghreb countries and Europe reciprocity in market rules shall be warranted. To this aim, a regional electricity market is going to be implemented in the Maghreb countries with the support of the EU.
Looking ahead into the future, further steps are envisaged related to extend the Euro-Mediterranean synchronously interconnected system, namely the extension to the south (a 220 kV line between Egypt and Sudan and an extension in the Middle East with the interconnections Syria-Iraq (400 kV line), Turkey-Iraq (400 kV line) and Jordan-Western part of Saudi Arabia (voltage level to be defined).
Even more appealing are the projects aimed at interconnecting Turkey with Georgia: a new 400 kV line is under study in view of the creation of an electrical ring around the Black Sea by means of the Trans-Caucasian lines as proposed in 1994 by RAO UPS of Russia.
To the East, regional interconnections between Turkey and their Eastern neighbouring countries are already in operation, namely the 154 kV lines built to feed the local load in Nahicevan and import power from Iran.MEE