1. HVDC TRANSMISSION F. M. Gatta, A. Geri, S. Lauria, M. Maccioni, G. M. Veca Università degli Studi di Roma “La Sapienza” Dipartimento di Ingegneria Elettrica Via Eudossiana n° 18, 00184 Roma, Italia Presented by Prof. Stefano Lauria 28/03/08 @ 14.30.00

2. INTRODUCTION In the past... Today... In Europe... 2 Introduction Applications Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca At the beginning of 20th century, DC (Direct Current) was superseded by AC (Alternating Current) for large-scale electrification. DC power did nevertheless survive, in applications like electric traction and drives. Today, bulk power systems are 3-phase AC, while utilization is either 1-phase or 3-phase AC. Continental Europe is actually a single AC power system, running synchronously at 50 Hz, spanning from Portugal to Poland and Greece! 28/03/08 @ 14.30.00

3. UCTE 3 Introduction UCTE History Applications Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca 28/03/08 @ 14.30.00

4. History of events 1930s 1940s 1954 1970s Today... 4 Introduction UCTE History Applications Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca Efficient static AC/DC conversion (mercury arc valves) was made possible. High Voltage DC (HVDC) bulk power transmission was studied in Germany. First commercial application in Sweden: submarine link bet- ween mainland and Gotland island (100 kV-20 MW-90 km). Thyristors (SCRs) took over; today, HVDC operation voltages attain 600 kV, transmitted power over 3000 MW. DC made its way back into bulk power systems! 28/03/08 @ 14.30.00

5. TYPICAL HVDC APPLICATIONS There are three typical HVDC applications: 5 Introduction Applications Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca 1. Interconnection of non-synchronous AC power systems, even at different frequencies. 2. Power transmission over long undersea cable links, if the AC solution requires intermediate compensation. 3. Point-to-point, long-distance transmission of large blocks of power. For entries 1. and 2., HVDC is the only practical solution. For entry 3. the choice of DC or AC transmission is a matter of technical-economic convenience. 28/03/08 @ 14.30.00

6. Submarine transmission 1 For a 380 kV-50 Hz AC submarine cable A bipolar HVDC submarine link can transmit 1000 MW over several hundreds of km, with 2 cables Several HVDC links in operation or planned in Italy 6 Introduction Applications Submarine 1 Submarine 2 Submarine 3 Long-dist. 1 Long-dist. 2 Long-dist. 3 Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca — maximum practical length is around 100 km —transmissible power over a single circuit (3 cables) is around 1000 MW — Sa.Pe.I., (Sardinia-Italy, under construction). 420 km,  500 kV, 1000 MW. —NorNed (Norway-Netherlands) 580 km,  450 kV, 700 MW 28/03/08 @ 14.30.00

7. Submarine transmission 2 7 Introduction Applications Submarine 1 Submarine 2 Submarine 3 Long-dist. 1 Long-dist. 2 Long-dist. 3 Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca GRITA – Monopolar link 400 kV – 500 MW (Italy – Greece)‏ SAPEI – Bipolar link, ±500 kV – 1000 MW (Sardinia – Latium)‏ SACOI – Three- terminals monopolar link, 200 kV – 300 MW (Sardinia – Corsica – Tuscany)‏ 28/03/08 @ 14.30.00

8. Submarine transmission 3 8 Introduction Applications Submarine 1 Submarine 2 Submarine 3 Long-dist. 1 Long-dist. 2 Long-dist. 3 Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca Algeria Tunisia Libia Italia SACOI (300 MW)‏ SAPEI (500+500 MW)‏ 500÷1000 MW 1 2 3 Francia Croazia Albania GRITA (500MW)‏ 4 5 6 Preliminary studies 1 - completed 2 - completed 3 - underway 4 - underway 5 - to be performed 6 - to be performed There are also other feasibility studies by TERNA 28/03/08 @ 14.30.00

9. Long-distance transmission 1 HVDC overland links are usually bipolar, on overhead lines. Compared to AC, DC transmission has several advantages As a consequence, less DC lines than AC lines are actually needed to transmit the same power 9 Introduction Applications Submarine 1 Submarine 2 Submarine 3 Long-dist. 1 Long-dist. 2 Long-dist. 3 Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca — HVDC overhead lines are less expensive and require narrower right-of-ways. —Line losses (Joule and corona) are also smaller. —Angular stability and reactive power balance are not a concern: there is no need of intermediate switching/compensating stations. 28/03/08 @ 14.30.00

10. Long-distance transmission 2 The main shortcomings lies in the AC/DC conversion stations due to their cost, large footprint and additional energy losses The comparison between all the above factors, dictates the convenience of AC or DC 10 Introduction Applications Submarine 1 Submarine 2 Submarine 3 Long-dist. 1 Long-dist. 2 Long-dist. 3 Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca © Siemens 28/03/08 @ 14.30.00

11. Long-distance transmission 3 The AC vs. DC break-even distance can be loosely estimated at 600-800 km for a 3000 MW power transfer Some existing links The current industry “standard” is  500 kV, 3000 MW for a single bipolar link In the midterm, operation of  800 kV links is expected, trasmitting 5-6000 MW on a single line 11 Introduction Applications Submarine 1 Submarine 2 Submarine 3 Long-dist. 1 Long-dist. 2 Long-dist. 3 Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca —Itaipu (Brazil):  600 kV, 2×3150 MW, 785 km. —‘Three Gorges’ (China, 2001-4): 2 links,  500 kV, 3000 MW each, 890 and 940 km 28/03/08 @ 14.30.00

12. INTEGRATION IN AC NETWORK There are several conditions to satisfy: 12 Introduction Applications Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca 1. The AC nodes at the HVDC line terminals must be able to supply/evacuate the rated power of the DC link. 2. The rated power of the DC link must be compatible with the TSO’s operation rules: f.i. UCTE takes at 3000 MW the largest single loss of generation in the European system. 3. AC short-circuit power at the conversion stations must be sufficiently larger than DC rated power (say, ESCR>3; depends on adopted technology). 28/03/08 @ 14.30.00

13. North Africa to Italy 1 13 Introduction Applications Integration NAIG links NA to Italy 1 NA to Italy 2 Italy to G 1 Italy to G 2 HVDC BERLIN '08 Prof.Giuseppe Veca There are no particular shortcomings aside from the cost of submarine cables: in the first stage (say, 3000 MW power transfer) up to 3 cables per pole are needed Other key points are: —Individuation and survey of cable routes in deep sea (see f.i. studies conducted for Sa.Pe.I. link)‏ —Identification of suitable EHV terminals in the Italian network (several powerful nodes on the Tyrrhenian coast, from Naples to Suvereto)‏ 28/03/08 @ 14.30.00

14. North Africa to Italy 2 14 Introduction Applications Integration NAIG links NA to Italy 1 NA to Italy 2 Italy to G 1 Italy to G 2 HVDC BERLIN '08 Prof.Giuseppe Veca SAPEI cable route attains 1600 m depth and required extensive surveys by means of Remotely Operated Vehicles (ROVs)‏ 28/03/08 @ 14.30.00

15. Italy to Germany 1 15 Introduction Applications Integration NAIG links NA to Italy 1 NA to Italy 2 Italy to G 1 Italy to G 2 HVDC BERLIN '08 Prof.Giuseppe Veca At an initial stage (e.g. 2-3000 MW) the existing 380 kV AC network could be used. Italy permanently imports 6000 to 7000 MW through the alpine interconnections, mainly from France and Germany; the new, northbound flow would be mainly virtual. This solution, however, potentially interferes with the Italian energy market, capping the transfer capability between network zones “Center”, “Center-North” and “North”. Network expansion could be required. Dedicated HVDC lines would solve Italian network problems. The key issue here, however, is the strong NIMBY attitude in Italy. 28/03/08 @ 14.30.00

16. Italy to Germany 2 16 Introduction Applications Integration NAIG links NA to Italy 1 NA to Italy 2 Italy to G 1 Italy to G 2 HVDC BERLIN '08 Prof.Giuseppe Veca The best long-term choice is probably represented by  800 kV overhead lines, despite their visual obtrusiveness. Routing is undoubtedly a problem. At a significant cost, HVDC underground cables could solve the public acceptance problem. If the auxiliary galleries of new railway tunnels are made available, cables would greatly simplify crossing the Alps. 28/03/08 @ 14.30.00

17. E-MAIL ADDRESSES 17 Prof. Fabio Massimo GATTA fabiomassimo.gatta@uniroma1.it Prof. Alberto GERI alberto.geri@uniroma1.it Prof. Stefano LAURIA stefano.lauria@uniroma1.it Marco MACCIONI marco.maccioni@uniroma1.it Prof. Giuseppe Maria VECA giuseppe.veca@uniroma1.it HVDC BERLIN '08 Prof.Giuseppe Veca 28/03/08 @ 14.30.00