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, Roma, Italia Presented by Prof. Stefano Lauria

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Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca At the beginning of 20th century, DC (Direct Current) was superseded by AC (Alternating Current) for large-scale electrification.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!

UCTE 3 Introduction UCTE History Applications Integration NAIG links HVDC BERLIN '08 Prof.Giuseppe Veca

History of events 1930s 1940s s 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!

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

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

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)‏

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 ( MW)‏ 500÷1000 MW Francia Croazia Albania GRITA (500MW)‏ 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

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

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

Long-distance transmission 3 The AC vs. DC break-even distance can be loosely estimated at 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 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, ): 2 links,  500 kV, 3000 MW each, 890 and 940 km

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)

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)‏

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)‏

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 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

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

ADDRESSES 17 Prof. Fabio Massimo GATTA Prof. Alberto GERI Prof. Stefano LAURIA Marco MACCIONI Prof. Giuseppe Maria VECA HVDC BERLIN '08 Prof.Giuseppe Veca