The Italy - Malta interconnection. Morris Brenna

Transcription

1 The Italy - Malta interconnection

2 Introduction The Republic of Malta has an area of 316 km2 and a population of about 410 thousands with an energy annual consumption of about 2.3 TWh. The Maltese electric network was an isolated system with a peak load around 424 MW while the power reserve margins shrunk below N-1 condition (in terms of power plants), relieved by the incumbent crisis, but still demanding prompt investments on the electric energy supply side. Moreover, some of the existing steam plants have been decommissioned both for ageing reasons and environmental compliances. As an alternative/integration to commissioning new small scale fossilfuel-fired generation plants, has been identified in an electrical submarine interconnection with the Italian grid to provide improvements in terms of security of supply, power quality; economic savings; possible integration of wind farms; CO2 and air pollutants emissions reduction.

3 Introduction Sicily has an area of km2 and a population of about 5 millions with an energy annual consumption of about 22 TWh (6.4 % of Italy s annual consumption). In 2008 in Sicily the electric demand was satisfied by local energy generation. The energy demand of the island was supplied by thermal plants (93%), hydro plants (3%) and renewable plants, mainly wind plants (4%). Currently, the average power requested by the loads is around 2,100 MW and the total installed power is so shared: 5,434 MW from thermal power plants 1,677 MW from wind farms 1,204 MW from photoviltaic generators Since 1973 the energy balance in Sicily has been always positive, with an average surplus of 5%. Up to a few years ago, the island was connected to the Italian national network by a 400kV singlecircuit submarine link. Currently, a new double-circuit 400 kv submarine cable line (Sorgente-Rizziconi) is in operation and connect the Sicily with the Italian mainland. The Sicilian primary network 380kV and 220 kv, operated in meshed way in all the substations of the network, can ensure the highest level of security during the normal operation of the transmission grid. The 150 kv network is operated partially in meshed way and partially in radial way. The dispatching is carried-out in compliance with the (N-1) static and dynamic security criterion.

4 Introduction The establishment of a link between Malta and Italy was the key step in the strategic reorientation of the Maltese energy system. This interconnection has unique regulatory features, since it is a "unilateral" public interconnection, ie realized, owned and operated by only one of the transmission operator (Enemalta), which has required special solutions for authorization processes and for capacity management. The project, which is part of the European Energy Program for Recovery (EEPR), as a public utility project, was partly funded by the European Union, which made available 100 million euros for its implementation (about half of the total cost of the project). Now this electrical connection is fully in the so-called Trans-European Energy Networks (TEN-E).

5 Introduction Three technical solutions are currently available for undersea transmission of electric power: AC (synchronous) links DC, line commutated (DC-LCC) DC, voltage source (DC-VSC) All three solutions are in principle applicable to the Malta Sicily link Performances, shortcomings and costs of these techniques differ widely. Factors to be considered for a comparison can be qualitatively outlined

6 Advantages of the Malta Sicily interconnection More stable power supply of the Maltese electrical system Lower electricity price in Malta Decommissioning of older thermal power plants with environmental benefits Flexible use of non-programmable renewable energies, especially wind power, in Sicily

7 The challenge Long submarine connection (about 120 km), maximum water depth: 160 m

8 Technical alternatives 132 kv transmission grid voltage in Malta 220 kv transmission grid voltage in Sicily AC LCC 100 kv VSC 200 kv DC

9 Comparison between AC and DC solutions AC Link capacity Power transmission limits Power transmission control Cables Transmission losses Substations Reactive power exchange with network Passive receiving network supply / blackstart capability Redundancy (upon loss of a cable) Stations Costs Cables (capital, for equal crosssection) Cables (laying) Reliability / availability (*) Dependent on cable ampacity and cable length Angular stability Requires external means. Only depends on dispatching if no parallel links exist Highest (extra joule losses due to skin effect and reactive current, plus dielectric losses) Minimum (less than 0.3% of rated power for each station ) Limited due to extensive shunt compensation (>95%) Possible, limited by voltage stability 3-Core cables: none(*) (requires 2nd cable) Single-core cables: 100% if fourth (spare) cable provided Minimal (standard devices), shunt reactors only extra cost Maximum (3 phase conductors, single-core cables also require Cu armour) Maximum for single-core cables; minimum for 3-core (possibly with shorter laying stretches) Higher DC-LCC DC-VSC Only dependent on cable ampacity (virtually unlimited length) AC short-circuit power at terminals Controllable (10-100)% of rated power. System frequency constraints if no parallel links exist Lower than AC (minimum with sea return or with use of EHVDC ) ( )% of rated power for each station Limited, if AC filter/capacitor switching is performed Not available Monopolar: none (requires 2nd cable) Bipolar: 50% if sea return electrodes available Maximum (large footprint, extensive AC filters); sea return also requires electrodes Minimum for monopolar with sea return or IMR(***) (only 1 cable). Otherwise 2 cables needed Minimum for monopolar or bundled laying None Controllable (0-100)% of rated power. System frequency constraints if no parallel links exist Lower than AC Maximum (( )% of rated power for each station) Controllable in amplitude and sign (subject to capability limits) Possible, limited by voltage stability (**) None (requires 2nd couple of cables) Significant, comparable with LCC Lower than AC due to higher part count (spares needed) Difficult, theoretically possible at reduced power. Further study needed. (**) Application under study. (***) IMR: Integrated Metallic Return. Comparable to bipolar (or external metallic-return monopolar) LCC, always 2 cables Minimum for bundled laying Further experience required (Comparable to LCC?)

10 Comparison between AC and DC solutions Steady-state performance assessment of HVDC links, either LCC or VSC, is quite straightforward and is usually performed by DC equipment manufacturers Working out the operating envelope of AC cable interconnections is more involved, and usually done at Utilities engineering level Extensive system impact studies are required in both cases, with regard to voltage/angular stability, insulation coordination, harmonics etc.

11 Power losses

12 Normalized costs

13 Normalized costs/mw

14 AC solution issues One of the most conspicuous features of HV and EHV cable lines (CLs) is shunt capacitance, times that of overhead lines (OHLs); series inductance is smaller (30 to 70% of OHLs depending on cable spacing) As a result, Surge Impedance Loading (SIL) of CLs is several times the apparent power transmitted at thermal limit (i.e. ampacity) HV/EHV CLs thus have a substantial reactive power surplus in any operating condition

15 Reactive power issues Reduction of maximum transmissible real power and/or of allowable cable length between compensating stations Underexcited operation of nearby generators Step voltage change at the energizing bus following line switching-on/off Voltage rise at the open receiving end, upon line switching-on, or load rejection Line-charging current must not exceed rated line-charging breaking current of circuit breakers (CBs)

16 Reactive power issues "System" effects are catered for by judicious application inductive shunt compensation of Oil-immersed shunt reactors are robust, well-proven devices without moving parts Theoretically, to reduce the charging current hunt compensation should be installed at intermediate locations along the CL. This is not possible in submarine links: shunt reactors are installed at the terminals of the cable line

17 Final choice After many studies, the final transmission system chosen for the Malta Italy interconnection is an AC 220 kv cable line The rated power is 200 MW, but the system is prearranged for a second cable line This interconnection started its commercial operation in April 2015 It is the longest HVAC submarine cable in the world The Italian connection is located in Ragusa, in the south-east of Sicily The Maltese connection is located in Maghtab, in north Malta

18 Single-line diagram of the final Sicily-Malta interconnection

19 Ragusa terminal station The Italian connection has been realized in an existing HV substation

20 Ragusa terminal station

21 Maghtab terminal station In Magthab has been realized a new substation that provide the connection with the Maltese HV system through 220/132 kv autotransformers. Maghtab station is located nearby the Maltese landing point of the Malta-Sicily interconnector. Due to proximity to the shore, all the equipment, including autotransformers, shunt reactors and 230 kv / 132 kv switchgears, have been installed indoor.

22 Main submarine cable parameters ( km) Main conductor 630 mm2, Cu Insulaton XLPE, 24 mm Water-blocking tape Semiconductng water-swellable tape Metallic sheath Lead alloy ½C, 3 mm, 29 ka for 1 s Polyethylene sheath Semiconductng PE, 2.6 mm Steel armor 1 layer, 120 GSWA wires x 5.6 mm Ø Nominal current 655 A (249 MVA) Overloadability 1 h 1,113 A (170%) Maximum operatng voltage 245 kv Basic impulse insulaton level (BIL) 1,050 kv DC resistance Ω/km AC resistance Ω/km Line inductance 0.42 mh/km Line capacitance F/km Positve sequence resistance Ω/km Positve sequence reactance 0.13 Ω/km External diameter 239 mm

23 Submarine cable Copper conductor Semiconductie watertggt compound Semiconductor screen XLPE Insulaton Semiconductor screen Pgase identier tape Watertggt ribbon Lead sgeatg Polyetgylene sgeatg Optcal iber cable Semiconductor iller Filler Semiconductor reinforcement ribbon Galianized steel wire reinforcement Polypropylene bandage Polypropylene yarn

24 Submarine cable

25 Land cable Aluminum conductor Semiconductor layer XLPE insulaton Semiconductor layer Hygroscopic layer Aluminum Sgeatg Polyetgylene Sgeatg

26 Main land cable parameters (19.1 km) Main conductor 1000 mm2, Al Insulaton XLPE, 19.1 mm Water-blocking tape Semiconductng water-swellable tape Metallic sheath Aluminium 1.2 mm, 50 ka for 0.5 s Polyethylene sheath HDPE, 4 mm Nominal current 763 A Overloadability 1 h 1,144 A (150%) Maximum operatng voltage 245 kv Basic impulse insulaton level (BIL) 1,050 kv DC resistance Ω/km AC resistance Ω/km Line inductance mh/km Line capacitance F/km Positve sequence resistance Ω/km Positve sequence reactance Ω/km External diameter mm

27 Shunt reactors Ragusa substation: 1 constant 220 Mvar SR directly connected to the 220 kv cable Maghtab substation 1 variable Mvar SR with on-load tap changer connected to the 220 kv cable 1 variable Mvar SR with on-load tap changer connected to the 220 kv busbars through a circuit breaker, therefore it can be disconnected

28 Maximum active transmissible power = = 1 Pc = surge impedance loading Sz = power at ampacity K" = cable propagation constant L = cable length

29 Capability curves

30 Capability curves Delivered active and reactive power in Malta, with attendant receiving end voltage at Maghtab busbars; 230 kv voltage in Ragusa, for different values of total inductive shunt compensation in Maghtab (from left to right, 240, 180, 120, 60 Mvar)

31 Transmission losses Actual value Percent value Overall transmission losses calculated for all operating points comprised inside the capability curve related to 120 Mvar shunt compensation in Maghtab. 230 kv voltage in Ragusa.

32 Power flows Half power By adjusting ATR tap and reactor it is possible to set the active and reactive power flows in the Maghtab substation at the desired values Full power

33 Final tests Final tests on the Malta-Sicily link started in January 2015; the link started commercial operation in April 2015; besides conventional high voltage tests, special tests have been carried out, in order to assess the impedance of the submarine cable line at 50 Hz and higher frequencies. Results, summarized in this figure, show that the first resonant frequency is about 375 Hz (around 7th harmonic); impedance magnitude at first resonant frequency is about 180.

34 Cable energization Current at the Sicilian (sending) end of the link during cable energization

35 Cable energization Phase to ground voltages in Maghtab substation during cable energization

36 Protection system The new 220 kv connection has a fully redundant monitoring, control and protection system in each substation, in order to ensure adequate safety margins for the operation of the two interconnected electrical networks. Given the importance of the new connection and terminal apparatuses, the protection system can detect the different types of failure that may occur on both the connecting cable and the electrical stations. More in detail, each of the different types of failure that may affect the interconnection system must be detected by at least two protections, based on different operating principles. A fault on the 220 kv cable or on the compensation inductors connected to the end of the cable must be eliminated in the base time by opening the line circuit breakers at the end of the HV cable (at Ragusa and Maghtab substations).

37 Protection system

38 Protection system To ensure the priority of one of the two protection systems, by automatically transferring (without delay) the control signal to the other in case of failure, both protectors use 24/48 optical fiber connection. Dielectric central element Grooied core in tgermoplastc material optcal ibers buffering Bandage witg syntgetc tapes Black polyetgylene sgeatg Aramid yarns Bandage witg syntgetc tapes Black polyetgylene sgeatg

39 Conclusions The new 220 kv AC cable connection between Sicily and Malta, with its 100 km (about km), represents the longest AC submarine interconnection in the world. The realization of the work has allowed for a more stable power supply to the Maltese electrical system and a lower cost of electricity while decreasing the environmental impact produced by the old Maltese power plants. In addition, there is a more flexible management of the Sicilian electricity production plant, favoring in particular the dispatching of non-programmable renewable sources, especially with regard to wind generation. However, it is necessary to point out some possible criticalities that may arise in the setting of the island of frequency of the Sicilian electrical system, especially in the event of a sudden trip of the new Sicily-Malta connection. These criticisms are partially resolved thanks to the new Sorgente-Rizziconi link.

40 Tgank you for your atenton