The record-breaking Italy-Greece HVDC link5 October 2002
The new Italy-Greece HVDC link is at 1000m the deepest submarine link in existence, and includes one of the longest underground sections in the world. Proving it involved an exhaustive series of factory and site tests which could be compared to main performance criteria reported during the commissioning stage, a process described in a paper delivered to Cigré Session 2002, Paris, in August this year.
The Italy-Greece bi-directional 500 MW HVDC link (known as GRITA) between Galatina and Arachthos consists of a mono-polar cable utilising a sea return and based on a grid commuted twelve pulse thyristor bridge. The design anticipates a possible 1000 MW bipolar extension and is characterised by state-of-the-art quality and reliability at the converter stations (DC cable, thyristor valves, converter transformers, AC and DC filters, smoothing reactors, AC and DC yard switching equipment), as well as by a modern control and protection system (converter firing, pole power control, frequency regulation, inter-station telecommunication, station control and monitoring).
The link, which was commissioned in 2001, connects the EU grid for the first time to Greece, and through Greece to producers in Albania and Turkey, reinforcing the Mediterranean ring and fostering east-west energy trading. Because strengthening the electrical network is an EU priority, the Union has provided 40 per cent of the total costs.
For the two networks the link has reduced grid operational costs, improved emergncy response and increased flexibility and reliability through active power reserve sharing.
Two 400 kV lines are connected to each converter station, with interconnection to the 150 kV distribution network. Both AC networks can supply 140 MVAr and absorb 100 MVAr. The maximum for single bank operation is 100 MVAr. The 400 kV nodes to which the converter stations are connected are 'strong' compared to the capacity of the DC link. In particular, the minimum short circuit ratio on the Greek side is 5-plus, and 7-plus in Italy.
The system has a minimum operational limit (50 MW) and can be operated at reduced voltage (320 kV DC at a current of 1000 A). At 500 MW and an ambient of 40°C the guaranteed conversion losses are about 7 MW while the DC line losses are about 14 MW. At the specified performance, link operation is allowed if at least one 400kV AC line is in service (with a minimum short circuit capacity of 3600 MVA in Italy and 2800 MVA in Greece). The system rating is automatically reduced if one electrode line goes out of service (the maximum current is reduced to 900A), at thyristor over-temperature (the transmitted power is progressively reduced with steps of 5 per cent); at ambient over-temperature or spare cooling system unavailability the current order is immediately frozen.
As well as conventional operational procedures, eg pole connect/isolate, power direction selection, these manoeuvres are specified:
• Slow inversion: this reduces the power at a fixed ramp speed (max 999 MW/min) to the technical minimum where the converters are blocked. Then an automatic de-ionisation interval of 10 min before de-blocking the valves and reaching a new power level with reversed flow. This condition is allowed up to 1000 times/year.
• Open line test: DC line voltage energisation from one station with the DC pole disconnectors of the other station opened. This procedure allows a check of the DC line insulation.
• Backup synchronous control (BSC) in case of a telecomms fault: once enabled (from either station) activation is automatic and allows a power or current change with auto-synchronisation of the stations.
•Restart attempts after DC line protective action: once enabled (from either station) this feature allows the automatic execution of three restart attempts (two at full voltage, one at reduced voltage) before blocking the link.
• Frequency control in case of islanding of the grid surrounding the converter station: once enabled the activation of frequency control is automatic.
• Fast inversion in case of islanding of the grid around the converter station: once enabled, activation is automatic. This procedure reduces the power at a fixed ramp speed (max 999MW/s) to the technical minimum. These are allowed up to 10 times per year.
Experience acquired in DC energy transmission, although not valueless, is not often re-usable, because HVDC technology evolves so fast, and because the few applications per year are so different, limiting standardisation.
A detailed review of HVDC system design together with in-depth performance testing was therefore necessary. Enel and PPC developed a detailed specification, the bidding was based on acceptance of a detailed design developed by the system manufacturer; and the factory and commissioning test plan required a thorough check of each converter station and of the overall link under normal, disturbed, and unusual conditions.
Marine survey and sea trial
At the planning stage two major difficulties were apparent; first, laying cable at a record water depth of 1000 m, and second, an operational mode with repeated inversions of power direction. Developing the transition joint between the oil-filled land cable and the mass impregnated submarine cable, and the huge porcelain insulators suitable for a very high salinity level, was also a challenge.
A marine feasibility study (in 1991) was followed by a complete sea-trial with 3.5 km of cable performed successfully in 1995. It consisted mainly of testing the embedding machine in shallow water (about 30 m) and at 150 m depth, cable laying (including junction repair) at a depth of 1000 m, cable recovery, and an electrical test at 600 kV DC for 15 min.
The following specific studies were conducted by the manufacturers.
Dynamic performance study
Parameters were defined after simulator runs tested the performance of the HV system with different AC configurations. The control functions relevant during disturbances were tuned to optimise power transfer during faults and to promote fast recovery, which minimises the probability of commutation failures caused by a disturbance. Some events were deeply analysed, such as the case of operation with the 150 kV network alone: the outage of one of the two incoming 400 kV overhead lines (for both stations) must not reduce the performance of the HVDC link; in this condition, if a further fault (transient or permanent) on the second line appears, the converter station will remain connected only to the weak 150 kV AC network through the 400/150 kV auto-transformers. In this case the feasibility of safely shutting down the link was checked, together with the possibilities for restart.
Operation with the 150 kV network was excluded due mainly to the possibility of 3rd harmonic critical resonance between the AC filter and the 150 kV network.
This investigation examined islanding transients occurring at the Galatina and Arachthos converters. The starting operating point was chosen to correspond to different working conditions of the link, with power flow from Galatina to Arachthos and vice versa. The frequency deviation during the first occurance of the islanding transients, as well as the frequency deviation following power modulation in steady-state islanded conditions, were examined. For each islanding separation, the short-circuit capacities of the residual grids were investigated and a suitable tuning of the frequency controller determined. The effectiveness of the fast inversion manoeuvre with frequency controller triggering was checked.
Total loss of AC network
The link has to stand the stresses caused by large perturbations, such as short circuits on the AC or DC side, and energisation and de-energisation of line components. In particular, the system has to stand total loss of the AC network.
Simulations were performed for different network states, eg. loss of the 400 kV ac network whether or not connected to the 150 kV network. The results indicated that the most critical cases are related to the total loss of the AC network when a converter station is working as an inverter at maximum power. In this case, the converter remains active, commutations do not cease, (since the resonant circuit between AC filters and converter transformers may support voltage in the station AC bus), so the pulsed converter continues to inject energy at the importing AC side. This creates an increase in the AC voltage leading to intervention of the AC bus arresters. Without protection, breaking the link in less than 100 ms would create stresses in the AC arresters beyond their capability (< 5 MJ).
With 400 kV and 150 kV networks connected, the time available to shut down the link in the case of total AC loss is greater. As a result of these studies a specific protection regime has been developed and tested.
Single and three phase auto-reclosure
In both Italy and Greece a single-phase fault triggers a single-phase auto-reclosure. And in Greece, for multiphase fault conditions, a three-phase auto-reclosure is also performed. With the inverter station connected to only one 400 kV line, a single-phase fault on this line with the opening of one phase of the breaker generates the same stress as total loss of the AC network. In the disconnected phase, voltage does not decrease, but, due to the delta winding of the converter transformer, it is maintained. Energy injected in the opened phase produces over-voltages that are limited only by the AC bus arrester. In this case, with total AC loss the energy limit of the AC arrester is exceeded in less than 100 ms. Several detailed simulations showed that speed of the protection logic circuits is dependent on the operational conditions; in particular, when the link is transmitting at a high power level, protection is very fast (<50 ms), limiting AC arrester stress at 60 per cent of its capacity.
Factory system tests
An unusual aspect of this interconnector is the high number of polarity reversals expected during the cable life. This fact has brought about an additional non-standard test, a polarity reversal ageing test consisting of 1000 cycles of polarity reversal (with load cycles), at 2 hour intervals between successive cycles.
As well as standard component and subsystem testing, system tests were performed, the most extensive part of the control and protection system validation because it was the primary test phase: in fact the tests performed at site were in some cases a repetition of those performed in the factory.
The cubicles for control, protection and operator communication were connected to each other and to the HVDC simulator used in place of the main circuits, the objective being mainly to debug the system. A number of test cases were conducted on control and protection functions and related circuit redundancies.
The results of the factory system tests were compared with specified performance criteria, mainly those not repeated during the on-site acceptance testing stage, to avert lifetime degradation of the connected main circuit equipment and/or cause a risk of instabilities in the surrounding AC networks.
Site system tests
At the end of installation of all submarine and land sections the whole system ws successfully subjected to an HVDC integrity test at a voltage of 500 kV for 15 minutes.
On site testing included full scale tests of all equipment, starting with high voltage energisation, through complete transmission tests, and the acceptance test of guaranteed performance. A major part of the process involved operating HVDC transmission under normal conditions with the automatic controls activated: several functions were partly or fully tested without affecting power transfer.
The site test plan was subdivided into:
Detailed on-site subsystems tests were carried out on the interconnected equipment to check the pre-conditions for energisation and the series of operational tests.
High voltage energisation
The involved sections were kept energised for several hours, to check corona or abnormal noise conditions by visual inspection and record surge arrester counters before and after energisation. The energisation of converter transformers, thyristor valves and DC yard was done first with blocked valves (AC-side) and later with valves de-blocked (DC-side). This test shows the converter controlling the DC voltage continuously.
Terminal /transmission test
This is the final test where everything works together: controls, main circuit equipment, operators and dispatchers (who planned power levels and system conditions during the transmission tests). The tests were at two levels: terminal tests with only one HVDC station (affecting one station/AC system mostly under high voltage conditions, without coordination between different stations/AC systems) and transmission tests with power transfer between AC systems. They covered current control (check of step response and firing symmetry), tap changer control (checking of firing and extinction angles within their limits), power control (power ramping normal, verification of power modulation and slow and fast inversion procedures), sequences (checking that the breakers, disconnectors and grounding switches operated safely), load and overload, and measurements of transmissible power, AC, DC and RI harmonics, and audible noise. Some of the system tests were devised as acceptance tests to demonstrate that performance requirements were met.
Towards future designs
The Italy-Greece HVDC project attained all its planned objectives, and on schedule, and factory and field tests demonstrated the accuracy of the pre-design studies. Significantly, some of the more radical characteristics of the GRITA link can be considered as advances on previous designs, for example the frequency regulation of the islanded AC network, linked with the automatic power fast reversal procedure, the algorithm which quickly and automatically recognizes the islanding state of the local AC grid, the current synchronous back-up which allows a reduced performance ALM operation remotely, even after a loss of telecommunication between the two terminals, and the fast and reliable recognition of total loss of the AC network through a new protection mechanism.
The GRITA link also represents an important stage in the development of submarine cable systems, particularly for DC. Mechanisation of the underground section was a powerful innovation. The maximum depth of 1000 m had never previously been accomplished with a power cable, the voltage and power ratings are among the highest achieved to date, the number of test load cycles with polarity reversal (1000) is a new record, and the Italian land power cable is at 43 km one of the longest worldwide.
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