Bi-metallic link re-think pays off

The SwePol link, which ties together the 400 kV grids in Sweden and Poland, is the seventh HVDC cable link to enter service between the power grids of continental Europe and the Nordic region (see table below), and the first with a metallic return. These links make it easier to optimise power generation in an area where different countries use various means of power generation and have different power demand profiles. For example, wet summers in the Nordic region, which relies heavily on hydropower, result in a considerable power surplus, which can be sold to countries that rely on more expensive fossil fuel-fired power plants. Conversely, in an exceedingly cold year when demand reaches a peak, utilities in the Nordic region can import surplus power from Poland rather than start their own oil-fired or gas turbine generation plant.

Power system reliability in the region is also increased by the addition of the new HVDC cable links. In the event of grid disruptions, the rapid power balancing ability of these links can be used to compensate for fluctuations in frequency and voltage. For example, it is technically feasible to reverse the entire 600 MW power throughput of the SwePol Link in just 1.3 seconds, although this is not a feature that will be used in practice. Nevertheless, a typical emergency power measure could call for a ramp-up of 300 MW within a few seconds to prevent grid failure if the voltage in southern Sweden were to drop below the 380 kV level.

Failure of a different kind occurred in the autumn of 2001 when repairs had to be carried out on the link's return cables, which had been damaged at two different locations by shipping traffic. This had been caused in one case by the snagging of an anchor and in the other probably by commercial fishing gear. Both occurred in sections where a rocky sea bed prevented the cables being buried to the normal one metre depth, so instead they rest directly on the sea bed.

Inspection with ROV (submarine camera) and divers showed that one of the return cables had suffered severe damage, while the second had only minor damage. The main cable was untouched. ABB carried out repairs by splicing in new sections of cable.

The whole repair operation (fault location, repair, mobilisation and demobilisation) took three and a half weeks including some weather waiting days. However, the HVDC link was only taken off line for short periods because the link could be operated on one return cable during most of the repair work.

SwePol Link AB is considering reinforcing the protection for the cables at points where they are not buried in order to reduce the risk of this kind of damage recurring.

Operating experience The SwePol link, which entered commercial service in June 2000, is a step towards the large-scale power distribution partnership that is known as the Baltic Ring. It is anticipated that the link will be primarily used by Vattenfall to export power to meet growing demand in northern Poland, where consumption is expected to rise by 10 per cent over the next five years. During the first year in commercial operation more than one million MWh was exported from Sweden to Poland, while only a small amount of energy was imported during the same period. Figure 1 shows a typical power exchange over a weekly period.

The predicted net export is some 1.7 TWh a year - around 1.5 per cent of Sweden's annual power production.

Environmental concerns All the previous links across the Baltic Sea use electrode stations off the coast to transmit the return current under the sea, and this has worked perfectly well (Table 1). The SwePol link was therefore planned originally as a monopolar HVDC system with earth return. However, in spite of the very low environmental impact exhibited by similar systems, the two return cable solution was eventually chosen as an alternative to electrodes in order to meet local resistance to the project, making the SwePol link the first monopolar system with a metallic return and adding around five per cent to the overall project cost.

The environmental issues that were raised during planning of this link may also apply to future installations. They covered three areas: • The magnetic field in the vicinity of DC cables.

• Chlorine generation by electrolysis of sea water at the positive electrode.

• Corrosion of metallic objects in the water or buried in the ground.

The introduction of the metallic return path provided by the cables eliminated concerns regarding chlorine formation or corrosion. While the magnetic field around DC cables is still present it has been reduced. It was not practical to lay the return cables at the same time as the high voltage cable, which meant that they could not be laid closely adjacent to each other. They also had to be separated because of the heat they generate.

At a distance of 10 metres from a single DC cable, the field produced is weaker than the earth's natural magnetic field. With the introduction of the return cables the magnetic field on the surface of the sea is typically reduced by 20 per cent in shallow waters and 50 per cent at a depth of 100 metres. So while the magnetic field still exists no adverse impact on marine life has been found. As modern seagoing vessels no longer rely on magnetic compasses they too are unaffected.

   Commercial arrangements SwePol Link AB was formed in 1997 to install, own and operate the cable link. It is a power transmission company that sells electricity transmission services across the link. ABB was awarded the contracts to supply, install and commission the submarine cable along with two converter stations.

A Polish subsidiary was formed in 1998 to handle the local business. On the Swedish side the link will be used primarily by state-owned Vattenfall, although other companies will be able to sign transmission agreements with SwePol Link. For the first ten years 50 MW is open for other electricity producers. The tariff for using this transmission capacity consists of a fixed network fee and a variable energy fee.

Cable and converter stations The link connects the 400 kV grids in the two countries by means of a 245 km mass impregnated cable operating at 450 kV DC, plus two XLPE polymer insulated cables rated 20 kV (in parallel) for the return current. It runs from Stärnö, just outside Karlshamn in Sweden, past the Danish island of Bornholm, and returns to land at the seaside resort of Ustka on the Baltic coast of Poland (Figure 2)(figures currently not available).

The Swedish converter station was sited at Stärnö because a 400 kV oil fired power station and the Swedish main grid are nearby. This avoided having to build new overhead lines that would have marred the Swedish countryside. The Polish converter station is connected to the Polish 400 kV grid at Slupsk, about 12 km from the coast (Figure 3).

The total cost of the link is estimated at about US$ 250 million and it involved around 2500 man-years of work for ABB, primarily at its plants in Ludvika and Karlskrona, Sweden.

DC circuit The land-based power grid is, of course, an AC system. However, for long underwater links DC is the only viable solution on account of the high capacitance of submarine cables - at distances greater than 40 km the charging current supplied from shore fully loads the cable and leaves no capacity for transmitting power. The SwePol Link is based on ABB's well known HVDC rectifier technology, which was first used in 1954. With this technology power is taken from one point in an AC network and converted to DC in a converter station, or rectifier. It is then transmitted over a line or cable and converted back to AC in another converter station, or inverter, before it is injected into the receiving network. As well as being the only viable technical alternative, DC cable is also much cheaper than AC cable.

The high-voltage cable used in the SwePol link (Figure 4) has an outside diameter of 140 mm, of which the central conductor takes up 53 mm.This is not solid but is constructed from copper segments to make it more flexible. The segments are shaped individually, then rolled as a unit to achieve an effective copper cross-section in excess of 99 per cent. The rest of the cable consists of various layers of insulation, sealant and double crosswind armouring to withstand the mechanical stresses during cable laying. Due to the great length of cable required it was manufactured in four sections and joined on board the laying barge.

The cables were buried around one metre beneath the sea bed to avoid damage caused by anchoring or trawling. The high voltage cable was laid separately from the two return cables with a separation of between five and 40 m depending on the depth of water.

The rated capacity of the link is 600 MW, with 20 per cent overload capacity at low ambient temperatures and 10 per cent overload at any ambient temperature when redundant cooling is available.

The visible parts of the link are the two converter stations (Figure 5). The converter valves (Figure 6) are located inside the 20 m high valve halls. There are 66 high voltage (9 kV) thyristors in each single valve and they are cooled by a water/glycol mixture in a single circuit system. The single phase three winding transformers are positioned just outside the building, with the valve side bushings protruding through the walls.

The high-voltage and return cables run underground almost all the way between the stations. On land this required clearing a five metre wide swathe through the landscape when the cables were being laid. This will soon be hidden, partly thanks to forest replanting.

Noise reduction Noise at a frequency of 100 Hz is generated by eddy currents in the power transformers. Converter stations also produce higher frequency noise that can be irritating to people living nearby. So the transformers and reactors were housed in special sound-proofing enclosures. The filter capacitor cans are also equipped with a noise-reducing device.

New transformer technology To avoid the presence of combustible material in the converter valve hall ABB has developed a dry type, high voltage transformer bushing. One SwePol link transformer unit has been equipped with two of these bushings as a reference installation. The insulating medium used on the valve side of the building is SF6. The operating experience to date has been excellent, endorsing the suitability of this bushing type for future projects.

One 95 Mvar filter meets the requirements for harmonic filtering with four branches, two of them tuned to the 11th and 13th harmonic with two high pass branches tuned to the 24th and 36th harmonics. To satisfy the demand for reactive power there are two 95 Mvar shunt banks. There is also a 117 Mvar shunt reactor on the Polish side.

The two sharply tuned filter branches employ the ConTune reactor. The control winding of this reactor allows continuous adjustment of the reactor inductance. Perfect tuning of the filter is therefore maintained at all times, irrespective of variations in network frequency and ambient temperature. The reactor has no moving parts, permitting a high quality factor and low filter loss.

The control system continuously monitors the filter current and AC bus voltage for the harmonic of interest allowing the regulating direct current fed to the control winding of the reactor to be adjusted to minimise the impedance of the filter.

Compact switchgear To save space in the switchyards ABB 'Compact' switchgear, consisting of conventional switchgear units mounted on a common platform, has been installed. The units for switching the reactive banks comprise one SF6 breaker, one disconnector, one earthing switch and one optical current transducer (shunt capacitor banks only). A core-and-coil assembly mounted at high potential monitors the bank current. It produces a digital signal which is transferred to ground by fibre optics. This provides a fast and accurate measurement to avoid the need for protective action at bank connection, which causes high frequency transients.

Control system ABB's MACH 2 Windows-based computerised control system provides fully integrated control, monitoring and protection for the link The main computers handle the operator interface, event recording and transient fault recording while controlling the converter process itself.

The normal control mode is power control. Power modulations such as frequency control and emergency power control (EPC) are included. The settings of these modulations can easily be changed by the utilities. The frequency control is typically activated outside the range 50 + 0.1 Hz. There are a number of activation criteria for the emergency power function. But typically a frequency drop below 49.5 Hz results in an EPC support, which at present is limited to 300 MW.

Normally, the converter stations are unmanned, and the operation of the link is controlled by despatch centres at Stockholm and Bydgoszcz. However, local control can be achieved when necessary in the converter station control rooms.

Tables

Table 1. HVDC links across the Baltic Sea