and North Islands. It uses thyristor-based line-commutated converters at each end of the link for rectifying and inverting between AC and DC. The link includes ground electrode stations that enable the use of earth return current. This permits operation with unbalanced current between the two poles, and monopolar operation when one pole is out of service. Converter stations The converter stations for each pole, at each end of the link include: converter valve hall, cooling system and control building converter transformers 220 kV AC switchyard equipment and connections 220 kV AC harmonic filters 350 kV DC switchyard equipment, including DC smoothing reactor The converter valves are twelve-pulse converters, arranged as three water-cooled quadrivalve assemblies. Both Pole 2 and Pole 3 use a design that suspends the quadrivalves from the roof of the valve hall. This provides superior seismic performance compared with a ground mounted arrangement, especially in New Zealand's highly seismic environment. There are three single-phase converter transformers for each converter valve (plus one spare transformer), and each transformer has two secondary windings connected to the valve. Each converter station requires power factor correction equipment to generate reactive power for the converters and provide voltage support to the surrounding AC grid. At the Benmore converter station, reactive power is provided by the generators at Benmore Dam. At the Haywards converter station, reactive power and power factor correction is provided by eight synchronous condensers, two shunt capacitors, two shunt reactors, and one static synchronous compensator (STATCOM). Details of the converter station equipment and ratings are given in the table below: Submarine cables The three submarine power cables installed in 1991 are each rated to carry 1430 A continuously at 350 kV operating voltage. They are constructed with a compacted multi-strand copper conductor as a central core, with mass-impregnated paper insulation surrounded by a lead sheath. Two layers of galvanised steel wire armour provides strength and mechanical protection. The outer layer of the cable is a serving made from polypropylene rope and the outer diameter is approximately 130 mm. The cables have a 30-minute overload capacity of 1600 A. To ensure security of the link's submarine power cables, a seven-kilometre wide Cable Protection Zone (CPZ) is enforced where the cables cross Cook Strait. Vessels are not permitted to anchor or fish in this area, and the area is routinely patrolled by sea and air. Anyone found anchoring or fishing in the area