A long distance HVDC transmission link between Central Operating Area and Western Operating Area in the Kingdom of Saudi Arabia was planned and designed and it is currently under development. This 770 km long point-to-point link consists of two LCC type converter stations and it is embedded in a powerful AC network, where the two areas are already interconnected through 380 kV series compensated lines.Both converter stations are connected to 380 kV existing substations, each one part of a meshed network.
The HVDC scheme is bipolar with neutral dedicated metallic return, and the possible operating configurations are: normal bipolar with metallic return, rigid bipolar (without neutral return) and monopolar with different possible connections of pole lines and metallic return. Due to environmental constraints, no return through ground is allowed.The nominal voltage is ±600 kV and the nominal power is 3500 MW with a considerable overload capability, both in short term and in continuous overload. These nominal values and the size of conductors of the overhead line were accurately selected on the basis of a least cost criterion, considering the investment and the operation costs (losses included) and assuming a load duration curve, over the expected life of the plant.
The line is fully overhead in desert area: the design of the towers is for application not only to this link but also to other links in Saudi Arabia, as a standard HVDC tower design up to 600 kV nominal voltage. The insulation withstand capability of the transmission line and of the DC open air part of the converter stations were carefully selected considering the pollution conditions, close to coastal area and in inland area.The link is able to operate at reduced DC voltage when the pollution conditions don’t allow the insulation withstanding the full voltage.
The requirement of the link in terms of harmonic disturbance and reactive power exchange is very stringent, thus requiring huge AC filter banks. Harmonic voltage background was evaluated through measurement campaigns in both winter and summer. An extremely detailed and sophisticated study was carried out, by means of simulations covering a large quantity of possible operating conditions of the AC system, in order to define the harmonic impedance sectors of the AC network seen from the existing 380 kV substations which either converter stations are connected to. The harmonic disturbance is a very sensible issue in Saudi Arabia, due to the presence of many SVCs in the 380 kV network.
The dynamic performance requirements of the link are demanding, taking into account the dynamic performance of the whole Saudi Arabian system on occasion of large perturbations, such as voltage recovery problems.From the environmental point of view, the converter stations are able to work properly at full power and even in overload conditions with an external ambient temperature up to 55°C, making the cooling of equipment a challenging issue keeping the losses at low values (within 0.7% in each converter station at 45°C ambient temperature). The valves are housed in valve halls and the HVDC equipment (switchgear, HV smoothing reactors, HV part of DC filters, etc.) are housed in DC halls.