In cases of very long distance ac transmission, including interconnection of separate power systems between regions or between countries, a number of power transmission issues have to be addressed. The main area of concern is the maintenance of synchronism as well as a stable voltage, particularly in the event of major system faults.
However, with series compensation, the viable distances of ac power transmission become sufficiently long in practice almost to nullify the question of distance as a limiting factor in the majority of cases. In fact, series compensated ac power corridors transmitting bulk power over distances of more than 1000 km are now a reality.
The benefits of power system interconnection are well established. It enables the participating parties to share the benefits of large power systems, such as optimization of power generation, utilization of differences in load profiles and pooling of reserve capacity. There can also be environmental benefits, such as when surplus of clean hydro resources from one region can help to replace polluting fossil-fuelled generation in another.
There are two main power systems in Brazil at present which are not interconnected, the North System and the South System. These are mainly hydroelectric, and account for more than 95 per cent of the nation’s total volume of power generation and consumption. Feasibility studies have been performed regarding an interconnection of the two systems, and a decision has been made to go ahead and build the transmission corridor.
Both ac and dc alternatives were assessed, and the ac option was chosen. It comprises a single 500 kV compact circuit (to be doubled at a later stage), more than 1000 km long and series compensated in several places along the line. The power transmission capability of the corridor will be 1300 MW and operation is scheduled to start at the end of this year.
The purpose of the interconnection will be to exploit the hydrologic diversity between the systems, so power flow will occur in both directions, depending on prevailing conditions. It will also help to reduce the risk of energy shortages at a time when the country’s power demand is growing rapidly, as well as reducing the amount of spinning reserve needed.
The ac option is attractive because it will make inexpensive hydro energy available to a rapidly growing economy, as well as bringing power to areas with a high potential for economic growth. To meet future demand, several hydroelectric plants are expected to be built along the same route in the coming two decades.
ABB Power Systems is installing and commissioning a total of six 500 kV series capacitors for the Eletronorte of Brazil project. Five are fixed and one is thyristor-controlled. In all, ABB will supply around 1100 MVAr of series capacitors.
TCSC for POD
The ABB Power Systems TCSC in the North-South Interconnection, the first of its kind to be installed in Latin America, will be located at the Imperatriz substation at the northern end of the interconnection. It will be used to damp low frequency inter-area power oscillations between the power systems on either side of the interconnection. These 0.2 Hz oscillations would otherwise constitute a hazard to power system stability.
Unlike the situation at Stöde, the TCSC at Imperatriz is not installed to avoid SSR but to increase the damping of power oscillations between the two systems. The Imperatriz TCSC has therefore been designed to handle conditions at the 60 Hz power frequency, and it will operate solely in the capacitive reactance range.
As the application is POD in this case, the boost level, defined as the ratio between the virtual reactance of the series capacitor and the physical capacitor reactance (XTCSC/ XC), is a key factor. It is a measure of the amount by which the reactance of the series capacitor can be virtually augmented in order to counteract system power oscillations.
The boost level can be varied continuously between 1 and 3. Expressed in terms of degree of compensation, k can be controlled over a range between 5 and 15 per cent. At rated line current, the nominal boost level has been set to 1.20.
TablesImperatriz TCSC technical data