SEL, a worldwide leader in the design, manufacture, supply, and support of products and services for power system protection, control, monitoring, automation, and integration is playing a crucial part in a new 500 kV, 1276 km (793 mile) link connecting the northern and southern electric power systems in Brazil. The link allows the Brazilian power companies to share generating resources more efficiently and eliminates the need to add approximately 600 megawatts of generation capacity to the Brazilian power system, saving an estimated $15 per MWhr of generated power.
Protective relays --- transmission, breaker, and transformer protection equipment --- are an integral part of the interconnection project. SEL is supplying the main protective relays for the project, which includes three new substations, additions to five existing substations, and five transmission lines with capacitive series-compensation. Two of the transmission lines include thyristor control series capacitors (TCSC).
Sharing electrical resources is important in Brazil because hydropower generation fluctuates according to the different rainy seasons in the country's northern and southern regions. The interconnection project links the Amazon River in the North to the southern Sao Paulo/Rio de Janeiro region via new 500 kV lines. The electric utility companies --- Furnas in the South and Eletronorte in the North -- will now be able to easily exchange electric power without the expense of an additional 600 MW of power.
In 1997, Furnas and Eletronorte released a competitive bid for supply of protective relays for the new line link. SEL first learned of the project through their Spanish distributor, PENTEX. Power system protection focuses on detecting line faults and tripping the correct circuit breakers needed to isolate only the faulted portion of the system. Following fault isolation, the protection devices must then restore the system to preserve system stability and restore load flow. The chances of line faults are very high considering the amount of line exposure on the 793-mile Brazilian interconnection project.
The project was designed around five sets of parallel transmission lines with capacitive-series compensation for reduced line impedance. Two sets of these transmission lines incorporate thyristor-controlled series capacitors (TCSC). While these features allow more power to flow through the transmission lines and improve overall power system stability, there was a concern they might complicate the protection scheme. In addition, the relays also must provide single-pole tripping, which allows two phases to remain in service in the event of a single-phase fault.
As part of the bid, Furnas and Eletronorte insisted that the power system protection components chosen for the system undergo extensive testing and real-time simulations to ensure reliability. The testing centered around the SEL-321-1 numerical distance relay, a phase to ground relay containing all the protective elements and control logic to protect any overhead transmission line, including EHV, HV, and subtransmission lines.
The SEL factory in Pullman, Washington, operates a model power testing facility where relays are subjected to thousands of simulated fault conditions and other disturbances in order to prove their relays would function properly for this particular application. Protection engineers in Brazil also subjected the relay to additional real-time simulations at Furnas facilities in Rio De Janeiro.
According to Jorge Kotlarewski, chief protection engineer at Furnas, SEL testing proved proper operation of the relays. His findings were summarized in the following written conclusion:
"In simulations done in a real-time simulator in Furnas, Rio de Janeiro, and in [model power] simulations in Schweitzer's laboratories in Washington, the SEL-321 line protection had very good performance, with correct operation in all of six hundred cases simulated," declared Kotlarewski. "These results gave us the confidence to install this protection in such an important system."
In addition to ensuring correct operation of the relay, these tests were required to prove the relay could handle all of the special challenges associated with series-compensated lines and single-pole tripping.
Single-pole tripping is required to ensure that power still flows through the two remaining healthy phases in the event of a single-phase fault. This is critical because more than 90 percent of all faults only involve a single phase. By removing only the affected phase from service, the relay allows 67 percent of the power flow to continue uninterrupted which, most importantly, keeps the system synchronized.
The relay was also required to distinguish between internal and external faults, since unwanted tripping for an external fault can cause a major blackout. To accomplish this task, the relay measures the apparent negative- sequence impedance during the fault in order to determine the direction of the fault. This technique ensures that the distance relay only trips for internal faults.
Thyristor-controlled series capacitors provide a mechanism for controlling series compensation and, subsequently, maximum power flow. By adding a control mechanism, it becomes possible to increase the loading on existing transmission lines and rapidly readjust power flow in response to various contingencies, such as heavy load conditions. It is often necessary to increase the amount of series compensation to get more power flow through the line during periods of heavy load.
The thyristor-controlled series capacitor can also be useful as a means for damping power oscillations that may occur as a result of a disturbance to the system, better enabling the power system to remain in synchronous operation.
Engineers at Furnas and Eletronorte performed exhaustive commissioning tests on the system including two, real short-circuit tests, oscillations due to strong load rejections, and many other stressing conditions for the purpose of evaluating the thyristor-controlled series capacitor lines. The interconnection project began successful commercial operation in January 1999 after a record construction time of 14 months.
Since the system has been in commercial operation, several faults have occurred, during which the relays performed flawlessly, as expected.