Control in cogeneration islanding systems for Saudi Aramco Processing Facilities

Abstract

The Saudi Aramco Processing Facilities (SAPF) are located in an eastern province of Saudi Arabia. While the plant has three industrial cogeneration units, and is connected to the Saudi national grid via two transmission lines, there is a possibility that both transmission lines will be isolated due to storms or other disturbances. If that happens SAPF would be in an isolated, or islanded, mode of operation. The three units would need to be coordinated to supply the required internal plant power demand in all cases where there initially was export to, or import from, the grid. To achieve this, a model is required that will ensure effective transition from interconnected to islanding mode. When islanding schemes function properly, the plant can continue operating under its own power generation despite interruptions in electrical supply from the utility grid. Without the implementation of the islanding mode of operation, any disturbance in the grid could cause a blackout, severely disrupting the plant operations, causing revenue loss and gas flaring. A fully functioning islanding scheme refers to the capability of the plant generators to withstand any disturbance to the power system and continue supplying the plant load. Continual balancing of active power generated and consumed is vital for power system security and stability, and to maintain frequency within an acceptable tolerance around nominal system frequency. Due to the large size of individual generators (150 MW each unit, a total of 450 MW) with respect to total in plant load (170 MW), the loss of a generator in a small island system can cause a large power imbalance and consequently a significant frequency excursion. Achievement of the above goal requires creation of an accurate model for the SAPF cogeneration and electrical network that includes gas turbine, governor, generator and power system. A simple model, the six-parameter linearized model, and the full nonlinear model have been used to represent the synchronous generator in different studies. The governor model is part of the turbine model in gas turbines. The excitation system and the power system stabilizer models have been based on the IEEE Standard. Specific models have been simulated in MATLAB®, and open inclusion of the transmission lines has been done in ETAP®. Critical scenarios that may lead to total blackout of the SAPF have been simulated and analyzed. This thesis provides a justification of future use of special protection systems on the SAPF power system to provide successful transition from interconnection mode of operation to islanding mode of operation during major disturbances. The special protection systems proposed are capable of handling cases of high export to the grid, and also import from the grid. Recommendations and future work are summarized at the conclusion of the thesis.