Investigation of Constrained Vsc-hvdc Tie Lines for Transient Stability Improvement of Interconnected Power Systems

Abstract

Voltage Source Converter (VSC) – High Voltage Direct Current (HVDC) tie lines have become popular for the interconnection of power systems owing to their ability to independently control active and reactive powers. This attribute has been harnessed for transient stability improvement through the control of active or reactive powers. The supporting control area in interconnected power systems through the VSC-HVDC tie lines can modulate the DC power to support a faulted control area to recover stable conditions. However, it has been traditionally assumed that the supporting control area is an infinite system able to support the faulted control area without compromising its stable operating conditions. On the contrary, power systems have operational limits which must be respected. Violation of stable conditions compromises the system's integrity and at worst can cause a total system collapse. Providing the required regulating power beyond the ability of the supporting control area should be prevented to avoid unstable conditions. It is on this gap that this thesis has studied and simulated two cases of a Kundur two-area power system interconnected through a VSC-HVDC tie line. The first case has the traditional unconstrained VSC-HVDC tie line assuming an infinite system for the supporting control area. The second case has a constrained VSC-HVDC tie line that monitors the amount of the required regulating power and compares it with the system strength of the supporting area to avoid violating its stable conditions. The transient stability response of the two cases of a Kundur two-area system connected through a VSC-HVDC tie line was simulated in MATLAB/SIMULINK software. Transient faults were performed on the two systems and their transient responses were plotted under different fault scenarios. The first fault scenario resulted in the required regulating power within the ability of the supporting control area and the second fault scenario resulted in the required regulating power above the ability of the supporting control area. The simulation results showed that the unconstrained VSC-HVDC system exhibits unstable conditions with its frequency response and rotor speed deviation deteriorating significantly without recovering stable conditions. With the incorporation of the Inter-Area Frequency Supplementary Controller (IAFSC), the modulated link power is reset to assist in recovering stable conditions once the stability threshold limit is violated. The IAFSC monitors the frequency of the control areas in real-time to mitigate violation of stable conditions. It is armed with a modulation release signal logic dependent on the iv frequency operating condition of the supporting control area. When the stability threshold limit is violated, the controller blocks the power ramp-up and resets the power modulation to the initial power order. In severe fault conditions, the IAFSC controller achieved transient stability improvement in the supporting control area thus mitigating unstable conditions that could jeopardize its stability