Voltage management system using hybrid voltage control to enhance voltage stability in Jeju power system

Voltage and reactive power control in power systems today is, in many counties, one of the major concerns in power system operations due to voltage instability and collapse. The problems are not an exception in the Korean power system grid. The Jeju power grid is characterized as follows: The system is a sub-system connected with the land systems of the Korean power grid through HVDC lines. With the aim of an economic and environmental operation, generators in the land system supply active power which contributes to half of the Jeju load demands. Thus, the power system can experience a black-out caused by a component failure such as the interconnection system or a generator on Jeju Island. In order to overcome these problems in the electrical utilities an Under Frequency Load Shedding scheme has been implemented. Another big problem is generator failure. If a generator is tripped in the system, to meet the loss of active power, the HVDC lines will transfer the active power to the island system. However, since the HVDC lines are current-based, they will absorb the reactive power in proportion to the increased active power. This makes the system react more severely from a voltage stability point of view. A constrained generator scheduling scheme is applied to the Jeju power system in order to control this problem. This paper introduces a preliminary work to install a Voltage Management System (VMS) in the Jeju power grid. The Jeju power system is modelled in the test-bed using a Real-Time Distal Simulator. The Data Base Server accumulates the test system data for the control algorithm in the VMS Server. The VMS Server maintains the main control algorithm proposed in this paper. The VMS consists of two parts. The first part regulates the voltage of the pilot bus of the power system by using the generators. The pilot bus voltage represents a level of the power system voltages. It can be maintained by using a sensitivity analysis with respect to the voltage and the reactive power. In addition, the controlled generators are selected by sensitivity between the pilot bus and generators. The second part controls the reactive power reserve of the generators by controlling the switched shunt devices and the ULTCs. The reference voltage of the pilot bus and the reference reactive power reserve of the generators are calculated by using optimal reactive power flow with the objective function of minimizing the active power loss. The first method is called the Continuous Voltage Control (CVC) and the second is called the Discrete Voltage Control (DVC) in this paper. The VMS Server coordinates the CVC and the DVC in a real-time closed loop. It makes the power system stable through the control of the reactive power reserve to solve the voltage and reactive power problems found in Jeju.