Tenaga Nasional Berhad (TNB) worked in close cooperation with Electricity Generating Authority of Thailand (EGAT) to fulfill the power demand of the consumer in both of the countries. TNB-EGAT HVDC interconnection system is the first HVDC scheme in Malaysia that links the power system networks of the northern part of Malaysia and the southern part of Thailand. The TNB-EGAT HVDC system was established in 2001, using monopolar type and rated at 300 kV DC with maximum 300 MW transferred capacity. The converter station is located in Gurun substation in Malaysia and Khong Ngae (KNE) substation in Thailand, connected with 110 km DC overhead line. Due to different AC voltage systems between Malaysia and Thailand, the HVDC line is used where different AC voltage rates, 275kV in Malaysia and 230kV in Thailand are converted to the same DC voltage range of 300 kVDC. The HVDC system also developed in order to control the stability during the power transfer, the HVDC system will block the operation of the power transfer immediately if there is a trip occur on the power station.
For the TNB-EGAT HVDC system, special pole controls functions that are employed in the HVDC interconnection system such as power run-up and run-back control functions, dual frequency modulation function, frequency limit function, and transient current order controller has been designed. The purpose of Power Run-Up and Power Run-Back controller function to increase or decrease the DC power by the predefined amount of power and to improve the DC power stability of the connected AC system. After achieving the power increase or decrease, operation continues at this power level until the operator selects a new power reference value.
The Power System Computer Aided Design (PSCAD) software is the digital simulator which can be used in simulating a power system and its controls. It comes with a graphical user interface for sketching electrical equipment and provides a fast and flexible solution. This PSCAD simulator represents and solves differential equations of the entire power system and its control in the time domain. The PSCAD software will be use to design the controller in the time domain and to model the TNB-EGAT HVDC interconnection system.
1.2 Problem statements
The HVDC system is one of the alternative ways for Malaysia and Thailand in order to supply enough power consumption as the rapid growth in the economy for both countries has increased the demand for electric power. In addition, the special controller has been designed to control the HVDC system during the power transfer. The Power Run-up and Power Run-Back controller are to increase or decrease the HVDC power capacity according to the requirement of the operator of both stations. In this research, the proper design of the controller is develop for the TNB-EGAT HVDC link and simulate in the time domain(t) using PSCAD software. The poor design of the controller will cause an impulsive transient in the TNB-EGAT HVDC. Impulsive transients are sudden high peak events that fluctuate the voltage and/or current levels in either a positive or a negative direction that will cause the big impact in the HVDC system. Thus, it is important to discover consequences of the impulsive transient towards TNB-EGAT HVDC interconnection system. These impulsive transient events can be categorized further by the speed at which the fluctuation occurs. If the impulsive transient occurs, not only might cause the poor quality of voltage and current but if fluctuations are too great, there is the possibility that a device being supplied with power will malfunction or be reset. To implement a flexible system, the investigation on how to minimize the impact of the impulsive transient during power transfer need to be carried out. If these impulsive transient events do not eliminate in the system, it might cause the digital process in the controller to misinterpret the signal and will send the signal to the operator, the operator will stop the power transfer, the result is the shortage in the power capacity for the station occur.