By Wong Chung Hoo and Ab’Llah Salleh
Tenaga Nasional Berhad, Malaysia,
V. Kasemsap and S. Sakdhnagool
B. J. Bisewski
Teshmont Consultants Inc., Canada
Despite the economic standstill that has hindered Asia, plans to reinforce a power interconnection link that can transfer 300 MW between Thailand and Malaysia have progressed. PEi catches up with the latest developments.
It has long been recognized that an electric power network interconnection between Thailand and Malaysia would be of benefit to each country. Interconnection of the two countries has been in place since 1982. This initial 132/115 kV AC interconnection is capable of a power transfer of 80 MW. However, the line is too weak to tie the two systems for synchronized operation. Import or export is achieved by isolating part of either network and connecting the isolated part as a block load to the Thailand or Malaysian side. The resulting interconnection did offer the benefit of transfer of energy up to 80 MW but was not very flexible operationally and did not allow for the two-way transfer of energy.
The Gurun converter station on the Malaysian side
In 1988, the two utilities, Tenaga Nasional Berhad, Malaysia (TNB) and Electricity Generating Authority of Thailand (EGAT) conducted a joint feasibility study for upgrading the existing interconnection through the ASEAN economic corporation platform. The study recommended a HVDC (High Voltage Direct Current) interconnection scheme which will resolve system stability problems as well as offer potential benefits in terms of sharing spinning reserve and conducting economic and emergency exchanges between the two utilities.
These studies showed that the lowest cost alternative would be a monopolar back-to-back HVDC transmission system with an AC line interconnection to each system. The disadvantage of this configuration is that the HVDC facility would have to be wholly owned and financed by one country (i.e. the country in which it was built). Therefore, a decision was taken to separate the two halves of the back-to-back station and build a converter station in each country connected by an HVDC transmission line. The siting of the stations was reviewed and it was decided that they be located at Gurun on the Malaysian side and Khlong Ngae on the Thailand side.
The new HVDC link has reinforced the interconnection between Malaysia and Thailand
The HVDC system will allow the deferment of additional generation in each country, which would be needed purely to meet peak loads. There is diversity in time between the peak loads in Malaysia and Thailand due to the different load patterns and also the one-hour time difference that exists. The differences in the loads means that the DC system can be used to offset peak loads in one country then reverse power and help offset peak loads in the other country. It is considered that the DC system will be used to supply peak demand for four hours each day in each direction of power flow. The net energy exchange on a day-to-day basis in this mode of operation would be nil.
The two converter stations are equipped with sufficient AC filters and shunt capacitors to allow for continuous operation at the ten-minute overload rating of 450 MW. Thus, under normal circumstances with operation at 300 MW or less there will be a surplus of capacitive reactive compensation which can be used to support the AC system voltage if required. The AC filters and shunt capacitors are under the control of an automatic reactive power control system, which can control the AC bus voltage, or the amount of reactive power exchanged between the converter station and the AC system. The AC filters and shunt capacitors can be connected even if the HVDC transmission system is not in service.
The nominal rating of the interconnection is 300 MW in both directions of power flow. There is a ten minute overload capability of up to 450 MW, which may be utilized once per day when all redundant cooling equipment is in service. At overloads of less than 450 MW it is possible to operate for longer than ten minutes due to the inherent capability of the equipment. The control system includes a function which monitors the level of overload to ensure that the full overload capability of the equipment will be available to enhance the system performance while at the same time ensuring that no equipment will be thermally overstressed from operation at higher than rated load. The overload capability will be used primarily for spinning reserve sharing, emergency assistance and if needed when operating in emergency frequency control.
In the event that either converter station or a portion of the AC system becomes isolated from the main part of the AC system, the frequency of the isolated system may drift due to the absence of a frequency setting station or imbalance between the generation and load. To help ensure the continued operation of the isolated AC system the HVDC system contains a frequency-limiting controller which becomes active if the frequency in either system goes outside a pre-set frequency error of ±0.5 Hz.
The interconnection project continued despite economic woes
The frequency limiter acts to increase or reduce DC power transfer to the extent possible of the equipment and maintain the AC system frequency at the upper or lower extent of the deadband. To enable resynchronization with the main part of the AC system the operator can manually adjust each of the deadband limits independently to bring the frequency of the isolated system in line with the frequency of the main part of the AC system.
The DC system response times for recovery from AC and DC system faults to 90 per cent of pre-disturbance power transfer levels were specified based on EGAT and TNB system data and past experience. For the interconnection, the point of import/export of energy is defined to be the Thailand-Malaysia border. The DC line is approximately 110 km long and the border is around 24 km from Khlong Ngae converter station and 86 km from Gurun converter station. For revenue metering on the Thailand side it is necessary to estimate to the best possible accuracy the losses in Thailand and then either subtract them from the power exported to Malaysia or add them to the power imported from Malaysia to determine the energy transfer at the border. Revenue metering on the Malaysian side requires similar calculations.
The HVDC converter station uses one 12-pulse thyristor valve group to perform the conversion from AC to DC and vice versa. The valve group multiple valve units (MVU) are designed as quadruple valves with associated components such as valve cooling system, damping circuit, firing electronics and other mechanical and structural items. Each single MVU is suspended from the valve hall roof, insulated to ground.
The main design features of the valves are:
•Direct cooling of all components in the valves, making possible a very compact valve structure.
•Optoelectronics firing and monitoring system from the ground to thyristor potential and vice versa without intermediate electronics.
•Monitoring of the status of all thyristor levels during operation.
•Protective firing of the thyristor as back-up firing.
The thyristors are of 100 mm diameter and there are 4×48 thyristors per quadruple valve.
Civil work and AC substation design was carried out locally in Thailand and Malaysia at the respective contractors’ offices in each country. Type tests on major equipment such as thyristor valves, converter transformers, smoothing reactors and surge arresters and the combined factory system test on the HVDC controls were witnessed by the consultant on behalf of EGAT and TNB. Where possible, TNB and EGAT representatives also witnessed the tests.
Technology transfer and training in HVDC technology are important aspects of this project in both Thailand and Malaysia. EGAT and TNB engineers worked closely with the consultant. There has also been cooperation between the consultant, EGAT’s and TNB’s engineers during the design, installation and commissioning phases.
There was a delay in completion of the project principally because of delay in delivery of converter transformers and because of problems with design and testing experienced at the transformer manufacturing plant. The converter transformers had to be remanufactured. Despite this, the Malaysia-Thailand interconnection was commissioned in October 2001, and has since exchanged energy on an emergency basis. Economic power exchange is now carried out on a commercial basis. So far, Malaysia has imported about 40 GWh and exported 20 GWh.
The interconnection project proceeded in spite of the economic downturn in Asia. This project is a major international interconnection and is of importance for the development of an ASEAN grid. It will enhance the performance of the electric power networks in each country as well as opening the door for commercial transactions between the respective utilities and/or power generators and consumers in the two countries.
The project was successfully implemented by awarding separate contracts to a common HVDC supplier in each country. This has permitted the use of local utility construction and project management practices and styles in each of the two countries.
In terms of technology, both utilities were able to acquire new experience in the field of HVDC technology during project implementation. Regular system studies and meetings took place between TNB and EGAT to prepare for the operational and commercial arrangements so as to exploit the full benefits of the interconnection.
Other links currently being studied in the region are plans for interconnections between Thailand-Cambodia, Thailand-Vietnam and Thailand-Laos along with Sabah-Sarawak, Brunei and Sarawak-West Kalimantan on Borneo.