Commissioning the Fire Suppression System for 500/230 kv Transformers of EGAT s Nongchok Substation S. Chaipitak, Electricity Generating Authority of Thailand, +6624362432, Chaipitak.som@egat.co.th ABSTRACT EGAT s Nongchok substation is one of the largest substations in Thailand, as it transmits electrical power from outbound power plants and supplies to metropolitan area where most of the country s industry and economy located in. Fire suppression with liner heat detector and foam-water spray system is installed in order to protect six single-phase 500/230-22 kv 200 MVA oil-filled transformers. Before the commissioning this fire suppression system, problems are found as the transformers are not de-energized before the foam-water spray system is activated and when the foam-water spraying, high possibility flammable spots of the transformers are not covered in the nozzles spraying area. This paper represents the problem found in the commissioning process which mainly about essential flammable spots which the oil-filled transformer s spraying area of the fire suppression system should be focused on. KEYWORDS: fire suppression system, fire protection system, automatic fixed foam, foam-water, transformer 1. Introduction When an oil-filled transformer catches on fire either by internal or external fault, the fire should be extinguished as soon as possible as it will spread and cause severe damage to the transformer. The damaged transformer may take from several weeks to months to replace and re-connect to the grid which depends on spare transformers available. Moreover, in the grid network reliability aspect, if this occurs to high-load utilization transformers, the grid will be weakened and may not support the N-1 scheme. EGAT s Nongchock substation is one of the largest transmission substations in Thailand. It consists of four voltage levels; 500 kv, 230 kv, 115 kv and 22 kv. There are sixteen oil-filled power transformers in total four are 230/115-22 kv (200 and 300 MVA) three-phase transformers and other twelve are 500/230-22 kv (3x200 MVA) single-phase transformers. In order to prevent the severe damaged that may cause by fire, six single-phase 500/230-22 kv 200 MVA transformers have been installed with a fire suppression system called an Automatic fixed foam system in the beginning of 2016. This system uses 3% concentrated foam-water solution to suppress the fire. Figure 1: Six single-phase 500/230-22 kv 200 MVA transformers Installed with the automatic fixed foam fire suppression system at EGAT s Nongchok substation 1
However, in order to maximize the fire suppression to full effectiveness, there are two recommendations in the installation of the automatic fixed foam fire suppression system that EGAT found during commissioning. Transformers are not de-energized before the foam-water spray system is activated and high possibility flammable spots of the transformers are not covered in the nozzles spraying area. 2. Commissioning the Fire Suppression System 2.1 The automatic fixed foam fire suppression system The automatic fixed foam fire suppression system consists of three main parts which are: 1) Foam and water supply and storage system this system consists of a diesel fire pump and an electrical jockey pump with a water storage tank and a concentrated foam bladder tank. In readyto-operate stage, the jockey pump is used to maintain the water pressure inside a fire suppression supply line (145-160 psi) if the pressure drops below a setting valve (causes from minor leakage or evaporation) the pump will start in order to make the water pressure back to working value. When the system operates, pressure will suddenly drop and the diesel fire pump will start operate and pump water from the storage tank to suppress fire at the transformer. Before reaching the transformer, water will pass through the foam-water bladder tank where a foam-water mixer is located. The mixer will mix the concentrated foam and water at a fixed ratio of 3% foam and 97% water. a) b) c) d) Figure 2: Foam and water supply and storage system; a) Diesel fire pump, b) Jockey pump, c) Water storage tank, d) Concentrated foam bladder tank 2) Fire detector and evaluation unit a unit consists of closed-loop copper tubes which are used for heat detection and an evaluation unit which is used to automatically send and trigger an operation of foam-water releasing system. When air pressure inside the closed-loop copper tube is increasing, which caused by rising temperature, an evaluation unit will evaluate a rate of temperature rise. If the rate of rise meets with the conditions, the unit will send an alarm to a fire protection panel. The panel will send the command to open a solenoid valve at a deluge valve in order to spray a foam-water solution. 2
a) b) Figure 3: Fire detector and evaluation unit; a) Evaluation unit connected with closed-loop copper tube, b) Fire control panel 3) Foam-water releasing system consists of a deluge valve and nozzles are used to spray a foamwater solution to suppress fire at the transformers. When a fire control panel commands a solenoid valve of a deluge valve to open, a diaphragm inside the deluge valve will be depressurized and opened, releasing foam-water solution through nozzles. The diaphragm can also be opened manually by manual station located at the deluge valve. 2.2 Commissioning process a) b) Figure 4: Foam-water releasing system a) deluge valves, b) spraying nozzles In a commissioning process, EGAT s commissioning team had found two major problems during the transformer spray test (the conducted test using only water in order to monitor a spray pattern of the fire suppression system) that should be put into a consideration. The first major problem found when the fire suppression system is activated as the evaluation unit sent the alarm and the fire control panel opened the solenoid valve to spray foam-water solution through nozzles, the transformer is still energized. This seemed to be normal for water spraying; however, for 3% foam concentrated solution might not. The more foam concentrated solution, the more conductivities the solution will have as shown in Figure 5 (at 3% foam concentrated, the solution s conductivity is approximately 710 µs/cm) Figure 5: The conductivities of foam-water concentrated solution at 1% (540 µs/cm), 3% (710 µs/cm), And 6% (1310 µs/cm), 3
From the measured conductivity result, it is shown that if the foam-water concentrated solution is sprayed while the transformer is energizing, there is a possibility that the transformer will break down and cause even more severe damages. It is recommended that the transformer should be de-energized prior to the foamwater spraying via a transformer self protection relay contact. The second major problem found when the foam-water solution is sprayed. High possibility flammable spots of the transformers are not covered in the nozzles spraying area (shown in Figure 6 and 7). The considerable spots are shown in Figure 8: 1) The pocket of the high side and low side bushings, and tertiary bushing pockets of transformers bushing have high flammable possibility as they are located higher from top of the transformer and separate the high voltage part and transformer s main tank. When the foam-water solution is sprayed, the solution might not cover the fire as it flows from top to bottom. 2) Diverter switch s pits of the on-load tap changer (OLTC) During and OLTC operation, there will be an arc occurring inside the diverter switch s pit if the arc quenching equipment is not in a good condition, there is a flammable possibility. 3) Manholes, oil pipes, and cables if fire breaks out, it will spread along oil pipes, manholes and cables. All of these spots are located on top of the transformer. However, if there is any leakage of the transformer tank, the fire will also spread at the transformer s bottom. Fire suppression pipe and nozzles layout design should consider these issues. Figure 6: High possibility flammable spots of the transformers at EGAT s Nongchok substation are not covered in the nozzles spraying area 4
Figure 7: Spraying layout design does not cover the high possibility flammable spots (Highlighted in red) Figure 8: Transformers high possibility flammable spots 5
3. Results Figure 9: A transformer on fire at a Salt River Project electrical substation on June, 1 st 2016. Retrieved from: http://www.fox10phoenix.com/news/150957887-story From the design layout of the EGAT s Nongchok substation fire suppression system, the commissioning team has found two major problems which are: 1. Transformer is not automatically de-energized before a foam-water solution is sprayed. 2. The nozzles spraying area is not covered high possibility flammable spots The first major problem may cause severe damages to the transformer if the fire suppression system is activated by either malfunction or manually operated by the operator. It is suggested that for system that using a conductive foam-water solution, should automatically de-energize the transformer prior spraying to prevent this problem. The second problem can be fixed by adjusting the nozzle s spraying angle as the most of high possibility flammable spots are located on top of the transformer, however, it is recommended to redesign the layout at the first hand. 4. Conclusion When the fire suppression system is triggered to operate, the catching fire transformer should be deenergized by the transformer s protection. However, in case of the malfunction or manually operation of the fire suppression systems whether there is fire or not, the transformers should be automatically de-energized in order to protect the transformer s equipment. As the fire suppression system is activated, foam-water enters the pipe line and sprays on the transformers through nozzles. Foam-water spraying should be designed for two functions which are: 1) Cooling down the transformers. 2) Covering the top of the transformers or oil to prevent combustion and separate oxygen from oil. The high possibility flammable spots of the transformers should be also considered in the design of the spraying layout. The high possibility flammable spots of the oil-filled transformer are: 1) The pockets of the high side and low side bushings, and tertiary bushing. 2) Diverter switch s pits of the on-load tap changer (OLTC). 3) Manholes, oil pipes, and cables. Fire suppression system is a need in preventing the spreading damage that may cause the severe damage to the transformer. Effective system should be carefully design, especially the spraying layout area of the foamwater. High side, low side, and tertiary bushing including diverter switch of the OLTC should be put into a design consideration as these are high possibility flammable spots of the oil-filled transformer. 6
5. Reference Associated Press. (2016). Substation fire sends smoke over west side of Phoenix area. Retrieved from http://www.fox10phoenix.com/news/150957887-story 7