During the three-day period from the July 26 to July 28, 2011, the city of Seoul in South Korea experienced heavy rainfall amounting to some 500 mm (20 inches), which represented 40% of the city's annual average rainfall. Meteorological records confirmed this was the heaviest rainfall over a period of three days since records began in 1903.
In Seoul, the Korea Electric Power Corp. (KEPCO) has four distribution control centers (DCC) that monitor and control 7,562 automatic switches, controlling 1,499 distribution feeders, in an area supplying 3.18 million customers. The rainfall on July 27 caused a total of 36 faults on the Seoul distribution network. However, the DCCs were able to isolate the fault sections and restore power to the healthy sections within 3 minutes of receiving notification of each fault by using KEPCO's distribution automation system (DAS). Among the 36 faults, 24 were processed within 3 minutes and eight within 10 minutes. The remaining four faults could not be handled within 10 minutes because the rainfall had damaged both the communications and distribution lines.
Network Performance During the Rainfall
Early in the morning on July 27, three fault-alarming indicators appeared simultaneously on the monitoring screen in the Gangnam DCC in Seoul. Triggered by faults, the current transformers installed in the switchgear on the source side of the fault position detected the fault currents, and the remote terminal units (RTUs) in the switchgear instantly sent messages to the DCC. Also included in the information from the fault-alarming indicators was the real-time load current on those feeders.
Once the operators in the DCC confirmed the faulty sections of the network, the remote-control function started to open the adjacent switchgear to isolate the faulty sections and close the normally open switchgear to restore power to the healthy sections of each feeder. This entire process was completed within 3 minutes on average, and KEPCO repeated this process for the rest of the faults throughout the day, thereby minimizing the number of customers experiencing supply interruptions.
Additionally, if this event had not been a disaster but a regular situation, KEPCO could have used the self-healing function that has a predetermined automatic sequence, and the complete restoration process might have been completed in an even shorter time.
Distribution Automation System Background
According to the economic growth in Korea, even a momentary fault on a distribution feeder can bring about a huge loss to connected customers. Considering the ever-increasing customer expectations for network reliability and power quality, the development of DAS was necessary for KEPCO. Originally, the concept of monitoring and operating switchgear in a distant site was quite sensational to engineers who were familiar with the old-fashioned manual process. The rapid expansion of qualified communications infrastructure in the 1990s turned this concept into a reality.
DAS has quickly improved KEPCO's system average interruption duration index (SAIDI) by reducing recovery time. Time spent in dispatching linemen to the fault location, manually switching on and off for isolation of the faulty section, and power restoration, which used to take more than 50 minutes, was reduced to just a few minutes. Considering the excessive travel time due to heavy traffic, the effect of DAS can be evaluated higher in urban areas like Seoul than in rural areas.
The DAS has three main features:
Remote monitoring and controlling of field devices, collecting real-time data (voltage, current) and the resetting of the minimum pick-up current of reclosers
Fault-handling for single and multi faults and load balancing with load transfer for scheduled outages
Optimal feeder reconfiguration for loss minimization and load balancing, detection of unbalanced phase current, calculation of short-circuit current and voltage drop calculation, automatic generation of single-line diagrams and a simulator for training purposes.
In the bigger picture, DAS consists of the DCC, automatic switchgear on the network and the remote communication system that links them together. DAS communication equipment transmits commands from the DCC to the automatic switchgear from which they receive data. In Korea, with optical fiber as the main medium, various wireless communication systems, such as digital trunked radio system and code division multiple access, are used for the DAS. However, even in foreign countries with different wireless systems, such as general packet radio service and global system for mobiles, the DAS has proven to be adaptable, as demonstrated by several pilot projects in China, Indonesia, Vietnam and Egypt.
State of Implementation
Initiated in 1983, KEPCO's DAS went through the development process and was finally adopted on real distribution networks in 1998. Since then, KEPCO has constantly upgraded the DAS, and now it is completely new. While KEPCO has been mainly focused on DAS software, many domestic manufacturers like ENTEC, Jin Kwang E&C Corp., Iljin Electric and KDN have contributed to the development of DAS field devices such as overhead switchgear, ring main units and RTUs, respectively. The total investment on DAS was some US$407 million. For more efficient DAS operation, KEPCO constructed 41 DCCs where operators could monitor and operate networks served by several branch offices simultaneously.
In 1996, KEPCO's SAIDI was 31 minutes. In 2010, 14 years later, it was reduced to 15.2 minutes — a figure comparable to that of top-level utilities in the world. Before DAS was introduced, it took 54 minutes on average for linemen to travel to the fault location and isolate the faulty section and even longer in urban areas because of heavy traffic. Nowadays, it takes just 3 minutes on average, a significant improvement in network reliability.
A utility's investment in new facilities increases in step with an increase in power consumption, which follows the economic development of a society. With the network load data acquired from DAS, KEPCO can decide exactly how much it needs to invest in the construction of new substations and feeders. Supportive data such as load current, real-time load characteristics and fault history of circuits helps engineers to make more accurate load density maps and predict future demand. On this basis, substations can be constructed in the right locations and the network reliability will be further improved.
Loss minimization is a key function of the DAS. Utilities have to make decisions on the interconnection of distribution lines, taking into consideration the daily and seasonal load variations to maintain system security. DAS provides the tools to relocate connecting points on the distribution network to optimize the feeder load and minimize losses by calculating the sectional loads and circuit impedances. For example, if feeder A is loaded with 5 MW and the connected feeder B is loaded with 7 MW, the loss minimization function would shift the connecting point of these two lines to the position on the network where it would minimize the total loss of both feeders. If expanded to the network level, this function can decide the right positions of the connecting points that will result in minimum power loss of the network.
Usually the social cost of power interruptions is heavily dependent on a society's economic level. The greater a society grows economically, the more rapidly the cost of a fault interruption to the society increases. For KEPCO, this is no longer a major problem because the DAS dramatically reduces the duration of fault interruptions.
A reduction in field work makes it possible for utilities to process routine work with fewer personnel. By replacing manual activities like facility monitoring and fault handling with an automatic system, the DAS has improved the utility's overall efficiency, which can lead to significant savings in the expenditure on resources.
The DAS provides many additional benefits including an increase in energy sales because of improved power quality and balanced electrical loads on distribution feeders. The quality of services for customers is improved because of a decline in the number of complaints, and the safety of field workers can be ensured as a result of the remote control of the switchgear. Furthermore, the DAS offers utility employees the opportunity to upgrade their skills with new technologies.
Sung Hwan Bae ([email protected]) received a BSEE degree from Konkuk University and a master's degree in electrical and computer engineering from Union Graduate College in the U.S. in 1990 and a Ph.D in public policy and information technology from Seoul National University of Technology in Korea. Since 1979, he has worked for the Korea Electric Power Corp., where he is currently head of Seoul District Division as a vice president. He is a senior member of the Korea Institute of Electrical Engineers.
ENTEC | www.entecene.co.kr
Iljin Electric | www.iljinelectric.com
Jin Kwang E&C Corp.
KDN | www.kdn.com
Korea Electric Power Corp.