By Conrad Oakey
When you push things into smaller and smaller spaces, they tend to get more and more complex. This is true both for electronics and the island-nation where so many electronics come from: Taiwan. The country is a study in efficiency. The city blocks created by avenues are further sliced by the many alleyways that convert every inch of the cities into housing and industry.
A typical aboveground FTU containing the AIM 6X6, mini-RTU, automated switches and modem. The final FTU before a fault will open its switch and allow the feeder to be re-energized within 20 seconds.
Accordingly, the power system that supports it all is similarly complicated, with many twists and turns both above and below ground.
A fault in such a dense, elaborate system can take hours of trial and error to locate and repair. This translates to hours of downtime for the homes and businesses along that faulted line. To correct this decidedly inefficient situation, Taiwan Power Co. proposed the implementation of a system capable of automatically detecting and isolating a fault on a feeder, and then restoring power to the feeder within 20 seconds of a fault.
A collaborative effort between Advanced Control Systems, Bitronics (now ALSTOM-Bitronics), Mitsubishi Electric Corp. (Japan), and Shihlin Electric Corp. (Taiwan) provided the answer. The new distribution automation/demand-side management (DA/DSM) system will be implemented first in the industrial Tai-Chung District, home of the third-largest city in the country. The application, which recently passed factory acceptance testing, utilizes high-speed measurement, communication, distributed processing and some formidable decision-making software to restore power within 20 seconds of fault detection.
Typically, when a fault occurs on a feeder, the breaker in the substation will open and the entire line will be de-energized until a crew can locate the fault, repair the line and restore power. The DA/DSM system installed in Tai-Chung improves on this scenario through use of intelligent nodes installed at line switch sites along the feeder. These feeder terminal unit (FTU) nodes are composed of an ACS mini-RTU, a three-phase automated switch, and an AIM6x6 (Analog Input Module; 2 bus, 6 Voltages and 6 Currents) custom power transducer, designed and manufactured by ALSTOM-Bitronics. One FTU is installed at each of 465 line switch sites that sectionalize 139 feeders. At each of the 16 substations, a high-speed network architecture RTU collects and concentrates data from the FTUs on feeders that the substation serves. This substation RTU communicates with FTUs by either conventional communications for overhead feeders or fiber links for underground feeders. All substation RTUs communicate with a system master station at the dispatch center by means of redundant communications networks.
When a fault occurs, the substation breaker will typically open within three to five cycles. Within that time, the AIM6x6 will have made several measurements (128 samples per cycle, RMS every quarter cycle). The AIM6x6 detects faults based on programmable current level and duration and sets targets indicating the faulted phases. The FTU records the fault indications from the AIM6x6 and transmits this to the substation RTU. If the breaker remains open after attempted auto-reclosing cycles, the substation RTU will immediately report the detected fault to the master station at the dispatch center. By this time, the feeder is de-energized, but not for long.
The master station uses the measurements from the FTUs to determine the section of the feeder in which the fault is located, just downstream from the last FTU to record the fault. The master then sends a command to the substation RTU to open switches on both sides of the fault, followed by a command to close the substation breaker. The feeder is then re-energized, restoring power to all customers short of the opened switch.
Perhaps the most remarkable part is that all of these actions occur within 20 seconds.
The system also allows the dispatcher to re-energize the section of the feeder that is downstream from the fault. The master station provides a recommendation for downstream power restoration within 20 seconds after the initial restoration of power upstream of the faulted section. Downstream power restoration is possible because, in many cases, a feeder terminates in one or more switches that can connect it to other feeders. Such switches are usually left open but can be closed to provide alternative paths for power sources. So when a fault has been isolated on both sides by open FTU switches, and after some computer modeling to determine system impact, the dispatcher can initiate supervisory control to close the terminal switch and provide power to all customers downstream of the fault as well. Since the FTUs measure voltage magnitude and phase angle on both sides of their switches, they also can act as permissive synch-check relays, ensuring that the re-closure is within tolerances.
By combining the SCADA information from the FTUs with geographically oriented facility maps, dispatchers are able to see graphical, real-time representations of the distribution system, and direct repair crews to a narrowly defined search area (between two line switches) to repair the feeder. Other integrated applications include capacitor bank control, trouble call analysis and a full array of distribution system analysis functions. Textual information, including alarm messages, event logs, and instructions, is presented in traditional Chinese characters, the native language of system dispatchers.
Despite the complexity of creating a real-time automated control system that speaks Mandarin, the project designers did have a few factors working in their favor. Taiwan Power in general prefers complete turnkey solutions, allowing the designers to select their preferred components. Because of the measurement and communication timing requirements of the system, interfacing with legacy equipment was necessarily side-stepped. The high-speed AIM6x6 unit is able to detect fault conditions before the substation breaker opens, a crucial piece of functionality unavailable in any other measurement device.
The end-result is an automated system that will reduce outage time by orders of magnitude and dramatically reduce time to repair, thereby increasing system reliability and customer satisfaction while reducing maintenance costs.
Conrad Oakey is a freelance technical writer and marketing consultant.