While Korea's transmission systems include 765-, 345- and 154-kV design voltages, the Korea Electric Power Corp. (KEPCO) has increasingly invested in the construction of 345-kV and 154-kV transmission lines to satisfy the rapid growth in demand. This large interconnected, complex transmission system exhibits a power-system reliability superior to radial systems; however, the increased fault-current level is a major disadvantage. As a result, the fault level at several of KEPCO's substations exceeds the short-circuit rating of the installed circuit breakers, requiring operational changes in areas such as open bus bars and transmission lines to limit the fault-current level.

To maintain the network design and its superior reliability, the Korea Electric Power Research Institute (KEPRI) developed the Automatic Power Restoration System (APRS) to alleviate the fault-current overloads on KEPCO's 154-kV transmission system.

Automatic Restoration System

The APRS performs actions or tasks for the automatic restoration of the 154-kV system in the event of a local disturbance. With the APRS installed, the faulty section is identified and separated from the system, and the normal operational mode is restored within a specified time period. While the developed APRS comprises one main system and four Data Acquisition Units (DAU), this system can be extended to include up to eight DAUs.

Each of the DAUs is installed in a 154-kV substation to acquire system information on the circuit breaker, disconnector switch status, protective relay operation, real-time 154-kV bus voltages, transmission line and main transformer power flows (MW), then transmit the data to the main system. The software in the main system automatically identifies the faulty section based on the change in circuit-breaker status, which results in the APRS rastoring the disconnected section.

APRS

The photos above show the APRS main system and the operator console displaying the data transmitted from the DAU for the four 154-kV substations — Banwel, Seoseoul, Ildong and Shihwa — to the main system. The DAU comprises Communication and I/O Control CPU Board, Analogue Input Board, Digital Input Board, Digital Output Board and Communication Modem (Channel Service Unit).

Installation and Operation

• Circuit-breaker ratings. In general, KEPCO has installed two types of circuit breakers in 154-kV substations having short-circuit ratings of 31.5 kA and 50 kA. Some 48 units rated at 31.5 kA are installed on 18 sections of the system where the fault level exceeds the circuit breaker rating. It would cost KEPCO US$5.76 million (US$120,000 per unit) to replace these units at present with 50 kA circuit breakers, and at the 154-kV system interconnection, 50 kA circuit breakers may not suffice. Therefore, KEPCO chose to apply the APRS as an economical alternative.

• The application area of APRS and its characteristic. The system simulated on page 60 shows the 154-kV system connected to Shinshiheung and Seoseoul 345-kV substations. KEPRI installed the APRS main system unit in Seoseoul Substation and DAU units in four 154-kV substations, which are connected via 1.544 MBps optical communication lines to improve the data transmission reliability. Banwel Substation is at the center of the selected 154-kV system, which, due to the system fault level, has a choice of three operational modes to reduce the available fault current: Shiwel transmission lines circuit breakers open; Banwel bus coupler open; or Banwel transmission lines circuit breakers open.

  The selected operational mode is to operate with the Shiwel transmission line circuit breakers open and the complete system remains firm.

• The APRS operation. In the event of a transmission line fault occurring between Ildong and Banwel substations, the latter substation will be blacked out (off supply). The APRS detects the fault and the location on the basis of information supplied by Banwel circuit breaker and protection-relay operations. To restore power to Banwel Substation, the Shiwel transmission line circuit breaker is closed within 10 seconds.

APRS Restoration Procedures

Step 1	Acquire operational status from the four DAUs.
Step 2	Identify the faulty section based on the circuit-breaker status and protective-relay operations.
Step 3	Isolate the faulted section from the healthy section of the 154-kV system.
Step 4	Restore the isolated section. If transmission line overloading is anticipated, perform a partial system restoration.

System Validation of APRS

During the development stages, KEPRI undertook a series of simulation studies to prove the performance on the APRS. The installation and field testing of Korea's first APRS at Banwel Substation was successfully completed on Dec. 26, 2000. The isolated area was automatically restored to normal state operation within 10 seconds. The customer outage duration probably would have been up to 10 minutes if an operator had made this restoration. Further testing of the APRS installed at Banwel has shown that it is capable of performing quick restoration of blackout areas such as Ildong, Banwel and Ahnsan.

Benefits of the APRS

The rapid increase in power demand and expansion of the 154-kV transmission system has resulted in a complex loop network with unbalanced load flows and fault levels that exceed the ratings of the controlling circuit breakers. The APRS will improve the reliability of KEPCO's 154-kV transmission system, and the US$280,000 invested in the development of the APRS is set to bring KEPCO significant savings as the number of APRS installations continues to increase, deferring the need to replace inadequate 154-kV circuit breakers.

Jin-Boo Choo attended Seoul National University and received the BS, MS and PhD degrees in electrical engineering in 1977, 1987 and 1994, respectively. As project leader, Choo has been involved with research projects on power system operation control and analysis. Currently, Choo is a group leader at the Power Systems Laboratory of KEPRI, located in Daejon, responsible for the power-system planning, operation and control, and FACTS technology.

Jeong-Boo Kim received the BS, MS and PhD degrees in electrical engineering from Seoul National University in 1986, 1989 and 1993, respectively. Kim, who joined KEPCO in 1971, is a KEPRI project leader in the R&D division, responsible for studies on insulation coordination and corona and field effects of transmission lines. Kim is a member of CIGR… Study Committee 22.

Soon-Chun Hong earned the MS degree in electrical engineering from Chungnam University, Taejon in 2001 and is the project leader for the development and installation of KEPCO's APRS.

Tae-Kyun Kim received the BS, MS and PhD degrees from Hanyang University, Seoul. Kim has been responsible for several research projects in the field of power system operation, control and analysis.