Kil-Young Ahn

Application of fault current limiter in 22.9 kV KEPCO grid

Watertight 25.8 kV/600 A/12.5 kA fault current limiter (FCL) was developed to realize the protection coordination by limiting fault current at 22.9 kV underground distribution systems. In order to maximize an effect of FLC, it is good to install the FCL near to the substation. This FCL is made of watertight structure with rupture disk to prepare for rainy season, and composed of fast fault detector (FFD), mechanical fast switch (FS) and current limiting resistor (CLR). When fault occurs between FCL and downstream protection device in distribution line, the fault is detected by FFD and the fault current is changing its direction from FS to the CLR. If the fault is resolved by downstream protection device such as circuit breaker (CB), the load current flows to the FS again. The limitation of fault current by FCL is perfectly completed within 1 cycle from the moment of fault occurrence. The general technical specification of Korea Electric Power Corporation (KEPCO) for FCL was established, and the performance of FCL was verified by type test and field test. Recently, five FCLs have been successfully installed in KEPCO power grid for the purpose of commercial demonstrations. The distribution lines installed with FCL were analyzed focusing on the protection coordination by over current relays (OCRs) in comparison with before and after application of FCL. And also, we estimate and propose the optimal setting value of the relays.

Analysis and experimental study for current break systems in a load break switch of RMU

Ring Main Unit(RMU) with a load break switch is designed to meet the growing requirements for oil - free, maintenance free, long life, maximum safety and future feeder automation application. SF6 gases as insulating and arc-quenching medium ensure the positive breaking of small current, main active load current, cable charging current, magnetizing current etc. The closing and opening motion of the switch is completed by a rotating shaft within several tens of milliseconds. Therefore, it is very difficult for product developer to observe the arc phenomena caused by the magnetic forces between two contacts. In this paper, to determine the characteristics of magnetic forces applied in the system of load break switch, electromagnetic analysis is performed on the load break switch contained permanent magnet. Experimental verification of arc phenomena using high speed camera is carried out as well.

Development of a fault current limiter for 22.9 kV distribution power line

A 25.8 kV/600 A/12.5 kA fault current limiter (FCL) for 22.9 kV distribution power line has successfully designed, manufactured, and field tested at KEPCO Power Testing Center for the protection coordination. FCL is composed of fast fault detector (FFD) developed as a key component, mechanical fast switch (FS), and current limiting resistor (CLR). The FFD plays a role in sensing a fault current, which is recognized when it passes over the certain fault current level. The FFD has 5 fault current levels between 0.9 kA and 3 kA, and the fault current levels can be selected according to the electrical power systems. This function makes FCL more simple, economical, manageable and useful in various electrical power systems in comparison with a superconductor of a superconducting FCL. The FS has a function of a current commutation from main circuit to the CLR when fault occurs by using of a vacuum interrupter (VI). The CLRs role is to reduce the fault current by its impedance, which is decided based on the power system analysis of an application site. The whole process is perfectly completed within 1 cycle after the fault occurs. Recently, short circuit test, impedance test, and the electrical insulation test are passed. And also the field test is performed on the 1.5 kA and 5.0 kA faults, which are made by connecting the Artificial Fault Generator (AFG) to the distribution line in test grid at KEPCO Power Testing Center. Various demonstration tests of the FCL have proved that the current limiting performance is satisfactory and the protection coordination with conventional relays is realized.

Study on back to back capacitive current switching of 36kV vacuum circuit breaker

Back-to-back capacitive bank switching is the most severe duty among capacitive current switching duties because the amplitude of the inrush current with high frequency is very large. In this paper, 36kV back-to-back capacitive current switching tests of test duty 2(BC2) were carried out with vacuum circuit breaker. And the tests were performed with a series of separate making tests of 120 operations followed by a series of CO tests of 120 operations due to limitations of the test plant according to IEC standards 62271-100. The two types of VCB were tested with single phase and the test results were compared in terms of pre-arcing time, break down voltage distribution, contact surface after tests, contacting area, and field enhancement factor.

Parameter study on temperature-rise prediction of a vacuum circuit breaker

Vacuum Circuit Breaker (VCB) is an electric control device that interrupts the abnormal currents from the over-loads or short-circuits. Recently, higher current-ratings and better thermal performance has become important in providing the reliability for the modern devices requiring compact-size and high performance. In the development of the VCB, one of the most important design factors is the temperature rise. This thermal effect in the VCB is caused by resistances in the connection and contact surfaces of main circuit. These resistances influence terminal surfaces and volume of Vacuum Interrupter (VI). Thus, it is crucial to find out the temperature design parameters for the effective thermal system design. In this paper, we verified the thermal distribution tendency by using ICEPAK simulation with a VCB system based on the thermal parameters such as the VI load diameter and terminal.

A study on electrical lifespan of VI by means of calculation of arc energy during arcing time in synthetic tests

This study focused on the evaluation of electrical lifespan of vacuum interrupter by means of calculation of arc energy during arcing time. The repetitive short circuit current breaking tests with synthetic test devices were conducted until the contact could not break the short circuit current to identify the limit of electrical life. As a result, the accumulative arc energy was calculated after tests. Moreover, arc energy in short circuit current breaking tests were calculated and compared with the results of synthetic tests.

A study on the dynamic behavior of multibody dynamics model on vacuum circuit breaker: Operating characteritics improvement of the VCB using Taguchi method

The VCB(Vacuum Circuit Breaker) have three main circuits which contain the VI(Vacuum Interrupter) and has the main frame unit as an operating mechanism. The operating mechanism generally uses a spring energy for movement of heavy linkages and electric contacts in the VI. In order to reliably cut off the breaking current, electric contacts are separated as fast as possible. Generally, the opening speed of the VCB after contact separation has to be a 0.9~1.1 m/s. Therefore, to satisfy the required opening velocity of the VCB contacts, the optimization of the design parameters of VCB is needed. In this study, firstly, to describe the dynamic responses of the VCB, the analytical multibody dynamics model using Recurdyn has been developed. Secondly, the prototype sample has manufactured to verify the analytical Recurdyn model. To improve the opening velocity of the VCB which has heavy loads, Taguchi design method is applied as optimization procedure.

A study on compact shunt release device of MCCB using a voltage detecting circuit with hysteresis

MCCB (Molded Case Circuit Breaker) is widely used in power distribution systems for protecting an electrical circuit from damage caused by overload or short circuit. The shunt release device, an internal accessory of MCCB, opens the mechanism in response to an externally applied voltage signal. The shunt release device consists of an actuator and a control circuit. The conventional shunt release device has two bodies for each part. In order to make it one, the compact actuator and a voltage detecting circuit with hysteresis are developed. To minimize the size and improve performance, the electromagnetic force of compact Actuator has been analyzed by means of Maxwell. This analytic model is validated by comparing the simulation results and the experimental results. The voltage detecting circuit with hysteresis consists of input, hysteresis, output, reference, signal circuit and transistors. It estimates the level of input voltage without additional power source. In this paper, compact shunt release device with voltage detecting circuit with hysteresis is introduced. The compact shunt release device makes it possible for installing maximum quantity of MCCB in a panel. We found that performance and operation reliability was improved compared to the conventional shunt release device.

Design study for decreasing impact forces of an axial type vacuum interrupter in vacuum circuit breaker considering the dynamic characteristics

Vacuum interrupters are now widely used for medium voltage switching up to 38kV in such applications as metal-clad switchgears, circuit breakers, etc. Vacuum interrupter minimizes the damage and loss of the contact points at the optimum zero point of current wave after separation of the contact points, due to its quick vacuum insulation recovery characteristics. The fixed and moving electrodes of vacuum interrupter are mated by a driving mechanism within several milliseconds. Therefore, considerable impact forces occur between the electrodes. These impact forces could cause structural damages to vacuum interrupters and shorten fatigue-life of the products. Also, rebound phenomena could occur between the contacts due to the impact forces. It could not only affect mechanical characteristics but also have a strong influence on electrical conditions. Therefore, it is very important for impact analysis to reflect the dynamic characteristics of materials that consists electrodes as well as the static characteristics. As the dynamic behavior of materials is different from the static(or quasi-static) one due to the inertia effect and the stress wave propagation, an adequate experimental technique has to be developed to obtain the dynamic responses for the corresponding level of the strain rate. In this paper, to determine the dynamic characteristics of materials, the Hopkinson bar experiment is carried out, and for decreasing impact forces of a axial type vacuum interrupter, design study is applied in impact analysis with explicit finite element method. The rebound phenomena between the electrodes during impact periods are compared with the one of modified model which is suggested for the improvements in specific parameters.