Application of a DC Distribution System in Korea: A Case Study of the LVDC Project

Abstract

With the rapid expansion of renewable energy and digital devices, there is a need for direct current (DC) distribution technology that can increase energy efficiency. As a result, DC distribution research is actively underway to cope with the sudden digitization and decentralization of load environment and power supply. To verify the possibility of DC distribution, Korea Electric Power Corporation (KEPCO) Research Institute made a DC distribution system connected with a real power system in Gwangju. The construction of the demonstration area mainly includes design of protection and grounding systems, operating procedures of insulation monitoring device (IMD), and construction of power converters. Furthermore, this paper goes beyond the simulation and the lab testing to apply DC distribution to a real system operation in advance. It is designed as a long-distance low-loaded customer for rural areas and operated by the DC distribution. In addition, safety and reliability are confirmed through field tests of DC distribution elements such as power conversion devices, protection and grounding systems. In particular, to improve the reliability of non-grounding system, the insulation monitoring device was installed and the algorithms of its operational procedures are proposed. Finally, this paper analyzes the problems caused by operating the actual DC distribution and suggests solutions accordingly.

Insulation Monitoring Method for DC Systems with Ground Capacitance in Electric Vehicles

“…The accuracy is the highest only when the three sampled voltages are in the climbing stage of the voltage variation curve. Equation (6) shows that when the difference of the three sampled voltages is close to zero, the calculated value of ∆E 1 is nearly infinite because of signal interference in the actual application unlike in the ideal situation. This condition seriously affects the measurement result.…”

“…Counter k is increased once every measurement period T C , as shown in Figure 5. To make kth measurements E 1 and E 2 close to the actual value, the average value of M 1 and M 2 phases are taken according to Equation (6). E 1 (k) and E 2 (k) can be rewritten as The estimated value of the kth E 1 and E 2 is set asÊ 1 (k) andÊ 2 (k), respectively, which can be obtained by the following first-order filter, where A is a filter coefficient that satisfies 0 < A < 1.…”

“…are set as the estimated voltage values. The estimated method can be derived from the following equation according to Equation (6).…”

“…When the insulation resistance of the system decreases to below a threshold value, the vehicle sends warning signals. If the situation is serious, then the high-voltage system must be cut off and stopped for troubleshooting [6,7]. The DC insulation monitoring (DC-IM) function is thus required before charging by the DC charging pile and during the process of driving the EVs [8].…”