• Proceedings of the KIEE Conference
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• 1996.07b
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• pp.797-799
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• 1996

This paper presents a simple method for evaluating of voltage stability using the line flow equation. Line flow equations($P_{ij}$, $Q_{ij}$) are comprised of state variable, $V_i$, ${\delta}_i$, $V_j$ and ${\delta}_j$, and line parameter, r and x. Using the feature of polar coordinate, these equations become one equation with two variables, $V_i$ and $V_j$. Moreover, if bus j is slack bus or generator bus, which is specified voltage magnitude, it becomes One equation with one variable $V_i$, that is, may be formulated with the second-order equation for $V_i^2$. Therefore, solutions are obtained with simple computation. Solutions obtained are used for evaluating of voltage stability through sensitivity analysis. Also, considering of reactive power source, method for evaluating the voltage stability is introduced. The proposed method was validated to 2-bus and IEEE 6-bus system.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.5
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• pp.17-26
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• 2018

If a photovoltaic (PV) system is installed in a primary feeder interconnected with the PV system, bi-directional power flow can occur, and then, the magnitude and direction of the fault current can change, depending on the fault location and point of common coupling (PCC) of the PV system, and the time current curve (TCC) cannot be properly coordinated between protection devices. Also, it is difficult to obtain a proper time interval for protection devices because the conventional setting approach is applied, even though the PV system is installed and operating. Therefore, this paper presents three operation modes considering the operational conditions of the PV system to obtain setting values for protection devices. Based on the mode, this paper proposes an algorithm to calculate the optimal protection coordination time interval according to the introduction capacity of the PV system. In addition, this paper performs modelling of a distribution system with the PV system and protection devices by using Off-DAS S/W, and analyzes the characteristics of the time interval between the protection devices, such as substation relays, reclosers, customer relays, and PV customer relays. The simulation results confirmed that the proposed operational modes and setting-value algorithms are useful and effective for protection coordination in a distribution system for a PV system.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.22 no.5
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• pp.22-33
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• 2021

In the case of open phase faults caused by the disconnection of distribution feeders interconnected to a PV system, many problems can occur depending on the core type and wiring method of the grid-connected transformers. Moreover, open phase faults are difficult to detect because the open phase voltage of the existing protection relay (Open Phase Relay (47)) can be maintained, even though a disconnection fault occurred, depending on the wiring method and the iron core type of the grid-connected transformer for a PV system. Therefore, this paper proposes a novel algorithm to detect open phase faults by comparing the currents and phases between the primary and secondary sides of a grid-connected transformer. In addition, this paper presents the modeling of a distribution system and protection devices for detecting open phase faults using PSCAD/EMTDC S/W, and implements a test protection device for detecting open phase faults based on the above-mentioned modeling. The simulation and test results confirmed that the proposed algorithm is useful for detecting open phase faults according to the wiring method and iron core type of grid-connected transformer for a PV system because operation slope and unbalance rate of the primary current exceed the setting value (30[%]) of the protection device.