Abstract
This paper presents the results of the simulations and their respective analyses corresponding to the power frequency overvoltages resulting from various fault types occurring inside a microgrid. During the islanded mode of operation, the analysed microgrid can be simultaneously fed by a diesel generator, a 1 MW wind power turbine, a small solar system and a 1 MW hydroelectric scheme. The operating voltage of the microgrid is 2.4 kV. During a fault in the system, the overvoltages normally occur in two remarkable instants. The first one occurs at the beginning of the fault itself. The second one occurs at the instant when the fault is cleared. The major concern here is the overvoltage during the fault period. Due to the travelling wave effect along cables and overhead lines composing the microgrid system, these overvoltages can be amplified, thus jeopardizing the insulation level of the microgrid transmission system and related equipment. Much of the work available now is dedicated to overvoltages present in high-voltage systems leaving a gap for the study and behaviour on low voltage microgrid systems. The overvoltage stress is characterized by the maximum low-frequency, short-duration (crest value) of the overvoltage. Both cables and overhead lines that constitute the microgrid transmission system are characterized by their R-L-C parameters. The simulations of the microgrid system are conducted using the ATP program. According to the international ANSI and IEEE standards, the minimum BIL (Basic Impulse Insulation Level) and BSL (Basic Impulse Switching Level) for the 2.4 kV voltage level are 20 kV and 10 kV, respectively; thus, care should be taken so that the healthy phases upon which commonly appear such overvoltages are not exceeded in their insulation level.
Highlights
Types of Overvoltages AnalysedThe faults to be considered here, and which give rise to overvoltages, are: - Line-to-ground fault; - Line-to-line fault; - Double line-to-ground fault; - Broken conductor
The establishment of microgrid systems is nowadays regarded by the power industry as one of the alternatives to assist critical loads and provide electricity to some regular consumers, especially during periods of prolonged unexpected interruptions
Various can be the causes for the presence of overvoltages in a microgrid, namely [1]. - Faults inside the microgrid: both line-to-ground (L-G) and double-line-to-ground (L-L-G) faults; - Resonance phenomenon: series resonant overvoltages and overvoltages resulting from ferroresonance; - Voltage regulation: due to microgrid connection to a large amount of distributed generators; - Lightning effect: overvoltages due to direct impact of lightning on the phase conductors and, overvoltages due to indirect impact of lightning; - Switching overvoltages: due to cable energisation or those due to shunt capacitor switching; - Load rejection
Summary
The faults to be considered here, and which give rise to overvoltages, are: - Line-to-ground fault; - Line-to-line fault; - Double line-to-ground fault; - Broken conductor. The other type of overvoltages (e.g. due to ferroresonance, line energisation, switching, etc.) are not studied here, as they need a somewhat different approach. Overvoltages due to lightning are not discussed either. During line-to-ground faults the system zero sequence impedance should be included in the analysis. The healthy phase(s) during fault periods normally exhibit a voltage increase
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