Oscillation Phenomena in Isolated High Voltage Networks Caused by Lightning Strokes

Isolated networks are common practice for operating high voltage networks up to a small earth fault current defined by standards. The reason for choosing this type of network are the saving of the arc suppressing coils by coexistence of a self-healing grid in case of an earth fault. During the starting up of a 110-kV-isolated network long-time measurement were installed. During these measurements several unexpected voltage oscillation phenomena were detected. Investigations show that lightning strokes in the area around the high voltage line are the sources of the voltage oscillation. Comparisons between the measurement of the voltages and currents of the high voltage line and the detection protocol of the Austrian Lightning and Detection System (ALDIS) proofed this hypothesis. This paper describes the phenomena based on measurement, the correlation with lightning strokes and will confirm the measurement with simulation examples. INTRODUCTION Under the aspect of the continuous growing electric energy consumption, the goal of the electricity supply is to provide electricity in high quality for every consumer. In order to meet the rising current consumption, besides establishing adequate power station reserves above all, the networks must be enlarged. In order to allow further extensions of these networks without endangering humans and animals in case of ground faults, considerations regarding the neutral point treatment of these networks in the sense of precaution are absolutely necessary [6,7]. One possibility of neutral point treatment for electrical networks up to a small earth fault current is the operation with an isolated neutral point [1,2,3,4,5]. One major advantage of isolated neutral point is the high power quality, because most of single line-to-ground fault, mainly lightnings, selfextinguish in a short period of time and does not require any switching activities on the operation side. Because of this outstanding feature grids with an isolated neutral point treatment can be classified as self-healing grids. During a start-up of such a network, measurements were performed because in isolated networks ferro resonance oscillation can occur and it should be proofed, that no ferro resonance oscillation cause problems. During these measurement no ferro resonance oscillation were measured but unexpected voltage oscillation phenomena. MEASUREMENTS As mentioned before, transient voltage measurements at the isolated network were performed in the first months of operation of a small 110-kV-network. During these measurements unexpected voltage oscillation phenomena, were detected. These oscillations were damped very well. It was interesting to observe that all three phases were affected in the same way and as a consequence the step could be seen also at the zero sequence voltage U0. These measurements show a step of all three voltages and a step of the zero sequence voltage up to 60kV. Several steps right after the step before were accumulating and voltage rises higher than 100kV were detected. Figure 1 shows the measured line-to-ground voltage. Figure 1: Measurements of the phenomena These measurements were discussed between several experts but no reason for these phenomena could be given. Energy of the voltage step One of the first steps was to calculate the energy of the voltage step, to get knowledge about a possible source. With the use of the zero sequence capacitance of the transmission line, 240nF, the energy of the voltage step C I R E D 22nd International Conference on Electricity Distribution Stockholm, 10-13 June 2013