engleski

Insulation coordination for wind power plants

Wind Power Plants (WPPs) have many things in common with traditional utility electrical systems, but they also have unique characteristics that require special attention. They are based on MV cable collection networks connecting tens of usually identical subsystems. Such arrangement implies a great number of electrical equipment (generators, converters, circuit breakers, step-up transformers) connected by cables at distances determined by the optimal tower positions. The main cause of overvoltages in the cable collector grids of WPPs are circuit breaker operations. On the other hand, WTs as very high structures with sharp blade tips attract lightning and suffer direct strokes. WPPs have proven to be challenging for insulation coordination because of its unusual characteristics. Extensive transient analysis and computer simulations in EMTP-RV are needed to properly size and protect the equipment insulation. Three events simulated in EMTP-RV will be presented: direct lightning stroke to WT blade, vacuum circuit breaker (VCB) switching overvoltage and isolated operation of a feeder. When direct strokes are considered, only the external parts are subjected to direct lightning strokes (LPZ 0A), according to the Lightning Protection Zones (LPZ) for WTs. The internal parts are subjected to indirect effects of lightning, but they are sensible because of their low insulation level. The EMTP-RV simulations of direct strokes to WTs are done to identify the effects of incoming surges to the internal components of the WT. When the lightning strikes directly to the blade, nacelle or tower, the surge travels from the stroke point, through the external components to the grounding system. If the transient ground potential rise reaches values higher than insulation levels, a back-flash from earth to phase will occur. Regarding the second example it is important to say that VCBs are nowadays frequently used in WPPs because they are compact, safe and require minimal maintenance. On the other hand, the multiple pre-strike, re-ignition and re-strike phenomena that occur in VCBs produce very fast front transient overvoltages. It is reported worldwide that many transformer insulation failures have occurred by switching operations of VCBs. In WPPs, the very fast transients cause by VCB operation will mainly stress the transformers connected to the collector grid. The most endangered transformer will be the one closest to the VCB in operation. Simulation of VCB switching requires advanced VCB model and high-frequency transformer model. The VCB model includes the random nature of arcing time, current chopping ability, the dielectric strength between contacts and quenching capabilities of the high frequency current at zero crossing. The advanced high-frequency transformer model enables to study both internal and transferred overvoltages. The third example concerns the isolated operation of a radial cable feeder. The collector system grounding is normally provided at the substation. If a single-line-to-ground fault (SLGF) occurs somewhere on the feeder the grounding impedance will determine the magnitude of the overvoltage. At the instance of feeder disconnection, the feeder loses its ground reference and the voltages in healthy phases rise to line voltage level. Field experience has shown that WTGs may continue to energize an isolated feeder due to enhancement of low-voltage ride-through capabilities of wind turbine generators. Energizing the isolated feeder will increase the amplitude of the existing overvoltage. Further overvoltage amplitude rise may occur if some WTGs trip and other remain operating due to change in the cable amount (capacitance) connected to the remaining WTG. One of the options used for reducing the amplitude of this overvoltage is installing the grounding transformer on each feeder so in case of feeder disconnection the ground reference will not be lost.

insulation coordination ; wind power plants ; temporary overvoltages ; vacuum circuit breaker switching ; high frequency power transformer model ; lightning overvoltages

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