Mätningar och undersökning av elkvaliteten i elnätet på reningsverket GRYAAB, i Göteborg
Examensarbete för masterexamen
Pettersson, Monica Vea
On a request from the water sewage plant Gryaab in Gothenburg a power quality survey was performed. The voltages and currents have continuously been sampled in the incoming 10 kV substation HSP01 from April-August 2008. Power quality measurements were also performed in Gryaabs other 10 kV substation HSP04 fed from HSP01 and at the terminals of the induction motor IN_PU4040 of 1.3 MW which controls one of the four large inflow pumps to the plant. The reason for the latter measurement was due to registered negative transients each time the motor was connected to the grid. The cause of the transients could be explained by the coupling in the substation, the ageing of the motor which could result in damage of the bearings or resonance in the motor circuit which amplifies high frequency harmonics. During the survey 58 transients was registered, of which 56 was caused from the coupling of IN_PU4040 in HSP01. The additional two transients could not be explained by any event at Gryaab. Two transients occurred during the measurements in HSP04. A comparison between the breaker actions in the substation HSP01 showed that these transients were caused by the breakers on the outgoing lines to the induction motor IN_PU2040 and IN_PU3040 respectively. Due to the fact that these two breaker actions never caused transients in HSP01, there are reasons to believe that the transients caused by the breaker actions are amplified in the substation HSP04. Twenty-one voltage dips occurred during the measurements, of which four had a retained voltage between 60 % - 75 % and a duration of 100 ms to 200 ms. These dips tripped the frequency converters, motors and active filters on the low voltage side of HSP04. A comparison between the measurement data at Gryaab and Gothenburg Energy’s measurement data at the feeding 10 kV station in Repesäll, shows that all of the dips below 90 % at Gryaab are caused by faults on higher voltage levels than 10kV. According to Gothenburg Energy mostly of the dips are caused by faults at 130 kV and upwards. In order to mitigate the effects of the voltage dips on the frequency converters, motors and filter at the low voltage side of HSP04 a dynamic voltage stabiliser (DVR) can be installed. But at this moment this cannot be recommended because of the low cost effectiveness. The DVR can compensate voltage sags, swells, voltage unbalance, phase angle jumps. All of the measured data from individual harmonics, THD, frequency variations and voltage unbalance fulfils the requirements set by the European EMC standard EN61000-2-12 with a large marginal. The prominent individual harmonics measured in HSP01 and HSP04 are equally distributed, these are the 5th, 7th, 11th and the 13th harmonic.
Elkraftteknik , Electric power engineering