Auto transformer tap changing
Dear Friends,
would like to share an interesting experience during my work as a commissioning Er. We were working in a 380 kV/132/13.8 kV transmission SS. The transformers were Areva make Autotransformers 502 MVA each ( 2 nos). The first charging of the transformer was done from 132 kV side. After 4 days, it was decided to switch on a 150 km 380 kV line on No- load on the 380 kV side bus of the above Tr. As soon as the line was energised, the voltage at the remote end was reported as around 405 kV ! (due to Ferranti effect). Now, the maintenance Ers. gave instructions to reduce the voltage by lowering the Tr. Tap ! Immediately, the voltage on 380 kV side increased further. There was some heated arguements between the maintenance and commissioning personnels as to why the voltage was increasing when the tap is lowered ! What followed was a beautiful explanation of the transformer output equation and concept of emf/turn from one of our seniors. The variation of the transformer output due to tap changing gets reversed based on which is the source side for the transformer. Thus, the general idea that "if the output voltage is low, raise the tap and if the output voltage is high , lower the tap" doesn't work when the voltage source side is on the secondary side. We had to raise the tap to lower the primary side voltage.
Thanks
bvc
Dear Friends,
would like to share an interesting experience during my work as a commissioning Er. We were working in a 380 kV/132/13.8 kV transmission SS. The transformers were Areva make Autotransformers 502 MVA each ( 2 nos). The first charging of the transformer was done from 132 kV side. After 4 days, it was decided to switch on a 150 km 380 kV line on No- load on the 380 kV side bus of the above Tr. As soon as the line was energised, the voltage at the remote end was reported as around 405 kV ! (due to Ferranti effect). Now, the maintenance Ers. gave instructions to reduce the voltage by lowering the Tr. Tap ! Immediately, the voltage on 380 kV side increased further. There was some heated arguements between the maintenance and commissioning personnels as to why the voltage was increasing when the tap is lowered ! What followed was a beautiful explanation of the transformer output equation and concept of emf/turn from one of our seniors. The variation of the transformer output due to tap changing gets reversed based on which is the source side for the transformer. Thus, the general idea that "if the output voltage is low, raise the tap and if the output voltage is high , lower the tap" doesn't work when the voltage source side is on the secondary side. We had to raise the tap to lower the primary side voltage.
Thanks
bvc