Measurement and Analysis of Neutral Point Currents in a 400-kV-Network

Neutral point treatment is a very important topic for protection and network operation. For different reasons 400-kV AC-power networks are operated with direct grounding of the neutral point. One major part of electrical networks are transformers. In transmission systems, usually at least one transformer per substation is operated with direct grounded neutral point, thus allowing earth currents to enter the transformer. New transformers are designed for very low noise emissions. During commissioning of a new transformer, unexpected noise was noticed and saturation due to DC currents respectively very low frequency currents (below 1Hz) was assumed to be the reason.
In a current research project of Austrian Power Grid and Graz University of Technology measurements of the neutral point current were performed. Analyses of these measurements revealed several frequency components in the spectrum, including DC, slow variations below 1Hz, 16.7Hz from railway system, 50Hz mains, harmonics and interharmonics.
This paper gives a description of the measurement setup, highlighting the problem of recording small DC currents in the presence of high AC currents. Furthermore results from a neutral point current measurement in the Austrian 400-kV-grid are presented and discussed.