Some industrial incidents are attributed to insulation degradation of electrical assets. The leakage current can be a crucial indicator for online insulation monitoring of asset. To realize per-phase monitoring, the measurement accuracy of milliampere-level leakage current is limited by the strong load current noises. This paper proposes an advanced current sensor with a dual-core topology based on differential-mode (DM) measurement. An analytical model of magnetic field of the dual-core sensor is originally presented. It helps clarify the significances of the inner core in filtering noises and reducing the influence of system operation and cable positioning. Then, a comprehensive design procedure is fully investigated by selecting the optimal position and approach for magnetic field detection, and a high-speed and low-noise signal conditioning circuit is presented. The proposed current sensor is tested in the laboratory. Experimental results indicate that measurement errors can remain within 0.1mA in the testing scope, where the load current is up to 100A and the DM current varies from 0-7mA. The proposed sensor is further validated with superior performance compared with two commercial industrial sensors. The contribution of this paper lies in providing the analytical analysis and design of a novel DM current sensor for leakage current measurement, which achieves online per-phase insulation monitoring for industrial electrical assets.
The voltage-regulating transformer (VRT) with on-load tap changer is specifically used to adjust the voltage on the medium-voltage side of the ultra-high voltage power transformer. In practice, during the process of on-load voltage regulation, it is difficult to acquire the tap changer's positon, which is necessary for the calculation of the VRT differential current. Therefore, a novel algorithm is proposed to identify the real-time turns ratio of the VRT, adaptively, according to the structure of the ultra-high voltage power transformer. On the basis of the real-time turns ratio, the adaptive differential current is calculated. Both theoretical analysis and simulation results indicate that the proposed algorithm can track and calculate the turns ratio accurately, thus preventing unbalanced currents during the tap-changing process. Moreover, the performance of the VRT differential protection is evaluated under inter-turn fault conditions. Simulation results verify that the proposed algorithm can overcome the disadvantages in the existing protection schemes.KEYWORDS differential protection, inter-turn fault, on-load tap changer (OLTC) position, ultra-high voltage (UHV), voltage-regulating transformer (VRT)
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