A Capacitance-to-Ground Measuring Method for Medium-Voltage Power Grid of a Ship Based on the Combination of High and Low Frequencies

With the increase in capacity of large ship electric power systems, medium-voltage electric power systems have gradually become an inevitable choice. Among China's large ships, the neutral point of the power system is usually grounded by high resistance, and its grounding parameters need to be determined, taking the system's capacitance to ground as a reference. Under different working conditions, the capacitance to the ground of the system will change, which requires online real-time measurement of the capacitance to the ground to provide a basis. However, the current flowing through the distributed capacitance and the capacitance itself cannot be directly measured by measuring instruments. Currently, the most commonly used method is the signal injection method, which can realize the secondary side measurement. This paper analyzed the traditional signal injection methods and found that all these methods are not suitable for real-time measurement of the capacitance to the ground of the medium-voltage electric power system of a ship. Among the current methods, this paper proposes combining the dual-frequency method and the high-frequency method. Through error analysis, for systems with different capacitances to ground, the frequency selection of the dual-frequency method will affect the measurement accuracy. To ensure the measurement accuracy, it is necessary to adopt the principle of "one low frequency + one high frequency". Therefore, based on the dual-frequency method and the high-frequency method, the paper proposed an improved dual-frequency method, taking a combination method of high frequency and low frequency for capacitance measurement of medium-voltage power systems with high resistance grounding. Then the paper studied the high- and low-frequency selection scheme by simulation comparison and finally determined the frequency selection scheme of 5000/120 Hz. The paper also carried out simulation and experimental verification and finally proved that under the selected frequency selection scheme, the proposed method can accurately measure the capacitance to ground in a medium-voltage power grid with high resistance grounding.