Conference Agenda

If the polarity of the PD impulses is accurately recorded and the polarity of the applied voltage at that time is known, it is possible to infer some information about the location of the PD. Specifically, in some situations, it can indicate whether the PD is originating from the insulation connected in series with the measuring impedance. In practical applications, this information can help determine if the PD is occurring within or outside the equipment being tested or, in some cases, if it is coming from a specific accessory.

This publication examines if and how this information can be utilized for PD diagnostic of shielded power cables, power transformers, and stator windings. It begins with practical measurements conducted on simple test circuits to explain the underlying theory of the direct and indirect PD measurement circuits. Later, it presents measurements taken from medium voltage cables, power transformers, and stator windings done to investigate if there are advantages of assessing PD polarity. Additionally, it discusses the challenges faced by PD instruments in achieving accurate detection of PD polarity.

Aged oil-filled (OF) cables are used in Japan; therefore, on-line condition monitoring for them becomes more important than before. A large-scale blackout occurred due to insulation breakdown of oil-impregnated paper insulation inside a joint of OF-cables in 2016. It is reported that the insulation breakdown was caused by partial discharges (PD) occurring in the oil-impregnated paper insulation.

We have been developing a radio interferometer system to detect PDs occurring in the joints of OF-cables. The sampling frequency, quantization bit rate, memory of acquisition of the system are 10 GHz, 10-bit, and 100 Mpts/channel, respectively. The acquired digital data can be stored directly into solid state drives (DDSs) and sent to the operation center through local area network (LAN) as well. The radio interferometer system can be controlled from the operation center through LAN.

In this study, the radio interferometer system was used to detect PDs generated in a hole of insulation papers set in a joint of 154 kV OF-cables and that of 275 kV OF-cables. Currents and electromagnetic (EM) waves of PDs generated in the joint were simultaneously measured using a high-frequency current probe and the radio interferometer system, respectively.

It is planned that the measurement for PDs is conducted on the upper floor than the floor on which underground cables arranged; therefore, the antenna-array of radio interferometer system was set behind a concrete wall with the width of 30 cm placed in a mock-up tunnel. This concrete wall was supposed to be the floor. The mock-up tunnel, of which size is 2 m×2 m×6 m, was used for consideration of actual usage of the radio interferometer system. All sides of the tunnel are made of aluminum plates; therefore, it is a severe environment for measuring the EM waves because the EM waves reflect on the surface of all sides, i.e., multipath effects appear. Although reinforced concrete and other metal materials are used in actual tunnels in which underground cables are arranged, it is thought that the environment in the mock-up tunnel is more severe than that in actual tunnels from the viewpoint of measuring the EM waves.

It was confirmed that the EM waves due to PDs leak from the flange of OF-cable joint by using the radio interferometer system. The characteristics of the PD currents and EM waves are also described.

Safety operation of superconducting magnetic levitation (SC Maglev) systems needs automatic insulation diagnosis for a large number of propulsion coils and levitation-guidance coils, which are arranged on both sidewalls of U-shaped guideways. We have been developing an on-board radio interferometer system with a vector-antenna to detect partial discharges (PD) occurring in propulsion coils and could locate a mock-up of propulsion coil with a PD source and/or insulation breakdown from a test bogie running at the speed of 212.3 km/h on the test track with the length of 500 m.

Two sets of an on-board radio interferometer system with a vector-antenna were newly developed in order to detect PDs simultaneously for both sidewalls of U-shaped guideways. The sampling frequency, quantization bit rate, memory of acquisition of the systems are 10 GHz, 10-bit, and 100 Mpts/channel, respectively. The acquired digital data can be stored directly into solid state drives (DDSs) and sent to the operation center through local area network (LAN) as well. The on-board radio interferometer systems can be controlled from the operation center through LAN.

In this study, the two on-board radio interferometer systems with the vector-antennas were used to detect PDs generated in a cylinder-shaped void composed of a cylinder-shaped electrode and a plane electrode, which was set in a mock-up of propulsion coil. As the previous studies, the cylinder-shaped electrodes with different diameters: 0.5 mm, 2 mm, 4 mm, 6 mm, 8 mm, and 10 mm were used; the gap length between the cylinder-shaped electrode and the plane electrode was set to 3 mm. Apparent charges, currents, and electromagnetic (EM) waves of PDs generated in the mock-up of propulsion coil were simultaneously measured by using a commercial PD measuring system based on the IEC60270 standard, a commercial current probe, and the two on-board radio interferometer systems with the vector-antennas, respectively.

This paper describes the characteristics of the EM waves received by using the on-board radio interferometer systems with vector-antennas in order to use them for automatic insulation diagnosis.