Toshihiro Takahashi

Insulation design method of cold dielectric type superconducting power cable and its verification tests

A high-Tc superconducting (HTS) power cable is considered to be a major candidate for the bulk power transmission device in the future. The HTS power cable with a cold dielectric type electrical insulation system can make the outer diameter of the HTS cable compact in order to replace the conventional XLPE cable laid in underground ducts. Therefore, establishment of the electrical insulation design, HV testing and evaluation methods is the most important in the realization of the HTS power cable. From these viewpoints, the electrical insulation design method and determination of testing voltages for a 500-m HTS power cable constructed for verification tests are discussed, and the result of the various tests are described in this paper.

Effective partial discharge measurement method for XLPE cables based on propagation characteristics of high frequency signal

Joints of power cable system have a possibility to include some defects due to long-term chemical reaction and deoiling of grease as well as on-site workmanship at their installation. The partial discharge (PD) measurement can be a powerful insulation diagnosis technology to confirm their soundness. PD is accompanied with a pulse current with its rise time of several nano-seconds, which propagates in the power cable. Thus its propagation characteristics are quite important to determine not only the objective frequency range but also the appropriate PD sensor installation. In this paper, the propagation characteristics of pulse signal in XLPE power cables are firstly discussed with empirical results. Then, the determining process of the objective frequency is proposed based on the frequency characteristics of the attenuation rate of the XLPE cable to measure the PD only at the both terminations.

Partial discharge characteristics of oil impregnated paper insulation system with oil gap under repetive impulse voltage superimposed on AC voltage

Partial discharge (PD) characteristics of an oil-impregnated paper insulation system with an oil gap under a lightning impulse voltage superimposed on an alternating current (AC) voltage were investigated. Prior to the voltage superimposition, insulation oil in the oil gap was degraded due to continuous PDs. Next, an AC voltage giving an electric field lower than designed one of 275 kV OF cable systems was applied to the insulation system, and then a lightning impulse voltage equivalent to switching surge voltage was repeatedly superimposed on the AC voltage. As a result, PDs progressed to continuous discharges from intermittent ones after the repeated superimposition of the lightning impulse voltage and the AC voltage. As a result, it is suggested that repeated applications of switching surge voltage may be one of the main causes of continuous PD occurrence in OF cable systems especially when insulation oil in the oil gap is degraded.

Noise discrimination from PD signal in pre-breakdown discharge detection test

In Japan, 22-77 kV cross-linked polyethylene (XLPE) cables have been put into practical use for about 40 years, and some of them have been in operation for more than 30 years, which exceeds their expected lifespans of 30 years. At these voltage levels, the degradation of the insulation of XLPE cables is due mainly to the water tree. Central Research Institute of Electric Power Industry (CRIEPI) has studied the dielectric strength of XLPE cables and the various aspects of water tree degradation using a pre-breakdown discharge detection (PBPD test). This tests success rate of whether to find the water tree of weakest point in cable specimen depends on a method that distinguishes small partial discharge (PD) signals caused by water trees from noises in noisy environments of the test field. It also determines the likelihood of establishing a robust PBPD test that incorporates the developed method for the discrimination of PD signals from noises.

Demonstration of a 500 m HTS Power Cable in the Super-ACE Project

A demonstration of a 500 m single-core high-Tc superconducting (HTS) cable has been carried out by Furukawa, CRIEPI and Super-GM at the CRIEPI site in Yokosuka, Japan. The cable was a cryogenic dielectric type rated at 77kV/1kA. In the test, critical currents (Ic) of HTS conductor and HTS shield at 73 K were 1910 A and 1620 A respectively, with heat invasion of 1.21 W/m, and a pressure drop of 100 Pa/m at an LN2 flow rate of 50 L/min. Rated loading tests corresponding to degradation of the insulation over 30 years were carried out, and there was no degradation in the cable. Moreover, overload and limiting tests were performed under the adverse conditions of over-voltage, over-current and stopping of refrigerators and pumps. After these events, the cable was removed and studied, revealing almost no damage.

Electrical Insulation of 500-m High-Tc Superconducting Power Cable

Electrical insulation is one of the essential technologies for the electric power apparatus. Determination of testing voltages and design method of the electrical insulation layer are inextricably linked each other, and are critical to developing and realizing a cold dielectric (CD) type high-Tc superconducting (HTS) power cable. The authors had proposed the electrical insulation design method with concepts of partial discharge-free designs for ac voltage condition. This paper discusses the testing voltages for a 77 kV 1000 A HTS power cable with a length of 500 m, and describes results of various voltage withstand test. As a result, it is concluded that the proposed electrical insulation design method is appropriate for the HTS power cable.

PD inception characteristics of polypropylene laminated paper/liquid nitrogen composite insulation system under local heating

A high-Tc superconducting (HTS) power cable is one of the major candidates for reinforcing power transmission grids dynamically. For operation of the HTS cable, a quench phenomenon would occur due to over-current operation, degradation of the HTS materials, and so forth. In that case, local heating would occur and local evaporation of liquid nitrogen would occur, which may lead reduction of the electrical insulation capability and generate partial discharge (PD) under operational voltage application. This paper deals PD inception characteristics with local heating simulating local quench of HTS material. As a result, it is revealed that there is a time delay from the start of heating to PD generation, and the delay depends on the pressure of liquid nitrogen.

Development of the new partial discharge measuring method and device for long power cable using foil electrode

Partial discharge (PD) measurement is one of the most significant method for assessing the insulation quality of power cables, particularly for cross-link polyethylene (XLPE) insulation material. The capacitive coupling method has been widely applied for PD detection in cable lines. However, the detection sensitivity of this method drops due to attenuation of partial discharge pulsed during propagation. Therefore, we proposed a new method that can be applied to detect PD in the normal part of the cable, by considering the impedance of the shield electrode of the cable itself. In this research, a low voltage communication cable with a coaxial structure was employed as a transmission line. PD signal with a fix intensity was simulated using a pulse generator in connection with the cable line. A capacitive electrode made of metallic foil was placed onto the plastic jacket of the cable. As a result, a significant voltage was detected.

Development of pre-breakdown discharge detection test for aged XLPE cable to find a deteriorating water tree

XLPE cables have been widely used in power transmission and distribution system especially in urban areas, since about 40 years ago. Nowadays, some XLPE cables have been operated for about 30 years, which is designed lifetime in their production. Therefore, understanding voltage withstand stress of aged XLPE cable is important for reliable operation and optimized replacement planning for the aged XLPE cable. CRIEPI has been investigating ac voltage withstand level and the degradation cause for aged XLPE cables that are removed from actual power grid due to replacement. This paper introduces the outline of the pre-breakdown discharge detection test system and some development of testing technique, and a characteristic result.

An integral equation for analysis of partial discharge data obtained with an asymmetric IEC(b) electrode system at various applied voltage frequency

Partial discharge (PD) phenomena are the major factors of insulation degradation to determine the life of high voltage power apparatus, and have been investigated from various viewpoints. In this paper, authors proposed an integral equation in order to explain the complex characteristics of partial discharge using very simple three-capacitance model with several numerical solutions at various applied voltage conditions. The f-n distribution patterns are calculated with the integral equation and are compared with those obtained experimentally with an IEC(b) electrode system at various applied voltage frequency (25–1000Hz).