Influence of load variations on PD-characteristics of MV-cable joints

Summary

Renewable energy shares are progressively growing in a worldwide seeking to provide a sustainable energy supply. Due to extensive constructions of e. g. wind turbines and photovoltaic plants across wide areas, new grid connections are necessary to transfer the power to the centers of consumption. Most of the decentralized on-shore facilities offer only a small installed plant capacity, whereby they are directly feeding into the MV-grid. A predominant portion of the German MV-grid consists of cable systems and their weak points, the MV-cable joints. This joints are proven to have a more inhomogeneous field distribution, leading to a premature PD-inception, jeopardizing a fault-free operation. This circumstances in combination with an on average higher capacity utilization and considerably fluctuating loads are leading to the suspicion of an accelerated aging, as grid operators observe a higher failure rate in the affected cable sections. The financial incentives, introduced by the Federal Network Agency makes it inevitable to take action and reduce the interruption duration to a minimum in applicable ways. Therefore, this paper investigates, whether and in how far the suspicion of an accelerated aging of MV-cable joints is reasonable. Previous studies suggest a relation between load fluctuations and partial discharges affecting the aging condition. In order to achieve a convergence to this issue, a thermal model of a MV-cable joint is evaluated by means of measuring the thermal behavior due to current pulses. The thermal model is subsequently used to simulate the thermal response to authentic load curves estimating the thermal inertia of the MV-cable joint system. Furthermore, defective MV-cable joints are placed in an appropriate configuration and simultaneously stressed by high currents and voltages. One loop contains a MV-cable joint equipped with several sensors to gather temperature, pressure and humidity data, while the other MV-cable joint in the second loop is connected to a PD-measuring system. Finally, the thermal cycling turns out to have a heavy impact on the PD-characteristics determining the aging.

Additional informations

Keywords

PD-characteristics, XLPE-cable, aging, diagnostics