*Article* **Partial Discharge Behaviour of a Protrusion in Gas-Insulated Systems under DC Voltage Stress**

#### **Thomas Götz \*,†, Hannah Kirchner † and Karsten Backhaus**

Chair of High Voltage and High Current Engineering, Faculty of Electrical and Computer Engineering, Institute of Electrical Power Systems and High Voltage Engineering (IEEH), Technische Universität Dresden, 01069 Dresden, Germany; kirchner.hannah@yahoo.de (H.K.); karsten.backhaus@tu-dresden.de (K.B.)

**\*** Correspondence: Thomas.Goetz1@tu-dresden.de

† These authors contributed equally to this work.

Received: 20 May 2020; Accepted: 5 June 2020; Published: 16 June 2020

**Abstract:** High reliability, independence from environmental conditions, and the compact design of gas-insulated systems will lead to a wide application in future high voltage direct current (HVDC) transmission systems. Reliable operation of these assets can be ensured by applying meaningful and robust partial discharge diagnosis during development tests, acceptance tests, or operation. Therefore, the discharge behavior must be well understood. This paper aims to contribute to this understanding by investigating the partial discharge behavior of a distorted weakly inhomogeneous electrode arrangemen<sup>t</sup> in sulfur hexafluoride (SF6) and synthetic air under high DC voltage stress. In order to ge<sup>t</sup> a better understanding, the partial discharge current is measured under the variation of the insulation gas pressure, the gas type, the electric field strength, and the voltage polarity. Derived from this, a classification of the different discharge types is performed. As a result, four different discharge types can be categorized depending on the experimental parameters: discharge impulses, discharge impulses with superimposed pulseless discharges, discharge impulses with superimposed pulseless discharges, and subsequent smaller discharges and pulseless discharges. Concluding suggestions for partial discharge measurements under DC voltage stress are given: recommendations for the necessary measurement time, the applied voltage and polarity, and useful measurement techniques.

**Keywords:** partial discharge; protrusion; gas-insulated system; HVDC; SF6; synthetic air
