Insulation stressed with fast rise time repetitive voltages from high voltage power electronics

The electric energy systems will undergo radical changes in the upcoming years due to changing consumer patterns and a large-scale introduction of renewable energy sources. As a result, the power system must handle new operation schedules with less inertia and more frequent starts and stops. Power electronic converters are central components for achieving seamless control of the power flow between the different suppliers and consumers in the power grid. The project focuses on three challenges particularly important for the development of generic test standards: 1) Partial discharge detection, 2) pulse propagation in networks, and 3) insulation ageing diagnostics. The project encompasses the full diversity of high voltage insulation systems, from internal insulation in power electronic modules, to insulation in connected apparatus such as cables, rotating machines, and transformers. However, the rapid voltage switching of power electronics converters affect connected equipment radically different from the slow-changing (50 Hz) voltage forms of traditional power sources. Sharp voltage pulses from power electronics can reduce service lifetime of power apparatus by igniting partial discharges that damage their electrical insulation. Despite the anticipated exposure to new operation schedules, high voltage electrical equipment is still factory tested using conventional, slow-changing 50 Hz voltages because there is a lack of reliable test standards for converter stresses. FastTrans addresses these issues by developing knowledge, equipment, models and techniques for testing high voltage electric power apparatus under power electronic stresses. FastTrans is highly relevant for high voltage DC transmission systems, reactive power compensation in power systems, for solar and onshore/offshore wind power production, in motors and drives for industry, in transformers for offshore platforms and subsea drives as well as for the electrification of the transport sector. A laboratory infrastructure for investigation of partial discharge occurrence in different insulation systems is established. This comprises: 1) a resonant test set-up for applying sinusoidal voltages up to 50 kV peak with less than 1 pC noise level, 2) two set-ups for applying switched unipolar or bipolar voltages with steep risetimes at voltages up to 40 kV, 3) a set-up for measuring field dependent conductivity in electric insulation, and 4) equipment for electrical, optical and acoustical measurement of discharges under various voltage waveshapes. Studies are done on propagation of voltage transients in small networks relevant for windfarms and drives. Transmission properties of relevant high voltage cables are mapped. Furthermore, algorithms are developed for modelling terminal properties of transformers and motors to allow simulation of voltage stresses for transients stressing inductive apparatus. Partial discharge inception in power electronic components under sinusoidal and switched voltages is investigated, and basis for defect identification in such components is developed. Space charge injection is important for discharge inception in converter insulation and has been studied. Test objects for studying partial discharges in transformer insulation has been designed, and investigation on discharge behaviour under varying voltage waveshapes is under way. Impact of changing the insulating liquids from conventional mineral oil to esters will be studied A set-up for studying partial discharges in high voltage motor insulation is established. A PhD on discharge ageing of low voltage motor insulation is close to completed. The project comprises several MSc students in various fields. A PhD student has started studying how voltage transients that hit motor terminals will propagate into the stator and stress the insulation.

High voltage power electronic converters are central elements in the physical infrastructure of any flexible energy system because they enable effective switching and seamless control of the power flow between the different suppliers and consumers in the electric power grid. However, the rapid switching of power electronic converters cause voltage surges with fast rise times that can inflict more severe stress on the insulation of electrical apparatus than traditional stress caused by pure sinusoidal voltages. Today, the industry's factory acceptance stress tests for high voltage electrical equipment used in converter-fed networks are still largely based on conventional, slow 50 Hz sinusoidal voltages because there is a lack of reliable international test standards for insulation coordination under power electronic stresses. FastTrans aims to address these issues by developing knowledge, equipment, models and techniques for testing and insulation coordination of high voltage electric power apparatus in networks stressed with fast rise time repetitive voltage pulses from power electronics. The project focuses on three specific R&D challenges that are particularly important for the development of generic test standards: 1) Partial discharge occurrence and detection, 2) pulse propagation in networks into components, and 3) ageing diagnostics. The project accommodates the full diversity of high voltage insulation systems, which range from the µm-sized and sharp-edged internal insulation in an integrated gate bipolar transistor (IGBT), to more uniform geometries in polymer cables, epoxy-insulated machines, and oil/cellulose-insulated transformers. The research documentation emanating from the project is expected to be sufficiently comprehensive to approach international organisations such as IEC and CIGRE for the development of generic and reliable test standards for high voltage equipment under the influence of fast transient stresses from power electronics.