Joachim Speck

Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters

Partial discharges (PDs) occur in the air-filled gaps of enameled wires that are touching. They erode the insulation and consequently lead to an interturn breakdown. This paper describes the failure mechanism of low voltage interturn insulation as a consequence of PDs and shows why and how it is influenced by the insulation design, temperature, and the applied voltage. Understanding the failure mechanism leads to better founded maximal permissible stresses and a proper design of the interturn insulation to avoid premature drive failures.

Electrical stress and failure mechanism of the winding insulation in PWM-inverter-fed low-voltage induction motors

The winding insulation of low-voltage induction motors in adjustable-speed drive systems with voltage-fed inverters is substantially more stressed than in line-powered motors. Consequently, this operation is subject to limitations depending on the electrical stress and on the failure behavior of the winding insulation. Actual recommendations do not consider sufficiently the physics behind these phenomena and contain large utilizable reserves.

Surface behaviour of epoxy castings during the early ageing period

This paper discusses the phenomena of the surface degradation in the early ageing period (EAP) under indoor conditions. The surface hydrophobicity of aromatic epoxy resin with different fillers was investigated. The lifetime of the insulating materials depends on their withstand against combined electric and climatic stress. The climatic stress is initiated by the humidity of the environment. To evaluate the surface hydrophobicity of insulating materials, a Modified Rotating-Wheel-Dip-Test (MRWDT) with different dipping liquids was used. To separate the influence of the humidity in the MRWDT from the combined stress, specimens were immersed in the used liquids. The processes of ageing are discussed by using the results of both methods and chemical analyses of the aged specimens.

Breakdown behaviour of polyethylene at dc voltage stress

The paper deals with breakdown voltage characteristics of polyethylene in homogeneous electric fields. The breakdown voltage characteristic is measured with foils in a sphere-sphere arrangement both with dc voltage and with impulse voltage superimposed on dc voltage. The electrical strength was found to be dependent on the pre-applied dc stress and the polarity of the impulse voltage at the instant of their superposition. The impulse breakdown voltage of dc prestressed samples was compared to impulse breakdown voltage superimposed on dc voltage. The influence of insulation thickness and a lamination of polyethylene foils on dc breakdown behaviour was investigated. Moreover, the breakdown voltage was measured in short circuit experiments. The influence both of fall time and of rise time of dc recovery voltage is discussed in connection with the space charge effect.

Flashover behaviour of insulators with inhomogeneous temperature distribution in gas insulated systems under DC voltage stress

Due to the increasing need for long-distance energy transmission and space saving installations, the focus is more and more shifted to DC operated gas insulated systems. Reaching the steady state of the electrical field distribution after energizing, the resistive field is controlled by conductivities of insulating materials and SF6. The heating of the inner conductor due to operating currents causes an inhomogeneous temperature distribution within the insulating material, influencing local conductivity. Hence, the location of the highest electrical field under DC stress is shifted with respect to the situation under AC or impulse voltage stress. Consequently, the flashover behaviour of insulators depends on the kind of the electrical stress (AC or DC). In this paper, a CFD model was developed based on experiments to precalculate the temperature distribution at various electrode temperatures. Following up, time-dependent dielectric calculations show the changing electrical field distribution. After reaching the steady state, its inverted character with a shifted high field region can be seen. Experimental results of long-term DC voltage tests confirm this general behaviour.

Influence of gas temperature on the breakdown voltage in gas-insulated systems

In the context of increasing transmission capacities during the last years, the investigation of thermal effects, especially for gas-insulated systems, is required. Higher operating currents decrease the gas density in the vicinity of the heated conductor significantly. Hence, the insulation strength of the system is reduced. Dimensioning for different load scenarios requires a comprehensive knowledge of the temperature influence on the insulation performance of AC and DC installations. This paper introduces an algorithm to predict the temperature-dependent breakdown voltage utilizing the streamer criterion and proves its functionality by experimental investigations. Necessary input parameters, like the electrical field strength and the gas density, can be gained from numerical calculations. The insulation strength of the investigated arrangement at a conductor temperature of 100 °C is decreased by (10 … 15) % in comparison to a conductor temperature of 20 °C. The accordance between measurement and calculation proves the quality of the model.

Influence of space charges on the field transition in gas-insulated DC systems

DC operated gas-insulated systems (GIS) play a key role in satisfying recent requirements of energy transmission. After energising, the initial electrostatic field is changing into the electric currents field, which is determined by conductivity processes based on charge carriers. Experimental results show the temperature-dependent effects of the changed field distribution under long-term DC stress and identify clear differences to calculations, which are solely based on conventional RC models. This work proves experimentally, what was exclusively calculated theoretically in literature so far: additional charge carriers, especially negative ions, accelerate the field transition after energising with DC.

Nonlinear dielectric behaviour of insulating oil under HVDC stress as a result of ion drift

The paper presents two nonlinear dielectric effects from literature in mineral insulating oil, when stressed with DC voltage. In the first part it is shown how current time measurements fit Kerr field strength measurements. In the second part, the nonlinear current time effect is noted for polarity reversal. Both effects are caused by oil intrinsic charge carriers and their special dielectric properties. The current-time-behaviour and the nonlinear field strength distribution are calculated solving the Poisson-Nernst-Planck-equation time dependently.

Effects of short-time voltage drops and polarity reversals on breakdown behaviour of cross-linked polyethylene

This paper deals with breakdown voltage characteristics of cross-linked polyethylene (XLPE) in homogeneous electric fields. The breakdown behaviour is measured with foils in a sphere-sphere arrangement under the influence of short-time voltage drops. The influence both of fall time and of rise time of dc recovery voltage is discussed in connection with the space charge effect. Moreover, the breakdown voltage was measured in fast polarity reversal experiments. The electrical strength was found to depend mainly on the rise time of dc recovery voltage. Secondly, with respect to service conditions, the influence of temperature on the dc breakdown behaviour was investigated.

Field transition in gas-insulated HVDC systems

DC operated gas-insulated systems play a key role in satisfying recent requirements of energy transmission. After energizing, the initial electrostatic field is changing into the electric currents field, which is determined by conductivity processes based on charge carriers. Experimental results show the temperature-dependent effects of the changed field distribution under long-term DC stress and identify clear differences to calculations, which are solely based on conventional RC models. Consequently, charge carrier based conduction processes have to be taken into account in order to calculate the transient and temperature-dependent behavior after DC switch on. This paper proves experimentally, what was solely calculated theoretically in literature so far: additional charge carriers, especially negative ions, accelerate the field transition after energizing with DC.