Frank Mauseth

A protocol for space charge measurements in full-size HVDC extruded cables

This position paper, prepared by the IEEE DEIS HVDC Cable Systems Technical Committee, illustrates a protocol recommended for the measurement of space charges in full-size HVDC extruded cables during load cycle qualification tests (either prequalification load cycles or type test load cycles). The protocol accounts for the experimental practices of space charge measurements in the thick insulation of coaxial cables in terms of poling time, depolarization time, heating and cooling of specimens, as well as for the experience gained very recently from such kind of measurements performed in the framework of qualification tests relevant to ongoing HVDC cable system projects. The goal of the protocol is not checking the compliance with any maximum acceptable limit of either space charge or electric field. Rather, this protocol aims at assessing the variation of the electric field profile in the cable insulation wall during poling time at the beginning and at the end of load cycle qualification tests for full-size HVDC extruded cables. Indeed, in the design stage the electric field distributions are determined by the cable geometry and by temperature gradient in the insulation. Thus, the design is based on macroscopic parameters conductivity and permittivity and how they depend upon temperature. Any disturbance of the electric field due to space charge accumulation will only be revealed during space charge measurements either in as-manufactured state or in the aged state after load cycle qualification tests.

Charging of dielectric barriers in rod-plane gaps

The electric properties of an air gap between a hemisphere-rod covered with 3 mm thick silicone rubber and plane metal electrode have been examined. Observation of discharge activity and measurement of the discharge current have been made under impulse voltage stress. When applying an impulse voltage close to the inception voltage, discharge activity is observed around the hemispheric tip of the rod. Observed light emission seems to be evenly distributed along the hemispheric tip comprising a number of independent discharges similar to discharges in a homogeneous electrode configuration. Applying a conductive layer at the rod tip reduces the discharge activity and measured discharge current considerably. The discharge activity was concentrated close to the tip end of the rod. Relaxation time constant for the deposited charge was measured to be about 4 h. By comparing conductivity of the insulation material, the dominating relaxation mechanism was found to be conduction through the solid insulation.

Streamer inception and propagation for air insulated rod-plane gaps with barriers

High voltage device dimensioning requires the prediction of the withstand voltage for test conditions like impulse, surges and AC. There is a large need of designers to have reliable design criteria and a well-defined simulation procedure for device development, which is able to predict the withstand voltage up to 10 % for arbitrary geometry. This task must be based on an appropriate model for streamer inception and propagation being at the outset of electrical breakdown. Investigations were done on a rod-plane gap without and with a barrier varying in position and size. The barrier used was parallel to the ground electrode. The streamer is initiated around the tip of the rod and will propagate in field direction until the barrier is reached. Depending on the background and local field, the streamer will find a way around the barrier towards the ground electrode. The experimental results are compared with results from numerical simulations, where the streamer inception voltage was determined from the background field. Depending on the position and size of the barrier, the withstand voltage either showed an increase or decrease compared to the withstand voltage of the air gap without a barrier.

Effect of water content on the conductivity of XLPE insulation

In HVDC cable insulation, the steady state electric field distribution is mainly determined by the following factors: DC conductivity, charge trapping, electron injection and ion formation. Factors which contribute strongly depend upon changes in morphology and chemical features within the insulation. In case of polymer insulated HVDC cables, nonuniform conductivity due to for example temperature gradients in the material will result in space charge formation and can yield an electric field distribution different from what will be expected in case of application of AC voltage. It is expected that the absorption of water will increase the formation of ions and thus increase the conductivity of XLPE insulation. The goal of this paper is to present results from conductivity measurements on XLPE cable insulation with varying water content. Measurements of charging and discharging currents were performed at applied electric stress in the range of 2-20 kV/mm, using Rogowski type test objects equipped with semiconducting polymeric electrodes. The measurements were carried out at temperatures of 40, 60 and 80 °C, at different relative humidities in the range from 10 % to 90 %. The test objects were conditioned in a climate chamber kept at the selected relative humidity and temperature of the subsequent measurement for up to 2 days prior to the conductivity measurements, thus ensuring that equilibrium water content was reached before the measurement started. In order to remove remnant space charge between each test, the test objects were kept grounded for a time 10 times longer than the high voltage charging time. The results show that an increase in water content increases the conductivity of the XLPE insulation. At the highest water content for a given temperature the conductivity was 1.8-3.4 times larger than at the lowest water content. The temperature- and field-dependency of the conductivity did not significantly change with increasing water content. The conductivity was found to increase proportionally to the square root of the water content, indicating dissociation of water as the origin of the increase.

The effect of DC superimposed AC voltage on partial discharges in dielectric bounded cavities

Voltage source converters is used in HVDC stations in offshore HVDC transmission systems, between the AC and DC power grid. The AC ripple voltage on the DC side of the HVDC stations can be in the range of 1-10 % of the nominal DC voltage, depending on the size of the filter employed. For offshore HVDC grids, there is a drive to use polymeric insulated cables on the DC side. This work investigates how an AC voltage at power frequency superimposed on DC voltage influence the partial discharge magnitude and repetition rate in artificial cylindrical cavities in polymeric insulation. The AC voltage is kept below the AC partial discharge extinction voltage, and the DC voltage is kept above the DC partial discharge inception voltage. A resistor-capacitor ABC-circuit model is used for prediction of partial discharge magnitude and repetition rate under combined AC and DC voltage. Measurements has been performed on a test object of 3 layers of PET film with 1 mm radius cylindrical cavity in the middle layer. The results indicate that an AC voltage ripple with an amplitude lower than the AC partial discharge extinction voltage will increase the number of large discharges, compared to a DC voltage without ripple, but the repetition rate will be several orders lower than the AC voltage frequency.

Electrical conductivity of medium voltage XLPE insulated cables

Improved knowledge on the charge transport phenomena in XLPE is important to control the field distribution in the cable insulation, and thus further improve the reliability of future HVDC cables. Polarization and depolarization currents was measured on two medium voltage cables at 40, 60, and 80°C with an applied average electric field varying from 2.5 to 19 kV/mm. The conductivity of the insulation was calculated based on the measured quasi steady state current. The two cables were identical apart from the XLPE material (95mm2 Cu conductor, 3.4 mm insulation). The field dependence of the conductivity was found to be non-linear for the entire field range. Measured currents were fit to the Poole-Frenkel and hopping conduction. The fit to Poole-Frenkel gave a relative permittivity of 2.9, while the fit to hopping conduction gave an average hopping distance of 19 nm. The best fit to the measurements was obtained with Poole-Frenkel conductivity. The temperature dependence follows an Arrhenius relation with activation energy of approximately 1.4-1.6 eV.

Diagnostic testing of thermally aged medium voltage XLPE cable joints

Current loading of wind farm distribution network is very different from that observed for traditional underground cable systems. Variations in wind power results in a significant number of current load cycles during service. The current can increase from zero to maximum rated load in a short time. This can cause high temperature gradients in the cable systems inducing significant thermo-mechanical forces during heating and cooling, and thermal oxidation at high load. Several premature service failures of cable joints in wind farm distribution networks have occurred, mainly due to the combined action of bad metallic connectors and high load current. Partial discharge measurements are today performed on-site using different frequencies and magnitudes of the test voltage. As the oxidation process causes the conductivity of the bulk and surfaces of the materials involved to strongly increase, this will also likely impact the frequency dependence of the partial discharges. This work includes laboratory high temperature ageing of rubber type heat-shrink MV XLPE cable joints including temperature gradients by application of high load currents. The ageing has been electrically characterised by variable partial discharge measurements in the frequency range of 10 mHz to 100 Hz up to 12 kV (2 U0). The joint material was examined by microscopy examinations after ageing. The results show that the partial discharges are strongly frequency and voltage dependent. For shorter ageing times of the rubber joints, the number of discharges per voltage cycle decrease to zero for frequencies below 10 Hz. At longer ageing times resulting in severe material ageing, the number of discharges per voltage cycle strongly increases by decreasing frequency. In this case the inception voltage at 0.1 Hz was equal or lower than that measured at higher frequencies. It is observed by SEM that especially the stress-control material was strongly degraded by the high temperature conditions.

Water tree growth of wet XLPE cables stressed with DC and high frequency AC voltage superimposed

Power electronics used for HVDC converters will stress a cable with a DC voltage with overlaid transients. The effect of these transients on the performance of the polymeric cable insulation is yet not known and the main purpose of this work has been to investigate the effect of this kind of voltage stress on XLPE cable insulation in water without metallic shield. Laboratory experiments were performed on a 12 kV XLPE cable aged wet at power frequency for one year at 36 kV - 6U0 before tested at DC with an overlaid transient simulating the voltage stress from a HVDC converter. During the experiments the active part of the cable samples was kept in water. The cables were examined for water tree growth. The results show an increase in the maximum length of water trees detected as a function of aging. One of the test samples broke down during the ageing.

Water tree growth of wet XLPE insulation stressed with DC and high frequency AC

Power electronics used for HVDC converters will stress cable insulation with a DC voltage overlaid transients. The effect of these transients on the performance of the polymeric cable insulation is yet not known. The main purpose of this work has been to investigate the effect of this kind of voltage stresses on XLPE cable insulation without any metallic watertight screen when exposed to moisture. Laboratory experiments were performed on Rogowski shaped test objects with an insulation thickness of 1.25 mm. At one of the semi-conductors, 20 salt particles were placed in order to facilitate initiation of water trees. The test objects were conditioned with water at 40 °C for 4 days ensuring saturation of water inside the insulation system before testing. The test objects were aged at DC stress with overlaid transients simulating the voltage stress from a HVDC converter. During the experiments the test objects were filled with deionized water. All experiments and conditioning of test samples were performed at 40 °C. Ageing tests were performed stressing the test objects with DC voltage combined with a high frequency AC voltage (15 kHz). Test objects were taken out and inspected for water tree growth regularly. The results show a rapid ageing caused by water treeing when exposed to the DC voltage overlaid a 13.7 % AC voltage with a frequency of 15 kHz.

Quality control of extruded HVDC cables: Low frequency endurance testing of model cables with contaminations

This paper presents results from laboratory experiments preformed in order to evaluate different methods for quality assurance of polymeric insulated HVDC cables. Samples of model cables were prepared, equipped with 1.5 mm thick XLPE insulation, containing insulating and conductive spherically shaped particle inclusions with diameters of 250, 140 or 80 µm, distributed at the inner semi-conductor interface. The results indicates that the 0.1 Hz VLF test voltage needs to be 2 to 3 times higher than at 50 Hz power frequency in order to eliminate the same defects within a comparable time.