Measurements of High Voltage DCFields in Air

PRESENTED BYASWANTH M RAJEEVANANDAVISHNU S

CONTENT

INTRODUCTION DC FIELD MEASUREMENT PROBES NEW ABB PROBES VERIFICATION OF PROBE IN A PLANE CONCLUSION

INTRODUCTION The global demand for electric power continues to increase in most

countries but in particular in non-OECD (Organization for Economic Cooperation and Development) countries.

For various reasons, high voltage direct current (HVDC) is often the preferred technology for transmission of electric power over long distances.

One of many challenges in the development of equipment for 800-kV DC is the proper design of the insulation on the air side.

The electric field distribution in the air surrounding the equipment is one of the key questions for designers. For traditional AC designs, the electric field distribution can be numerically computed from the geometry and dielectric permittivity of the materials involved.

A HVDC field probe measurement system recently developed by ABB is presented, with a description of its basic design and properties.

The main reason for ABB to develop a HVDC field probe is to be able to measure the DC field around full-scale HVDC insulators.

Conduction in Air Air is widely used as the natural insulation medium in the electrical

industry. Overhead lines, converter stations, live tank breakers, etc., all rely on the insulating properties of air.

In air, electrical conduction happens mainly due to the drift of singly charged ions in the electric field. Such ions are produced naturally in air due to cosmic radiation and the radioactive decay of the gaseous elements of the decay chain of long lived radioactive elements.

As shown in Figure 1, there is a critical value of the electric field above which small increases in the field lead to a rapid increase in current.

For air at atmospheric pressure, the transition to the so-called corona regime occurs for fields around 2.5 to 3 kV/mm. While coronas are known to occur in the vicinity of high voltage equipment, especially overhead lines, typical HVDC equipment is designed to operate below the corona inception level.

Voltage-current characteristics in air

Current density j of an air gap as a function of the applied DC field E. In the Ohmic region, at low fields, the current density

increases linearly with the applied field, until a saturation field Es is reached.

For fields between Es and the corona inception field Ec , the current density carried by the air remains constant.

Above the corona inception field, electron-impact ionization of neutral air molecule becomes possible, leading to a rapid increase in the current density when increasing the field

Many approaches can be used to calculate the electric field distribution around air-insulated HVDC apparatus. The simplest one uses a so-called “RC model,” where air is modeled simply using an ohmic conductivity and an electric permittivity.

For a more accurate description of the field distribution, a full simulation of the drift of ions in air is required.

DC Field Measurement Probe System

Basically two types of devices are described in the literature: probes based on electro-optic sensors and probes with self-contained electronics .

Probes of the electro-optical type use the linear electro-optic Pockels effect. The main components are an optically active crystal, for example lithium niobate (LiNbO3), together with a polarizer and quarter-wave plate.

The second type of probe described in the literature is the optically insulated cylindrical field mill . This type of probe is generally smaller than the ones of the electro-optical types. This has the advantage of being able to measure very close to an object without disturbing the field.

The New ABB Probe To be able to measure DC electric fields around HVDC insulators , ABB decided

to develop in house a new DC field probe. A specification was established and different measurement principles evaluated. To

be able to measure close to the surface of an object under potential, the diameter of the probe should be less than 20 mm and it should be corona free over the relevant field range.

The detection level and resolution should be 0.01 kV/mm or better. To obtain high quality results, the aim was to achieve a minimum accuracy of 5% at a field of 0.5 kV/mm.

Probe (26 mm long and 12 mm diameter) held in place by optical fiber to the left and fiber loop and Kevlar thread to the right.

Large-Scale Probe Positioning System

To use the DC field probe for measurements around full-scale HVDC insulators, a positioning system was developed. The system consists of two 10-m-long glass-fiber-reinforced epoxy masts fastened to supporting concrete foundations .

The positioning of the two carriages and tilting of the masts are controlled by a motion controller located in a control cubicle at the bottom of the masts.

Each carriage carries a dielectric box, containing a camera and servo for rotation of the probe. The probe is connected between the two dielectric boxes by a Kevlar line and an optical fiber, the latter transmitting the signal from the probe.

Schematic showing the large-scale probe positioningsystem consisting of 10-m-long glass-fiber-reinforced epoxymasts (1 and 2), optical fiber and Kevlar thread with attachedmeasurement probe in the center (3), and test object (4).

Verification of the Probe in a Plane Gap

1. Finite Element Model of the Probe Two objectives must be met in a useful simulation of the probe.

First, the model should permit the evaluation of how the presence of the probe affects the electric field around the probe, and second, it should be possible to simulate the response of the probe for a given electric field.

2. Proximity Effect The FEM model of the probe can quantitatively explain this

observation. The analysis of the simulation shows that the discrepancy is due to the distortion of the field created by the probe, which acquires a position dependence when the probe is in the vicinity of the plates.

Plate-to-plate configuration used to calibrate the probe and to study the factors that affect the field measurements.The two rectangular electrodes (250 × 300 mm) are much larger than the gap distance d, which is varied from 50 to 100 mm. The probe’s position is varied from the grounded plate to the plate at voltage U, and the measured field is compared with the uniform field value U/d.

3.PROBE CHARGING

Field Measurements Around a HVDC Wall Bushing The test setup comprised a HVDC wall bushing and a ground plane. The

ground plane was a vertical metallic disc at the end of the bushing to simulate a wall.

The field distribution along different paths perpendicular to the bushing surface were measured as well as the time for development from voltage application. The measurements were performed at three different voltage levels; 0.5U0, U0, and 1.5U0, where U0 represents the rated voltage for the bushing.

The nonlinear effect has a significant influence on the field but only locally around the bushing. At a distance of 1 m or more from the surface, the increase in the field caused by the charging of the bushing is no longer detectable.

Simulation results indicate field stresses around the shed tips in the range of 2 kV/mm, which could be enough to explain ionization of the air. Another possible explanation of the observed nonlinearity is corona from the ground potential of the bushing, test setup, or at other points in the test room.

Field Measurements Around a DC Cable Termination

•Cross section of development downscaled prototype 525-kV DC cable termination for field measurements and simulation studies.• Green indicates the cable. •The red arrow indicates where the semicon of the cable connects to the field grading layer.

The termination basically consists of an outer hollow core composite tube with silicone rubber sheds that is filled with SF6.

The cable semicon connects to the nonlinear field grading layer at 1,500 mm, and it extends all the way to the top of the termination, where it is in electrical contact with the applied voltage. The top of the termination is graded with a toroid.

Comparing the measurements with those simulated from an RC model, good agreement is found for short times after voltage application, but significant deviations are noted for longer times.

The field increases along the termination from the bottom to about 2,000 mm and then decreases slightly to 3,700 mm. Above 3,700 mm, the field increases along the termination to the top.

At 1,500 mm, the cable semicon connects to the field grading layer that is applied to the inside of the termination.

The nonlinear conductivity of the field grading layer likely contributes to a more complex field distribution than may be explained by the RC model, although a varying conductivity is applied in the model.

Conclusion

The DC field probe system described in this article allows the measurement of the field surrounding full-scale HVDC apparatus and has proven to give robust and credible results.

The measurements have shown that the RC model used in designing equipment gives reasonable results as well as how large the discrepancies can be during certain conditions.

In addition, this work has given valuable input to the ongoing development of more advanced simulation models for HVDC apparatus and provides the ability to describe the nonlinear effects seen in the measurements.