The document describes a new HVDC field probe developed by ABB to measure electric fields around HVDC equipment. It discusses the probe's design, which uses optical fibers and is less than 20mm in diameter to measure close to surfaces. The document also details calibration of the probe in a plane gap, measurements of fields around a HVDC bushing and cable termination, and findings that an RC model agrees with measurements at short times but deviations occur over longer times likely due to nonlinear conductivity effects. In conclusion, the probe provides robust field measurement data that helps design HVDC equipment and develop more accurate simulation models.

1. Measurements of High Voltage DC
Fields inAir
PRESENTED BY
ASWANTH M RAJEEV
ANANDAVISHNU S

2. CONTENT
 INTRODUCTION
 DC FIELD MEASUREMENT PROBES
 NEW ABB PROBES
 VERIFICATION OF PROBE IN A PLANE
 CONCLUSION

3. 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.

4. Conduction inAir
 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.

5. 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

6.  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.

7. 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.

8. The NewABB 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.

9. 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.

10. Schematic showing the large-scale probe positioning
system consisting of 10-m-long glass-fiber-reinforcedepoxy
masts (1 and 2), opticalfiber and Kevlar thread with attached
measurement probe in the center (3), and test object (4).

11. 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.

12. Plate-to-plateconfigurationusedtocalibratetheprobeandtostudythefactorsthataffect
thefieldmeasurements.
Thetworectangularelectrodes(250×300mm) aremuchlargerthanthegapdistanced,
whichisvariedfrom50to100mm.
Theprobe’spositionisvariedfromthegroundedplatetotheplateatvoltageU,andthe
measuredfieldiscomparedwiththeuniformfieldvalueU/d.
3.PROBE CHARGING

13. Field MeasurementsAround 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.

14. Simulationresults indicate fieldstresses around the shed tips in the range
of 2 kV/mm, which couldbe enough to explainionizationof the air.
Another possibleexplanation of the observed nonlinearityis corona
from the ground potentialof the bushing, test setup, or at other points in
the test room.

15. Field MeasurementsAround 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.

16.  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.

17. 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.