Land electrode for a high voltage direct current...

Details Land electrode for a high voltage direct current... Land electrode for a high voltage direct current...

1999-06-21

2001-06-12

Reichard, Dean A. (Department: 2831)

Electricity: conductors and insulators

Earth grounds

C174S068100, C174S07000A, C174S07000A

Reexamination Certificate

active

06245989

ABSTRACT:

TECHNICAL FIELD
The invention relates to a land electrode for grounding of a high voltage direct current (HVDC) transmission system.
BACKGROUND OF THE INVENTION
Ground electrodes, in this context, means devices used to connect an electrode line of a power network comprising an HVDC transmission system, via one or more feeder cables, to a conducting medium such as soil or sea water.
HVDC transmission systems usually have DC voltages above 5 kV and a transmitted power above 10 MW.
As compared with alternating current (AC) transmission systems, HVDC transmission systems require only two conductors. At least one of those conductors is implemented as an overhead line or a high voltage cable. For bipolar transmission another conductor of the same kind is used under normal operating conditions, but in monopolar transmission, the ground, that is soil and/or sea water, is used as a return conductor for the transmitted DC-current. However, also in HVDC transmission systems intended for bipolar transmission, ground electrodes are required to transfer unbalance currents and, under operation in monopolar mode, the whole DC current transmitted by the HVDC system.
The ground electrodes can operate as anodes, that is, delivering current to the conducting medium, or as cathodes, that is, receiving current from the medium.
Depending on the location of the HVDC-system, the ground electrodes can be located in soil or in sea water. Ground electrodes located in soil usually have certain advantages as compared to ground electrodes located in sea water. Thus, they are comparatively easy to mount and to have access to in the case of maintenance or repair. At least under normal operating conditions they are also better protected against mechanical damages and will usually not be subject to varying mechanical stresses. The risk that human beings or animals will come into direct contact with the electrode is very small.
This application is concerned with land electrodes, that is, ground electrodes located in soil.
The land electrode, via one or more feeder cables, transfers the DC current from an electrode line of the HVDC system to the soil or vice versa. The soil, in this context, is generally to be regarded as a conducting, however, inhomogeneous medium.
For a general description of ground electrodes in connection with HVDC systems, reference is made, for example, to E. Uhlmann: Power Transmission by Direct Current, Springer Verlag 1975, in particular pages 255-273.
The land electrodes are-apart from the requirements as to current and resistance-also required to be electrically safe, to have high operational reliability and sufficiently long service life, and in addition, to not cause any harmful environmental effects, such as for instance drying up of the soil in the vicinity of the electrode.
The resistance of ground electrodes has to be low, usually well below one ohm. In particular for land electrodes, the step voltage at the surface of the soil in the vicinity of the electrode, which creates a danger to human beings, should be less than a specified level. The step voltage Vs is calculated according to an expression Vs=(5+0.03* &rgr;
s
) volts, where &rgr;
s
is the minimal local specific resistivity (expressed in ohm*m), of the soil located above the electrode.
A conventional land electrode comprises an active part, herein called the electrode body, which is in electric contact with the soil and through which the current is transferred, interconnection cables for internal connection of parts of the electrode body as described below, and additional parts performing mechanical functions, including mechanical protection.
The average current density on the surface of the electrode body is usually not higher than a few A/m
2
.
In order to reach a sufficiently low grounding resistance, a land electrode usually comprises a large number of sub-electrodes, each sub-electrode being fed from a separate sub-electrode feeder cable. A sub-electrode comprises a backfill, usually a bed of coke, and an active sub-electrode element, the feeding element, embedded in the backfill. The feeding element is in electric contact with the sub-electrode feeder cable and has an active part of its surface which is in electric contact with the backfill. In cases where the sub-electrode comprises more than one such feeding element, these elements are coupled to each other by feeding element interconnection cables.
The backfill occupies a certain volume around the feeding element and is in its turn embedded in the soil. The active part of the surface of the backfill is that part of its surface which is in electric contact with the soil.
The sub-electrodes are usually arranged in sections, each section being fed from a separate section feeder cable, which is in electric contact with the electrode line. Each section of sub-electrodes may comprise a plurality of sub-sections, each sub-section being fed from a separate section interconnection cable which is in electric contact with the section feeder cable.
The sub-electrodes are arranged along contour lines. A contour line is to be understood as the trace, as seen from above, of a section or a sub-section of sub-electrodes. The contour line of a section or a sub-section of sub-electrodes can typically have the shape of a circular arc, in which case the sections and/or sub-sections of sub-electrodes can be arranged in such a way that the contour lines of the electrode coincide with a circle.
FIG. 1
illustrates schematically an electrical configuration typical for an HVDC transmission system with land electrodes at both ends. An electric alternating current (AC) power network N
1
is via a transformer T
1
coupled to the AC-side of a thyristor converter SR
1
and an AC power network N
2
is via a transformer T
2
coupled to the AC-side of a thyristor converter SR
2
. On the DC-sides of the converters, an overhead line LO connects one of their respective poles, and the ground return comprises two electrode lines LE
1
, LE
2
, two land electrodes
15
of similar structure, and the soil (not shown) between the electrodes. The land electrode at the converter SR
1
comprises a plurality of sub-electrodes
16
, each of which is coupled to the electrode line via a feeder cable
29
. Each subelectrode comprises a plurality of feeding elements
161
,
162
,
163
, interconnected by interconnection cables
2
′,
2
″,
2
′″ respectively. The electrode body comprises all the feeding elements
161
,
162
,
163
comprised in all the sub-electrodes coupled to the electrode line.
FIG. 2A
shows schematically a typical layout, as seen from above, of a land electrode
15
for an HVDC transmission system. The contour line of the electrode is in the form of a circle and the electrode is fed from an electrode line LE
1
via three section feeder cables
29
a
,
29
b
,
29
c
.
FIG. 2B
shows a side view of a part of the electrode, comprising three series connected rod-shaped feeding elements
161
,
162
,
163
, with their longitudinal direction in a horizontal direction. Each feeding element is embedded in a layer
170
of backfill in the form of coke, which layer in turn is embedded in a soil layer
28
at some distance below the surface
10
of the soil. All parts of the electrode are similar to the part illustrated in FIG.
2
A.
FIG. 2C
shows a cross section through the electrode along the section IIC—IIC in FIG.
2
B. The diameter of the ring can typically be in the order of 1 km. The material of the feeding elements is typically silicon iron or graphite (for electrodes operating as cathodes, also mild steel).
Alternatively, the feeding elements may be arranged with their longitudinal direction in a vertical direction. This is illustrated in
FIG. 3A
, showing a side view of a part of an electrode of similar ring form as the electrode illustrated in
FIG. 2A
, with three parallel connected rod-shaped feeding elements
161
,
162
,
163
.
FIG. 3B
shows a cross section through a sub-electrode along the section IIIB—IIIB in FIG.
3
A. Each feeding element is embedded

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