IEEE PES PowerAfrica 2007 Conference and Exposition Johannesburg, South Africa, 16-20 July 2007
Geophysical and geological pre-investigations for HVDC-electrodes H. Thunehed GeoVista AB P.O.Box 276, SE-97108 Luleå, Sweden Phone: +46-920-38388, Email:
[email protected]
U. Åström ABB SE-77180 Ludvika, Sweden Phone: +46-240-782000, Email: urban.astrom@ se.abb.com
B. Westman ABB SE-77180 Ludvika, Sweden Phone: +46-240-782000, Email:
[email protected]
Abstract – The potential differences caused by the current injected in the ground by a HVDC-electrode can cause corrosion of e.g. pipelines and operational problems in transformer stations. Also, hazardous electric fields can arise in the vicinity of HVDCelectrodes. High potentials and strong fields from electrodes are, in principle, related to high-resistivity rock in the ground. The presence of such potentially problematic rock volumes is not routinely checked before the construction of an electrode site. However, geophysical methods are available that can estimate the electrical properties of the bedrock down to considerable depths. Such surveys, in combination with geological knowledge and modeling can be used to predict the environmental impact of an electrode. In this paper we present modeling results that shows the relevance of geo-electrical surveys in a HVDC-electrode project. Some methods that can be used to estimate the electrical properties of the ground around an electrode site are also presented. Good electrode sites can thus be selected at an early stage of a power transmission project.
Problematic high potentials and strong electric fields are related to high-resistivity rock volumes in the ground. The resistivity of rocks can vary over several orders of magnitude [1]. Most commonly, high resistivities are found for old crystalline bedrock composed of granite, gneiss and similar rock types. However, high-resistivity rock can sometimes also be found in fairly young rock sequences as for example basaltic lavas. Today, pre-investigations are limited to, at most, finding a location with near-surface low-resistivity material to act as a host to the electrode itself. This might be sufficient for making sure that the electrode works technically, i.e. is able to inject the current into the ground. However, other problems can be related to high-resistivity volumes at some distance from the electrode or at large depths.
I. INTRODUCTION
We will first have a look at the situation around an electrode situated on the surface of a horizontally stratified earth (Figure 1). Although real geology is more complex than the examples, they will give some insight into the influence of rocks situated at different depths. All examples illustrate possible situations in an area with old crystalline bedrock covered by younger sediments. The electrical properties assigned to each geological unit are however quite typical for most areas around the world. We will assume that the electrode has been placed in a material of reasonably low resistivity, like clayey sediments with a resistivity of 10 Ωm. This unit is underlain by older, more consolidated sediments with a resistivity of 40 or 200 Ωm. The next layer in the sequence consists of granitic rock where the resistivity takes values of 2500 or 25000 Ωm in the examples. Such large differences in resistivity are not uncommon between different variants of these kinds of rock.
High-voltage DC current (HVDC) is an efficient method for power transmission. Most land schemes operate in bipolar operation mode with no return current, but the systems are also designed to operate in monopolar operations that require electrodes where significant amounts of current is injected into the ground. In a high-resistivity environment, the injected current will create large potentials and hence electric fields in the vicinity of the electrode. The strong electric fields can create safety problems close to the electrode for humans, grazing cattle etc. The effect of the electrode can also be noted at quite large distances as electric potential differences of more moderate magnitude. These potential differences can give rise to corrosion of pipe-lines and other installations and also interfere with e.g. transformer stations.
II. POTENTIALS AROUND AN ELECTRODE
Finally, we introduce a layer of fairly low resistivity at the bottom of the sequence. This corresponds to rocks in the lower part of the earth’s crust or in the mantle that are known from deep-probing geophysical surveys to be present in many parts of the world, although not everywhere. Two types of possible problems can be identified due to an electrode. Firstly, there might be near-source problems related to hazardous electric fields in the immediate vicinity of the electrode. Areas where the electric field reaches levels of tens of Volts per meters might put humans and animals in danger. Secondly, there might be far-distance problems related to fairly moderate field strengths. Electric fields of the order of a few Volts per km can in some cases create corrosion problems. The amount of corrosion is however also a function of the design and orientation of the object in question. Constructions oriented parallel to the current flow in the ground will be most affected. The dashed line in Figure 1 indicates roughly a level where corrosion problems might arise. The thicknesses and resistivities of the different layers in the examples are listed in Table I. Model 1 corresponds to an electrode that has been placed in a favorable environment. The thicknesses and low resistivities of the two uppermost layers prevent strong electric fields near the source and the fairly low resistivity of the granitic layer prevents strong fields at large distances. The distance where the field strength falls below 10 V/km is slightly more than 800 meters. I 10000 Clay Consolidated sediment Granite 1000
Lower crust/mantle
Electric field (V/km)
100
10
2 3
1 1
4 0.1
0.01 0.01
0.1
1
10
100
Distance from electrode (km)
Fig. 1. Electric field due to 1000 A current injected by a point electrode on the surface of a layered earth. Each curve is labeled with the corresponding model number. Model parameters are according to Table 1.
Model 2 is identical to model 1 except for the resistivity of the second layer. The layer has a resistivity that is five times higher than for the first model. This has the consequence that the distance at which the electric field falls below 10 V/km is increased to 1.8 km. Even though this is not a very large distance, it means that the area with corrosion risk is almost five times as large compared to the first model. Table I. Layer parameters for the models in Figure 1. The resistivities of layers 1 and 4 are 10 and 500 Ωm respectively.
Model
Thickness layer 1 (m)
1
40
2
3
4
Thickness layer 2 (m) Resistivity layer 2 (Ωm)
Thickness layer 3 (m) Resistivity layer 3 (Ωm)
1000
10000
40
2500
1000
10000
200
2500
50
100000
200
25000
50
5000
200
25000
40
10
10
Model 3 presents a situation with a considerably worse geological situation compared to the two first models. The two uppermost layers are thinner and the third, granitic layer has a quite high resistivity. The distance at which the curve falls below the 10 V/km level has now been increased to over 9 km. This means that the area with corrosion risk has been increased by a factor of around 120. Model 4 is identical to model 3 with the exception that the depth to low-resistivity rock in the lower crust has been reduced to 5000 meters. This has the effect that the electric field strength falls off very rapidly at distances greater than 10 km. This is a larger distance than the 10 V/km distances for both models 3 and 4, but problems have occurred with real HVDC-electrodes at such large distances. The modelling results thus imply that it is necessary to estimate the electrical properties of the bedrock to depths of many km in order to properly predict the environmental impact of an electrode. The modelling above is based on a simplified model of the resistivity structure of the ground. Real geology contains e.g. sub-vertical contacts, faults, folds and anisotropy. Even though such structures must be considered, the conclusions from the modelling are valid in a more complicated environment. What is important to point out is that the four models represent very similar basic geology. They would be more or less impossible to distinguish from each other without a geophysical survey. Such a survey must also be capable of estimating the electrical properties of the ground from the surface down to a depth of several km.
III.
GEOPHYSICAL SURVEY METHODS
It was concluded from in the previous section that it is necessary to estimate the electrical parameters of the ground with reasonable accuracy from the surface down to several km depth. Both the geometry of different rock units and their resistivity must be determined. This is however hardly possible to accomplish with one type of geophysical method alone. The choice of methods is also a function of the geological environment in the project area. Some areas may require quite extensive field investigations, whereas others can be covered with reconnaissance surveys only. Airborne methods can be used to cover large areas. The electric resistivity of the ground is estimated by inducing currents in the ground with the help of an A.C. current in a coil mounted on an aircraft or towed underneath a helicopter. The secondary magnetic field due to the currents in the ground is measured with the help of a receiver induction coil. Measurements can be made either in the time domain or in the frequency domain. The depth of investigation is an effect skin effects so that high frequencies can be used to investigate near-surface layers and low frequencies can be used to investigate rocks at larger depth. It is usually possible to estimate the resistivity of the ground down to depths of 100 to 200 meters and in some cases even more. Airborne methods are very cost-effective but mobilization costs can be considerable and they are hardly an alternative if just a small area needs to be covered. The same type of measurements can be done on the ground as the type of measurements described above for airborne surveys. It is however also common to perform ground measurements with galvanic methods. A lowfrequency current is then injected into the ground with the help of two electrodes. The resulting potential difference between two other locations is then measured with a pair of potential electrodes connected to a high-impedence voltmeter. The depth of investigation is dependent upon the distances between the electrodes. This type of survey can therefore be performed for both detailed near-surface investigations and for deep-probing investigations. There is however a practical limit for the investigation depth since very large amounts of connecting cable needs to be put out in the field. The most common choice of geophysical method for very deep investigations is magneto-tellurics. Naturally occurring electromagnetic fields are measured. The sources to such fields are particle radiation from the sun captured in the ionosphere and distant thunder storms. It can be shown [1] that the magnetic and electric fields from such sources become horizontal and perpendicular to each other over a layered earth at a sufficient distance from the source. Also, the ratio of the electric to the magnetic field strength is directly related to the resistivity of the ground. The electric field strength is therefore measured with the help of two pairs of electrodes perpendicular to each other and two induction coils, also perpendicular to each other. Figure 2 shows the
principal setup for the measurements. Normally, more than one station is measured simultaneously so that fields from close by sources can be filtered out. Magnetotelluric measurements can be made for frequencies down to fractions of mHz. This means that the depth of investigation can be more than 100 km.
Ex Ey
Hx
Hy
Fig. 2. Principal setup for a magnetotelluric survey. Electric and magnetic fields are measured in two perpendicular, horizontal directions.
It is not always possible to perform geo-electric measurements everywhere and with the desired resolution and depth of investigation. It is therefore also important to be able to use auxiliary information to support the modelling. The geometry of a geological unit might be determined from other types of geophysical data like magnetometry or gravity surveys. Geological mapping and modelling will also assist in reducing ambiguities in the geophysical models.
REFERENCES [1]
G. V Keller and F. C. Frischknecht, Electrical Methods in Geophysical Prospecting., Pergamon press. 1966.
