Impact of HVDC Cable Configuration on Compass Deviation Sander MEIJER, Roald DE GRAAFF, Frank DE WILD; DNV GL, Netherlands,
[email protected],
[email protected];
[email protected] Stephen HEMPHILL, Mick MCGUCKIN; Mutual Energy, Ireland,
[email protected],
[email protected]
ABSTRACT Due to four recent faults in the low-voltage return conductor of the Moyle Interconnector cable, alternative solutions to re-establish the dual monopole operation have been searched for. One of the consequences of changing the configuration of the high-voltage conductor and low-voltage return conductor is a change in the induced magnetic fields, which has an impact on the compass deviation. Without mitigation, deviations of up to 180° could occur near the shore and deviations up to 5° as far as 1.7 km from shore. Theoretical studies, backed up and verified by ship surveys, have been used to assess the impact of laying new replacement low-voltage conductors alongside the existing high voltage conductors has on the compass deviation caused by both.
return conductor
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KEYWORDS HVDC, Compass Deviation, Magnetic Fields
INTRODUCTION The Moyle Interconnector between Ireland and Scotland is a 500 MW Dual Monopole HVDC link. Due to four cable faults on the Moyle Interconnector, all caused by the same type of failure of the integrated return conductor (IRC) insulation, and all occurring within a period of 21 months, Moyle examined three options to either replace or remove the need for the low voltage integrated return conductors: 1.
2.
3.
application of one of the HV conductors as LV return conductor, achieving a single 250MW monopole (as an emergency fall back in the event of simultaneous LV cable faults on both poles); installation of two new new separate LV Cables to re-establish 500 MW dual monopole operation (using the existing HV and new LV); amendment of the convertor station controls for bipole operation, at 500MW.
Besides many other aspects, the impact of such configuration change on the magnetic fields was assessed for each of the above options. This contribution discusses the implication of those changes to EMF characteristics regarding onshore and offshore legislation and regulations, in particular compass deviation.
MAGNETIC FIELDS The Moyle cable has a special coaxial design. In particular, the insulated HVDC conductor is surrounded by the integrated return conductor, see figure 1. As a result, the magnetic fields induced by the current in the highvoltage and low voltage conductors mostly cancel each other.
HVDC conductor Fig. 1: Simplified cross-section of the Moyle cable. However, by changing the cable system configuration to a situation with a new external return conductor, this will affect the magnetic fields around the cables. The basic formula to calculate magnetic field amplitude at a distance r from a straight conductor is given in the following:
B
0 r I 2r
The resultant magnetic field originates from the current flowing through the high-voltage conductor, the return current flowing through the return conductor and the earth magnetic field. To study the effect of DC magnetic fields introduced by the DC current flowing through the submarine cables on the use of magnetic compasses, three compass deviation limits will be assessed: 3, 5 and 10 degrees. To evaluate the occurring compass deviation, the resulting magnetic field above the cables needs to be determined, and from this the compass deviation can be calculated. Only the horizontal component of the magnetic field is relevant for compass orientation. In the geographic area of the Moyle interconnector the horizontal component of the earth magnetic field has a magnitude of around 17.9 µT, with a declination of around -3.8 degrees [2] (so pointing eastwards against the north direction), also see figure 2.
-5
x 10
Compass deviation, threshold of 3 degrees |Bcalc | 6105
6
MAG
6100 4
Y (km)
Magnetic field (F)
8
2
0
6095 0.5 6090
1
1.5
2 2.5 distance (m)
3
3.5
< 3 degrees
6085
4 4
x 10
> 3 degrees Fig. 6: Comparison of measured and calculated 6080 magnetic330 field for340 survey350 on Scotland side. 360 370 X (km) Compass deviation, threshold of 5 degrees 6105
Y (km)
6100 6095 6090 < 5 degrees > 5 degrees
6085 6080 330
340
350 360 370 X (km) Compass deviation, threshold of 10 degrees 6105 6100
Y (km)
Fig. 2: Top: Horizontal component of the earth magnetic field in nT (2010) and bottom: Earth magnetic field declination in degrees (2010) [1].
6090 < 10 degrees > 10 degrees
6085
The simulation software package MATLAB has been used for the calculations in this contribution. All the relevant materials for the model have a relative magnetic permeability of 1.0. This is the case not only for the copper core and IRC, the lead sheath, but also for the air, the sea water and the sea-floor. The distance between the two existing pole cables is between 1 km and 2 km. This is a much larger distance than the distance at which the magnetic field of a single cable diminishes, typically only 1% of the magnetic field is left after 10 m, considering maximum magnetic field at the
6095
6080 330
340
350 X (km)
360
370
Fig. 4: Comparison of compass deviation for a counter clock-wise current and three threshold values surface of the conductor with a diameter of approximately 10 cm. It is also a much larger distance than the average depth of the sea floor. Therefore, we can consider both cables as individual sources of magnetic fields. There is negligible mitigating effect from one cable to the other. The magnitude of the magnetic field has been calculated at sea level, both in the X (West-East) and Y (SouthNorth) direction, along both DC cables. The magnetic field in the Z direction (so perpendicular to the sea) will have no effect on the compass reading and is therefore not considered. From this, the resulting magnitude and direction of the magnetic field can be determined along the cables. Together with the magnitude and inclination of the earth magnetic field, the compass deviation for each point along the cables at sea level is calculated, figure 3. From the calculated magnetic field the compass deviation is determined and compared with the threshold values of 3, 5 and 10 degrees, see figure 4. There are some sections where the compass deviation exceeds the threshold value of 3 or 5 degrees. This can be explained by the following reasons:
Fig. 3: Compass deviation due to Moyle’s magnetic field
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due to the shallow water near the coast (shore crossings).
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the magnetic field has almost the same orientation as the earth magnetic field. Since the resulting magnetic field at sea level is perpendicular to the cable, this results in increased compass deviation and even 180° deviation was estimated.
Ireland side. The cause for this shift was not investigated as the shape and amplitude are the most important parameters, and as it only occurs on one side, but it is likely a transformation error during processing of the geographic coordinate data of the ship survey, or an inaccuracy of the cable location data.
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due to hairpin repairs which means that there is a pronounced angle of approximately 40 degrees between cable sections. Hence the magnetic fields generated by both sections are not parallel, but also have this angle. The resulting magnetic field near the segment joint has a higher magnitude than the individual fields. This effect is absent if the angle between segments is zero degrees (i.e. straight cable) and maximal for an angle of 90 degrees (i.e. perpendicular cable segments).
Secondly the compass deviation is measured and simulated when crossing the cables, see figure 7. The amplitude and direction of the calculated compass deviation are in agreement with the measurement results.
MODEL VERIFICATION To validate the software model several ship surveys were performed in the shallow waters around the landing points on both sides of the two poles. During these surveys the interconnector was in bipole mode utilising the high voltage conductors in pole 1 and pole 2 , meaning that the current in pole 2 is in East-West direction, and in pole 1 in West-East direction, resulting in a counter-clockwise circulating current. During the surveys the integrated return conductors did not carry current.
It can be concluded that the theoretical model is verified by the ship survey results.
LOCATION OF THE RETURN CABLE Mutual Energy decided to install new replacement lowvoltage return cables as the solution to restore the original transmission capacity of the Moyle interconnector. The fact that a separate cable will be used offers the possibility to locate it in such a way, that it will contribute in the mitigation of the magnetic fields and thus the compass deviation. For that purpose, using the previously described and verified theoretical model, the impact of the return conductor location on the compass deviation was assessed for a number of configurations, see figure 8: 0.
‘Bipole’, in this configuration there is no return cable, and this configuration is included for reference only.
During these ship surveys, recordings were made of the ship location, the magnetic field and of compass deviations. The compass deviations were obtained by comparing the readings from a magnetic compass with those from a GPS compass. For comparison, the magnetic fields and compass deviations were calculated along the recorded ship trajectories for the situation that the Moyle interconnector is in bipole configuration. Figure 5 shows the trajectory of the ship on the Scottish side of the Moyle cable. The recordings started at the square (left bottom) and ended at the circle (right top). The total length of the travelled distance was around 35 km. Figure 6 compares the measured with the calculated results using the model discussed in the previous sections. The figures show a match between the magnetic field amplitude and wave shape. For the Scotland side the peaks in the magnetic field pattern are shifted by approximately 1 meter. This is not seen on the Northern
Fig. 5: Ship trajectory on Scotland side.
Fig. 7: Comparison of measured and calculated compass deviation at a cable crossing near the Irish side.
cable next to the rock-dump. This means there will be a distance between both cables and thus less efficient cancelling of magnetic fields. The rock-dump bank has different widths, from 2.5m to 7.5m. The HV cable will be buried 0.5m deep. Figure 9 shows that at the locations where the bank is less wide, the LV cable will be closer to the HV cable and the mitigation will be more efficient. For all situations, the compass deviation is less than 1° at about 900 meters from shore. Fig. 8: Evaluated return cable locations. 1.
‘Co-trenching’, with the cables touching (diameter HV cable: 116mm, diameter return conductor: 99mm)
2.
‘Next to rock dump’ for distances d of 2.5m, 4m and 7.5m, and
3.
‘Buried apart’ at distances d of 10m and 15m
For all situations, the return cables are placed south of the high voltage cables. Figure 9 shows the results for the North Cable near the Irish side. Several observations can be made, which will be discussed below. Close to shore the unmitigated compass deviation can be as high as 180°. This can be explained by the facts of shallow waters and the cables are laid almost perpendicular to the earth magnetic field. Depending on the direction of the current, this means that the earth magnetic field will either be increased when the induced magnetic field is pointing in the same direction, or can be decreased when the induced magnetic field is pointing in the opposite direction. Once the induced magnetic field is greater than the earth magnetic field, the net magnetic field will point in the opposite direction and thus the compass will also point in the opposite direction. It can also be seen that the temporary bipole configuration always results in the highest compass deviation. This can be explained by the fact that the cables are separated far apart and the induced magnetic fields will not cancel each other. All locations of the low-voltage conductor will help in reducing the magnetic fields and thus the compass deviation. Only the co-trenching solution will actually totally eliminate the effect due to the highest efficiency in cancelling the induced magnetic field by the high-voltage conductor. However, co-trenching will be a delicate process to realize. The existing cable has to be located accurately and excavated or de-buried. Then the new lowvoltage conductor needs to be installed in the same trench as the HV conductor and somehow attached to it. Then the bundle has to be buried again. This might lead to significant risks to the existing cable. To minimise risks during installation of the new replacement LV conductor, but still benefit from the cancelling effect, it could be decided to locate the LV
A final studied configuration for the LV cable is to have a separate trench at 10m or 15m from the HV cable. Probably this is the easiest and less risky approach. However, as can be seen from figure 9, this will also have less effect on the reduction in compass deviation. Still the compass deviation will be less than 5° when around 500 meter from shore.
CONCLUSIONS Based on the investigations described in this study, the following conclusions can be drawn: 1.
2.
3. 4. 5.
6.
7.
Due to the change from coaxial cable arrangement to separate high-voltage and lowvoltage return conductors, a net magnetic field will be generated by the Moyle interconnector. The net magnetic field may lead to a deviation in the compass readings, depending on the current magnitude and direction. The theoretical model has been verified by performing ship surveys. The unmitigated compass deviation can get as large as 180°. Without mitigation estimated compass deviation would be greater than 5° for the first 1.7km from shore. Depending on the position of the new lowvoltage return cable, the region with a compass deviation can be reduced to 400-900m from shore. Quantified risk assessment was applied to assess the potential health & safety risks associated with any sections which, post mitigation, compass deviation was sticll considered too high.
REFERENCES [1] British Geological Survey, "The Earth's Magnetic Field: An Overview", paragraph 3.2, URL: http://geomag.bgs.ac.uk/education/earthmag.html#_T oc2075556 [2] “Magnetic Field Calculators,” National Oceanic and Atmospheric Administration, National Geophysical Datacenter, http://www.ngdc.noaa.gov/geomagweb/#igrfwmm
125
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Compass deviation [°]
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1 LV cable 2.5m to the south, 0.5m above cable
LV cable 4m to the south, 0.5m above cable
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LV cable 7.5m to the south, 0.5m above cable Bipole
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Fig. 9: Compass deviation for evaluated return cable locations.
