When the Sun is active and the Northern Lights are swirling overhead, engineers have learned to watch their systems with extra care. At these times, magnetic disturbances are causing induced currents to flow through the conducting networks that mankind has stretched across the Earth's surface during the last 150 years. The early telegraph system was the first to be affected and there are many reports of the "celestial voltages" making it impossible to send messages. In more recent times, the geomagnetically induced-currents (GIC), or telluric currents, as they are also called, have been observed in high voltage power systems and in steel pipelines. In pipelines, the currents are a possible source of corrosion, and also interfere with the pipeline potential surveys that are part of the corrosion prevention work. On electric power systems, the currents cause partial saturation of the transformers and distortion of the ac waveform. This can cause overheating of the transformer, misoperation of relays and system blackouts. The worst effects occurred on March 13, 1989, when a severe magnetic disturbance caused a blackout of the Quebec power system leaving 6 million residents without power for over 9 hours.

GEOMAGNETIC INDUCTION

Telluric activity is produced by variation of the earth's magnetic field. Although several phenomena are involved, there are two primary causes. The first is solar heating. As well as illuminating and heating the day-side of the earth, the Sun also heats the upper atmosphere. The resulting convection currents carry electrons and ions across the earth's magnetic field lines, thereby generating an electric current on the sunward side of the earth. This electric current system is centred over the equator and extends horizontally in the ionosphere at a height of 100 km. It creates a magnetic field fixed in space and pipelines or other long conductors carried through this magnetic field by the earth's rotation experience a regular change in the magnetic field each day.

The second is solar eruptions. The Sun continually radiates particles outward into space, which is termed solar wind. Eruptions on the Sun's surface produce sudden high-speed clouds of charged particles (plasma) that travel through the solar wind to interact with the Earth's magnetic field. When the solar wind reaches the Earth it is deflected by the magnetic field, which itself is compressed on the day-side of the Earth and drawn into a comet-like tail on the night-side. This forms a cavity in the solar wind called the magnetosphere. The arrival of a high-speed plasma cloud from the Sun enhances compression of the magnetic field and increases electric current activity on the outer edges of the magnetosphere. This causes a sudden change or Geomagnetic Disturbance (Storm) in the magnetic field observed at the earth's surface. The solar particles guided into the high-latitude ionosphere give rise to the Northern Lights (Aurora), and to intense electric currents called auroral electrojets.

TELLURIC CURRENTS IN PIPELINES

Telluric current effects have been observed on oil and gas pipelines in many countries around the world. The greatest impact has been observed on pipelines at higher latitudes, such as in Alaska, Canada and Norway, owing to proximity to the auroral electrojets. The magnetic field variations induce an electric field in well-coated pipelines, which in turn drives electrical charges (current) along the pipe. These telluric currents fluctuate in intensity and direction as the magnetic field strength increases and collapses.

For pipelines the significant thing about telluric currents is not the current induced in the pipeline but the variations in pipe potential that they produce. Prevention of pipeline corrosion requires that the electrical potential of the pipe surface relative to the surrounding soil (pipe-to-soil potential) be maintained within a narrow range, typically 300 to 500 mV. This is accomplished by impressing a controlled DC current onto the pipe surface (cathodic protection). Telluric activity, however, cause erratic potential swings often in excess of ±2V. Not only does this make it difficult to accurately determine and control the level of cathodic protection it increases risk of current discharge from the pipe surface (corrosion) during peak swings in the positive half of the telluric waveform.

Pipeline engineers can reduce the effect of this interference by draining the telluric currents off the pipeline during periods of high activity. This can be accomplished to some degree using low resistance sacrificial anode grounding beds distributed along the length of the pipeline, but can be more efficiently controlled using forced current drainage systems. The latter involves use of Ôpotential controlled' DC power supplies at strategic locations along the pipe route to provide both cathodic protection current (Icp) and discharge of telluric mitigation current (It). The output of these units vary in response to changes in the pipe potential, which is continually measured by a permanent reference electrode and compared to a pre-set value. When positive telluric currents (telluric discharge) causes the pipe potential to exceed this set value, the current output is automatically increased to compensate. When the pipe potential exceeds the set value in the more negative direction (telluric pickup) the output reduces to zero. The pre-set value is the potential normally required for steady-state cathodic protection in the prevailing soil conditions.

Potential controlled rectifiers greatly reduce the risk of corrosion due to telluric discharge but they do not eliminate telluric potential swings. When telluric current is present an additional voltage drop in the earth introduces error into pipe-to-soil potential readings which can seriously compromise cathodic protection performance surveys. Since the geomagnetically induced current cannot be interrupted, alternative methods are required to ensure accuracy. These typically include interruptible steel coupons installed next to the pipe to simulate coating defects, combined coupon/reference probe arrangements to record the polarized potential with time, and complex survey techniques utilizing synchronized data loggers and an applied correction factor.

Predictions of geomagnetic activity are available and can be used as an aid in planning pipeline potential surveys and in the design of telluric current mitigation systems. These techniques have now been successfully used for long gas and oil transmission lines in both western Canada and in the Maritimes.