What is Geomagnetic Induced Current?
Geomagnetic Induced Current (GIC) is the result of coronal mass ejections (CMEs) from the sun. CMEs interact with the earth’s magnetosphere through a process called electromagnetic induction. This results in geomagnetic disturbances (GMD).
There are two kinds of solar activity that have the potential to disrupt our lives on earth: solar flares and CMEs. Watch this video from NASA to learn the difference.
When CMEs occur, magnetic particles eject from the sun’s surface and transport in the solar winds. This generates rapidly changing magnetic fields as they move across the earth’s surface. The magnetic fields interact with the earth’s conductivity resulting in ground voltage differences. The ground voltage differences result in the flow of GIC current. Finally, the current flows through the neutral conductor of wye-connected transformers, interconnected by high-voltage transmission lines.
Where do Geomagnetic Disturbances occur?
- GMD events are just as likely to occur in the southern hemisphere as the northern hemisphere.
- Industry studies have identified locations with latitudes above 45-50 degrees may be susceptible to GMD events.
- The probability of a GMD event increases based on the sunspot count. Yet GMD events can occur at any time during the solar cycle.
To better evaluate whether your transformers are at risk, read through the NERC standard TPL-007.
What is the effect of GIC on a Transformer?
The flow of GIC in transformers is the root cause of all GMD related power system problems.
- The magnitude of the GIC current and the associated DC offset is superimposed on the excitation current which forces the transformer into part-cycle saturation.
- GIC can result in increased reactive power requirements and large harmonic currents during GMD events.
- The harmonics can result in tripping of VAR compensation devices at times when additional VARS are needed most. This results in system disturbances and instability. Large GMD events are often associated with a variety of system alarms.
What standards are available in reference to GIC?
The increase in awareness of geomagnetic disturbance events has led to the creation of new standards in the US, including FERC order 830. This resulted in the creation of NERC standard TPL-007. The standard establishes guidelines on planning for geomagnetic disturbances.
All transmission and generator owners with grounded wye connected transformers operating with a high side voltage of 200kV or greater are subject to the requirements of TPL-007. The standard requires owners to perform an assessment of their assets and collecting GIC data to validate system modelling of GIC flow during GMD’s.