What is Geomagnetic Induced Current (GIC)?
Geomagnetic Induced Current (GIC) is the result from our sun’s constantly occurring solar flares that in turn produce Coronal Mass Ejections (CME) that interact with the earth’s magnetosphere. Following Maxwell’s Equations, the charged particles from the CME induces a current in overhead as well as underground power transmission conductors. The resulting phenomena is known as a Geomagnetic Disturbance (GMD).
A GMD event may occur at any time during a solar cycle that repeats every 11 years. In the event of a partially or fully Earth directed CME, the net effect is a reactive power flow that diminishes the performance of the connected power transformers.
Why is GIC Monitoring Important? What does a GIC Sensor do?
Geomagnetic Induced Current may cause serious thermal issues in power transformers leading to a major economic loss. A reliable GIC sensor will monitor GIC fluctuations in real time giving proper alerts and reports aiding in the prevention of an overheating power transformer, protecting, and lowering the risk of a power transformer failure.
Geomagnetic Monitoring (GMD Monitoring): An Introduction
There are many factors that influence the magnitude of the solar storm starting with the volume of charged particles in a CME, the storm’s trajectory towards the Earth, a substation’s Geomagnetic latitude, the transmission line voltage, and the power transformer core types: shell-form, five-limb, three-limb, or single-phase. To complicate matters further, the soil resistance plays a role in whether the DC resistance from one power transformer to another allows Geomagnetically Induced Current (GIC) to flow freely or is impeded. Since GIC may be induced in large bodies of water, GIC may be induced in an Ocean and flow inland. The NERC GMD task force has developed many interesting white papers that can provide additional information about geomagnetic disturbances.
Geomagnetic Induced Current Sensor Solutions – Advanced Hall Effect Sensors
A “Hall Effect sensor” (aka GIC sensor) transducer is used for Geomagnetically Induced Current monitoring because a traditional current transformer measures only AC while Hall Effect sensors measure both AC, DC as well as quasi-DC. This sensor is best installed on the neutral conductor from the H0 or X0 bushing. The GIC sensor chosen should have an intrinsically wide dynamic current range of +500 to -500 Amps while having good accuracy to measure 10’s of Amps and have a bandwidth from DC to 1 Hz. Also, environmentally the sensor should have a wide temperature range and have at least an IP65 rating.
Optical fiber sensors can be used to measure Geomagnetic Induced Currents on the transformer phase conductors as well as the neutral. However, these sensors are quite expensive and add little value to understanding how a specific transformer is affected by the magnitude of GIC a transformer is subjected to. It is no better than a well-designed and robust Hall Effect CT that has the proper dynamic range and accuracy that monitors the GIC neutral current.
Advanced Power Technologies’ GIC Monitoring Solutions
We offer two solutions:
- Total ECLIPSE for GIC patented core saturation detection for GIC, harmonics, and alarming on the potential internal winding, core and flitch plate heating based on encrypted thermal modeling (Figure 1).
- ECLIPSE HECT for GIC measurement reporting only (Figure 2).
Figure − 1 Total ECLIPSE GIC Monitor with Core Saturation Detection and Thermal Modeling
Figure − 2 ECLIPSE HECT for GIC Monitoring
The selection of the ECLIPSE HECT or the Total ECLIPSE will depend on the operating voltage of your transmission systems (200 kV and above), your geomagnetic latitude on the globe, and a specific transformer’s vulnerability to GIC. First, concerning operating voltage, it has been found that the impedances at lower transmission voltage is high enough to attenuate GIC from flowing through the transmission line. Regarding geomagnetic latitude, because there are three Electrojets: Aurora Borealis (Northern lights), Equatorial, and Aurora Australis (Southern Lights) and if your transmission lines are in these areas, the quantity of charged particles from the solar flare in these areas will induce a varying magnitude and polarity of GIC in these transmission lines and in Wye connected power transformers. Finally, and most importantly, more vulnerable power transformer core types will exhibit a higher level of heating than other core types. For example, shell-form, single-phase, five-limb and seven-limb core types may be more vulnerable making the ECLIPSE or Total ECLIPSE is a good choice for these applications.
If the levels of GIC are low and/or the vulnerability of the transformer is not too high, as the case of power transformers with three-limb cores, the ECLIPSE HECT may be the appropriate choice.
Advanced Power Technologies’ Part-Cycle Core Saturation Detection Solution
Our patented solution provides accurate power transformer part-cycle core saturation detection when Geomagnetic Induced Current is flowing. Unlike inrush and overexcitation, part-cycle core saturation due to GIC and DC will produce harmonics where the even harmonic currents will be greater than the odd harmonic currents. This unique harmonic signature while GIC is present is an effective tool for Control Center personnel to have an understanding when a specific power transformer is being thermally affected by taking into consideration the transformer’s loading, top oil, and winding hotspot temperatures. In addition, Geomagnetic Induced Currents of a significant magnitude shall be present to trigger a sequence of three alarms:
- Minor GIC when the GIC level is over a pre-programmed threshold.
- Major GIC when part-cycle core saturation is detected and a Minor GIC alarm is asserted.
- Critical GIC when there is a major alarm and when loading or winding hotspot is over a pre-programmed threshold.
These three alarms provide critical information whether a power transformer is at risk from the effect of Geomagnetic Induced Currents and thus allow the operators to shed load off the transformer or take it out of service.
Encrypted thermal modeling during a GMD event
Advanced Power Technologies, in partnership with Hitachi ABB Power Grids, has added the ability to both the ECLIPSE and Total ECLIPSE to track additional heating of the transformer’s windings and structural parts caused by GIC. This is achieved by programming Hitachi ABB Power Grids’ GIC Thermal models. Thermal models applied would be specific to the power transformer that the ECLIPSE or Total ECLIPSE is fitted with. Now you will know the level of GIC as well as predicted temperature increases in the tank because of GIC. Hence, this thermal modeling solution along with our patented core saturation detection algorithm provides a complete Geomagnetic Monitoring solution to provide situational awareness to GMD events.
