Effects on power transformer
The main area of concern regarding geomagnetic induced current is the large power transformers. Power transformers today have been optimized to the level of only requiring a few amperes of exciting current in order the produce the necessary magnetic flux for the voltage transformation.
Therefore problems arise during a geomagnetic induced current event that can induce quasi DC current that may exceed up to 20 times the peak value of the magnetising current.
There are three main areas that affect the power transformer during an Geomagnetic induced current event:
- Increased reactive power consumption
- Increased even and odd harmonics
- Stray flux heating causing hotspots
Half cycle saturation
Transformers use iron cores in order to reduce the reluctance of the flux path. This is desired in order to keep the circulating current as low as possible. However, by using an iron core non-linearity is introduced into its operation.
This is illustrated in Figure 1.
The DC flux adds to the AC flux in half a period and subtracts in the other. This causes the transformer to operate in the non-linear range of the transformer core leading to saturation for a half cycle. When the transformer operates with a saturated core it causes it to draw a large asymmetrical exciting current full of even and odd harmonics.
Since the excitation current is purely reactive (lags the system voltage by 90°) this results in an increase in reactive power loss.
A test made on a 3 phase bank of single phase 500/230 kV transformers showed an increase in reactive power loss from 1 MV ar during normal operation to 40 MV ar during the geomagnetic induced current event with 25 A induced in each phase. Since a geomagnetic storm effects large parts of the power system simultaneously, the increased demand in reactive power from the transformers can be overwhelming and in worst case lead to a voltage collapse.
Therefore when performing analysis of the impact of GIC it is important to take into account the different kinds of transformers in the power system.
Increased even and odd harmonics
The even and odd harmonics produced by the increased and distorted excitation current creates problem for the other apparatus in the power system. Specifically the protection relays where they can cause unwanted operation.The harmonics usually includes the first 10 orders with varying levels depending on the geomagnetic induced current.
However a large geomagnetic induced current may result in lower Total Harmonic Distortion (THD) since the transformer consumes more of the fundamental frequency current.
With a saturated core the flux will no longer be contained within the transformer. Instead the flux will travel through adjacent paths which might involve the transformer tank or core clamping structures. This stray flux will induce eddy currents that is converted to heat.
Since a geomagnetic induced current event can last for hours this can severely damage the transformer. These hotspots have been noted to damage the paper winding insulation, create impurities in the insulation oil and so on.
|Title:||Geomagnetic induced current as a severe threat to power systems – Linus Khosravi, Erik Johansson – Master’s Thesis in Electric Power Engineering programme at Department of Energy and Environment, Division of Electric Power Engineering, CHALMERS UNIVERSITY OF TECHNOLOGY|
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