This chapter gives a brief overview of how geomagnetic currents arise, the processes that occur from Sun to earth and how they affect the power system. The propagation of coronal mass ejections through space affects the magnetic field of the earth, giving rise to variations in potential on the Earth’s surface. This potential difference results in geomagnetically induced currents (GIC) flowing and affecting transformers with grounded neutrals.
1.2 BACKGROUNDGeomagnetic induced currents (GIC) are quasi-dc currents with frequencies ranging from 0.001~0.1Hz 1 that flow through the transformer neutrals into the power system network. The peak values could be as high as 200A, lasting for several seconds to hours. The peak value ever calculated in Southern Africa is 108A, at Alpha substation in South Africa 2.
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In brief, geomagnetic induced currents are a result of solar storms. The Sun goes through 11-year solar cycles, with solar activity increasing towards the end of each cycle 3. The last solar cycle ended in the years 2008-2009 and it was the 23rd solar cycle since the first recorded cycle in 1755 4. Although the GIC activity peaks towards the end of the cycle, they are not limited to occurring at peak times only.
The chain of events that lead to geomagnetic disturbances begins from the Sun’s activities and ends when geomagnetic induced currents interfere with technological systems such as telecommunications and power systems at the Earth’s surface as shown in Figure 1.1. Figure 1.1: The space weather or GIC chain 3The hot, outer layer of the Sun is known as the corona 5. The corona is made of hot plasma, reaching temperatures between ?1×10?^6K to 6x?10?^6K. Periodically, the Sun loses mass in the form of coronal mass ejections (CMEs). The CMEs are ejected into the space towards the earth.
The occurrence of a CME is solar cycle dependent. Each directed CME hit the earth’s magnetosphere and causes distortion of its magnetic field. CMEs stretch the magnetosphere on the night-side of earth causing it to release energy through magnetic reconnection (see Figure 1.2). Figure 1.2: Reconnection of the magnetosphere due to interference of coronal mass ejections (CMEs)The perturbations in the magnetosphere have an impact on the stability of the ionosphere 6. The dynamic changes in the magnetosphere link with the ionosphere through the ionosphere’s polar regions.
During the magnetosphere-ionosphere interactions, the magnetosphere’s current system transfers energy to the ionospheric particles. These variations and couplings result in auroral and other electrojets in the ionosphere, which are horizontal electric currents flowing in the D and E layers of the ionosphere 7. The variation of the magnetosphere-ionosphere electric fields result in temporary variation of the earth’s magnetic field at the earth’s surface. The potential difference, gives rise to the flow of geomagnetic induced currents. The conductivity profile of the earth’s surface determines the surface impedance which in turn determines the characteristics of the resultant geo-electric field.
The magnitude of the induced electric field depends upon the rate of change of magnetic field, and the earth’s conductivity. The relationship between the changing magnetic and electric fields are given by the Maxwell-Faraday equations:?xE=-?B/?t where ?x is the curl operator (1.1)?_?E^ ??E.dl=?-?/?t ?_?? ^ ??B.ds? (1.2)V=-??/(?t ) Faraday^’ s Law (1.3)Countries close to the earth’s poles such as Canada, Norway, Sweden and Russia are more vulnerable to geomagnetic induced currents. The risk of geomagnetic induced currents is larger in networks located at highly resistive regions as on igneous rocks 3.
A detailed mathematical model which confirms that a more resistive earth gives higher electric fields is given in 4. 1.3 OBJECTIVESThis project seeks to have an in depth understanding to the transformer responses to geomagnetic induced currents. The work will be centred on a 3p-5L core structure. In brief, the objectives of this thesis are: To investigate through experiments and FEM simulations, the reactive power consumption of 3p-5L power transformers under the influence of geomagnetic induced currents. To investigate the thresholds of geomagnetic induced currents that may cause noticeable degradation to power transformers. 1.
4 HYPOTHESISTwo hypothesis are to be tested:These tests will be used to prove statistically two hypotheses;a) H2a Tests on model transformers and extension of the results to power transformerswith suitable transformer equivalent circuit and FEM simulations will improve theconventional models of the reactive power requirement in transformers conductingGICs.b) H2b Thresholds of GICs initiating damage in transformers, based on identifiablemechanisms of degradation, can be determined from the practical records oftransformer degradation leading to relatively early failure, and calculation of theassociated GICs.1.5 RESEARCH QUESTIONSThe following research questions have been set up to assess the project hypothesis: How does reactive power increase in transformers saturated by the flow of GIC affect power system stability? What is the role of installing GIC monitoring devices in order to fully understand the phenomenon behind the risk of quasi-dc current to transformers? How does different structure of transformers affect their response to GIC? What are the different levels of GIC that cause noticeable degradation in power transformers? How does the reactive power consumed by a power transformer vary with respect to GIC? What is the implication of general power theory in determining reactive power absorbed by the transformer as opposed to conventional methods of calculating power?CHAPTER 2: LITERATURE REVIEW2.1 INTRODUCTIONThis chapter introduces details of past GIC events and the extent of damage they caused to transformers. Records of currents that flow in transformers that were damaged by the Halloween Storm (2003) and the Hydro Quebec events are also given. In addition, a detailed explanation of the transformer electrical responses to geomagnetic induced currents, with particular reference to 3p-5L transformers that form the basis of this study is reviewed. 2.
2 HISTORICAL EVENTSThere are three main events that occurred since the geomagnetic induced currents were discovered. The first event being, the Carrington event (1859) discovered by the British astronomer Richard Carrington which, for the very first time in the history, observed a solar flare. The second event in the history of GICs was the Hydro-Quebec event (1989). This had devastating effects on the entire Quebec power system, causing a blackout and an estimated loss amounting to $13.2 million 4. The most recent event was the Halloween Storm (2003) that ravaged a couple of transformers in Eskom’s network (South Africa), Sweden and England.
This event cleared the popular belief that Africa was not prone to the geomagnetic induced currents