Post date: Jul 21, 2014 7:33:5 AM
Power systems can suffer heavily from disturbances in the earth’s magnetic field. Changes in the magnetic field induces quasi-dc currents (one with frequency much less than 1 Hz) in high voltage, AC transmission lines [1]. As mentioned in [2], this causes half-cycle saturation in the transformers, causing harmonics and increase in reactive power demand. The loss of reactive power support can lead to voltage collapse, and/or transformer overheat, potentially resulting in equipment damage.
The impact of these geomagnetically induced currents (GICs) on the power grid can be modeled using elementary voltage/current sources. Example modelling can be found in [3-5] for power flow and [6] for transient/stability applications. The electric field induced by geomagnetic disturbance (GMD) can be modeled as a DC voltage source in series with the transmission lines [7]. The voltage source can be calculated as follows.
(1)
where the elements of the DC voltage vector V includes substation neural voltages and bus voltages. Let s denote the number of substation neural voltages and m the number of bus voltages. Thus V has size s+m, with the first s elements being the substation neural voltages. GIC is then calculated by dividing V over the substation grounding resistance.
The parameters of (1) are I and G. I captures the impact of GMD-induced dc line voltage as Norton equivalent dc current injections.
G is power system bus admittance matrix with special properties: 1) it is a real matrix with just conductance values, 2) conductance values are determined by the parallel combination of the three individual phase resistances, 3) G is augmented to include the substation neutral buses and substation grounding resistance values, 4) transmission lines with series capacitive compensation are omitted since series capacitors block dc flow, and 5) transformers are modeled with their winding resistance to the substation neutral in the case of autotransformers.
Practically, G is not completely measurable. The determination of G depends on several factors, some of which are readily available or easily estimated from standard power flow models. These includes network topology, transmission line resistance, transmission line series compensation, and transformer’s resistances and configuration (wye or delta) [5]. However, the substation grounding resistance [2,9], which is an important factor for an accurate estimation of G, is hard to measured. The reason is because it depends more on the local soil and earth condition than the construction of the substation grounding grid. Furthermore, the grounding resistance can vary by more than an order of magnitude, from 0.05 to 1.5 ohms. It is those characteristics of the substation grounding resistance that makes it an interesting subject in the study of GIC sensitivity.