Research On Effects Of Geomagnetically Induced Current On Large Power Transformers

Geomagnetically induced current (GIC) will affect the secure operation of large power transformers. Along with the development of UHV power grids, the China's power grids will be larger and next fastigium of solar activities is coming, so the probability of transformers influenced by GIC will increase. The study of the effects of GIC on transformers is necessary. With the support by the National High Technology Research and Development Program ("863" Program) of China (2007AA04Z425 Analysis and control technology of the effects of geomagnetic storm on large power grids and transformers), this paper studied the impact mechanism of GIC on large capacity transformers and developed a simulation system for studying the effects of GIC on transformers. This paper also proposed the evaluation method for the risk of transformer influenced by GIC and evaluated the risk of transformers in 750kV planning power grid at China's Northwest district influenced by GIC. The main research contents are as following:(1) The impact mechanisms of GIC on the harmonic current, reactive power loss, vibration, noise of transformer and transformer loss and temperature rise are studied, and the transformer influence affairs caused by the GIC in the world are analyzed. Based on the study, the harmonic current, reactive power loss of transformer and the transformer temperature rise caused by the leakage flux are the major reason of GIC influencing the secure operation of power grids and transformers.(2) The transformers withâ–³connected windings are chosen as the study objects. The variety of the magnitude of every order harmonic current and the total distorted rate of harmonic current from the transformer normal operation to the deep dc bias are simulated and analyzed. The distribution and circulation of transformer harmonic current in different connected windings are analyzed by the transformer equivalent model under GIC.(3) The distributions of transformer main flux and leakage flux under GIC are analyzed and the influence factors of leakage flux are discussed. The effects of load current on the leakage flux of 500kV shell-form transformer and 500kV core-form transformer are simulated and analyzed by the two-dimension finite-element models of transformer, and the effect of leakage flux on the transformer tank loss is also analyzed.(4) The transformer model based on the J-A theory and the three-dimension finite-element model of transformer are developed. Based on the transformer models, a simulation system for studying the effects of GIC on transformers is developed. Using the simulation system, the effects of GIC on the 500kV shell-form transformer and 500kV core-form transformer are studied and the acceptable GIC limits in transformers are discussed based on the transformer operation guide.(5) The evaluation method for the risk of transformer influenced by GIC is proposed and the existing dc equivalent model of power grid for calculating the GIC level of transformer is improved. The GIC level of transformers in 750kV planning power grid is calculated. Based on the calculation result, the risk of transformers in 750kV planning power grid is evaluated and the defending measure is suggested.