273_manana - Effects of Magnetizing Inrush Current on Power...

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Effects of Magnetizing Inrush Current on Power Quality and Distributed Generation Ma˜nana, M.; Egu´ ıluz, L.I.; Ortiz, A.; D´ ıez, G.; Renedo, C.; P´erez, S. Department of Electrical Engineering E.T.S.I.I.T. University of Cantabria Avda. Los Castros s/n 39005 Santander, Cantabria Spain Phone: +34942201378 Fax: +34942201385 Email: mananam@unican.es Abstract. When a power transformer is energized there is an important transient inrush of current that it is necessary in order to establish the magnetic field of the transformer. Some power transformers exhibit peak current demand up to eight or ten times the nominal value. In addition, during the first cicles high values of the homopolar components of current are also requested by the transformer. If the power transformer is placed in a substation that works as a common coupling point for distributed generation facilities some specific power quality problems can be found. This paper analyzes the problem from a general point of view considering not only the theoretical approach but also the results obtained in a real case including two 180 MVA power transformer and a 120 MW distributed generation. Keywords . Power Quality, distributed generation, protec- tions, inrush current, homopolar components 1. Introduction From a general point of view, the magnetizing of power transformer can be the origin of some power quality problems. First at all, it should be underlined that the transient of current that is required to establish the magnetic field of the transformer during the mag- netization can not be considered a fault condition so it should not cause protective relays to operate. A basic approach to the problem can be done considering a coil surrounding a ferromagnetic material which is supplied from a sinusoidal voltage [1]. The steady-state flux can be computed as the integral of the supplied voltage v ( t ) , φ ( t )= 1 N Z v ( t ) dt = 1 N Z cos( ωt ) dt = 1 ωN sin( ωt ) (1) Equation 1 highlights the fact that flux lags the voltage by 90 degrees. Considering the coil as a linear inductance, the current will have exactly the same waveform as the flux, that is, i ( t )= 1 L Z v ( t ) dt (2) However, it is well known that the ratio between the flux density φ and the magnetic field intensity H in a ferromagnetic material is linear while H is lower than a value that define the knee of the saturation curve. Transformers are designed to operate close the knee of the saturation curve of the core hysteresis loop when the machine is working under normal conditions. When a transformer is connected after a period out of service, the conditions of the analysis are different. Considering that the transformer is energized when the voltage is zero and there is not residual flux or in the worst case, the residual flux has a value that is the opposite to the theoretical means that the resulting flux will be up to twice its normal maximum value. In this case the magnetizing current will
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This note was uploaded on 09/28/2010 for the course EPS 090229 taught by Professor Nxhviet during the Winter '10 term at Hanoi University of Technology.

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273_manana - Effects of Magnetizing Inrush Current on Power...

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