上海意兰可电力电子设备有限公司shanghai.docVIP

上海意兰可电力电子设备有限公司Shanghai InducTek Electric Electronic Equipment Ltd. Shanghai InducTek Electric Electronic Equipment Ltd. Building 5#,ZhenZhongXin Rd. No.599, XiaoKunShan Town, SongJiang, Shanghai, China Tel：(021 Selection and Rating of Transformers A). Introduction The number of non-linear loads – which draw non-sinusoidal currents even if fed with sinusoidal voltage - connected to the power supply system is large and is continuing to grow rapidly. These currents can be defined in terms of a fundamental component and harmonic components of higher order. In power transformers the main consequence of harmonic currents is an increase in losses, mainly in windings, because of the deformation of the leakage fields. Higher losses mean that more heat is generated in the transformer so that the operating temperature increases, leading to deterioration of the insulation and a potential reduction in lifetime. As a result, it is necessary to reduce the maximum power load on the transformer, a practice referred to as de-rating, or to take extra care in the design of the transformer to reduce these losses. To estimate the de-rating of the transformer, the load’s K-factor may be used. This factor is calculated according to the harmonic spectrum of the load current and is an indication of the additional eddy current load losses. It reflects the excess losses experienced in a traditional wire wound transformer. Modern transformers use alternative winding designs such as foil windings or mixed wire/foil windings. B). Transformer losses Transformer losses consists of no-load (or core loss) and load losses. This can be expressed by the equation below. where: PC = core or no-load loss, PLL = load loss PT = total loss. Core or no-load loss is due to the voltage excitation of the core. Although the magnetizing current does include harmonics, these are extremely small compared with the load current and their effect on the losses is minimal. Load losses are made up of I2 R loss