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Simulation EMT PowerFactory- Transformer model
  25 4.3.4.   Power Transformer Power Transformer is the most importance device in the power system, used to transfer the power from high voltage level to lower voltage level and vice versa. The model makes special consideration for auto-transformers. In this section, two-winding transformer (3-phase) and three-winding transformer (3-phase) have been taken to descript the EMT Simulation modelling in PowerFactory v15.1 for this report.   Two-winding Transformer (3-phase) The two-winding transformer model is a very detailed model for various kinds of three-phase, two-winding transformers in power systems. This section describes the general model and is valid for all PowerFactory calculation functions. Particularly, saturation or capacitive effects, which are only relevant for some calculation functions such as high frequency EMT-applications, Harmonics Simulation and so on. For simulating nonlinear, electromagnetic transient such as transformer inrush currents or ferroresonance, core saturation needs to be included into the transformer model. Furthermore, depending on the frequencies involved in the transient simulation, the transformer model has to account for the stray capacitances between windings and winding to ground. The non-linear magnetizing reactance X m   represents the saturation characteristic of the transformer and it is defined in the transformer type ( TypTr2n  /EMT simulation page ). The model supports the following options: - Linear:   no saturation considered - Two slopes:   the saturation curve is approximated by a two linear slopes Figure 4.22 General diagram model of the 2-winding transformer    26 - Polynomial:   the saturation curve is approximated by a polynomial of user-defined order. The polynomial fits asymptotically into the piecewise linear definition. - ,  Msat  i  Magnetizing Current pu -  M  Ψ  Magnetizing Flux pu -  M   L  Linear Reactance pu - 0 Ψ  This parameter is automatically calculated to that the polynomial characteristic fits the saturated reactance in full saturation and transits steadily in to the piece-wise linear characteristic at the knee point.  Pu - ksat   Saturation exponent, i.e. polynomial degree pu Figure 4.23 Two slope and polynomial saturation curves    27 Parameter Description Unit Knee Flux Knee-point of asymptotic piece-wise linear characteristic. Typical value around 1.1 to 1.2 times the rated flux.  p.u. Linear (unsaturated) Reactance Magnetizing reactance for unsaturated conditions  L unsa  .  In p.u. values, the linear reactance is equal to the reciprocal of the magnetizing current (reactance part of the exciting current)  p.u. Saturated Reactance Magnetizing reactance for saturated condition  L  sat  .  p.u. Saturation Exponent Exponent of polynomial representation (  k  sat ). Typical values are 9, 13, and 15. The higher the exponent the sharper, the saturation curves. - - Current/Flux values:   The user can also define the saturation curve in terms of measured current-flux values and select between a piecewise linear  or spline   interpolation. The base quantities of the p.u. values in the current-flux table are also referred to the peak values of the corresponding nominal variables: 3 [][]2103[] basebasebase SMVA IAUkV  = × ⋅⋅   3 []/3[]2102[] basebasebase UkV Vs fkHz π   Ψ ⋅ = × ⋅   The zero sequence magnetizing reactance  strongly depends on the construction characteristic of the transformer core (three-legged, five-legged, shell-type, etc.) and its vector group. Figure below shows the equivalent circuit for the zero sequence. Table 4.2 Basic data of the two-slope and polynomial saturation characteristics Figure 4.24 Equivalent circuit for the zero sequence    28 Transformer with delta-connected windings If the transformer has delta-connected windings, then any zero sequence excitation approximates a zero-sequence short-circuit, as the delta-connected winding short-circuits the zero sequence current. In that case there is no need to represent zero sequence saturation. Transformer without delta-connected windings If the transformer type does not have delta-connected windings, then the zero-sequence excitation current results generally higher than the positive-sequence excitation current and strongly depends on the core type. To account for the higher zero-sequence linear exciting current when no delta-connected winding is available, PowerFactory allows for the definition of linear (unsaturated) zero-sequence magnetizing impedance. This zero-sequence magnetizing impedance and its R/X ratio is defined in the load flow page (TypTr2nLoad flow); the parameters are made available depending on the vector group (i.e. hidden in case of delta-connected winding). To account for the core type dependency of the zero-sequence saturation characteristic, the transformer model supports the following two options in the EMT-simulation page: 3 Limbs core:  use this option for three-legged core designs. In this core type, the fluxes are roughly equal in the three legs and must therefore return outside the core through the air-gap and the tank. Because of the fact that the air-gap and the tanks are no-magnetic, the zero-sequence magnetizing current is nearly linear and therefore the model uses the linear zero-sequence magnetizing impedance defined in the load flow page. In other words, it does not consider zero-sequence saturation effects. 5 Limbs core:  use this option for five-legged and shell-type cores. As the zero-sequence fluxes return inside the core, the model uses the saturation characteristic (of the  positive sequence) in the zero-sequence magnetizing reactance as well.   Three-winding Transformer (3-phase) In PowerFactory   each winding of a transformer can have taps, however only one of the tap changers can be controlled in the load-flow calculation. The adjustment of the taps
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