a 2 R ja 2 x l2 R c jx m1

Transcription

1 Electric Machinery Labs NQN_version.0 Lab Exercise 6: Siulation of Inrush Currents in Three-Phase Transforers Students ID Group Date I. Introduction: Transforer plays an iportant role in the power transission and distribution syste, for transferring energy between different voltage levels. Transission systes norally transfer large aount of energy, hence the use of three-phase transforers. Definition: Three-phase transforer is a static electrical achine, used for transferring energy fro one three-phase voltage/current syste to another three-phase voltage/current syste, due to electroagnetic induction phenoenon. Transforers are constructed fro agnetic cores and windings. The lab exercise is constructed to help students build atheatical odels of threephase transforer in the siulation environent of MATLAB/Siulink, used for investigating inrush currents when energising the transforer. Magnetic uration will be considered in the transforers odel. II. Model of urated three-phase transforers. Model of a two-winding transforer (Fig. -) + i i /a + v a Z v R jx l Fig. - R c a R ja x l jx i = x l = ωbv ωbr dt xl + x l xl x = xm un i = x l = ωbv ωbr dt x l = + + x x x x M un l l = ωbλ = ω λ b ω b is the base frequency for inductance calculation Page

2 Electric Machinery Labs NQN_version.0 A look-up table can be used to ipleent the uration characteristic of the agnetic core, where flux linkage is calculated at any tie step fro the unurated value and the difference between it and the urated value. The difference between to unurated and urated flux linkages can be deterined fro the open-circuit test. For three-phase transforers, depending on the agnetic core structure and three-phase connections, one can use the single-phase equivalent circuit above, or the full three-phase equivalent circuits. Consider a Y-Y connection for a three-phase transforer (Fig. -), a single-phase equivalent circuit can be developed, as shown in Fig. -3. Fig. - A three-phase transforer with Y-Y connection Fig. -3 Single-phase equivalent circuit of a Y-Y three-phase transforer Three single-phase equivalent circuits can be put together for a full equivalent circuits, for investigating inrush currents in all three phases, or unbalanced operating conditions. Consider now a three-phase transforer with -Y connection, the full odel of the achine can be seen in Fig. -4. Note the way three single-phase equivalent circuits are linked to for a full equivalent circuits. Page

3 Electric Machinery Labs NQN_version.0 Fig. -3 Single-phase equivalent circuit of a Y-Y three-phase transforer Fig. -4 Full odel of a -Y three-phase transforer Page 3

4 Electric Machinery Labs NQN_version.0 A Look-Up Table was used to siulate agnetic uration (Fig. -5), and the load circuit can be used to link secondary current to the secondary voltage (Fig. -6). Dpsi table will be set in the initialization script. Fig. -5 Look-Up Table for agnetic uration siulation Fig. -6 Load circuit III. References:. A.E. Fitzgerald, Charles Kingsley, Stephen D.Uans, Electric Machinery, McGraw Hill.. Chee-Mun Ong, Dynaic Siulation of Electric Machinery: Using MATLAB/ Siulink, Prentice Hall. IV. Lab exercise goals:. Build a three-phase transforer odel in MATLAB/Siulink.. Study paraeters of the odel, and an approach to incorporate agnetic uration into the odel. 3. Siulate inrush currents when energising transforer under no-load conditions. V. Lab equipent: - PC with MATLAB/Siulink installed, pen, and ruler. VI. Siulation of inrush currents in three-phase transforers: Page 4

5 Electric Machinery Labs NQN_version.0 - Build a single-phase odel for a two-winding transforer, with agnetic uration and load circuit. - Build a three-phase transforer s odel. - Connect three-phase voltage sources to input terinals, note that the phase displaceent for all three-phase voltages are 0. - Modify the para. file to reflect the achine s paraeters as given to the group by the instructor. - Run the initialization procedure (to siulate open-circuit condition) and then the siulation. - Obtain all three input currents for difference starting conditions: different initial phase angles for source voltages, different level of agnetic reanence in phase legs. Coent on the effect of initial phase angle and level of agnetic reanence on inrush currents. - Run an FFT analysis on the collected current wavefors and give coents on the results. VII. Lab report: - Hand in the report no later week after all the lab exercices have been finished. - Note clearly student s naes, ID nubers, group nuber, date of exercise. - Lab results should be clear, concise, and coplete as required. - Plagiaris ight result in zero arks being given to all partners. Page 5