Research On Short Circuit Calculation Method Of Distribution Network With Induction Generator

This thesis was supported by National Natural Science Foundation of China(51277184) and independent research project of State Key Laboratory of PowerTransmission Equipment&System Security and New Technology(2007DA10512709209).Connecting distribution generation(DG) with power grid is an effective way tosolve the current power shortage and environmental problems. Induction generator(IG)is widely used in the fixed speed wind power generation system due to lower cost,smaller size, and less maintenance requirements. Without considering theelectromagnetic transient characteristics of IG, using the voltage source equivalentmethod for induction motor to calculate the short circuit current of IG will result inlarge errors. When a short circuit fault occurs, IG will inject the positive and negativesequence components of the short circuit current into the network. The integration oflarge scale IGs will affect the relay protection and reclosing in the distribution network.The calculation of short circuit impulse current and other limit electrical parameters alsoprovides guidance for the rational design and reliable operation of IG. Correctcalculation of the sequence components of the short circuit current for multiple IGs andeffective evaluation of the three phase short circuit current peak for a single IG in thedistribution network are important. The main contents are as follows:The fault responses of IG under different short circuit ratio(SCR) are analyzed, andthe dynamic current source model based on terminal voltage of IG is established. Basedon the space vector expression of the stator short circuit current under asymmetric fault,this model reveals the interaction between the sequence components of the short circuitcurrent and the sequence networks of the distribution network. The injection of the shortcircuit current changes the distribution of nodal voltages in distribution network, and theterminal voltage of IG changes with the decaying current, then it affects the short circuitcurrent in reverse.An iterative algorithm to determine the sequence and phase components of shortcircuit current for distribution network with IGs is proposed. Based on the symmetricalcomponent method, the formula of short circuit current sequence components at thefault bus considering its negative sequence voltage is derived according to its compositesequence network. The formula to determine the initial value of the short circuit current sequence components is derived according to the constant flux linkage theorem, andthen the formula to calculate the RMS of the short circuit current sequence componentsin half a cycle is derived. Based on the proposed dynamic current source model, theiterative principle in each iterative step is formed. The proposed iterative algorithm issuitable for the short circuit current calculation of distribution with multiple IGs.On the basis of the analytic time domain formula of three phase short circuitcurrent, an evaluation method to determine current peak value of three-phase shortcircuit for IG in distribution grid is proposed. The impacts of rotor speed change on thedirect and periodic components of the short circuit current are analyzed. The electricaltorque of IG is calculated according to its static equivalent circuit, and the rotor speed iscalculated according to the equation of rotor motion, and then the moment of currentpeak can be determined according to change law of rotor speed.Simulation models are built on PSCAD/EMTDC platform to verify the correctnessof those two proposed methods. Dynamic responses of IG are analyzed, both thecorrectness of the proposed iterative algorithm and the evaluation method are verifiedby the comparisons between the simulation and calculation results.