| The short-circuit calculation of distributed generations(DG) in power systems is a basic method for the selection and settings of electric equipments and their protection devices. The short-circuit current of DG depends on the control and protection strategy employed during grid faults. However, the traditional models based on the voltage source equivalent do not consider the fault ride-through features of DG and cannot calculate their short-circuit currents in power grid accurately. Supported by the National High Technology Research and Development of China(863 Program)(2011AA05A107) and National Nature Science Foundation of China(51277184), the paper investigates the fault ride-through behaviour and control of induction generator, inverter interfaced distributed generator and doubly fed induction generator, and constructs the short-circuit calculation models in consideration of the fault ride-through characteristics. And then the iterative and practical algorithms for the short-circuit calculation of DG are proposed sequentially. The studies are shown as follows:① The transient stability problem of multiple induction generators during fault ride-through period in distribution networks is investigated, and an analytical method to determine the critical speed and critical clearing time of multiple induction generators system is proposed. Based on the transient flux characteristics and rotor speed of induction generator during a grid fault, the short-circuit calculation model of induction generator based on symmetrical components is established. The transient stability region and critical clearing time are defined based on the theory of an induction generator. Considering the constraint of network equation and rotor speed variations of induction generators, an analytical method based the steady-state equivalent circuits to determine the transient stability of distribution network with multiple induction generators is proposed. We investigate the relationship between the stator flux and the rotor flux by their improved positive-sequence and negative-sequence static equivalents, and the short-circuit calculation model of induction generator based on symmetrical components is established. For the study of the relationship between distribution networks and the short-circuit currents of units, the fault characteristics of units under different conditions are analyzed.② The instantaneous power of current-controlled inverter interfaced distributed generator is investigated under unbalanced grid faults, and the improved power control of current-controlled inverter interfaced distributed generator during fault ride-through period is proposed. On the basis of those, the short-circuit calculation model of current-controlled inverter interfaced distributed generator is constructed. The analytic formula of its current harmonic distortion, three-phase current peak, DC voltage fluctuation in terms of the control factor of its output current reference are derived. In consideration of the above constraints, the improved power control of current-controlled inverter interfaced distributed generator during fault ride-through is proposed. The differences of short-circuit current among inverter interfaced distributed generators using dead-beat control, proportional- integral and proportional-resonant in current inner loop are compared. According to its sequence currents control and reactive power support during the fault, the short-circuit calculation model of inverter interfaced distributed generator using dead-beat current control is established. Considering the fault transient process of it using proportional-integral and proportional-resonant current control, their short-circuit calculation models are constructed using the transfer function of closed loop system.③ The voltage characteristics of voltage-controlled inverter interfaced distributed generator under an unbalabced grid fault are derived, and the improved voltage control inner loop during fault ride-through period is designed, and the short-circuit calculation model of voltage-controlled inverter interfaced distributed generator is established. The droop control principle of voltage-controlled inverter interfaced distributed generator and its modified control scheme to overcome the effects of nontrivial feeder impedance are studied. The improved voltage control inner loop of inverter interfaced distributed generator during fault ride-through period is designed by introducing the negative sequence and third harmonic voltage control loop. The fault response characteristics of voltage-controlled inverter interfaced distributed generator and the relationship between the output power and the positive- and negative-sequence networks are analyzed. Then the sequence components model of voltage-controlled inverter interfaced distributed generator is established to calculate the short-circuit current.④ A generalized formula of rotor current reference for the fault ride-through operation of doubly fed induction generator under a unbalance grid fault is presented, and the flexible power control strategy of doubly fed induction generator is designed. Taking into account the rotor excitation and crowbar activation time, the short-circuit calculation model is bulit. Considering the rotor phase current limit, the flexible power control strategy of doubly fed induction generator is presented to reach the predefined active and reactive power fluctuation. For the transient feature differences of doubly fed induction generator during the crowbar activation and deactivation, using the rotor current peak to judge the activation, the relation between crowbar activation and post-fault terminal voltage is studied. And the calculation models are derived to determine the short-circuit currents under three-phase and unbalanced grid faults by taking into account the rotor excitation and crowbar activation time.⑤ A mathematical model for short-circuit calculation of DG in power systems is built based on the short-circuit current analysis. And the iterative solution method for multi-DG integrated into power systems is studied using the node-splitting principle. The short-circuit fault responses of DG with different capacities are analyzed for the study of the interaction of the fault currents with power system and the ride through time. An iterative algorithm considered the fault ride-through control and time to obtain the sequence components of short-circuit current for multiple DG in unbalanced distribution grid is proposed. The correctness of proposed iterative algorithm is verified by the test systems with multiple induction generators and inverter interfaced distributed generators using PSCAD/EMTDC simulation software.⑥ The pre-computed curve to calculate the periodic values of fault current is developed to the pre-computed surface for meeting the requirement of the practical application of short-circuit calculation. The voltage support from conventional source to DG is analyzed by Thevenin equivalent of the system outside the coupling point of DG. A short-circuit calculation method based on pre-computed surface was proposed by developing the surface of short-circuit current changing with the calculated reactance and the open circuit voltage. The positive-sequence augmented networks for short-circuit calculation in consideration of the negative-sequence current injection of DG is formed using the positive-sequence equivalent rule, and the pre-computed surface method for unbalance grid faults is also presented. The correctness of proposed practical algorithm is verified by the multiple doubly fed induction generators system and the Luxi Island microgrid using PSCAD/EMTDC simulation software. |
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