This paper proposes a new software design method of dynamic equivalents for the AC/DC hybrid power system. First, some critical issues of dynamic equivalents for the AC/DC hybrid power grid have been intensively investigated. Then, in consideration of the function requirements, this paper builds a general framework for the dynamic equivalent software, which follows the design routine of the separateness of modules and data switching, and flexible extensibility. Finally, a new software tool for dynamic equivalents is developed with OOP technics and unified modelling, and a case has been studied to validate the performance of the newly developed software.
When fault occurs in high wind power proportion grid, the control strategy and external characteristics of the wind power plant are affected by voltage drop of grid, so the traditional short-circuit current engineering calculation is difficult to converge. A short-circuit current engineering calculation method for high wind power proportion grid, considering grid node importance evaluation and crowbar protection of DFIG(double-fed induction generator), is proposed. This paper proposes the external equivalent model of the DFIG when fault occurs in grid, considers voltage drop degree and node importance indexes, and determines the weight of each index through Delphi method. The DFIG crowbar operation priority is evaluated to ensure the successful convergence of calculation and the maximum voltage support of wind power plants. The study case shows that proposed method is superior to the traditional method.
The slow coherent groups recognition method is one of the dynamic equivalence techniques, which is based on the singular perturbation principle. Its algorithm performance is widely recognized. In order to reduce the solving difficulty and time consuming, this method ignores the generator stator damping effect. But during a fault, the generator stator damping effect directly affects the oscillation amplitude and frequency of the rotor angular curve. If the damping characteristics of the original system are ignored, when the damping torque coefficients of each generator vary greatly, the coherence between generators will be affected, and the accuracy of the simplified equivalent system will inevitably decrease. This paper proposes a new method based on the traditional slow coherent groups’ recognition method. The new method, which is based on the second-order model of the generator, can consider the generator stator damping effect without increasing the algorithm complexity. The active and reactive power of the system are decoupled. The eigenvalues of the system model and the general solution of the second-order differential equations are obtained through rigorous derivation. It also proposes a two-scale coherent groups recognition criterion. Finally, the effectiveness of the new method is verified based on the IEEE-39-Bus system.
With the continuous growth of load and the enhancement of grid structure, the problem of excessive short circuit current has become a key factor affecting the safe operation of the China Southern Power Grid. The comparison of measured data shows that the short circuit current calculation results of China Southern Power Grid are optimistic based on the existing short circuit current calculation method. It brings difficulties to the arrangement of the power grid operation mode. This paper first briefly introduces the calculation principle of the IEC60909 Standard (short for IEC60909 Short Circuit Current Calculation Standard). And based on this standard, an improved algorithm for short-circuit current calculation is proposed. FCAP (short for Fault Current Analysis Program), a newly developed software based on the improved algorithm, is compared with other similar software being popular in domestic and foreign to prove its engineering practical value.
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