Abstract
Hybrid ac/dc microgrids (MGs) integrated with traditional diesel generators, distributed energy storage systems (ESSs), and high penetration of renewable energy sources (RESs)-based distributed generators (DGs) have become an attractive power supply solution for isolated remote areas and islands, which can effectively reduce environmental protection pressure and improve power supply reliability. However, in such inherent low-inertia systems, randomness and fluctuation of the output power of RESs and uncertain load consumption can easily incur dynamic stability issues, such as transient power impact, unacceptable frequency deviations, and operation mode transitions for security. To solve the above problems and enhance the dynamic stability of the system, an enhanced dynamic stability control (EDSC) scheme with locally measured signals only is proposed in this paper. In this control scheme, the bi-directional interlinking dc-ac converter uses the ac frequency in ac MG as the reference value of dc voltage and adopts the current feedforward control to control the dc voltage in the dc MG to be consistent with the ac frequency. By electrically coupling dc voltage and ac frequency, power disturbances in ac or dc sides will cause almost identical variation degrees in both ac frequency and dc voltage. Under the proposed EDSC scheme, the distributed ESSs in both ac and dc sides are then automatically coordinated and controlled by the unified droop control to balance transient power disturbances and smooth output of diesel generator under normal condition, which can effectively improve stability and controllability of such low-inertia systems. Furthermore, in an emergency such as failure of diesel generator, the operation mode can be switched seamlessly with the proposed EDSC scheme. The detailed theoretical analysis including control system design, small signal model analysis of key parameter influence on system dynamics, and simulation verifications in the PSCAD/EMTDC environment is presented to verify the effectiveness and practicality of the proposed EDSC scheme.
Highlights
In existing remote or islanded areas where traditional diesel generators are commissioned for main power supply, fuel replenishment, high fuel emissions and pollutions, and system power supply unreliability have become prominent challenges
Common issues associated with such MG systems, which is deemed as low-inertia ‘‘weak’’ grids, are the transient power disturbances resulted from intermittent renewable distributed generators (DGs) and the uncertainties of load consumption, which if not properly addressed can significantly compromise the system dynamic stability and operation reliable
The proposed enhanced dynamic stability control (EDSC) scheme in this paper aims to address the issues of transient power disturbance smoothing under normal condition and seamless operation mode transition in emergency only with conventional local control schemes which can be designed, implemented and respond autonomously
Summary
In existing remote or islanded areas where traditional diesel generators are commissioned for main power supply, fuel replenishment, high fuel emissions and pollutions, and system power supply unreliability have become prominent challenges. Power support loops of distributed ESSs and bi-directional interlinking converters with complicated design of key control parameters such as virtual inertia or capacitance may not be properly coordinated and applied As these distributed ESSs adopted normal power dispatch mode with additional ac frequency and dc voltage based power support loops, if diesel generator failed, ESSs’ seamless transition from grid-following mode to grid-forming mode would be a challenging issue [19], unless fast operation mode detection and control system switching were adopted for distributed ESSs. The proposed EDSC scheme in this paper aims to address the issues of transient power disturbance smoothing under normal condition and seamless operation mode transition in emergency only with conventional local control schemes which can be designed, implemented and respond autonomously. It focuses on MG system level to enhance dynamic stability of low-inertia hybrid ac/dc microgrid with high penetration of renewable DGs, diesel generators, and multiple distributed ESSs. The main features of the EDSC scheme are as follows: 1) The interlinking dc-ac converter controls the dc bus voltage, collecting the real-time local ac frequency (from phase-locked-loop, PLL) as the input for the dc voltage.
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