Abstract
Instabilities related to sequence-couplings of current controlled inverters (CCIs) have been recognized recently, but limited to accurate stability analysis of CCIs synchronized to the grid by the phase-locked loop (PLL). As for CCIs, another feasible grid-synchronization method is based on the instantaneous power (IP) theory, while instability recognitions of the IP-based CCI (IP-CCI) are lacking. Therefore, this paper reveals the instability mechanism and mitigation of the IP-CCI by comparison with the PLL-based CCI (PLL-CCI). Specifically, sequence-admittances of the IP-CCI are modeled, and interactions between sequence-couplings and circuit-coupling of the CCI-grid system are analyzed further. Simultaneously, influences of different system parameters on instabilities of the IP-CCI are thoroughly studied. Studies indicate that sequence-admittances of the IP-CCI exhibit closer sequence-couplings in comparison to that of the PLL-CCI, and negative damping of the IP-CCI is resulted from sequence-couplings, which clarifies interactive instabilities of the grid-connected IP-CCI are coupling-related. Therefore, corresponding instability mitigation can be achieved via sequence-decoupling methods, like the studied controller parameters regulation methods and the filter-based methods. Whereas, the stability deterioration level of sequence-couplings is grid-impedance-dependent, which means the weak grid would limit the decoupling performance. Nyquist plots, simulations and experiments are carried out to verify the studied coupling-related instability of the IP-CCI.
Highlights
The three-phase voltage source inverter (VSI) has been widely used in the evolving distributed generation systems because of its flexible control features [1]–[3]
In this paper, instability mechanism and mitigation of the instantaneous power (IP)-current-controlled inverters (CCIs) are analyzed from the coupled-sequence-admittance models by comparison with the phase-locked loop (PLL)-CCI
The instability of the IP-based CCI (IP-CCI) is firstly studied from its terminal admittance characteristics, and the corresponding controller regulation and filter-based instability mitigation mechanism is clearly clarified
Summary
The three-phase voltage source inverter (VSI) has been widely used in the evolving distributed generation systems because of its flexible control features [1]–[3]. Sequence-couplings in single-phase grid-connected CCI have been proposed in [18] and [19], where the coupling effect of the PLL is expressed by the multi-frequency impedance matrix. Considering sequence-couplings, many studies have tried to convert the admittance or impedance matrix to its equivalent SISO models to simplify the stability analysis [20]–[23]. In view of the above considerations, by means of the sequence-admittance models plotted in Bode diagrams, this paper will recognize the coupling-related instability mechanism and mitigation of the IP-CCI by comparison with the PLL-CCI. The inner loop current reference of the traditional studied PLL-CCI contains signals mainly at the perturbed frequencies fp and fn, and only small coupling terms observed at (fp−2f1) and (fn+2f1) [16], [17], [21]. FPLL (s) represents the transfer function of the PLL, it is described as: FPLL (s)
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