Precise modeling and analysis of DQ-frame current controller for high power converters with low pulse ratio

Discrete time modeling, implementation and design of current controllers

“…z -1 | αβ → z -1 e -jω e T s | dq (8) The transfer function of the system in the synchronous frame (9) exhibits complex coefficients due to the crosscoupling …”

“…This model transformation can be done back shifting the coordinate transformation (6) and applying the ztransformation on the difference equation (7) resulting in (8).…”

“…based on the phase margin [8], the root locus method [5] or the technical or symmetrical optimum (SO) [19]. A quasi continuous control design based on the symmetrical optimum is a common control design procedure.…”

“…state space control [6] or the root locus method [7]. This is not possible working with continuous time models where typically the open loop transfer function is used to design the controller optimizing the phase margin and the crossover frequency [8], as thereby only approximations of the delays and the latch can be taken into account. This still guarantees the stability of the closed loop system but phase margin cannot be directly related to certain closed loop characteristics e.g.…”

Performance evaluation of current control strategies in LCL-filtered high-power converters with low pulse ratios

“…Firstly, considering the low switching frequency, a precise signal modeling of D&H is necessary before any analysis and design of the current controller. Based on the precise model, two methods are usually adopted to improve the phase drop characteristic caused by the D&H. One is to use the angle compensation to eliminate a phase bias [19] [20]; the other one is to use a state-estimator to compensate the phase drop caused by the computation delay in the loop [2].…”