Grid Impedance Identification and Structured-h2 Optimization Based Controller Design of Active Front-end in Embedded AC Networks

Abstract

The increasing use of power electronics systems and distributed generation in modern power networks has raised important issues regarding the effects that inter-connected converters systems can have on the performance and stability of the whole grid. Interactions between locally optimized converters, designed not taking into account the external influence of the system in which they are operating, may lead to performance degradation or even system instability and failure. This paper will deal with the problem of active-front end control design taking into account of system interactions with other power converters in the grid. A globally optimized local converter control tuning method, which is based on a state space model identification of the unknown grid system seen from the converter at its point of common coupling (PCC), is here proposed. The plant for the control design will then be not only represented by the local converter dynamics, but will also include the identified system at the PCC. In order to describe and validate the concept, a simplified notional system, comprising of two interconnected converter systems coupled on the same AC bus is analyzed. The effectiveness and advantages of the proposed method is validated by experiments with a traditionally designed PI controller for comparison.

Decentralised Optimal Controller Design of Variable Frequency Three-Phase Power Electronic Networks Accounting for Sub-System Interactions

“…Since, both the VSI and AFE have cross-coupling terms in their stateequations, misalignment of the PLL to the grid, can cause interactive effects between both sub-systems. Equations (20) and (21) presented how each of the cross-coupling terms found in each converter's state equations can be redefined, as to attribute the angle θ e of misalignment between the two converters dq-frames during disturbance events. Therefore, by substituting (20) into the AFE state equations (5) and 6, where cross-coupling with VSI is present, the states can then be defined as those in (27) and (28) respectively, to correctly account for misaligned cross-coupled VSI states in the AFE model.…”

“…Similarly, in the VSI state equations (2) and (4) include AFE cross-coupling terms I ad and I aq . To correctly model these cross-coupled terms in events of frame misalignment, one can substitute (21) into these state equations in order to derive redefined VSI state-space equations taking into account PLL interactions:…”

“…H 2 control had therefore been used, which is less conservative than H ∞ , at the cost of reduced robustness. It was shown in [20], [21] that H 2 does allow for an optimal, and fully decentralised control for the power network. In addition, since the H 2 optimisation uses the whole system model for the development of decentralised controllers, each independent controller is optimised with the knowledge of every other converters closed loop dynamics, and crosscoupled interactive effects.…”