Current regulation plays an important role in modern power electronic AC conversion systems The most direct strategy to regulate such currents is to use a simple closed loop proportional-integral (PI) regulator, which has no theoretical stability limits as the proportional and integral gains are increased, since it is only a second order system However, pulsewidth modulation (PWM) transport and controller sampling delays limit the gain values that can be achieved in practical systems Taking these limitations into account, this paper presents an analytical method to determine the best possible gains that can be achieved for any class of practical linear AC current controller The analysis shows that the maximum possible proportional gain is determined by the plant series inductance, the DC bus voltage and the transport and sampling delays, while the maximum possible integral gain is determined primarily by the transport and sampling delays The work is applicable to stationary frame PI regulators, stationary frame controllers with back electromotive force compensation, stationary frame P+ resonant (PR) controllers, and synchronous d- q frame controllers, since they all have identical proportional and integral gains that must be optimized for any particular application

Optimized Design of Stationary Frame Three Phase AC Current Regulators

This paper investigates the capacitor-current-feedback active damping for the digitally controlled LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor due to the computation and pulse width modulation (PWM) delays. The LCL-filter resonance frequency is changed by this virtual impedance. If the actual resonance frequency is higher than one-sixth of the sampling frequency (fs/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed; thus, high robustness against the grid-impedance variation is acquired. Experimental results from a 6-kW prototype confirm the theoretical expectations.

Capacitor-Current-Feedback Active Damping With Reduced Computation Delay for Improving Robustness of LCL-Type Grid-Connected Inverter

This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results.

Generalized Design of High Performance Shunt Active Power Filter With Output LCL Filter

The interconnection stability of a grid-connected voltage-source converter (VSC) can be assessed by the passivity properties of the VSC input admittance. If critical grid resonances fall within regions where the input admittance acts passively, i.e., has nonnegative real part, then their destabilization is generally prevented. This paper presents an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived. Design recommendations for minimizing the negative-real-part region are given as well.

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Passivity-Based Stability Assessment of Grid-Connected VSCs—An Overview

Resonant controllers have gained significant importance in recent years in multiple applications. Because of their high selectivity, their performance is very dependent on the accuracy of the resonant frequency. An exhaustive study about different discrete-time implementations is contributed in this paper. Some methods, such as the popular ones based on two integrators, cause that the resonant peaks differ from expected. Such inaccuracies result in significant loss of performance, especially for tracking high-frequency signals, since infinite gain at the expected frequency is not achieved, and therefore, zero steady-state error is not assured. Other discretization techniques are demonstrated to be more reliable. The effect on zeros is also analyzed, establishing the influence of each method on the stability. Finally, the study is extended to the discretization of the schemes with delay compensation, which is also proved to be of great importance in relation with their performance. A single-phase active power filter laboratory prototype has been implemented and tested. Experimental results provide a real-time comparison among discretization strategies, which validate the theoretical analysis. The optimum discrete-time implementation alternatives are assessed and summarized.

Effects of Discretization Methods on the Performance of Resonant Controllers

Current regulation plays a key role in modern power electronic AC conversion systems. The most obvious strategy is to use a simple closed loop PI regulator. However it is well known that this approach cannot achieve zero steady state error because of the instability that inevitably occurs as the PI gains are increased to reduce this error. Linear analysis identifies that an AC load controlled by a PI regulator is a second order system with no theoretical stability limits as the gains are increased. Hence other factors must be responsible for the gain limits that are known to occur in practice. This paper revisits the use of a PI regulator to control AC currents and identifies that it is control loop delays that limit the PI regulator s gains. It is this limit that then constrains the controller s performance, particularly when the system feeds into backemf type loads. Methods are then presented to analytically determine the best possible gains that can be achieved within this constraint, and to improve the performance of a PI regulator by incorporating feedforward compensation of the load backemf into the control loop.

Improved Stationary Frame AC Current Regulation using Feedforward Compensation of the Load EMF

Current regulation is an essential element of modern power electronic AC conversion systems. However it is well known that simple stationary frame PI current regulators cannot achieve zero steady state error for AC systems because of the instability that inevitably occurs as the PI gains are increased in an attempt to reduce this error. Recent work has shown for single phase systems that the gains are in fact limited by the second order effects of sampling and transport delays, rather than any fundamental theoretical constraints. This paper extends this understanding to three phase systems, presents a deterministic strategy to maximize the PI gains for practical AC current regulation systems, and proposes some additional strategies to further enhance the performance of a basic PI regulator. The conclusions presented are confirmed by detailed simulation investigations and matching experimental results.

Enhanced three phase ac stationary frame PI current regulators

Despite its inherently modular hardware structure, the control system of most cascaded H-bridge converters is usually highly centralized, and relies on a high bandwidth intra-converter communication system to transmit time critical control signals to the module controllers. In contrast, this paper presents a decentralized control strategy for a modular cascaded converter, where each module controller determines its own switching actions based on local sensors, a local current regulator and a local modulator. The system achieves the same performance as a centralized control system, with a low bandwidth requirement for the intra-converter communication system. Controller high voltage isolation requirements are also reduced because most of the system measurement sensors can be embedded within each converter module slave controller.

W.Y. Kong

Papers

Optimized Design of Stationary Frame Three Phase AC Current Regulators

Improved Stationary Frame AC Current Regulation using Feedforward Compensation of the Load EMF

Enhanced three phase ac stationary frame PI current regulators

Decentralized control of a cascaded H-Bridge multilevel converter