Inverters In Distributed Generation Systems Research Articles

Coordinated control of parallel operated renewable‐energy‐based DG systems

This study proposes a coordinated control algorithm for two parallel-operated distributed generation (DG) systems fed from different renewable energy sources. The control algorithm ensures accurate control of reference active power generation from the inverters in the DG systems. In addition to reference power generation, the control algorithm also offers compensation of harmonics and reactive currents of the local loads by utilising the remaining capacities of the inverters in the DG systems. For this purpose, the control algorithm estimates compensating factors for each inverter based on the distributed harmonic and reactive currents. The inverter with a higher surplus capacity contributes to higher compensation and vice versa. The harmonic and reactive currents of the load are extracted using least mean mixed-norm adaptive algorithm for effective compensation. Furthermore, the control algorithm automatically switches to low-voltage ride through (LVRT) on sensing the grid faults. During LVRT, the active power generation by the inverters in DG systems is reduced and reactive currents are injected to support the grid recovery. The proposed control scheme is thoroughly investigated using computer simulations and hardware-in-the-loop-based experiments to show its feasibility and effectiveness.

Model Predictive Control of a Voltage-Source Inverter With Seamless Transition Between Islanded and Grid-Connected Operations

Inverter-based distributed generation (DG) system is becoming an attractive solution for high penetration of renewable energy sources to the main grid. DG system should be able to supply power to the local loads whenever necessary even in case of utility power outage. Thus, the inverters in DG systems are expected to operate in both grid-connected and islanded mode, where they are acting as a current source for the ac grid and a voltage-source for the load, respectively. Transition between modes of operation is nontrivial and can cause deviations in voltage and current, because of mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. Thus, it is necessary to have seamless transition between grid-connected and islanded mode. This paper presents a new control strategy with seamless transfer characteristics for a grid-connected voltage-source inverter using model predictive control (MPC) framework. The main objectives of the proposed predictive controller are: 1) decoupled power control in grid-connected mode, which enables the proposed power electronics interface to provide ancillary services such as reactive power compensation; 2) load voltage control in islanded mode; and 3) seamless transition between modes of operation through proposed synchronization and phase adjustment algorithm. The proposed controller features simplicity to implement since only one cost function should be minimized for all modes of operation, and hence no ambiguity in the control algorithm that could cause mode transition problems. An autotuning strategy for weight factors in MPC cost function is proposed to simplify the weight factor tuning strategy. The stability analysis of the proposed controller is provided. Simulation and experimental results validate the expected performance and effectiveness of the proposed control strategy.

A Wireless Controller to Enhance Dynamic Performance of Parallel Inverters in Distributed Generation Systems

This paper presents a novel control strategy for parallel inverters of distributed generation units in an AC distribution system. The proposed control technique, based on the droop control method, uses only locally measurable feedback signals. This method is usually applied to achieve good active and reactive power sharing when communication between the inverters is difficult due to its physical location. However, the conventional voltage and frequency droop methods of achieving load sharing have a slow and oscillating transient response. Moreover, there is no possibility to modify the transient response without the loss of power sharing precision or output-voltage and frequency accuracy. In this work, a great improvement in transient response is achieved by introducing power derivative-integral terms into a conventional droop scheme. Hence, better controllability of the system is obtained and, consequently, correct transient performance can be achieved. In addition, an instantaneous current control loop is also included in the novel controller to ensure correct sharing of harmonic components when supplying nonlinear loads. Simulation and experimental results are presented to prove the validity of this approach, which shows excellent performance as opposed to the conventional one.