A Power Calculation Algorithm for Single-Phase Droop-Operated-Inverters Considering Linear and Nonlinear Loads HIL-Assessed

Jorge El Mariachet,Jose Matas,Mingshen Li,Yajuan Guan,Helena Martín,Josep M Guerrero

doi:10.3390/electronics8111366

Jorge El Mariachet, Jose Matas + Show 4 more

Open Access

https://doi.org/10.3390/electronics8111366

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Abstract

The active and reactive powers, P and Q, are crucial variables in the parallel operation of single-phase inverters using the droop method, introducing proportional droops in the inverter output frequency and voltage amplitude references. P and Q, or P-Q, are calculated as the product of the inverter output voltage and its orthogonal version with the output current, respectively. However, when sharing nonlinear loads these powers, Pav and Qav, should be averaged by low-pass filters (LPFs) with a very low cut-off frequency to avoid the high distortion induced by these loads. This forces the droop method to operate at a very low dynamic velocity and degrades the system stability. Then, different solutions have been proposed in literature to increase the system velocity, but only considering linear loads. Therefore, this work presents a method to calculate Pav and Qav using second-order generalized integrators (SOGI) to face this problem with nonlinear loads. A double SOGI (DSOGI) approach is applied to filter the nonlinear load current and provide its fundamental component to the inverter, leading to a faster dynamic velocity of the droop-based load sharing capability and improving the stability. The proposed method is shown to be faster than others in the literature when considering nonlinear loads, while smoothly driving the system with low distortion levels. Simulations, hardware-in-loop (HIL) and experimental results are provided to validate this proposal.

Highlights

The parallelization of single-phase inverters without communications between them has usually been performed using the droop method, which drives the sinusoidal references of the inverters for sharing the common loads [1,2,3]
The main parts considered in this work can be seen: the power stage with inner controller, the pulse width modulation block (PWM), the LC output filter, the vo and io sensing, the Pav and Qav calculation block, and the droop generator producing the voltage reference νref [24,25]
In the simulations and experiments, a diode bridge rectifier (DBR) supplying a RC load is used as a nonlinear load ZNL

Summary

Introduction

The parallelization of single-phase inverters without communications between them has usually been performed using the droop method, which drives the sinusoidal references of the inverters for sharing the common loads [1,2,3]. The method introduces proportional droops in the inverter frequency ω* and voltage amplitude V* references, respectively, according to the P and Q load consumed powers. These powers are usually obtained as the product of the measured output current io (t) with the measured output voltage vo (t) and its quadrature version, vo⊥ (t), respectively. The LPFs provide the average powers consumed by the load Pav and Qav and removes the double frequency components

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