Control of Grid-Following Inverters Under Unbalanced Grid Conditions

Abstract

This paper proposes a new control scheme to eliminate the 3 rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3 rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3 rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup.

Unbalance mitigation strategies in microgrids

“…The modelling and experimental validation of another strategy for DFIG systems under distorted and balanced systems is discussed in [354]. Zarei et al [355] proposed a control scheme to remove the third harmonic component in the output currents of grid-following inverters, under unbalanced grid conditions. Islam et al [356] attempted to address short-term voltage instability in modern distributed networks under asymmetrical faults, while Ghahderijani et al [357] proposed a voltage support control strategy to mitigate the voltage imbalance based on a minimum current approach.…”

Voltage support control strategy of grid‐connected inverter system under unbalanced grid faults to meet fault ride through requirements

“…Elimination of third-order current harmonic component: the DC-link voltage oscillations are the main cause of the third-order harmonic component at the grid side [11]. The orthogonal current signals, dq are DC components including are expressed by (46)…”

“…Using inverse Park transform block implements, the reference currents of (46) is transferred to the 'abc' frame as given in (47). Equation of (48) includes three parts, which are PS components, NS components and third harmonic component [11].…”

“…Consequently, these oscillations result in overvoltage, which damages the power converter. Beside this, voltage oscillations lead to third-order current harmonic component at the grid side [10,11]. Therefore, better control algorithms have to be reacting to eliminate these adverse effects on the inverter side under unbalanced grid conditions.…”

A Simple Self-Tuning Resonant Control Approach for Power Converters Connected to Micro-Grids With Distorted Voltage Conditions

“…This highlights one of the most important features of RC: the harmonic cancellation, where the resonances are set in order to minimize the harmonic effects of the system. In [20] the RC is extended to reduce the second harmonic at the dc voltage produced by the unbalanced grid voltage, and in consequence, reducing the third harmonic ac current. Even though none of these control techniques are designed or considered for variable frequency conditions, they give the background to build and extend RC to new applications.…”