A Review on Grid-Connected Converter Control for Short-Circuit Power Provision Under Grid Unbalanced Faults

Abstract

Abstract-As an increasing amount of converter-based generation on power electronics is connected to power systems, transmission system operators (TSOs) are revising the grid connection requirements to streamline the connectivity of the devices to maintain security of supply. Converter-based generation can behave significantly different from the traditional alternators under grid faults. In order to evaluate the potential impact of future converter-based power systems on protective relays, it is necessary to consider diverse current control strategies of voltage source converters (VSC) under unbalanced faults as the performance of converters primarily depends on their control objectives. In this paper, current control strategies of VSC under unbalanced faults for short circuit power provision are reviewed in two groups, namely power-characteristic-oriented and voltage-support-oriented control strategy respectively. As the fault current provided by converters should be restricted within secure operation limits considering semiconductor capabilities, converter current limit issue is also discussed.

Voltage Support under Grid Faults with Inherent Current Limitation for Three-Phase Droop-Controlled Inverters

“…Furthermore, an inherent current limitation is achieved at all times without using any saturation units but based on the ISS property of the closed-loop system. However, apart from the desired current limitation, according to the voltage support concept, a grid-connected inverter should have support capability when faults occur at the grid in terms of positive sequence voltage increase and negative sequence voltage elimination, aiming to restore the voltage to its pre-fault conditions [18,20]. Since controlling the negative sequence powers is inevitable to mitigate the negative sequence voltage at the PCC under unbalanced faults, a current limitation should be also applied at the negative sequence current while a more sophisticated algorithm is required to optimally allocate the maximum current of each sequence.…”

“…However, when unbalanced faults appear at the grid, the selection of the appropriate strategy to optimally provide grid support is a complicated problem [17,18]. Significant amount of research has addressed the inverter response through current controllers that inject both positive and negative sequence currents, in order to provide voltage support in terms of positive sequence voltage support and negative sequence voltage elimination [19,20]. The voltage support concept is thoroughly presented in [20], where current-controlled inverters are reviewed to employ symmetric sequence components and reduce the voltage unbalance factor (VUF).…”

“…Significant amount of research has addressed the inverter response through current controllers that inject both positive and negative sequence currents, in order to provide voltage support in terms of positive sequence voltage support and negative sequence voltage elimination [19,20]. The voltage support concept is thoroughly presented in [20], where current-controlled inverters are reviewed to employ symmetric sequence components and reduce the voltage unbalance factor (VUF). The way current limitation is achieved under unbalanced grid conditions still represents a challenging task, especially when droop controlled inverters are considered, since their task is to regulate the grid voltage and frequency.…”

Fault Analysis Method Considering Dual-Sequence Current Control of VSCs under Unbalanced Faults

“…Therefore, the instantaneous active and reactive powers measured at the point of common coupling (PCC) can be expressed by [29]: • -lagging version of the original vector; the superscripts "+" and "−" refer to the positive-and negative-sequence components; the operator "·" refers to the dot product of vectors; P + and Q + are positive-sequence powers originating from positive-sequence components, while P − and Q − are negative-sequence powers resulting from negative-sequence components; P and Q are the oscillating power terms, whose average value is zero. The dual-sequence current control strategy of a VSC mainly depends on how its current references are generated [5]. Even though the current references can be formulated in various ways, a common feature of these strategies is that they can feed positive-and negative-sequence short circuit currents simultaneously.…”

“…However, the short circuit response of a current-controlled voltage source converter (VSC) is mainly governed by its control system and can be significantly different from that of a synchronous generator. Under grid unbalanced faults, a VSC can be controlled to provide both positive-and negative-sequence currents with advanced control techniques [5]. Therefore, the conventional method [6] for static fault analysis is inadequate as it does not take the dual-sequence current control of VSCs into consideration.…”