###### Abstract  
This paper analyzes the influence of inverter-interfaced distributed generations' (IIDGs) response during transmission network faults. The simplest and safest solution is to switch IIDGs off during network faults without impacting the network voltages. A more elaborate and efficient concept, required by national grid codes, is based on controlling the IIDGs' currents, involving positive- and negative-sequence voltage measured at the connection point. In this way the magnitude and phase of the injected currents can be adjusted, although the generated power will depend on the actual line voltages at the connection point. Therefore, an improved concept is proposed to adjust IIDGs' fault current injection through the required active and reactive power, employing the same voltage characteristics. The proposed, i.e., power-based concept, is more definite than the current-based one, since the required power will always be generated. The discussed concepts for the fault current injection by IIDGs were tested in different 110-kV networks with loop and radial topologies, and for different short-circuit capabilities of the aggregated network supply. Based on extensive numerical calculations, the power-based concept during transmission networks faults generates more reactive power compared to the current-based concept. However, the voltage support by IIDGs during transmission networks faults, regardless of the concept being used, is influenced mainly by the short-circuit capability of the aggregated network supply. As regards distance protection operation, it is influenced additionally by the network topology, i.e., in radial network topology, the remote relay's operation can be delayed due to a largely seen impedance. distributed generation; fault current injection; voltage support; distance protection 2021  
<cc> <concept>  
a large body of literature proposes control approaches for unbalanced FFC (UFFC) injection during asymmetrical network faults [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]; a comprehensive review is given in [7]. These control approaches all have several objectives, i.e., the quality and limitation of the injected FFC, mitigation of the inverter's DC-link voltage ripple, damping of instantaneous power oscillations, control of phase voltages within predefined safety limits, and computation of the reactive power reference, depending on the resistive-inductive network impedance model and reference voltage sequences. A commonly used flexible voltage support during network faults is proposed in [8], applying instantaneous symmetrical component-based FFC injection. Ref. [9] investigates an FFC dynamic response, which can reach up to five times the nominal current in cases of sub-optimal control parameters. To prevent the inverter's transient overcurrents, Ref. [10] proposes a model-predictive control, whereas [11; 12] propose flexible power control. The approach proposed in [13] can balance positive and negative-sequence active and reactive powers while keeping the injected FFC to a predefined maximum value. The FFC injection proposed in [14] uses a mixed-potential function to control the DC-link voltage compensation term to reduce power oscillations. Ref. [15] proposes an approach for controlling phase voltages within predefined safety limits by setting the positive and negative reactive power references based on an equivalent impedance network model. The IIDG control proposed in [16] boosts short-term voltage stability based on the negative-sequence voltage at the point of common coupling, and controls the phase voltages within predefined safety limits. Ref. [17] adjusts the FFC injection to network conditions, whereas [18] proposes unified reactive power support with an active power curtailment controller, which can distinguish between islanding and voltage transient events, and provides effective voltage unbalance compensation along with zero active power oscillations and FFC with a controlled peak.