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Providing trustworthy and accurate multi-frequency (or harmonic) models for Renewable Energy Generators (REG) is an ongoing challenge for harmonic studies. There have been effective attempts to propose and design a test device to validate the harmonic models, mainly based on shunt current perturbations. However, using additional devices for perturbations is costly for converter-based test sites. This paper provides the test specifications to extend the application of the grid emulators for voltage perturbations and appropriate harmonic model validation. Besides, the effects of the sequence couplings, initial emissions, and power set-points on the test results have been overlooked in the literature. Considering these effects, this paper proposes a generic test methodology to obtain more accurate models in the sequence domain. The experimental verification of the proposed methodology is demonstrated using a 7 MVA grid emulator for testing of a 2 MVA photo-voltaic converter and a 2 MVA Type 3 wind turbine. This way, the test challenges, specifications, and recommendations are presented using the MW-scale experiments on different REGs. Furthermore, the effects of sequence couplings and initial emissions on the calculation results are investigated and compared. The proposed methodology is applicable for harmonic model validation as well as empirical modelling.

The electrical test and assessment of wind turbines (WT) are going hand in hand with standards and network connection requirements. In this paper, the latest developments in the testing of the electrical characteristics of WTs, including IEC standards, compliance test methods, and industrial test benches, are studied. In this paper, the general structure of advanced electrical test benches, including grid emulator or controllable grid interface (CGI), wind torque emulator and device under test (DUT), has been proposed to harmonize the available industrial test sites. The AC and HVDC transmission systems impose different electrical characteristics on wind power plants (WPP). HVDC connection leads to a converter-based grid, yet AC connection has different grid characteristics in terms of grid impedance, short circuit ratio (SCR), inertia, and background harmonics. Therefore, this paper recommends performing compliance tests in two divisions as AC- and HVDC-connected WTs using a converter-based CGI and emulate the corresponding AC grid for DUT. Also, this paper recommends the additional tests for the current version of IEC 61400-21-1. The additional tests consist of test options for new features of modernWTs, such as Grid-forming, system restoration, and black start capabilities, as well as emulation of different grid characteristics, such as detailed power system, inertia, SCR, and different grid connection emulations for DUT. This way, the possibility of research, development, and demonstration studies on WTs and WPPs would increase. The proposed additional tests can be implemented using the available advanced test benches by adjusting their control systems. Since most of the industrial test benches are based on converters, the characteristics of a real power system can be emulated by a CGI coupled with real-time digital simulator (RTDS) systems through high-bandwidth power-hardware-in-the-loop (PHIL) interface.

The short circuit response of voltage source converters (VSCs) could be significantly different from that of synchronous condensers (SCs) under unbalanced faults. This paper investigates the combined effect of VSC-based sources and synchronous condensers under grid unbalanced faults considering two groups of VSC fault-ride-through (FRT) control strategies. It aims to provide an evaluation of the different control strategies and to explore the impact of equipping a synchronous condenser at the point of common coupling (PCC). By examining the combined short circuit currents, PCC voltages, DC-side voltages and system frequency responses, this study points out the merits and demerits of the examined control strategies; illustrates the benefits of synchronous condensers on mitigating the disadvantages of certain control strategies; and provides suggestions on selecting the control strategies for the VSC when incorporating a synchronous condenser at the PCC.

With converter-based renewable energy sources increasingly integrated into power systems and conventional power plants gradually phased out, future power systems will experience reduced short circuit strength. The deployment of synchronous condensers can serve as a potential solution. This paper presents an optimal synchronous condenser allocation method for improving system short circuit ratio at converter point of common coupling using a modified short circuit analysis approach. The total cost of installing new synchronous condensers is minimized while the system short circuit ratios are maintained above a certain level. The presented method is implemented on the simplified western Danish power system for a future scenario.