This paper presents a voltage sizing methodology for a Single Delta Bridge Cell (SDBC) converter applied to a Static Synchronous Compensator (STATCOM). As renewable energy penetration increases, grid codes have become more demanding, requiring flexible and accurate voltage control strategies. To address these challenges, this work proposes an iterative algorithm that simultaneously determines the optimal number of submodules and DC bus voltage reference across the full range of operating conditions defined by the Spanish grid code. Unlike conventional analytical methods that rely on fixed design parameters, the proposed approach enables the joint sizing of all relevant variables. A case study demonstrates the algorithm’s ability to adapt to the complete operating envelope, providing information about the interrelationships between design variables and offering enhanced flexibility in STATCOM converter development.

The contribution of virtual inertia through grid-connected converters is essential to tackle the stability issues that arise from the substitution of traditional synchronous generators by converter-interfaced systems. In the last decade, different adaptive virtual inertia controllers have been proposed, but the contribution of negative inertia to the transient response of grids has not been addressed yet. In this paper, an adaptive positive-negative inertia (A-PNIE) control is proposed, where the inertial contribution of the converter is optimised according to grid conditions. In addition, a Dual A-PNIE control is proposed for interlinking converters (IC), which adapts the bidirectional inertia support by switching from positive to negative inertia. A key advantage of the proposed controller is that it can be used for ac and dc grids interchangeably, using a per-unit representations of the frequency and voltage. The simulation results at a single-grid and two-grid scenario, and the experimental results on the single-grid scenario emulated at a Power-HIL testbench demonstrate the effectiveness and feasibility of the proposed controller, as well as the role of the negative inertia concept in accelerating the recovery of the frequency towards its nominal value after a power perturbation.

A more renewable energy generation model entails the replacement of synchronous generator-based power plants by converter-interfaced renewable sources, resulting in more susceptible power systems in terms of frequency and voltage oscillations under sudden power perturbations. In addition, dc-based power systems are becoming very popular thanks to the advantages they offer compared to classical ac systems. This has created a need for developing new time-domain simulation tools to represent the dynamic behaviour of converter-dominated, hybrid ac/dc power systems. This paper presents a Simulink® based simulation tool, named DFPF, that can be used to study the dynamic performance of ac, dc and hybrid power system scenarios. The DFPF tool consists of combining the dynamical models of grid-connected elements with the iterative solving of a static power flow algorithm of each part of the power system. The Simulink interface simplifies the implementation of element models and power system test scenarios, and since fast electromagnetic transients are not considered, medium-term simulations can be carried out rapidly for relatively complex scenarios. To test the tool, a hybrid scenario comprised by four ac grids interconnected by a multi-terminal dc grid is implemented and simulated. The results demonstrate that the DFPF tool can be employed to evaluate the transient response of interconnected ac and dc systems under different grid conditions.

This paper analyzes and compares the reactive power limits of single- and three-phase configured dc-link Voltage Source Converter (VSC) structures for Static Synchronous Compensator (STATCOM) to operate with negative-sequence voltage and current. The three-level Neutral Point Clamped (3L NPC) is used as an example of a three-phase dc-link topology. Likewise, the Cascaded H-Bridge (CHB) is used as a single-phase dc-link case, in both star and delta configurations. Modular Multilevel Converter is also considered as a combination of both dc-link configurations. The conclusions drawn are generic whichever the Power Electronics Building Block (PEBB) used in each VSC structure.

The evolution of self-commutated semiconductor devices, development of fast digital controllers and the emergence of new multilevel converter topologies have allowed to increase the voltage and power ratings of power electronic equipment. This has accelerated the use of these devices in MV levels. As multilevel Voltage-Source Converter (VSC) topology-based equipment prevails in MV applications, the operating range of these converters is getting increasingly wider. A common choice to increase the power rating is to combine basic Power Electronics Building Blocks (PEBB). There is a demand of new operating requirements for power converters, such as unbalances in the grid or fault operation. This paper presents a methodology to evaluate different MV multilevel VSC structures working under any operating scenario. The methodology is integrated into a software-based tool. This tool enables to obtain valuable information to compare, select and design the converter PEBB combination that better suits certain operating requirements. In order to evaluate the potential, the proposed methodology is applied in two operating case studies.

This paper presents a comparative eigenvalue analysis of the stability characteristics and small signal dynamics of four different control strategies for Synchronous Machine Emulation (SME) by power electronic converters, considering a Synchronous Machine (SM) as the benchmark system. The four SME techniques are selected to represent the most established general approaches for emulating the inertial characteristics of SMs in the control of power electronic converters. The small-signal stability assessment is based on the analysis of system eigenvalues, including evaluation of participation factors and parametric sensitivities. All the investigated techniques can be tuned to obtain similar inertial dynamics under grid frequency variations, but exhibit differences in other small-signal characteristics due to the distinct control system implementations. Among the analyzed cases, the current-controlled virtual synchronous machine has the highest damping of the most oscillatory mode. However, the study shows that the most oscillatory modes of the other techniques are associated with the LCL impedance, and could be further attenuated by active damping techniques.

This paper presents a passive method to balance the voltage of the split capacitors in Neutral Point Clamped (NPC) based Voltage Source Converters (VSC). The voltage of the two capacitors is equated by connecting a low VA transformer at the AC side of the converter. The transformer is connected in wye configuration and its neutral point is connected to the DC side midpoint through a resistance and an inductance. By doing so, it is assured that the mean voltages of the DC side capacitors are balanced. In addition, this method is independent to the load and therefore the balancing is assured under any working point of the converter. The VSC and the balancing circuit have been simulated in the simulation tool Saber and the good performance of the balancing circuit has been validated experimentally in a 6.7KVA test bench.