The development of multiterminal VSC-HVDC grids poses a new challenge: a fast and selective protection algorithm for such grids needs to be implemented. Therefore, this paper addresses a novel protection algorithm, which proves to be accurate, fast and selective for various combinations of fault distance, impedance and power flow scenarios in a multiterminal VSC-HVDC environment. It relies on the wavelet transform and principal component analysis to retrieve information from the local DC current measurement, generating a highly informative input for a genetic fuzzy system. Based on previously simulated fault scenarios, the genetic fuzzy system is able to decide if the input signal refers to an internal or external fault, generating the trip signal when needed. Furthermore, the method has the advantage of using local measurements to avoid communication delay. Finally, the algorithm was implemented in hardware and tested in a hardware-in-the-loop simulation, which emphasizes the practical use of the proposed protection scheme.

In this paper, a sliding mode plus proportional-integral (PI) controller for a boost converter in a photovoltaic system is proposed. The proposed controller is characterized by being easy to implement and by operating with constant switching frequency. The parameters of the proposed controller are calculated using the weighted particle swarm optimization technique, ensuring low percentage of overshoot and short setting time. The use of this optimization technique allows one to ensure the stability of the controller. A linear lead-leg controller is considered in order to compare the performance of the proposed controller. Finally, experimental results using a solar kit are presented to verify the performance of the proposed controller.

Reactive power support of photovoltaic farms has been under discussion in several countries. This kind of operation has been proposed because the photovoltaic farm converter is an expensive asset that is often used well below its power rating. This paper proposes a methodology for estimating the reactive power support capability and the associated technical cost of operating a photovoltaic installation as a STATCOM at nighttime. The technical cost is related to the need for buying active power from the grid to compensate for power losses. A precise estimation of this cost is of interest to both photovoltaic farm owners and grid operators to be able to evaluate the economic feasibility of this kind of operation. In order to illustrate the proposed methodology, a 1.5-MWp photovoltaic farm is considered. By evaluating the losses of each component (converter, tie reactor, filter, and transformer), a reactive power support capability map integrating the technical cost for each point of operation is obtained. The analysis outlines key points to operate a PV farm as a STATCOM at nighttime: the system must include a blocking diode, the capability map is asymmetric, an operation with variable dc-link voltage is desirable and can lead to savings of up to 8.9% in comparison with operation at fixed nominal dc-link voltage.

High-voltage dc (HVdc) transmission has been largely used to interconnect asynchronous systems and integrate renewable energy resources into electrical grids. However, the high short-circuit currents and low-tolerance of power electronics equipment impose new challenges for these systems’ protection. To address these challenges, a new distance protection algorithm for HVdc grids with modular multilevel converters (MMCs) is proposed in this article. The proposed algorithm identifies the faulty line/cable using the resonance frequency of a dc capacitor installed in each terminal. The technique was tested in a four-terminal HVdc grid with five cables and ten circuit breakers. The algorithm was highly selective for faults in the lines and provided fast identification, in less than 1 ms, without communication amongst terminals. The algorithm was tested in hardware under high-noise conditions and provided reliable results.

The power system is going through a change in its very foundations. More and more power converters are being integrated into the electric grid to interface renewable energy resources and in high-voltage direct-current (HVDC) transmission systems. This article presents a discussion on the stability of power systems when HVDC transmission systems based on modular multilevel converters (MMC) are connected in grid-forming (GFM) mode to the legacy power system using concepts of energy functions and Lyapunov stability theory and considering aspects of the interoperability between GFM converter technologies. As a base for the stability analysis, we review the main GFM converter technologies (droop and virtual synchronous machine), highlighting their differences. Then, we present a model using the center-of-inertia formulation for a multi-machine/multi-GFM converter power system representing a close future scenario of power systems where GFM converters might adopt different technologies. To illustrate the theoretical Lyapunov-based stability analysis, simulations performed in Matlab/Simulink showed the behavior of a 12-bus test system during a frequency disturbance that originated from the sudden connection of a load. To reflect the interoperability of different GFM technologies and the power system, scenarios with one single GFM technology and a scenario with mixed technologies were investigated. For the test system considered, the frequency response with fewer oscillations and a higher frequency nadir was obtained when all GFM converters were operated as VSMs that have a higher inertial response contribution.