A naive battery operation optimization attempts to maximize short-term profits. However, it has been shown that this approach does not optimize long-term profitability, as it neglects battery degradation. Since a battery can perform a limited number of cycles during its lifetime, it may be better to operate the battery only when profits are on the high side. Researchers have dealt with this issue using various strategies to restrain battery usage, reducing short-term benefits in exchange for an increase in long-term profits. Determining this operation restraint is a topic scarcely developed in the literature. It is common to arbitrarily quantify degradation impact into short-term operation, which has proven to have an extensive impact on long-term results. This paper carries out a critical review of different methods of degradation control for short-time operation. A classification of different practices found in the literature is presented. Strengths and weaknesses of each approach are pointed out, and future possible contributions to this topic are remarked upon. The most common methodology is implemented in a simulation for demonstration purposes.

There is a growing interest in the parallel operation of Voltage Source Converters (VSCs) both in an isolated microgrid or connected to the utility grid. The most common solution in the literature for the paralellization of VSCs is the so-called droop control, which brings about a relationship between active power and frequency. In this paper, a different approach is proposed where reactive power is used instead of active power to ensure synchronous operation. Active and reactive power are independently controlled using a dq-frame representation based on the vector oriented control, which inherently provides current limitation capability. A detailed dynamic model of the system is used to demonstrate the relation between reactive power and frequency. Due to the intrinsic synchronizing mechanism, the proposed scheme can operate in both isolated and grid-connected modes. As opposed to droop control schemes, active power is not used for synchronization and thus synchronization is possible even if active power is not controllable. Simulation and experimental results, for a case study where a VSC is connected to a host grid, are presented to validate the proposal.

Due to the increasing penetration of distributed renewable energy generation systems, their participation in power system restoration is gaining a growing interest. For the black start of a system without synchronous generation, Wind Energy Conversion Systems (WECSs) must be able to control voltage and frequency. However, WECSs have been traditionally controlled as current sources. This paper proposes a decentralized control system for the black start of WECSs operating in parallel. In comparison with known methods for operation of distributed energy resources without synchronous generation, this paper specifically addresses the requirements of WECSs. Total wind power is regulated to match the demand during the black start process. This power is shared between WECSs according to the wind power available in each of wind turbine using a droop control characteristic between the voltage and active power. Using the so-called reactive power synchronization method, the WECSs can operate in synchronism even if wind power is significantly different in each wind turbine. Simulation results for a case study where two WECSs are connected in parallel are presented to validate the proposal.

This paper presents a comprehensive analysis of the effect of the converter synchronizing methods on the contribution that Battery Energy Storage Systems (BESSs) can provide for the support of the inertial response of a power system. Solutions based on phase-locked loop (PLL) synchronization and virtual synchronous machine (VSM) synchronization without PLL are described and then compared by using time-domain simulations for an isolated microgrid (MG) case study. The simulation results showed that inertial response can be provided both with and without the use of a PLL. However, the behavior in the first moments of the inertia response differed. For the PLL-based solutions, the transient response was dominated by the low-level current controllers, which imposed fast under-damped oscillations, while the VSM systems presented a slower response resulting in a higher amount of energy exchanged and therefore a greater contribution to the support of the system inertial response. Moreover, it was demonstrated that PLL-based solutions with and without derivative components presented similar behavior, which significantly simplified the implementation of the PLL-based inertia emulation solutions. Finally, results showed that the contribution of the BESS using VSM solutions was limited by the effect of the VSM-emulated inertia parameters on the system stability, which reduced the emulated inertia margin compared to the PLL-based solutions.

This paper addresses the design and analysis of a voltage and frequency control (VFC) strategy for full converter (FC)-based wind energy conversion systems (WECSs) and its applicability for the supply of an isolated load. When supplying an isolated load, the role of the back-to-back converters in the FC must change with respect to a grid-connected application. Voltage and frequency are established by the FC line side converter (LSC), while the generator side converter (GSC) is responsible for maintaining constant voltage in the DC link. Thus, the roles of the converters in the WECS are inverted. Under such control strategies, the LSC will automatically supply the load power and hence, in order to maintain a stable operation of the WECS, the wind turbine (WT) power must also be controlled in a load-following strategy. The proposed VFC is fully modelled and a stability analysis is performed. Then, the operation of the WECS under the proposed VFC is simulated and tested on a real-time test bench, demonstrating the performance of the VFC for the isolated operation of the WECS.