Damping low-frequency oscillations in power systems using grid-forming converters

The increasing incorporation of renewable energy in power systems is causing growing concern about system stability. Renewable energy sources are connected to the grid through power electronic converters, reducing system inertia as they displace synchronous generators. New grid-forming converters can emulate the behavior of synchronous generators in terms of inertia provision and other grid services, like power-frequency and voltage-reactive regulation. Nevertheless, as a consequence of synchronous generator emulation, grid-forming converters also present angle oscillations following a grid disturbance. This paper proposes two novel power stabilizers for damping low-frequency oscillations (LFOs) in the power system. The first power stabilizer provides power oscillation damping through active power (POD-P), and it is implemented in a grid-forming converter, using the active power synchronization loop to damp system oscillations by acting on the converter angle. The second one provides power oscillation damping through reactive power (POD-Q), and it is implemented in a STATCOM, using the voltage control loop to damp system oscillations. Both proposals are first assessed in a small-signal stability study and then in a comprehensive simulation. Moreover, two cases are considered: damping the oscillations of a single machine connected to an infinite bus through a tie-line, and damping the inter-area oscillations in a two-area system. Simulation results, as well as the stability study, demonstrate the ability of both stabilizers to damp power system oscillations, being the POD-P more effective than the POD-Q, but at the cost of requiring some kind of energy provision at the DC bus.This work was supported by the Spanish Research Agency under Project PID2019-106028RB-I00/ AEI/10.13039/501100011033

Black-start capability of PV power plants through a grid-forming control based on reactive power synchronization

Power system restoration is a critical process for any power system. As synchronous generators are being replaced by power electronic converters used in renewable energy generation, the contribution of renewable energy power plants to power system restoration (PSR) after a black-out is becoming more relevant, the so-called black start capability. Existing solutions for providing black start capability to photovoltaic (PV) power plants rely on the use of energy storage systems (ESS) in a hybrid PV plant. In contrast, this paper proposes a solution for the contribution of PV power plants to the PSR that allows a completely autonomous black start process. Reactive power synchronization is used for controlling the PV inverters as virtual synchronous generators (VSG), providing grid-forming control and ensuring synchronism. During the black start process, the PV power is regulated to match the demand using a decentralized solution to share the load between multiple PV inverters. The solution has been validated to handle the most critical situations during the black start process such as the variation on the power source, i.e. irradiance, or on the supplied load and the connection to the main grid.This paper was supported by the Spanish Research Agency under project reference PID2019-106028RB-I00/AEI/10.13039/501100011 033

Control of Variable Speed Wind Turbines with Doubly Fed Asynchronous Generators for Stand-Alone Applications

This paper addresses the design and implementation of a novel control of a variable speed wind turbine with doubly fed induction generator for stand-alone applications. In opposition to grid-tied applications, in stand-alone systems the voltage and frequency must be generated by the doubly fed induction generator. Therefore, a voltage and frequency controller is required for supplying the load at constant voltage and frequency. This controller is implemented by orientation of the generator stator flux vector along a synchronous reference axis. In this way, constant voltage and frequency is obtained and the generator will supply the active and reactive power demanded by the load, while the wind turbine will be responsible for achieving power balance in the system. Then, power control is assumed by the pitch actuator controlling the rotational speed of the wind turbine for power balancing. A load shedding mechanism is needed if the load power exceeds the maximum available wind power. Detailed simulation results are presented and discussed to demonstrate the capabilities and contributions of the proposed control scheme.This work has been supported by the I+D program for Research Groups of the Autonomous Community of Madrid under ref. S2013/ICE-2933.PublicadoPublicad

Decentralized Control of Offshore Wind Farms Connected to Diode-Based HVdc Links

This paper presents a novel decentralized control for offshore wind farms connected to the onshore grid through a high-voltage dc link by means of a diode rectifier. The proposed control system is implemented in each wind turbine generator system (WTGS). The capacitor placed at the filter of the wind turbine front-end converter is used for the proposed control implementation. Frequency control is achieved by aligning the capacitor voltage vector along a reference axis rotating at the reference frequency. Then, a frequency-reactive power droop control allows the synchronization of all the WTGSs. On the other hand, this droop strategy also leads to total reactive power sharing among WTGSs without relying on communications. An additional secondary frequency control is also implemented to compensate the frequency deviation caused by the droop control. The proposed control system has been validated by simulation and results demonstrate the appropriate performance even during start-up and faults

Sequence Control Strategy for Grid-Forming Voltage Source Converters Based on the Virtual-Flux Orientation under Balanced and Unbalanced Faults

Renewable power generation has increased in recent years, which has led to a decrease in the use of synchronous generators (SGs). These power plants are mainly connected to the power system through electronic converters. One of the main differences between electronic converters connected to power systems and SGs connected to the grid is the current contribution during faults, which can have an impact on protection systems. New grid codes set requirements for fast current injection, but the converters' maximum current limitation during faults make it challenging to develop control strategies for such current contribution. This paper presents a positive and negative sequence current injection strategy according to the new Spanish grid code requirements for the novel grid-forming converter control algorithm based on virtual-flux orientation. The behavior of the proposed strategy is tested in a hardware in the loop (HiL) experimental set-up under balanced faults, meaning that the fault is symmetrically distributed among the three phases, and unbalanced faults, where the fault current is distributed asymmetrically between the phases.This paper was supported by the Spanish Research Agency under project reference PID2019-106028RB-I00/AEI/10.13039/501100011033

Control of the Parallel Operation of VSC-HVDC Links Connected to an Offshore Wind Farm

This paper introduces the control of the parallel operation of two voltage source converter (VSC)-HVdc links interconnecting an offshore wind farm. The aim of the study is to propose and validate a control system that allows the parallel operation of two VSC-HVdc links by controlling the currents injected by the VSC converters. The currents set points are established by a voltage controller in order to maintain constant voltage and frequency in the capacitor of the output filter and therefore within the offshore wind farm (OWF). It is demonstrated that the decoupled control of the d-q component of the voltage at the capacitor allows achieving the direct control of voltage and frequency, respectively. The voltage and frequency control is implemented by orienting the capacitor voltage toward a synchronous axis that is generated within the controller and therefore is not subjected to any grid disturbance. Both converters collaborate therefore in maintaining constant voltage and frequency, achieving in this way the parallel operation of the converters. The validation of this approach is demonstrated by simulation where the OWF and the VSC-HVdc rectifier have been modeled. Simulation results demonstrate that the proposed control system allows the parallelization of the converters while maintaining constant voltage and frequency within the OWF, even during transient faults

Analysis of the converter synchronizing method for the contribution of battery energy storage systems to inertia emulation

This article belongs to the Special Issue Energy Storage for Grid Integration of Renewable Energy.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

Comparison of Two Energy Management System Strategies for Real-Time Operation of Isolated Hybrid Microgrids

The propagation of hybrid power systems (solar–diesel–battery) has led to the development of new energy management system (EMS) strategies for the effective management of all power generation technologies related to hybrid microgrids. This paper proposes two novel EMS strategies for isolated hybrid microgrids, highlighting their strengths and weaknesses using simulations. The proposed strategies are different from the EMS strategies reported thus far in the literature because the former enable the real-time operation of the hybrid microgrid, which always guarantees the correct operation of a microgrid. The priority EMS strategy works by assigning a priority order, while the optimal EMS strategy is based on an optimization criterion, which is set as the minimum marginal cost in this case. The results have been obtained using MATLAB/Simulink to verify and compare the effectiveness of the proposed strategies, through a dynamic microgrid model to simulate the conditions of a real-time operation. The differences in the EMS strategies as well as their individual strengths and weaknesses, are presented and discussed. The results show that the proposed EMS strategies can manage the system operation under different scenarios and help power system operator obtain the optimal operation schemes of the microgrid.This work was supported by the Autonomous Community of Madrid under the PROMINT-CM project (S2018/EMT-4366)

Reactive power synchronization method for voltage-sourced converters

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

Modeling and Control of LCC Rectifiers for Offshore Wind Farms Connected by HVDC Links

This paper presents a voltage and frequency control (VFC) and an average-value model (AVM) of a line-commutated converter for a rectifier station in an offshore wind farm (OWF) connected by a high-voltage direct current link. A capacitor bank is placed at the AC terminals of the rectifier station to perform VFC within the OWF. The proposed model uses the active and reactive power generated by the OWF as inputs, while the state variables are the voltage magnitude and phase angle at the capacitor bank bus. The proposed VFC is based on the orientation of the voltage vector at the capacitor bank bus toward a synchronous reference axis. It is then demonstrated that frequency control is achieved by regulating the reactive power balance at the capacitor bank bus, while voltage control is carried out by regulating the active power balance. Moreover, it is demonstrated that in a diode rectifier, although voltage cannot be controlled as in a thyristor rectifier, it is bounded within acceptable limits. In addition, small-signal study is performed to facilitate controller design and system stability analysis. VFC and the accuracy of the proposed AVM are validated by simulation, using both the proposed AVM and a detailed switching model