Control of Transient Power during Unintentional Islanding of Microgrids

In inverter-based microgrids, the paralleled inverters need to work in grid-connected mode and stand-alone mode and to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using the droop control. Inverters can have different power set points during grid-connected mode, but in stand-alone mode, they all need their power set points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set points to inverters with low power set points, which can raise the dc-link voltage of the inverters causing them to shut down. This paper investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the dc-link voltage and adjusts the power set point in proportion to the rise in the voltage. A small-signal model of an islanded microgrid is developed and used to design the controller. Simulation and experimental results are presented to validate the design

Line-Interactive UPS for Microgrids

Line interactive Uninterruptable Power Supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery. Power can also be exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. In this paper, a line interactive UPS and its control system are presented and discussed. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The drooping coefficients are chosen to limit the energy imported by the USP when re-connecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60kW line interactive UPS systems are provided to validate the design

DC microgrid power coordination based on fuzzy logic control

The power coordination in DC microgrids has a vital role in enhancing the performance and management of multi generation units. Renewable Energy Sources (RES) are limited to their available power with intermittent nature. Battery-based energy storage sources have limitations in the charging and discharging capabilities to avoid depleting the battery and preserve the State of Charge (SOC) within its satisfactory limits. The battery balances the power difference between RES and loads. However, in severe cases where the SOC is very low, load shedding is crucial. In this paper, a Fuzzy Logic Controller (FLC) has been proposed to coordinate the power flow of PV unit and battery to satisfy the load by full use of the available PV power. It controls the PV’s output power and keeps the SOC and charging / discharging power of the battery within their required margins regardless of the variations in load. Furthermore, load shedding of low priority load has been implemented when the battery couldn’t balance the microgrid power flow. Simplicity in managing multi input-multi output system by FLC is the main merit. Matlab/Simulink results are presented to validate the performance of the proposed controller

Supervisory control for power management of an islanded AC microgrid using frequency signalling-based fuzzy logic controller

In islanded AC microgrids consisting of renewable energy sources (RES), battery-based energy storage system (BESS), and loads, the BESS balances the difference between the RES power and loads by delivering/absorbing that difference. However, the state of charge (SOC) and charging/discharging power of the battery should be kept within their design limits regardless of variations in the load demand or the intermittent power of the RES. In this paper, a supervisory controller based on fuzzy logic is proposed to assure that the battery power and energy do not exceed their design limits and maintaining a stable power flow. The microgrid considered in this paper consists of a PV, battery, load and auxiliary supplementary unit. The fuzzy logic controller alters the AC bus frequency, which is used by the local controllers of the parallel units to curtail the power generated by the PV or to supplement the power from the auxiliary unit. The proposed FLC performance is verified by simulation and experimental results. IEE

Control Strategy for Uninterrupted Microgrid Mode Transfer during Unintentional Islanding Scenarios

This paper presents a microgrid control strategy to unify the control topology for energy storage systems (ESS) and renewable energy sources (RES) inverters in an AC microgrid and to protect the microgrid reliability from unintentional islanding instability using control loops which use the DC link voltage as a feedback. This bounds the DC link voltage and provides reliable operation in the microgrid. Simulation validates the proposed control strategy, and experiment results extol the concept

Control of a Realistic Wave Energy Converter Model using Least-Squares Policy Iteration

PublishedThis is the author accepted manuscript. The final version is available from Institute of Electrical and Electronics Engineers via the DOI in this record.An algorithm has been developed for the resistive control of a non-linear model of a wave energy converter using least-squares policy iteration, which incorporates function approximation, with tabular and radial basis functions being used as features. With this method, the controller learns the optimal PTO damping coefficient in each sea state for the maximization of the mean generated power. The performance of the algorithm is assessed against two on-line reinforcement learning schemes: Q-learning and SARSA. In both regular and irregular waves, least-squares policy iteration outperforms the other strategies, especially when starting from unfavourable conditions for learning. Similar performance is observed for both basis functions, with a smaller number of radial basis functions underfitting the Q-function. The shorter learning time is fundamental for a practical application on a real wave energy converter. Furthermore, this work shows that least-squares policy iteration is able to maximize the energy absorption of a wave energy converter despite strongly non-linear effects due to its model-free nature, which removes the influence of modelling errors. Additionally, the floater geometry has been changed during a simulation to show that reinforcement learning control is able to adapt to variations in the system dynamics.This work was supported partly by the Energy Technologies Institute and the Research Councils Energy Programme (grant EP/J500847/), partly by the Engineering and Physical Sciences Research Council (grant EP/J500847/1), and partly by Wave Energy Scotland

Hybrid Generators-based AC Microgrid Performance Assessment in Island Mode

Achieving an accurate steady-state averaged active power sharing between parallel inverters in islanded AC microgrids could be realized by a traditional droop control. For identical inverters having the same droop gains, it is assumed that the transient average power responses will be similar, and no circulating current will flow between the units. However, different line impedances could influence the instantaneous power significantly and thus circulating power flows among the inverters particularly during sudden disturbances such as load changes. This power, if absorbed by an inverter, will lead the DC link voltage to rise abruptly and trip the inverter, thus, degrading the performance of the whole microgrid. The problem becomes worse when hybrid generators are serving as unidirectional power source. This paper assesses the performance of hybrid generators within an islanded microgrid against the mismatch in line impedances. Two schemes to stabilize the microgrid are proposed. In addition, a participation factor analysis is developed to select the most effective controller scheme to bound the DC link voltage and minimize the circulating power. Simulation and experimental results are presented to verify the analysis and the capability of the proposed controller

A review of recent control techniques of drooped inverter‐based AC microgrids

As the penetration of distributed generation (DG) systems in the grid is increasing, the challenge of combining large numbers of DGs in the power systems has to be carefully clarified and managed. The control strategy and management concept of the interconnected systems should be flexible and reliable to handle the various types of DGs. This can be suitably met by microgrids. This paper introduces the microgrid structure and elements and states the main objectives that should be achieved by the microgrid controllers and each DG controller in both operation modes (grid-connected and island mode). It also presents the challenges of having multiple DG units in a microgrid in terms of accurate power control/sharing, voltage and frequency regulation, power management between DGs, different renewable energy sources integration and deployment, seamless mode transfer, and the modeling issues. The centralized and decentralized control techniques as potential solutions have been discussed and compared by highlighting the advantages and disadvantages of each. Furthermore, the recent control techniques for drooped alternating current microgrids and the main proposed solutions and contributions in the literature have been exposed to finally overcome the droop control limitations and obtain a flexible and smart distributed power system

Hybrid, multi-megawatt HVDC transformer topology comparison for future offshore wind farms

With the wind industry moving further offshore, High Voltage Direct Current (HVDC) transmission is becoming increasingly popular. HVDC transformer substations are not optimized for the offshore industry though, increasing costs and reducing redundancy. A suggested medium frequency, modular hybrid HVDC transformer located within each wind turbine nacelle could mitigate these problems, but the overall design must be considered carefully to minimize losses. This paper’s contribution is a detailed analysis of the hybrid transformer, using practical design considerations including component library minimization. The configurations investigated include combinations of single phase H-Bridge and Modular Multilevel Converter topologies operating under minimum switching frequency control strategies. These were modelled in the MATLAB/Simulink environment. The impact of the minimum switching control strategy and converter topology on power transfer stability and overall efficiency is then investigated. It was found that the H-Bridge converter generated the lowest overall losses, but there was a trade off with power flow sensitivity due in part to the additional harmonics generated