Virtual Impedance Impact on Inverter Control Topologies

The different nature of the energy resources requires high reliable power inverters to supply regulated power to the end customer and to ease its integration within the microgrid. In this paper, modeling, design and control of inverters are presented for two different topologies. The study addresses the feasibility of the single loop and double loop control of inverters. The bode plot technique is used to analyze the system behavior when the inductor and the capacitor currents are used as feedback signals. The different output impedance natures affect the power sharing between inverters and stability. Therefore, a proposed virtual impedance is implemented to enhance the control performance. Simulation results are presented to show the validity of the control strategy

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

Impedance interaction between islanded parallel voltage source inverters and the distribution network

In an islanded microgrid consisting of parallel-connected inverters, the interaction between an inverter’s output impedance (dominated by the inverter’s filter and voltage controller) and the impedance of the distribution network (dominated by the other paralleled inverters’ output impedances and the interconnecting power cables) might lead to instability. This paper studies this phenomenon using root locus analysis. A controller based on the second derivative of the output capacitor voltage is proposed to enhance the stability of the system. Matlab simulation results are presented to confirm the validity of the theoretical analysis and the robustness of the proposed controlle

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

Stability Analysis and Control of a Microgrid against Circulating Power between Parallel Inverters

In grid-connected mode, the grid normally absorbs all the power generated by each inverter in a microgrid. Droop control-based microgrid power management employs the frequency as a wireless communication to determine the power outage. However, in the cases of grid loss, each inverter should receive, from a supervisory controller, new settings of the output power suitable to the microgrid load. Because of the supervisory controllers are slower than the droop control loops, this might produce unstable dynamics caused by the excess generated power circulating between the inverters if the microgrid load is low. This case degrades the microgrid stability leading the DC link voltage of each inverter to rise to trip point. In this paper, a PD voltage control loop is proposed to stabilize the system and minimize the circulating power so providing more time for the supervisory control to respond without tripping any inverter. A detailed small signal model is developed and stability analysis is performed to tune the controller’s gain. Matlab/Simulink results validate the performance of the proposed controller

Design and modelling of permanent magnet fault current limiter For electrical power applications

As the electrical power grids are extending in capacity with connection of distributed generations, the fault current level is increasing and approaching the capacity limits of the circuit breakers. In this paper, a saturated inductor fault current limiter (FCL) based on permanent magnet biasing has been developed to overcome the inherent disadvantages associated with many previous technologies such as superconducting based techniques. A 3D Finite Element Modeling (FEM) is used to develop and validate the proposed design and compared it with air-cored inductor. A lab-scale prototype was built to verify the design. Furthermore, a scaled up model which could be introduced to 11 kV network is introduced and its electromagnetic performance is evaluate

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

DC link voltage control during sudden load changes in AC microgrids

Parallel inverters in AC microgrids can achieve accurate power sharing using droop control. However, different grid line impedances will result in different transient power and thus different energy being delivered or absorbed by the inverters during sudden load changes. This might lead the DC link voltage to rise beyond the trip limit causing the inverter to shut down, which reduces the reliability of the whole microgrid. This paper investigates the effect of the mismatch in line impedances on the stability of the DC link voltage during a sudden load changes and proposes a scheme to control the DC link voltage during disturbances. Simulation and experimental results are presented to verify the efficacy of the proposed controller

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