Tevfik Sezi

Conditions for stability of droop-controlled inverter-based microgrids

We consider the problem of stability analysis for droop-controlled inverter-based microgrids with meshed topologies. The inverter models include variable frequencies as well as voltage amplitudes. Conditions on the tuning gains and setpoints for frequency and voltage stability, together with desired active power sharing, are derived in the paper. First, we prove that for all practical choices of these parameters global boundedness of trajectories is ensured. Subsequently, assuming the microgrid is lossless, a port-Hamiltonian description is derived, from which sufficient conditions for stability are given. Finally, we propose for generic lossy microgrids a design criterion for the controller gains and setpoints such that a desired steady-state active power distribution is achieved. The analysis is validated via simulation on a microgrid based on the CIGRE (Conseil International des Grands Reseaux Electriques) benchmark medium voltage distribution network.

Voltage Stability and Reactive Power Sharing in Inverter-Based Microgrids With Consensus-Based Distributed Voltage Control

We propose a consensus-based distributed voltage control (DVC) that solves the problem of reactive power sharing in autonomous inverter-based microgrids with dominantly inductive power lines and arbitrary electrical topology. Opposed to other control strategies available thus far, the control presented here does guarantee a desired reactive power distribution in steady state while only requiring distributed communication among inverters, i.e., no central computing nor communication unit is needed. For inductive impedance loads and under the assumption of small phase angle differences between the output voltages of the inverters, we prove that the choice of the control parameters uniquely determines the corresponding equilibrium point of the closed-loop voltage and reactive power dynamics. In addition, for the case of uniform time constants of the power measurement filters, a necessary and sufficient condition for local exponential stability of that equilibrium point is given. The compatibility of the DVC with the usual frequency droop control for inverters is shown and the performance of the proposed DVC is compared with the usual voltage droop control via simulation of a microgrid based on the Conseil International des Grands Réseaux Electriques (CIGRE) benchmark medium voltage distribution network.

Synchronization of droop-controlled microgrids with distributed rotational and electronic generation

We consider the problem of frequency synchronization and power sharing in a lossy droop-controlled autonomous microgrid with distributed rotational and electronic generation (MDREG). At first, we establish equivalence of the dynamics of a regulated synchronous generator and a droop-controlled inverter with low pass filter. We then give a necessary and sufficient condition for local synchronization of the microgrid by using ideas from graph theory and second order consensus algorithms. In addition, we show that sources in an MDREG can achieve a desired active power distribution via frequency droop control and provide synchronization conditions for a lossless microgrid as a special case. Our analysis is further validated via a simulation example of a lossy microgrid based on the CIGRE benchmark medium voltage distribution network.

A survey on modeling of microgrids-From fundamental physics to phasors and voltage sources

Microgrids are an increasingly popular class of electrical systems that facilitate the integration of renewable distributed generation units. Their analysis and controller design requires the development of advanced (typically model-based) techniques naturally posing an interesting challenge to the control community. Although there are widely accepted reduced order models to describe the dynamic behavior of microgrids, they are typically presented without details about the reduction procedure|hampering the understanding of the physical phenomena behind them. The present paper aims to provide a complete modular model derivation of a three-phase inverter-based microgrid. Starting from fundamental physics, we present detailed dynamical models of the main microgrid components and clearly state the underlying assumptions which lead to the standard reduced model representation with inverters represented as controllable voltage sources, as well as static network interconnections and loads.

On power sharing and stability in autonomous inverter-based microgrids

We consider the problem of voltage and frequency stability for an autonomous inverter-based microgrid. An LMI-based decentralized feedback control design is derived that stabilizes the system under the consideration of droop-like controllers aiming to achieve power sharing among the different generation units. We provide a design procedure that accounts for uncertainties in line impedances and loads while guaranteeing zero steady-state frequency deviation.

A consensus-based distributed voltage control for reactive power sharing in microgrids

We propose a consensus-based distributed voltage control (DVC), which solves the problem of reactive power sharing in autonomous meshed inverter-based microgrids with inductive power lines. Opposed to other control strategies available thus far, the DVC does guarantee reactive power sharing in steady-state while only requiring distributed communication among inverters, i.e. no central computing nor communication unit is needed. Moreover, we provide a necessary and sufficient condition for local exponential stability. In addition, the performance of the proposed control is compared to the usual voltage droop control [1] in a simulation example based on the CIGRE benchmark medium voltage distribution network.

Distribution system state estimation using unsynchronized phasor measurements

Accurate monitoring of the distribution system is performed using state estimation methods. The purpose of these methods is to estimate the most likely state of the grid given various types of redundant measurements. In this paper, we propose a three phase state estimation method that can handle accurately unsynchronized three phase phasor measurements. Unsynchronized phasor measurements, as opposed to synchrophasor measurements, consist in phasor measurements that do not have accurate time stamps. The use of such measurements could be very valuable in unbalanced distribution networks. To handle these measurements, we add unknown synchronizing operators to the state variables. The identification of these additional state variables allows considering any configuration of unsynchronized phasor measurement in a simple and intuitive way. The proposed method is illustrated on a simulated distribution network.

Stability of synchronized motions of inverter-based microgrids under droop control

We consider the problems of frequency stability, voltage stability and power sharing in droop–controlled inverter–based microgrids with meshed topologies and dominantly inductive power lines. Assuming that the conductances in the microgrid can be neglected, a port–Hamiltonian description of a droop–controlled microgrid is derived. The model is used to establish sufficient conditions for local stability. Furthermore, we propose a condition for the controller parameters such that a desired steady–state active power distribution is achieved. The robustness of the stability condition with respect to the presence of conductances is analyzed via a simulation example based on the CIGRE benchmark medium voltage distribution network.

Protection scheme for a new AC railway traction power system

The new, 160 mile long Amtrak Northeast Corridor railway electrification project has four supply substations that connect 115 kV utility power to the 55 kV single-phase, AC traction power system. Eighteen autotransformer-paralleling stations are spaced along the route. Each autotransformer connects the overhead catenary to an auxiliary feeder line, with the rails tied to an intermediate point of the autotransformer. Each train draws power from two adjoining autotransformers. The protection scheme includes distance protection for the catenary and feeder lines, transformer protection for the power transformers and autotransformers, plus bus protection and backup protection. The protection scheme uses a specialized type of numerical distance protection relay optimized for railway traction systems. In the event of a fault on the protected part of the line, a trip occurs and the relay calculates the distance to the fault. All the autotransformers within that distance are automatically disconnected, and the relay attempts a reclose. If the reclose is successful, the autotransformers are reconnected.

Evaluation of current limiting methods for grid forming inverters in medium voltage microgrids

In this work, different current limiting methods for grid forming inverters are presented and theoretically analyzed. A transient non-linear virtual impedance is introduced and compared with the standard current limiting methods in synchronous rotating reference frame. Their performance is evaluated by means of real time simulation for standalone and parallel operation of an inverter with a synchronous generator in medium voltage microgrids.