Eleftherios O. Kontis

A simulation tool for extended distribution grids with controlled distributed generation

In this paper, a new software tool is presented for the simulation of electrical networks under steady-state conditions. Its distinct advantage is the robust integration of distributed generation droop controls, while offering the ability to simulate extended networks fast and reliably. The proposed simulation tool is based on the combination of two well-known software products, namely MATLAB and OpenDSS. The latter is employed as an unbalanced power flow solver, whereas the former implements the droop control of DG units. Simulation results for a simple and extended low-voltage network show the effectiveness of the proposed tool and mainly the reduction in the execution times over other conventional time-domain-based software products.

Measurement-Based Hybrid Approach for Ringdown Analysis of Power Systems

System identification methods have been widely used for the study of low frequency electromechanical oscillations and the development of low order dynamic models. This paper introduces a hybrid frequency/time-domain approach to estimate the dominant modes contained in ringdown responses of power systems. Practical issues and solutions encountered in the application of the hybrid method are discussed. The performance of the proposed technique is evaluated by applying the Monte Carlo method to synthetic signals and simulated responses from a large-scale power system, as well as to measurements recorded in a microgrid laboratory test facility. Results in all cases proved to be very accurate, verifying the robustness of the proposed method.

Development of measurement-based generic load models for dynamic simulations

In this paper the development of robust measurement-based load models for dynamic simulations is discussed. Load model parameters vary significantly, due to different loading conditions, thus, load models obtained from measurements are valid only for a specific operating condition and cannot be easily generalized. Scope of the paper is to develop a generic load model, suitable for dynamic simulations over a wide range of operating and loading conditions. In order to derive the proposed generic model, three methodologies are thoroughly investigated. Several simulation scenarios of different operational conditions are examined with NEPLAN software and are used to validate the accuracy of the proposed models.

Measurement-Based Dynamic Load Modeling Using the Vector Fitting Technique

Accurate load modeling is essential for power system stability analysis and control. This topic has regained interest, due to the high penetration of new types of loads and the increased availability of measurements in extended power grids. In this paper, an aggregated load model based on measurement data is formulated for dynamic simulations of large power systems. The proposed model employs variable-order transfer functions, enabling the accurate simulation of complex load dynamics. A complete methodology for the automatic derivation of the minimum-required model order is proposed with the model parameters calculated via a robust multisignal identification procedure. For this purpose, the vector fitting method is introduced as a technique for measurement-based load modeling. Several simulations are performed using the NEPLAN software to investigate the accuracy and the generalization capabilities of the proposed model. The model performance is thoroughly compared with other conventional load models, also using measurements recorded on a laboratory-scale microgrid.

A Coordinated Droop Control Strategy for Overvoltage Mitigation in Active Distribution Networks

In this paper, a centralized control strategy for low-voltage networks is proposed, aiming to coordinate the droop characteristics of the distributed renewable energy sources. The main objectives are the overvoltage mitigation and the optimal network operation in terms of maximum power injection and uniform power curtailment. The distinct features of the proposed method are the incorporation of droop-based control techniques into the conventional optimal power flow formulation and the use of the sensitivity theory as a means to significantly decrease the computational burden. The validity of the developed method is justified using a genetic algorithm, while extensive comparisons with conventional centralized and decentralized control strategies indicate its superior performance in the operation of low-voltage networks.

Dynamic equivalencing of active distribution grids

The increased penetration level of distributed renewable energy sources (DRESs) into the existing distribution grids changes the dynamic properties of conventional systems, creating new challenges for power system operators. Therefore, new dynamic equivalent models are required to evaluate the dynamic behavior and stability margins of active distribution networks. For this purpose in this paper, a generic, input-output equivalent model is formulated, suitable for dynamic simulations of active distribution grids with high penetration level of DRESs. The developed model employs variable-order transfer functions, enabling the accurate representation of complex power system dynamics. The optimal order of the transfer functions is automatically determined through an iterative procedure, while the required model parameters are identified from measurements using the Vector Fitting method. Various network topologies and a diverse range of DRESs are considered to validate the accuracy of the developed model and demonstrate its generic properties. Finally, the performance of the proposed model is thoroughly compared with other conventional equivalent models, using error indexes.

On the Applicability of Exponential Recovery Models for the Simulation of Active Distribution Networks

This letter delivers a method to extend the use of the exponential recovery model (ERM) for the dynamic simulation of active distribution networks (ADNs). The conventional ERM uses exponential functions to describe the steady-state and transient characteristics of the grid, failing to replicate reverse power flow phenomena which may occur in modern ADNs during or after voltage disturbances. To solve this issue a new formulation employing polynomial functions instead of the original exponential is proposed.

Coordinated phase-based voltage regulation in active unbalanced LV distribution networks

In this paper, a phase-based control algorithm is proposed to effectively address the voltage regulation problem in unbalanced low-voltage (LV) networks with high photovoltaic (PV) penetration. The proposed method is based on a singlephase droop characteristic of the injected power with respect to the phase-to-neutral voltage at the point of common coupling for each PV unit. The main objective is to regulate efficiently the grid phase voltages, as well as to ensure a uniform active power curtailment among the PV units connected in the same phase. Simulations of a highly unbalanced LV network justify the validity of the proposed method and its enhanced performance over the conventional droop control in terms of overvoltage and unbalance mitigation.

Modeling of nonlinear dynamic power system loads using the vector fitting technique

In this paper, a generic load model is presented, suitable for dynamic simulations over a wide range of loading conditions. The proposed model is based on the well-established exponential recovery load model. Contrary to the conventional first-order form of this model, the proposed formulation employs variable-order transfer functions, enabling the accurate simulation of high-order load dynamics. A complete methodology is proposed for the identification of the required load model parameters from real field measurements. For this purpose, the Vector Fitting technique is introduced for the first time as a method for measurement-based load modeling. Several simulation scenarios are performed using the NEPLAN software to validate the accuracy of the proposed model. Numerical studies indicate that the developed load model captures accurately the load dynamic behavior over a wide range of loading conditions.

Long-term evaluation of DRES penetration in LV networks using droop control techniques

The main objective of this paper is to investigate the maximum penetration level of distributed renewable energy sources (DRESs) in a real radial low-voltage network. For this purpose, the business as usual (BAU) scenario, where no control scheme is applied to the installed DRESs, is compared with two conventional droop control techniques, namely the active power curtailment and the uniform power curtailment. Appropriate key performance indicators are introduced to evaluate the long-term performance of the examined control strategies. Simulation results show that, compared to the BAU scenario, a higher penetration level can be attained by exploiting the above-mentioned droop control schemes.