Georgios C. Kryonidis

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.

A Nearly Decentralized Voltage Regulation Algorithm for Loss Minimization in Radial MV Networks With High DG Penetration

In this paper, a nearly decentralized voltage regulation algorithm is proposed that effectively coordinates the operation of distributed generation (DG) units in order to minimize the active power losses and the total reactive power consumption in medium-voltage (MV) networks with radial topology. This control requires minimum communication infrastructure, whereas decisions are individually taken by each DG unit based on local and remote measurements. Furthermore, a new cooperative on-load tap changer control is employed to further reduce the network losses. The proposed controls are validated by time-domain and time-series quasi-static simulations in a radial MV network. The former highlights the fast response and the robustness of the proposed voltage regulation algorithm, while comparisons with well-known decentralized control schemes and an optimal power flow method show the improved performance of the proposed controls.

A Control Method for Balancing the SoC of Distributed Batteries in Islanded Converter-Interfaced Microgrids

In a low-voltage islanded microgrid powered by renewable energy sources, the energy storage systems (ESSs) are considered necessary, in order to maintain the power balance. Since a microgrid can be composed of several distributed ESSs (DESSs), a coordinated control of their state-of-charge (SoC) should be implemented, ensuring the prolonged lifespan. This paper proposes a new decentralized control method for balancing the SoC of DESSs in islanded microgrids, without physical communication. Each DESS injects a current distortion at 175 Hz, when its SoC changes by 10%. This distortion is recognized by every DESS, through a phase-locked loop (PLL). In order to distinguish the origin of the distortion, each DESS injects a distortion of different time duration. This intermediate frequency has been selected in order to avoid the concurrence with the usual harmonics. The DESSs take advantage of this information and inject a current proportional to the SoC. Implementing this strategy, a comparable number of charging/discharging cycles for each DESS are achieved. Furthermore, an active filter operation, implemented in the rotating frame for each individual harmonic, is integrated in the control of the distributed generation units, supplying nonlinear loads with high-quality voltage. The effectiveness of this method is verified by detailed simulation results.

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.

Performance of solid-state transformers on voltage regulation of active distribution networks

This paper deals with the recently emerging technology of solid-state transformers (SSTs), focusing on the technical benefits provided towards the optimal network operation. More specifically, the contribution of the SST to the well-known Volt/Var optimization problem is examined in terms of voltage regulation, minimization of network losses, and reduction of the computational burden. For this purpose, the mathematical formulation of the optimization problem is properly modified to incorporate the distinctive characteristics of SST. Deterministic and probabilistic simulations are performed on an extended radial medium-voltage network with high penetration of distributed generation to demonstrate the improved performance of SST against conventional transformer.

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.

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.

An enhanced decentralized voltage regulation scheme for the reduction of tap changes in HV/MV transformers under high DG penetration

In this paper, a decentralized control scheme is proposed that achieves two objectives: To mitigate overvoltages and to reduce the frequency of tap changes in the high/medium-voltage transformer. The proposed control is based on the reactive power capability of the distributed generation units, while actions are individually taken by each unit based only on local measurements. The validity of this method is examined through time-domain and time-series simulations. The former highlights the quick response and the robustness of the proposed control, whereas in the latter the performance and effectiveness of the proposed method in the long term is presented, focusing on the reduction of the transformer tap changes.