Andrea Savio

A novel measurement-based procedure for load dynamic equivalent identification

The distribution network modeling is of great importance on power system analysis, considering both power quality and network stability. Computational effort reduction is one of the main targets for the network equivalent representation, even if results reliability remains a key point to be verified. In this paper an improved model is proposed, based on the most commonly used composite load models. Its main property is the generalization ability: the equivalent model can be applied to networks in various configurations, e.g. with different power absorption or different share of load types. A measurement-based procedure for load modeling is presented and discussed with simulations. Thanks to the combination of two optimization algorithms, namely the genetic algorithm and the grey-box approach, the procedure reaches high accuracy levels with a huge variety of load representations.

Contribution of MV static distributed generation to voltage unbalance mitigation

Focusing on medium size and large size photovoltaic plants typically connected to Medium Voltage distribution networks, their electronic power converters are usually three-phase devices. Depending on the sun irradiation profile, influenced by weather conditions and seasonal variations, these generators usually work at partial loading. As a consequence, the operating point of a PV inverter could be quite far from its current limitation, especially if a rectangular or a circular capability curve is provided. So there are margins for intentionally injecting additional currents according to network requirements without altering the converter rated size and the active power generation.

Risk of MV distribution systems uncontrolled islanding supplied by LV embedded generators

The increasing penetration of distributed generation has lead international authorities to integrate the rules for the connection of active end-users to distribution networks. Thus, power regulating functions aiming at supporting the voltage and frequency stability of the main grid have been recently introduced in European grid codes. In this paper, the consequences of adopting such regulating functions have been examined in detail. In particular, the risk of uncontrolled islanding on a Medium Voltage network with increasing amount of embedded generation connected to Low Voltage subsystems has been studied. Both existing regulating requirements, such as active and reactive power modulation according to locally measured frequency and voltage, and additional stabilizing functions, such as Inertia Emulation and Fast Voltage Support, have been investigated. Results of dynamic simulations, carried out making use of DIgSILENT PowerFactory 2016 software, demonstrate the interactions between stabilizing functions, generators protection systems and network automatic reclosing procedures.

Influence of load characteristics and main settings of distributed plants stabilizing functions on the undesired islanding detection

Distribution networks are increasingly hosting embedded generators, most of them converting renewable energy primary sources. These units are usually interfaced with the main network by means of static power converters. Grid codes have recently introduced stabilizing functions to engage distributed plants in supporting the main system stability during network events and consequent voltage/frequency perturbations. The paper highlights the risk of unintentional islanding, which may be concrete depending on the embedded plant operating conditions, the load characteristics and the stabilizing functions main settings. Firstly, the stabilizing effect of rotating loads is demonstrated. Secondly, the positive impact of the recently introduced intentional delay on the reactive power response in the case of voltage variations is studied. Non-detection zones in terms of generator active and reactive power are reported as final results with reference to different scenarios.

Undesired islanding of MV networks sustained by LV dispersed generators compliant with present grid code requirements

As a consequence of the increasing penetration of distributed generation from renewable energy sources into power systems, international authorities have provided the rules for the connection of active end-users to distribution networks. Power regulating functions have been recently introduced in European grid codes aiming to support voltage and frequency stability. In this paper, the effects of the adoption of such regulating functions have been widely examined, emphasizing the risk of uncontrolled islanding on a Medium Voltage network as the amount of static generators connected to Low Voltage subsystems increases. The study investigates the performances of both existing regulating requirements, such as active and reactive power modulation according to local frequency and voltage, and the additional contribution of further stabilizing functions, such as Inertia Emulation and Fast Voltage Support. Furthermore, the interaction between power regulating rules, generators protection systems and automatic reclosing procedures applied by the Distribution System Operator to circuit breakers at sending end of Medium Voltage feeders, are investigated. The presented results, carried out making use of DIgSILENT PowerFactory software, well demonstrate the correlation between regulating functions applied to generators and the islanding phenomenon.

Cableway Storage System regulation to supply ancillary services to the distribution and transmission network

Storage systems are becoming more and more interesting in supplying ancillary services to electrical networks, in particular in presence of distributed generation. In this paper, a dedicated Cableway Storage System, connected to the MV grid, is presented and characterized under both the mechanical and the electrical points of view. A mechanical 2D model of the system has been developed to represent its behaviour over time, without external controllers. Subsequently, an induction machine with variable frequency drive has been considered to obtain a smooth and adjustable profile of the active power exchanged with the main network. Electrical simulations on a realistic case study confirm the effectiveness of the proposed CSS system in providing ancillary services to the network, with a very fast electrical response.