Massimiliano Coppo

Voltage management in unbalanced low voltage networks using a decoupled phase-tap-changer transformer

The paper studies a medium voltage-low voltage transformer with a decoupled on load tap changer capability on each phase. The overall objective is the evaluation of the potential benefits on a low voltage network of such possibility. A realistic Danish low voltage network is used for the analysis. The load profiles are characterized by using single phase measurement data on voltages, currents and active powers with a 10 minutes resolution. Different scenarios are considered: no tap action, three-phase coordinated tap action, single phase discrete step and single phase continuous tap action. The effectiveness of the tapping capability is evaluated by comparing the Voltage Unbalance Factor and the voltage levels on the neutral cable.

Analysis of the Italian distribution system evolution through reference networks

This paper presents initial results of a three-year research project entitled ATLANTIDE, which is aimed at developing a comprehensive digital archive of reference models of Italian distribution networks, including forecasted load and generation development evolving towards future smart grid scenarios. Such reference network models and evolutionary scenarios should provide a useful benchmark for testing and comparing different control methodologies, distribution schemes and operation strategies for dealing with the new challenges caused by the envisaged widespread diffusion and integration of distributed generation, renewable generation and distribution storage devices. The paper focuses on the criteria adopted for defining a set of evolutionary scenarios for the reference networks, accounting for current drives and future trends, valuable to a wide range of the stakeholders of the distribution business.

Coordinated voltage control in MV and LV distribution networks with inverter-interfaced users

In the last years there has been a progressive growing of distributed generators connected to the LV distribution networks, leading to possible depletion of power quality levels. For this reason, standards and grid codes are under revision and several studies have been conducted investigating the possibilities offered by grid connected inverter-interfaced generators to participate to network voltage regulation and unbalance correction. In this paper a control strategy is proposed applicable to a DMS coordinating the MV network regulation, applied through a novel adaptive regulation area criterion, with local LV network controllers which, in turn, coordinate the downstream inverter-interfaced generators. The aim is to demonstrate the benefits coming from a correct coordination of the distributed LV resources with the DMS located at the primary substation.

Temporary islanding operations of MV/LV active distribution networks under fault conditions

In the last twenty years the Distributed Generation (DG) increased till a high level that introduced several issues to the classical management system of distribution networks. In many countries different technical rules have been introduced in order to integrate the DG units in the medium voltage (MV) and low voltage (LV) distribution network. New standards state requirements for all generators, included inverter based ones, imposing a Fault Ride Through (FRT) philosophy and new active and reactive power regulation actions with the aim of maintaining the transmission system stability at the level it was with traditional rotating generators, or even increasing it. These technical rules may not be fully compliant with the automatic reclosing cycle usually performed from the CB at the beginning of MV feeders and/or with the automatic selection procedure of faulty section selection and supply restore to healthy ones. In this paper the effects of DG units connected at MV and LV voltage level in relation to different fault conditions are analysed. The possibility of temporary uncontrolled islanded operations are investigated in order to evaluate the role of new Standards or Regulatory or Law requirements in the automatic reclosing procedure and MV network automation.

Voltage control for unbalanced low voltage grids using a decoupled-phase on-load tap-changer transformer and photovoltaic inverters

This paper presents modeling and analysis of the potential benefits of joint actions of a MV/LV three-phase power distribution transformer with independent on-load tap-changer control on each phase and photovoltaic inverters provided with reactive power control capability, in terms of accommodating more renewable generations in the LV grid. The potential benefits are investigated in terms of voltage unbalance reduction and local voltage regulation. 24-hours root-mean-square dynamics simulation studies have been carried out with time-step of 1 second using 10-mins resolution consumption and production profiles. A totally passive real Danish low voltage distribution network is used for the grid topology as well as for the characterization of loads profiles, while the production ones are empirically defined under assumptions in scenarios with different level of photovoltaic penetration and grade of unbalance.

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.

Representative distribution network models for assessing the role of active distribution systems in bulk ancillary services markets

Nowadays, renewable energy sources feed significant shares of energy demand, resulting in a reduction of market energy price but also a significant increase of expenditures for ancillary services due to inherent not complete predictability and programmability. Thus loads and generation in the distribution network are more and more required to play an active role in supporting grid stability by participating to ancillary service markets, either directly or through aggregators. The paper presents a novel procedure to estimate distribution networks capabilities to support grid stability, by making use of synthetic representation of HV/MV substations and their relevant distribution networks with a GIS based clustering procedure. The impact of active distribution networks to the bulk ancillary service market is then evaluated by applications to the Italian electric system and by assuming different regulatory frameworks for transmission/distribution network market interactions.

Dynamic control of inverter-connected generators for intentionally islanded MV distribution networks

The increasing diffusion of embedded generators on distribution systems leads to the idea of managing portions of the network operating in islanded mode for overcoming emergency conditions. To this aim, it becomes essential to have reliable control strategies for the different types of generator and loads. In this paper some issues regarding the dynamic control of frequency and voltage in MV islanded networks supplied by inverter interfaced DGs are investigated. The aim is to identify opportunities and constraints in light of national regulatory provisions on active power reduction and reactive power compensation. Appropriate models for the inverters control have been developed and systematic simulations of a realistic network switching to islanding operation have been performed.

Application of a correction current injection power flow algorithm to an unbalanced 4-wire distribution network incorporating TN-C-S earthing

Power flow analysis of distribution networks incorporating Low Voltage (LV) consumer representations needs to be cognisant of both a highly unbalanced load structure as well as the provision of the grounding network between the consumer and grid operator (TN-C-S earthing). In this paper, the asymmetrical 3-phase (plus neutral) power flow problem is solved by a correction current injection method using a complex Y matrix approach in consideration of a representative Irish urban distribution network. This methodology offers a much improved and more robust alternative for ill-conditioned asymmetrical network scenarios compared to the standard power flow methodologies such as the Newton-Raphson methods or the forward-backward sweep approach (which encounters convergence issues as a consequence of the complex earthing arrangements). The model refers to a 4-wire representation of a suburban distribution network within Dublin city, Ireland, which incorporates consumer connections at single-phase (230V-N). Investigations relating to a range of network issues are presented. More specifically, network issues considered include voltage unbalance/rise and the network neutral earth voltage (NEV) for increasing levels of micro/small wind generation technologies with respect to a modeled urban wind resource.

Use of single-phase inverter-interfaced DGs for power quality improvement in LV networks

The growing diffusion of single-phase distributed generators in LV distribution networks, already stressed by unbalanced loadings, is likely to give rise to further power quality problems, mainly voltage magnitude and unbalance issues. This paper first focuses on some recent normative decisions regarding the reactive power management for LV active users, then investigates the effects of a stronger participation of the latter to the network voltage regulation, enabled by a suitable local control strategy based on these standards. Simulations on a case study LV network are conducted for a 24 h time window, using a Backward-Forward sweep solution method.