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Modeling and management of active electric distribution networks
Full text linkIn modern electricity distribution networks the vertically integrated grid paradigm is rapidly changing due to the ever growing presence of generation sources connected at lower voltage levels. These units, although having each a small size respect to the main generation plants connected to the bulk grid, in the last years are becoming more and more relevant due to their high number.
If on one hand the connection of distributed generators is encouraged by the reduction of their cost and the incentives for renewable energy policies, on the other hand this process is resulting in serious concerns on the power system's stability and security.
From the bulk grid standpoint, for example, the increasing share of distributed generation in the power generation mix is becoming a key issue as regards the system's frequency regulation.
Concerning the operation of the distribution system, the presence of renewable intermittent generation resources (e.g. photovoltaic) and new storage-capable loads (e.g. plug-in hybrid electric vehicles), commonly referred to as Distributed Energy Resources (DERs) is often cause of undesired voltage and current unbalances and higher network losses.
To face the mentioned issues, several national and international standard bodies stated new technical requirements for the generators connected to the distribution network with the aim of improving their integration in the grid regulation.
A remarkable share of generators being connected to the distribution grid use static converters as an interface to the system: with the new standards these units need to be capable of changing their operating point supporting the grid regulation either on a local basis (e.g. frequency and Volt/Var control), as a response to remote signals from the DSO or remaining connected in presence of transient fault conditions (fault-ride-through).
As clearly appears from the decisions being taken updating the grid codes, the integration of DGs and in general of distributed energy resources (DERs) will have a key role in the future electrical grids, both for security reasons and to improve the system's efficiency. For this reason, other than the technical requirements for the grid regulation support, decisions at regulatory level are going to be taken in order to path the way towards the ``smart-grid''.
The mentioned challenges related with the integration of distributed resources in the grid operation highlight the importance of modelling with higher detail a distribution network in order to represent correctly all the active users which may participate to its regulation. Having suitable simulation tools, scenarios of integration of these resources may be studied proposing strategies for their management. These aspects are addressed in this thesis in which active distribution networks are studied both from the representation and management points of view.
This thesis presents a methodology to represent distribution systems and, in general, multi-conductor networks enabling the consideration of asymmetrical systems, even in presence of specific grounding options and of circuits with different number of phases.
From the management point of view, this thesis proposes a decentralised energy management system suitable for Medium Voltage networks aiming at involving DERs in the network's regulation. A coordinated control is also presented for the management of DERs connected in Low Voltage distribution networks, aiming both at limiting the voltage unbalance and aggregating services to be provided to the upstream MV grid.
Both the modelling methodology and the management strategies are simulated in several case studies to demonstrate the applicability of the proposed tools to different power systems
Distributed voltage control strategy for LV networks with inverter-interfaced generators
Full text linktLow voltage distribution networks are characterized by an ever growing diffusion of single and threephase distributed generators whose unregulated operation may deplete the power quality levels, inparticular as regard voltage profiles and unbalances. This issue is at present under discussion by sev-eral national and international standardization bodies and the general trend is to require, for the newconnections of generators to medium and low voltage grids, their participation to the reactive powernetwork management. In this paper a novel strategy proposes to control the network voltage unbalancesuitably for coordinating single and three-phase inverter interfaced embedded generators, concurrentlywith a local volt/var regulation action as foreseen by the new grid connection requirements. Simulationsconducted on case study network representing a typical Italian 4-wire LV distribution system underdifferent load/generation conditions, demonstrate that the coordinated action of single-phase and three-phase inverters may considerably reduce the degree of unbalance thus improving the network powerquality levels
Aggregati di veicoli elettrici per la fornitura di servizi ancillari nelle reti di distribuzione
No full textPer il pieno sfruttamento delle molteplici risorse distribuite sul territorio si stanno sviluppando reti elettriche intelligenti (smart grids) dotate di nuovi sistemi di comunicazione e di misura in grado di garantire la gestione e il coordinamento del sistema di distribuzione. In questo scenario, aggregati di veicoli elettrici come stazioni di ricambio di batterie (swapping station) e parcheggi, si presentano come un’interessante opportunità nel prossimo futuro, potendo svolgere anche un ruolo chiave nel mercato dei servizi ancillari. La tecnologia Vehicle-to-grid (V2G), infatti, permettendo la bi-direzionalità dei flussi di potenza sulle batterie a bordo dei veicoli, può consentire di rispondere in maniera dinamica ad una di gestione evoluta del sistema elettrico basata ad esempio su segnali di prezzo variabili.
In questo articolo vengono presentati e analizzati i servizi ancillari offribili alla rete da differenti tipi di aggregati di veicoli elettrici, concentrandosi in particolare sulla regolazione di tensione
The participation of small-scale variable distributed renewable energy sources to the balancing services market
Full text linkThe paper considers different market settings for the participation to the balancing services market of small scale variable renewable energy sources connected to the distribution grid. By mixing an economical and a technical approach, it evaluates the efficiency of participation to the market under two distinct approaches to resources' aggregation: a commercial scheme and a technical one. In the former, the supply of the small scale variable distributed renewable energy sources is grouped on a purely commercial basis; in the latter, the distribution system operator is responsible of the imbalances that may possibly arise in the distribution grid and aggregates the sources from a technical perspective. By considering a reference distribution network and designing scenarios for the forecast uncertainty about supply and demand of power profiles, the impact of different market frameworks is assessed. The upward and downward balancing services provided by variable distributed energy resources and controllable units connected to the high voltage grid are both considered. Moreover, the power supply curtailments, that endogenously arise due to the violation of technical constraints of the distribution grid and the random nature of energy supply by renewables, are addressed, for each specific market model. As a general outcome of this research, it is shown that providing balancing energy based on a commercial approach is preferable as long as renewables' curtailment penalty is low and local generators have correlated forecast errors (as in the case of photovoltaic units) with a large reserve capacity. High penalties for curtailment and lower correlation among generation schedule deviations, along with a lower reserve by distributed units, make the technical approach more convenient
Risk of unintentional islanding in LV distribution networks with inverter-based DGs
Full text linkThe number of small power Distributed Generation
(DG) units connected to the distribution networks has increased
particularly for photovoltaic (PV) applications. According to
recently issued Italian standards, each DG unit is allowed to
participate in the active and reactive power regulation, using
specifically defined P-f and Q-V droop characteristics.
Furthermore, these standards have introduced wider frequency
and voltage thresholds allowed during normal operation of DG
units. Therefore the risk of unintentional islanding operations is
expected to increase, due to the possibility of the DG units to
supply the loads with a wider voltage and frequency range
without the protections interventions. In this paper, the impact
of the loads’ voltage and frequency dependence on the isolated
grid’s frequency and voltage deviation, during unintentional
islanding operations, has been considered. The system has been
modeled and analyzed through detailed dynamics simulations.
The results show that, using a classical grid-connected inverter
control system, the loads’ voltage and frequency dependence
may play a crucial role in determining the unintentional
islanding operation and possible failure of the protections
intervention
A Simplified Algorithm for OLTC Control in Active Distribution MV Networks
Full text linkThe increasing penetration of Distributed
Generation (DG) in distribution electrical systems may cause
alteration of voltage profiles on the lines. Traditionally,
distribution networks with radial configuration are designed
as a passive top-down architecture where voltage regulation
is mainly performed by an On-Load Tap Changer (OLTC)
transformer located at the Primary Substation. Therefore it
becomes rather difficult to compensate lines radiating out
from the same bus-bar where some of these are subject to
overvoltages due to the power injection by DGs connected
whereas others are subject to voltage drops due to the
presence of passive loads only. A first step in regulation and
managing the voltage levels, in order to be consistent with the
contingent needs of the network, may be a decentralized local
control strategy performed by the DG units, however better
results will be obtained with a coordinated OLTC
intervention.
In this paper an OLTC control strategy, based on only few
remote measurements, is proposed. This procedure allows the
tap changer controller to infer the state of network node
voltages and to act accordingly. The performance of the
proposed control method is demonstrated through
simulations on a realistic MV distribution network
Use of Single-Phase Inverter-interfaced DGs for Power Quality Improvement in LV Networks
No full textThe 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
Modeling the Control of Islanded Networks Supplied by Inverters: the Case of MV and LV Systems
Full text linkThe increasing number of inverter-based
Distributed Generator (DG) units connected to both MV and
LV distribution networks can be used as a resource in order to
overcome some emergency conditions. The presence of
different inverter-based DG units could introduce issues
regarding the management of intentional islanded networks,
therefore reliable inverter control strategies for islanded
operation assume an essential role.
In this paper an inverter operating scheme has been
implemented suitable in presence of networks supplied by a
single DG or multiple DGs and for both grid tie and grid off
operation. This inverter control scheme has been modeled and
tested through detailed dynamic simulations in LV and MV
islanded networks. Moreover the dynamic behavior of the
proposed inverter control strategy has been studied and
analyzed in light of the recently issued Italian connection grid
codes for active users.
The results show that the control scheme allows operation
of intentional islanded networks supplied by single or multiple
DG units. In case of multiple DG units, simulations show that,
grid connection rules influence the dynamic of islanding
operations depending on the power balance existing before the
islanding forming
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