AFFIDABILITA’ E QUALITA’ DEL SERVIZIO ELETTRICO PER UTENTI

POLITECNICO DE TORINO-Ingegneria Elettrica-
relatore: Prof. Roberto Napoli
Negli ultimi anni l’Italia è stata caratterizzata da un incremento dei consumi di
energia elettrica sia nel settore industriale che nel settore terziario, inoltre si è
riscontrato, grazie allo sviluppo tecnologico, un aumento sostanziale degli
utilizzatori che fruttano componenti elettronici molto sensibili, come vedremo, ai
disturbi della rete in grado di provocare mal funzionamenti o addirittura la rottura
di tali componenti.
Un altro cambiamento radicale che ha sconvolto il panorama elettrico per quanto
riguarda la produzione e la vendita di energia elettrica, è stato la liberalizzazione
del mercato che ha portato all’introduzione di soggetti terzi come distributori di
energia ,a differenza di quanto capitava in passato, dove questo mercato era
gestito interamente da ENEL.
Queste condizioni, cioè una maggiore qualità del servizio e la comparsa di tanti
piccoli distributori, ha evidenziato la necessità di avere delle linee guida da
seguire per regolare i rapporti tra le varie parti; tale necessità è stata recepita sia da
A.E.E.G. che ha emanato le proprie regole in campo legislativo, sia dal CEI che
ha stabilito le regole tecniche per uniformare tutte quelle regole che erano state
emanate da ogni singolo distributore.
In questo elaborato faremo inizialmente una panoramica sulle principali cause che
inficiano la qualità dell’alimentazione, per poi passare ad una analisi dei principali
aspetti delle leggi e delle regole tecniche vigenti, evidenziando alcuni problemi e
proponendo alcune soluzioni non prese in considerazione da queste.

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State of the Art in Medium Voltage Power Semiconductors

Recent technology advances in power electronics have been made by improvements in
controllable power semiconductor devices. Figure 2-3 and Figure 2-4 summarize the most
important power semiconductors on the market and their rated voltages and currents today [14],
[21]. The device characteristics for medium voltage power semiconductors are shown in Table
2-2 [28].
Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and IGBTs have replaced
Bipolar Junction Transistors (BJT) almost completely. A remarkable development in
MOSFETs took place during the last years. Nowadays MOSFETs are available up to a
maximum switch power of about 100kVA [21].
Various new concepts of MOS-controlled thyristors such as the MOS-controlled thyristor
(MCT) and the MOS turn-off thyristor (MTO) have been presented but they do not have any
commercial applications.
Conventional GTOs are available with a maximum device voltage of 6kV in traction and
industrial converters (Table 2-2) [21], [28]. The high on state current density, the high blocking
voltages, and the possibilities to integrate an inverse diode are considerable advantages of
these devices. However, the requiring of bulky and expensive snubber circuits [92], [93] as
well as the complex gate drive are the reasons that GTOs are being replaced by IGCTs and
Gifts [21], [28]. Like GTOs, IGCTs are offered only as a presspack device. The symmetrcial
IGCT is offered by Mitsubishi with a maximum device voltage of 6.5kV (Table 2-2) [21], [28].
An increase of the blocking voltage of IGCTs and inverse diodes to 10kV is technically
possible today [21].
Due to the thyristor latching, a GTO structure offers lower conduction losses than an IGBT of
the same voltage class. To improve the switching performance of classical GTOs, gatecommutated
thyristors (GCTs) with a very little turn-off delay (about 1.5μs) have been
developed [90], [91]. New asymmetric GCT devices up to 10kV with peak controllable
currents up to 1kA have been manufactured but only those devices with 6kV and 6kA are
commercially available.
IGBTs were introduced on the market in 1988. IGBTs from 1.7kV up to 6.5kV with dc current
ratings up to 3kA are commercially available today (Table 2-2) [21], [28]. They have been
optimized to satisfy the specifications of the high-power motor drives for industrial and
traction applications. They are mainly applied in a module package due to the complex and
expensive structure of an IGBT presspack [28].
In IGBT modules, multiple IGBT chips are connected in parallel and bonded to ceramic
substrates to provide isolation. Both IGCTs and IGBTs have the potential to decrease the cost
of systems and to increase the number of economically valuable applications as well as the
performance of high-power converters, compared to GTOs, due to a snubberless operation at
higher switching frequencies (e.g. 500-1000Hz).
Figure 2-5 represents the typical converter voltage as a function of power ratings for both IGBT
and IGCT applications [28], [30], [31]. It can be seen that LV-IGBT modules are commercially
available with a maximum device voltage of 1700V on the entire low-voltage drive market (i.e.
up to 690V). On the other hand, MV-IGBT modules enable converter designs in a voltage range
from 1kV up to 7.2kV with a power range from 200kVA up to 7MVA (Figure 2-4) [28]. MVIGBT
modules have replaced GTOs in recent traction applications.
“IGBT presspacks are applied mainly in self-commutated High Voltage Direct Current
(HVDC) converters (e.g. HVDC light) where a redundant converter design is a main
requirement and each converter switch position consists of a series connection of many IGBTs
(e.g. n ≥ 10) [28].”

Comparing Transformer-based and Transformerless Uninterruptible Power Supplies

Robin Koffler is the General Manager for Riello UPS Ltd the UK subsidiary of Riello UPS (RPS S.p.A)

Choosing between transformer-based or transformerless uninterruptible power supplies may not be a simple ‘either/or’ decision, particularly above 10kVA. Both technologies have their place in today’s power protection scenarios but the key differences between them are: physical size, efficiency, noise output and the levels of input harmonic distortion that they generate.
Both uninterruptible power supply designs produce a tightly regulated source of uninterrupted power but they differ in the way they generate the dc voltage required by their inverters and their output stages.
Transformer-based Uninterruptible Power Supplies: until the early 1990s, the only design of online uninterruptible power supply was transformer-based. Nowadays, the design is still available but generally in larger sizes for UPS from eight to 800kVA. The most common applications for this are large industrial sites.
This type of UPS has a robust transformer-isolated inverter output, which makes it more suitable for the type of application where there is a likelihood of electrical noise; spikes, transients, and potentially, a high degree of short-circuit currents.
The inverter generates an ac supply from its dc power source, which is fed into a step-up transformer. The primary function of the transformer is to increase the inverter ac voltage to that required by the load. The transformer also protects the inverter from load disruption, whilst also providing Galvanic isolation (a method of isolating input and output).
Modern inverter designs use IGBTs (Insulated Gate Bipolar Transistors) in place of more traditional switching components (such as power transistors and thyristors). IGBTs combine the fast-acting and high power capability of the Bipolar Transistor with the voltage control features of a MOSFET gate to form a versatile, high frequency switching device. This in turn has given rise to more powerful, efficient and reliable inverters.
Transformer-based UPS are also supplied with a dual input option as standard, which can be selected at installation by simply removing a linking connector from its input terminal. This allows it to be powered from two separate ac supply sources thus adding further resilience. A transformerless UPS can be installed with dual input capability, with supplies derived from the same source, but this is typically a factory-fit option.
Transformerless Uninterruptible Power Systems
Transformerless UPS is a newer design, commonly available from 700VA to 120kVA. The primary purpose behind the introduction of transformerless units was to reduce the overall physical size and weight thus making an uninterruptible power supply unit more suitable for smaller installations and/or computer room/office type environments, where space may be limited. It also generates far less noise and heat than its transformer-based cousin and has far lower input harmonic distortion levels making it compatible with environments where electronic equipment (such as computers) may be more sensitive to this type of distortion.
In place of the step-up transformer, a transformerless UPS uses a staged process of voltage conversion. The first stage combines a rectifier and booster-converter to generate a dc supply for the inverter. An uncontrolled, three-phase bridge rectifier converts the ac supply into a dc voltage. This is passed through a mid-point booster circuit to step the dc voltage up to typically 700-800Vdc from which a battery charger and inverter are powered. In the second stage, the inverter takes the supply from the booster-converter and inverts it back to an ac voltage to supply the load.
An added benefit of this method is that the rectifier can operate from either a three or single-phase input supply. This can be configured at installation for systems up to 20kVA. A control system ensures a stable, regulated dc voltage is supplied to the inverter at all times and the inverter can operate regardless of UPS output load variations or mains power supply fluctuations or disturbances.
Choosing between Transformer-based or Transformerless Uninterruptible Power Supplies
In many applications the choice between the two may be clear. It is where the two ranges overlap, in terms of power rating, that the decision is more complicated. Consideration needs to be given then to: initial purchase cost, physical size, running costs, the installation environment, and in particular, the levels of input harmonic distortion they generate. Both designs can be operated in parallel to achieve higher levels of availability and resilience.
Over the last decade, the gap between these two uninterruptible power supply technologies has reduced as manufacturers have applied common techniques and research & development efforts to both designs. The driving force behind this has been cost and size, alongside demands to improve operating efficiency and reduce harmonic generation. In terms of online performance, both designs provide the same level of performance and are classified as VFI systems (voltage and frequency independent - in accordance with EN/IEC 62040-3). Their principal differences are their effects on upstream supplies and the operating environment.
Transformerless UPS are generally recognised as more efficient and having a higher power factor than an equivalent transformer-based design, therefore operating costs can be lower.
Below 10kVA, the transformerless UPS design dominates the online uninterruptible power supply market and has become the standard within data centre environments as they offer a more compact footprint, higher operating efficiencies and lower noise output. However, the strengths of the transformer-based design come into play in the industrial environment.

EXCELENTE ARTIGO DO ENG. ROBIN KOFFLER,ELE E REALMENTE VALIDO NOS MERCADOS EUROPEUS,NORTEAMERICANO,ASIATICO,MAIS NO MERCADO ESPECIFICAMENTE BRASILEIRO ,TERIA QUE TOMAR EM CONTA NOSSA REALIDADE ESPECIFICA,ISTO E,BRASIL E O CAMPEAO MUNDIAL DOS RAIOS,AS DESCARGAS ATMOSFERICAS NAS REGIONES SUDESTE E MAIOR PARTE DO TERRITORIO,SAO DE NATURALEZA INTENSA QUE OS UPS COM TRANSFORMADOR TEM MAIOR VIDA UTIL E ROBUSTEZ QUE OS QUE SAO PROJETADOS SEM TRANSFORMADOR,NESTE CASO OS CIRCUITOS SUPPRESSORES DE SURTOS E DESCARGAS ATMOSFERICAS TRABALHARAM A SUA MAXIMA CAPACIDADE,POR ISSO TEM QUE SER BEM ANALIZADO A ESCOLHA MAS CERTA PARA CADA NECESSIDADE.

SEMINARIO BASICO DE UPS

SEMINARIO SOBRE UPS , TENDENCIAS TECNOLOGICAS Y COMERCIALES.

DISPERSÃO DE TRANSFORMADORES DE BAIXA FREQUÊNCIA COMO FILTRO EM UMA UPS

V SEMINÁRIO NACIONAL
DE CONTROLE E AUTOMAÇÃO
INDUSTRIAL, ELÉTRICA E DE
TELECOMUNICAÇÕES


ESTUDO DE VIABILIDADE DA UTILIZAÇÃO DA DISPERSÃO DE TRANSFORMADORES DE
BAIXA FREQUÊNCIA COMO FILTRO EM UMA UPS DE 6kVA COM MODULAÇÃO PWM 3
NÍVEIS
Trabalho TT - 050 Página 1 de 5
Página 1 de 5
Halisson Alves de Oliveira *, Lucas Maciel Menezes *, Raphael Amaral da Câmara *, Cícero
Marcos Tavares Cruz *, Armando Walter Cavero Miranda **
* Universidade Federal do Ceará
** Microsol


RESUMO – Este artigo apresenta um estudo de
perdas no núcleo em um transformador de baixa
freqüência sendo alimentado por uma tensão
senoidal e por uma tensão em formato PWM
senoidal. As perdas no núcleo são comparadas
nos dois casos, sendo verificada a possibilidade
da utilização da dispersão do transformador como
elemento de filtro para um inversor com
modulação PWM senoidal de 6KVA. O estudo
visa também à redução de custos na UPS com a
retirada do indutor de filtro e utilização do
transformador como elemento filtrante,
isolamento galvânico e meio de elevação da
tensão de saída do inversor da UPS.
ABSTRACT – This paper presents a study of low
frequency power transformer losses with a
sinusoidal and a Pulse Width Modulation
sinewave input voltage in a commercial UPs. Two
cases are compared, the first one is a inverter
with PWM control technique with a LC filter and
after a low frequency power transformer. The
second case is the transformer connected directly
in the inverter output without the LC filter. The
study motivation is lower the UPS cost with the
LC filter removed, without any output voltage distortion.
READ FULL PAPER HERE