POWER ELECTRONICS for SMART GRIDS

(1)

POWER ELECTRONICS

for SMART GRIDS

A. Del Pizzo

(2)

Outline

Preliminary considerations

Objectives of Smart Grids

Basic power electronics elements

Power electronics apparatuses

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Preliminary considerations

In the last decades the energy demand is continuosly increasing (

both in industrialized and in emerging countries

) and electrical loads are becoming more and more sophisticated.

[20,300 terawatt-hours today to 33,000 terawatt-hours by 2030 in the world]

Electrical drives and power electronics apparatuses for energy conversion are widely used;

as a consequence, big problems of

power quality

occur on the modern distribution grids

.

(4)

Preliminary considerations

In addition to the increased requirements and needs of end-users, the

Distributed

Generation (DG)

has introduced very high levels of complexity in grid operation and

management [

even if well-accepted by the market

]

Together with :

- Power quality

-

Efficiency of energy management

a real problem is the

Stability

of networks having prevalent Distributed Generation

architecture, especially when renewable energy sources are used.

(5)

One preliminary question is:

Today’s Grids

Are they “stupid”

or “passive”?

Tomorrow’s Grids

Will they be“smart”

or “active”?

Answer:

Today, some intelligence levels are already implemented.

For example, ENEL considers its network the largest “smart-grid” currently active in the world

Automatic Meter Management Telegestore is operating on about 32 Million of Customers Network automation

- HV and MV network remotely operated. - More than 100.000 MV substations

remote controlled

- Automatic procedures for fault clearing Asset Management

Cartographic census of network assets Database of network events

Optimization of network investments based on a risk analysis.

We are going towards

smarter grids

(6)

Shift from today’s to tomorrow’s power grids

Traditional structure

Hierarchical power systems

Expected transition:

unidirectional energy flow, from central

source to the distributed end-users

Smart Grid

(future structure)

two-way power flow of

distributed generation

(7)

Shift from today’s to tomorrow’s power grids

The Smart Grid is not a “thing”, not an “object”

but it is an “

idea

”, a “

vision

”.

Nevertheless, the Smart Grids could represent a revolution with respect to the

traditional concept of a power system. This revolution will be made through a

gradual transformation towards a more intelligent, more effective and

environmentally sensitive network to provide for our future needs.

The active management of power electrical networks needs large investments

of Governements in

Research and Development Projects

,

in order to accelerate the grid transformation process.

Smart-Grids European Technology Platform

(

) is now the European effort in that

(8)

Main feautures (and requirements) of a Smart Grid

(

the future electric network

)

• Capacity:

the demand for electrical energy has to be satisfied.

• Accessibility:

the Renewable Energy Sources should have access to the Grid

.



Reliability:

high quality electricity must be always available; no interruptions must occur

.

• Efficiency

:

production, transportation and consumption of electricity must be efficient;

efficiency is necessary in order to reduce gas emission (CO

₂

) and to obtain

lower costs.

• Sustainability:

Low-carbon energy-sources must be integrated into the system.

• Flexibility

:

it is necessary in order to meet the new consumers requirements,

(e.g., their active participation in the electric energy generation or

the fast and easy recharging procedure for road electrical vehicles).

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Traditional Systems

Smart Grids

◊ Centralized and distributed power generation

◊ Intermittent renewable power generation

◊ One-directional power flow

◊ Multi-directional power flow

◊ Generation follows load

◊ Loads follows generation

◊ Operation based on historical experience

◊ Operation based on real-time data

◊ Full and efficient grid accessibility

◊ Consumers participate in the market

◊ Centralized power generation

◊ Limited grid accessibility for new producers

An optimal smart grid should be able to:



accept any kind of generation source;



deliver power of any quality on demand;



diagnose itself;



heal itself through intelligent use of

redundancies.

(10)

MICROGRIDs

A microgrid comprise

medium

- and/or l

ow-voltag

e distribution systems with distributed

energy sources, storage devices and controllable loads.

They can operate either if connected to the main power network or if isolated

(islanded) in a controlled and coordinated way.

Frequently we refer to a selfsufficient interconnection of distributed generation,

residential and industrial load in a low-voltage network without a persistent connection

to a larger grid.

Protection is a key challenge of Microgrids

.

When a fault occurs on the grid, the microgrid should be isolated from the main utility

as quickly as possible to protect the microgrid loads.

The creation of

ad hoc

microgrids by islanding pockets of a larger network has the

potential to stop cascading outages while critical loads are online.

There is a project, supported by EU (“More Microgrids”), finalized to identify and

address the challenges of proliferation of microgrids in Europe.

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TECHNOLOGIES used in SMART GRIDS

In order to fulfill the above listed requirements, a suitable automation system is needed.

It should be intelligent enough to correctly take into account generation profiles that may change

with the weather and the time (like wind or photovoltaic generation).

The result is a continuously changing distribution of power flow and direction, instead of the relatively stable, unidirectional power flow of a today distribution network.

All these functions requiremany different technologies at the same time:

◊

Power Electronics Apparatuses

for

filtering

and for the operations devoted to

maintain prefixed levels of

power quality

.

◊

very effective

Sensors and Transducers

together with

Metering Systems

in general;

◊fast and reliable

Information and Communication Technologies

(ICTs);

◊

Power Conversion Systems

able to rapidly and efficaciously adapt the values of

voltage/current/power/energy according to the requests

(these systems include

(12)

Power Conversion Systems for Smart Grids

◊

_{electric generators}

◊

_{energy storage units}

◊

_{static power electronic converters}

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Electric Generators

Power Electronics

Traditional electromechanical rotating generators:

◊

_{Synchronous machines (alternators) with excitated rotor;}

magnetically isotropy or “salient pole”, depending on the rotor

speed

◊

_{Induction (Asynchronous) machines with squirrel-cage rotor,}

mainly operating on grids of prevalent power, with impressed

voltage

In the last year the attention has been mainly devoted to:

◊

“

Double-fed” Induction (Asynchronous) machines with wound

rotor, for medium-power wind generators

◊

Permanent Magnet (PM) synchronous generators for wind

generators of small power and of very high power, for

micro-combined heat and power units [micro CHP], for UPS

(Uninterruptible Power Systems)

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Electric Generators

Power Electronics

Main Advantages of PM Synchronous Generators:

◊

High power density (kW/kg and kW/m

3

₎

◊

Absence of ring-brush contacts (they are brushless)

◊

Possibility to be operated as “Direct Drive” (no-gear) or to

maintain good performance at low speed

◊

High efficiency

Main drawbacks:

◊

No variability of rotor exciting field

◊

Constructive problems to fix the magnets on the rotor

◊

Cost

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Magnetic configurations of PM Synchronous Generators:

◊

The magnets can be mounted either externally or internally to the rotor (correspondently, we have the “Surface mounted PM generators” or “Interior PM Generators”)

◊

The stator is mainly “three-phase” with low and/or high pole-pair number, as requested by the specific application

◊

In some cases the stator can be multi-phase; for small power (few kW), the stator can be single-phase

◊

The topology can be “Radial flux” (most part of solutions) or “Axial flux”

Power Electronics N N N S S S N N N N N N S S S S S S NUCLEO STATORICO AVVOLGIMENTI DISCO ROTORICO ALBERO MAGNETI N S S N NUCLEO STATORICO AVVOLGIMENTI DISCO ROTORICO ALBERO MAGNETI N S S N NUCLEO STATORICO AVVOLGIMENTI AVVOLGIMENTI DISCO ROTORICO ALBERO ALBERO MAGNETI MAGNETI N S S N N S S N

(16)

Main components:

◊

Batteries

◊

High-Speed Flying Wheel

s

◊

SuperCapacitors

Power Electronics

Energy Storage Units

Further needed components:

◊

Power electronic converters to drive and control

the storage unit (e.g. a DC-DC bi-directional

converter for connect he battery to a dc-link).

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◊

Power switching devices

◊

Basic Converter Topologies

◊

Main power electronic apparatuses for smart grids

Power Electronics

Static Power Electronic Converters

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continua continua alternata

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Power Electronics

Static Power Electronic Converters

Power switching devices

◊

MOSFET (

Metal Oxide Semiconductor Field-Effect Transistor

) for low

voltage, low power (until some tens of kW)

100V/200A or 500 V/20A

◊

IGBT (

Insulated Gate Bipolar Transistor

) for a wide range of power

(some kW until some MW);

until 4.5 kV

◊

GCT

(Gate Commutated Turn-off Thyristor)

or IGCT

(Integrated Gate

Commutated Thyristor)

for high voltage, high power (several MW),

5kA, 10 kV

.

Source Drain Gate D S G N N-Channel - MOSFET P G D S vDS vGS +  Isolante G D S v_DS vGS +  (a) (b) (c) Source Drain Gate D S G N N-Channel - MOSFET P G D S vDS vGS +  Isolante G D S v_DS vGS +  G D S v_DS vGS +  G D S v_DS vGS +  (a) (b) (c) IGBT v_GE Collettore Emettitore Gate G C E v_CE Collettore Emettitore Gate G C E v_CE v_GE IGBT v_GE Collettore Emettitore Gate G C E v_CE Collettore Emettitore Gate G C E v_CE v_GE

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Power Electronics

Basic Converter Topologies

DC-DC Conversion

◊

Usually the dc-dc converters are called “chopper”

◊ Chopper step-down (“buck”)

◊ Chopper step-up (“boost”)

◊ Chopper buck-boost

◊ With respect to the energy-flow, Choppers can be uni-directional (one quadrant) or bi-directional (four-quadrant)

◊ Application fields of choppers:

 Connecting a supercap to a d.c.-bus;  Connecting a battery to a d.c.-bus;

 Connecting a fuel-cell to a d.c.-bus at a fixed voltage;

 Connecting a PhotoVoltaic plant to a d.c.-bus at impressed voltage;

 Connecting a stabilized d.c.-bus to the output of a rectifier placed on the armature of a synchronous generator in wind plants;

 Supplying d.c.-motors at variable speeds;

 All the connections of two lines in d.c., even when one (input or output)voltage has to be constant;

alternata



alternata



alternata



alternata



v₁ v₂ Load i₂ i1 T D v₁ v₂ Load i₂ i1 T D v₁ _v 2 Load i₂ i₁ T D L C v₁ _v 2 Load i₂ i₁ T D L C

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Power Electronics

Basic Converter Topologies

DC-AC Conversion

◊

Usually the d.c.-a.c. converters are called “inverter”

◊ Today, the most part of inverters are VSI (Voltage Source Inverter); CSI-Current Source Inverters are not frequently used.

◊ PWM (Pulse Wide Modulation) Inverters are practically always used, instead of the

six-step inverters with rectangular voltages; frequently PWM-VSI inverters are three-phase.

◊ With respect to the energy-flow, Inverters are intrinsically bi-directional

◊ Application fields of inverters:

 Connecting a d.c.-bus to an a.c. grid (e.g. in P.V. plants, or in Wind plants, …);

 Supplying a.c.-motors at variable speed (induction motors, synchronous motors, brushless motors);

alternata



alternata



alternata



alternata



+

-M



V _C T1 T2 T3 1 D D₂ D3 i1 3 i 2 i 1 ' D D_{2 '} D3 ' T1 ' T2 ' T3 ' A B C K H

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Power Electronics

Basic Converter Topologies

DC-AC Conversion

◊

in the last years sometimes inverters are multi-phase (n_ph>3); this configuration can be used either for supplying multi-phase loads, or to supply different three/single-phase loads at reduced losses, or to improve reliability when this is very important;

◊ moreover, there are many “multi-level” inverters, i.e. inverters with more than 2 voltage levels; these configurations have been introduced especially for the cases where requested voltage and/or power are over the limits of the switching devices available on the market

◊ now “multi-level” inverters are used also to improve energy performance in terms of reduction of ripples in currents and voltage, with the aim to improve some important power-quality indexes;

◊ some multilevel topologies are also fault-tolerant, i.e. able to improve reliability of the conversion unit, because in case of fault they continue to supply the load, even if at reduced power

◊ there are many different topologies of multilevel inverters; the “diode-clamped” ones are now the most common; the “cascaded H-bridge” appear to be more interesting for control the power quality

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

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

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Power Electronics

Basic Converter Topologies

DC-AC Conversion

Multilevel topologies of PWM-VSI Inverters

V C₁ C₂ T_A1 T_A2 T_A1' T_A2' O D_b D_b' A B C A B C Diode-Clamped C1 C2 A O V_AO = E V₁ = E V₂ = E V_m-1 C_m-1 V_A1 V_A2 V_A(m-1) Cascaded H-Bridge T T/2 0 VA0 4E T 0 VA1 T T/2 0 T/2 0 VA2 VA3 VA4 0 T T/2 T T/2

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Power Electronics

Basic Converter Topologies

AC-DC Conversion

◊

A direct a.c./d.c. converter is generally called “rectifier”; it can be “not controlled” (using only diodes); partially-controlled (diodes+thyristors) or totally-controlled (all thyristors).

◊ The a.c./d.c. conversion can be made also in two stages, using a not-controlled rectified followed in cascade by a chopper in order to vary the output voltage level.

◊ Instead of traditional rectifiers, now we can use also the “Voltage Source Rectifier (VSR)” which is composed by controlled switching devices (e.g. IGBT); the structure is equal to the one of an inverter (VSI, for this reason it is not shown here), but the power flow is in opposite sense. They are also called “Active Front End” (AFE).

◊ These VSRs have not only the basic function of conversion a.c./d.c., but they can have additional features: they are able to keep constant the voltage on the capacitors in the dc-link, to ensure a power-factor very close to 1, to sensibly reduce the harmonic content of the currents; frequently these active front-ends have multi-level topology.

◊ The VSRs are more economical used in a range of medium-low power (until some hundreds of kW)

alternata



alternata



alternata



alternata



~

M T₃ T₂ T₁ L_s V_a I_a ~ T₃ T₂ T₁ L_s V_a I_a T₆ T₅ T₄ M V

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24/28

One Day Workshop SAE-NA- Istituto Motori CNR, Napoli - -Power Electronics for Smart Grids- - A. Del Pizzo

Power Electronics

Basic Converter Topologies

AC-AC Conversion

◊

Usually the a.c./a.c. conversion is made in two (or more) stages in cascade; i.e. an a.c./d.c. conversion followed by a d.c./a.c. one.

◊ In the last years there is a growing interest for “Matrix converters”, which are a.c./a.c. converters of ”direct” type, because they carry out the conversion in only one step; they have the advantage that can avoid the passage in d.c., especially important when the environmental conditions are dangerous for capacitors.

alternata



alternata



alternata



alternata



~

(a)

~

C

=

raddrizzatore+ -V.S.I. 6 - step controllato M 

=

C + -M  raddrizzatore

a diodi chopper 6 - step V.S.I.

(b) a) Convertitore diretto (monostadio)

~

C

=

~

C

~

frequenza fissa frequenza variabile

Cicloconvertitore Convertitore a matrice

b) Convertitore indiretto (pluristadio)

=

~

=

Raddrizzatore controllato + C.S .I.

C.S.I. commutazione forzata commutaz. da rete secondaria

(sincroconvertitore)

Convertitore attivo + V.S .I. Entrambi i convertitori a commutazione comandata V.S.I.

Raddrizzatore + V.S .I. commutaz. forzata (SCR) commutaz. comandata (GTO,

IGBT, BJT, M OSFET, ...)

~

L

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One Day Workshop SAE-NA- Istituto Motori CNR, Napoli - -Power Electronics for Smart Grids- - A. Del Pizzo

Power Electronics

Basic Converter Topologies

Filtering and Power Quality improvement

◊

Together with classical “passive” filters (capacitor banks or reactors), we can use “active filters” which are based on the use of power electronic converters together with inductors and/or capacitors.

◊

Active filters can be placed in series or in parallel to the line.

◊ In a.c. grids the FACTS (Flexible AC Transmission Systems) increase the capacity of the grid, improve quality indexes and improve stability.

◊ STATCOM (Static Compensators of reactive power).

◊ Static Compensators of reactive power (SVC – Static VAR Compensator); they are based on the presence of Li-ion batteries that can dinamically storage the energy.

◊ Voltage and VAR Optimized control (VVO) is performed by apparatuses which includes transformers with proper tap changers (in order to regulate the voltage) and compensators of reactive power; the control algorithms implemented in the microcontroller try the optimum value of voltage that can be combined with the VAR data.

~

(a)

~

C

=

raddrizzatore+ -V.S.I. 6 - step controllato M 

=

C + -M  raddrizzatore

a diodi chopper 6 - step V.S.I.

(b) a) Convertitore diretto (monostadio)

~

C

=

~

C

~

Cicloconvertitore Convertitore a matrice

b) Convertitore indiretto (pluristadio)

=

~

=

Raddrizzatore controllato + C.S .I.

C.S.I. commutazione forzata commutaz. da rete secondaria

(sincroconvertitore)

Convertitore attivo + V.S .I. Entrambi i convertitori a commutazione comandata V.S.I.

Raddrizzatore + V.S .I. commutaz. forzata (SCR) commutaz. comandata (GTO,

IGBT, BJT, M OSFET, ...)

~

L

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Power Electronics

Filtering and Power Quality improvement

L_c C T1 T2 L CF T1 T2 TCR TSC

Thyristor Controlled Reactor Thyristor Switched Capacitor

L_c C T1 T2 L_c C T1 T2 L_c C T1 T2 L CF T1 T2 T1 T2 TCR TCR TSCTSC

Thyristor Controlled Reactor Thyristor Switched Capacitor

L_c

C

L

TSC TSC

Fig. 3 - StatVar combinato con filtri LC. Lc C TCR CF LF CF L_F L_c C L L TSC TSC

Fig. 3 - StatVar combinato con filtri LC. Lc C TCR CF LF CF LF CF L_F CF L_F

Fig. 6- Schema di principio di uno Statcom AT MT

VSC Voltage Source Converter

C

Fig. 6- Schema di principio di uno Statcom AT MT

VSC Voltage Source Converter

C

Fig. 7- Schema unifilare di uno Statcom a 3 livelli Fig. 7- Schema unifilare di uno Statcom a 3 livelli

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Power Electronics

Filtering and Power Quality improvement

Fig. 11 – SSSR – Static Synchronous Series Compensator

Fig. 11 – SSSR – Static Synchronous Series Compensator Fig. 11 – DVR – Dynamic Voltage RestorerFig. 11 – DVR – Dynamic Voltage Restorer

Fig. 12 – UPFC – Unified Power Flow Controller Fig. 12 – UPFC – Unified Power Flow Controller

Fig. 13 – IPFC – Interline Power Flow Controller Fig. 13 – IPFC – Interline Power Flow Controller

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Power Electronics

Power Electronic Transformer in Medium Frequency

(a) PV PV PV H-Bridge - 1st_module H-Bridge - 10thmodule H-Bridge - 1st_module H-Bridge - 10th_module H-Bridge - 1st_module H-Bridge - 10th_module LV Inverter MV Rectifier-Inverter MF Transformers 2 p v vs2 1 p v vs1 3 p v vs3 1 p i 1 s i 2 s i 2 p i 3 p i 3 s i ,1 dc v ,3 dc v ,2 dc v GRID L R LL 1 L i 2 L i 3 L i 1 L v 2 L v 3 L v O PV PV PV PV PV PV H-Bridge - 1st_module H-Bridge - 10thmodule H-Bridge - 1st_module H-Bridge - 10th_module H-Bridge - 1st_module H-Bridge - 10th_module LV Inverter MV Rectifier-Inverter MF Transformers 2 p v vs2 1 p v vs1 3 p v vs3 1 p i 1 s i 2 s i 2 p i 3 p i 3 s i ,1 dc v ,3 dc v ,2 dc v GRID L R LL 1 L i 2 L i 3 L i 1 L v 2 L v 3 L v O (b) PV PV PV H-Bridge – 1st _module H-Bridge -10th_module H-Bridge - 1st _module H-Bridge - 10th module H-Bridge – 1st _module H-Bridge – 10th module LV Inverter MV Rectifier-Inverter MF Transformer GRID L R LL 1 L i 2 L i 3 L i 1 L v 2 L v 3 L v O ,1 dc v ,3 dc v ,2 dc v p v vs PV PV PV PV PV PV H-Bridge – 1st _module H-Bridge -10th_module H-Bridge - 1st _module H-Bridge - 10th module H-Bridge – 1st _module H-Bridge – 10th module LV Inverter MV Rectifier-Inverter MF Transformer GRID L R LL 1 L i 2 L i 3 L i 1 L v 2 L v 3 L v O ,1 dc v ,3 dc v ,2 dc v p v vs

Fig. 3. Schematic representation of a PET with a N-level inverter on LV side

PV vdc,N _H-B N LV Inverter MV Rectifier MF Transformer GRID C1 PV vdc,1 _H-B 1 vp,1 vp,N vp vs NPC single-phase C_Ns NPC three-phase MV Inverter L R LL 1 L i 2 L i 3 L i 1 L v 2 L v 3 L v O ip PV vdc,N _H-B N LV Inverter MV Rectifier MF Transformer GRID C1 PV vdc,1 _H-B 1 vp,1 vp,N vp vs NPC single-phase C_Ns NPC three-phase MV Inverter L R LL 1 L i 2 L i 3 L i 1 L v 2 L v 3 L v O ip

POWER ELECTRONICS for SMART GRIDS