power electronics for smart grids

8/4/2019 Power Electronics for Smart Grids

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POWER ELECTRONICSfor SMART GRIDS

A. Del PizzoDept. of Electrical Engineering – University of Naples Federico II

One Day Workshop SAE-NA -- Istituto Motori CNR - Napoli, November 8, 2010 1/28



Outline

One Day Workshop SAE-NA -- Istituto Motori CNR - Napoli, November 8, 2010 2/28

Preliminary considerations

Objectives of Smart Grids

Basic power electronics elements

Power electronics apparatuses



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


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.

3/28

Load complexity




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.

4/28

Grid complexity due to Distributed Generation

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



One preliminary question is:

5/28One Day Workshop SAE-NA - Istituto Motori CNR, Napoli - - Power Electronics for Smart Grids- - A. Del Pizzo

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 MeterManagement

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(in a progressive way)




Shift from today’s to tomorrow’s power grids

Traditional structure

Hierarchical power systems

Expected transition:

unidirectional energy flow, from centralsource to the distributed end-users

Smart Grid (future structure)

two-way power flow of

distributed generation


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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

(sponsored by the European Commission) is now the European effort in that

direction.



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 (CO2) and to obtainlower 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 orthe fast and easy recharging procedure for road electrical vehicles).



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.



MICROGRIDs

A microgrid comprise medium - and/or low-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 thepotential 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.



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 require many different technologies at the same time:

◊Power Electronics Apparatuses for filtering and for the operations devoted tomaintain 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 ofvoltage/current/power/energy according to the requests (these systems includeelectric generators , energy storage units , static power electronic converters )



Power Conversion Systems for Smart Grids

◊ electric generators

◊ energy storage units

◊ static power electronic converters

Power Electronics



Electric Generators

Power Electronics

Traditional electromechanical rotating generators:

◊ Synchronous machines (alternators) with excitated rotor;magnetically isotropy or “salient pole”, depending on the rotorspeed

◊ 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 woundrotor, for medium-power wind generators

◊ Permanent Magnet (PM) synchronous generators for windgenerators of small power and of very high power, for micro-combined heat and power units [micro CHP], for UPS(Uninterruptible Power Systems)



Electric Generators

Power Electronics

Main Advantages of PM Synchronous Generators:

◊ High power density (kW/kg and kW/m3)

◊ Absence of ring-brush contacts (they are brushless)

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

◊ Temperature at rated operations (demagnetization can occur)



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-pairnumber, 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

NN

NN

NN

SS

SS

SS

NUCLEO STATORICO

AVVOLGIMENTI

DISCO ROTORICO

ALBERO

MAGNETI

N

S S

N

NUCLEO STATORICO

AVVOLGIMENTI

DISCO ROTORICO

ALBERO

MAGNETI

N

S S

N

NUCLEO STATORICO

AVVOLGIMENTIAVVOLGIMENTI

DISCO ROTORICO

ALBEROALBERO

MAGNETIMAGNETI

N

S S

N

N

S S

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Main components:

◊ Batteries

◊ High-Speed Flying Wheels

◊ 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-directionalconverter for connect he battery to a dc-link).



◊ Power switching devices

◊ Basic Converter Topologies

◊ Main power electronic apparatuses for smart grids


Power Electronics

Static Power Electronic Converters

alternata continua continua alternata

alternata alternata continua continua

alternata continua continua alternata

alternata alternata continua continua




Power Electronics

Static Power Electronic Converters

Power switching devices

◊ MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) for lowvoltage, 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

v DS

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

v DS

vGS

+

Isolante

G

D

S

v DS

vGS

+

G

D

S

v DS

vGS

+

G

D

S

v DS

vGS

+

(a) (b) (c)

IGBT

vGE

Collettore

Emettitore

Gate

G

C

E

vCE

Collettore

Emettitore

Gate

G

C

E

vCE

vGE

IGBT

vGE

Collettore

Emettitore

Gate

G

C

E

vCE

Collettore

Emettitore

Gate

G

C

E

vCE

vGE




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 asynchronous 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 beconstant;

continua continuacontinua continua

v1 v2

Load

i2i1

T

Dv1 v2

Load

i2i1

T

D

v1 v2

Load

i2i1

T

D

L C v1 v2

Load

i2i1

T

D

L C




Power Electronics


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 SourceInverters 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, brushlessmotors);

continua alternata

continua alternata

+

-

V C

T1 T2 T3

1D 2D D3 1i

3i

2i

1'D 2'D D3'

T1' T2' T3'

A

BC

K

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Power Electronics


DC-AC Conversion

◊ in the last years sometimes inverters are multi-phase (n ph >3); this configuration can be used eitherfor 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 ofripples 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 conversionunit, 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 themost common; the “cascaded H-bridge” appear to be more interesting for control the powerquality

continua alternata

continua alternata




Power Electronics


DC-AC Conversion

Multilevel topologies of PWM-VSI Inverters

V

C 1

C 2

T A1

T A2

T A1 '

T A2'

O

Db

Db'

A B C

A B C Diode-Clamped

C 1

C 2

A

O

V AO

= E V 1

= E V 2

= E V m-1

C m-1

V A1

V A2

V A(m-1)

Cascaded H-Bridge

T

T/20

V A0

4E

-4E

T 0

V A1

T T/2

0

T/2

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V A2

V A3

V A4

0

T T/2

T T/2

Power Electronics





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 incascade 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)” whichis composed by controlled switching devices (e.g. IGBT); the structure is equal to the one of aninverter (VSI, for this reason it is not shown here), but the power flow is in opposite sense. Theyare also called “Active Front End” (AFE).

◊ These VSRs have not only the basic function of conversion a.c./d.c., but they can have additionalfeatures: 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; frequentlythese 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 continuaalternata continua

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T3

T2

T1

L s

V a

I a

~

T3

T2

T1

Ls

V a

I a

T6

T5

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Power Electronics





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 theadvantage that can avoid the passage in d.c., especially important when the environmentalconditions are dangerous for capacitors.

alternata alternata

alternata alternata

~

~

C

=raddrizzatore +

-

V.S.I.6 - stepcontrollato M

= =C+

-

M

raddrizzatorea diodi

chopper6 - stepV.S.I.

Convertitore a matrice

Power Electronics





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/orcapacitors.

◊ 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 thepresence of Li-ion batteries that can dinamically storage the energy.

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

~

~

C

=raddrizzatore +

-

V.S.I.6 - stepcontrollato M

= =C+

-

M

raddrizzatorea diodi

chopper6 - stepV.S.I.

Convertitore a matrice

Power Electronics





Lc

C

T 1 T 2

L

C F

T 1 T 2

TCR TSC

Thyristor Controlled Reactor Thyristor Switched Capacitor

Lc

C

T 1 T 2

Lc

C

T 1 T 2

Lc

C

T 1 T 2

L

C F

T 1 T 2 T 1 T 2

TCRTCR TSCTSC

Thyristor Controlled Reactor Thyristor Switched Capacitor

Lc

C

L

TSC TSC

Fig. 3 - StatVar combinato con filtri LC.

Lc

C TCR

C F

LF

C F

LF

Lc

C

LL

TSC TSC

Fig. 3 - StatVar combinato con filtri LC.

Lc

C TCR

C F

LF

C F

LF

C F

LF

C F

LF

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

2-level STATCOM

Power Electronics





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

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

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

power electronics for smart grids

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