POWER ELECTRONICS for SMART GRIDS A. Del Pizzo Dept. of Electrical Engineering University of Naples Federico II One Day Workshop SAE-NA -- Istituto Motori CNR - Napoli, November 8, 2010 1/28
Outline Preliminary considerations Objectives of Smart Grids Basic power electronics elements Power electronics apparatuses One Day Workshop SAE-NA -- Istituto Motori CNR - Napoli, November 8, 2010 2/28
Preliminary considerations Load complexity 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
Preliminary considerations Grid complexity due to Distributed Generation 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
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 (in a progressive way) 5/28
Expected transition: unidirectional energy flow, from central source to the distributed end-users Shift from today s to tomorrow s power grids two-way power flow of distributed generation Hierarchical power systems Smart Grid (future structure) Traditional structure 6/28
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. 7/28
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: Efficiency: high quality electricity must be always available; no interruptions must occur. 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). 8/28
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. Traditional Systems Centralized power generation One-directional power flow Generation follows load Operation based on historical experience Limited grid accessibility for new producers Smart Grids Centralized and distributed power generation Intermittent renewable power generation Multi-directional power flow Loads follows generation Operation based on real-time data Full and efficient grid accessibility Consumers participate in the market 9/28
MICROGRIDs A microgrid comprise medium- and/or low-voltage 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. 10/28
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: 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 electric generators, energy storage units, static power electronic converters) Power Electronics Apparatuses for filtering and for the operations devoted to maintain prefixed levels of power quality. 11/28
Power Electronics Power Conversion Systems for Smart Grids electric generators energy storage units static power electronic converters 12/28
Power Electronics Electric Generators 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 microcombined heat and power units [micro CHP], for UPS (Uninterruptible Power Systems) 13/28
Power Electronics Electric Generators 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 Temperature at rated operations (demagnetization can occur) 14/28
Power Electronics 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 ) S N N S S N 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 AVVOLGIMENTI DISCO ROTORICO N S S ALBERO MAGNETI N NUCLEO STATORICO 15/28
Power Electronics Energy Storage Units Main components: Batteries High-Speed Flying Wheels SuperCapacitors 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). 16/28
Power Electronics Static Power Electronic Converters Power switching devices Basic Converter Topologies Main power electronic apparatuses for smart grids continua continua continua continua 17/28
Static Power Electronic Converters Power Electronics 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 D Drain N-Channel - MOSFET D + D + Collettore C IGBT C Collettore Gate G Isolante N P G v DS G v GS v GS S Source S S (a) (b) (c) v DS G Gate v GE Emettitore E v CE G Gate v GE E v CE Emettitore 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. 18/28
DC-DC Conversion Basic Converter Topologies Usually the dc-dc converters are called chopper Chopper step-down ( buck ) Chopper step-up ( boost ) Chopper buck-boost i 1 T i 2 Load v 1 D v 2 continua continua Power Electronics continua continua i 1 D T Load v 1 L C v 2 i 2 With respect to the energy-flow, Choppers can be uni-directional (one quadrant) or bidirectional (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; 19/28
Basic Converter Topologies Power Electronics DC-AC Conversion Usually the d.c.-a.c. converters are called inverter continua continua continua continua 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 sixstep inverters with rectangular voltages; frequently PWM-VSI inverters are three-phase. + K T 1 T 2 T 3 V C A D1 2 B D D 3 i 1 C T 1' T 2' T 3' i 2 i 3 M H 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, ); - D1' D 2' D 3' Supplying a.c.-motors at variable speed (induction motors, synchronous motors, brushless motors); 20/28
Basic Converter Topologies Power Electronics 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 continua continua continua continua 21/28
Basic Converter Topologies Power Electronics DC-AC Conversion Multilevel topologies of PWM-VSI Inverters = E V 1 C 1 V A1 A T A1 C 1 D b T A2 = E V 2 C 2 V A2 V AO V O D b' T A1' A B C C 2 T A2' C m-1 V m-1 = E V A(m-1) O Diode-Clamped A B C V A1 0 V A2 T/2 T Cascaded H-Bridge 0 V A3 T/2 T 0 V A4 0 V A0 4E T/2 T/2 T T 0 T/2 T -4E 22/28
AC-DC Conversion Basic Converter Topologies Power Electronics 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). I a V a V M a ~ ~ L s T T 2 T T T T 1 3 1 2 3 L s I a V M continua contin contin T 4 T 5 T 6 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) 23/28
Basic Converter Topologies Power Electronics continua cont 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. ~ raddrizzatore + controllato - = C V.S.I. 6 - step M (a) cont ~ raddrizzatore a diodi = = In the last years there is a growing interest for Matrix converters, which are a.c./a.c. converters of direct type, frequenza because fissa they carry out the frequenza conversion variabile in only one step; they have the advantage that can avoid the passage in d.c., especially important when the environmental conditions are dangerouscicloconvertitore for capacitors. Convertitore a matrice + chopper - a) Convertitore diretto (monostadio) ~ ~ C V.S.I. 6 - step M (b) b) Convertitore indiretto (pluristadio) 24/28
Basic Converter Topologies 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/or ~ = capacitors. raddrizzatore + V.S.I. controllato C 6 - step (a) 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. raddrizzatore + chopper C (b) STATCOM (Static Compensators of reactive power). frequenza fissa ~ a diodi - a) Convertitore diretto (monostadio) 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. Cicloconvertitore ~ ~ - frequenza variabile V.S.I. 6 - step Convertitore a matrice M M 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. b) Convertitore indiretto (pluristadio) 25/28
Filtering and Power Quality improvement Power Electronics TCR TSC AT L L c MT C F T 1 T 2 VSC T 1 T 2 C Voltage Source Converter Thyristor Controlled Reactor Thyristor Switched Capacitor C Fig. 6- Schema di principio di uno Statcom L c L c L L F L F Fig. 7- Schema unifilare di uno Statcom a 3 livelli C F C F TSC C TSC C TCR Fig. 3 - StatVar combinato con filtri LC. 2-level STATCOM 26/28
Filtering and Power Quality improvement Power Electronics Fig. 11 SSSR Static Synchronous Series Compensator Fig. 11 DVR Dynamic Voltage Restorer Fig. 12 UPFC Unified Power Flow Controller Fig. 13 IPFC Interline Power Flow Controller 27/28
Power Electronic Transformer in Medium Frequency Power Electronics LV Inverter i s1 MV Rectifier-Inverter H-Bridge - 1 st module (a) MV Rectifier-Inverter H-Bridge 1 st module (b) v dc,1 PV v dc,2 PV i p1 v p1 v s 1 i p2 v p2 v s 2 i s2 H-Bridge - 10 th module H-Bridge - 1 st module i L1 R L GRID L L v L1 LV Inverter v dc,1 PV v dc,2 MF Transformer H-Bridge -10 th module H-Bridge - 1 st module i L1 R L GRID L L v L1 v dc,3 i L2 i L3 v L2 v L3 O PV v p v s i L2 i L3 v L2 v L3 O PV i p3 v p3 v s 3 v dc,3 H-Bridge - 10 th module H-Bridge - 10 th module MF Transformers i s3 H-Bridge - 1 st module PV H-Bridge 1 st module H-Bridge - 10 th module H-Bridge 10 th module LV Inverter MF Transformer PV v dc,1 v H-B1 p,1 i p MV Rectifier C 1 MV Inverter i L1 R GRID L L v L L1 v p v s NPC singlephase NPC threephase i L2 i L3 v L2 v L3 O C Ns PV v dc,n H-B N v p,n Fig. 3. Schematic representation of a PET with a N-level inverter on LV side (cascaded H-bridge) and a N -level NPC converter on MV side. 28/28