A New Device for the Control and the Connection to the Grid of ...

A New Device for the Control and the Connection to the Grid of Combined RES-based Generators and Electric Storage Systems M. G. Ippolito*, E. Telaretti*, G. Zizzo*, and G. Graditi** *DEIM – Università di Palermo, viale delle Scienze, Palermo, (Italy) **ENEA, P.le Enrico Fermi 1, Portici - Napoli (Italy) Abstract—In this paper a new device for the control and the connection to the supply utility grid of combined RESbased generators and electric storage systems is presented. The device is a bidirectional converter that can be controlled so as to facilitate the interface between the low voltage grid and photovoltaic or wind generators combined with batteries, contemporary addressing the requirements of the reference technical standards for users connection and providing different ancillary services. In the paper the functioning of the device and its circuit diagram are described, the single parts are shown and the potentiality of its control system are commented. Index Terms-- bidirectional converter; energy storage system; lithium-ion

I. INTRODUCTION In Italy, according to the most recent regulations and technical standards dealing with the connection of active and passive users to the low voltage (LV) distribution grid [1]÷[3], all the electric power generators must fulfill well-defined requirements and participate to the voltage regulation, injecting reactive power according to specific capability curves. Moreover, these systems have to participate to frequency transients regulation: - reducing the generated power, if frequency exceeds a maximum value; - after a stop, gradually injecting power into the grid, minimizing in this way the effects on the electrical system. Considering that the most of the local generators connected to the LV utility grid are fed by not controllable renewable energy sources (RES), and as a consequence they are not always able to respect the above mentioned requirements, the coupling of such systems with electric storage systems can be a way for providing the required services. This coupling is able to increase the efficiency of the generation of electric energy from RES, to provide a higher management flexibility and power quality level, to assure voltage and frequency regulation inside the range defined by the national technical standard even if in presence of perturbations that require rapid interventions.

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In this optic it is necessary to devise new interfaces and control systems working following both remote and local control signals. Therefore, it is necessary to realize a double interface, towards the grid and towards the telecommunication systems, with bidirectional flows both for power and for transmitted data. In this paper a new device for the control and the connection to the supply utility grid of combined RESbased generators and electric storage systems is presented. The device has been designed and realized by a collaboration between ENEA, the Italian National Institute for Energy and Environment, and the DEIM (Department of Energy, Information Technology and Mathematics) of the University of Palermo within the project “Advanced Energy Storage Systems” financed by the Italian Minister for the Economic Development, through the program RdS (Research on the Electric System). The device and the related control strategy have been thought with the purpose of facilitating the interface between the national LV grid and RES-based generators combined with Lithium-Ion (Li-Ion) storage systems. For this reason, a bidirectional converter has been realized, composed by a DC/DC converter coupled with a DC/AC converter, that can inject energy into the grid from the batteries or from the RES generator or can absorb energy from the grid for charging the batteries. The rated power of the power converter is 20 kW and the generators that can be used are both photovoltaic (PV) and wind generators. During the functioning, the bidirectional converter can provide ancillary services or it can absorb and inject current so as to minimize the electric energy purchase cost of the user, following to specific price signals from the utility grid. The control system has been designed according with the national technical rules for the connection of active and passive users to the LV grid, CEI Standard 0-21 [2]. It implements voltage and frequency monitoring,

maximum and minimum voltage and frequency protection, Low Voltage Fault Ride-Through (LVFRT) function, limitation of the DC current injected into the grid, power quality control according to the technical standard CEI EN 50160 [4], reactive power regulation, active power injection control. In the following the functioning of the device and its circuit diagram are described and the single parts are shown and commented. II. OPERATING MODES OF THE DEVICE In Fig. 1 is represented the block diagram of the connection of the bidirectional converter to the LV grid. The DC/DC converter is named Converter 1, the DC/AC converter is named Converter 2. The bidirectional converter is connected directly to the batteries (electric storage system) and through an external DC/DC converter receiving energy from a PV system. Downwards Converter 2 and upwards the interface protection relay (IPR) the privileged loads are connected. Downwards the IPR, the LV grid and the standard loads are present. The bidirectional converter is able to work both in stand-alone and in grid-connected mode. During the gridconnected mode operability, the grid signals are elaborated by the Smart Grid Controller that, through the Power Electronic Controller, gives commands to Converter 1 and 2. In particular, Converter 1 is an IGBT chopper that can work both in step-up and in step-down mode, realizing in this way the possibility of inversion of the power flow from the and towards to the batteries. In step-up mode the main function of Converter 1 is to maintain the DC-link voltage close to the rated value (800 V) and to guarantee the injection into the grid of the power produced by the RES-based generator. Moreover it controls the value of the charge/discharge current of the batteries and establishes the charging and discharging period on the basis of the signals coming from the Power Electronic Controller Micro TI. Converter 2 is a three-phase bridge with three couple of IGBT. Each couple of IGBT receives control signals from a specific driver. Converter 2 has three different operating modes: - grid-connected mode; - stand-alone mode; - battery charging mode. In the grid-connected mode, Converter 2 works as a grid-on inverter, transferring power from the DC-link to the load or the grid. In this case all the sources (grid, generator and storage system) can contemporary supply the load. The stand-alone mode is activated in presence of a malfunctioning of the grid, after the IPR intervention. In

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this situation only the privileged loads are fed by the generator and the storage system. In the battery charging mode the grid supplies the loads and, contemporary, thanks to Converter 1 working in step-down mode, also the batteries. In this situation a part of the energy required by the batteries can be provided by the RES-based generator. The device can commutate its operating mode from “Grid-connected” (or vice versa), under the control of remote signals (from the grid), having the purpose of maximizing the economic advantage for the user, making it implementing load shifting strategies. The control system of the device can operate both independently and receiving input signal from the grid, according to CEI 0-21 Standard [2]. For this reason the device can be controlled in order to follow various objectives described in the following. A. Local generator connection The device can be used for injecting into the grid the electric power generated by a RES (PV or wind system). In this case all the specifications of CEI standard 0-21 must be accomplished. In particular the device is able to: - limiting the DC current injected into the grid; - regulating the reactive power taking part to the voltage regulation; - limiting the generated active power taking part to the frequency regulation or following a command from the grid; - disconnecting the source supplying only the privileged loads in presence of voltage or frequency outside specific ranges. B. Island supply of privileged loads The device can supply privileged loads in presence of an outage of the grid. The passage from the grid-on mode to the grid-off mode is activated by an intervention of the IPR. In this case the VSI mode is activated and the output voltage is generated constant and equal to 400 V. When the grid is restored the CSI mode is activated by the Power Electronic Controller. C. Harmonics compensation The device can operate as an active filter compensating the harmonics eventually present in the voltage supply. The control system acting on Converter 2 is able to create current waveforms that can superimpose perfectly on the external harmonics obtaining in this way a perfect sinusoidal signal. D. Load shifting and peak-shaving The control system can be set in order to follow load shifting programs.

Load shifting operations offer to the customers a way to more effectively manage the cost of their electricity bill. This can be accomplished letting them take advantage of the relatively low cost of the electricity during off–peak demand periods and generate their own power (through the storage) during peak periods, when electricity energy prices are high, also avoiding high demand charges [5], [6]. These decisions are based on financial/cost considerations and not on Utility considerations, such as supply and grid stability. A secondary effect of load shifting is peak-shaving. Peak-shaving has been practiced, until now, by using gas turbines or diesel generators. However, today industrial users or public facilities can take advantage of battery systems capable of discharging for short periods of time during on–peak hours and charging during off–peak demand periods, also reducing peak demand charges.

III. CIRCUIT DIAGRAM In Fig.2 and Fig.3 the circuit diagrams of Converter 1 and 2 are shown. The MICRO TI controller in Fig.2 regulates the values of the duty cycle of IGBT1 and IGBT2 using a PWM modulation and assuring the required output voltage and current. The MICRO TI controller is located in the converter motherboard indicated as "CTR 2012" in Fig. 3. The control signals of Converter 2 are, instead, generated by the "MDL 2012" controller, that is the modulator of Converter 2. Converter 2 includes also a second order tri-phase low-pass filter, for attenuating the harmonic voltages generated by the same converter. The bidirectional converter includes also the in-rush circuit installed upwards Converter 1 and downwards Converter 2 and a EMC filter towards the grid.

BIDIRECTIONAL CONVERTER

LV GRID

IPR

POWER ELECTRONIC CONTROLLER

RS 232

SMART GRID CONTROLLER

PLC

(MICRO ST))

(MICRO TI) STORAGE SYSTEM DC-link 800V 400V CONVERTER 1 (DC/DC)

CONVERTER 2 (DC/AC)

DC/DC CONVERTER

PV GENERATOR 400V

PRIVILEGED LOADS

800V

LOADS

Fig. 1. Connection of the bidirectional converter to the grid, the generator and the batteries.

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STANDARD LOADS

I DC-link

C

IGBT1

D1 E

G

I Battery

800V

C2

I Switch

L1

400V BATTERIES

D2

IGBT2 G

E

C1

C

INVERTER

Driver Boost

Driver Buck

MODULATOR

From MICROTI Fig. 2. Circuit diagram of Converter 1.

L2 L3 MD 2 C1

MD 3

3

MD 4

3

4

4

DC-link

4 1

1

1

C5 6

6

Driver

6 2

2

Driver

Grid

L4

3

2

Driver

MDL 2012

CTR 2012

Fig. 3. Circuit diagram of Converter 2.

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C6 C7

batteries. Indeed, no limitations are present on the type of batteries that can be connected to Converter 1. The main technical specifications of Converter 1 and 2 are summarized in Table I and II, respectively. The main technical specifications of the Li-Ion storage system are reported in Table III.

IV. REALIZATION OF THE DEVICE The inside of the bidirectional converter built according to the circuit diagrams in Figures 2 and 3 is shown in Fig. 4. In Fig. 5 is represented the Smart Grid Controller and in Fig. 6 is represented the Power Electronic Controller. The device uses a storage system composed by Li-Ion batteries but it has been tested also with Lead-acid

Fig. 4. Inside of the bidirectional converter.

Fig. 5. Smart-grid controller.

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Fig. 6. Power electronic controller.

been presented. In a future work the results of the calculations of the savings for the end–users will be evaluated.

TABLE I TECHNICAL SPECIFICATION OF C ONVERTER 1 Technical Specifications Rated power 20 kW Battery-side input voltage 400V ± 1% Rated DC-link voltage 800 V ± 10% Maximum output current (step-down mode) 50A Maximum output current (step-up mode) 25A

ACKNOWLEDGMENT This work has been developed within the project “Advanced Energy Storage Systems” financed by the Italian Minister for the Economic Development, through the program RdS (Research on the Electric System). .

TABLE II TECHNICAL SPECIFICATION OF C ONVERTER 2 Technical Specifications Rated power 20 kW Rated Frequency (stand-alone mode) 50Hz ± 0.1Hz Rated Output (grid) voltage 400V Rated DC-link voltage 800 V ± 10% Maximum output current 29A (grid-connected mode) Maximum output current 25A (battery charging mode) Power factor (grid-connected mode) -0.9 ÷ + 0.9 Power factor (battery charging mode) 0.99

REFERENCES [1] Deliberation ARG/elt 187/11 “Modifiche e integrazioni alla deliberazione dell’Autorità per l’energia elettrica e il gas ARG/elt 99/08, in materia di condizioni tecniche ed economiche per la connessione alle reti con obbligo di connessione di terzi degli impianti di produzione (TICA), per la revisione degli strumenti al fine di superare il problema della saturazione virtuale delle reti elettriche”, December 2011. [2] Italian Standard CEI 0-21 “Reference technical rules for the connection of active and passive users to the LV electrical Utilities”, December 2012. [3] Deliberation 84/2012/R/EEL “Interventi urgenti relativi agli impianti di produzione di energia elettrica, con particolare riferimento alla generazione distribuita, per garantire la sicurezza del sistema elettrico nazionale”, March 2012. [4] European Standard EN 50160 “Voltage characteristics of electricity supplied by public distribution systems”, May 2011. [5] A. Oudalov, R. Cherkaoui and A. Beguin, “Sizing and Optimal Operation of Battery Energy Storage System for Peak Shaving Application”, In Int. Conf. IEEE Powertech 2007, Switzerland, 2007, pp. 621–625. [6] L. Dusonchet, M.G. Ippolito, E. Telaretti, G. Graditi, “Economic Impact of Medium-Scale Battery Storage Systems in Presence of Flexible Electricity Tariffs for EndUser Applications”, In 9th International Conference on the European Energy Market EEM 2012, Italy, pp.1-5. [7] L. Dusonchet, G. Graditi, M.G. Ippolito, E. Telaretti, G. Zizzo, "An optimal operating strategy for combined RES– based Generators and Electric Storage Systems for load shifting applications", In 4th IEEE International Conference on Power Engineering, Energy and Electrical Drives 2013, Turkey, pp.1-6.

TABLE III TECHNICAL SPECIFICATION OF THE LI-ION BATTERIES Technical Specifications Technology LiFePO4 Capacity (Typical) 20 Ah Capacity (Minimum) 19.5 Ah Rated Voltage 3.2 V Internal resistance ” 3 mŸ Battery charge (Std charge) 0.5 C Battery charge (Quick charge) 1C CC charge cut-off voltage 3.65 ± 0.05 V Discharge (Std discharge) 10 A, 2.00 V Discharge (Max discharge) 40 A, 2.00 V Pulse discharging 8 seconds 100 A Charging temperatures 0 ÷ 45 °C Discharging temperatures -20 ÷ 60 °C Storage temperatures -10 ÷ 60 °C BMS Passive cell balancing Storage life One year Battery size 225 x 150 x 10 mm Battery weight 535 ± 15 g

V. CONCLUSION The device presented in this work has been tested implementing an optimal operating strategy for its management in small/medium–scale public facilities that has the aim of maximizing the arbitrage benefit for the end–user, by shifting the energy consumption from on– peak to off–peak hours. In [7] the implemented load shifting algorithm has

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