Control and energy management of a wind-photovoltaic...

Control and energy management of a wind-photovoltaic hybrid system

Mehdi Dali (1), Jamel Belhadj(1), Xavier Roboam (2), J.M. Blaquiere(2)

(1) LABORATOIRE DES SYSTEMES ELECTRIQUES-LSEEcole national des ingénieurs de Tunis-ENIT, BP 37, Le Belvédère 1002 Tunis, Tunisia

E-mail: [email protected], [email protected](2) LAPLACE, UMR CNRS, INPT, UPS

ENSEEIHT, BP 7122, 2 rue Camichel, 31071 Toulouse Cedex 7, France,E-mail: [email protected]

Keywords«Hybrid system», «Wind energy», «Photovoltaic energy», «Test bench», «Energy management».

AbstractIn this paper, we have designed a micro-generation energy system obtained by interconnecting a windturbine with a photovoltaic generator and characterized by Very Low Voltage (VLV) node coupled to abattery storage and DC loads. The control strategies of the sources are based on power, voltage andcurrent control with Maximum Power Point Tracking (MPPT). Experiments based on a physical emulatorof the hybrid system with a lead acid accumulator and its DC load is considered. The development ofexperiments is necessary to validate theoretical studies. The real time supervision and the energymanagement of the hybrid system is presented and detailed. Experimental results are presented anddiscussed showing the relevant behavior of the energy management strategy.

IntroductionDue to the development of renewable energy systems, sustainable development issues (pollutant emission,rarefaction of fossil energy resources), hybrid systems are one efficient solution for electrical energygeneration, especially for isolated sites or for a micro-generation unit connected to weak AC grid [1]. Thehybrid systems combine several conventional or renewable energy sources interconnected via a DC bus.For remote area, the association of an electrochemical storage with the hybrid system allows eliminatingthe diesel generator (which is commonly required in generation systems based on a single renewableenergy source). In this context, we define a hybrid generation system obtained by combining photovoltaicpanels and wind turbines (taking advantage of their complementary nature) with storage batteries toovercome periods of scarce generation and for the system control. Many topologies are available forhybrid systems, depending on interface converters between sources and the interconnection method. Theoptimum design of the hybrid system constitutes one of our objectives [2], [3], [9] and [11]. Ourinvestigation concerns the definition of the system topology, the interconnection of the sources withmaximum energy transfer, the analysis of energy losses involved with power conditioning converters,optimum control and energy management. We have chosen to design a hybrid system with Very Low DCVoltage (VLV) of 48V [11]. With such DC bus, stand-alone systems can be implemented into rural orurban areas. A 48V battery and the DC load are also connected to the DC bus. The system supervisionallows managing all power devices (converters, protection) [5], [6].

Hybrid System topologyThe studied system represented in Fig. 1 is composed of two sources:

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- a 600W wind generator equipped of a direct driven permanent-magnet synchronous generator(PMSG), a diode rectifier and a (DC/DC) buck converter for the tracking of the maximum powerpoint [4];

- a photovoltaic source (400W), interfaced by a (DC/DC) buck converter used for the MPPT byregulating the output current Ipv. Choosing the buck converter for the two sources is related to theDC bus value fixed at 48V.

Fig. 1: Wind-photovoltaic hybrid system bloc diagram

Fig. 2 and Fig. 3 show respectively the bloc diagram of the proposed MPP Tracking for the wind and PVsubsystems.

Fig. 2: Current control of wind-side DC-DC chopper

By dividing the reference wind power Pref.w by the battery voltage Vbat, the reference current is calculated.The optimal reference value of the current Iref.w is imposed to the current controller of the DC-DC chopper[12]. The error resulting from the comparison between desired and actual values of the output current Iw isprocessed by the microcontroller through a PI regulator, issuing a value VL of the inductance voltage (L).The sum of the inductance voltage VL and the battery voltage Vbat is divided by DC voltage Vbus to issuethe duty-cycle w required for the IGBT switching operation, as shown in Fig. 2.

For the photovoltaic subsystem, the aim of the control strategy is to adjust the voltage Vpv to have themaximum power extraction. It can be deduced that two controllers must be considered to controlrespectively the voltage Vpv and the PV output current Ipv which are the two state variables [6].

Iw

PMSG Buck

Buck

Battery48 V

Ipv

Rectifier

LOAD

IL

PV ArrayIB

Wind Turbine

w

pv

DC Bus

L

Rectifier

w

VbatVL

Iw

Vbat

PMSG Load

PI+_

+ +

_

+

1 Vbus

Buck

CbusC

Wind turbine

Pref.w

Iw

Vbus

Iref.w


L

LoadCpv

Vpv Vo Vbat

Ic

Iin IpvPV modules

Battery

Ie

C

Vref.pv

Vpv

- PIIref.c

-+

Iin

Vbat

Ie Iref.pv

+ - PIVL

++ + Vs

1 Vpv

Vbat

Ipv

pv

Current loop

Vref.pv

current Iref.pv ref.pv bat as inequation (1).

Wind generator emulator

PV emulator

VLV DC node

48 V Battery

Dspace

Pref.pv

Pref.w

[V ; I]

[Switch]

Pw

Ppv

Pbat

controller.


Energy management strategyGeneral description of the supervisor control

bus(Vbat_nomcontroller.

BatteryCharge

BatteryDischarge

V bat min

V bat max

MPPT=0

V BUS

V

t

V nominal bat = 48

= 54.4

= 45

MPPT=1Vdc, low

Vdc, highV BUS

V bat Security

V bat Security

H

L

V

solar subsystem as a secondary generator.

different modes:

Mode 1:pv

Mode 1bus < Vbat

min


Contrarily, if the load demand is lower, the battery can be charged until its full charge state which istypical of the maximum voltage limit. Operation Mode 2, is reached if this voltage limit (Vbus > Vdc, high) isexceeded

Mode 2: the supervisor continue to manage the solar subsystem for maximum generation (i.e., operationat the point of maximum energy conversion MPPTpv=1) but the wind power subsystem is set to track acomplementary power reference (MPPTwind=0). This reference corresponds to the power required tocomplement the solar generation and together satisfies the total power demand. It should be remarked that,in Mode 2, the battery bank is by passed and is consequently not requested to supply the power of theload.If, during this Mode 2, the battery is discharged decreasing the bus voltage below its minimum limit (Vbus< Vdc, low), the system return to the Mode 1 of operation.

Mode 3: it corresponds to periods for which the solar power is sufficient to satisfy the total load demand(MPPTpv=0). Therefore, the solar subsystem has to track the total demand while the wind subsystem isswitched off (MPPTwind=0, Pref

wind = 0). A charge cycle is then applied to the battery.This situation is maintained until the total power demand exceeds the available solar power reference; forsuch a case the battery bank is transitorily able to provide the complement of energy flow until itsdischarge limit for which the supervisor control switches to Mode 2, activating the wind generator.

A reliable criterion to decide when to switch from maximum power generation (Mode 1) to powerregulation (Mode 2) must be deduced of photovoltaic and wind subsystems.

Fig. 6: Schematic description of the operation modes

To summarize, the supervisor control is responsible of switching from one operation mode to anotherdepending on the climatic conditions, the load demand, and the battery charge. Moreover, it is in charge ofsetting the reference values of the generation modules and the battery current for each mode of operation.A schematic description of the operation strategy is depicted in Fig. 6.

Operation strategyThe energy management program communicates the values of reference powers to the sources. Thesupervisor controller determines in real time the operation mode of both generation subsystems, switchingfrom power regulation (MPPT=0) to maximum power conversion (MPPT=1). Decision criteria for thesupervisor are based on measurable system variables (voltages, currents).

Supervision of the PV generating unit

MPPTpv=1

MPPTwind=1

MPPTpv=1

MPPTwind=0

MPPTpv=0

MPPTwind=0

Mode 1 Mode 2 Mode 3

PV subsystem: maximumenergy conversionWind subsystem: maximumenergy conversionBattery: charge cycle and/orsupply power to the load

PV subsystem: maximumenergy conversionWind subsystem: powerregulationBattery: charge and/ordischarge cycle

PV subsystem: powerregulationWind subsystem: offBattery: charge and/ordischarge cycle


Mode 1/ Mode 2 or Mode 3respectively). In Mode 1 and Mode 2

:

bat

PI =

V(1)

Mode 3

ref pv load bat L BP = P = V (I +I )

L B

bus

Pref pv = Pload

Vpv[V]Ppv= f (Vpv)

Popt

pv =1

operationPref pv

C

B

Mode 1 and Mode 2)

bus

(Mode 3Mode 2:

opt

Supervision of wind generating unit

=1) or=0) (Mode 1 or Mode 2 /Mode 3 respectively).

Mode 1=1).


In Mode 2

(3)

[rpm]P = f ( )

P = Pload pvPopt

=1

operation

P

Mode 1

Mode 2).

Mode 1

In Mode 3=0).

Experimental results

is

Vbus


Fig. 9: Experimental waveforms of the output voltage and current of the PV buck converter

(a) (b)Fig. 10: I-V and P-V characteristics representing the transient during the start-up of the PV generationsystem emulator.

The related MPPT control locus is shown in Fig. 10(a), 10(b); the maximum power point is about 310 Wat 84 V and 3.8A. These Figures show the behaviour of the proposed MMP algorithm during the start-upof the whole PV generation system. The output voltage of the power generator at the initial instant, whenthe converter is disabled, corresponds to the open circuit voltage (94 V). When the system is enabled thereference power moves from the starting value (94V) towards the maximum power point (84 V). Theseresults confirm that the MPPT control is successfully achieved.This second set of results is obtained by using the whole hybrid network as experimental set up. Thedspace control desk experiment software provides functions for monitoring experiments and capturingdata files that can be plotted using Matlab. Experiments were conducted to evaluate the performance ofthe proposed hybrid system supervision strategy. In Fig. 11, the time behavior of different systemvariables is displayed. The features of the proposed operation strategy are examined through severalvariations, which combine variations of wind speed and load.The wind turbine speed is presented in Fig 11(a). In Fig 11(b), (d), (f) and (g), the wind generated power,the PV generated power, the total power demand, and the battery powers delivered to the load aredepicted. In Fig 11(c), (e) and (i) are shown the system variables used to manage the operation mode. Fig.11(c) depicts the reference power of the wind subsystem (i.e., Pref w). Fig. 11(e) and 11(i) shows thedecision variables for the solar subsystem (i.e., Pref pv and Iref.pv).The supervisor control determines the operation modes of the remote grid by processing the informationpresented in Figs. 11(c), (e) and (i). Fig (h) and (j) show respectively the PV generator voltage Vpv and theDC bus voltage Vbus. The initial conditions of experiments are: null inputs sources (speed wind =0m/s andirradiation =0w/m²). The parameters of this experience are: Vbat_nom=48V, hysteresis band =1V,Pbat_tolerence=10 W, Vbat_max = 50V, Vbat_min = 45V.

Cur

rent

I pv (

2A/d

iv)

Vol

tage

Vo (

20V

/div

)

Time (10µsec/div)

Vo

Ipv

Ip

Initialpoint

PPM

Vpv

Ppv

Initialpoint

PPM

20 V/div

20 V/div

1 A/div

200W/div

Vpv


0100230300

0147

360

0150

440

0

190250

0190

440

0190250380

-2100

210360

808794

0

3.55.7

0 100 200 300 400 500 600 7004546474849

t1 t2 t3 t4 t5 t6 t7 t8 t9 t10

t ( s )

W [ r p m ]

( a )

( b )

( c )

( d )

( e )

( f )

( g )

( h )

( i )

( j )

P w [ W ]

P.ref. w [ W ]

P pv [ W ]

P.ref. pv [ W ]

P load [ W ]

P bat [ W ]

V p [ V ]

V bus [ V ]

I.ref. pv [ A ]

Fig.11: Experimental curves of the hybrid system energy management

At time t1 the energy management starts operating: the reference powers of PV (current) and windsubsystems are at their maximum levels (Mode1). The load is only supplied by the batteries.Consequently, the battery voltage decrease to Vbat_min. At t2, the wind speed being Swind =14m/s, the windsubsystem produce the total power demand. So, the battery voltage increases to Vbat_nom. At t3, the windspeed decrease Swind =10m/s, the wind subsystem cannot satisfy the demand, the battery bank is sufficientto complement the generation to satisfy the load requirements. At t4, the photovoltaic emulator isconnected and generates, together with the wind subsystem, the total demand (load and required charge ofthe battery): then the DC bus voltage increases. At t5 the load decreases Pload=190W, the battery becomesfully charged. Therefore, the DC bus voltage continues to increase. At t=t6, the voltage limit (Vbus>Vbat

max) is exceeded, the supervisor control switches from Mode 1 to Mode 3, the PV power subsystem is set totrack a power reference (MPPTpv=0) and the wind subsystem is switched off (MPPTwind=0). Consequently,the photovoltaic voltage and the aerogenerator speed both increase and the DC bus voltage decrease. Fromt7 to t8, the load is increased and decreased (Pload=250W and Pload=190W respectively), Mode3 ismaintained while the PV generation subsystem is set to regulate the power regulate. At t9, the powerdemand become equal to 380W, the PV subsystem restrains its generation at 250 W. This situation leads


the system to operate in Mode 2, being the wind subsystem responsible to complement the PV generationto satisfy the total power demand. At t10, with the wind speed established at Swind =8m/s, the wind poweris decreased and then the battery supplies the lack of power. The DC bus then decreases.

ConclusionIn this paper, the authors investigate an experimental study of a hybrid energetic system with an energymanagement strategy. The system is obtained by the interconnection of the Photovoltaic emulator andWind turbine emulator with converter interfaces. For such system an energy management is necessary tocontrol the energy fluxes between the load, the storage elements and both sources in order to guarantee astable operation in adequation with remote grid specifications. The authors have defined internal andexternal control loops with an energy management strategy to satisfy the balance between the loadconsumption, the battery state of charge and the behavior of the renewable energy sources interconnected:in fact, the principle consists in keeping the DC bus voltage inside a bandwidth around its rated value. Theperformance of electric generation hybrid systems relies heavily on the existence of a custom-madesupervisor control capable to efficiently administrate the diverse energy resources involved.The comprehensive supervisor system developed in this paper proved to be highly competent to manageand coordinate the operation of the subsystems that constitute the hybrid system. It provides a versatiledecision framework to determine the operation mode of each electrical subsystem. One of its mostattractive features is that reliable and unambiguous criteria only based on easily measurable systemvariables (namely, voltages and currents) have been proposed for the decision algorithms of thesupervisor. Experimental results show the good performance of the designed hybrid system.

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