Energy management optimization of a hybrid power production unit based renewable energies

Electrical Power and Energy Systems 62 (2014) 1–9

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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes

Energy management optimization of a hybrid power production unit based renewable energies Achraf Abdelkafi, Lotfi Krichen ⇑ National Engineering School of Sfax, Control and Energy Management Laboratory (CEMLab), BP 1173, 3038 Sfax, Tunisia

a r t i c l e

i n f o

Article history: Received 20 May 2013 Received in revised form 24 March 2014 Accepted 5 April 2014

Keywords: Power supervision strategy Wind generator Fuel cell Electrolyzer Supercapacitor

a b s t r a c t Hybrid power production units seem to be an interesting alternative for supplying isolated sites. This study proposes a new supervision strategy in order to ensure an optimized energy management of the hybrid system. The considered hybrid unit includes a wind generator (WG), a fuel cell (FC), an electrolyzer (EL) and a supercapacitor (SC). An overall power supervision approach was designed to guarantee the power flow management between the energy sources and the storage elements. The aim of the control system is to provide a permanent supply to the isolated site by adapting production to consumption according to the storage level. A mathematical analysis of the hybrid system using models implemented in Matlab/Simulink software was developed. Simulation results illustrate the performance of the control strategy for an optimal management of the hybrid power production unit under different scenarios of power generation and load demand. Ó 2014 Elsevier Ltd. All rights reserved.

Introduction In recent years, the energy infrastructure has undergone a great change especially for the electricity production. This event results from the growth of the energy needs in the world and also the decrease of the primary energy resources from fossil fuels i.e. coal, oil and natural gas. Actually, the integration of a conventional energy production unit will be hampered not only by the cost rising of the primary energy but also by the massive release of the pollutant gases. To ensure global energy needs, the trend converges to the use of new decentralized renewable resources [1]. Renewable energies such as wind generators (WG) and photovoltaic (PV) systems can generate electricity using clean unlimited resources. Wind energy records the fastest growth rate in the world. The technology of this energy resource has led to a great improvement in wind turbines which convert wind into electricity with a competitive installed cost. Wind power is classified into two broad categories: fixed speed and variable speed wind turbines (VSWT). The VSWT adjusts the rotational speed in order to capture the maximum power and then allows the increase of the aerodynamic efficiency compared to fixed speed [2,3]. Renewable energy sources can either be integrated in the power electrical network, or participate to resolve problems of isolated site electrification because of financial constraints for the difficult ⇑ Corresponding author. Tel.: +216 74 274 418; fax: +216 74 275 595. E-mail address: lotfi[email protected] (L. Krichen). http://dx.doi.org/10.1016/j.ijepes.2014.04.012 0142-0615/Ó 2014 Elsevier Ltd. All rights reserved.

access areas. The WG is able to generate electricity in an appropriate way, technically and economically. Because of the fluctuating and unpredictable characteristic of the wind, it is necessary to include a storage system to ensure energy needs and to adapt the production to consumption in a stand-alone operating [4]. The combination of different renewable sources can reduce the availability problem of the resource, but it cannot prevent the use of a storage system. The hybrid production systems are a very interesting solution which allows optimizing the energy sources, increasing the energy efficiency and reducing the storage system capacity. These systems require a good sizing to increase energy efficiency and to ensure working continuity. Indeed, the study of various renewable potentials, the knowledge of load profiles and the power production management ensure a correct sizing of the hybrid system. The use of a hybrid system to supply an isolated site allows meeting the energy needs during all the year. A hybrid system is mainly made up of renewable sources, a long-term storage unit and a short-term storage unit used for fast dynamic energy needs. The possible structures of multi-sources power station differ according to the organization of these three elements [5]. The hydrogen storage system, based on a hydrogen fuel cell (FC) and an electrolyzer (EL), is considered as a good means of long-term storage. Indeed, it facilitates the integration of a WG to ensure a permanent supplying of an isolated site in spite of production fluctuations and load variations [6,7]. When the WG produces more power than consumption needs, the EL must absorb the power

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A. Abdelkafi, L. Krichen / Electrical Power and Energy Systems 62 (2014) 1–9

Nomenclature Vw

q Rw

x Rs Ls /m p Vsd, Vsq Isd, Isq Tem

wind speed (m s1) air density (1.22 Kg m3) blade radius (m) rotational speed (rad s1) stator winding resistance (X) stator winding inductance (H) permanent magnetic rotor flux (Wb) number of pole pairs d–q components of the stator voltages (V) d–q components of the stator currents (A) electromagnetic torque (N m)

excess by producing hydrogen which can be stored in a tank. When the WG produces less power than consumption, the FC generates the power difference by using the stored hydrogen. The slow dynamics of hydrogen system requires the use of a storage system that allows a fast dynamics in order to provide the missed power and to circumvent the delay due to the response time of the FC and the EL [8–10]. Many hybrid power production structures were studied in Refs. [6,11–13]. Ref. [6] presents an investigation on a stand-alone hybrid system based on an FC and an EL. This work analyses the performance of integrating the hybrid system for supplying an isolated site without taking into consideration the slow dynamics of the hydrogen system. Refs. [11–13] propose a stand-alone WG/ PV/FC/EL hybrid energy system with a battery bank used for short-time backup and storage system. This structure presents a good performance, but requires a high installation cost and imposes the availability of two main sources: the wind and the sunlight. In this study, we suggest a stand-alone hybrid power production unit made up of a WG and an FC as energy sources, an EL as a hydrogen storage system and a supercapacitor (SC) as a fast dynamics storage system. The main contribution of this work is to propose an energy management algorithm according to the following constraints: (i) wind speed fluctuations, (ii) load demand, (iii) storage level of hydrogen system and SC, (iv) hydrogen slow dynamics and (v) optimal operation of each element. The considered approach offers many power flow possibilities according to six operating modes of the hybrid system in order to meet the energy constraints. Therefore, the proposed control strategy ensures an optimal energy management and increases the overall system reliability. The remainder of this paper is organized as follows. The structure of the considered hybrid system is detailed in section ‘Hybrid system structure’. Sections ‘Modeling of the hybrid system’ and ‘Control of the hybrid system’ are devoted to the modeling of each component and the control design are detailed, respectively. Section ‘Proposed power supervision strategy’ gives a description of the proposed power supervision strategy for the coordination of the hybrid system elements. The simulation results are provided in section ‘Results and discussion’. Finally, in the last section a conclusion of this work is presented.

Urev r1, r2 si, ti Ael Tel Nel It1, It2It3 IL1, IL2IL3 Uc1, Uc2 n

xn

reversible cell voltage (V) parameters for ohmic resistance of electrolyte parameters for overvoltage across electrodes electrode area (cm2) electrolyzer temperature (°C) number of electrolyzer cells inverter output current (A) load currents (A) line-to-line voltages (V) damping ratio undamped natural frequency

type BMOD0094 rated at 2 kW used as a fast dynamic storage system in order to circumvent the slow dynamics of the hydrogen system. The different elements of the production unit are connected to a common DC bus through four power converters. These converters are considered as adaptation modules placed between the multisource unit and the DC bus. The AC/DC converter (1) ensures the maximum power extraction from the PMSG and so the operating at variable speed. The boost DC/DC converter, (2) adjusts the PEMFC voltage to the DC bus voltage. The buck DC/DC converter, (3) adapts the DC bus voltage to the EL voltage. The reversible DC/ DC converter, (4) controls the SC current to adapt production to consumption. A DC/AC converter is used to transfer the generated power by the hybrid system to a three-phase load through an LC filter. This filter ensures a three-phase voltage source reducing the harmonics generated by the inverter. Modeling of the hybrid system Wind generation system The aerodynamic power captured by the wind turbine is defined by the following expression:

Paer ¼

1 qpR2w V 3w C p 2

ð1Þ

The power coefficient C p expresses the wind turbine efficiency for converting energy from kinetic to mechanical. This coefficient is related to the tip speed ratio (TSR) k and the pitch angle b. The TSR is the ratio of the tangential velocity Rx to the wind speed.

k¼

Rx Vw

ð2Þ

The main advantage of the VSWT is the rotation speed adjustment to extract the maximum power at each wind speed. As a matter of fact, the TSR should be fixed at its optimum value kopt , therefore, the ratio x=V w should be constant. This wind turbine is coupled directly to a smooth pole PMSG to convert mechanical energy into electrical energy. The PMSG equations in the Park reference frame are expressed as follows:

Hybrid system structure

dIsd  pxLs Isq dt dIsq ¼ Rs Isq þ Ls þ pxLs Isd þ px/m dt

V sd ¼ Rs Isd þ Ls The structure of the hybrid system is shown in Fig. 1. The WG consists of a VSWT with a rated power of 3.85 kW, based on a permanent magnet synchronous generator (PMSG). The hydrogen system is made up of a proton exchange membrane fuel cell (PEMFC) with a rated power of 1 kW and an EL as a hydrogen storage system rated at 1 kW. The considered hybrid unit is equipped with a SC

V sq

T em ¼ p/m Isq The WG and the PMSG parameters are given in Table 1.

ð3Þ

ð4Þ

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WG PMSG

Vw β

Cdc Udc

Ifc L fc

Compressor

H2

Electrolyzer

H2

Um1

Uc1

N

Um 2

Uc 2

DC/AC

AC/DC

Im_ fc

Conv.2

Ct

Ct N

3Ct

Ufc Um_ fc

Iel Lel I Lel

DC/DC

Ct

Im _ el

Conv.3

Cel U m _ el

DC/DC

Im _ sc

Conv.4

Csc Lsc Rsc Um _sc

Usc

PEMFC

Hydogen tank

O2

RL LL

IL1

It1 Rt Lt

Conv.5

Conv.1

H2

Load

I m _ g I m _ inv

DC/DC

SC

Control Control Control Control Conv.4 Conv.3 Conv.2 Conv.1

Control Conv.5

Control and Power management of the hybrid system Fig. 1. Structure of the hybrid system.

Table 1 WG and PMSG parameters. Value

Symbol

Value

R kopt Cp max Pn

2m 8 0.473 3.85 kW

p Rs Ls /m

4 0.82 O 15.1 mH 0.5 Wb

Vfc (V)

1.5

Symbol

1 0.5 0 100

Proton exchange membrane fuel cell The FC is an important energy source used to convert hydrogen into electricity from chemical reactions. The voltage of a PEMFC elementary cell is defined by the following expression [14–17]:

V fc ¼ ENernst  V act  V ohm  V conc

ð5Þ

where ENernst is the Nernst potential, Vact is the activation voltage drop, Vohm is the ohmic voltage drop, Vconc is the concentration loss. The FC is characterized by a high current and a low voltage. The combination of several cells in series increases the voltage and subsequently increases the power. The output voltage of the stack is determined by the number of cells in series nfc.

V stack ¼ nfc V fc

ð6Þ

The FC modeling shows that this model depends essentially on the temperature T fc , the hydrogen pressure P H2 and the oxygen pressure PO2 . The FC energy efficiency is improved by increasing the temperature at constant hydrogen and oxygen pressures as it is shown in Fig. 2.

80

60

Tfc (°C)

40

20 0

20

40

60

80

Ifc (A)

Fig. 2. Influence of temperature on a PEMFC elementary cell voltage.

duction and supplies FCs by hydrogen during peak times. This allows a more stable production and improves energy efficiency of the hybrid system. The water electrolysis into hydrogen and oxygen is ensured by a DC current between the two electrodes separated by an aqueous electrolyte with high ionic conduction. The electrical behavior of an EL is obtained empirically by measurements. The voltage expression of an EL cell at a given temperature is as follows [6,12,18]:

0 1 t 1 þ Tt2 þ Tt32 r1 þ r2 T el el 2 el @ U cellel ¼ U rev þ Iel þ ðs1 þ s2 T el þ s3 T el Þ log Iel þ 1A Ael Ael ð7Þ The combination of several cells in series increases the EL voltage and power.

Electrolyzer

U el ¼ Nel U cellel The EL stores the energy indirectly in the form of hydrogen through water electrolysis. It has an important role in the field of renewable energy: stores energy during the times of excess pro-

ð8Þ

Similarly to the PEMFC, the voltage of an EL cell depends on the temperature. Fig. 3 shows that any increase in temperature T el

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The control of the generated power by the WG is ensured by the control of the PMSG electromagnetic torque. This control is ensured by the regulation of the stator currents Isd and Isq and the knowledge of the speed [2,19]. The determination of the reference electromagnetic torque T em ref is made up of two parts: the first ensures the operation in the maximum power extraction area to improve the turbine performance and to work with a maximum power coefficient C p max . In this area, the reference electromagnetic torque is proportional to the square of the speed as it is defined in the following equation:

Ucell -el (V)

2.5 2 1.5 1 80 60 40

Tel (°C)

20 0

40

20

60

80

T em

Iel (A)

ref

¼ kopt x2

with kopt ¼

Fig. 3. Influence of temperature on an electrolytic cell voltage.

Table 2 EL parameters. Symbol

Value

Symbol

Value

r1 r2 s1 s2 s3

7.3  105 O m2 1.1  107 O m2 °C1 0.16 V 1.38  103 V °C1 1.6  105 V °C2

t1 t2 t3 Ael Tel

1.6  102 m2 A1 1.3 m2 A1 °C 4.12 m2 A1 °C2 0.25 m2 25 °C

qpR5 C p 2k3opt

ð10Þ max

.

The maximum power extraction technique consists in applying a vector control to the PMSG. This control results in the regulation of the direct current component to a null reference, and the regulation of the quadrature current component to a reference Isq ref which is directly related to the reference electromagnetic torque by the following expression:

Isq

ref

¼

T em ref p/m

ð11Þ

When the rotational speed of the generator reaches the rated speed

xn , the reference electromagnetic torque is limited to its nominal Table 3 SC Parameters. Parameter

Value

Capacitance Rated voltage ESR Leakage current Operating temperature Maximum DC current

94 F + 20%  0% 75 V 12.5 mH 0.15 A, 72 h, 25 °C 40 °C to +65 °C 50 A

decreases the EL performances i.e. the energetic efficiency of the EL decreases by the temperature. The EL parameters are given in Table 2. Supercapacitor The use of a storage system with a fast dynamics is necessary in many applications and especially with hybrid systems. Therefore, a SC is a storage system that ensures a better convenience between the intermittent generated power by the WG and the consumer demand which increases the performance of the hybrid system. The SC model includes a capacity Ct, an equivalent series resistance ESR and an equivalent resistance in parallel EPR. The combination of SC units in series and parallel realizes a SC bank. The total resistance and the total capacitance of this SC bank are given by the following set of equations [8,9]:

Rsc C sc

tot

tot

ESR np Ct ¼ np ns

¼ ns

value. The limitation of the electromagnetic torque and the speed is performed by the pitch control [19]. The FC temperature is assumed to be constant and the hydrogen and oxygen pressures depend on the molar flow consumed by the PEMFC. These two molar flow rates are directly proportional to the FC current. The FC output voltage is injected into the DC bus through a filter Lfc and a unidirectional current boost chopper. To adjust the FC current to its reference value, a PI controller is implemented. The EL is supplied since the wind generated power is greater than the required load power. This power transit is done from the DC bus to the EL through a buck chopper and a filter Lel C el . The EL control is made by two PI controllers: the first is used to achieve the regulation of the inductor Lel current and the second one is used to ensure the regulation of the EL voltage. The SC smoothes the WG fluctuations and circumvent the slow dynamics of the hydrogen system. The SC system is composed of three parts: an SC, an inductive filter Lsc and a reversible current chopper. A PI controller is used to adjust the SC charge or discharge currents to their reference values. These reference currents are imposed by the control algorithm of the hybrid system. The DC/AC converter ensures the energy transit from the DC bus to the three-phase load through an RLC filter and an inverter. Ref. [19] gives a clear description of these elements modeling. To regulate the voltages across the three load phases and according to the second order transfer function of the RLC filter, a resonant controller is used [20]:

CðsÞ ¼ ð9Þ

The SC parameters are given in Table 3. Control of the hybrid system The control of the hybrid system is accomplished by the control of the different power converters. This control is recognized by the voltage adjustment of each power source to the DC bus voltage.

c0 þ c1 s þ c2 s2 þ c3 s3 ðd0 þ d1 sÞðs2 þ x2p Þ

ð12Þ

c0, c1, c2, c3, d0, d1 are the parameters of the resonant controller and

xp is the angular frequency. The use of the resonant controllers ensures the control of the alternative voltages U c1 and U c2 . This control allows the obtaining of an autonomous production system that can supply a remote site with approximately ideal three-phase voltages at the nominal frequency 50 Hz. Fig. 4 summarizes the control structure of each device and the control principle of the load voltages.

A. Abdelkafi, L. Krichen / Electrical Power and Energy Systems 62 (2014) 1–9

5

Fig. 4. Hybrid system control.

Proposed power supervision strategy In order to ensure an optimal management of the hybrid production system, a power supervision strategy is proposed. This strategy, depicted in Fig. 5, allows power dispatching between the different sources and manages the energy flow distribution. In the considered approach, a power management algorithm is developed according to the constraints of each element during an abrupt load demand. The role of the hybrid system energy management is to ensure the supplying continuity of the load within the regulation constraints of the DC bus and the load voltages. The actual energy production is always different from its reference

one, so the storage units should compensate or absorb the difference of power. The controlled devices are the SC, the FC and the EL. These devices are monitored by the reference powers set by the energy flow management algorithm. Since the FC and the EL are slow dynamics components, six operating modes of the hybrid production unit are considered to provide the reference powers of the SC, the FC and the EL and achieve dynamic balance in power. Fig. 6 gives a clear description of each operating mode in accordance with the control algorithm. This control improves the reliability and increases the energy efficiency of the overall system. The FC and the EL should not operate at the same time in order not to degrade the overall performance of the hybrid system.

Fig. 5. Proposed energy supervision algorithm.

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Fig. 6. Description of the operating modes.

Mode 1

Mode 4

This mode corresponds to a wind generated power greater than the load required power and the difference between these two powers is greater than the nominal power of the EL 00 P 00el n . In this case, there is an excess of energy to be stored and the reference powers of the EL and the SC are given by the following equations:

This mode corresponds to a wind generated power less than the load required power and the difference between these two powers is greater than the FC rated power 00 Pfc n 00. In this case, there is a lack of energy that should be provided by the PEMFC. The reference powers of the FC and the SC are given by the following equations:

P el ref P sc ref

Pfc Psc

¼ P el n 1 ¼ P sto  P el

mode 1 mode

ð13Þ n

ref mode 4 ref mode 4

¼ Pfc n ¼ Psto þ Pfc

ð16Þ n

Mode 2

Mode 5

It corresponds to a wind generated power greater than the load required power and the difference between these two powers is less than the nominal power of the EL. Then, the excess energy is stored in the EL which is characterized by a slow dynamics. The SC has to store the difference of power during the EL response time. If Psto > 0 and Psto 6 P el n and Pel ref > P el , reference powers of the EL and the SC are given by the following equations:

This is the case where the wind power generated is less than the load required power and the difference between these two powers is less than the FC rated power. The lack of energy is provided by the PEMFC, which is characterized by a slow dynamics. So, the SC must circumvent the difference of power during the FC response time. If Psto < 0 and jP sto j 6 P fc n and P fc ref > Pfc , reference powers of the FC and the SC are given by the following equations:

P el

ref mode 2

¼ P sto

Pfc

ref mode 5

¼ jPsto j

P sc

ref mode 2

¼ Pel

Psc

ref mode 5

¼ Pfc  Pfc

ref

 Pel

ð14Þ

ð17Þ ref

Mode 3

Mode 6

Similarly to the second operating mode and if Pel ref < P el the SC is discharged during the EL response time, the reference powers of the EL and the SC are given by the following equations:

Similarly to the fifth operating mode and if P fc ref < Pfc the SC should be charged during the FC response time, the reference powers of the FC and the SC are given by the following equations:

P el

ref mode 3

¼ P sto

Pfc

ref mode 6

¼ jPsto j

P sc

ref mode 3

¼ Pel

Psc

ref mode 6

¼ Pfc  Pfc

ref

 Pel

ð15Þ

ð18Þ ref

7

The sum of the six reference powers of the SC gives the total reference power to charge or to discharge the SC.

Psc

ref

6 X ¼ Psc

Pg (W)

A. Abdelkafi, L. Krichen / Electrical Power and Energy Systems 62 (2014) 1–9

2000 0

ð19Þ

ref mode i

4000

10 5

β (°)

0

20

40

60

80

100

120

10 5 0

Cp

0.6

0

20

40

60

80

100

120

Pfc (W)

40

60

80

100

120

0

20

40

60

80

100

120

2000 0 1000

Pfc ref Pfc

500 0

ω (rad s-1 )

Psc (W)

60

80

100

120 Pel ref Pel

500 0 0

20

40

0

20

40

60

80

100

120

60

80

100

120

1000 0 -1000

Fig. 8. Power assessment of the hybrid system.

1000

Psc-mode1

500 0 0

20

40

60

80

1000

100

120

Psc-mode2

500 0 0

20

40

60

80

0

20

40

60

80

100

120

0 -500 -1000

Psc-mode3 100

120

0 -500 -1000

Psc-mode4 0

20

40

60

80

0

20

40

60

80

100

120

0 -500 -1000

Psc-mode5

1000

100

120

Psc-mode6

0 0

Paer (W)

40

500

0.4 0.2

20

1000

Time (s)

Reference powers for the different SC modes (W)

Wind (ms-1)

15

20

4000

Results and discussion The simulation of this hybrid production unit was performed using Matlab–Simulink software. This simulation is investigated to validate the energy supervision algorithm and to confirm the different power flow paths. The WG is the main source of this hybrid production unit. Fig. 7 shows the various curves demonstrating the good operating of the wind turbine. The WG produces a nominal power when the wind speed reaches its nominal value fixed at 10 ms1 . Beyond this speed, the wind turbine protection limits the speed to its rated value. This limitation is done by increasing the pitch angle and therefore, reducing the power coefficient. The power assessment of the hybrid production unit is shown in Fig. 8. This figure shows that the FC and the EL powers are deferred compared to their references. The FC/EL system cannot follow the rapid fluctuations of the wind power and the change in the power demand. The hydrogen system should be accompanied by a fast dynamic storage system such as an SC in order to participate in smoothing the transmitted power to the load. According to the supervision algorithm, six modes of the SC are recognized. Three modes are used to charge the SC (mode 1,2,6)

0

0

Pel (W)

The control technique of the SC state of charge (SOC) requires the knowledge of the electricity incoming and outgoing the SC in both charge and discharge modes. In the case of an SC charge resulting from the three operating modes (1,2,6), the SOC is less than the maximum state of charge ðSOC max Þ. Otherwise, and to avoid the problem of an SC overloading, a dumping load is added. In the case of an SC discharge resulting from the three operating modes (3,4,5), the SOC is greater than the minimum state of charge ðSOC min Þ. Otherwise, and to ensure the balance between production and consumption, one solution is to disconnect the lower priority loads.

PL (W)

i¼1

0

20

40

60

80

100

120

150

20

40

60

80

100

120

Time (s) Fig. 9. SC reference powers for the six modes.

100 50

0

20

40

0

20

40

60

80

100

120

60

80

100

120

4000 2000 0

Time (s) Fig. 7. Operating curves of the WG.

and three other modes for the discharge (mode 3,4,5) as shown in Fig. 9. The first mode of the SC is the case of an overproduction of energy greater than the EL nominal power. The difference between the excess energy and the EL nominal power corresponds to the reference power to charge the SC. The second operating mode of the SC ensures the storage of the difference between the EL reference power and its actual one. This mode allows the SC charge during the EL response time. Similarly to the second operating mode and in the case of a decrease in the excess of energy

A. Abdelkafi, L. Krichen / Electrical Power and Energy Systems 62 (2014) 1–9

Mode

8

6

3

5

2.5 2 104

4

104.005

6

3

5.5

2

5 50

50.005

1

0

20

40

60

80

100

120

Time (s) Fig. 10. Mode transitions.

Im g (A)

10

PL ref (W)

5 0 40

60

80

100

Im fc (A)

4

Im el (A)

4

2000

40

60

80

100

120

0 0

20

40

60

80

100

0 0

20

40

2000 82

84

86

80

100

120

88

90

92

94

2600

2200

0 20

40

60

80

100

120

Time (s)

5

Fig. 12. Active and reactive load powers.

0 0

20

40

60

80

100

120

Vc1 (V)

420 400

10*IL1 (A)

200 0

20

40

60

80

100

0

120

Time (s) Fig. 11. Different modulated currents and DC bus voltage.

from the previous time; a third operating mode is designed. At this moment, the EL reference power is less than its actual power. The difference between these two powers defines the reference power to discharge the SC during the EL response time. The fourth mode of the SC is the case of an energy deficit greater than the FC rated power. The sum of the energy deficit with the FC nominal power corresponds to the reference power to discharge the SC. The fifth mode of the SC overcomes the difference between the FC actual power and its reference one. This mode is used to discharge the SC during the FC response time. Similarly to the fifth operating mode and in the case of a decrease in the energy deficit compared to the previous time, a sixth mode is expected. The FC actual power becomes greater than its reference power. The difference between these two powers defines the reference power to charge the SC during the FC response time. Fig. 10 shows the transitions between the six modes during the whole sequence and it is noticed that only one mode is active at each moment as it is presented in the two zooms of the figure. The interconnection of the different devices is made at the DC bus through different power converters. The control of these converters requires the DC bus voltage regulation. The balance between the sum of the different modulated currents on the left of the DC bus and the inverter modulated current allows the regulation of the DC bus voltage as shown in Fig. 11. This regulation is ensured using a PI controller.

Load voltage and current

380

60

2400

120

2 0

Im sc (A)

20

2

-5

Q L ref (VAR)

4000

120

10 5 0 0

Udc (V)

20

Power demand

Im inv (A)

0

PL ref mod (W)

-200 0

20

40

60

80

100

120

200 0 -200 19.9

19.95

20

20.05

20.1

40

40.05

40.1

200 0 -200 39.9

39.95

Time (s) Fig. 13. Single-phase load voltage and current.

The first curve in Fig. 12 represents the active and reactive reference powers required by the load and the computed active power PL ref mod given by the supervision algorithm. This computed power follows the load reference power when the hybrid production unit is able to provide the required power. Otherwise and where the SC reaches the minimum of the SOC, the lower priority loads are disconnected with a step of 100 W. This disconnection ensures the balance between production and consumption. The second curve in Fig. 12 is a zoom of the first one during the interval [82,94s]. This curve shows the change in the load required power proposed by the power supervision algorithm and demonstrates the power management usefulness in the case of autonomous operating.

A. Abdelkafi, L. Krichen / Electrical Power and Energy Systems 62 (2014) 1–9 Table 4 PI controllers’ parameters.

References

Controlled systems

Process variables

Kp

Ki

PMSG DC bus

Isd; Isq Udc

2nLsxn  Rs 2nCdcxn

Ls x2n

FC

Ifc

2nLfcxn

Lfc x2n

EL

Uel

2nCelxn

IL

2nLel xn

C el x2n Lel x2n

2nLscxn

Lsc x2n

SC

el

Isc

PI parameters

C dc x2n

Fig. 13 illustrates the single-phase voltage and current of the load with two zooms at different moments. The first zoom at the time t = 20 s corresponds to an application of a resistive load. This curve shows that the voltage and the current are in phase. The transition from a resistive load to an inductive load at the time t = 40 s introduces a phase difference between voltage and current as shown in the second zoom. These figures justify the use of a resonant controller for load voltage regulation. Conclusion In this paper, a hybrid power production unit made up of WG/ FC/EL/SC and supplying a three-phase load was presented. The aim of this study was to develop an energy management algorithm with respect to wind speed fluctuations, load power variations, slow dynamics of the hydrogen system and SOC of the SC. The fast response of the proposed control strategy allows an optimized energy management of all elements of the hybrid unit and ensures a high quality energy supply with a permanent feeding of the three-phase load. Simulation results were presented to illustrate the effectiveness of the considered hybrid unit structure and to prove the reliability of the developed supervision algorithm. Appendix A. PI controllers’ parameters The used PI controllers have the following form:

CðsÞ ¼ K p þ

Ki s

ðA:1Þ

The determination of the controllers gains consist in identifying the closed loop transfer functions denominators with a second order polynomial which has the following form:

D¼1þ

2n

xn

sþ

1

x2n

9

s2

Table 4 resumes all controllers’ parameters.

ðA:2Þ

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