reactive energy control scheme for grid-connected fuel cell system with local inductive loads

Journal Pre-proof Active/Reactive Energy Control Scheme for Grid-connected Fuel Cell System with Local Inductive Loads

Mustafa İnci PII:

S0360-5442(20)30298-X

DOI:

https://doi.org/10.1016/j.energy.2020.117191

Reference:

EGY 117191

To appear in:

Energy

Received Date:

09 January 2019

Accepted Date:

17 February 2020

Please cite this article as: Mustafa İnci, Active/Reactive Energy Control Scheme for Grid-connected Fuel Cell System with Local Inductive Loads, Energy (2020), https://doi.org/10.1016/j.energy. 2020.117191

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Journal Pre-proof Active/Reactive Energy Control Scheme for Grid-connected Fuel Cell System with Local Inductive Loads 1Mustafa 1İskenderun

İnci

Technical University, Engineering and Natural Sciences, 31200, İskenderun, Hatay, Turkey E-mail: [email protected]

Abstract: In the grid interconnection of fuel cells, local reactive loads connected to the electrical systems generate reactive power which deteriorates the power factor in the utility-grids. This situation causes energy losses, excess electrical energy demand, overloading and bill penalties for consumers. In grid-connected fuel cells, conventional energy management methods are unable to compensate reactive powers in the utility-grid due to reactive loads. For this purpose, in the current study, a new energy management method is developed to reduce electrical energy demand with the elimination of reactive energy consumption from the electrical grid. In this regard, the developed method implemented in a grid-connected fuel cell power plant with reactive loads presents an effective way to prevent the demand charge penalties. In the performance stage, the electrical energy flow is analyzed in a single-phase system under different inductive load banks and compared to the conventional method in order to clarify the validity of the proposed control method. The case studies show that the proposed method based system can not only supply active power to consumers but also provides smooth power factor between grid voltage and current. Keywords: Fuel cell, grid connection, energy management, power control, power factor, reactive load. NOMENCLATURE A

: Tafel slope

pu

: Per-unit

AC

: Alternating current

PWM

: Pulse-width modulation

Cdc

: Dc-link capacitance

Pref

: Reference active power value

D

: Duty cycle

Q

: Reactive power

DC

: Direct current

Qref

: Reference reactive power value

Ddc

: Diode in boost converter

RL

: Resistive-inductive

F

: Faraday fixed value

RLC

: Resistive-inductive-capacitive

h

: Hysteresis band value

Rc

: Gas constant

I dcc

: Current value in boost converter

R

: Resistance value in fuel cell

I fc

: Fuel cell current

S

: Apparent power

I grid

: Grid current

S dc

: Switch in boost converter

I grid

: Load current

S1 , S 2 , S 3 , S 4

: Switches in inverter

io

: Exchange current

SRF

: Synchronous Reference Frame

i nom

: Fuel cell current at nominal operation

T

: Temperature in Kelvin

Ir

: Reference current for switching

V dc

: Boost converter output voltage

kW

: Kilo-watt

V dc,ref

: Reference value of boost converter output

kVA

: Kilo-volt-ampere

V fc

: Output voltage of fuel cell stack 1

Journal Pre-proof Ldc

: Inductor in boost converter

V filter

: Filtered voltage at inverter-side

N

: Number of cells

V grid

: Grid-side voltage

Np

: Primary winding of transformer

Voc

: Open-circuit voltage of fuel cell

Ns

: Secondary winding of transformer

V

: Resistive voltage loss of fuel cell

P

: Active power

Vd

: Absolute polarization overvoltage

PEMFC

: Proton exchange membrane fuel cell

Vo

: Fuel cell voltage at nominal operation

pfc

: Power factor correction

V system

: System voltage

PI

: Proportional-integral

ω

: Angular velocity

PLL

: Phase-locked loop

z

: Quantity of electrons in fuel cell

PQ

: Active-reactive power

1.

INTRODUCTION In the last few decades, renewable energy sources are of great interest due to the environmental factors and rapid reduction

of fossil fuels. It is widely known that renewable energy sources are sustainable, cost-effective and reliable. Among renewable energy technologies, fuel cells have recently increased the popularity thanks to flexibility, silent operation, high performance and modular construction [1-3]. Furthermore, the fuel cells enhance the system performance in terms of power regulation and stabilization in comparison with the discontinuous operation of wind and solar energy sources [4]. Recently, fuel cell energy generation units have been implemented in the grid integration applications for single-phase and three-phase electric grid connections [5-7]. The integration of fuel cells and electric grids is defined as a grid-connected fuel cell system. This system structure is employed to diminish the electrical energy demand supplied from the electric grids. This means that the electricity produced by the fuel cell system can either be used directly by local loads (which is suitable for illumination, heating and other devices in buildings) or be sold to the electric distribution companies [8, 9]. Fuel cell energy systems are strategically located near a distribution line to reinforce the electric grid, thereby minimizing the instantaneous changes in the electric grid and improving the electric grid safety/efficiency [10, 11]. In this context, the electrical energy generation from fuel cell stacks is delivered to the electric grids through the controlled interfacing elements. The grid-connected fuel cell system ensures a collective power exchange between the local load and the grid supply [12]. These systems should effectively provide the energy flow, low-distortion and safety operation for consumers. However, several local loads such as transformers, electrical machines, inductors and capacitors whose currents have phase inequality compared to voltage, cause the reactive power and power factor worsening in the electrical systems [13-15]. In distribution systems, the power factor is defined as the ratio of active energy to apparent power and it must be greater than 0.95 power factor according to the United States Department of Energy’s voluntary Energy Star guidelines [16]. Therefore, reactive energy causes inessential power demand, and thereby users are penalized by energy suppliers. But, it is known that local inductive loads are more common, and they cause overheating and overloading risks for transmission components in the utility-grid. Also, they induce unneeded power flow and bill penalties, as shown in Fig. 1 (a). For this purpose, the reactive power at grid-side should be compensated and power factor correction (PFC) has to be provided between grid voltages and grid currents. In the grid-connected fuel cell system, energy management is employed to provide optimal active/reactive energy flow between fuel cells and grids. Thus, the fuel cell interfacing system is located in an electrical system to control the compensation of reactive energy in addition to active energy injection, as shown in Fig. 1 (b).This process is used to ensure the approximately unity grid-side power factor. For this purpose, in this work, the grid-connected fuel cell system is designed to provide the active/reactive energy for a single-phase system under 2

Journal Pre-proof resistive/inductive load groups. Compared with existing grid integrated fuel cell energy generation, the proposed study aims to achieve unity grid-side power factor together with power flow control.

power factor < 0.95

Load

Active Energy (kWh)

Active Energy (kWh)

Reactive Energy (kVArh)

Reactive Energy (kVArh)

Grid Voltage Grid Current Electricity Supply (a) No power factor correction power factor ≈ 1 Active Energy (kWh) Saving Grid Voltage Grid Current

Load Active Energy (kWh) Reactive Energy Saving (kVArh) Interface

Electricity Supply

(b) Power factor correction with fuel cell system

Fuel Cell

Fig. 1 Energy flow between the electricity supply and local load (a) no power factor correction and (b) power factor correction with a fuel cell interfaced system In order to enhance the energy quality of a grid-connected system, a fuel cell is integrated into the point of common coupling through interfacing elements in the current study. Also, an original power flow control scheme is developed to reduce the electrical energy demand with the elimination of reactive power consumption under reactive load conditions. The minimization of reactive power increases the ratio of the active power to apparent power. In this way, it is aimed to satisfy the low phase angle shift between the grid voltage and current. 1.1. Literature Review Energy generation units are integrated into the electrical grids and load groups through interfacing elements. At the present time, fuel cells are very attractive energy generation units which require a significant development for practical applications in grid integration. In this regard, the requirement of high-quality electrical energy plays an important role for researchers and scientists dealing with the integration of energy generation units and utility-grids. In this context, there are several energy generation methods to ensure high-quality energy supply from the fuel cell unit to the demand-side. The main purpose of energy management is to supervise the power exchange between fuel cell stack and an electrical grid. In literature, it is utilized to arrange smooth power exchange from the fuel cell to grid/load or vice versa. Hence, the energy management methods associated with grid integration are considerable for (1) improvement of energy qualification and the minimization of grid interferences (2) optimal energy supply, (3) sustainable and reliable operation. The functions and objectives of grid-connected fuel cell systems related to energy management are presented in Table 1.

3

Journal Pre-proof In recent studies, various energy management methods based on PQ theory have been considered to improve the energy quality of grid integrated fuel cell systems for different purposes. The researchers in Refs. [17-19] deal with the current quality issue in order to diminish current harmonics at the grid-side. Chen et al. [17] use a resonant current control scheme based on PQ control theory. Also, it is proposed to reduce the current harmonics by using a control scheme in Ref. [17]. In Ref. [18], vectorproportion integration control based energy management has developed basis on PQ control theory. It is also examined to compensate low order harmonics in the designed system [18]. In Ref. [19], a coordinated control scheme, which assures low total harmonic distortion, is reported in addition to PQ control in grid-inverter. Also, system performance is analyzed to fulfill load tracking [19]. Sergi et al. [20] have purposed to eliminate the current ripples in the utility-grid integrated fuel cell system. In the designed system, the specific point is that ripple filters are employed to mitigate the switching components [20]. However, these studies are tested under nonlinear [17-19] and resistive [20] load conditions. Hence, it is clear that the phase inequality is not investigated in addition to power flow. In Ref. [21], researchers have considered load regulation to obtain high energy efficiency for steady-state situations. For this, an economic model predictive control has addressed in this study. The system has been tested under resistive-inductive load without examining phase inequality [21]. Raoufat et al [13] investigated system structure to improve the overall robustness of grid inverter during the transient. In order to optimize energy efficiency during transient situations, robust nonlinear control is applied to the system in [13]. But, no-load condition is analyzed in this study. In addition, researchers deal with the system structure of the grid-connected system in Ref. [16]. Cossutta et al. [16] have analyzed the reduction of line current harmonic distortion using a multilevel converter. Bayrak and Cebeci [22, 23] deals with anti-islanding issue for grid-failure situations. Also, researchers aim to model a micro-grid system including hybrid energy generation units in Ref. [22]. In the related work, Bayrak and Cebeci [22] have specified to develop a control module in the designing program for the integration of energy units. In another study, an islanding detection technique is presented to protect the grid-connected fuel cell structure [23]. The aim of the study is to investigate a robust and fast response time against grid disconnections [23]. The grid-connected fuel cell has been also analyzed for over-voltage and over-frequency conditions [23]. Bayrak and Cebeci [22, 23] use resistive-inductive-capacitive loads at the receiving end. Still, the researchers don’t pay attention to the phase angle shift between grid voltage and current. Grid faults are critical and hazardous in energy systems that result in various harmful impressions such as economic implications and malfunction of equipment. In Refs. [24-26], the authors are interested in energy management to provide faultride through capability. Among these studies, Hajizadeh et al. [24] use a positive-negative sequence extractor to investigate fault ride-through capability under unbalanced voltage sag situations. In this regard, it remains connected to the grid during faults [24]. Sabir [25] has reported that an additional phase-locked-loop (PLL) is eliminated to provide low voltage ride-through capability under balanced/unbalanced faults. Roy et al. [26] have remained fault ride-through capability by using a fuzzy logic control with feed-forward. Also, the performance results are compared to conventional PQ control theory in [26]. Nonetheless, the phase angle shift is not the subject of these scientific studies [24-26]. In comparison with Refs. [24-26], the system has also been tested under-voltage swell conditions in Ref. [27]. Mosaad et al. [27] have analyzed the dynamic performance of voltage sag/swell situations by using PI control. Mojallal et al. [28] have presented a proxy-sliding mode control method to improve the post-disturbance recovery of fuel cells. The researchers also aim to improve the performance efficiency of fuel cell plants during asymmetrical voltage reductions [28]. The impacts of short circuit faults are reported in [29]. Stewart et al. [29] use a PI control to observe the effects of faults on hydrogen pressure, voltage and current. Load-following with the fault-handling capability of a grid-connected system has been reported in Ref. [30]. PQ control theory with two-loop current control is employed to manage active/reactive power from fuel cell to the mains electricity. Wand et al. [30] specify that the system can maintain stable-situation under serious voltage sags. Short-time overloading capability is achieved through PQ control theory in inverter control [31]. 4

Journal Pre-proof Wang et al. in Ref. [31] aims at the improvement of the system stability for temporary faults. Although the systems are performed for resistive-inductive load in Refs. [30]-[31], power factor correction is not observed at the grid-side. Table 1. Functional capabilities of conventional & proposed studies in the grid-connected fuel cells dealing with energy quality Issue Current quality Load regulation

Objective Reduction of current harmonics at grid-side Elimination of current ripples Optimization and energy efficiency during transient regulation

Load Nonlinear R load RL load

Ref [17-19] [20] [21]

System structure

Performance robustness of converters

No-load

[13]

Reduction of the line current harmonic content using multilevel inverter Operation of the system under grid-failure mode Detection of grid-failure time Energy management to provide fault-ride through capability Energy management to provide fault-ride through capability To prevent the effects of faults under voltage sag/swell Improvement of fuel cell response against symmetrical & asymmetrical voltage sags Load-following with the fault-handling capability Short time overloading capability Management of hybrid generation units Optimization and analysis of dynamic fuel cell Improvement of the overall system reliability Optimization and active-reactive power oscillations Energy management to maximize power injection capability under unbalanced voltage situations Stabilization of the dc-link voltage and providing a robust operation against the nonlinear situation Energy management to obtain optimal energy stability at gridside Energy management to diminish reactive power and provide power factor correction at grid-side

RL load

[16]

RLC RLC No-load Sensitive No-load Sensitive

[22] [23] [24, 25] [26] [27] [28, 29]

RL load RL load Thermal load RL load Sensitive No-load R load

[30] [31] [4, 32] [33] [34] [35] [36]

No-load

[37]

RL load

[38]

RL load

Proposed

Anti-islanding Grid faults

Stabilization

A significant number of previous studies point out the stabilization of fuel cell systems with grid integration. Among these studies, the management of hybrid energy generation units is specified in Refs. [4, 32]. Bornapour et al. [4] have proposed a stochastic model through optimal coordinated method for a micro-grid system including heat and power units. Also, the system has been tested under thermal load situations [4]. Hongbo et al. [32] have suggested an optimization model for a residential energy system. The main mission of the study is to identify the optimal operation planning for cost and low-emission. Bicer et al. [33] have introduced an artificial neural network for grid-connected fuel cell systems. In the designed system, the dynamic response of fuel cells has been analyzed to maximize the system performance [33]. But, in this study, the optimization is realized without the phase shift angle under resistive-inductive load. Ayyappa and Gaonkar [34] use PQ control theory to improve the system reliability for hybrid operation. Besides, the coordination between energy generation units has been addressed in Ref. [34]. Current control based energy management has been investigated in Ref. [35]. Celik and Meral [35] use positive-negative sequence extraction based PQ control theory for the regulation of power oscillations in the grid-connected system. But, in this paper, only active/reactive power oscillations are compensated through the used PQ control theory. Celik et al. [36] deal with maximum power delivering capability using a positive-negative sequence extraction PQ method in a grid-connected hybrid energy system. Wu et al. [37] have used synchronous reference frame control in order to control active-reactive power. Also, 5

Journal Pre-proof active disturbance rejection control integrated to PQ control theory is employed to improve the system performance [37]. Tejwani and Suthar [38] deal with a dc-dc converter control to obtain optimal energy stability in addition to PQ theory used in gridinverter. Even though there are energy flow control studies for the grid integration of fuel cells dealing with energy quality, there are no studies on the compensation of reactive power flow seen on the electrical grid under reactive loads. This implies that the reactive power compensation issue still needs to be improved for grid-connected fuel cell systems. 1.2. Key contributions In comparison with conventional grid-connected fuel cell studies, this work presents an active/reactive energy flow control strategy with reactive power elimination capability. As a result of this study, the elimination of reactive power guarantees power factor correction in the electric grid. The objective of the current study is to advance an effective control structure for grid interface of fuel cell system that has the following functions: (1) smooth active/reactive power control (2) dynamic reactive power support under reactive loads (3) power factor correction at grid-side (4) dc-link voltage stabilization. To this end, a power factor correction based PQ (PFC-PQ) control method is introduced for a grid-connected fuel cell. In comparison with the conventional PQ control method, it is capable to compensate the grid-side reactive power due to reactive load bank groups. The control method enhances the performance of the traditional PQ control structure, herein referred to as PFC-PQ control method. In the current study, the proposed control structure is designed and tested in a grid-connected fuel cell in order to provide optimal energy flow through the grid-line. The proposed control method is based on a synchronous reference frame with αβ/dq transform. In order to demonstrate the validity of the proposed control, performance analyses have been further given in comparison with traditional PQ control. In regard to the performance comparison results, the various case studies have been analyzed by using the Simulink environment program. In this research work, the key contributions of the study are expressed as follows: i. The proposed control scheme is implemented to regulate reactive power between the fuel cell energy unit and electrical grid. In comparison to conventional PQ control theory operated in grid-connected fuel cell structures, it prevents the reactive power flow in grid-transmission lines. Hence, it reduces the electrical energy demand with the elimination of reactive power consumption from the utility. ii. With the reduction of reactive power at grid-side, inessential energy demand due to reactive loads is reduced from the grid-supply. This condition makes the reduced cost to the consumers and energy losses on the transformers, transmission lines and cables. iii. Compared to conventional studies, the controllability of reactive power compensation eliminates the phase inequality between grid current and grid voltage. It prevents the deterioration of the power factor at AC utility grids and reduces the amount of electrical power consumed for industrial and/or residential applications. Power factor correction aims to enhance the power quality that increases energy efficiency and system stability. This paper is arranged as follows: First, the design and system structure of the grid-connected fuel cell are described in part 2. Then, the proposed system, which includes the phase estimation, power computation, reference generation and hysteresis control, is presented in part 3. The various case studies are given to demonstrate the effectiveness of the proposed control method in part 4. Performance results are also compared to the conventional method. Finally, a conclusion section is briefly presented in part 5.

6

Journal Pre-proof 2.

THE SYSTEM CONFIGURATION In the grid integration, the dc energy generated from fuel cells has to be converted to ac energy form [20]. For this purpose,

the energy conversion is achieved through interfacing sub-components. In these systems, dc-ac conversion and the energy management control of fuel cells with the grid are fundamentally based on dc-ac converters called inverters [20]. In a gridconnected fuel cell system, the connected loads significantly affect the electrical output values. In the operation of the gridconnected fuel cell, the purpose of a dc-dc converter is to keep the desired voltage stable at its output within ±5% of its nominal voltage value [27]. In this way, the main duty in the integration of fuel cells to the grid is accomplished through the inverter part. Both the active power supplied from the fuel cell to the grid and the reactive power flow between the fuel cell and the grid is realized by an inverter [39]. The scheme in Fig. 2 introduces the circuit arrangement of the grid-connected fuel cell designed in the current study. According to the system, it includes a fuel cell generation unit, dc-dc boost converter, dc-ac converter with switching ripple filter and step-up transformer in order to supply active power for connected local loads and provide PFC at grid-side. Fuel Cell Voltage (V)

60 40 20 0 -20 -40 -60 0.0

100

Inverter Voltage (V)

50 0 -50 0.02

0.04

-100 0.0

0.08

0.06

Ifc

L dc V fc

DdcI dc Sdc Cdc

Vdc

DC-DC Control

S3

S2

S4

Filter

TR

0.02

Lf Cf

Vfilter

0.04

0.06

Vsystem

I load

0.08

Grid

I system I grid

PWM

PWM Voltage Controller

S1

0.08

Vgrid

Pref Qref

Reference Generation

Vsystem Isystem

V system I load Inverter Control

Lload Rload

Vdc,ref

0.06

Grid Voltage (V)

S1 S2 S3 S4

Sdc Vdc

0.04

Full-bridge Inverter

DC-DC Boost Con.

PEMFC

0.02

600 400 200 0 -200 -400 -600 0.0

Local Loads

Fig. 2. The scheme of the power circuit of tested and modeled single-phase grid-connected fuel cell 2.1. Fuel Cell In this study, proton exchange membrane fuel cell (PEMFC) is employed as a fuel cell. PEMFC is the most well-known fuel cell implemented in power generation applications. It is also known as low-voltage less than 50 Volts and high-current energy supply. The equivalent electrical circuit of PEMFC is presented in Fig. 3, and its behavior is expressed in Eqs. (1-4).

7

Journal Pre-proof

N

Voc

A

io

NAIn(ifc /io ) RΩ

ifc

D

ifc V

Vfc

Fig. 3. The equivalent electrical scheme of a PEMFC The equivalent output potential value of fuel cell is defined as [40]:

V fc  Voc V Vd

(1)

where, V fc is the output voltage of fuel cell stack, Voc is the open-circuit voltage, V is the resistive voltage loss and Vd is the absolute polarization voltage loss, respectively [41]. The open-circuit potential value of fuel cell is considered as [24, 41]: 1 / 2     44,43 RcT  PH 2 PO2   Voc  K c Vo  (T  298)  ln  zF zF  PH 2O    

(2)

The value of resistive overvoltage is [40]:

V  i fcR

(3)

The absolute polarization overvoltage is described as:

Vd  N  A  ln(i fc / io )

(4)

In which, N indicates the quantity of cells in the fuel cell. Also, A is Tafel slope and io is exchange current detailed in [41]. In this study, the maximum power of PEMFC is 6.02 kW. In the structure of PEMFC, the number of cells is 65 and Nerst voltage is 1.015 Volts for each cell. According to the PEMFC operating characteristics, it generates 133.7 A and 45 V in nominal operating conditions. Dynamic parameters are also detailed in Table 2. Table 2. Dynamic parameters of fuel cell used in the proposed system Parameters Nominal Power Number of cells Maximum operating point ( Imax (A),

Values 5200 W 65 (133.7A, 45V)

Vmax (V)) Maximal Power Fuel cell resistance Nerst voltage of each cell

6020 W 0.07833 ohm 1.015 V 8

Journal Pre-proof Nominal usage of H 2

99.56 %

Nominal usage of O2 Nominal consumption of fuel Air Exchange current ( io ) System temperature ( T )

59.3 %

Pressure ( Pfuel ) of fuel supply

60.38 slpm 143.7 slpm 0.29197 A 338 Kelvin 1.5 bar

Pressure ( Pair ) of air supply

1 bar

2.2. Interfacing Depending on the fuel cell dynamic characteristics, the dc-dc converter has to boost and stabilize the output voltage with effective conversion efficiency [42]. The principal aim of a dc-dc converter is to maintain the constant voltage at the input of the inverter. In the proposed system, the supervision of dc-link voltage is provided by using a PI control. The input of this control method is characterized as the change between actual voltage and the reference [13]. The PI output is used to trigger switch by using pulse width modulation. According to the switching duration or duty cycle (D), the dc-link voltage is expressed in Eq. (5). The boost converter is designed for continuous current, and its parameters used in the proposed model are an inductor ( Ldc  0.6mH ) and a dc-link capacitor ( C dc  10mF ).

Vdc  V fc /(1  D)

(5)

The stabilized dc power at the converter output has to be converted to ac power so that it can be integrated with the grid [10, 42]. To this end, a dc-ac converter is utilized to convert the dc power to ac power [28]. Inverters are critical interfacing components, and they are positioned after the dc-dc converters. A full-bridge inverter is used in the system, and the output potential value of full-bridge inverter is expressed using the Fourier series. In terms of Fourier transformation, the output voltage contains only odd harmonic components in terms of the square wave control strategy [43]. The value of full-bridge inverter is given as follows [44]: Vo t   Vo t  

  Vn sin n 1  n  odd

not   n 

(6)

4Vdc sin not  n

(7)

The fundamental control is implemented based on inverters. For this purpose, a full-bridge inverter is used to supervise power flow in the proposed construction. Therefore, the main target of the inverter is to manage energy movement from the fuel cell and mains-grid, and vice versa. LC filter is used to diminish the ripples as a result of the switching operation in voltage source inverters [45]. Also, a transformer is employed to link the inverter to an electrical grid [29, 46]. The system parameters of interfacing materials in the designed model are introduced in Table 3. Table 3. Parameter values of power electronics and sub-interfacing elements Boost converter Type Inductance value ( Ldc )

The parameter values of interfacing elements Inverter Boost Type 0.6 mH Filter inductor ( L f )

full-bridge 1.5 mH

DC-link capacitance value ( C dc )

10 mF

Filter Capacitor ( C f )

5 uF

Reference

100 V

Step up- transformer( N p / N s )

1/4 9

Journal Pre-proof 2.3. Grid and Local Load In the designed system, the fuel cell energy generation unit is connected to a single-phase electrical grid in the ratings of 220 Vrms/50 Hz. Three local parallel resistive-inductive (RL) load banks are used to consume active/reactive power and create a phase inequality between grid and voltage, as shown in Fig. 4. Table 4 exhibits the rating values, impedance values and operating durations of load banks. To Grid Grid Bus

Load Bank #1

Switch 1

Switch 2

Switch 3

7Ω

5.2 Ω

6.6 Ω

Load Bank #2 2.6 mH

Load Bank #3 4.4 mH

3 mH

Fig. 4 Local resistive-inductive load banks Table 4. The specifications of local RL load banks Power Rating 6.3+0.73 kVA 8.1+1.92 kVA 6.9+0.64 kVA

Load Bank - I Load Bank - II Load Bank - III 3.

Impedance (R+jωL) 7.0+j0.82 5.2+j1.38 6.6+j0.94

Activation At t=0 s At t=1.2 s At t= 2.2 s

PROPOSED CONTROL The controller arrangement of the improved energy management method is presented in Fig. 5. This method is applied in

the inverter part of the grid-connected fuel cell system. The main functions of the proposed method are to supply active power from the fuel cell and to regulate reactive power exchange between fuel cell system and local inductive loads in order to improve power factor at grid supply. According to the control scheme, there are four main parts that are phase estimation, power calculation, the reference signal and switching generation. Phase Estimation Vgrid

Vsystem I system I load

SRF-PLL Power Calculation Power (Eq. 15 and Eq. 18)

Reference and Switching

ɵ

P ref Psystem Q

load Psystem Q Q system system P load (Not used) Isystem Q load ɵ

Reference Signal Generation

Hysteresis Current Controller

Switching Signals

Fig. 5 The general scheme of the proposed power flow control 3.1. Phase Estimation In the proposed control, phase estimation is accomplished through PLL and it is utilized to synchronize the system angular velocity  in accordance with grid voltage ( Vgrid ) [47]. In the proposed control, SRF-PLL is applied to evaluate the phase information of an electrical signal, and its structure is introduced in Fig. 6. In order to detect the frequency and angle information 10

Journal Pre-proof of single input grid voltage, Park Transformation is applied to generate dq components from α and β [48]. In this conversion, α and β are orthogonal signals each other and β is lagging signal (π/2 rad) of α component. For this purpose, the components of α and β are indicated in Eqs. (8-9).

V  Vgrid  Vm cost 

(8)

V  Vgrid e  j  / 2 t  Vm sin t 

(9)

By using α and β components, d and q components in park transformation are generated as: V d   cos  t  sin  t  V   cos  t  sin  t  V m cos  t  V           q    sin  t  cos  t  V     sin  t  cos  t   V m sin  t 

SRF-PLL

(10)

ϴ

Vgrid π/4 delay

α

d

β

q

ωo

KP

V αβ/dq q

KI∫

ω

∫

Δωt

Fig. 6. SRF-PLL to estimate phase information of grid-side voltage in PFC-PQ method Assuming that V are V are have magnitudes in 1 pu, it is obtained that Vq =0 for the ideal condition. Nonetheless, the minor alteration in the frequency of grid voltage creates a non-zero voltage in Vq signal. This is dependent on the change in the orthogonal voltage component ( V ) under instantaneous voltage variations. Thence, a minor alteration in the grid frequency forms a small phase error. This event can be expressed analytically as follows: Vq  cos(t ) sin(t   e )  cos(t ) sin(t )  cos(t )sin(t   e )  sin(t )

 cos(t )sin t  cos  e  cos(t ) sin  e  sin(t )

(11)

It is expressed as sin  e   e and cos  e  1 for small values of error angle (  e ). If the phase angle produced by the PLL is close to the grid voltage, the error angle t   e  is small or about zero. Thus, for a balanced three-phase system, it appears that the q-axis component in the rotating reference frame falls to zero when the PLL is locked. Thus, Vq  cos(t )sin t   cos(t ) e  sin(t )   e cos 2 (t )

(12)

  e 1  cos(2t )  2 The phase angle generated by SRF-PLL is defined in Eq. (13).





   V q ( t ) K P    t   0 dt

(13)

t   Vq (t )KI dt

(14)

Where

11

Journal Pre-proof 3.2. Power Calculation In the tested system, the consumed energy by the load is ensured via utility-grid and/or fuel cell energy unit. The phasor diagram of power flow in the system is introduced in Fig. 7. According to the operational principle, it is clear that a significant part of the reactive power demand at the load-side is provided through the fuel cell system in order to prevent the absorption of reactive power from the grid supply. In this condition, the reactive power must be zero in the grid line, and the reactive power controlled by fuel cell energy unit is equal to load-side reactive power for an ideal condition.

Sold ϴ1

Qsystem

Snew

Qload Qgrid

Qgrid = Q load -Qsystem Pload= Pgrid + Psystem

Pgrid Qsystem

ϴ2 Psystem Ssystem

Fig. 7. The phasor diagram of power flow in the proposed test system In the power triangle, the grid-side power factor is improved and corrected by injecting reactive power of installed fuel cell energy structure. The power triangle shows that the reactive power supplied by the fuel cell system is utilized to reduce the phase angle between grid active power and grid reactive power. It is evident that the phase inequality of grid voltage and grid current reduced from 1 to  2 according to the power relationship. If it is assumed that the old apparent power is S old , Pload and Qload are written as follows:

Sold  Sload  Vload  Iload  Vgrid  Iload  Pload  jQload

(15)

Active and reactive powers at load-side are specified using Clarke’s Transformation. The relations of Pload and Qload are defined as follows:

Pload 



1 Vsystem Iload   Vsystem  Iload   2

Qload 





1 Vsystem  Iload   Vsystem Iload   2

(16)



(17)

where α and β components are orthogonal signals of voltage and currents. According to Clarke’s transformation, the instantaneous power values at the fuel cell system are calculated as below:

Ssystem  Vsystem I system  Psystem jQsystem

(18)





(19)





(20)

Psystem 

1 Vsystem I system  Vsystem  I system  2

Qsystem 

1 Vsystem  I system  Vsystem I system  2

where Psystem and Qsystem are dependent on α and β components that are orthogonal signals of voltage and currents. The amount of load-side power is equal to the sum of system and grid-side powers. The reduction in the grid-side reactive power is the inequality between Qload and Qsystem. Reference ( Qref )and reactive power error ( Qerr ) are defined as: 12

Journal Pre-proof Qref  Qload

(21)

Qerr  Qload  Qsystem Qerr 



(22)

 

1 1 Vsystem I load  Vsystem I load  Vsystem I system  Vsystem I system 2 2



(23)

3.3. Reference Generation and Hysteresis Control Conventional active reactive (PQ) power methods are based on actual and reference powers, and the calculated powers are exploited to manage only power flow control from fuel cells to the grid [49, 50]. In a conventional method [49-51], the reference active power ( Pref ) is compared to the actual active power value supplied grid-side inverter to supervise the power flow from the energy generation unit to the grid-side. However, it is requested that no reactive power is supplied to the electrical demandside. To this end, the reference value of reactive power ( Qref ) is given as zero (0) in the inverter control. But, local inductive loads induce unneeded reactive power flow and the phase inequality between voltage and current. For this purpose, the reactive power at the grid-side should be compensated and PFC must be provided between grid-side voltages and currents. Thus, the fuel cell interfacing system is located in an electrical system to control the compensation of reactive power and ensure approximately unity power factor at grid-side power. Fig. 8 shows an improved dq based fuel cell inverter control method called PFC-PQ control. According to the structure of the proposed controller, it is designed to handle active and reactive power movement between fuel cell system and local loads. Also, it aims to provide a unity grid-side power factor together with power flow. Therefore, the amount of reactive-power at the load-side is calculated and used as a reference signal in the proposed scheme. Generation of α-β Signals

Power Calculation ∑

Vsystem-α Vsystem Isystem I load

Vsystem-β Orthogonal Isystem-α Signal Isystem-β Generation Iload-α Iload-β

∑

∑

1/2

Psystem

PFC-PQ Control

Kp ∑

Id,ref ∑ KI ∫ P ∑ ref αβ/dq Id I d,actual Isystem-α α d q Isystem-β β Iq,actual Iq ϴ 1/2 Qsystem Kp Iq,ref Qerr Q load 1/2

ϴ d q

α β

inverse dq/αβ

Ir

S1 S4 S2 S3

Fig. 8. The scheme of proposed PFC-PQ control for grid-connected fuel cell energy system The supervision of the full-bridge inverter is implemented based on dq reference frame. In the control scheme, the calculated power values are utilized to generate d- and q- components, respectively [52]. I d , ref  K p ( Pref  Psystem )  K i  ( Pref  Psystem )dt

(24)

I q , ref  K p (Qref  Q system )  K p (Qload  Q system )

(25)

Then, the system current ( I system ) is divided into d and q components by using αβ/dq transform. In this step, orthogonal signals of Isystem are generated and applied to inputs of αβ/dq transform. Subsequently, actual d- and q- components of Isystemare calculated expressed in Eq. (26). 13

Journal Pre-proof  I d , actual   cost  sin t   I   cost  sin t   I system   I         q, actual   sin t  cost   I    sin t  cost   I system   

(26)

I d ,actual and I q,actual are clarified as follows: I d , actual  I system  cos  t   I system   sin  t 

(27)

I q , actual   I system  sin  t   I system   cos  t 

(28)

Final d- and q- components ( I d , I q ) are determined using d- and q- components of reference and actual values. The difference between reference and actual components specify the final components given in Eqs. (29-30). I d  I d , ref  I d , actual

(29)

I q  I q , ref  I q , actual

(30)

In order to calculate the reference signal used in the switching process, final d- and q- components ( I d , I q ) are employed to generate a reference signal. In the reference generation part, d- and q- components are converted to α- and β- components [52].  I    cos  t   sin  t   I d  I          sin  t  cos  t    I q 

(31)

The generated reference signal is equal to α-component I r  I  . Because it is a single-phase and only α-component is used for the designed system. I r  I   I d cos  t   I q sin  t 

(32)

The generated reference value ( I r ) is used in the hysteresis current control method to trigger switches in the full-bridge inverter. The generation of switching signals is executed from the transitions between upper and lower bands of hysteresis control [52]. The switching rules of hysteresis control are summarized according to Eqs. (33-34). If I r  h THEN S1 is ON,

(33)

If I r  h THEN S2 is ON,

(34)

Where h is the hysteresis band which is equal to 0.01 in the designed configuration. 4.

PERFORMANCE RESULTS In the current work, the proposed control method is implemented in a 6 kW PEMFC based grid-connected system, and the

dynamical behavior of the power plant is tested in the Simulink environment. The designed system and method have been examined in the case of inductive loads and performed to compensate the reactive power under different inductive load conditions. Also, the grid-side power factor values are analyzed for both the proposed method and the conventional method. In order to analyze the performance of the proposed scheme, the grid-connected inverter is set in the values of 220Vrms/50Hz, and it is tied to PEMFC with a rating of 6.02 KW. The voltage/power characteristics curve of the fuel cell is introduced in Fig. 9, and it presents that the rating of voltage is 45 V, the rating of current is 133.3 A for maximum power operation.

14

Journal Pre-proof

Voltage (V)

Voltage vs Current 66 60

Operating Temperature

54 48

(133.7,45)

42 36 30

0

20

40

60

80 100 Current (A)

120

140

160

Power vs Current 7.2

Power (kW)

6.0 4.8 (6.02 kW) Maximum Operating Point

3.6 2.4 1.2 0.0

0

20

40

60

80

100

120

140

160

Current (A)

Fig. 9. P/V characteristic curve of 6 kW/45V PEMFC The dc-side waveforms of the designed power plant are introduced in Fig. 10. According to the waveforms, the duty cycle of dc-dc converter switch is generated as approximately 0.42. The input voltages of boost converter are also shown in the waveforms. The fuel cell generates nearly 42-43 V at the output. According to the operational values, the current value of i fc is calculated as i fc  P / V  5.2k / 42  123.8 A for grid-connected fuel cell system. In addition, the performance results introduce that dc-link voltage is kept at reference value (100 V) by using the dc-link control mechanism.

DC Waveforms Duty Cycle

0.5

0.41-43

0.4 0.3

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

2.8

3

2.8

3

Vfc

60

42-43 V

50 40 30

1

1.2

1.4

1.6

1.8

110 100 90 80

2

2.2

2.4

2.6

Vdc

98-101 V

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

Fig. 10. The waveforms of duty cycle, fuel cell voltage and dc-link voltage The grid-connected PEMFC system is used to feed resistive-inductive loads connected in parallel to the grid. Fig. 11 presents the current waveforms for state 1, state 2 and state 3. Between t=0 s and t=1.2 s, the system is operated in state 1 and local load 1 (R=7 ohm, L=2.6 mH) is active. At t=1.2 s, the operation of the grid-connected system is passed to state 2. In this state, local 15

Journal Pre-proof load 2 (R=5.2 ohm, L=4 mH) is connected to the system while load 1 is disconnected. In the last period called state 3, local load 3 (R=6.6 ohm, L=3 mH) is in active mode and load 2 is disconnected from the system. The transitions from state 1 to state 2 and from state 2 to state 3, are given in waveforms. It is shown that the values of load currents are 44.12 A, 58.67 A, 46.63 A according to the state conditions. In power flow control of installed inductive loads, the active and reactive powers supplied by the fuel cell power plant are introduced in Fig. 12. It is shown that active powers at fuel cell system, grid and load are identical for both conventional and proposed systems. In the system, the fuel cell plant generates approximately 5.2 kW power supply to the load groups. But, the active power values consumed at load-side are 6.3 kW, 8.1 kW and 6.9 kW for state 1, state 2 and state 3, respectively. So, utility-grid meets the additional demand of inductive load which is 1.1 kW, 2.9 kW and 1.7 kW for three states, respectively. In the reactive power part, the conventional method cannot achieve the compensation at grid-side, and the inductive load draws all the amount of the reactive power from the utility grid. In local loads, 0.73 kVAR at state 1, 1.92 kVAR at state 2 and 0.94 kVAR at state 3 are measured for conventional control based system and it causes the increment of apparent power. These values are reported as 0.72 kVAR, 1.91 kVAR and 0.93 kVAR for the proposed method based system. The detailed information of performance results is introduced in Table 5. As shown in performance results, the proposed method supplies needed reactive power into local loads and ensure nearly zero reactive power from the electrical grid. The results show that the conventional method cannot supply a significant reactive power to the grid-side. The reactive power values supplied by the fuel cell system are 0.04 kVAR, 0.05 kVAR and 0.05 kVAR for the conventional method. But, performance values are 0.70 kVAR, 1.78 kVAR and 0.89 kVAR for the proposed method. At the grid-side, the proposed method presents damping results of reactive power. The reactive power values are mitigated to 0.02 kVAR, 0.13 kVAR and 0.04 kVAR for three states. However, the conventional method cannot prevent the reactive power flow from load-side to grid-side. The reactive power values are 0.69 kVAR, 1.87 kVAR and 0.89 kVAR for the conventional method. The numerical results show that the proposed study can be used to reduce the electrical energy demand from utility-grid in order to prevent the demand charge penalties for real-time implementations. Table 5. Reactive power values of load, system and grid for different load group states

Time interval Load Group

Load FC System Grid

State 1 0.0 s < t < 1.2 s R=7 Ω & L=2.6 mH State 1 0.73 kVAR 0.04 kVAR 0.69 kVAR

Conventional State 2 1.92 kVAR 0.05 kVAR 1.87 kVAR

States State 2 1.2 s < t < 2.2 s R=5.2 Ω & L=4.0 mH Reactive Power Values State 3 0.94 kVAR 0.05 kVAR 0.89 kVAR

State 1 0.72 kVAR 0.70 kVAR 0.02 kVAR

State 3 2.2 s < t < 3.0 s R=6.6 Ω & L=3.0 mH Proposed State 2 1.91 kVAR 1.78 kVAR 0.13 kVAR

State 3 0.93 kVAR 0.89 kVAR 0.04 kVAR

16

Journal Pre-proof Transition from State 1 to State 2 Isystem (A)

36.08 A

40 20 0 -20 -40 0.9

0.95

36.97 A

1

1.05

Igrid (A) 20

Transition from State 2 to State 3 40 20 0

1.1

-20 -40 2.1

23.44 A

36.97 A Isystem (A)

36.64 A

2.15

2.25

2.2

23.44 A

8.83 A

11.55 A

20

0

2.3

Igrid (A)

0

-20

-20

0.9 0.9

0.95

1

44.12 A

50

1.05

Iload (A)

1.1

2.1

58.67 A

2.15

2.2

58.67 A

2.25

Iload (A)

46.63 A

50

0

2.3

0

-50

-50

0.9

0.95

1

1.05

1.1

2.1

2.15

2.2

2.25

2.3

At t=2.2 s, Load power demand changes to State 3

At t=1 s, Load power demand changes to State 2

Fig. 11. The system, grid and load currents for transitions from state 1 to state 2 and from state 2 and state 3 Conventional Method 10

Proposed Method

8.1 6.3

8

Load FC Grid

6.9 5.2

6 4

0

0.5

1.7

1

1.5

2.5 2.0

2

2.5

1.92

1.5 1.0

5.2

5.2

5.2

2

1.1

Load FC Grid

0.5

1.7

1.1

0

0.5

1

1.5

2.5 2.0

2

2.5

1.91

1.5 1.0

0.94

0.73

2.9

0 3

Load FC Grid

6.9

6.3

8

4

5.2 2.9

0

8.1

6

5.2

2

10

0.72

1.78

0.02

0.13

3 Load FC Grid

0.93

0.5

0.0 -0.5

0.04

0

0.5

0.05

1

1.5 (a)

0.0 -0.5

0.05

2

2.5

3

0

0.5

1

1.5

0.04

2

2.5

3

(b)

Fig. 12. The values of active/reactive power according to the conventional and the proposed method For tested local inductive loads, power factor values are far from desired values without compensation. Fig. 13 shows power factor values for conventional and proposed control method based grid-connected fuel cell systems with local inductive loads. As shown in the results, power factor values are 0.81, 0.77 and 0.80 for three states, respectively. When the proposed method is performed to supervise the reactive power flow between the load and installed the system, it is shown that the unity power factor is provided at the grid-side. The power factor values are equal to approximate unity under steady-state conditions.

17

Journal Pre-proof Conventional Method

Proposed Method

Power Factor (pu)

1.08

Power Factor (pu)

1.08

1.00

1.00

0.92

0.92

unity 0.81

0.84

unity Transition to State 3

0.84 0.76

0.76 0.68

unity Transition to State 2

0.80

0.77

0

0.5

1.0

1.5

2.0

2.5

3.0

0.68

0

0.5

1.0

1.5

2.0

2.5

3.0

Fig. 13. Power factor values in the grid for conventional and proposed methods Fig. 14 introduces phase deviations between grid voltage and grid currents for different time intervals. In waveforms, the grid-side voltages are ideal and pure. But, grid-side currents include switching ripples due to switching operation of the fullbridge inverter. At the output of the full-bridge inverter, switching ripple filters have achieved to reduce the significant part of switching ripples in system current. A small part of the switching ripples that cannot be reduced is seen as noise in the system current. The phase deviations for both conventional and proposed systems are also detailed in Table 6. In state 1, local load values are 7.0 Ω and 2.6 mH for resistance and inductance, respectively. According to the conventional controlled inverter, a 29.2 degrees phase inequality occurs between voltage and current. In the proposed system, the phase inequality is only 0.13 degrees when local load 1 is connected to the system. In the second condition, when local load 2 is connected (load 1 is disconnected), resistance/inductance values are defined as 5.2 Ω and 4.0 mH. In state 2, the phase inequality increased to 32.1 degrees. Also, the proposed method achieves to reduce this value from 32.1 degrees to 0.21 degrees. In a final statement, local load 3 is active when local load 2 is passive. The values of resistance & inductance are 6.6 Ω and 3 mH for state 3. When local load 3 is in operation, the phase inequality of grid voltage and current is 28.7 degrees. In this case, the proposed control based system makes the current in phase with the grid voltage and phase inequality is 0.17 degrees. The results show that grid-side power may have dramatical results on energy demand for real applications. The proposed method significantly reduces the phase inequality between grid voltage and grid current as a result of reactive power injection by the fuel cell system. In addition, the improvement of the power factor also reduces the electricity bills together with less reactive power consumption.

18

Journal Pre-proof Conventional Method

Proposed Method

State 1

State 1 Grid Voltage Grid Current

1 0.5

0.5 29.2°

0

pu

pu

Grid Voltage Grid Current

1

29.2°

-0.5

0.13°

0

0.13°

-0.5

-1

-1 0.7

0.71

0.72

0.7

0.73

0.71

0.72 State 2

State 2 Grid Voltage Grid Current

1

0.5 32.1°

0

pu

pu

Grid Voltage Grid Current

1

0.5

32.1°

0.21°

0

0.21°

-0.5

-0.5

-1

-1 1.42

1.43

1.44

1.42

1.45

1.43

1.44

1.45

State 3

State 3 Grid Voltage Grid Current

1

Grid Voltage Grid Current

1

0.5

0.5 28.7°

0

pu

pu

0.73

28.7°

0

-0.5

-0.5

-1

-1 2.76

2.77

2.78

2.79

0.17° 0.17°

2.76

2.77

2.78

2.79

Fig. 14. Phase inequality between grid voltage and grid current according to the conventional method and the proposed method Table 6. Phase inequality in degree for different local load groups State

Time interval

State 1 State 2 State 3

0.0 s < t < 1.2 s 1.2 s < t < 2.2 s 2.2 s < t < 3.0 s

Local Loads R L 7.0 Ω 2.6 mH 5.2 Ω 4.0 mH 6.6 Ω 3.0 mH

Phase Difference Conventional Proposed 29.2 degree 0.22 degree 32.1 degree 0.27 degree 28.7 degree 0.29 degree

Power Factor Conventional Proposed 0.81 0.997 0.77 0.994 0.80 0.996

In addition to voltage/current waveforms, Fig. 15 introduces the PLL responses based on conventional and proposed systems. The PLL responses are given in degree and the values change from 0 to 360 degrees. It is presented that the phases of grid currents deviate in a conventional controlled system. As the number of inductive load increases, the phase inequality between voltage and current increases from state 1 to state 3. However, when reactive power is compensated by the proposed system, the phases of currents are in phase with grid voltages.

19

Journal Pre-proof Conventional Method State 1

State 2

State 3 Grid Voltage Phase (degree)

Grid Voltage Phase (degree)

Grid Voltage Phase (degree) 300

300

300

200

200

200

100 0

100 0

100 0

0.7

0.705

0.71

0.715

0.72

1.42

0.725

1.425

1.43

1.435

1.44

2.76

1.445

300

300

300

200

200

200

100

100

100

0

0 0.7

0.705

0.71

0.715

0.72

0.725

0 1.42

1.425

1.43

1.435

1.44

2.765

2.77

2.775

2.78

2.785

Grid Current Phase (degree)

Grid Current Phase (degree)

Grid Current Phase (degree)

1.445

2.76

2.765

2.77

2.775

2.78

2.785

Proposed Method State 2

State 1

State 3

Grid Voltage Phase (degree) (degree)

Grid Voltage Phase (degree) 300

300

200

200

100

100

Grid Voltage Phase (degree) 300 200 100

0

0 0.7

0.705

0.71

0.715

0.72

0.725

0 1.42

Grid Current Phase (degree)

1.425 1.43 1.435 1.44 Grid Current Phase (degree) (degree)

1.445

300

300

300

200

200

200

100

100

100

0

0

0.7

0.705

0.71

0.715

0.72

0.725

0 1.42

1.425

1.43

1.435

1.44

1.445

2.76

2.765 2.77 2.775 2.78 Grid Current Phase (degree)

2.785

2.76

2.765

2.785

2.77

2.775

2.78

Fig. 15. PLL responses of grid voltage and grid current in the conventional method and the proposed method 5.

CONCLUSION The use of fuel cells in the field of grid connection is still being researched and developing day by day. With the reduced cost

of fuel cells, these energy generation units are expected to become more widespread as a new energy source in practical applications of grid integration in the following years. Moreover, the power rating values of fuel cells are developing rapidly, and their applications in the grid integration are of great significance for solving traditional energy problems. In this regard, the research studies in the grid-connected fuel cell energy systems are going to be interested in the following headlines: (1) integration with other renewable energy systems, (2) solution of high-order energy quality problems for transient situations, and (3) optimization and maximum power extraction from fuel cells. In the proposed grid-connected energy system, the resistive/inductive loads are employed to consume electrical energy, which changes the characteristics of the system currents. In conventional grid-connected energy systems, the installed loads create the reactive power flow & phase displacement between grid voltage and grid current. However, classical PQ control methods cannot achieve to prevent the reactive power flow from the utility grid. In order to overcome the drawback of the traditional PQ method, an improved power flow control scheme called the PFC-PQ method has been presented in this work. Also, it is implemented in the grid-connected fuel cell system under resistive/inductive load groups. The proposed method has been performed to control energy management through the inverter part of the fuel cell interfacing power plant. In this research study, it is used to control supplied active power into loads, and to control reactive power exchange through interfacing system in order to prevent the flow in the utility-grid. For this purpose, the design, analysis and performance results of the proposed method have been tested under dynamic resistive-inductive load groups. In performance results, a comprehensive set of case studies has been tested to show the enhancement of the energy quality in the distribution system. Furthermore, the detailed analysis of energy demand reduction is reported by numerical results. In the 20

Journal Pre-proof energy demand reduction, the designed energy system considerably lessens the consumption of reactive power from utility-grid. In comparison with conventional PQ control, the utilization of the proposed control facilitates the stabilization and quality of electrical energy at the grid-side. When the grid-connected fuel cell system is controlled by a conventional method, low power factor values at the grid-side seem as 0.81, 0.77 and 0.8 for state 1, state 2 and state 3, respectively. When the proposed method is performed in the grid connection of fuel cell, it achieves the control of reactive power between the fuel cell and local loads in addition to the active power supply. Thus, it keeps power factor values at approximately the unity factor under reactive load conditions. It should be noted that the power factor values are observed as 0.997, 0.994 and 0.996 for PFC-PQ controlled energy system. Also, the phase inequality values between grid voltage and grid currents are presented for two methods. It is clear that the PFC-PQ control method shows good results to decrease phase inequality between grid current and grid voltage. In addition, reactive power values are diminished to 0.02 kVAR, 0.13 kVAR, 0.04 kVAR from 0.69 kVAR, 1.87 VAR, 0.89 kVAR. The obtained results demonstrate that the PFC-PQ control based system shows satisfactory performance to control active/reactive power flow control and power factor correction for reactive load groups. REFERENCES [1] Inci M, Turksoy O. Review of fuel cells to grid interface: Configurations, technical challenges and trends. J Clean Prod. 2019;213:1353-70. [2] İnci M. Interline fuel cell (I-FC) system with dual-functional control capability. International Journal of Hydrogen Energy. 2020;45(1):891-903. [3] Bizon N, Thounthong P. Real-time strategies to optimize the fueling of the fuel cell hybrid power source: A review of issues, challenges and a new approach. Renew Sust Energ Rev. 2018;91:1089-102. [4] Bornapour M, Hooshmand RA, Khodabakhshian A, Parastegari M. Optimal coordinated scheduling of combined heat and power fuel cell, wind, and photovoltaic units in micro grids considering uncertainties. Energy. 2016;117:176-89. [5] Lu YZ, Cai YX, Souamy L, Song X, Zhang L, Wang J. Solid oxide fuel cell technology for sustainable development in China: An over-view. International Journal of Hydrogen Energy. 2018;43(28):12870-91. [6] Argyrou MC, Christodoulides P, Kalogirou SA. Energy storage for electricity generation and related processes: Technologies appraisal and grid scale applications. Renew Sust Energ Rev. 2018;94:804-21. [7] Yu SL, Fernando T, Chau TK, Iu HHC. Voltage Control Strategies for Solid Oxide Fuel Cell Energy System Connected to Complex Power Grids Using Dynamic State Estimation and STATCOM. Ieee T Power Syst. 2017;32(4):3136-45. [8] Maleki A, Hafeznia H, Rosen MA, Pourfayaz F. Optimization of a grid-connected hybrid solar-wind-hydrogen CHP system for residential applications by efficient metaheuristic approaches. Appl Therm Eng. 2017;123:1263-77. [9] Ali ES, Abd Elazim SM, Abdelaziz AY. Optimal allocation and sizing of renewable distributed generation using ant lion optimization algorithm. Electr Eng. 2018;100(1):99-109. [10] Sharma RK, Mishra S. Dynamic Power Management and Control of a PV PEM Fuel-Cell-Based Standalone ac/dc Microgrid Using Hybrid Energy Storage. Ieee T Ind Appl. 2018;54(1):526-38. [11] Caballero JCT, Roffiel JA, Marino MAL, Lievana ORM, Pouresmaeil E, Vechiu I. A control method for operation of a power conditioner system based on fuel cell/supercapacitor. Electr Eng. 2018;100(2):857-63. [12] Emami K, Ariakia H, Fernando T. A Functional Observer Based Dynamic State Estimation Technique for Grid Connected Solid Oxide Fuel Cells. Ieee T Energy Conver. 2018;33(1):96-105. [13] Raoufat ME, Khayatian A, Mojallal A. Performance Recovery of Voltage Source Converters With Application to GridConnected Fuel Cell DGs. Ieee T Smart Grid. 2018;9(2):1197-204. 21

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Declarations of interest: none Dr. Mustafa İnci

Journal Pre-proof HIGHLIGHTS  Active/reactive energy control strategy is proposed for a grid-connected fuel cell.  Grid connected fuel cell system is designed for a single phase system.  The control strategy is performed to provide power factor correction at grid-side.  The system is tested under different dynamic resistive-inductive load changes.