Series active power filter supplied by fuel cell for mitigating power quality issues

Received Nov 25, 2019 Revised Feb 19, 2020 Accepted Mar 3, 2020 In this paper, the combination of the series active power filter (SAPF) with a fuel cell (FC) source is deliberated. The FC based on the SAPF aims to compensate voltage deviations or disturbances that occur in the system caused by power quality issues. The proposed system consists of a fuel cell source connected to the DC link through two DC-DC converters, the first extracts the maximum power of the FC source through pulse width modulation (PWM) signals generated from the maximum power point tracker (MPPT) controller. Thus, the second converter is used to regulate the high voltage side of the converter through closed control loops, in addition to a voltage source inverter (VSI) and a series injection transformer. Despite of fluctuations of the DC link during the compensation of the needed energy, MPPT and closed control loops generate PWM signals to the switching devices of DC-DC boost converters in order to extract maximum fuel cell power and to maintain the bus voltage within its limits and around its reference values respectively. The proposed topology is simulated in MATLAB/Simulink software, where simulation results show that the proposed FC based SAPF can efficiently reduce problems of voltage sags-wells and harmonics.


INTRODUCTION
Given the demand of improving the power quality especially in industry, where the critical equipment and sensitive loads are widely used and their power supply should not interrupted. Here, uninterrupted, clean and regulated power supply is required when feeding loads that have important tasks. In other hand, most common voltage disturbances appear in alternative current appliances AC providing a reduction in voltage amplitude known as sag. Thus, sag and interruptions provide most of the industrial problems (90%) that affect their supply quality [1]. Other, swell problem is also among disturbances that affect the power quality, which is defined by a rising in voltage amplitude above its nominal value [2]. In addition, harmonic distortion can provide huge problems in the whole of the power conversion chain such as heating the system components, mechanical oscillations, unpredictable behavior of protecting devices, and may cause damage [3,4].
Due to these problems, it is necessary to approve protecting devices and effective solution to solve such disturbances. In this paper, we focus on series active power filter SAPF in order to mitigate sag-swell and harmonics [5][6][7][8] that affect our system. SAPF have the same topology as the active filter, with an excellent dynamic capability to restore the load voltage to i nominal value within a few milliseconds as

SYSTEM DESCRIPTION
The objective of this study is to supply the SAPF with a SOFC source through dc-dc boost converters to extract the maximum power of the FC and stabilize the output operating voltage of the DC link, while compensating voltage fluctuations. The module framework is shown in Figure 1, where a sensitive three-phase load is supplied by a three-phase source through a three-phase impedance. The injection mode of the SAPF is achieved through a three-phase transformer, where line voltages of the load are restored around its reference values during the applied voltage disturbance at the grid side. The storage unit can be replaced by the SOFC with its conversion chain as viewed in the Figure 1.

Modeling and characteristics of SOFC
The operating principal of the FOFC device is based on the electrochemical reaction between hydrogen and oxygen across a solid electrolyte, in order to generate electricity. Exactly, the O² ions are constructed at the cathode side, where the oxygen accepts electrons from the load circuit. The produced ions are transferred afterward to the anode through the electrolyte, and then combined with hydrogen to form water. The results of this reaction are transferred through the load circuit to the cathode [13]. The essential reactions of the SOFC are the following, at the cathode: at the anode: 175 the overall reaction is: the dynamic model of the SOFC is detailed in references [13][14][15][16][17][18]. Following expression represents the output voltage of the stack device: The ohmic losses V ohm is caused by electrical resistance of electrodes, the resistance to the flow of ions in the electrolyte, which can be deduced by Ohm's law.

Ohmic losses
These losses are due to the opposition of electrons through the anodes and to the relocation of particles through the electrolyte. In addition, the interconnections or bipolar plates of the FC source are added to the ohmic losses [13][14][15].
where r is the internal resistance and ifc is the fuel cell stack current.

Activation losses
This loss is caused by the sluggishness of chemical reaction that takes place on the surface of electrodes. A drop voltage produced by the fuel cell is carrying the reaction forward, which transfers the electrons to or from the electrode [13]. The activation loss vact is given by: where R is the ideal gas constant, T is the temperature, and F is the Faraday constant. The coefficient α represents the electron transfer of the reaction at the electrode, and ifc0 is the exchange current of the fuel cell stack.

Concentration losses
These losses are known as mass transport losses and are caused due to the reduction in the concentration of reactants in the region of the electrode when the fuel is consumed. The consumption of reactants at respective electrodes, i.e. hydrogen at the anode and oxygen at the cathode leads to a slight reduction in the reactants concentrations, which cause a drop in partial pressure of gases resulting in a drop voltage that portion of the electrode can produce [13]. The concentration loss vconc is defined by (7): where ifcl is the limiting current. The Nernst equation is: where v standard is the open-circuit potential at the standard pressures. pH2 , pO2 and pH2O are the partial pressure of hydrogen, oxygen and water respectively. Thus, these pressures are defined in (5), which are as follow:

Modeling and characteristics of SAPF
The control strategy used to extract the reference voltages of series active power filter (SAPF) is based on the PQ method [15,19], and the control scheme is shown in Figure 2.  Figure 2. Series active power filter strategy control

C32
The grid three-phase voltage source is assumed to be symmetric and not distorted: where Un and θn are respectively the root mean square (rms) voltage and initial phase angle, n is the harmonic order. When n=1, it means three-phase fundamental voltage source.
The (12) is transformed into (α-β) reference frame, The three-phase positive fundamental current template is constructed as: The (16) is transformed to (α-β) reference frame: According to the instantaneous reactive power theory [13], then: where AC and DC components are included: where p and q are passed through low pass-filter (LPF) and DC component is gotten by: According to [15], transformation is made: The DC components of and : The fundamental voltages in (α-β) reference frame are: The three-phase fundamental voltages are specified by: where;

Modeling of DC-DC boost converter
DC-DC boost converter or step-up voltage converter, steps up input voltages, step down input currents of the DERs through the circuit as shown in Figure 3. Vlow and Vhigh are the voltage at low and high voltage sides respectively and d(t) is the switching duty cycle. Generalized equations of the power converters can be derived for continuous conduction mode CCM due to its simplicity, where two states are used. Starting from the state space equations during the ON-OFF switching states, the converter model can be linearized using an averaging method. For the used dc-dc converters, when the switching device is turned on, it conducts for a ratio D of a period (26), and when the switching device is turned off, the diode conducts for a ratio of (1-D) of the same period (27).
The (28) is used to linearize the above state-space equations of the buck converter, where X is the steady-state component and D is the steady state or DC component duty-ratio. (28)

Perturb and observe P&O
This MPPT method is the widely common and used due to its simplicity and its ease of implementation. Thus, this MPPT method perturbs the system variables (voltage and current), and consequently observes the output generation of the FC source. The principal of this technique is to adjust the operating voltage of the system around the maximum power point (∆P=∆V>0), (∆P=∆V<0). The mathematical expression that resumes the operating principal of the P&O algorithm is: The following Table 1 can illustrate the variation of this MPPT technique around the MPP operating point of the system.

RESULTS AND ANALYSIS
In this section, MATLAB/Simulink simulation results are presented and discussed in details. The proposed SAPF-FC model is simulated by MATLAB/Simulink as shown in Figure 4. Thus, five phenomena are studied in this paper, where the SAPF with the SOFC compensate disturbances of the voltage that appear across the source to avoid damage of the connected load. Such disturbances are applied by a programmable AC source, which is defined by the following specifications:  Root mean square RMS of the phase-phase voltage: 380 V  Phase angle: 0°  Frequency: 50Hz Next section deals with a discussion of the applied phenomenon independently.

Voltage sag
This phenomenon is applied between [2-2.3] (s), where the voltage magnitude is selected 50% equivalent to 0.5 pu in the three phases. Figure 5 shows waveforms of the source, injected and the load voltages. As illustrated in the Figure 5, the SAPF has injected the complementary voltage of 0.5 pu during the specified period [2-2.3] (s) of the three phases in the same direction of the source and load voltages. In addition, the curves of the injected currents of the SOFC, and its DC-DC converters are viewed in the Figure 5.
In normal mode, the SAPF necessitate a constant DC voltage of 750V DC across the capacitance equivalent to the input voltage of the inverter 380V AC. Two stage DC-DC boost converters are sized to step up the SOFC voltage to the DC link reference voltage value, where the first boost converter extract the maximum power of the SOFC through the MPPT controller, whereas the second is a regulated output voltage to maintain the dc link voltage constant as shown in Figure 6. As observed, during the applied sag voltage, currents of the SOFC conversion chain have increased because the DC link capacitance has intervened to compensate the sag voltage disturbance as viewed in Figure 7. Therefore, the capacitance is recharged afterward through the generated energy of the SAPF with the SOFC. Figure 8 represents the active power of the SOFC, which is close to the demanded power by the SAPF to compensate the capacitance energy, taking into consideration converter losses due to the switching devices and resistive elements.

Voltage interruption
Interruption phenomenon appears when the system voltage decrease with 99% of the reference fundamental value during a specified period do not exceed 60 seconds. In-our case, shown in Figures 9 (a)-(c) respectively the grid voltages, injected voltage and load voltage. The SAPF compensate the lack of energy caused by the interruption of the grid power supply. As seen in previous sag voltage phenomenon, the injected currents of the SOFC conversion chain increased due to discharging the DC link capacitance. In other hand, the injected power was remained constant despite the increasing of currents and decreasing of voltages during the compensation, i.e. at the dc link, the injected power was about [4007.5-4349.5] (W), equivalent to the product

Harmonics elimination
Using the AC programmable source, two main harmonics are generated of the order 5th and 7th with the magnitude and the angle phase of [0.2 pu, 35°] and [0.3 pu, -25°] respectively. At the instance t=2 (s), Figures 13 (a), (b) and c represent the source voltage, injected voltage and load voltage respectively. The FSPA compensate and correct the voltage harmonics by injecting the complementary voltage value through the injection transformer as viewed in the Figure 13. Thus, during the compensation, the total harmonic distortion THD value has ameliorated, where the source THD=10.61% , and the load THD=1.84%. As a remark, the SOFC current did not affected by the applied voltage harmonics, where the voltage harmonics phenomena is applied and mitigated. Hence, the selected data shown in Figures 14-16 respectively describes the voltages, currents and powers of the two boost converters of the PV conversion chain during normal and perturbed operation modes.

Unbalanced voltage
In this case, the AC programmable source is programmed to create a voltage unbalance between the three phases in term of phase angle parameters, where following specifications are used:  Time variation: Phase  Type of variation: Step  Figures 17 (a), (b) and c represent the source voltage, injected voltage and load voltage respectively. The simulation result of this disturbance is viewed in the Figure 17. As seen, after unbalancing the system voltages with an angle phase of 60°, the SAPF has compensated this disturbance during the specified period between [2-2.3] (s). The estimation of the unbalance value is evaluated by the total of unbalance, which is about 18% in the source side and 0.5% at the load side. Moreover, the load was protected against the applied voltage unbalance, where its voltage waveform was maintained purely sinusoidal. The selected data shown in Figures 18-20 respectively describes the voltages, currents and powers of the two boost converters of the PV conversion chain during normal and perturbed operation modes.     Figure 21 illustrated the phenomenon simulation results. As the other created disturbance, the SAPF inject compensating voltage through the injection transformer, which is synchronized and in opposition of phase. The reaction of the SAPF has provided acceptable THD values, where its values at the source side were about 3.54% and 1.54% at the load side. The selected data shown in Figures 22-24 respectively describes the voltages, currents and powers of the two boost converters of the PV conversion chain during normal and perturbed operation modes.

CONCLUSION
Through this brief study, a SAPF relies on FC source was used in order to mitigate voltage disturbances such as sag-swell and harmonics. In this paper, a conversion chain is connected to the renewable source FC, where the control strategy of the DC side is based on two combined controllers. First, The MPPT controller is applied in the first DC-DC boost converter to extract the maximum power of the FC, while the high side voltage of the second DC-DC converter is maintained constant during the applied disturbances using closed control loops. Exactly, duty cycles performed by the proportional integral PI compensator are adjusted in order to mitigate DC bus variations and extract the maximum by generating PWM signals to the switching devices of the DC-DC converters. Among the obtained results, renewable energies can be an efficient solution to solve power quality issues.