An Optimization Approaches and Control Strategies of Hydrogen Fuel Cell Systems in EDG-Integration Based on DVR Technology

The adoption of HFCS has been widespread within the EDG infrastructure, particularly in the context of Distributed Flexible AC Transmission System (D-FACTS) Technology. Nevertheless, the assessment of HFCS performance presents a formidable challenge, largely attributable to the pronounced divergence in evaluation software among different EDG aspects. Figure 1 illustrates the interfacing diagram between HFCS and DVR technology within the EDG framework. The components that can be combined to interact with the EDG infrastructure include DC-DC and DC-AC converters, filtering systems, and step-up transformers.

2.1 Types and utilization of FCs in EDG

The fuel cells (FCs) demonstrate an electrochemical device characterized by the presence of two dis-tinct electrodes, denoted as the anode and cathode, that facilitate the generation of electrical energy via tandem reduction-oxidation (redox) reactions. These redox processes harness the chemical energy resident in hydrogen or other suitable fuels, typically in the presence of an oxidizing agent, conventionally oxygen. The classification of fuel cells is based on many factors such as the kind of electrolyte, temperature range for operation, and the specific fuel used [24-26]. Regarding this matter, the most extensively studied fuel cell varieties include PEMFCs, alkaline fuel cells (AFCs), solid oxide fuel cells (SOFCs), and molten carbonate fuel cells (MCFCs). Table 1 proposes a summary of the primary FC types used in EDG. Figure 2 depicts a graphical representation of the PEMFC power technology, including additional components such as a hydrogen tank operating under elevated pressure, a compressor for air, valves for both cathode and anode inlets, an output manifold, a nozzle, as well as pressure and humidity sensors.

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Figure 1. HFCS with EDG interfacing to DVR technology

Table 1. The most prevalent types of FCs employed in EDG [24-30]

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Figure 2. A diagram of PEMFC

In order to enhance the ability of fuel cells to withstand corrosion and operate efficiently, especially with regards to the proton-exchange membrane, novel low-cost bipolar plates have been created. These developments enhance the overall durability of the PEMFC by optimizing operational settings to suit the changing features of the system [27-30]. The hydrogen oxidation reaction (HOR) described here is the essential partial redox process occurring at the anode in a PEMFC.

$H_2 \rightarrow 2 H^{+}+2 e^{-}$                 (1)

where, e knows an electron.

Simultaneously with another partial redox process occurring at the cathode, hydrogen reacts with oxygen, resulting in the production of water, electricity, and heat, as shown in Eq. (2) and Eq. (3) hereunder.

$\frac{1}{2} \mathrm{O}_2+2 \mathrm{H}^{+}+2 e^{-} \rightarrow \mathrm{H}_2 \mathrm{O}$                 (2)

$\mathrm{H}_2+\frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{O}+$ Electricity + Heat                      (3)

Figure 2 illustrates the functional behavior of a PEMFC, including the levels of electric current and the resulting output voltages. In this scenario, polarization refers to the difference between the maximum voltage that can be achieved theoretically and the actual output voltage. It can be quantified using the Eq. (4) as follows:

$E=-\frac{\Delta G}{n F}$                       (4)

where, $\Delta G\left(\mathrm{~mol}^{-1}\right)$, refers to the Gibbs free energy; $n$, knows as to the number of electrons transferred into a hydrogen molecule, and F illustrates to the Faraday's constant, 96,485.0 Coulombs $\mathrm{mol}^{-1}$. Hence, the total energy (the difference in enthalpy, $\Delta \mathrm{H}$) can be computed using Eq. (5) as shown. The symbol $\mathrm{T}(\mathrm{K})$ denotes the temperature in Kelvin, whereas $\Delta S\left(\mathrm{~J} \mathrm{~mol}^{-1}\right)$ indicates the entropy difference between the product and reactant for one mole of hydrogen. The highest possible energy efficiency $\left(\eta_{\max }\right)$ of FC can be determined using Eq. (6). In its entirety, the PEMFC has unequivocally augmented efficiency performance by over 83% under conditions set at 25℃ [31]. Consequently, the voltages exhibited by the PEMFC, denoted as $\left(E_{\text {cell }}\right)$, may be expressed through the formulation outlined in Eq. (7).

$\Delta H=\Delta G+T \Delta S$                      (5)

$\eta_{\max }=\frac{\Delta G}{\Delta H}$                  (6)

$E_{cell }=E_{ocv }-\eta_{\text {act }}-\eta_{\text {ohm }}-\eta_{\text {trans }}$            (7)

where, $E_{O C V}$ indicates the open-circuit voltage, while $\eta_{\text {act }}$ deals with activation loss. $\eta_{\text {ohm }}$ shows the loss of electrical resistance, and $\eta_{\text {trans }}$ refers to the mass transport loss.

2.2 DVR technology controller techniques

The emergence and advancement of DVR technology have been propelled by its efficacy in optimizing energy utilization, stabilizing voltage levels, exerting control over power demand, rectifying power factor discrepancies, mitigating voltage sags, and alleviating harmonic distortions, as elucidated in references [32, 33]. Consequently, the foundational underpinnings of DVR technology are rooted in the comprehension and application of high-voltage power electronics to regulate both transmission and distribution grids [33-36], Table 2 provides an overview of recent DVR technology controller techniques, serving as a reference point for understanding the variations in this technology's implementations.

Table 2. Recent controller techniques of DVR technology

To date, several studies have investigated DVR technology which is a widely recognized, low-cost beneficial properties to mitigate severe sag and swell and other PQ issues [42]. A DVR's mitigation capability depends on the highest voltage it can inject and active power involvement. However, Standard DVRs utilization batteries as their DC input are difficult to obtain, costly, and dangerous to dispose of after usage. This study recommends boosting the DVR's VA assessment by 1.5 times by boosting the DC interconnection voltage to 1.5 times with the microgrid regulator's voltage and quality enhancement approaches. In this direction, the primary objective of the present investigation [43] is to improve the transient responses of 3-phase load voltage and DC-link voltage during immediate grid voltage distortion scenarios. For the DC-link voltage regulation of the DVR technology, an outstanding recurrent compensation petri fuzzy neural network (RCPFNN) controller is initially demonstrated as an alternative to the existing proportional-integral (PI) and fuzzy neural network (FNN) controllers. The identified RCPFNN controller's detailed network topology and online learning method are derived. Furthermore, some results from tests are shown to validate the potential and efficacy of the DVR technology employing the suggested RCPFNN controller for enhancing load transient voltage quality.

Moreover, a great deal of previous research into DVR technology has focused on PQ issues in EDG [44]. This article recommended a transformer less Packed U Cell (PUC)-based DVR technology solution with single-phase solar power systems. The explored topology is distinguished by its more affordable price and size due to the absence of the injection transformer. During the voltage line disruptions, a reliable finite control set model predictive control (FCS-MPC) technique was presented to ensure the generation of the appropriate compensatory voltage while managing the filtering current and the PUC capacitor voltage within their references. Besides that, theoretical analysis, simulation, and actual implementation results were provided under normal and disturbed operation situations to illustrate the suggested solution's high dynamic efficacy and efficient operation. To address PQ issues in EDG, the sliding variable is proposed [45] for each converter leg that is a function of the respective filter-capacitor voltage and a fictional voltage that relies on the converter neutral-point voltage (NPV). The suggested approach permits control of both the compensator voltages and the converter NPV. This article additionally demonstrates methods to calculate the optimal sliding coefficient for a four-leg DVR technology in order to achieve the optimum sliding existence region. To conclude, the suggested control scheme's performance is experimentally confirmed within several operating situations employing a laboratory prototype of a four-leg DVR technology.

2.3 PQ related to the challenge of EDG

In order to address PQ-related issues such as voltage sag, current disturbance, harmonics, and unbalancing, PQ needs to bring together technical approaches. To be able to provide an excellent power energy supply, which in its purest form necessitates symmetry, balance, and access to clear, noise-free sinusoidal wave appearance, the electrical system as a whole need to fulfill certain requirements [46, 47]. Whenever frequency and voltage are brought into account, DVR technology is a typical solution that forms stability of EDG in PQ issues. The IEEE and IEC Standards are rapidly developing definitions to describe power quality. The PQ issues were the primary cause of the EDG failure. Table 3 shows the most frequently PQ-related issues in EDG.

Table 3. The most frequently PQ-related issues in EDG [48-51]

2.4 The converter topology of DVR technology

Essentially, EDG is able to obtain power from HFCS employing DVR technology. The DC converters generate demand power by HFCS and then support the required power into EDG based on injection transfer. Concerning this, DVR technology at the utility-scale involves the following elements:

2.4.1 DC-DC converters topology

Due to their significant DC output voltages, DC-DC converter's topology has various industrial uses for generating large step-up voltage gain, such as in DVR technology and energy-efficient systems, as well as other alternative sources of energy [52]. A traditional boost converter, on the other hand, cannot achieve such high-voltage benefits due to its restricted duty cycle [53, 54]. It is possible to achieve boost converters with higher voltage gains by employing an extreme duty-cycle design and utilization. The voltage and power output in high-frequency current source (HFCS) applications exhibit variability and a substantial dependence on the operating parameters. Therefore, DC/DC power converters are frequently utilized to ensure a consistent voltage that aligns with the subsequent power bus. Due to the nonlinearity of HFCS and load pat-terns. In this point, power converters must be designed to be more resilient. The EDG continues to rapidly create such applications [54-56]. Bidirectional power flow is supported by the DC-DC converter topology. Figure 3 depicts a standard boost converter (BC) topology connected to HFCS whereas Figure 4 depicts an interleaved boost converter (IBC) topology coupled to HFCS.

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Figure 3. The standard BC topology

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Figure 4. The IBC topology

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Figure 5. The MISO topology

The isolated boost converter (IBC) configuration is used in conventional design approaches for DC-DC converter topologies, largely to mitigate potential problems related with the operation of high-frequency semiconductor switches. The interleaved DC-DC converter, on the other hand, is used delib-erately to achieve two goals: reducing input current ripples and minimizing switching losses. This novel tech-nique contributes greatly to the overall efficiency of the converter system, distinguishing it from typical converter topologies. However, The IBC topology uses in small electronic components. Meanwhile, some recent studies of DVR technology applied multi-input single-output (MISO) converters topology interfaced with HFCS as shown in Figure 5.

Nevertheless, the impact of passive circuit components, which include inductors and capacitors, on cir-cuit performance is crucial in DC-DC power converter architecture circuits. The voltage that produces ripple is substantially influenced by characteristics such as duty ratio values and the switching frequency of the ac-tive power semiconductor. This ripple, in turn, can exhibit variation contingent upon the prevailing load con-ditions. Furthermore, in the realm of HFCS, voltage doubler boost converters (VDBC) topology is charac-terized by the utilization of four parallel phases as depicted in Figure 6 which have gained widespread adop-tion. Figure 7 illustrates the VDBC topology that incorporates an isolation transformer.

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Figure 6. The configuration of a voltage doubler, boost converter, and dual buck converter

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Figure 7. The VDBC topology, which relies on the utilization of an isolation transformer

Bridge converters are frequently employed in the analysis of isolated DC-DC converter topologies to enhance the stability of the output voltage. Figure 8 depicts the configuration of a dual half-bridge converter (DHBC) in a high-frequency current source (HFCS) system utilizing an isolation transformer. While the dual full-bridge converter (DFBC) topology is more intricate and costly, the power regulation in this converter is less complicated than in other DC-DC converters. This article discusses two types of bridge-based DC-DC converters: the current-fed DHBC and the voltage-fed DHBC. Furthermore, the dual active bridge is a type of bridge converter commonly employed in high-frequency communication systems for interconnecting energy delivery grids. Figure 9 depicts the schematic diagram of a dual-active bridge converter (DCBC) topology. Figure 10 depicts the circuit of a forward converter topology (FCT), which utilizes a transformer to achieve galvanic isolation. On the other hand, Figure 11 illustrates the topology of a high step-up coupled inductor converter (HSUCIC). In addition, the high step-up coupled inductor converter and zero-voltage transition are two other topologies that offer significant voltage conversion ratios. Figure 12 depicts the configuration of zero-voltage transition converters (ZVTC) coupled to high-frequency current sources (HFCS).

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Figure 8. The DHBC topology

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Figure 9. The circuit of DFBC topology

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Figure 10. The circuit of FCT

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Figure 11. The HSUCIC topology

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Figure 12. The ZVTC topology

2.4.2 The DC-AC inverter

The voltage-source inverter (VSI) of DVR technology operates as a power interconnection between the HFCS stack and an EDG network. It converts DC electricity to sinusoidal AC [57], and can produce three-phase waveforms employing the DC/AC inverter. The inverter controls energy by modulating it through six semiconductor switches. Each of them has a controllable on-off state determined by the duty ratio of PWM [58]. The network characteristics will undoubtedly alter with the integration of several power inverters, posing additional technical issues such as sophisticated control systems, sophisticated stability analyses, and systematic protection. The primary function of this VSI-DVR technology is to supervise the real and reactive power flowing through the HFCS and EDG interactions.

2.4.3 Filters

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Figure 13. Several types of passive filters at EDG connected with HFCS

DVR technology relies on filters to ensure effective harmonic protection, reap reactive power ad-vantages, achieve load balancing, compensate for neutral current, and reduce flickering. The inclusion of filters in VSI in DVR technology is linked to improved noise performance, leading to a decrease in size, volume, and cost [59, 60]. Passive filters that make employing DVR technology are commonly used in ELG systems to enhance power quality. The inclusion of voltage harmonics reimbursement, current harmonics re-imbursement, three-phase mains voltage balancing, and three-phase mains current balancing renders it a highly attractive and widely adopted an approach even in the present day. Harmonic distortion is particularly problematic due to the line current produced by non-linear loads. Total harmonic distortion (THD) can cause a multitude of issues, including conductor excessive heat, voltage distortion, transformer failure, unneeded protective device trip, and network interference. As to the IEEE Standard 519-2014, if the THD level at the point of common connection (PCC) falls below 8% at low voltage levels, it is necessary to assess the system. Moreover, IEEE Standard 1547 imposes a restriction on the THD of the injected current, setting the maximum allowable value at 5%. Figure 13 depicts various types of passive filters for DVR technology attached to EDG alongside HFCS.

2.4.4 Transformer

Currently, the benefits of improving the energy efficiency of EDG coupled with HFCS are becoming more evident. This includes enhancing the reliability of power utility systems, reducing operational expenses, improving signal PQ, and minimizing negative environmental impacts. Due to these factors, DVR technology transformer stations have become a crucial component in all EDG systems. Furthermore, distribution transformers are strategically placed at the final stations in order to provide electrical feeders, hence enhancing the efficiency of the distribution chain [61]. Attaining targeted and efficient advancements in standards plays a crucial role in shaping the equilibrium of demand and supply, as well as the success of the network process. This, in turn, determines the smooth functioning of the system. An in-depth analysis of the electrical power requirement of the Emergency Diesel Generator (EDG) has been conducted utilizing distribution transformers, which also provides precise forecasts for the EDG. In this regard, transformers are subject to extremely harsh operating conditions and have inflicted significant harm to unwanted high-frequency impulses as a result of direct lightning impulses, electrical circuit breaker functioning, and system malfunctions [62].

The significant benefits of improving the energy efficiency of EDG coupled with HFCS have become more readily apparent in recent years. This increased efficiency not only improves the performance of mod-ern utilities systems, but it also reduces costs related to operation, improves PQ, and reduces negative environmental impacts. As a direct consequence, transformer stations that employ DVR technology have become essential components of all EDG systems. Furthermore, the well-chosen placement of distribution trans-formers at terminal stations entrusted with delivering electrical feeders improves the overall EDG’s efficiency [61]. In this situation, achieving precise and effective developments in standards emerges as a critical determinant impacting the demand-supply balance. The success of this network process, in turn, dictates the system's smooth operation. Through the use of distribution transformers, a thorough examination of the EDG's consumption is methodically carried out. This approach not only offers precise estimates for the EDG, but it also emphasizes the vital function of transformers, which collaborate in difficult situations. They have been critical in preventing substantial damage resulting from high-frequency impulses generated by circuit breakers, direct lightning strikes, and failures in systems [62].

Tripling the capacity of renewable energy sources constitutes the primary mechanism for achieving emission reductions by 2030 within the context of the Net Zero Emissions by 2050 Scenario. Advanced economies and China are anticipated to attain approximately 85% of the requisite renewable capacity by the year 2030 under existing policy frameworks. However, to realize similar advancements in other developing nations, the implementation of more vigorous policy measures and enhanced international support are imperative. This strategic expansion is essential for aligning global energy practices with the ambitious targets set forth in the NZE Scenario, highlighting a pivotal shift towards sustainable energy systems. Following this, reducing methane emissions from the energy sector yields substantial climate advantages. Within the frame-work of the Net Zero Emissions by 2050 Scenario, it is estimated that approximately USD 75 billion in cumulative investment is needed by 2030 to implement all methane abatement strategies in the oil and gas industry. Remarkably, this investment represents merely 2% of the net income that the oil and gas sector recorded in 2022. This juxtaposition underscores the economic feasibility of significant methane reduction measures relative to the industry's financial capabilities, highlighting an efficient pathway towards mitigating one of the most potent greenhouse gases [76-80].

Emerging technologies like hydrogen and carbon capture, utilization, and storage (CCUS) are projected to play a pivotal role in emissions reduction primarily post-2030. In the context of the Net Zero Emissions by 2050 Scenario (NZE Scenario), if all currently announced projects for hydrogen electrolysis capacity come to fruition, they are anticipated to fulfil approximately 70% of the requisite capacity by 2030. Similarly, the CCUS projects that have been announced, predominantly within advanced economies, are expected to meet nearly 40% of the global requirements by the same year. These figures underscore a critical need for enhanced policy measures aimed at fostering demand for low-emission products and fuels, thereby ensuring these technologies achieve their potential in reducing greenhouse gas emissions effectively and sustainably. In this direction, the realization of hydrogen deployment as outlined in the Net Zero Emissions by 2050 Scenario (NZE Scenario) hinges critically on significant investments in the production, transmission, and distribution of low-emissions hydrogen and hydrogen-based fuels. Current annual investments approximate USD 1 billion; however, this figure must escalate to around USD 150 billion by 2030 to align with NZE tar-gets. Additionally, achieving these goals requires an investment of at least USD 100 billion in dedicated renewable electricity capacity. This substantial increase in funding is essential to establish the infrastructure needed to support a hydrogen economy that contributes effectively to reducing global emissions [81-84].

The advantages associated with the integration of HFCSs within EDGs represent a subject that transcends the confines of the present discourse. HFCSs exhibit the potential to ameliorate power quality (PQ) concerns, notably in addressing voltage sag mitigation. Furthermore, HFCSs have demonstrated their proficiency in load peak shaving through energy storage during off-peak periods and subsequent energy discharge into the distribution grid feeders during peak demand intervals. By employing HFCS-equipped EDGs for this purpose, demand profiles can be rendered more uniform, thereby enhancing PQ. This comprehensive assessment takes into consideration all pertinent capital outlays and economic dividends arising from HFCS adoption, drawing upon the substantiated competencies of this technology. The utilization of HFCSs engenders substantial benefits for the EDG and can be enumerated as follows:

(i) HFCS-equipped EDGs possess the inherent capacity to deliver peaking power without exerting a significant impact on CO2 emissions reduction.

(ii) HFCS-equipped EDGs exert a substantial influence on the enhancement of distribution grid infra-structure, ameliorating the resilience of the network against power quality challenges.

(iii) HFCS-equipped EDGs significantly contribute to the fortification of the EDG system, thereby enhancing its robustness against power quality issues, including voltage sag.

(iv) HFCS-equipped EDGs play a pivotal role in providing support for both reactive and active power, facilitating the maintenance of voltage levels within the EDG.

(v) The HFCS-equipped EDG represents a pivotal technological enabler for the dependable and adaptable integration of various renewable energy sources into diverse applications within the power grid.

(vi) HFCS-equipped EDGs offer an elevated degree of flexibility in the context of static power electronic devices such as DVR, and DSTATCOM.

(vii) The HFCS-equipped EDG is capable of storing substantial quantities of energy, further enhancing its utility and versatility.