The aim of the thesis is the modeling and control of a three-level converter with diode coupling for the grid connection of a megawatt stack of fuel cells. Boost Inductor DC Side Inductor Output Filter Fuel Cell Inductor Mains Current Side Inductor Inverter Side Inductor.
Introduction
They are seen as reliable and one of the most efficient technologies as their efficiency varies between 40 and 60% (Ramezanizadeh et al., 2019). These harmonics adversely affect sensitive equipment above several kilowatts of connected loads (Jeong et al., 2010).
Statement of the Research Problem
Several configurations of HAWTs can be found based on the orientation of the rotors (downwind or downwind), their flexibility (rigid or bent) and the number of blades (two or three). To reduce the size of the whole unit, two inductors as shown in Figure 2.29(B) can be combined. However, the active power controllability through DC link voltage control can be detailed based on the power balance.
This chapter focused on describing the megawatt fuel cell power system and modeling the components involved in it. In addition, the overshoot and undervoltage at the beginning of the simulation were 0.324% and 1.985. In addition, the overshoot and undershoot of this voltage were 0.312 at the beginning of the simulation.
The voltage and current of the grid received from the megawatt fuel cell system were as shown in Figure 4.31. Chapter three focused on the description of the grid connected megawatt fuel cell system and the modeling of components involved on it.
Aim and objectives of the research
Outline of the thesis
The megawatt fuel cell stack consists of several stacks of proton exchange membrane fuel cells (PEMFC) connected in series and parallel. A total of twelve stacks were connected in parallel to obtain the megawatt fuel cell stack.
Publications
International Journals
The final part of this chapter is focused on modeling the control system for the inverter to properly deliver the generated power from the megawatt fuel cell stack to the grid and local load. The megawatt fuel cell system is considered to operate in grid-tied mode, and then a simulation was performed to evaluate the system performance in off-grid mode.
Conference proceedings
The system was developed based on the model of each component, and the designed parameters of the fuel cell, inverter, LCL filter were also presented. It was assumed that the hydrogen and oxygen composition, system temperature, hydrogen and oxygen pressure were kept constant during the simulation, whereas the hydrogen and oxygen flow rates were dependent on Equation 4.1 and Equation 4.2.
Introduction
Alternative Energy Systems
- Solar Power System
- Trough systems
- Power-towers
- Dish engine technologies
- Photovoltaics
- Wind turbine
- Vertical-Axis Wind Turbines (VAWTs)
- Geothermal Power
- Hydropower
- Biomass Energy
- Fuel cell
- Fuel cell stack
- Air subsystem
- Fuel subsystem
- Thermal subsystem
- Electrical subsystem
- Types of fuel cell
A conversion technique uses steam from the reaction in the fuel cell stack to obtain hydrogen from fuel reforming. Cooling relies on the operating temperature of the fuel cell and the external environment of the fuel cell.
Power Quality Requirement for Grid-Connected Alternative Energy Systems 25
- Reactive power control
- Frequency regulation
- Harmonic compensation
- Dynamic grid support of grid-tied alternative energy systems
- Reliability and efficiency control
In the case of more inductive feeders, the grid-connected alternative energy systems can change the exchange of reactive power with the grid to improve the voltage profile of the feeder. Based on Equation 2.1, a power reduction operation of the alternative energy systems allows more reactive power support.
Power Converter Technologies for Alternative Energy Systems
- Modular converters for alternative energy systems
- String inverter topologies
- Single stage inverters
- Double stage inverters
- Centralised inverters
- High power and voltage converters
In addition, as shown in Figure 2.28, DC-to-DC converters with high step-up gains can be used as the front end when using microinverters. In case DC to DC converters are used for multiple alternative energy systems (photovoltaics, fuel cell, etc.) in string configuration, the energy collection can be maximized at the string level, then the power can be collected at the DC output (Figure 2.40) . The typical three-phase centralized inverter is shown in Figure 2.41, coupled through a grid.
Among the multi-level inverter configurations, the three-level neutral point clamped (NPC) inverters shown in Figure 2.42 are the most attractive.
Summary
Introduction
System Description
Centralized and string configurations (Figures 3.1A and 3.1B) can provide the same output power without a converter to increase the fuel cell output voltage. In such a case, a DC to DC boost converter can be associated with each fuel cell and the constant DC outputs from these converters can be connected in parallel. The topology used in this study is the centralized configuration where the megawatt fuel cell stack is connected to the utility grid directly through the inverter without the use of DC/DC converters.
A neutral-clamped three-level inverter converts the fuel cell's 1400 DC voltage to AC to supply a local load and feed excess into the utility grid depending on production conditions.
System Modelling
Modelling the Fuel cell
- Cell reversible voltage
- Activation voltage drop
- Ohmic voltage drop
- Concentration voltage drop
- Cell dynamics
- Power generation
- Designed parameters of the megawatt fuel cell stack
Ψ is a variable parameter with a possible maximum value of 23 which is influenced by the preparation procedure of the membrane. Reduction in the pressure of oxygen and hydrogen is a function of the electric current and the physical properties of the system. The value of the capacitance is only a few Farads, while the resistance 𝑅𝑎 is determined from the cell output current and the calculated activation and concentration voltages.
A typical PEMFC stack is shown in Figure 3.3; 𝑉𝑆 represents the stack output voltage obtained by multiplying the voltage 𝐹𝐶 and the number of cells.
Modelling the Three-level diode clamped inverter
- Input – output characteristics
- Three-Level inverter voltage
- Space Vector PWM (SVPWM)
- Carrier-Based PWM
- Fluctuation of Neutral Point Voltage
- Design parameters of the inverter
If the first switch (𝑆𝑋1) and the second switch (𝑆𝑋2) are on and the third switch (𝑆𝑋3) and the fourth switch (𝑆𝑋4) are off, the output is coupled to the top of the DC side and the resulting output voltage is 𝑉𝑑𝑐⁄2. Therefore, it is possible to represent the output voltage with three legs by using the switching states of the three legs. Nevertheless, the space vector PWM technique involves complex equations and processes prior to generating the output voltage.
In addition, the large vector does not affect the neutral point voltage modification by producing a similar voltage change in the two DC capacitor voltages.
Modelling the Filter
- LCL Filter
- Designed LCL Filter parameters
The main function of the LCL filter is to reduce high order harmonics on the output side of the inverter. The demand for reactive power can cause a resonance of the capacitor interacting with the grid. The LCL filter should reduce the current ripple to 20%, resulting in a ripple current of approximately 5% of the output current (Reznik et al., 2014).
Where 𝐾𝑎 is the expected attenuation and r refers to the ratio of the inductance at the inverter side to that of the grid side.
Modelling the Inverter Control
- Reference frame transformations
- Outer control loop
- Inner current control loop
- Harmonic compensation
- Phase-Locked Loop synchronisation method
- Designed control parameters
Consequently, one alternative is to control the active power of the inverter by controlling the DC switching voltage across the capacitor (Blaabjerg et al., 2006), as depicted in Figure 3.18. In which 𝑣𝑑𝑐, 𝐼𝑑𝑐 and 𝑃𝑑𝑐 are the DC link voltage, inverter input current and power, respectively, 𝑃𝑎𝑐 is the grid active power, and 𝑣𝑑𝑞 is the 𝑣𝑑𝐑𝑞 and 𝑣𝐑𝐑 of the grid voltage and current. When the d-axis of the synchronous rotating frame is aligned with the grid voltage, the q-axis voltage is equal to zero, (𝑣𝑞= 0), so the d-axis voltage is equal to the phase voltage magnitude (𝑣𝑑 = 𝑣𝑚 where 𝑣𝑚 denotes the phase voltage magnitude).
Therefore, the bandwidth of the current control loop is closed to 𝟏 𝟐𝟎⁄ of the sampling frequency (Firdaus et al., 2017).
Summary
Introduction
The results from the simulation are divided into two parts, namely the DC and AC parts. The DC side includes the voltage, current and power generated by the megawatt stack and then fed into the DC link, while the AC side consists of the inverter, filter and control.
DC side Results
The rise time of the voltage across each capacitor was about 4.711 milliseconds which corresponds to the time required for the voltage to rise from 0 to 100% of its final value. Moreover, the voltage overshoot and undershoot at the beginning of the simulation were 120.86% and 1.989%, respectively.
Inverter and AC side results
The phase-to-ground voltage rise time was approximately 2.11 milliseconds and the fall time was 2.141 milliseconds. The rise time of the interphase voltage was approximately 5.853 milliseconds, while the fall time was 5.837 milliseconds. Similarly, the rise time of the phase current was about 5.819 milliseconds, while the fall time was 5.823 milliseconds.
In addition, the voltage overshoot and undershoot at the start of the simulation were 1.99% and 1.99% respectively.
Inverter control
Phase Locked Loop
Similarly, the phase generated by the PLL is depicted in Figure 4.10. The PLL measured the grid voltage and phase angle used to synchronize the dq frame current control. It can be observed that the PLL produced an appropriate phase angle to allow synchronization of the inverter with respect to the grid.
Voltage control
Moreover, the voltage overshoot and undershoot at the beginning of the simulation were 21.341% and 1.994%, respectively.
Current control
This voltage was required for the generation of Ud_ref, which is used to obtain md. This voltage is generally equal to zero, since the reactive power from the network is always controlled. Ud_ref are the output signals for the current errors used in Equation 3.71, along with current errors in Appendix A.11; as can be seen in Figure 4.17(a), its RMS value is about 500 V, however, the signal included an overshoot of about 4.578%, while the undershoot was 9.897%, the corresponding rise and fall times were 16.509 milliseconds and 12.463 milliseconds, respectively. milliseconds.
At the start of the simulation, this active power response was characterized by an overshoot of 14.935% between time t = 0 and t.
Case studies
Case study 3: 900 kW purely resistive load
On the other hand, the rise time of the phase current was about 5.858 milliseconds, while the fall time was 5.855 milliseconds. In addition, the overshoot and undershoot of the currents at the beginning of the simulation was 1.984. Furthermore, the over- and under-estimation of the voltage at the beginning of the simulation was 0.211 % and 1.987 % respectively.
In addition, the overvoltage and undervoltage at the beginning of the simulation were 6.19% and 1.983%, respectively.
Case study 4: 1.26 MW purely resistive load
Moreover, the over and under pressure of this voltage at the beginning of the simulation was 0.323 % and 1.987 % respectively. On the other hand, the rise time of the phase current was about 5.858 milliseconds, while the fall time was 5.856 milliseconds. In addition, the overshoot and undershoot of the currents at the beginning of the simulation were 1.986 % and 0.306 % respectively.
Furthermore, the overshoot and undershoot of the voltage at the beginning of the simulation were 0.206% and 1.986%, respectively.
Case study 5: Off-grid operation of the megawatt fuel cell system without the
The phase-to-phase voltage response had a rise time of approximately 5.867 milliseconds, while the fall time was 5.866 milliseconds. On the other hand, the current response had a rise time of about 0 seconds, and a fall time was 0 seconds since no current was fed into the grid.
Case study 6: Megawatt fuel cell operating in off-grid mode and connected to
Summary
Conclusion
In such a case, the goal of the control measure is to correctly supply the obtained energy to the network. The last part of this chapter was focused on modeling the control system for the inverter to properly deliver the generated power from the megawatt fuel cell stack to the grid and the local load. It was stated that the control of the supply current is important for the nominal operation of the network.
Chapter four dealt with the simulation results of the developed system; a total of twelve PEMFC stacks were connected in parallel to obtain the megawatt fuel cell stack.
Recommendation for further research
Design of proton exchange membrane fuel cell grid-connected system based on resonant current controller. Sensitivity Analysis, Adaptability Improvement and Control of Grid Connected Photovoltaic Power Plants under Grid Frequency Variations. Investigation of water management dynamics on the performance of a Ballard-Mark-V proton exchange membrane fuel cell stack system.
Step-by-step design and control of three-phase LCL filter-based grid-connected inverter.
