Energy

Let us first discuss the global energy perspective [1–6]. Figure 1.1 shows the global energy generation and the generation of US energy in the same perspective. About 84% of global energy is generated by fossil fuels, 3% from nuclear plants and the remaining 13% comes from renewable sources, such as hydro, wind, solar, biofuels, geothermal, wave and tidal power. US energy generation follows a similar pattern. About 41% of US energy comes from oil, of which a significant part is imported. The present availability of shale oil (and natural gas) is decreasing this dependence, and according to the forecast of the International Energy Agency (IEA), the United States will be energy independent in the future. The global per capita energy consumption is highest in the United States. With nearly 4.5% of world’s pop- ulation (313 million out of 7 billion), the United States consumes nearly 28% of global energy and this reflects a very high standard of living. In comparison, China, now the world’s second largest economy, with nearly 19% of the world’s population (1.34 billion), consumes nearly half the total energy con- sumed by the United States. Of course this scenario is changing fast because of the rapid industrialization of China.

Figure 1.2 shows the idealized energy depletion curves of fossil and nuclear fuels throughout the world, which are somewhat Gaussian in nature. The world has enormous reserves of coal, and at the present consumption rate it is expected to last approximately 200 years. From the oil depletion curve, it appears that we are presently near the peak and is expected to become exhausted in 100 years. The oil price has been increasing recently because of the rising demand and dwindling supply. The natural gas reserves are expected to last around 150 years.

The recent availability of large amounts of shale oil and gas is creating an economic boom in some countries, particularly in the United States, as mentioned previously. Uranium (U-235) has a very low reserve and is expected to become exhausted in around 50 years. With adequate conservation, the curves in the figure can be flattened in order to last for a longer period. Exploration of new fuel sources, particularly offshore resources, can provide new sources of oil and gas. Renewable energy resources (not included in the figure) will theoretically extend the energy depletion curve to infinity. It is no wonder that due to their competitive costs, extensive availability and environ- mentally clean nature, renewable energy sources are now being extensively explored all over the world. Recent studies show that renewable energy alone (with adequate storage) can supply all the energy needs of the world.

Environmental Pollution: Global Warming Problem

Unfortunately, fossil fuel burning generates gases (SO2, CO, NOX, HC and CO2) that cause environ- mental pollution. The most dominant effects of fossil fuel burning are climate change or global warming problems [3, 4], which is mainly caused by CO2 and other gases – called greenhouse gases (GHG). GHG trap solar heat from the atmosphere (called the greenhouse effect). The United Nations (UN) Intergov- ernmental Panel on Climate Change (IPCC) has ascertained that the man-made burning of fossil fuels causes the global warming problem. It may be mentioned here that nuclear power does not have the traditional environmental pollution problem, but the safety of nuclear plants with regard to the radiation hazard is of serious concern. Another problem with nuclear power is that the waste from nuclear plants remains radioactive for thousands of years, and we do not know how to safely dispose of such waste. It is possible that in the future this waste will cause considerable damage to our society.

Control Scheme

The control scheme for PTC is shown in Figure 19.11. The observer uses the load currents to generate the rotor fluxes, stator fluxes and a delay-compensated version of the sampled currents. These variables are used to generate the torque and stator flux predictions for each converter state. As shown in the earlier section, the predictions are evaluated in the cost function using the flux and torque references, and the optimal state is selected and applied directly to the converter. In this control scheme, the torque reference is generated by an external PI controller that controls the speed. Figure 19.12 shows the experimental waveforms of a speed reversal using PTC. The results show a dynamic similarity to DTC in both torque and current. As with the case of DTC, the control of the switching frequency has also been addressed in the literature [46, 47]. One of the main advantages of PTC over DTC is the simplicity of control implementation when the circuit topology changes [48, 49]. For example, a three-level neutral-point-clamped inverter requires the use of several switching tables and an algorithm to select one of these tables for each sample time. Another advantage of PTC is its flexibility to incorporate different control objectives, such as minimization of the switching frequency [50], reduction of the common-mode voltage [16], or control of the input reactive power [51, 52]. The next section will address both concepts: topology changes and additional control objectives.

Simulation and Experimental Results

UnA Matlab/Simulink model can be used to verify the PWM algorithm presented above. A simulation example is presented here. The input voltage is kept at 100 V peak to show the exact voltage transfer ratio at the output side, and the switching frequency of the devices is kept at 6 kHz. (Different parameters can be set in the Matlab/Simulink model.) The load connected to the Matrix converter is R–L with parameter values R=15 Ω and L=10 mH. The above parameters are utilized for the carrier-based mod- ulation scheme. Similarly, the DDPWM parameters are R=15 Ω, L=12 mH, output frequency =40 Hz and switching frequency=10 kHz. In the case of the space vector modulation technique, the parameters are R=12 Ω, L=40 mH, output frequency=40 Hz and switching frequency=6 kHz. The operation of the topology of the matrix converter is tested for a wide range of frequencies, from as low as 1 Hz to higher frequencies, considering deep-flux weakening operation. The simulation results are shown for different modulation techniques (Figures 15.15 to 15.23).

PV Power Generation

In past designs, a centralized converter-based PV system was the most commonly used type of PV system. As shown in Figure 5.16, in this system, PV modules are connected to a three-phase voltage source inverter. The output of each phase of the inverter is connected to an LC filter to limit the harmonics. A three-phase transformer, which steps up the voltage and provides galvanic isolation, connects the inverter to the utility [18–26]. Low-frequency transformers are considered poor components mainly because of their large size and low efficiency. To avoid the need for low-frequency transformers, multiple-stage converters are widely used in PV systems. The most common topology, which is represented in Figure 5.17, includes a volt- age source inverter and a dc-dc converter. Commonly, the dc-dc converter contains a high-frequency transformer. Despite offering a high boosting capability and galvanic isolation, this converter consists of multiple power processing stages, which lower the efficiency of the overall system. Moreover, bulky electrolytic capacitors are required for the dc link. Electrolytic capacitors, which are very sensitive to temperature, might cause severe reliability problems in inverters, and an increase by even 10 ∘C can halve their lifetime. Therefore, PV inverters containing electrolytic capacitors are not expected to pro- vide the same lifetime as the PV modules. Consequently, the actual cost of the PV system involves periodic replacement of the inverter, which increases the levelized cost of energy extracted from the PV system. Considering the aforementioned problems, it is essential to support the design of alternative inverter topologies with higher reliability and lower cost [18–26]. The ac-link universal power converter can overcome most of the problems associated with existing PV inverters. As mentioned earlier, the control scheme of this converter guarantees the isolation of the input and output. However, if galvanic isolation is required, the link inductor can be replaced with a single-phase high-frequency transformer, which eliminates the need for low-frequency transformers employed in traditional centralized converter-based PV systems. With or without a transformer, the proposed inverter is capable of stepping up or stepping down the voltage. The other merits of the proposed inverter are the elimination of the dc link and the replacement of the bulky electrolytic capacitors employed in the multiple-stage conversion systems with an LC pair having alternating current and voltage. Because the direction of power flow in a PV inverter is always from the PV toward the load, the PV-side switches do not need to be bidirectional. Figure 5.18 shows the soft switching ac-link PV inverter. Although this inverter does not have a dc link, it can inject reactive power into the grid during voltage sags. To provide the low-voltage ride-through (LVRT) feature, the PV-side switches should be replaced by bidirectional switches, as illustrated in Figure 5.19. The principle of operation of the inverter dur- ing the grid fault is slightly different from that of normal operation. During mode 1, the link will be charged through the PV up to a certain level. Then, similarly to normal operation, it will be discharged into the output phases; however, no net energy is taken from the link in this case. After the output-side switches are turned off, the energy stored in the link is discharged into the PV-side capacitor. In this case, the PV-side filter capacitor absorbs the energy discharged into the input. Therefore, in order to provide the LVRT feature, the PV-side filter capacitor needs to be designed based on the reactive power rating of the inverter. Once the link is completely discharged, it will be recharged from the PV with current flowing in the opposite direction. In fact, this method of control can also be used for normal opera- tion. In the case of normal operation, the energy remaining in the link after turning off the output-side

Principle of Operation of the Soft Switching ac-Link Universal Power Converter

The principle of operation of the soft switching ac-link universal power converter is similar to that of the hard switching ac-link universal power converter. The main difference is that between each charging and discharging mode there is a resonating mode during which none of the switches conduct and the link resonates to facilitate the zero voltage turn-on and soft turn-off of the switches. To explain the principle of operation, a three-phase ac–ac converter is considered. The basic operating modes and relevant waveforms of this converter are represented in Figures 5.7 and 5.8. Each link cycle is divided into 16 modes, with 8 power transfer modes and 8 partial resonant modes taking place alternately. The link is energized from the input phase pairs during modes 1, 3, 9, and 11 and is de-energized to the output phase pairs during modes 5, 7, 13 and 15. Modes 2, 4, 6, 8, 10, 12, 14 and 16 are resonating modes. The following are the details of various operating modes: Mode 1 (energizing): Before the start of mode 1, input switches that are supposed to conduct during modes 1 and 3 are activated (S6, S10 and S11 in Figures 5.7 and 5.8); however, they do not immediately conduct because they are reverse biased. Once the link voltage, which is resonating before mode 1, becomes equal to the maximum input line-to-line voltage that is supposed to charge the link (VAB in Figures 5.7 and 5.8), the proper switches (S6 and S10) become forward biased, initiating mode 1. Therefore, the link is connected to the input voltage pair having the highest voltage via switches that charge it in the positive direction. Owing to the high frequency of the link, VAB can be assumed constant during mode 1. The link current (i Link) during mode 1 can be calculated using the following equations: VAB = L di Link(t) dt (5.1) i Link(t) = 1 L∫ t 0 VABdt = VABt L + iLink(0) (5.2) In the above equations, L is the link inductance. During this mode, the link voltage is equal to VAB, as shown in Figure 5.7. The link charges until the current of phase B on the input side, when averaged over a cycle, meets its reference value. It is assumed that phase A carries the maximum input current; hence, it will be involved in charging the link during both modes 1 and 3. At the end of mode 1, switch S10 is turned off. As mentioned earlier, the link capacitor acts as a buffer across the switches during their turn-off, which results in negligible turn-off losses. Mode 2 (partial resonance): During this mode, none of the switches conduct and the link resonates until its voltage becomes equal to that of the other input phase pair, which is supposed to charge the link (VAC in Figures 5.7 and 5.8). The voltage across this phase pair is lower than the voltage across the

Aurther

Kamal Al-Haddad

Downlaod Book