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Comparisons of the multilevel solutions for wind turbines.

Table 2 Comparisons of the multilevel solutions for wind turbines.

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Fig. 1. Annual and cumulative installed Wind Power Capacity from 1985 to 2009 (source: BTM Consult).
Fig. 1. Annual and cumulative installed Wind Power Capacity from 1985 to 2009 (source: BTM Consult).
Fig. 2. Wind turbine market share distributed by manufacturers in 2009. (source: BTM Consult).
Fig. 2. Wind turbine market share distributed by manufacturers in 2009. (source: BTM Consult).
Fig. 3. Cumulative PV installed capacity from 2000 to 2009 (source: EPIA, http://www.epia.org) [78]. The annual growth rate is still very high (>30%) especially for the last 3 years. As in the previous years the vast majority o f new capacity was installed in EU with Germany as dominating the market followed by Spain and Italy. USA market is also growing fast According to Pho ton Magazine, the prices for PV modules are decreased by the end of 2009 to around 1.5-2 €/Wp with stronger trends for using thin film technology in order to reach the psychological threshold of 1 €/Wp, which really will trigger the mass- penetration of PV as an energy and power source. In addition to the PV module cost, the cost and reliability of PV inverters are also important issues. The inverter cost share represents about 10-15% of the total investment cost of a grid connected PV system.
Fig. 3. Cumulative PV installed capacity from 2000 to 2009 (source: EPIA, http://www.epia.org) [78]. The annual growth rate is still very high (>30%) especially for the last 3 years. As in the previous years the vast majority o f new capacity was installed in EU with Germany as dominating the market followed by Spain and Italy. USA market is also growing fast According to Pho ton Magazine, the prices for PV modules are decreased by the end of 2009 to around 1.5-2 €/Wp with stronger trends for using thin film technology in order to reach the psychological threshold of 1 €/Wp, which really will trigger the mass- penetration of PV as an energy and power source. In addition to the PV module cost, the cost and reliability of PV inverters are also important issues. The inverter cost share represents about 10-15% of the total investment cost of a grid connected PV system.
The prices for PV inverters in the 1-10 kW range are show this n in Fig. 4. It can be seen that the inverter cost of power class has decreased by more than 50 % during the last decade. The main reasons for this reduction are the increase of the production quantities the implementation of new system technologies . string-inverters). A further 50 % reduction of the ific cost is anticipated during the coming decade h requires the implementation of specific measures in the development and the manufacturing processes.
The prices for PV inverters in the 1-10 kW range are show this n in Fig. 4. It can be seen that the inverter cost of power class has decreased by more than 50 % during the last decade. The main reasons for this reduction are the increase of the production quantities the implementation of new system technologies . string-inverters). A further 50 % reduction of the ific cost is anticipated during the coming decade h requires the implementation of specific measures in the development and the manufacturing processes.
rotational speed will decrease as the radius of the blade increases. For multi-MW wind turbines the rotational speed is 8-13 rpm. So far the most weight efficient way to convert the low-speed, high-torque power to electrical power is to use a gear-box and a standard fixed speed generator as illustrated in Fig. 5
rotational speed will decrease as the radius of the blade increases. For multi-MW wind turbines the rotational speed is 8-13 rpm. So far the most weight efficient way to convert the low-speed, high-torque power to electrical power is to use a gear-box and a standard fixed speed generator as illustrated in Fig. 5
Fig. 6. Roadmap for wind turbine technology from wind to electricity [7].
Fig. 6. Roadmap for wind turbine technology from wind to electricity [7].
betters oe yi Fig. 7. Power characteristics of different wind turbine systems. Passive stall is based on fixed speed operation. The development in wind turbine systems has been steady for the last 35 years [3]-[35] where four to five generations of wind turbines have been developed and the power capacity has increased by a factor of 100. In a wind turbine it is essential to be able to control and limit the converted mechanical power at higher wind speeds, as the power in the wind is a cube of the wind speed. It is also important to maximize the energy capture from the wind below maximum power production. The power limitation is done either by stal control (the blade position is fixed but stall of the wind appears along the blade at higher wind speed), active stall (the blade angle is adjusted in order to create stal along the blades) or pitch control (here the blades are turned out of the wind at higher wind speed). The basic output characteristics of a wind turbine using these three methods of controlling the power are summarized in Fig. 7.
betters oe yi Fig. 7. Power characteristics of different wind turbine systems. Passive stall is based on fixed speed operation. The development in wind turbine systems has been steady for the last 35 years [3]-[35] where four to five generations of wind turbines have been developed and the power capacity has increased by a factor of 100. In a wind turbine it is essential to be able to control and limit the converted mechanical power at higher wind speeds, as the power in the wind is a cube of the wind speed. It is also important to maximize the energy capture from the wind below maximum power production. The power limitation is done either by stal control (the blade position is fixed but stall of the wind appears along the blade at higher wind speed), active stall (the blade angle is adjusted in order to create stal along the blades) or pitch control (here the blades are turned out of the wind at higher wind speed). The basic output characteristics of a wind turbine using these three methods of controlling the power are summarized in Fig. 7.
Fig. 9. Partial variable speed wind turbine with variable rotor resistance. ~ Tt uses a wound rotor induction generator (WRIG) and it has been used by since the mid 1990's. The generator is directly connected to the grid. The rotor winding of the generator is connected in series with a controlled resistance, whose size defines the range of the variable speed (typically 0-10% above
Fig. 9. Partial variable speed wind turbine with variable rotor resistance. ~ Tt uses a wound rotor induction generator (WRIG) and it has been used by since the mid 1990's. The generator is directly connected to the grid. The rotor winding of the generator is connected in series with a controlled resistance, whose size defines the range of the variable speed (typically 0-10% above
Fig. 10. Variable speed wind turbine with partial scale power converter. This configuration is known as the doubly-fed induction generator (DFIG) concept, which gives a variable speed controlled wind turbine with a wound rotor induction generator (WRIG) and partial power- scale frequency converter (rated to approx. 30% of nominal generator power) on the rotor circuit. The topology is shown in Fig. 10. The stator is directly connected to the grid, while a partial-scale power converter controls the rotor frequency and thus the rotor speed. The power rating of this partial-scale frequency converter defines the speed range (typically +30% around synchronous speed). Moreover, this converter performs the reactive power compensation and a smooth grid connection. The control range of the rotor speed is wider compared to the variable rotor resistance type. The smaller frequency converter makes this concept attractive from an economical point of view. In this case the power electronics is enabling the wind turbine to act as a dynamic power source to the grid. However, its main drawbacks are the use of slip-rings and the protection schemes/controllability in the case of grid faults.
Fig. 10. Variable speed wind turbine with partial scale power converter. This configuration is known as the doubly-fed induction generator (DFIG) concept, which gives a variable speed controlled wind turbine with a wound rotor induction generator (WRIG) and partial power- scale frequency converter (rated to approx. 30% of nominal generator power) on the rotor circuit. The topology is shown in Fig. 10. The stator is directly connected to the grid, while a partial-scale power converter controls the rotor frequency and thus the rotor speed. The power rating of this partial-scale frequency converter defines the speed range (typically +30% around synchronous speed). Moreover, this converter performs the reactive power compensation and a smooth grid connection. The control range of the rotor speed is wider compared to the variable rotor resistance type. The smaller frequency converter makes this concept attractive from an economical point of view. In this case the power electronics is enabling the wind turbine to act as a dynamic power source to the grid. However, its main drawbacks are the use of slip-rings and the protection schemes/controllability in the case of grid faults.
Fig. 8. Fixed speed wind turbine with directly grid connected squirrel-cage induction generator. See See ee eS Se he ee ee Pe This configuration corresponds to the so called Danish concept that was very popular in 80’s. This wind turbine has a fixed speed controlled machine, with asynchronous squirrel cage induction generator (SCIG) directly connected to the grid via a transformer as shown in Fig. 8. This concept needs a reactive power compensator to reduce (almost eliminate) the reactive power demand from the turbine generators to the grid. It is usually done by continuously switching capacitor banks following the power production variation (5-25 steps Smoother grid connection occurs by incorporating a soft-starter as shown in Fig. 8. Regardless the aerodynamic power control principle in a fixed speed wind turbine, the wind fluctuations are converted into mechanical fluctuations and further into electrical power fluctuations. These can yield to voltage fluctuations at the point of connection in the case of a weak grid.
Fig. 8. Fixed speed wind turbine with directly grid connected squirrel-cage induction generator. See See ee eS Se he ee ee Pe This configuration corresponds to the so called Danish concept that was very popular in 80’s. This wind turbine has a fixed speed controlled machine, with asynchronous squirrel cage induction generator (SCIG) directly connected to the grid via a transformer as shown in Fig. 8. This concept needs a reactive power compensator to reduce (almost eliminate) the reactive power demand from the turbine generators to the grid. It is usually done by continuously switching capacitor banks following the power production variation (5-25 steps Smoother grid connection occurs by incorporating a soft-starter as shown in Fig. 8. Regardless the aerodynamic power control principle in a fixed speed wind turbine, the wind fluctuations are converted into mechanical fluctuations and further into electrical power fluctuations. These can yield to voltage fluctuations at the point of connection in the case of a weak grid.
System comparison of wind turbine configurations.
System comparison of wind turbine configurations.
Fig. 12. Two-level back-to-back voltage source converter for wind turbines. (2L BTB).
Fig. 12. Two-level back-to-back voltage source converter for wind turbines. (2L BTB).
Fig. 16. Three-level Neutral Point Clamped and five-level H-bridge converter for wind turbines. (3L-NPC + 5L-HB).
Fig. 16. Three-level Neutral Point Clamped and five-level H-bridge converter for wind turbines. (3L-NPC + 5L-HB).
Fig. 14. Three-level H-bridge back-to-back converter for wind turbines. (3L-HB BTB).
Fig. 14. Three-level H-bridge back-to-back converter for wind turbines. (3L-HB BTB).
Fig. 15. Five-level H-bridge back-to-back converter for wind turbines. (5L-HB BTB). With the same voltage rating switching devices, 5L- HB BTB can achieve five level output voltage, and doubled voltage amplitude compared to the 3L-HB BTB solution. These features enable smaller output filter and less current rating in the switching devices as well as in the cables [38], [45].
Fig. 15. Five-level H-bridge back-to-back converter for wind turbines. (5L-HB BTB). With the same voltage rating switching devices, 5L- HB BTB can achieve five level output voltage, and doubled voltage amplitude compared to the 3L-HB BTB solution. These features enable smaller output filter and less current rating in the switching devices as well as in the cables [38], [45].
The DC/DC converters with medium frequency transformer (MFT) operate at several kHz to dozens of kHz, the transformer size is thereby reduced. Because of the cascaded structure, this configuration can be directly connected to the transmission power grid (10 kV-20 kV) with high output voltage quality, filter-less design, and the redundancy ability [47]-[49]. Fig. 17. Cascade H-bridge back-to back converter for wind turbines with Medium Frequency Transformer. (CHB-MFT).
The DC/DC converters with medium frequency transformer (MFT) operate at several kHz to dozens of kHz, the transformer size is thereby reduced. Because of the cascaded structure, this configuration can be directly connected to the transmission power grid (10 kV-20 kV) with high output voltage quality, filter-less design, and the redundancy ability [47]-[49]. Fig. 17. Cascade H-bridge back-to back converter for wind turbines with Medium Frequency Transformer. (CHB-MFT).
Fig. 18. Converter cell of the Cascade H-bridge back-to-back converter with Medium Frequency Transformer.
Fig. 18. Converter cell of the Cascade H-bridge back-to-back converter with Medium Frequency Transformer.
Another solution for the electrical power control is to use the multi-pole synchronous generator and a full scale power converter. A passive rectifier and a boost converter can be used in order to boost the voltage at low speed. The system is industrially used today and it is shown in Fig. 21.
Another solution for the electrical power control is to use the multi-pole synchronous generator and a full scale power converter. A passive rectifier and a boost converter can be used in order to boost the voltage at low speed. The system is industrially used today and it is shown in Fig. 21.
Fig. 23. Regulation functions for active power control implemented in wind farm controller required by the Danish grid codes. a) delta control, b) balance control, c) absolute production constraint and d) system protection.
Fig. 23. Regulation functions for active power control implemented in wind farm controller required by the Danish grid codes. a) delta control, b) balance control, c) absolute production constraint and d) system protection.
All considered grid codes require fault ride-through capabilities for wind turbines to overcome grid faults. Voltage profiles are given specifying the depth of the voltage dip and the clearance time as well. One of the problems is that the calculation of the voltage during all types of unsymmetrical faults is not very well defined in some grid codes. The voltage profile for ride-through capability can be summarized as shown in Fig. 26. Fig. 26. Voltage profile for fault ride-through capability in European grid codes for wind power [61]. Ireland’s grid code is very demanding in respect to the fault duration while Denmark has the lowest short circuit time duration with only 100 ms. However, the grid code in Denmark requires that the wind turbine shall remain connected to the electrical network during successive faults, which is a technical challenge.
All considered grid codes require fault ride-through capabilities for wind turbines to overcome grid faults. Voltage profiles are given specifying the depth of the voltage dip and the clearance time as well. One of the problems is that the calculation of the voltage during all types of unsymmetrical faults is not very well defined in some grid codes. The voltage profile for ride-through capability can be summarized as shown in Fig. 26. Fig. 26. Voltage profile for fault ride-through capability in European grid codes for wind power [61]. Ireland’s grid code is very demanding in respect to the fault duration while Denmark has the lowest short circuit time duration with only 100 ms. However, the grid code in Denmark requires that the wind turbine shall remain connected to the electrical network during successive faults, which is a technical challenge.
Fig. 25. Requirements for reactive power provision of generating units without limiting the active power output in the German transmission grid code [55].
Fig. 25. Requirements for reactive power provision of generating units without limiting the active power output in the German transmission grid code [55].
Fig. 24. Danish grid code demands for the reactive power exchange in the PCC [51], [52].
Fig. 24. Danish grid code demands for the reactive power exchange in the PCC [51], [52].
Some typical curves of a PV cell current-voltage and power-voltage characteristics are shown in Fig. 29.
Some typical curves of a PV cell current-voltage and power-voltage characteristics are shown in Fig. 29.
Fig. 28. Ideal equivalent circuit of a PV cell.
Fig. 28. Ideal equivalent circuit of a PV cell.
Fig. 27. Reactive current support during faults as specified in the German grid code [61].
Fig. 27. Reactive current support during faults as specified in the German grid code [61].
Fig. 30. Different grid-connected PV inverter structures. a)C entral inverter; b) String inverter; c) Module inverter; d) Multi-string inverter. The Multi-String concept is a flexible solution, having a high overall efficiency of power extraction, due to the fact that each PV string is individually controlled.
Fig. 30. Different grid-connected PV inverter structures. a)C entral inverter; b) String inverter; c) Module inverter; d) Multi-string inverter. The Multi-String concept is a flexible solution, having a high overall efficiency of power extraction, due to the fact that each PV string is individually controlled.
Fig. 31. Comparison of commercial PV inverters in terms of efficiency, weight and volume [79].
Fig. 31. Comparison of commercial PV inverters in terms of efficiency, weight and volume [79].
Fig. 32. Parasitic capacitance in PV panels [73].
Fig. 32. Parasitic capacitance in PV panels [73].
Fig. 33. Transformer-less PV system showing the parasitic capacitance between the PV and the grounded frame of the array anc the path of the alternating ground leakage current.
Fig. 33. Transformer-less PV system showing the parasitic capacitance between the PV and the grounded frame of the array anc the path of the alternating ground leakage current.
Fig. 35. Experiments on full-bridge topology. 1) bipolar output of the converter, 2) the voltage to ground (DC- terminal of the PV array), 3) the leakage ground current for the single-phase inverter with bipolar PWM.
Fig. 35. Experiments on full-bridge topology. 1) bipolar output of the converter, 2) the voltage to ground (DC- terminal of the PV array), 3) the leakage ground current for the single-phase inverter with bipolar PWM.
Fig. 34. Full-bridge topology with bipolar PWM. The H-Bridge is a well-known topology and it is made up of two half bridges. This topology has also been used in motor drives and UPS applications. To control the four switches in this topology several PWM techniques can be implemented. The simplest one is the bipolar PWM, which modulates switches T1-T4 (see Fig. 34) complementary to T2-T3, resulting in a two evel output voltage. The conversion efficiency is reduced due to the fact that during the free-wheeling period the grid current finds a path and flows back to the DC-link capacitor. Fig. 35 shows some measurements on such a converter.
Fig. 34. Full-bridge topology with bipolar PWM. The H-Bridge is a well-known topology and it is made up of two half bridges. This topology has also been used in motor drives and UPS applications. To control the four switches in this topology several PWM techniques can be implemented. The simplest one is the bipolar PWM, which modulates switches T1-T4 (see Fig. 34) complementary to T2-T3, resulting in a two evel output voltage. The conversion efficiency is reduced due to the fact that during the free-wheeling period the grid current finds a path and flows back to the DC-link capacitor. Fig. 35 shows some measurements on such a converter.
Using the H-Bridge topology with different PWM schemes than the bipolar one, will result in unipolar output vo switching the unipo the freew. tage (+Vdc, 0 and -Vdc) that has twice the frequency. The advantage of this method is that the grid side filter needs to be much smaller due to ar output voltage of the converter and also due to the fact that the frequency of the output voltage is twice the switching frequency. Furthermore, during heeling period, the grid current finds a path through the short-circuited output of the converter, either through T1-T3, as shown in Fig. 36 or similarly through T2-T4. On the other hand, there is a significant disadvantage using unipolar PWM transformer-less PV systems, regarding the voltage to ground of the PV array and the ground leakage currents. As shown in Fig. 37, the modulation strategy generates a varying common-mode voltage. An FFT of the voltage to ground, as detailed in Fig. 38, shows that voltage components at the switching frequency have very high amplitudes in the range of Vac.
Using the H-Bridge topology with different PWM schemes than the bipolar one, will result in unipolar output vo switching the unipo the freew. tage (+Vdc, 0 and -Vdc) that has twice the frequency. The advantage of this method is that the grid side filter needs to be much smaller due to ar output voltage of the converter and also due to the fact that the frequency of the output voltage is twice the switching frequency. Furthermore, during heeling period, the grid current finds a path through the short-circuited output of the converter, either through T1-T3, as shown in Fig. 36 or similarly through T2-T4. On the other hand, there is a significant disadvantage using unipolar PWM transformer-less PV systems, regarding the voltage to ground of the PV array and the ground leakage currents. As shown in Fig. 37, the modulation strategy generates a varying common-mode voltage. An FFT of the voltage to ground, as detailed in Fig. 38, shows that voltage components at the switching frequency have very high amplitudes in the range of Vac.
Fig. 37. Experiments on a full-bridge topology with unipolar PWM. 1) unipolar output of the converter, 2) voltage to ground (DC- terminal of the PV array), 3) leakage ground current for the single- phase inverter with unipolar PWM.
Fig. 37. Experiments on a full-bridge topology with unipolar PWM. 1) unipolar output of the converter, 2) voltage to ground (DC- terminal of the PV array), 3) leakage ground current for the single- phase inverter with unipolar PWM.
Fig. 39. Highly Efficient and Reliable Inverter Concept (HERIC) showing path of the current during the zero voltage state, for positive load current [104]. To keep high efficiency and all the advantages given by the unipolar PWM, but still have the common-mode behavior as in case of the bipolar PWM, the H-Bridge topology may be modified [104], the Highly Efficient and Reliable Inverter Concept (HERIC). The modification includes two extra switches (T5-T6) each connected in series with a diode. During the zero voltage vector, depending on the sign of the reference voltage, either T5 of T6 are tured ON, while T1, T2, T3 and T4 are all in their OFF state and the PV array is disconnected from the grid. In this way there is a possibility to achieve the zero voltage state and the output voltage will be unipolar, having the same frequency as the switching frequency and there will be no high frequency fluctuations present at the DC terminals of the PV array. Furthermore, the efficiency of the inverter is still kept high, because during the freewheeling period, the load current is short-circuited through T5 or T6, depending on the sign of the grid current.
Fig. 39. Highly Efficient and Reliable Inverter Concept (HERIC) showing path of the current during the zero voltage state, for positive load current [104]. To keep high efficiency and all the advantages given by the unipolar PWM, but still have the common-mode behavior as in case of the bipolar PWM, the H-Bridge topology may be modified [104], the Highly Efficient and Reliable Inverter Concept (HERIC). The modification includes two extra switches (T5-T6) each connected in series with a diode. During the zero voltage vector, depending on the sign of the reference voltage, either T5 of T6 are tured ON, while T1, T2, T3 and T4 are all in their OFF state and the PV array is disconnected from the grid. In this way there is a possibility to achieve the zero voltage state and the output voltage will be unipolar, having the same frequency as the switching frequency and there will be no high frequency fluctuations present at the DC terminals of the PV array. Furthermore, the efficiency of the inverter is still kept high, because during the freewheeling period, the load current is short-circuited through T5 or T6, depending on the sign of the grid current.
Fig. ‘40. H5-topology for high efficiency. Path of the current in the case of positive voltage state, for positive load current.
Fig. ‘40. H5-topology for high efficiency. Path of the current in the case of positive voltage state, for positive load current.
Fig. 41. Single-phase topology with DC decoupling used for transformerless PV systems. Another topology using the DC decoupling method is the one presented in [106], which adds two extra switches and two extra diodes to the H-Bridge topology as shown in Fig. 41. The modulation strategy is also a hybrid type. The active voltage vector is achieved by switching T5-T6 with high frequency. Switches T1-T4 are switched with the grid frequency and in anti-parallel to T2-T3, depending on whether the reference voltage is in the positive or negative half period. In this way the output voltage of the converter will be a unipolar voltage, like in case of the HERIC and H5 topologies.
Fig. 41. Single-phase topology with DC decoupling used for transformerless PV systems. Another topology using the DC decoupling method is the one presented in [106], which adds two extra switches and two extra diodes to the H-Bridge topology as shown in Fig. 41. The modulation strategy is also a hybrid type. The active voltage vector is achieved by switching T5-T6 with high frequency. Switches T1-T4 are switched with the grid frequency and in anti-parallel to T2-T3, depending on whether the reference voltage is in the positive or negative half period. In this way the output voltage of the converter will be a unipolar voltage, like in case of the HERIC and H5 topologies.
Another solution for generating the zero voltage state can be done using a bidirectional switch made of one GBT and one diode-bridge [109]. The topology is shown in Fig. 42, using the bidirectional switch, as an auxiliary component with a grey background. This bidirectional switch is clamped to the midpoint of the DC-link capacitors in order to fix the potential of the PV array also during the zero voltage state when T1-T4 and T2-T3 are open. An extra diode is used to protect from short-circuiting the lower DC-link capacitor.
Another solution for generating the zero voltage state can be done using a bidirectional switch made of one GBT and one diode-bridge [109]. The topology is shown in Fig. 42, using the bidirectional switch, as an auxiliary component with a grey background. This bidirectional switch is clamped to the midpoint of the DC-link capacitors in order to fix the potential of the PV array also during the zero voltage state when T1-T4 and T2-T3 are open. An extra diode is used to protect from short-circuiting the lower DC-link capacitor.
Fig. 44. Patented topology from Refu Solar [111] for single-phase PV -inverter.
Fig. 44. Patented topology from Refu Solar [111] for single-phase PV -inverter.
The NPC topology is suitable for transformer-less PV systems, since the voltage to ground is constant in case of both terminals of the PV. The only drawback for the single-phase NPC topology is the high DC-link voltage, which has more than twice the grid peak voltage and might reach voltages higher than the allowed maximum system voltage, therefore a boost stage is needed before the inverter, which decreases the overall efficiency of the whole PV system. The Neutral Point Clamped (NPC) topology was introduced some years ago in [110] and has mostly been used for applications in AC drives. The voltage to ground measured at both PV array terminals is constant, when the switching ripple is not taken into consideration. This is due to the connection of the neutral line of the middle point of the DC-link that fixes the potential of the PV array to the grounded neutral [108]. Fig. 43 shows this topology.
The NPC topology is suitable for transformer-less PV systems, since the voltage to ground is constant in case of both terminals of the PV. The only drawback for the single-phase NPC topology is the high DC-link voltage, which has more than twice the grid peak voltage and might reach voltages higher than the allowed maximum system voltage, therefore a boost stage is needed before the inverter, which decreases the overall efficiency of the whole PV system. The Neutral Point Clamped (NPC) topology was introduced some years ago in [110] and has mostly been used for applications in AC drives. The voltage to ground measured at both PV array terminals is constant, when the switching ripple is not taken into consideration. This is due to the connection of the neutral line of the middle point of the DC-link that fixes the potential of the PV array to the grounded neutral [108]. Fig. 43 shows this topology.
Fig. 45. Conergy topology used for single-phase converter [112].
Fig. 45. Conergy topology used for single-phase converter [112].
It is also shown in Fig. 47, where the voltage to ground varies with the switching frequency and changes according to the PWM strategy. Fig. 47. Measurements for 3FB topology. 4) shows the voltage to ground of the DC+ terminal [250V/div]. Fig. 46. FFT of voltage to ground for the 3FB topology.
It is also shown in Fig. 47, where the voltage to ground varies with the switching frequency and changes according to the PWM strategy. Fig. 47. Measurements for 3FB topology. 4) shows the voltage to ground of the DC+ terminal [250V/div]. Fig. 46. FFT of voltage to ground for the 3FB topology.
frequency and multiples of it, having high amplitude using a 3FB topology.
frequency and multiples of it, having high amplitude using a 3FB topology.
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