28-01-2013, 11:54 AM
Battery Energy Storage Systems (BESSs)
[attachment=48573]
ABSTRACT
For larger systems over 10kW, three-phase inverters are most often used. All the configurations described for single-phase utility connection can be used for the three phase. Again, the isolation from the grid can be done either by using a line-frequency transformer or a high-frequency transformer. In the later case, an additional converter is required for converting DC from PV into high-frequency AC.
Figure above shows a typical topology for a three-phase PV inverter utilizing a line-frequency three-phase transformer. The DC output of the PV array is connected across a filter capacitor. The output of the capacitor connects to the input of a voltage-source three-phase inverter. The output of each phase of the converters is connected to an inductor and capacitor to limit the high-frequency harmonics injected into the AC system. A synthesized AC output voltage is produced by appropriately controlling the switches. A three-phase transformer is then used to connect the power to the utility.
Different configurations without transformers can be used for the PV system. However such topologies are more for the European countries and Japan where system grounding is not mandatory for the PV inverters. In the United States, the National Electrical Code (NEC) Article 690 requires that the PV modules be system grounded and monitored for ground faults when the maximum output voltage of the PV modules reaches a certain level (e.g., 50 V) (Kjaer et al. 2005). Only a few transformer-free, high-input voltage topologies that can be grounded both at the input and at the output are yet known. Modern inverters tend to use a high-frequency transformer for galvanic isolation. This technology results in entirely new designs, such as the printed circuit board (PCB) integrated magnetic components (Kjaer et al. 2005). One such design is shown in Figure 9, where the transformers are embedded in high-frequency DC-DC converters. Such a topology is also very useful for multi-string configurations, where each of the strings can be connected to a common DC bus and then converted to grid compatible AC by using a single DC-AC inverter.
[attachment=48573]
ABSTRACT
For larger systems over 10kW, three-phase inverters are most often used. All the configurations described for single-phase utility connection can be used for the three phase. Again, the isolation from the grid can be done either by using a line-frequency transformer or a high-frequency transformer. In the later case, an additional converter is required for converting DC from PV into high-frequency AC.
Figure above shows a typical topology for a three-phase PV inverter utilizing a line-frequency three-phase transformer. The DC output of the PV array is connected across a filter capacitor. The output of the capacitor connects to the input of a voltage-source three-phase inverter. The output of each phase of the converters is connected to an inductor and capacitor to limit the high-frequency harmonics injected into the AC system. A synthesized AC output voltage is produced by appropriately controlling the switches. A three-phase transformer is then used to connect the power to the utility.
Different configurations without transformers can be used for the PV system. However such topologies are more for the European countries and Japan where system grounding is not mandatory for the PV inverters. In the United States, the National Electrical Code (NEC) Article 690 requires that the PV modules be system grounded and monitored for ground faults when the maximum output voltage of the PV modules reaches a certain level (e.g., 50 V) (Kjaer et al. 2005). Only a few transformer-free, high-input voltage topologies that can be grounded both at the input and at the output are yet known. Modern inverters tend to use a high-frequency transformer for galvanic isolation. This technology results in entirely new designs, such as the printed circuit board (PCB) integrated magnetic components (Kjaer et al. 2005). One such design is shown in Figure 9, where the transformers are embedded in high-frequency DC-DC converters. Such a topology is also very useful for multi-string configurations, where each of the strings can be connected to a common DC bus and then converted to grid compatible AC by using a single DC-AC inverter.