As noted earlier, only level 1 and level 2 are feasible within the confines of a residential setting. This can be accomplished through integrated chargers when available or an external EVSE. In the latter case, the most obvious circuit configuration is a single-phase bi-directional rectifier/inverter powered by a 60A/ 240 VAC circuit that is readily available from the distribution transformer. The DC-link voltage is then processed by a bi-directional DC-DC converter that performs the isolation function. This simple topology shown in Fig. 14.8 can be called the canonical topology as will be repeated, with minor changes, for most grid-tied system irrespective of power rating.
In North America, the 240 V from the residential distribution transformer is in the form of a split 120 V supply, suggesting small modifications to the canonical topology. Figure 14.9 shows two possibilities:
The two topologies in the figure are similar, but the one on the right has better voltage utilization and is better equipped to counter unbalanced loads on the split supply .
For the DC-DC converter, many bi-directional isolated circuit topologies have been proposed . Typical circuits are shown in Fig. 14.10.
When the two controlled bridges are independently driven in phase-shift modulation (PSM), these are generally referred to as dual active bridge (DAB) topologies. In their simplest operation mode, when power needs to be transferred from the left-side circuit to the right-side circuit, for instance, the right-side IGBT switches are left undriven, leaving their antiparallel diodes in the form of a regular diode bridge. Under these circumstances, the topology becomes identical to a regular PSM converter, which is simple to operate, but not very flexible in terms of voltage gain. On the other hand, when both bridges are modulated, power transfer
Fig. 14.11 Configuration with isolation at the grid
can be accomplished in both directions and with great variability ranges on the input and output voltages. In addition, zero voltage switching (ZVS) can be assured for all switches for reduced switching loss and generated electrical noise (EMI).
Other topologies [37, 38] based on the DAB have been proposed with purported additional benefits, such as better switch utilization, extended ZVS operating range, and more flexible voltage amplification.