Ungrounded PV Power Systems in the NEC: Page 5 of 12
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While modern transformers are exceptionally efficient, losses that can never be entirely eliminated occur in the core and in the windings. These losses are dissipated as waste heat, which is one of the reasons that 60-Hz transformer-based inverters often have relatively large heat sinks.
Since large transformers are generally more efficient than smaller ones, it is not uncommon to see a higher efficiency differential between isolated and non-isolated inverters at smaller inverter capacities. For example, SMA America’s 8 kW transformerless inverter (SB 8000TL-US) has a CEC efficiency of 98%, which is a full 2% higher than the efficiency of its 8 kW transformer-isolated inverter (SB 8000US). Generally speaking, non-isolated inverters are 1%–2% more efficient than equivalent isolated inverters.
Output filtering can easily be accomplished with the addition of properly designed filter elements, such as an additional inductor, capacitor or both. Voltage step-up can be addressed with higher dc utilization voltages. For single-phase inverters, operating voltages in the 330–600 Vdc range are generally adequate for utility interconnection. However, interconnecting a single-stage, non-isolated inverter to a 480 Vac 3-phase system requires higher utilization voltages, such as 1,000 Vdc. Alternately, a boost stage can be added to a 600 Vdc non-isolated inverter to allow for 480 Vac interconnection or to allow for lower minimum input voltages and a wider MPPT window. Decoupling is the critical issue when moving from grounded to ungrounded systems.
Figures 2a and 2b show an inverter schematic with and without an isolation transformer. In both diagrams, the dc system is grounded. With the isolation transformer (Figure 2a), there is no direct path between the dc and ac grounds. However, once the transformer is replaced with line inductors (Figure 2b), there is a direct short through the coupled grounds.
To avoid the coupled-ground problem, either the ac or the dc ground must be removed. It is not possible to remove the ac system ground for premises wiring of less than 1,000 V because the NEC does not permit this. However, the removal of the dc system ground is addressed in NEC Section 690.35, “Ungrounded Photovoltaic Power Systems.” —Adapted with permission from SolarWorld Engineering Bulletin 1002–2010 (see Resources)
Improved economics. Since non-isolated inverters are more efficient, they have the potential to increase a PV system’s specific yield and improve a customer’s return on investment as a result. In theory, non-isolated inverters should also cost less to purchase, ship and install than isolated inverters. As Wiles explains in Home Power magazine, “The transformer is usually heavy, costly and bulky—decreasing efficiency and increasing the inverter’s size and shipping costs.”
Eliminating the isolation transformer in a utility-interactive inverter may also enable additional savings. Verena Arps is the director of technical sales at REFUsol, an inverter manufacturer with a line of transformerless 3-phase string inverters ranging in capacity from 16 kW to 24 kW. Arps points out that REFUsol’s transformerless inverter topology does not require active cooling: “Because the inverters are more efficient, the internal heating losses are decreased, which allows for the elimination of active cooling components.”
Even when active cooling components are included, non-isolated inverters generally have a lower parts count than their isolated counterparts. A reasonable claim can be made that they have less embodied energy than transformer-isolated inverters since they are smaller and lighter. These same attributes could make them easier to install.
The extent to which the raw material reductions associated with non-isolated inverters translates to up-front cost savings still remains to be seen. In today’s market, a non-isolated inverter may cost about the same as an equivalent isolated inverter from the same manufacturer. However, non-isolated inverters have yet to achieve manufacturing efficiencies of scale. They are still a specialty or niche product compared to isolated inverters. Most industry experts agree that transitioning to non-isolated inverters will eventually drive inverter costs down in North America.