PWDickerson
Senior Member
- Location
- Clinton, WA
- Occupation
- Solar Contractor
We installed a 12.5 kW PV system at a commercial site last year, and we are having transformer problems. The system looks like this: The output of three 3.84 kW 240V single phase (no neutral) inverters are connected in a 3-phase 240V delta configuration. The AC output of each inverter has OCP consisting of a fused disconnect with 20 amp fuses. The combined output of the three inverters connects to the primary side of a 15 kVA 240D primary/277-480Y secondary transformer. There is OCP on the 277/480 side of the transformer in the form of a 3-phase fused disconnect with 20A fuses (initially), and there is another 3-phase fused disconnect with 20 amp fuses (initially) at the utility service 600' distant from the PV system. Note that we are not back-feeding a step-down transformer. The transformer was designed as a step-up unit.
The 20A fuses on the 480V side were not sized correctly initially. They were sized for the PV system output, but the transformer manuf. suggested larger fuses to accommodate the inrush current that occurs when the transformer is turned on.
I was at the site last week and noted that one of the inverters was not operating and showed "Grid NA" error. Upon investigation our electrician found that two of the 20A fuses at the 480V 3P disconnect at the transformer were blown, and the neutral wire connecting to the transformer XO conductor had overheated. The transformer manuf. thought the transformer might still be OK, but when we replaced the blown 20A 480V fuses with 30A fuses and powered up the transformer from the 480V side with the inverter disconnects open, we immediately blew a 30A fuse indicating damage to the transformer. The 240V fuses protecting the inverters never blew.
At this point it seems like an inverter disconnected for some unknown reason, which fed imbalanced current into the transformer, and that led to excessive neutral current on the 480 side of the transformer, excessive current on some of the 480V phase wires, which blew the fuses, and eventually to overheating and damage to the transformer.
These events highlight a design flaw in the system. We should have installed common trip OCPD's at the transformer. I have much more experience in designing smaller residential system, and I would very much appreciate comments on best practices for installing a system in this environment.
BTW, I would have loved to spec an inverter with an output that matched the utility voltage, but due to very generous state incentives for locally made equipment, the single-phase inverters were required.
The 20A fuses on the 480V side were not sized correctly initially. They were sized for the PV system output, but the transformer manuf. suggested larger fuses to accommodate the inrush current that occurs when the transformer is turned on.
I was at the site last week and noted that one of the inverters was not operating and showed "Grid NA" error. Upon investigation our electrician found that two of the 20A fuses at the 480V 3P disconnect at the transformer were blown, and the neutral wire connecting to the transformer XO conductor had overheated. The transformer manuf. thought the transformer might still be OK, but when we replaced the blown 20A 480V fuses with 30A fuses and powered up the transformer from the 480V side with the inverter disconnects open, we immediately blew a 30A fuse indicating damage to the transformer. The 240V fuses protecting the inverters never blew.
At this point it seems like an inverter disconnected for some unknown reason, which fed imbalanced current into the transformer, and that led to excessive neutral current on the 480 side of the transformer, excessive current on some of the 480V phase wires, which blew the fuses, and eventually to overheating and damage to the transformer.
These events highlight a design flaw in the system. We should have installed common trip OCPD's at the transformer. I have much more experience in designing smaller residential system, and I would very much appreciate comments on best practices for installing a system in this environment.
BTW, I would have loved to spec an inverter with an output that matched the utility voltage, but due to very generous state incentives for locally made equipment, the single-phase inverters were required.