Wiring (from the BOS to the tower and up to the turbine)
Generator, slip rings & brushes
Voltage Tests. With the power on, safety gear donned, and all systems enabled, we checked for proper voltage with a multimeter on the battery bank (48 VDC) at the OutBack power center that’s interconnected with two PV systems and the grid (240 VAC), and both the DC and AC sides of the OutBack inverter. We also scrolled through the parameters programmed in the inverter to verify that they matched what was recorded in the MREA operation logs and that nothing had been changed.
Resistance testing. We then furled the machine and turned off the power from both the grid and the battery bank, since resistance checks cannot—and should not—be done while a system is energized. Still armed with a digital multimeter, now set to ohms, we checked the components in the controller box: fuses, a diode, a circuit breaker, a voltmeter, and an ammeter. The fuses and circuit breakers had continuity, the diode (electrical check-valve) was showing one-way conductivity, and the needles on the analog meters were free to move. The BOS components were verified to be in good working order and not the cause of Little Jake’s problems.
Conductor continuity & insulation testing. Next, we tested the underground wiring from the BOS to the junction box at the tower base. We disconnected both ends of the two wires (DC positive and negative) for continuity and insulation tests, using the multimeter to verify low resistance in the copper conductors and a megaohmeter or “megger” to test the THWN conductor insulation. Our megger was very old, with a hand crank and an analog display, but still accurate. Meggers are simple: The operator applies 500 or 1,000 volts to the conductors (in our case, with a hand crank), and the needle should display infinite resistance. Anything else signals insulation breakdown and/or a high-resistance fault. Our conductors passed both the continuity and insulation tests.
It was time to send a crew up the tower to test the wires from the junction box at the tower base to the slip-ring assembly near the tower top. (These were the same tests we just conducted from the BOS to the tower base.) We measured low resistance in the copper wires with a multimeter and saw infinite resistance with the megger. We also checked for short-circuits and insulation breakdown by connecting one test lead of the meter to the metal tower and the other test lead to each of the two power output conductors, one at a time. We were looking for infinite resistance from each conductor to the grounded tower on both the multimeter and the megger, and our tests confirmed that the wiring was good from the BOS to the top of the tower. This meant that our primary suspect was now the wind turbine itself.