Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Troubleshooting
Aircraft Equipment Problem Critical
Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Human Factors
During pushback I started the right engine. The engine generator did not take over from the APU generator after start valve closure. I reset the [engine] generator; but it was still not coming 'online.' We gate returned and Maintenance swapped out a Generator Control Unit (GCU) which did not fix the problem. We deferred the engine driven generator and conducted the flights with the APU operating and supplying power to the right side bus. After arrival back in ZZZ; the Ramp agents connected external power and I tried to swap power from the APU to ground power. The right side [electrical] bus would not go 'offline' on the APU generator. I wrote up the problem and contacted Dispatch and Maintenance. Recurrence of this event is impossible to predict or avoid. Aircraft electrical failures are hard to predict. Maintenance should be complimented on their handling of this event. The system was troubleshot; tested; and deferred in less than 45 minutes. An outstanding job.
An air carrier Captain reports about a recurring Right Bus Electrical System malfunction on their MD-82 aircraft. The right engine generator did not take over from the APU generator after start valve closure and later; the right bus  would not allow the APU generator to go 'off-line' and swap to ground power.