What must be true for two alternators to be paralleled successfully?

For two alternators to be paralleled successfully, they must indeed have the same voltage. This is crucial because if the voltages are not equal, it can result in circulating currents between the alternators, which can cause overheating and potential damage. When alternators are paralleled, they need to share the load equally without any issues of load imbalance. If one alternator has a higher voltage than the other, the one with the higher voltage will attempt to deliver power to the lower voltage alternator, leading to instability and potential failure in operation.

Having the same voltage ensures that both alternators can contribute to the load effectively and maintain system stability. In addition to the same voltage, other conditions like having the same frequency and phase sequence are also necessary, but for the question focused on what must be true specifically for paralleling, matching voltage is the essential requirement.