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Bus voltage angles in PSSE are always in absolute angles and are not affected by set_relang option.

The reason for the drift in voltage angles are frequencies deviating from nominal frequency (50 or 60 Hz). At underfrequency all bus voltage angles are decreasing and vice versa at overfrequency.

Below there is an example of two bus voltage angles after simulation of a trip of a generator.

The bus bus voltage angles are constantly decreasing due to the underfrequency in the system. Any oscillatons in the angles are not seen here.

However, when plotting the difference between the two channels the oscillations are seen clearly, as shown below. Here channel 12 minus channel 2 is plotted:

So, by choosing one of the bus angles as reference and plotting the other against that channel, the relative bus voltage angles can be plotted!!!