Hmm, although you say the TCP connection didn’t work, you didn’t provide any error message or description of what happened so I cannot provide any insight there. However, based on your description of what happens when using UDP with a TCP command channel, I wonder if the device is simply ignoring the PMU Connection Tester’s configuration frame requests because the Device ID Code doesn’t match what the device is expecting? Can you locate the PMU Hardware ID setting in the device and make sure the Device ID Code in the PMU Connection Tester matches?
As for your last question, I’m not that familiar with how the SEL-735 works. If you can get the device to provide you with a configuration frame, then you’ll be able to interpret the data frames and see exactly what type of data the device is sending. If there are no phasors in the stream, then perhaps you are only receiving frequency, delta frequency, a set of status flags, and maybe some time quality flags.