If the measured values (the seqData) are actually phsMeas, or if at least their internal structure is the same, you can right-click the line saying IEC61850 Sampled Values in the dissection pane and from the context menu choose Protocol Preferences -> force decoding of seqData as phsMeas. Otherwise you'll have to write your own dissector. By a quick scan of the dissector tables, I haven't found a place where to hook a dissector for the seqData alone, but check the SV dissector source to confirm this.