Wow, lots of flapping e-peens here. Please note I specificaly mentioned at the SUBSTATION for a reason. Anyone that thinks that a substation has anything at all to do with generation, please go away: while there CAN be generators at substations, if one's in use at the substation, the chances of there being enough current to do a TV interview anywhere within the substation's reach are vanishingly small, they typically call times when substation generators are active "brownouts". One of the bits of equipment at a substation, however, IS an isolation transformer, specificaly designed so asynchronicities induced downstream of the substation don't propagate back up the line to the generators and blow them out, even if an embedded signal had to be such a gross change that it affected the base 60 Hz signal (if you're dealing with 50 Hz power, again, go away, because all 50 Hz operators also have their own intelligence agencies that are decidedly NOT the NSA). Typically, you won't see even a need for that with embedded signals that are extreme-order harmonics of the base 60 Hz (6 kHz is an off-the-cuff example), which is what the entire point of an embedded signal IS: a signal that doesn't effect the existing signal in any negative fashion (you're still going to want the embedded signal to not travel upstream though, so you can actually use differing embeded signals for different substations, or the whole "locate the mook" thing falls prety flat, you already know to within a 20-block area if you can figure out which specific substation to inject the signal to)
I should apologize for one bit here: I really should have inserted a paragraph break before the "As for the hum..", apparently many of the flapping e-peens thought that TEMPEST was somehow interconnected with the inserted signal (it's not). There's an entire career path in the US Navy dedicated to the fact that individual electronic devices react in increasingly individual ways to data (EWs, if you must know) as they get older, and with multiple devices in the area to get signatures from, you can easily determine which devices are being used and from that and a general knowledge of where the devices may be, you can get a location on them. In fact, NCIS ACTUALLY PORTRAYS AN EW SPECIALIST, it's literally on your TV every week. So while TEMPEST can't really be used in real-time (well, it can, but a SLQ-25 isn't really manportable), it can certainly tell you if you have the right spot