The word carrier implies that the conductor of a transmission line is being used to transmit an electric signal at a frequency other than 60 Hz to convey certain information about the current state of the electric system. In the case of DCB, only two bits of information can be conveyed by one terminal letting the other terminal know if fault current is either flowing away from the line or into the line. This is accomplished by either the presence or absence of a signal. A terminal that receives a signal implies that the other terminal sees fault current away from the line and a terminal that does not receive a signal implies that the other terminal sees fault current towards the line. This is where you get the words "directional comparison" in DCB. A terminal gets to know which out of two different directions that fault current is flowing at the other terminal. The reason why a terminal needs this information is because it is difficult for the terminal's line relaying to determine fault location at the other terminal. A location on the line and just outside the substation is just a short distance away from a location that is away from the line but also just outside the substation. Line relaying is very good at differentiating locations that are close to it but not so well at locations that are many miles out. Therefore, the purpose of DCB is to prevent over-tripping of additional lines. If a terminal happens to see fault current onto his line and receives a carrier signal, then he is essentially being told by the other terminal, "Hey buddy, you just sit tight, don't do anything, I see the fault current as well, and I got things under control on my end." As a result, the terminal receiving the signal ensures that tripping of its line breaker is blocked. This is where you get the word "blocking" in DCB.
This year I was asked to investigate a 115 kV line that over-tripped twice for a fault that took place two lines out. The carrier signal operates at 96 kHz. Sequence of event (SOE) information that was gathered shortly after these incidents seemed to show that Station A tripped because it did not have the presence of a carrier signal from Station B. SOE data can come from digital fault recorders (DFR), microprocessor relaying, and signal processing equipment (carrier set). Relay and electronic crews gather this data and protection engineers analyze it. This was the first time I have ever gotten involved in an investigation of an over-trip due to carrier issues, so I had to consult equipment vendors, design engineers, protection engineers, and electronic technicians.
The first step in troubleshooting an over-trip due to a carrier issue is to ensure that a terminal sends a signal when it is supposed to. A line relay that sees current flow toward the line and away from the substation is considered forward direction. A line relay that sees current flow away from the line and towards the substation is considered reverse direction. Therefore, a test set can be used in the line relaying circuitry to simulate reverse direction to prove that carrier signal is sent. A relay crew and a field engineer would typically perform this work utilizing a test plan that is created by a protection engineer.
The second step in troubleshooting is to perform manual check-back testing of the signal. This is done by having an electronic technician at each terminal. Signals are sent and received to verify that a terminal receives a signal when the other terminal sends it. Signal power levels are measured to ensure that the carrier set will operate at the signal power received and to determine if there is any considerable attenuation of the signal in the various parts of the circuitry that the signal travels through. This is where I come in.
The third and final step in troubleshooting is to check for timing and discontinuities of the signal and verify the functionality of signal processing and relaying equipment by performing what is called end-to-end testing. As in the first step, a relay crew and a field engineer would typically perform this work utilizing a test plan that is created by a protection engineer.
It is important to understand how a signal is sent from one terminal to the other. There is a lot of equipment to consider. You basically need a way to generate a strong enough signal to overcome any losses it will suffer during transmission (transmitter), match the different characteristic impedances between coaxial cable circuits and transmission lines (impedance matching transformer), select for the particular signal frequency (LC series pass filter), couple it to the line (coupling capacitor potential device), ensure is travels only on the line (line or wave trap), and then couple it back to the carrier set at the other terminal. It is possible that signal power levels can be severely attenuated at certain parts in this signal path. If levels are too low, then the carrier set may not actuate during signal transmission. The picture provided shows an example of typical attenuation that takes place on a transmission line. You can see that the transmitter output of 10 Watts has already been cut by more than half when leaving the coaxial circuit!
In this example you can see that the power level of the carrier signal where it enters the RFL 6785P carrier set was +16 dBm, using the PowerComm PCA-4125 Power Communications Analyzer. This was -15 dB compared to carrier set's nominal level. There is a 12 to 15 dB margin that the carrier set will respond to. So the -15 dB reading was just at the edge of this margin. Two years ago, the signal receive strength was +28 dBm. So, something changed on the transmission line and carrier equipment to cause more power loss of the carrier signal. After extensive testing, this factor could not be identified. Therefore, the new power level was accepted as the norm and the carrier set was recalibrated to accept this level as the nominal. To explain what happened during the system event, the vendor stated that the carrier set actuates for a certain dB margin and that fault current may compromise the carrier signal. EPRI states that "during the occurrence of a fault, there may be additional noise generated by the impulsive voltages and currents associated with the fault" (2017). Therefore, it is very likely that Station A received the carrier signal when it was supposed to but unfortunately the signal power level and signal to noise ratio was too low until after the fault current was dissipated by the tripping of the breaker. This resulted in a over-trip and misoperation because the breaker operated when it was not supposed to, due to the carrier set being late in actuating the block function of the relays. Now, with the carrier set calibrated to the new signal power level, the carrier set and relaying should respond appropriately. This was confirmed by performing end-to-end testing at a later time. A project was created to install a carrier signal meter to alert transmission operators when signal power levels are too low during the daily, automatic check-back tests of the carrier signal.
I highly recommend the use of the PowerComm 4125 by technicians when it comes to troubleshooting signal levels! Performing routine signal strength testing allows a utility to ensure its carrier equipment is properly calibrated.
Article by Dan Scrobe
References:
EPRI AC Transmission Line Reference Book-200 kV and Above, 2017 Edition. EPRI, Palo Alto, CA: 2017. 3002010123.
