Signalcrafters Model 60 Tunes Line Traps in Minutes
Many Signalcrafters customers are in the business of protecting electric transmission lines from fault damage. Among the methods used to accomplish this is one wherein radio signals are coupled directly onto the high voltage lines themselves. This type of communication is known as "POWERLINE CARRIER" (PLC). The information communicated is called "PROTECTIVE RELAYING", where relays at the ends of the transmission line are instructed to operate, de-energizing the line whenever conditions on that line exceed predetermined values.
A LINE TRAP IS
As you can imagine, it takes some unusual engineering to couple a radio signal onto an antenna carrying 500,000 volts. Nonetheless it is done everyday. One of the devices employed in Powerline Carrier is the LINE TRAP, sometimes called a wave trap. A line trap is a parallel resonant circuit installed on the powerline at the local substation. It is tuned to resonate at a specific frequency or frequencies. These frequencies are equivalent to the TxRx frequencies of the local powerline carrier transceiver. Properly tuned, the line trap shows its highest magnitude of impedance, ç Z ç , at these carrier frequencies while permitting the 60 Hz power frequency to pass.
OLD TUNING METHOD IS COSTLY
Tuning these devices in the past has been an expensive, time consuming task prone to error. The accepted method to accomplish this task employed a signal generator, a frequency counter, a VTVM, a frequency selective level meter and a resistor plus all the interconnecting leads and transitions. Furthermore, the trap had to be removed from the powerline. What one did in this method was to place the known value resistor in series with the trap and measure the voltage across it with the level meter at a specific frequency. Using Ohms law, a current value can be obtained. Now, with this data, one could calculate the ç Z ç the trap showed at that frequency. One then would have to repeat this procedure over and over as required to determine the traps impedance vs. frequency characteristics over its bandwidth.
THE MODEL 60 SOLUTION
Signalcrafters Model 60 Impedance Magnitude Meter eliminates all of that complex data taking/calculating. In doing so, the time required to perform the task is reduced from hours to minutes. Furthermore, using the Model 60 for the job involves far fewer expensive instruments. With the Model 60, one simply connects the two test leads across the wave traps mounting ears (the trap neednt be removed from the line). Then one provides a 10 Vrms signal on a frequency of interest at the "Sig In" port of the instrument and it instantly reads the magnitude of the traps impedance displayed in Ohms directly on the analog front panel meter. If the signal source driving the Model 60 is capable of a swept output, one can see the traps ç Z ç displayed dynamically.
WHAT HAPPENS IS
The Model 60 contains an internal current transformer to convert the traps input voltage to current through the trap. The Model 60 also measures the voltage across the trap. An on board analog computer then does all the ratio-taking mathematics to produce the displayed magnitude of impedance.
If a change in the line traps resonant frequency is desired, one simply adjusts the signal generators output to the new frequency, then adjusts the traps inductance or capacitance to again produce a peak reading on the Model 60.