Utility Keeps the Juice Flowing
Muir does contract and project work in Montana and is chief engineer for
STARadio Corp.’s operations in Great Falls, Mont. In December, he embarked on
an interesting project prompted by an expected loss of local utility power.
1: Utility workers prepare to break the feeder.
started when STARadio was notified that construction of a 230 kV electric
transmission line — linking Great Falls to Lethbridge in Alberta, Canada — would
require the temporary shutdown of the local 7.2/14.4 kV circuit that supplies
power to its Benton Lake transmitter site.
location is home to STARadio’s KINX(FM) and KWGF(FM), both Class Cs, plus a
low-power DTV tenant, all on an 800-foot tower believed to be the tallest in
the state. The new transmission lines would be run close to the lower-voltage
ones, and the contractor planned to use helicopters to assist in stringing the
the interest of safety, the local utility, via its contractor, decided to shut
off nearby active circuits that could cause harm to contractor personnel during
installation. The local circuit also provides power to a competitor’s Class C
FM transmitter site a mile away, a railroad microwave relay site and several
farms and ranches.
that the interruption would last for a considerable amount of time, the utility
contractor decided to employ a generator to power all users on the local
circuit until service was restored.
transmitter site contains a 480 V, three-phase pad transformer, originally used
to power an analog klystron television transmitter that has since been
removed. It was now highly underutilized, so he decided to connect the
generator to the secondary side of the transformer and backfeed power into the
primary side of the local distribution system.
To do so, line crews had to break the circuit
towards the utility power source, as shown in Fig. 1 and 2. Then the 300
kW diesel generator was connected to the pad transformer secondary.
What people thought would only a few days of
generator operation turned into nine, during which time the generator was
loaded to approximately 75 percent capacity with an estimated diesel
consumption rate of 15 gph. When it was shut down, the unit had consumed
close to 3,200 gallons of diesel fuel.
2: A lineman makes the connections to connect the generator.
rental of the generator, installation and fuel cost were borne by the utility
company. Who says engineers are always spending money?
This is a great example of cooperation between
a broadcaster and its local utility.
* * *
looks like a lot of readers are building Frank Hertel’s RF MOSFET tester (Workbench,
Dec 20, 2012).I’ve gotten
comments and questions from as far away as Brazil. So let’s refer to Frank’s schematic.
The first question concerns the capacitor
used. It’s a 0.01 MFD (microfarad) capacitor, but the value isn’t particularly
critical, as it just holds the MOSFET gate charge in the “on” state when the “Charge”
button is pressed and released.
the gate isn’t leaky, the LED will extinguish quickly. If the LED fails to illuminate
when the “Charge” button is pushed, the MOSFET is bad. If the LED stays on all
the time, this indicates a bad MOSFET. Alternately pressing the “Charge” and
“Discharge” buttons should turn the LED alternately “on” and “off.”
Another question concerned whether circuit
modifications were necessary to test an “N” Channel MOSFET. Most RF power
MOSFETS are “N” Channel, so no modification is needed.
3: The utility’s generator kept all the stations on the air.
how will the tester work with an IRFP350? Since this power MOSFET is an “N”
Channel type, the conduction curves are similar to other power MOSFETS, and the
tester should function properly.
Contribute to Workbench. You’ll help your fellow
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John Bisset has spent 43 years in the broadcasting industry and is still
learning. He is SBE certified and is a past recipient of the SBE’s Educator of
the Year Award.