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Utility Keeps the Juice Flowing

In Montana, a power company helps keep customers on the air

Fig. 1: Utility workers prepare to break the feeder. Greg 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.

It 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.

That 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 cables.

In 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.

Knowing 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.

Greg’s 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.

Fig. 2: A lineman makes the connections to connect the generator. 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.

Fortunately, 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.

* * *

It 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.

If 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.”

Fig. 3: The utility’s generator kept all the stations on the air. 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.

Finally, 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 engineers and qualify for SBE recertification credit. Send Workbench tips to [email protected]. Fax to (603) 472-4944.

Author 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.