by Bill McNulty, P.E., HD Electric Company
A 69kV transmission line snakes through what used to be farm land but is now a mix of new subdivisions, shopping malls and office parks. Most of these new customers are now served by this transmission line through two new substations.
Starting this spring, the C phase on this transmission line has been going down about every other day. Today was the ninth outage so far. The source dispatcher says the problem isn't due to any overload on the C phase but is due instead to a fault on the line. Fault recorders in the two new downstream substations say that the problem isn't on the load side. It's your job to keep the transmission line up and this month your job has gotten a lot harder.
Naturally, the customers are starting to get fed up with these too frequent outages and the phone calls are pouring in. The shopping malls are the loudest complainers; even with their expensive automatic transfer switches, they are finding both sources down since both of the two new substations are served by your transmission line. You've already done a visual inspection of all 278 poles, the insulators hanging from the cross arms and the conductors hanging from the insulators. You've tested every lightning arrester on the line, there's no evidence of any wildlife causing these outages and no storms that coincide with the times of the outages. It's just starting to get hot outside but it's already a lot hotter in your office.
You've already run through your list of the usual causes of these types of problems but so far nothing has stopped the outages. A second complete inspection of the line shows no visible flashover damage on any of the porcelain suspension insulators. Still, you can't help but suspect a bad insulator somewhere on that C phase. You don't even want to try telling your boss that you want to replace all the insulators. After all the unplanned outages, the last thing you need is a long planned outage. What you really need is a way to test the insulators quickly and easily and without a planned outage.
Does this situation sound all too familiar? Fortunately, there is now a solution with the Hi-Test Insulator Tester by HD Electric Company. The Hi-Test Insulator Tester tests energized or deenergized suspension insulators for non-visible internal failures at any line voltage. The model IT-4 tester is portable, self contained and designed for use on the end of a hotstick. Built in test probes contact the metal ends at the top and bottom of each skirt and the front panel display readily identifies the high leakage current typical of bad insulators. A loud buzzer also signals the lineman that a bad insulator has been found.
The IT-4 Insulator Tester works by applying a 10kVDC voltage across each insulator in a string and then measuring the DC leakage current. Most importantly, it does this while the insulators are still live. Insulators with non-visible internal failures will show a high DC leakage current which is then displayed in lights on the front of the tester. A loud buzzer also signals high leakage and helps to ensure that nothing is missed. The tester is battery operated with a rechargeable battery that lasts for several days of testing. Recharging takes place overnight or in the line truck during lunch.
Back in the field, testing of the insulators on your 69kV line is underway. As long as you're stopping at each pole, you decide to test the insulators on all three phases. In about two weeks, with some overtime thrown in, you manage to test every insulator on the line. Here's what you find: two bad insulators on C phase and one insulator each on A phase and B phase that weren't completely bad but looked like they were on their way to failure. After locating the bad and questionable insulators you go ahead with a planned outage to replace the four insulators.
After about two quiet months you realize that you've solved the problem. You also notice that those nagging TVI complaints that were pushed to the back burner while you were tracking down the outages have also disappeared. It's the middle of summer now and plenty hot outside but things have finally cooled down in your office.
The Hi-Test Insulator Tester is now used regularly to test your glass and porcelain suspension insulators on both new installations and existing lines. It's great for finding defects before they become problems and is the only way to find the types of defects that cause those nagging RF/TVI (Radio Frequency/Television Interference) complaints. You're thinking that once you finally catch up you might even lend your IT-4 Insulator Tester to another crew.
The Hi-Test Insulator Tester is supplied complete with long life rechargeable battery, carrying case and universal spline for attachment to a hotstick.
Copyright © 2000 HD Electric Company