7.2 kV Distribution Shunt Reactors

[kmh1]

We are working with a client who is experiencing failures of their single phase air core shunt reactors applied at 7.2 kV. The system consists of extremely long, lightly loaded, underground cable runs along railway corridors. Reactors fairly regularly burn up without necessarily causing an immediate short circuit on the system. Weather conditions do not seem to be a factor. Could there be a problem with resonance in this case? The reactors are switched manually. They are installed on wooden stands and suitable grounding may also be of concern. I have seen mention of adding surge arresters at the reactor terminals in this type of application, however there are arresters are installed in the line side of the isolation switches. Photo is attached of failed reactor.

 

[magoo2]

Presumably these reactors are intended to control the voltage rise on the long runs of UG cable. Is that correct?

The cables are operated at 7.2 kV phase to ground, correct?

How often are the reactors connected or what is the spacing between reactors?

What is the ampere rating of the reactors? Do you know the kvar or uH rating?

What cable size are you using? Presumably this is 15 kV cable, correct?

Once you answer some of these, we can see if resonance is a possibility or not.

 

[kmh1]

Yes, the reactors were installed to control voltage on the long lightly loaded cable runs. The cables and reactors are operated at 7.2 kV to ground. The reactors are rated 140 kVAR, 20 Amps, 370 Ohms, 983 mH, 115 deg temp rise, 185 deg insulation, copper wound. The concentric neutral cables are 15 kV, #1 AWG. Installation distances vary, one set is installed 11 miles from an upstream set of Cooper voltage regulators, another set is 20 miles from an upstream set of voltage regulators.

 

[pie314]

KMH1
What are the dimensions of the reactor? You mentioned a single phase reactor, but I see (3) reactors in the picture. It looks like a (2) layer design reactor. I assume the the layers are in parallel not series. It could be that the volts per turn are too high. Also shunt reactors are subjected to vibration type forces so the turns must be constrained.

Good luck

 

[kmh1]

It is a 3-phase system with 3 reactors at each location (one per phase) connected line to ground.

 

[magoo2]

The pictures looked like they were burned up - possibly from overload. The design looks a lot like a carrier line trap. They have very little overload capability.

Were the failures associated with the 11 mi or 20 mi run? Or both?

If you can get a current reading at the reactor end and see if you're pulling more than 20 A, that would be worth doing. Or set a small distribution transformer and check the voltage.

Had you checked how much charging current was drawn at the source end before you connected the reactors?

I don't think too much about the vibration theory. If they had a magnetic core, I might buy that, but not with an air core.

 

[pie314]

I disagree with Magoo2. Air core reactors are more prone to vibration problems than Iron core. As the reactor traverses from zero to its peak current the reactor forces are trying to expand the diameter and shrink the height. Since a shunt reactor sees only rated current these forces are not very high, however they occur twice every cycle. (120 times per second on a 60 hertz system). When large dry type air core reactors were first made in early 1960 there were vibration type problems. The modern Epoxy fibre glass reactors solved this problem.

If I knew the dimensions It would help

 

[kmh1]

After further investigation it appears that several factors were at play here. Firstly, the reactors only appear to have been designed to rated current at nominal voltage, as there is no maximum operating current or voltage listed on the nameplate. Secondly, the units are 115 deg C rise units designed with 185 deg C insulation based on 40 deg C ambient temperature, however, they are installed outdoors in direct sunlight and are dark red in colour. Essentially operating the units at overload and above the insulation temperature rating is leading to their demise. Short of installing some sort of forced air cooling or shielding from the sun, a redesign of the units to be applied is in order.