Surge Arresters on Distribution Transformers 2

[suco]

Hey all,

I've recently gone to some training for distribution system engineering where one of the instructors told me that one of our practices was "absolutely stupid". We double up arresters on all of our distribution transformers (residential and commercial 3-ph banks) as well as underground cable dips. The instructor said he'd read many studies about the lack of cost-benefit to this practice; it may have been at the bar after class when he told me, but I've heard that's where you learn the most.

I was wondering what the theory was behind this and if doubling up on arresters is a waste of time and money. I can't find any papers on the subject.

Thanks.

 

[bacon4life]

If you have access to the IEEE Guide for Direct Lightning Stroke Shielding of Substations, it explains the cost benefit discussion about how much to spend on lightning protection, given the keraunic level and the expense of the equipment that could be damaged by a lighting stroke. My region has very low keraunic levels, so there are debates as to whether the economics can justify using any arrestors. Why do you double them up instead of just buying ones with twice the capacity?

Where are you located? What voltage do you use for underground? There are a number of studies that have looked at whether to place arrestors at every pad mount transformer, just the underground dip, or both the dip and the normal open. When the wave reflects at the end of a cable, the magnitude doubles, hence the use of arrestors at ends & normal open locations.

 

[magoo2]

I think the concept of parallel arresters or doubling the arresters at each location was around in the days of silicon carbide arresters. The SiC arresters had internal gaps and these would degrade with time, leading, in some cases, with the arresters destroying themselves by operating on lower and lower voltages until they conduct at normal voltage. By having two arresters at each location, you could have one fail and you'll still be protected.

I think the modern metal oxide or zinc oxide arrester is much simpler - it doesn't have internal gaps - and is much more reliable. As bacon4life suggests, maybe you ought to look at arresters with higher capacity. In North America, we have normal duty and heavy duty ratings for distribution arresters. We're in a moderate keronic area (~5-6 flashes/sq km/yr) in my area so we elect to go with heavy duty.

I don't know of any company other than yours that parallels arresters today as a standard practice. I personally think it's a waste of money but you have to look at your own experience.

 

[suco]

We're located in South-Central Kansas; Our system voltage is 7200/12470. Right now, we just double up arresters on the underground dip.

Our Operations Manager swears by it and has said he hasn't seen a transformer doubled up on arresters go bad. I haven't seen any record of that, and I don't know if it exists. I was just curious as to others practices and if there was actually a benefit. I have talked to our linemen and they always say it's all or nothing - either both arresters have blown or neither have blown and arresters down line have blown, or the Homeguard Unit(surge protector behind the meter) has blown. Also they complain about the additional labor to add the bracket to the pot.

Another thing I've been hearing is CSP transformers are junk. Do you guys buy mainly CSPs or Conventionals?