D-DT-1860 A redundant Pole-Mounted Transformer connection specification

D-DT-1860 :

The current configuration of surge arrestors at pole mounted transformers in Southern African utilities is D-DT-1860. Download D-DT-1860 SPEC.pdf (File Size:108kb).

In the development of Live Line Technology products, this configuration was identified as a problem, where the surge arrestor was connected behind the fuse (see diagrams 1-2 below). In this configuration the surge arrestor was taken out of the circuit every time the fuse blew, removing the over voltage protection during lightning storms. Live Line Technology changed this configuration from a series connection into a parallel configuration in the Combi unit.

The Problems are discussed below.

Problems with the D-DT-1860 specifications :

A distribution networks pole mounted transformers are protected from over-voltage by means of the MOV (metal oxide varistar) surge arrestor. The function of the fuse at the pole mounted transformers is to protect the line (feeder) from over current when there is a fault at the transformer and is a point of isolation.

The practice of placing the fuse in front of the surge arrestor is a problem at transformers. Surge arrestors should be connected to deal with incoming transients and should be placed in front of the fuse and transformer to protect both units. The surge arrestor & fuse and their individual functions should never be confused with each other.

This problem needed rectification and is summarised below :

Fuse problems with configuration D-DT-1860 :

Nuisance fuse blow rates were high. (during lighting storms)
Field staff under pressure during excessive fuse blows, causing unplanned outages.
High rates of field staff overtimes.
Staff exposed to hazardous conditions, during stormy conditions.
High transport costs.
Outage Time High.

Surge arrestor problems with D-DT-1860:

Surge arrestors that conduct over voltage, blow fuses which are connected in series with them.
No Adequate way of knowing the when the surge arrestor is blown.
Surge arrestor has no live line capabilities. (+/- 3 Hrs to change the Arrestor.)
Changing of surge arrestors has caused fatalities in the past.
Arrestors dont get maintained in D-DT-1860. (reactive maintenance level 1.)

Properties of the fuse versus the surge arrestor :

When the properties and purpose of both the fuse and surge arrestor are compared, the results are glaring. These two products have two very different functions and need to work in conjunction with each other, and not oppose each other. Because the fuse cannot handle the Ka ratings of the surge arrestor, it blows when it is placed in front of the surge arrestor.

Some engineers will try increase the size of the fuse to prevent nuisance fuse blows during storms, but this will be short lived, as the fuse degrades over time. Unfortunately the fuse just cannot compare to the physical characteristics of the arrestor.

Analysis The biggest fuse (100A for reticulation lines) can dissipate a maximum of 1600A2s, whilst the surge arrestor starts at 10000 A2s. The surge arrestor must be the master in the circuit, and the fuse,the slave.

Characteristics Graph :

Properties of the two units :

Fuse properties
Low current dissipation capabilities.
Rated from 1A- 100A
Isolates over current faults.
Cannot be rated for lightning.

versus

Surge Arrestor properties
High current dissipation capabilities
Standard ratings start at 10Ka
Discharge excess voltages
Designed for lightning protection.

Conclusion :

Analysis of DDT-1860 reveals that excessive fuse blows at transformers will occur during lightning storms due to the fact that the fuse cannot be graded for lightning.

The fuse and arrestor characteristics can never be matched, so the surge arrestor needs to be placed in front of the fuse.

The Combi unit, allows the surge arrestor to deal with lightning, protecting the fuse and transformer from lightning damage.
Contact sales at :
Int: (0027) 468 3344
Tel: (018) 468 3344
Fax: (018) 468 3303
E – sales@liveline.co.za

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