Surge arrester connection 3

[KillBill7]

I recently got in debate with a coworker on the right way to connect the surge arrester across the terminal of the equipment it is suppose to protect. I have attached a sketch for both options which came under discussion. To me both options are electrically same with option 1 offers a shorter lead and direct path to transformer bushing which is better for the surge protection. However, my colleague seems to be convinced that the option 2 is the right way to make the connection. I am interested to see what you guys think?

 

[9wearSuave]

Both connections are the same. The only time option 2 is used depends on how the equipment is laid out (ie, making sure jumpers are trained properly)

 

[che12345]

I have the following opinion for your consideration.
1. Both options 1 and 2 are the same at 50-60Hz power frequency. This is NOT the issue in this discussion.
2. The bonding conductor Z=R+jX is about the same at either 50 or 60Hz power frequency.
Attention: for surge arrester (including for lightning protection); the frequency of interest is very much higher. At very much higher frequency, Z=R+jX . But X increases proportionally with frequency, to a very much higher value. The R increases very little, mainly due to skin effect.
3. Taking above 2 into consideration, the aim is to have the bonding conductor as short and as direct as possible.
4. I am of the opinion that option 1 is better.
Che Kuan Yau (Singapore)

 

[wcaseyharman]

The utility I work for uses #1. A coworker of mine looked at this a few years ago. The goal was to reduce the length of the lead to the surge arrestor to as short as possible to reduce the impressed voltage onto the equipment during a transient - as was mentioned before, the X for the lead is quite high for very high frequency transients and the difference was several thousand volts if I recall correctly.

 

[RRaghunath]

Longer lead will mean increased voltage that appears at the transformer terminals during Lightning activity.
This may result in compromise of lightning protection for the equipment or if not, reduced protection margin.

R Raghunath

 

[prc]

The two connection diagrams offer the same performance for surge voltages and currents. There is one more option: the jumper first comes to the transformer terminal and then goes to the arrester terminal. This is also perfectly acceptable for surge voltages.

 

[wcaseyharman]

This site has a good discussion of the topic:

http://www.arresterworks.com/Arrest...orks_ArresterFacts-001_ArresterLeadLength.pdf

Option #1 appears to effectively have “zero lead length” which reduces the severity of the transient.

 

[stevenal]

In most cases, we find Option 3 (described by prc above) provides the shortest path. The conductor running back to the arrester does not need to be sized for the load, and can be easily disconnected if needed without disturbing the load circuit.

 

[jghrist]

The option described by prc is probably the most common. You don't show the ground connections, but they also are important because the surge has to flow through the ground connection. Best is to have the arrester mounted on the transformer tank, which is typical for CSP type (completely self-protected).

 

[bacon4life]

The best choice also depends on whether the picture show a distribution transformer (15-35 kV) or a substation transformer (115 kV-500 kV). An approximate rule of thumb is that the arrestor lead adds 10 kV per foot of lead length.

For a 15 kV transformer with a 95 kV BIL, the voltage drop from the arrestor lead is a relatively large portion of the allowable BIL.

For a substation transformer with a BIL of at least 450 kV, adding voltage drop from several feet of lead length is still a relatively minor portion of the overall BIL. HV arrestor terminals typically only have a 2-bolt NEMA connections, so they often cannot carry rated current for the substation transformers. When we configure them as as figure 1, we typically need to add a section of busbar so that load current does not flow through the arrestor terminal. Also, it is important to verify the cantilever strength of substation arrestors.