MV Switch vs Breaker for Unit Substations

[rockman7892]

We've been involved in a lot of replacements of Unit substations recently that involve the replacement of MV Switch, Transformer and LV SWGR/SWBD with new equipment.

With these replacements the obvious economical solution is to provide a MV switch on the primary but with consideration for Arc Flash it may make better sense technically to provide a Breaker (metal enclosed) or a VFI pad mount type transformer both of which have CT's on transformer secondary for AF mitigation purposes. Obviously these solutions for mitigating AF come with an increased cost and was curious to hear the experience of others for providing such solutions considering economics?

With LV SWGR an MM switch can be installed on the main breaker which will mitigate AF on secondary of main but will still leave the line side of the main exposed for AF. This may or may not be a concern based on operating procedures (always opening primary switch first before close/rack breaker) however if it is a concern mitigation on the primary is the only solution. With a SWBD in most cases because of the open construction of the SWBD the MM on the main breaker cannot provide AF mitigation benefit for entire board so mitigation on the primary is necessary.

I appreciate any experiences or insight others have with consideration for cost and situations where the customer may not be aware of potential hazards.

 

[MatthewDB]

I'll start with I'm glad to see people recognizing the specific danger found in the MV to LV transformer secondary prior to the downstream LV breaker. The high transformation ratio found at this location, especially at higher MV voltages means that clearing time on a secondary fault is measured in seconds. A fuse value that doesn't result in nuisance fuse blowing on transformer inrush is incapable of providing meaningful protection. My employer's safety training includes information at an event that occured in the 1980's when an employee accidentally caused the fault between a 2.5MVA 13.8kV:480V transformer upstream of the secondary breaker. He was 3 feet away and it left little of him afterwards. Another employee at 18 feet away was also killed by severe burns. Two more who were beyond 30 feet also received severe burns but recovered.

One of the reasons this specific risk is so bad is that even verifying absence of voltage is risky. Doing it "by the book" means a 40 cal/cm2 suit and still requires long distances. Getting access is a challenge and verifying absence of voltage means using a long hotstick to reach in and contact the conductors.

That said, the primary fused switch is perfectly safe provided disciplined procedures are followed. In unit substation form, the primary switch is right there providing close location and avoidability of the concern about mis-identification. Standard MV switches have viewing windows and visible breaks so you can see that the primary is open. Once verified open locked, the risky absence of voltage verification step is unnecessary and the lineup is safe to work on.

The discipline required is that nothing can be done in the transformer or secondary breaker compartments without that primary switch being verified open and locked. That includes, as you mention, racking the breaker in and out.