Grounding & Protection Medium Voltage

[Krusty]

Being I normally do not design medium voltage systems, I could use some input as to whether the following design meets code for protection and grounding.

A pumping station in a rural area is replacing a 150hp, 480V motor with a 400hp, 2300V motor. The utility will provide (3) 7200V ? 2400V pole mounted transformers and the 2400V, 3 phase 3W service to the customer NEMA 4X, 5KV pad mounted switchgear. The utility system is 12,470Y/7200V.

The (3) section switchgear has a main disconnect (requested by the utility) at 600A, 20A fused disconnect to feed a 2400V-480Y/277V pad mount transformer (replacing the existing 480V 3 phase, 3W service to the MCC and control panels in the control room), and a 150A fused disconnect to feed the motor starter for the new 400hp pump motor.

The service switchgear metal enclosure will be grounded to the concrete pad rebar as well as (2) ground wells. The 75KVA transformer is 10? away from the switchgear and will be grounded and bonded at the secondary to an ufer ground and (2) ground wells. The neutral will be run to the control room 100A main disconnect and bonded to the ground terminal.

The motor starter will have a new Multilin 269 Plus relay with 50/5A ground fault CT to protect against ground faults.

The question is does this meet code and / or see any obvious problems? I appreciate any responses.

 

[rcwilson]

Re: Grounding & Protection Medium Voltage

IMO it sounds like your system is per Code and will work. It is similar to many others that I have seen.

I'll leave it to others to comment on the grounding and bonding of the 480V secondary. I think you may have grounded it twice.

Does the 2400V system have any ground reference like a set of wye connected voltage transformers or surge arrestors? If not, the system will operate ungrounded which is per code, but not necessarily desirable. The only return path for ground fault current is through the unfaulted phases' capacitance to ground from the cables, utility transformer windings, etc. There may not be enough system capacitance to ground to trigger the 269 ground fault device. But I would leave it in service with a low setting.