Bushing Seperation vs. Bus seperation

[TomScott]

I've had a question posed by one of my construction crews regarding a 115kV to 69kV station we are converting to all 115kV. All the bus work is designed to meet 115kV standards already, but the low side switch currently on site is a 115kV switch spaced at the 69kV separation of 6'. We are in the process of moving these switches to allow a separation of 7'6" based on ANSI C37.32-2002 (7' minimum spacing between conductors). We are also increasing the separation of the PT's and station service.

However, we have noticed that the arrester and bushing phase separation of the high side of the existing 115kV to 69kV auto transformer is approx. 59". So the question has been asked, why can the transformer bushings and arresters on the transformer be spaced less than the open air bus they feed? I've been scouring the ANSI resources we have on hand but cannot find the section that governs phase separation on equipment.

Can you all point me int he right direction?

Thanks,
Tom

 

[jghrist]

See IEEE Std C57.12.00-2010 Section 6.8, Table 11 Minimum external clearances between transformer live parts
of different phases of the same voltage

 

[prc]

As per C57.12.00 the phase to phase clearance required is only 41 inches for 115 kV & 49 inches for 138kV. What can be the reason for this? I thought like this- in case of conductors, the spacing is decided based on a rod-rod electrode consideration. When electrodes are smooth or of high dia or the field around it is a controlled one (in case of transformer condenser bushings that is the case) the clearances can be drastically reduced. Table 10 of above standard (in latest edition of 2015,it is table 10) shows such reduced values.
What should be the consideration for the gap between bushing and lightning arrester of same phase? Any standard available?[pre][/pre]

 

[cuky2000]

Allow me to offer a few comments:
1) The bushing clearance of electrical apparatus, including transformer and breakers is verified by laboratory testing proven for many years.
2) Air insulated substations are typically composed of assemblies of insulators, bus, and connectors not tested in a factory setting.
3) If reduced clearance is decided, the engineer should prepare an insulation coordination study considering a representative bus gap factor similar to the one published in the enclosed file.
4) For most stations layout, the disconnect switch phase-to-phase clearance is a determinant factor to drive rigid bus metal-to-metal separation.
5) Disconnect switch phase-to-phase is determined by the potential arcing that could be expected on the switch blade (vertical, center, double break, etc.)
5) If flexible cable (string bus) is used instead rigid bus, usually the phase to phase separation is larger except for short run connecting bus to breaker or transformers.

 

[stevenal]

I cannot think of an instance where hot line work would be required on a bushing, with the exception of testing for the absence of potential. Hot line work on a bus may be needed at some point, so wider spacing will make the task safer.