Clearances Between Differing HV System Voltages

[stretched2thin]

NEMA gives guidelines for clearances between live parts and ground and also live parts between phases for system BIL ratings and system voltages. (SG 6 - 1990 Table 32-1)

However, as far as I can tell, it does not give clearances between live parts of different system voltages. I'm mainly concerned about clearances for HV bus in a substation.

My first impression is that you would consider the lowest system voltage to be grounded and use the phase-to-ground clearance requirement for the highest system voltage. In other words the clearance between the LV and HV bushing should be the Phase-to-Ground clearance given for the HV Bushing Voltage and BIL.

I base this assumption on my history of transformer testing. In testing HV-to-LV clearances for transformers, the lower voltage terminals are grounded during Hipot and Impulse tests. These are the main two tests that test HV-to-LV clearances.

I know the easiest answer is to use the larger of the two distances. However, if it is safe, it would be advantageous for us to go with the smaller distance requirement.

 

[stevenal]

Voltage from one system to another can exceed the voltage from either system to ground. Why not base clearances on the phasor voltage difference? Worst case would be a 180 degree phase difference so that phase A to phase a clearance would be the sum of each systems clearance to ground.

 

[cuky2000]

Your post require further clarification for apparent conflicting reasons:

a- LV and HV bus are typically segregated in different areas in the substation for safety reasons. It is unusual to see LV and HV in low and high bus.

b- ANSI/IEEE Std C2 (NESC) covers safety clearance between different voltages applicable to T&D main and under build lines.

c- NEMA Std SG-6 not applicable to flexible bus. Common engineering practice usually increases the published clearance by 50 %.

d- NEMA SG-6 is not a mandatory standard even though is commonly used and accepted by industry and utilities up to 230 kV. Above 230 kV (UHV) NEMA do not apply, the clearances typically is based by rob-gap and plane-to-plane and other dielectric criteria.

e- The NEMA clearances are more conservative than IEC Std. for similar voltage level.

f- I cannot see the relation between Hipot and impulse test in apparatus with the AIS clearance in substation busses.

g- Bushing clearances are much lower than the NEMA requirement for the same voltage level (ex. Transformer, circ breaker, disconnect switches, etc). Apparatus pole clearances are determined by test

h- “…. the clearance between the LV and HV bushing should be the Phase-to-Ground clearance given for the HV Bushing Voltage and BIL“ . Please clarify this statement since most cases clearances had been determine for maximum voltage difference.
- Transient condition: The surge reflecting traveling waves surge will doubling the voltage at open bus.
- Steady-state conditions: LV and HV difference could be maximum during 180 degree phase shift.

 

[stevenal]

Cuky,

Regarding item b, Part 2 of the NESC doesn't apply within the station. However, some of these clearances are based on phasor voltage differences (235A3a). Doesn't seem that much of a stretch to apply the same principal inside the station where there is no express rule.

 

[mersin33]

assuming rules of O/H line design somewhat applies to this case:

separation (m)= 1.2 + 0.0102 (kV1 + kV2 - 50)

where:

kV1: is line to ground potential of first system in kV,
kV2: is line to ground potential of second system in kV