ACSS in Substations 1

[Mbrooke]

What is typically the maximum permissible temperature rating of bushing cable terminations (breakers, isolators, CTs, trafos) in substations? I'd like to bring ACSS into the substation.

 

[cuky2000]

All power connectors including those for bushing cable terminations (breakers, isolators, CTs, trafos, etc), are rated for carrying the ampacity of the largest conductor(s) for which the connector was designed to connect. For reference, see NEMA Standard CC-1, ANSI/IEEE Standard C119.
For Instrument Transformers, the following applies:
[a]PTs and CCVTs: are not thermal in-line devices and as such do not affect connector ratings.
b)Surge arrester: Similar to PT & CCVT as described above
[c]Free standing CT’s The max. current that can carry depends on its connected ratio & the thermal rating factor – “k”. Connector selection should take this in consideration

In general, equipment nameplate is based on a 40°C ambient temperature. This current rating can be adjusted for specific ambient temperature and other environment and material parameters.
Below is a suggested source to follow. I hope this helps you select the right connector for the cable termination with the Aluminum Conductor Steel Supported (ACSS).

 

[Mbrooke]

First part makes sense, but with ACSS we are talking about a much smaller conductor running at 200*C instead of 100*C, meaning the terminations are in theory going to be much hotter.

 

[cuky2000]

NEMA Standard CC-1, ANSI/IEEE Standard C119 :
a) The ratings on Table are given for a wind velocity of 2 ft/s, an ambient air temperature of 40C, a conductor temperature of 70C (30C rise), and emissivity 0.35. Sun effect was not taken into consideration because of the wide variation depending upon the location.
b) The designer should recalculate these values to take sun effect into consideration once the installation location is determined.
c)The temperature rise shall be determined at 100, 125, and 150% of the rated current
d) is not intended to qualify the connectors for service higher than the normal rating.
e) Connectors that are supplied in equipment are covered by the equipment standards and are excluded from this standard.
f) If the user desires to operate the conductor and connectors at a higher temperature rating, the test currents shall be raised to a value agreed upon between the manufacturer and user.

Although the hardware for other bare OH conductors such as the traditional ACSR has similarities with the hardware for ACSS conductor, there are a few important differences to be noted: a)the hardware for ACSS conductors must handle higher temperature and higher current densities. b) Corrosion inhibitors should be specified as suitable for high-temperature use with ACSS.

It is a good idea to check with your hardware manufacturer for a recommendation on the use of these types of hardware on ACSS.

See the link below for hardware applications suitable for ACSS conductors:

http://preformed.com/images/pdfs/En...er_Terminal_CMPTM_for_ACSSTW_TransCatSec4.pdf

 

[Mbrooke]

Big help, thank you!


What about the bushing on isolators and SPS2 breakers?

 

[cuky2000]

For bushing on isolators and breakers the Allowable Continuous Current Class (ACCC) for connectors and bushings provided with the equipment follow the nameplate which is based on a 40°C ambient temperature. This current rating can be adjusted for specific ambient temperature without exceeding the normal allowable maximum temperature that can withstand.
Particularly the Siemens SPS2 dead-tank circuit breaker is normally rated for 55C ambient temperature that is above the 40C minimum required by the IEEE Standard.
I am not sure the ACSS or regular ACSR or rigid bus will be a better choice for jumpers and short connections inside the substation equipment.
Below is a general short guideline for the Allowable Continuous Current Class (ACCC) for equipment that will include bushing on isolators and breakers.

 

[Mbrooke]

Maybe I'm missing the obvious- but doesn't a smaller conductor get hotter? As such the point where the jumper (and its termination hardware) connects to the bushing will get hotter thus the bushing will need to be rated for the higher temps?

 

[cuky2000]

If the conductor temperature is higher than the standard continuous rating (normal & emergency), the equipment needs to be derated accordingly. A few options are available:

a) Oversize the equipment and work with the manufacturer to derate the continuous rating (I do not know any acceptable standard procedure to derate equipment bushing-connector assembly operating the jumpers at higher temperature).
b) Using hotter and smaller conductor could be a more expensive proposition than use a larger or multiple conductors per phase.

In general, using new ACSS or reconducting an existing transmission line could be an acceptable solution to increase the line capacity at a reasonable cost. However, It is not clear if there is any advantage of using or replacing jumpers and other string conductors inside the substation. Usually the jumper or other string busses the sagging is less critical than the use of ACSS of a transmission line.