OPGW Thermal Rating Estimator

Calculates the short circuit kA²-sec rating of optical ground wire including heating from DC offset. Replace default input data, then click "Check Inputs and Compute".

BACKGROUND

Single line-to-ground faults can inject significant current in overhead ground wires. Resistive losses then produce heating of the conductors. If temperature gets too high, aluminum can anneal and fiber optic strands or their cladding can be damaged. In the U.S., OPGW manufacturers typically publish fault current capacity in terms of a kA²-sec rating which is based on the energy that will raise conductor temperature from 40℃ to 210℃. A simple calculation of the required rating involves multiplying the square of the symmetrical short circuit current (rms) by the fault clearing time. However, some considerations which can complicate the calculation include:

Power system faults often include DC offset (an asymmetrical fault component). The DC component introduces additional heating in the conductor. The severity of the offset depends on the fault X/R ratio and the point-on-wave initiation time of the fault. The above calculator assumes a fully offset waveform which decays at a rate based on the entered X/R ratio. To eliminate DC offset in the calculations set the X/R ratio to a very small number; however, the number must still be greater than zero (0.001 for example).
Fault currents used in the calculation of OPGW thermal ratings should consider possible future increases. Using present-day fault currents could leave the OPGW vulnerable if future system build-out increases fault current levels.
Normally, not all the fault current will return on a given OPGW. Some fault current will flow to ground through the tower or will flow on other shield wires if present. Users can enter the percentage of current flowing on the OPGW of interest. Detailed information regarding fault current split can be obtained using EMTP type programs such as PSCAD or the Alternative Transients Program (ATP).
OPGW cables often provide a path for high-value communications. Consequently, it may be worth selecting a thermal rating that can withstand failure of primary protective relay systems. The above calculator include inputs and results for primary and backup clearing times.
It can take minutes for OPGW to cool after a fault. If high speed reclosing is used, the OPGW could be exposed to multiple shots of fault current with negligible cooling between reclose attempts. The calculator includes an input for the number of automatic reclose attempts used for the transmission line. Per typical industry practice, auto reclosing is assumed to be disabled for failure of primary protection.

Calculations use numerical integration to compute the root-mean-square [1] to find the equivalent symmetrical current that produces the same heating as that of the fully offset current as described by equation (1) from [2].

I_asymmetrical(t) = |I_symmetrical|{sin(ωt+θ-φ) - sin(θ-φ)e^-αt} (1)

where ω is the frequency in rad/sec, α = ωR/X, θ is the closing angle, and -φ is the phase angle of the current with respect to the voltage. Maximum DC offset occurs when θ-φ = ± π/2.