The Customary IEC 60287-2-3  particulars a strategy to calculate the steady-state present score for cables of all voltages put in in tunnels with pure or pressured air flow. The tactic is predicated on the calculation of the temperature of the cable floor, the air within the tunnel and the tunnel wall as a perform of the warmth generated by the cables. Consideration of longitudinal temperature gradients is concerned because the transferring air contained in the tunnel (assumed to be both laminar or turbulent relying on the air velocity) removes warmth from the cables. Longitudinal warmth switch throughout the cables and the environment of the tunnel then again is assumed to be negligible and warmth generated by a cable is assumed to be fixed alongside its size.
The tactic is relevant to any kind of cable and for a number of circuits. The place a number of circuits are concerned, their traits are assumed to be equivalent. Solely regular state circumstances are thought-about, the place the inlet air temperature and the cable loading are fixed for a adequate time for regular temperatures to be achieved.
The present score of cables inside a tunnel relies on the air temperature. The longitudinal air motion within the tunnel dissipates warmth from the cable floor alongside the tunnel axis. This creates a rise in air temperature alongside the tunnel axis as we transfer away from the inlet. The next warmth switch mechanisms are thought-about:
- Radial conduction throughout the cable.
- Radiation from the cable floor to the tunnel wall.
- Convection from the cable floor to the air contained in the tunnel.
- Convection from the air contained in the tunnel to the tunnel wall.
- Longitudinal warmth switch by convection ensuing from air stream (laminar or turbulent relying on velocity).
A thermal community representing slices of the tunnel cross part, is taken into account and the slices are linked by the longitudinal warmth switch of the air stream alongside the tunnel. A delta-star transformation is utilized on ever slice to derive a thermal community with one thermal resistance every between the star level and the cable floor, the tunnel wall, and the ventilating air respectively. This ends in a fictitious enhance of the ambient temperature to account for the air flow. The equal thermal resistance of the cable surrounding is calculated (iteratively) and used within the regular state calculations to acquire the present score.