A Method of Power Loss Assessment in Low Voltage Cables for Industrial Electrical Grids

: pp. 17 - 26
Lviv Polytechnic National University

Current harmonics in industrial cable lines are accompanied by harmful phenomena that cause additional active power losses in cable cores. The phenomena are an increasing the insulation temperature and reduction cable lifetime. Failures are often noted in the cable electrical grids due to improper accounting for the impact of harmonics in the cable sizing. Therefore, the correct consideration of the active power loss in cables carrying distorted currents allows for the correct sizing its types and avoid potential problems in the power grid operation. Through considering the concern on estimating additional power losses caused by cable current harmonics the generally accepted method for the loss estimation is analyzed in the paper. The distortion of the load current causes increasing the active power loss in the cable due to cable core resistance gain through increasing the current harmonic frequency. This is due to skin and proximity effects in cable cores with distorted currents.
In low-voltage electrical grids, as a rule, four-core XLPE cables are used and the grid loads are variable. Consequently, the magnitude of the currents and their distortion in individual phases change over time, causing a time-varying asymmetry. This operating condition causes harmonic currents in the neutral conductor of the cable, not only multiples of three, but also the entire spectrum of harmonics existing in the phase currents. The paper presents a practical method of cable ampacity assessment for a low-voltage grid supplying a variable nonlinear load.
The results of the industrial grid load analysis show that under nonsinusoidal conditions each type of cable can be characterized by a certain frequency coefficient for calculating the linear frequency dependence of the cable core resistance. Using the example of the power supply of CHPP electrostatic precipitators, the impact of the current distortions on power losses in cable lines was analyzed, and a comparison was made between the normative and proposed methods for calculating power losses. The results of the analysis confirm the high accuracy of the proposed method for calculating losses. This example shows that under sizing cables supplying nonlinear loads, the impact of the current harmonics of these loads on the increase in cable operating temperature due to a significant increase in active power losses must be taken into account. The correct cable sizing for non-sinusoidal conditions will avoid possible damage to cables or increased aging of their insulation during operation.

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