With the gradual increase of proportion of cables in 10 kV power lines for railway.The power factor is low due to big distribution capacitance of cable in case of light load.For the reactive power compensation

the distribution c apacitance of cable needs evaluation.The c alculation formula of distributed capacitance of single-core cable is given.Capacitance for three-core cable line is calculated by finite element method.A kind of reactive power compensation device based on reactance voltage regulation is proposed.The voltage regulation is completed by a vacuum on-load tap-changer（OLTC） with auto transformer.The device changes the gear of the OLTC in accordance with the electrical load so to achieve reactive power regulation of railway line.Both the feature and operation time sequence process of the OLTC is presented.Compensation capacity for the three-core cable line is designed and the compensation device is made by the use of FEM and the sample is subject to site test after operation in the network.The calculation method of distributed capacitance and the suitability of the reactive power compensation proposal are proved by the site operation data.