Research On On-site Heating Measures For UHV Transformers

With the large-scale construction and operation of UHVDC transmission,the insulation problem of converter transformer has caused great concern due to its special structure,complex insulation and high cost.The valve-side winding of converter transformer runs under the influence of AC-DC compound voltage,which and DC voltage accounts for a large proportion.The distribution of AC component in oil-paper composite insulation is determined by material size and dielectric coefficient,and the change of humidity or temperature will not cause the obvious variation of AC voltage distribution.While the DC component in composite insulation is of resistive distribution,which is determined by material size and resistivity,and the change of humidity or temperature will cause the variation of resistivity of insulation material.Therefore,for converter transformers,the risk of insulation damage increases with the decreasing of temperature and the increasing of water content.At present,hot-oil circulation method is widely used in transformer field to dry treatment.This method is limited by the low power of oil filters,so that the heating cycle takes a lot of time,and the temperature rise is limited,specially,in low temperature climatic environment.In this paper,based on the analysis of the existing problems of various on-site heating methods for transformers,the short-circuit method is innovatively proposed for on-site heating.Based on the principle of transformer short-circuit test,the valve side winding of converter transformer is short connected and rated current is applied to the network-side winding.The transformer insulation medium is heated from inside by using the active power generated.Firstly,the process of heat exchange during heating is analyzed.Through the derivation of heat flux formula and the substitution calculation of hypothetical parameters,it is concluded that the short-circuit method can achieve designed temperature rise and greatly shorten heating time by contrast with hot-oil circulation method,when the external conditions are similar.Thereby,the feasibility of this method is verified in theory.Secondly,because of the high voltage and current required for short-circuit heating and the large demand for power capacity,the power supply in converter station cannot be directly used,therefor,a short-circuit heating device is developed.The device consists of a voltage regulator,an intermediate transformer,a compensation capacitor bank,a measurement system and a control and protection system.It can steadily increase the network-side voltage of the converter transformer,make the rated current of the network-side winding output,and accurately compensate the reactive power,thus reducing the requirements for the voltage and capacity of the power supply in the station.The field-heating test of UHV converter transformer with this heating device is carried out.By comparing and analyzing the experimental data with the theoretical calculated data,it is proved that the heating effect of the device is obviously betterthan that of the hot oil circulation method,and the device has stable performance,safety and reliability,which can be used in common AC transformer.The field drying treatment method of UHV large transformer is innovated,and an effective method of transformer on-site insulation treatment is added.It is of great significance to ensure the insulation performance of large transformers and safe operation after commissioning.