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The AC and DC magnetization characteristic test platforms are built, respectively. By analyzing the mechanism of magnetization process of ferromagnetic materials, the differences between AC and DC magnetization characteristics of single-phase four-limb transformer are compared. It is found that the AC and DC magnetization characteristics of single-phase four-limb transformer present similar laws. At the same magnetic field strength level, the magnetization curve has a good degree of coincidence in the linear segment. The degree of AC magnetization in the knee-point segment is slightly higher than that in DC magnetization. Meanwhile, the AC magnetization characteristics of the saturation segment are similar to the DC magnetization characteristics, but under DC conditions, it is easier to make the core into deep saturation.

An experimental investigation of magnetization characteristics of single-phase four-limb transformer in laboratory is presented in this work. The waveform distortion properties of induced voltages and exciting currents during magnetization process are discussed. Using recursive least squares method, the adverse effects of trend term on flux calculation are effectively eliminated. The comparisons of magnetization characteristics of eight transformer models with single-phase four-limb cores which are produced by various grades of steel sheets are performed. In addition, the characteristics of hysteresis curves and basic magnetization characteristics of different transformer models relevant to various grades of steel sheets are analyzed. The experimental results show that odd harmonic is the main factor causing the distortion of exciting current and induced voltage. Additionally, the transformers composed of steel sheets with high magnetic induction such as 27RK090, 27SQGD090, etc. have better magnetization characteristic than of steel sheets with large no-load loss coefficient such as B27P100, 30QG105, 30SQG105, etc. The obtained results can provide valuable information for transformer core design.

±1100kV converter transformer is the key equipment of ±1100kV HVDC project. Due to the voltage level and capacity is greatly improved, the volume and weight of converter transformer is beyond the transportation boundaries, so it’s necessary to use the disintegration transport scheme. This paper introduces four kind of on-site assembly schemes, which includes single temporary tank, double temporary tank, component transportation-field assembly scheme and series or parallel of two converter transformers. After comparing the feasibility and applicability of the 4 schemes, it is considered that the component transportation-field assembly scheme is the best scheme for the on-site assembly of ±1100kV converter transformer.

HVDC converter transformer is a highly nonlinear transmission of harmonic source, which produces large amounts of harmonic current during the operation. The harmonic losses caused by harmonic current share more than half of converter stations’ losses, and affect temperature rise and load capacity of converter transformer. For safe and economic operation of the converter transformer, accurately measuring the converter transformer’s harmonic losses has a great significance. This paper propose a method of converter transformer measurement based on harmonic power source by applying the 1/2 rated harmonic current on the AC side of converter transformer. According to comparison of calculation and test result, this paper finds that the calculation method recommended by IEC 61378.2 and GB/T 18494.2 can’t give out the accurate harmonic losses of converter transformer. With the test data, this paper gets the harmonic losses’ frequency characteristic of converter transformer and the harmonic loss coefficient $K_{r,h}$.

The distortion of the electric field in the oil–pressboard composite insulation caused by the accumulation of the interface charge is detrimental to both the insulation design and operation of converter transformers. The influence of moisture content on the surface charge accumulation of oil–pressboard insulation under DC voltage was studied in this study. In accordance with the Kerr electro-optic effect, the electric field strengths in transformer oil and the surface charge density were acquired after applying the positive and negative DC voltages in three oil–pressboard insulation models with different moisture content, respectively. The resistivities of the oil and pressboard in three models, namely Model 1# with 3.8–4.2 ppm moisture in oil and 0.35–0.37% moisture in pressboard, Model 2# with 7.6–7.9 ppm moisture in oil and 0.79–0.82% moisture in pressboard and Model 3# with 14.9–15.4 ppm moisture in oil and 1.39–1.42% moisture in pressboard, was also measured. The results indicate that: (i) as negative charges in oil accumulated on the pressboard surface in a much greater speed than the positive ones, the electric field in transformer oil under negative DC voltage decreases more rapidly with time than that under positive DC voltage; (ii) the increase of the moisture content in both oil and pressboard, under either positive or negative DC voltage, leads to the decrease of both the electric field strength in transformer oil and the charge density with time; and (iii) the increase of moisture content could not only decrease the resistivity of both oil and pressboard, but also the ratio of the resistivity between the pressboard and the oil. On the basis of the Maxwell–Wagner theory, the decrease of the ratio between the pressboard and oil could lead to the decrease of the interfacial charge density, leading to the slow transient process of the electric field in transformer oil under DC voltage.