HBFEM for a DC-Biased Problem in HV Power Transformers

A typical DC transmission system consists of a DC transmission line connecting two AC systems. A converter at one end of the line converts AC power into DC power, while a similar converter at the other end reconverts the DC power into AC power. One converter acts as a rectifier, the other as an inverter. The basic purpose of the converter transformer on the rectifier side is to transform the AC network voltage to yield the DC voltage required by the converter. Three-phase transformers, connected in either wye-wye or wye-delta, are used.

The model has a voltage-driven source connected to the magnetic system, which is always coupled to the external circuits. The current in the input circuits will be unknown, but saturation of the current waveform occurs because of the nonlinear characteristic of the magnetic core. Considering a three-phase transformer, connected in wye-wye, a computer simulation model with a neutral NN (and external circuits for both primary and secondary windings) is obtained using the HBFEM technique. According to the Galerkin procedure, system matrix equations of HBFEM for the HVDC transformer can be obtained through Faraday’s and Kirchhoff’s laws for the transformer with external circuits [23, 24].

During geomagnetic disturbances, variations in the geomagnetic field induce quasi- DC voltages in the network, which drive geomagnetically induced currents (GIC) along transmission lines and through transformer windings to ground wherever there is a path for them to flow. The flow of these quasi-DC currents in transformer windings causes half-cycle saturation of transformer cores, which leads to increased transformer hotspot heating, harmonic generation, and reactive power absorption - each of which can affect system reliability. As part of the assessment of geomagnetic disturbances (GMDs) impacts on the Bulk-Power System, it is necessary to model the GIC produced by different levels of geomagnetic activity [28-30]. The HBFEM can be used for solving geomagnetically-induced currents (GIC) and harmonic problem directly, while commercially available GIC modeling software packages cannot solve the harmonic problem. The detailed theory and numerical model for GIC modeling will be explained later in Chapters 3 and 6.