Electrical steels and power transformer cores in deep saturation

Under normal circumstances, power transformers are designed to predominantly operate within the linear portion of the transformer core magnetization curve. The basic goal of this design is to retain a reasonable amount of linearity and to stay below flux density values that would incur excessive losses. In recent years, however, we are facing more challenges of saturation, such as the half-cycle saturation problem caused by geomagnetically induced currents (GIC) and overflux issues in quadrature boosters (QB). In deep saturation, hot spots may be observed, which can damage the adjacent insulations and even lead to transformer failure. Therefore, it is vital that understand the performance of electrical steels in deep saturation well to take up those challenges. Unfortunately, there has been far too little knowledge about it from experiments. This paper conducts a literature survey on electrical steels as well as power transformer cores, followed by a discussion about the current series of standards for measuring the magnetic properties of electrical steels. The knowledge gap in deep saturation measurement is highlighted, and corresponding solutions are proposed.