The transformer core provides a magnetic path to channel flux. The use of highly permeable material (which describes the material’s ability to carry flux), as well as better core construction techniques, helps provide a desirable, low reluctance flux path and confine lines of flux to the core. The core is constructed of numerous thin strips of grain-oriented silicone steel, called laminations, which are electrically isolated (yet still magnetically coupled) from each other by thin coatings of insulating material. This is important to reduce the no-load losses of the transformer. The core is a source of heat in the transformer and as a core increases in size, cooling ducts within the core may become necessary. Problems such as short-circuited core laminations will result in increased losses and possibly overheating of the transformer core.
The core is insulated from the grounded mechanical structures that hold it together and support it and is then intentionally grounded to a single point. Larger transformer cores that have multiple core sections isolated from one another by cooling ducts may implement core jumpers to bond the sections of the core together and a single lead to solidly connect the bonded group to earth. The core, which is effectively a conductor that is not intended as a current-carrying path, may acquire some potential through capacitive coupling with the innermost winding when the transformer is energised (resulting in partial discharges that may damage the transformer) and induced potential when the transformer is carrying load unless the core is solidly connected to earth. The core ground also assures protective device operation in the event of a winding to core insulation failure. For such a fault to be recognised by the protective system of the power supply/line (and quickly disconnect the line), the transformer core must be grounded to provide an electrical (fault) path back to the source. A core is typically grounded at a single point only, as multiple core grounds may result in circulating currents and overheating (and gassing) in the core.
The magnetic health of a transformer is of paramount importance to a transformer’s proper operation. Most common core problems encountered in the field include core ground problems, poor core construction, shorted laminations and overheating. The following electrical field tests used in conjunction with our range of transformer test equipment provide information about the integrity of the transformer core.
- Exciting current: detects most transformer core problems including shorted laminations and other problems that significantly affect the reluctance of the flux in the core, such as a partially shifted or open core joint, poor workmanship in the assembly of the core, etc.; sensitive to core magnetisation
- DC insulation resistance (core ground): checks for unintentional core grounds (best tool for this) and problems involving the core ground insulation. Low insulation resistance values between the core and ground can be caused by shifting of the core laminations and by conductive contamination or foreign objects that bridge the core-to-ground insulation.
- Capacitance/power factor/dissipation factor: the capacitance of the low voltage winding (CL) measured during a power factor/dissipation factor test is sensitive to the deterioration of or complete loss of the core ground connection
- Sweep frequency response analysis (SFRA): sensitive to changes in the magnetic core and core magnetization in the lower frequency range while loss of core grounding may be detected in the higher frequencies (e.g., ≥ 50 kHz).