TABLE OF CONTENTS A core fault is a rare event, which usually only occurs in the largest turbine-driven generators where the laminated steel cores are sufficiently massive, and carry a sufficiently high magnetic and electric loading (see Table 2.4). They can occur anywhere in the laminated steel core of large rotating electrical machines but are more common in the stator core [2]. A core fault initiates when core plates are electrically connected together, either because of an insulation failure between them, or as a result of physically imposed short circuits due to various causes including foreign bodies. These connections circulate additional currents in the core, coupling the main flux, leading to additional losses and heating, further weakening core plate insulation and expanding the fault. The fault is then at a zone in the core that is overheated, buckled and electrically interconnected. This can expand, melting material, leading to the catastrophic runaway of the fault and creating a cavity in the core, continuing until the main conductor insulation is damaged and the machine is disconnected for an earth fault. Core faults generally occur some distance from the main conductors and final disconnection frequently occurs after irreparable damage has been done to the core and conductors. A number of large generators worldwide have experienced this problem (see Figure 2.17) mainly as a result of damage to the their stator cores during manufacture or rotor insertion, when laminations became shorted together, and the application to the core of...
