IEEE Std 1129-2014 pdf download – IEEE Guide for Online Monitoring of L .arge Synchronous Generators (10 MVA and Above).
4.2.4 Circulating core currents Problem Description: Local overheating of the core could be the result of a short circuit between adjacent core laminations allowing axial (longitudinal) flow of current between core laminations. Usually, the insulation breaks down, creating an electrical short circuit due, for example, to mechanical damage or a metallic foreign object shorting a number of laminations. Instrumentation: Core circulating currents are not directly detectable during operation of the machine, although modern shaft-voltage monitoring devices may be able to detect anomalies in the core, such as shorted laminations producing circulating currents. 4.2.5 Volts per hertz (V/Hz) Problem Description: Inter-laminar current flow and consequent severe local overheating can also be the result of an over-fluxing incident, caused by excessive V/Hz applied to the machine (due to operation at overvoltage and/or under-frequency). In almost every case, this over-fluxing is an over-excitation event occurring while bringing the unit online as it ramps up from zero speed to full speed with the excitation on. Instrumentation: V/Hz protective relays are designed to remove the excitation in a short time; however, serious damage to the core insulation could have occurred. This damage may show up later during normal operation as hot spots in the core. 4.2.6 End-tooth heating Problem Description: Core-end iron can overheat when magnetic flux enters in the axial direction the ends of the stator core during under-excited operation of the generator. The problem worsens when a unit is also operating at rated megawatts while under-excited.
4.2.8 Core vibration Problem Description: The core vibration of the generator results chiefly from the unequal magnetic pull in the airgap (magnetostriction adds to magnetic noise but very lttle to measurable vibration). Large salient- pole machines have cores with relatively large diameters and smaller depth than generators with cylindrical rotors. This, together with a large number of poles, results in more bending modes and vibration induced stresses. The vibration is mainly at twice line frequency. Loose cores may respond dramatically to the vibration driving force from the rotor. Kceping the core tight by torqueing the axial bolts and circumferential bands (when incorporated) help reduce core vibration. Designs developed in order to have the natural frequencies far from twice line frequency and from line frequency also favor low vibration amplitudes. A loose core can result in higher unit noise or, in extreme cases, the breakage of laminations near the bore surface. A loose core also can force vibration of the end-turns and cause winding failure. The detection online of a loose core may be performed by a sound analysis and more readily by vibration- pattern analysis by trending frame vibration, particularly when the core is not spring-mounted as with most four-pole smaller units. Most two-pole units have cores that are spring-mounted. Instrumentation: Core vibration is monitored online by mounting accelerometer probes on the back of the core in strategic locations. The measurement also includes phase measurements. Phase measurement is critical for understanding the relative movement between two components, in this case, the core and the frame.