b'34 Partner News Summer 2022CorporateModel Based Voltage and Current Analysis (MBVI) used to diagnose rotor faults in synchronous machinesFor engineers, the desire for reliable data onThe next step, according to conventionalbetween the techniques. Phase current which to base decisions endures. Innovationwisdom, would have been significantanalysis is dependent on measuring the and the adaption of emerging technology todisassembly. Where this would have givendifference between a machine rotor and a specific end allows tighter control of keya definitive answer, it was not an entirelystator waveforms, and will not therefore work equipment and the potential of substantialdesirable approach, owing to the potentiallyon a machine where there is none, such as cost savings.destructive nature of the tests requiredsynchronous motors or generators. and the potential long-term outage of the Recently, in collaboration with a specialistmachine which was still essentially availableWhere the equipment had not been condition monitoring company, an alternative if a poor spare. previously used on generators, the supplying approach that can be used to diagnosecompany were quick to accept the challenge and manage rotor issues on synchronousTesting and diagnosis to do so and collaborated to develop a machines has been identified. The discoveryAs an alternative, the potential Modelprogramme of testing.resulted from investigations into a vibrationBased Voltage and Current (MBVI) analysis issue manifesting on a North Sea productionas a diagnostic tool for this scenario wasThe resulting tests carried out online platform main generator. investigated. The MVBI system builds adetected discernible and measurable signals, model of the relationship between voltagewhich suggested the magnetic fields Power on the installation is provided by twoand current waveforms.strength created by the generators four generators, each with greater than 20 yearspoles was not balanced. Otherwise, they in service, and manufactured to the sameThe equipment connects to existingwould be in a healthy machine, indicating specification. The machines are installed onlow-voltage monitoring circuits, andlikely faults in the rotor windings. an external location with no fixed systemsthe algorithms within the software are to facilitate overhaul or change out of thesufficiently powerful to present an initialThis information was sufficient confirmation rotating equipment elements.diagnosis on a graphical interface for theto commit to strip down and overhaul and technician or inspector on site.gave a benchmark from which any further The affected generator was exhibiting higherdeterioration could be measured on the than normal vibration, which was particularlyThe test output is similar to that obtainedfaulted machine and point to a signature, evident when starting large inductive drives.from phase current analysis, but cruciallywhich could indicate any deterioration in the The vibration was then, on occasion, significantthe way the diagnosis is made differsother. enough to trip the unit out of service. A rotor issue was suspected, and a range of tests had been carried out, including partial discharge and vibration analysis.Despite suggestions as to the cause, there was no diagnosis.'