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Installing a new or replacement generator requires careful consideration during installation, offline acceptance testing and increased diligence during initial start-up and operation. After replacing the first of four 132 MVA, 15 kV, 85.7 RPM Allis-Chalmers hydrogenerator stators, Chelan County PUD personnel were confronted with an unexpected distribution of generator stator RTD temperatures during the initial unit start-up. A correlation was established between the location of the higher stator winding RTD readings and the region of overheating shown on a continuous infrared thermal imaging display provided by data from a permanently installed array of fast-response, infrared sensors mounted on the rotor. The thermal map pattern provided a classic indication of circulating currents in a phase circuit. This condition is often seen in windings that have cutout coils. The generating unit operated in this condition for fewer than three weeks before a coil-to-coil jumper failed causing a severe arc that finally went to ground, tripping the unit on a ground fault.

A distributed array of sensors mounted between poles on the rotor of an operating salient-pole hydrogenerator continuously provides the powerhouse control room with position-resolved, noncontact surveillance of both the stator core and the end turns. As the rotor spins, the sensors scan the stator for various symptoms of distress, including infrared, acoustic, and RFI (radio-frequency interference) emissions, which are sensitive indicators of hot spots, corona and/or partial discharges, and loose coils and/or wedges. The installation of the rotor-mounted scanner (RMS) greatly reduces the chances of forced outages resulting from catastrophic generator failure. As a diagnostic system, the RMS enables the utility to make realistic decisions relative to corrective actions, such as reducing machine load or scheduling a repair outage that has minimum impact on the economic dispatch of system generation. The RMS is described in detail along with a reliability verification. >

Simultaneous stator winding insulation monitoring techniques are compared during on-line, failure-mode testing of a large hydroelectric generator. Three systems that are capable of detecting RF activity on the stator winding are operated during these tests: (1) RFI (radio frequency interference) measurements made at the neutral connection of the generator winding, (2) partial discharge analyzer (PDA) measurements made using capacitive couplers installed at certain locations on the high voltage circuit ring bus, and (3) RF, location-specific measurements made from probes mounted on the generator rotor. Those segments of the failure-mode tests pertaining to the comparative measurements and analysis performed with the three RF monitoring techniques are described. An assessment is made of how each of the three RF monitoring systems can be used to determine the extent to which partial discharge and corona activity are prevalent on the winding, how the location of the RF activity can be used for winding condition diagnosis, and how the extent of the RF activity can be used with other, unrelated generator measurements to determine corrective action, and when needed.

The hydrogenerator rotor-mounted scanner uses its complement of sensors to measure the critical operating parameters on the stator. The sensors sweep by the active stator elements measuring radiant thermal energy, the clearance between the rotor and the stator (air gap), radio frequency radiation, and magnetic fields. This combined information, measured over the entire generator, permits a real-time evaluation for generator operation, and long term analysis for scheduled maintenance planning. Successfully measuring parameters from a high speed rotating frame, transferring the information to a central processor, and presenting useful information to the user, requires the orchestration of many system elements: (1) a platform for the sensors, (2) a power supply for the on-rotor electronics, (3) telemetry for bi-directional communications, (4) a central process controller, and (5) software for control, data conversion, analysis and user interface. Additionally, much of the monitoring system must successfully operate in a harsh operating environment that includes the combination of high centrifugal forces, large temperature variations, mechanical vibration, adverse electrical and magnetic fields, and high relative speeds between the sensors and their targets.

The authors describe how a four-year RD (charge) maintenance current; internal temperature; electrolyte fluid level; reservoir specific gravity; (rated) current-path integrity; and electrode utilization (state-of-charge) for all cells plus current, voltage, and temperature for the overall bank. The system employs multi-sensor cell-monitor modules affixed to each cell which transmit condition data via fiber-optic cable to a supervisory bank monitor. As many as 256 cells can be supervised per bank monitor, and an unlimited number of bank monitors can be supervised by a (remote) centralized PC employing modems and public telephone lines. The PC can periodically interrogate all active bank monitors, download and process stored data, and archive and display both state and trend battery-condition information down to the level of individual parameters on individual cells. >

Comprehensive rotor-mounted scanning of hydrogenerator stators has attained commercial status on the basis of prototype development and extensive beta-site testing on four generators and two pumped-storage units ranging in capacity from 22 to 450 MVA. Thermal, RFI, acoustic, air-gap, magnetic, and rotor vibration sensors are monitored in real-time to provide early warning of impending failures. Resulting data are continuously processed for alarm conditions, analyzed for anomalous technology, and archived for future trend assessment. This technology is now being extended to two and four-pole turbine-driven generators in which the environment is far more severe than in salient-pole hydro machines. The authors provide graphic results obtained using hydrogenerator scanners and describe techniques under development to achieve comparable results with turbogenerator scanners. >

The operating and maintenance evaluation of hydroelectric generators is undergoing radical change. New sensor technology and the application of expert systems are altering the way generator inspections are scheduled, altering the way in which generators are operated and maintained, and facilitating the implementation of new economic demands for generator operation with near-zero forced outages.

The authors describe the application of a trajectory sensitivity method to the identification of synchronous generator reactances and excitation system time constants. The evaluation of the identification method is performed using different types of disturbances in a single-synchronous-generator, single-line-to-infinite-bus power system. This generator is modeled in great detail to provide an accurate reference model. The robustness and performance of the method are tested by identifying the d and q-axis reactances of a less detailed synchronous machine model. The performance of the identification method is evaluated by comparing time responses and transfer functions of the reference and the identified models using different types of faults. >

This paper describes operational advantages of a 300 MVA Superconducting Generator being developed and jointly funded by EPRI and Westinghouse. This unit is scheduled to be on line at a U.S. utility in 1985 for an extended test and demonstration period.