In this paper we describe simulations, using EPRI's Extended Transient Midterm Stability Program (ETMSP), which were performed to understand the nature of a failed load rejection test on a nuclear unit, The failure was a result of large induction motors stalling, causing protective relays to operate, Potential remedial measures were simulated and a final solution, using a temporary voltage boost on the AVR, adopted and implemented to prevent further failures.

This paper presents a brief review of Ontario Hydro's perspective and experience with power system under excitation limiters (UELs). The purpose of the UEL is reviewed along with the results of field testing and modelling. It is concluded that users of these devices must define the dynamic performance requirements of the limiters, by examining the purpose of their installation, and then setting and testing them accordingly. >

The hottest spot temperature allowance to be used for the different insulation system temperature classes is a major unknown facing IEEE Working Groups developing standards and loading guides for ventilated dry type transformers. In 1944, the hottest spot temperature allowance for ventilated dry type transformers was established as 30/spl deg/C for 80/spl deg/C average winding temperature rise. Since 1944, insulation temperature classes have increased to 220/spl deg/C but IEEE standards continue to use a constant 30/spl deg/C hottest spot temperature allowance. IEC standards use a variable hottest spot temperature allowance from 5 to 30/spl deg/C. Six full size test windings were manufactured with imbedded thermocouples and 133 test runs performed to obtain temperature rise data. The test data indicated that the hottest spot temperature allowance used in IEEE standards for ventilated dry type transformers above 500 kVA is too low. This is due to the large thermal gradient from the bottom to the top of the windings caused by natural convection air flow through the cooling ducts. A constant ratio of hottest spot winding temperature rise to average winding temperature rise should be used in product standards for all insulation temperature classes. A ratio of 1.5 is suggested for ventilated dry type transformers above 500 kVA. This would increase the hottest spot temperature allowance from 30/spl deg/C to 60/spl deg/C and decrease the permissible average winding temperature rise from 150/spl deg/C to 120/spl deg/C for the 220/spl deg/C insulation temperature class. >

Some of the torsional modes of oscillation of turbine-generator multimass shaft systems can be destabilized through the control action of high speed electrohydraulic turbine governing systems. This form of destabilization surfaced during the commissioning of one of Ontario Hydro's 665 MVA nuclear units. The causes of the problem were identified by measurements taken during the commissioning process and a detailed study of the control system. Conclusions reached as to the causes were supported by computer modelling. More accurate valve linearization circuits and filtering of shaft torsional components in the governor speed signal resolved the problem. A recommendation is made for direct monitoring of shaft torsional components, at least during the commissioning process.

Further development of a power system stabilizer based on measurement of unit accelerating power, where the mechanical power component is derived from speed and electrical power signals, is described. Techniques to facilitate the implementation of such a controller are discussed and the ability of this type of stabilizer to remove some of the limitations inherent in other stabilizer designs is demonstrated. The design, testing and commissioning of this stabilizers is covered and field experience with it on large fossil-fired and nuclear units is documented.