Tva Hydro Generator Trip On Current Unbalance
A TVA hydro generator tripped by negative sequence overcurrent due to system voltage unbalance three times on 1/22/2025 and once on 1/25/2025. A similar generator trip occurred several years prior to the most recent event at a different hydro plant.
Prior transmission system reconfiguration led to the system voltage unbalance. A few days prior to the trip, there was a temporary transmission system reconfiguration which redirected the Hydro plant into an alternate substation instead of the normally connected substation. This reconfiguration was implemented to avoid creating a three-terminal line while work was being done at a nearby substation to replace a breaker.
TVA has had issues in the past with voltage unbalance problem due to untransposed transmission lines. The issue of voltage unbalance especially manifests itself on long heavily loaded transmission lines, particularly at the end of long radial lines or in the middle of long network transmission lines.
System voltage unbalance at the high side of the generating plant manifests itself in current unbalance produced by the generator which may lead to generator rotor damage due to high temperatures. Generator current unbalance limits are defined by IEEE Standard for Salient-Pole 50Hz and 60Hz Synchronous Generators and Generator/Motors for Hydraulic Turbine Applications Rated 5 MVA and Above C50.12-2005. These limits are expressed as a fixed ampere limit as a percentage of rated machine MVA.
Some questions arose around interpreting the limits provided in the standard such as is the permissible negative sequency current a percentage of the actual load or the rated load. In subclause 4.1.6.1 it is stated that “negative-sequence current is expressed as a percentage of rated stator current”. In addition, “These values also express the negative-sequence current capability at reduced generator MVA capabilities, as a percentage of the stator current corresponding to the reduced capability”. Some interpreted the last sentence in the quoted subclause to mean the I2 capability is not a fixed value in amperes, rather it should be a percentage of calculated positive sequence current at whatever output the machine is operating at. The other way of interpreting this is that the I2 capability is a fixed value in amperes based on the full MVA rating of the machine and that current limit applies regardless of how the machine is loaded.
The main theme of the paper is to determine the expected amount of negative sequence current given a certain level of voltage unbalance and clarifying the interpretation of the subclause in the standard that addresses negative sequence limits. The authors would like to review the guidelines for utilizing negative sequence generator protection per industry established guidelines. It was determined that the generator relays at the Hydro plant were properly set and were correctly protecting the machine from rotor heating due to current unbalance for all four trip events. The paper will also highlight changes introduced in the 2025 version of C50.13.