The design, installation, testing, and operating experience of a transient excitation boosting (TEB) system at Grand Coulee Third Powerplant on the Columbia River in Washington state is described. The installed TEB system provides significant reactive power support to the northwest power system from this 4000 MVA hydroelectric powerplant. A microprocessor-based TEB module that interfaces to existing solid-state regulators on the six Third Powerplant generators and physically fits into the limited space available was designed. Redundancy, fail-safe circuitry, and complete status indication and alarms were designed into the TEB system from the start. Installation of the TEB system involved significant changes to the regulator wiring. Also significant were the amounts of control cabling and wiring changes needed in the existing plant control systems. Power system stabilizer (PSS) control system changes were necessary to make TEB and PSS control philosophies similar. >