Excitation System Failure Modes: How They Manifest and When to Call a Specialist

Excitation system failures are among the most technically complex failure modes in turbine-generator systems. Unlike mechanical failures that often provide visible or audible warning signs, excitation system failures can manifest as subtle control system anomalies, protection system trips, or — in the worst cases — failures that only appear under specific operating conditions that are difficult to reproduce.

This article examines the primary excitation system failure modes, how they manifest in operating data and protection system records, and when the complexity of the failure warrants specialist engineering support.

Loss-of-Excitation Failures

Loss of excitation (LOE) is one of the most serious excitation system failure modes. When a generator loses excitation, it begins absorbing reactive power from the system rather than supplying it, and if the condition persists, the machine will lose synchronism. LOE can be caused by AVR failure, field breaker failure, excitation transformer failure, or SCR bridge failure.

LOE protection is designed to detect and respond to loss of excitation before the machine loses synchronism. However, LOE protection settings that are not properly coordinated with the generator capability curve can cause unnecessary trips or, more dangerously, allow the machine to operate in the loss-of-excitation region without tripping.

SCR Bridge Failures

The SCR bridge is the power conversion element of the static excitation system. SCR failures can cause field forcing (if the SCR fails in the conducting state) or field suppression (if it fails in the non-conducting state). Field forcing is the more dangerous failure mode, as it can drive the generator into the overexcited region and cause overheating of the rotor winding.

PSS Instability

Power system stabilizer (PSS) instability occurs when the PSS is tuned to provide positive damping under one set of system conditions but provides negative damping under different conditions. PSS instability can cause low-frequency oscillations that are difficult to diagnose and can destabilize the interconnected power system.

Protection Coordination Failures

Protection coordination failures occur when excitation system protection settings are not properly coordinated with the generator capability curve, the transformer protection, or the system protection. These failures can manifest as unnecessary trips (settings too conservative) or as failures to trip when the machine is in a damaging operating condition (settings too permissive).

When to Call a Specialist

Specialist excitation engineering support is warranted when: a protection trip cannot be explained by reviewing the event data; PSS tuning is required after significant network changes; excitation system modifications are being considered that affect protection coordination; or the OEM's recommended solution involves significant capital expenditure that the plant wants to independently evaluate.