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Beckwith Electric Home

Impact of Voltage Transformer Connection and Grounding on Generator Protection Security and Selectivity

Introduction

Voltage-based elements applied in generator protection depend on voltage signals sourced from generator-zone voltage transformers (VT). These VTs reduce the potential form the high generator terminal voltage to a lower value used by the protective system. This value is typically 120 VLL, which is 69VLN. Other values may be used provided the protective system is designed to handle the nominal rated voltage, as well as overvoltages that may be encountered.

Of concern to designers and operators of protective systems is the impact of ground faults on the secondary side of the VT. These may occur from faulty insulation, fault of protective device, meter or transducer connected to the VT secondary and careless maintenance procedures ("screwdriver shorts"). VT secondary side ground faults may manifest themselves as a generator ground fault trip or other voltage-based element trip, resulting in the machine being taking out of service, as well as possible incorrect target information. VT secondary windings are typically grounded for safety reasons.

Several connection and grounding strategies have been developed to improve protective system security as well as provide proper targeting information. In addition, digital protection systems may employ line-to-line quantities in systems with line-to-ground VTs to obtain proper targeting and segregated phase voltage-based metering and oscillography.

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