Flexible Coil Sensor for Generator Stator Current

Why Generator Stators Need Flexible Coils

Standard donut current transformers cannot fit around the bus bars of a 5–20 MW generator output, especially when buses are paralleled or routed through tight enclosures. A flexible Rogowski coil wraps around any conductor, regardless of shape or position, and measures up to 30 kA AC with linearity better than 0.5% across the full range. For commissioning measurements, troubleshooting, or temporary metering, the flexible coil is often the only option.

Installation Steps

1. De-energize and ground the generator output if regulations require
2. Open the coil and wrap around one phase conductor
3. Lock the connector and orient the arrow toward the load
4. Plug the integrator output into a power quality analyzer or oscilloscope
5. Re-energize and verify polarity matches the analyzer phase reference

Accuracy and Bandwidth

EK flexible coils achieve Class 0.5 accuracy from 1% to 100% of rated current, with bandwidth from DC blocking to 20 kHz. This captures both fundamental current and significant harmonics generated by exciter ripple or non-linear loads on the bus.

Dynamic Range Advantage

Unlike iron-core CTs, Rogowski coils do not saturate. A coil rated 3000 A measures fault currents up to 30 kA without distortion, making the same sensor useful for metering, protection development, and post-event analysis. The integrator electronics have built-in overload protection and recover within milliseconds of the fault clearing.

Common Applications

• Acceptance test of new generators against the manufacturer test report
• Verification of parallel load sharing between multiple generators
• Harmonic analysis on neutral-grounded generators feeding VFD loads
• Temporary metering during load bank tests for emergency power systems

Safety

Although Rogowski coils are inherently safer than CTs because they do not have an open-circuit secondary voltage hazard, installers must still follow all medium-voltage safety protocols when working near 4.16 kV or 13.8 kV generator buses. The coil insulation rating must match the system voltage, or a dedicated insulating sleeve must be used if the coil is rated only for 1000V.

Maintenance and Calibration

To maintain 0.5% accuracy, Rogowski coils should be re-calibrated every two years. Calibration checks the integrator gain and ensures the coil loop connector hasn't developed resistance that would cause phase shift or amplitude errors at higher frequencies.