Transformer bushings play a crucial role in ensuring the safe and efficient operation of transformers. These bushings provide electrical insulation and support for high-voltage conductors, allowing the transfer of electricity from the transformer to external circuitry. To ensure the reliability of transformer bushings, it is important to conduct regular testing, including capacitance and dielectric dissipation factor (C&DF) tests.
What is a C&DF Test?
Capacitance (C):
Measures the bushing's ability to store electrical charge. Any deviation in capacitance may indicate internal defects such as voids, cracks, or moisture ingress.
Dielectric Dissipation Factor (DDF / Tan δ):
Also known as the power factor, it measures the energy loss within the insulation. An increase in DDF signals insulation deterioration.
C1 and C2 Measurements
C1 – Main Insulation:
Represents the capacitance and DDF of the core insulation of the bushing. It reflects the overall health of the main insulation system.
C2 – Stress Cone/Grading Region:
Focuses on the region more prone to moisture and surface contamination. C2 is highly sensitive for detecting localized problems early.
Why Are These Measurements Important?
1. Early Fault Detection – Identifies insulation degradation before it escalates into catastrophic failures. Predictive Maintenance – Tracking C1 & C2 trends over time helps optimize maintenance schedules and budgets.
2. System Reliability – Ensures transformers continue to operate safely, reducing unplanned outages and extending equipment lifespan.
To conduct C&DF tests on transformer bushings, specialized equipment such as a tan delta test set is required. The test involves applying a test voltage to the bushing and measuring the capacitance and dissipation factor values. These values are then compared to the manufacturer's specifications or previous test results to assess the condition of the bushing.
