What is a capacitance and loss tangent tester?-News-Huazheng Electric Manufacturing (Baoding) Co., Ltd-

A capacitance and loss-tangent tester is a diagnostic instrument used by power engineers to judge the health of transformer insulation. It simultaneously measures two quantities:

Capacitance (C) – the dielectric’s ability to store electric energy.
Loss tangent (tan δ) – the ratio of resistive to capacitive current in the insulation, also called the dissipation factor or power factor under sinusoidal stress.

Together these numbers reveal moisture, voids, thermal ageing or contamination long before breakdown occurs.

HZ-2000H Transformer Tan Delta Tester.jpg

Why transformer capacitance matters

The winding-to-winding and winding-to-ground structures form distributed capacitors. If these change—because oil level drops, paper dries or moisture enters—the voltage distribution shifts, producing hotspots and extra losses. Tracking C from one maintenance cycle to the next therefore flags mechanical or chemical changes inside the tank.

What the loss tangent adds

A perfect dielectric draws only capacitive current. Real insulation also draws a small resistive component; the phase angle between the two is 90°–δ. Tan δ is the tangent of that lost angle. A clean, dry oil-paper system gives tan δ ≈ 0.001–0.005 at 20 °C; values above 0.01 usually call for drying or oil reclamation. Because the measurement is made at rated frequency (50 Hz/60 Hz) and at a chosen voltage (typically 2 kV–12 kV for field work), it reflects the actual operating stress.

How the test is performed

Modern portable units combine a precision high-voltage source, a reference capacitor and a phase-sensitive detector. The operator attaches three leads—high-voltage, measurement and guard—then presses start. The instrument raises voltage automatically, compares the specimen current with the internal reference, and displays C and tan δ within seconds. Most testers also record watts loss and oil temperature so results can be normalised to 20 °C for trending.

Interpreting the data

A 5 % jump in capacitance often indicates floating screens or oil level loss.
A doubling of tan δ almost always means excess moisture (>2 % by paper weight).
Rising tan δ with increasing voltage points to partial discharge activity; flat tan δ suggests conduction loss.

Routine measurement—yearly for critical transformers, or after every fault—lets crews schedule oil processing or internal inspection instead of running to failure. The payoff is longer insulation life, lower losses and fewer unplanned outages.

What is a capacitance and loss tangent tester?

Why transformer capacitance matters

What the loss tangent adds

How the test is performed

Interpreting the data

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