SMC ETP-3 Three-phase Winding Resistance Meter

• High immunity to external perturbations
• Extremely fast measurement cycle
• Automatic calculus of the resistance per phase

The ETP-3 is designed to measure very low resistance values on transformer windings or on any load with a strong inductive component. The unit applies the necessary DC current to magnetize the winding and stabilize the current, in order to provide an accurate, repeatable measurement.

The software takes into account the connection group and the temperature to calculate simple (u, v, w) and compound (uv, uw, vw) ohmic resistance values referenced to a normalized temperature (temperature-corrected). The ohmic values obtained allow verifying the correct insulation between turns, and to assess the transformer’s specified electrical and mechanical characteristics.

Winding Resistance Graphics

The following problems can be detected with the ETP-3:

• Loose connections
• Tap changer malfunction
• Shorted turns
• Hot spots on the windings

Complete three-phase system

The ETP System is composed by four independent measurement modules controlled from a common measurement & analysis software in a PC. Diagnostics are performed by co-relating the measurements done by each module:

• Turns Ratio: three-phase measurement of the turns ratio and the excitation current in ever y position of the tap changer. Results are compared with the theoretical values and the error is shown graphically.
• Recovery Voltage: evaluation of the solid and liquid insulation condition through electrical measurements.
• Winding Resistance: three-phase measurement of the resistance values in every winding and for every position of the tap changer.
• Short-circuit Impedance: evaluation at low voltage of the short-circuit impedance and voltage.
• The above measurements are used to evaluate the condition and then predict possible failures at the four main circuits in a transformer:
• Electrical Circuit: checking the continuity of the windings, the connections and the tap changer.
• Geometrical Circuit: checking the symmetry between the columns of the core, between the windings and the core and between these and the transformer's enclosure.
• Magnetic Circuit: checking the condition of the magnetic core (loose plates, short-circuits, detachment of the magnetic shunt, etc.).
• Dielectric Circuit: checking the degradation and the ageing of the solid and liquid insulation. Testing is a simple, automatic and fully software-guided task. The operation is divided into to main steps: 1) Measurement, which guides the operator to the connections and wait operations, eliminating human errors and ensuring repeatability, and 2) Analysis, by immediately displaying the measured values on the screen as they are acquired by the software. The information can be immediately converted into a complete report and interpreted by an expert to determine whether maintenance operations should be planned or not.