The ETP-4 module measures the short-circuit impedance in distribution and power transformers by applying a test voltage on the high voltage winding and measuring the resulting current and phase angle. The reading is automatically compensated by the transformer's temperature value and normalized to 75º C / 165º F.

The following problems can be detected with the ETP-4 module:

• Shorted turns
• Interrupted windings
• Tap changer dirt or damages
• Magnetic core problems

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, wich 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.

The unit performs the power transformer testing in a fully automatic way. The test voltage is generated through a motor driven "variac". This assures the repeatability of the successive tests in terms of the voltage application mode and avoids undesired risks due to sudden transients.
 

Characteristics

• The software program guides the correct connection of the test cables on the primary winding and the short circuits that must be done in the secondary (or tertiary) side, according with the specific connection group the transformer under test has.
• The software program recommends the tap changer positions that should be tested according with the maximum regulation % and the nominal tap.
• Displays and records the current and voltage waveform. This improves the diagnostic capability for further analysis.

 

Applications

From the information supplied by this test unit it is possible to have an idea about the physical integrity of the transformer under test. The typical problems that are easily detected with this unit are the following:

• Physical displacement of the transformer windings, due to electromagnetic efforts.
• Problems in the iron core.
• Transport damages.
• Fixation elements which may be broken.

The program obtains from the measured values the components of the impedance triangle in short circuit. From this information, and its evolution in time it is possible to determine which part of the transformer is being deteriorated.
The test also confirms any incipient problem, which may have been detected with other test equipment that make up the ETP system.
The following types of voltage transformers can be tested with the ETP-4, despite the size and power:

• Power Transformers (single or three phase).
• Distribution Transformers (single or three phase).
• Power Auto transformers (single or three phase).
• Instrument Voltage Transformers.