The ETP-2 module measures the recover y voltage (polarisation spectrum) on power and distribution transformers and autotransformers of any type with oil-paper dielectric, calculating the degree of humidity contained in the paper and evaluating the overall condition of the oil- paper system.

These calculations are performed by applying various DC voltage levels through a programmable output during specific time inter vals. Calculated parameters are:

• Recovery voltage and Time Constant
• Insulation Resistance
• Polarisation index

The following problems can be detected with the ETP-2:

• Solid dielectric degradation
• Liquid dielectric degradation
• Contaminated insulation

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.

Characteristics

• Fully automatic Recovery voltage test in all the test cycles of voltage application, discharge of the insulation and measurement acquisition.
• Automatic generation of the measured values on each test cycle graphic representation.
• Total test time of 40 minutes using the time base of 100 ms.
• The connection to the transformer is guided from the computer to avoid connection errors.
• All measurement processes are automatic and the results do not depend of the user personal criteria.
• Results values are automatically corrected to a reference temperature enabling, standard results.
• Numerical and graphic presentation give the results of the following parameters:
• Recovery Voltage.
• Raising time of the capacity.
• Insulation resistance.
• Time constant.
• Polarization Index.

The graphic curves gives very important information about the evolution of the obtained values, which makes the diagnostic of possible problems fast and easy.

The control program allows selecting the following possibilities to perform the insulation tests:
HV + LV against Ground.
HV against LV + Ground.
HV against LV + Tertiary + Ground.
 

Applications

The user-friendly software, quick measuring speeds, and the ability to get instantaneous organized results (on the screen, recorded or printed), the ETP-2 has great advantages when performing the following tasks:

• Quality Control after the manufacturing process.
• Reception of new or repaired transformers.
• Detection of possible problems during the maintenance task itself.
• Integrates the data inside a predictive maintenance program.

The recovery voltage test, designed for oil insulated transformers, is a new outstanding technology which allows advanced diagnostic of the paper insulation status and ageing, without considering the status of the insulation oil. It is a great advance, due to the fact that until now, there is no other method to know the paper insulation status, without extracting it from the inside of the transformer.

At the same time that the recovery voltage test occurs, the insulation resistance test, the polarization index and the time constant test are performed as well, delivering all the necessary complementary information for a proper and accurate evaluation of the general insulation status.

The automatic recording of the successive test results creates a database, which allows a trend analysis and an early detection of possible problems in the insulation. The following types of voltage transformers can be tested with the ETP-2, 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.
• Current Transformers.