Capacitance & Tan δ Measurement

Transformers are critical pieces of equipment which play a vital role in a variety of different industries. Ensuring this asset remains in top-notch, healthy, and reliable condition is important. Transformers are well-maintained without causing them unnecessary outages and big financial losses. Power Factor or Tangent Delta TEST is one of the recommended and most popular diagnostic tests for power transformers. It is also called the Capacitance and Dissipation Factor Test.

The overall power factor measurement is used to assess the integrity of the insulation system within a power transformer. This insulation system is mainly comprised of cellulose insulation and an insulating fluid (such as mineral oil, natural ester, and silicon, among others), which may become compromised due to one or more of the following reasons,

• Natural aging and deterioration
• Overheating
• Moisture Ingress
• Localized defects (such as partial discharge, voids, cracks, and partial or full short-circuits)

This test should be used to indicate the dielectric losses in insulating liquid when used in an alternating electric field and to indicate the energy dissipated as heat. The DF is the ratio of the power dissipated in the insulating liquid in watts to the product of the effective voltage and current in volt-amperes when tested with a sinusoidal field under prescribed conditions. A low DF indicates low dielectric losses. It is useful as a means to maintain sample integrity and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling. Insulating liquid samples that are defective often pass other standard electrical and chemical tests, yet fail this test. This test may be satisfactorily performed in the field and in a laboratory environment.

Partial Discharge Analysis

A partial discharge is an electrical discharge bridging a portion of the insulation between two electrodes. The discharge may occur at any point in the insulation system when the electric field strength exceeds the breakdown strength of that portion of the insulation system. In transformers, the insulation system is comprised of the winding insulation material as well as the dielectric fluid. “Localized electrical discharge that only partially bridges the insulation between conductors and which can or cannot occur adjacent to a conductor.”

PD can be initiated by voids or cracks within a solid dielectric, at interfaces within solid or liquid dielectrics, in bubbles within the dielectric fluid, or along the boundary between different insulation materials. Partial discharges in transformer windings can begin as small voids in the paper insulation. As PD activity progresses, the repetitive discharges eventually cause permanent changes within the affected solid insulation and impregnating dielectric fluid which reduces the insulation qualities of either or both. Over time, partially conducting carbonized trees are formed. This places greater stress on the remaining insulation, leading to further growth of the damaged area, resistive heating along the tree, and charring also known as tracking. This eventually culminates in the complete dielectric failure of the transformer.

There are three different types of partial discharges

• Surface PD Discharge
• Internal PD Discharge
• Corona PD Discharge

Partial Discharge tests can be done under different methods

Ultrasonic methods are particularly effective at finding surface PD and corona PD. Acoustic emissions from PD activity are normally at a frequency too high for the human ear, i.e. ultrasonic. As the PD gets worse, the frequency sometimes decreases into the audible range. Using an airborne ultrasonic microphone is the most sensitive way to detect PD where there is an air path between the source and the microphone. Contact ultrasonic sensors can be used for sealed chambers.

UHF is ideal as a non-contact detection method for electromagnetic PD signals in open terminal switchyards, cable systems and distribution networks.

Coupler method

Sweep Frequency Response Analysis (SFRA)

A sensitive diagnostic technique for detecting changes in the electrical characteristics of power transformer windings. Such changes can result from various types of electrical or mechanical stresses (shipping damage, seismic forces, loss of clamping pressure, short circuit forces, insulation failure, etc.). The test is non-destructive and non-intrusive and can be used either as a stand-alone tool to detect winding damage, or as a diagnostic tool to pinpoint damages detected in other tests (e.g., insulation power factor, dissolved gas analysis, or short circuit impedance tests). FRA consists of measuring the admittance or impedance of the capacitive and inductive elements comprising the transformer windings. The measurement is performed over a wide range of frequencies and the results are compared with a reference “signature” or “fingerprint” of the winding to make a diagnosis.

Mechanical stresses may cause displacement of transformer windings from their position and may also cause deformation of these windings

Winding faults                                     Mechanical fault/changes

• Deformation                                Clamping structures 
• Displacement                              Connections
• Shorts

Core-related faults                                Electrical faults

• Movements                                   Shorted turns/open circuit windings
• Grounding                                     Bad ground connection of transformer tank
• Screens

The test involves measuring the frequency response of each winding. The frequency is measured by injecting a sine wave signal w.r.to earth at one end winding to be tested and measuring the signal amplitude there at the other end of the winding. Transformer issues can be detected in different frequency ranges

Low frequencies

Middle frequencies

High frequencies

Turns ratio test

Transformer Turns Ratio (TTR) is one of the most common tests used to assess the condition of the transformer's windings and core. It is performed as a part of the acceptance and maintenance test procedure to determine any problems due to poor design, assembly, handling, overloading, fault conditions or poor maintenance.

The test ensures the correct ratio of both primary turns and secondary turns and guarantees the proper functioning of the power transformer. Turns Ratio measurements of a transformer are extremely important for several reasons, such as Quality verification, Validation of design specifications, Assessment of possible damage, Establishment of transformer condition and condition trends. During the test, if the Turns Ratio results differ from the expected target values, the probability of a defect or error is high. Untargeted results could indicate a defect in windings or a tap-changer.

Voltage/ Magnetic Balance test

A magnetic balance test is conducted only on three-phase transformers to check the imbalance in the magnetic circuit. The result of this test indicates the uniform distribution of flux insulation. This test is carried out by applying 3 phase 400V AC to one winding, measured induced voltages on the other two winding on the same side. In general, the magnetic balanced test is carried from the HV side.

The transformer is connected in

• Star
•  Delta

This test is performed to check:

• The imbalance in the magnetic circuit
• Any fault or defect in the magnetic core
• To identify inter-turn faults in the transformer

Winding resistance

Winding resistance measurements are used to perform for assessing possible damage in windings or contact problems, such as from the bushings to the windings, the windings to the tap changer, etc. This test is a verification that the proper size of conductors has been used and that the joints have been made properly. Since this test is indicative, there is no tolerance applicable to the measured resistances. Resistances of the windings are measured by using a 'resistance bridge'

Insulation resistance

The insulation resistance (IR) test is performed to evaluate the insulation resistance of transformers. Insulation Resistance shows the resistance value of the insulation between copper conductors to earth Based on the IR value, we can reveal the degradation of insulation due to contamination, moisture, and severe cracking. The test can be performed through a megger by applying a constant DC voltage of 2500 volts to 5000 volts for 10 minutes between HV and LV windings, LV windings to earth, and HV windings to earth. An electrical power transformer's total insulating system must pass this test to be deemed healthy. This test is run between the LV and HV windings, as well as between the HV and LV windings.