Dissipation Factor (Tan Delta) Testing in High Voltage Current Transformers (CTs)

Importance of Insulation Diagnostics in High Voltage CTs

Current transformers (CTs) are among the most critical components of power networks, playing a vital role in protection and measurement systems. One of the most important considerations in the operation and maintenance of these devices is the continuous monitoring of their insulation condition.

These transformers are installed inside substations, and any insulation failure in them may lead to high-current short circuits. Such faults can escalate and damage more expensive equipment in the substation, such as power transformers.

High-voltage CTs are typically built with oil-paper insulation systems. Over time, this insulation deteriorates due to electrical, mechanical, and thermal stresses. Eventually, the degradation may reach a point where the insulation can no longer withstand these stresses, resulting in dielectric failure.

Insulation aging is generally a slow process. However, with regular monitoring, it is possible to detect early signs of degradation and take corrective action—such as repair or replacement—before a failure occurs and causes a wider system fault.

One of the most effective methods for assessing the insulation condition of CTs and CVTs is the dissipation factor (tan delta) test, which evaluates both the capacitive value and the dielectric losses of the insulation system.

Internal Structure and Capacitive Grading of CTs

In a bottom-core current transformer (CT), the high-voltage primary conductor passes through multiple magnetic cores as shown in the figure below. Since this conductor operates at high voltage, its insulation system must be designed to withstand the electric stress within a very limited space.

The insulation structure of high-voltage CTs is similar to that of high-voltage bushings. In both cases, capacitive grading is employed to control the electric field. This is achieved by wrapping the high-voltage conductor with alternating layers of oil-impregnated paper and aluminum foils, forming a multi-layer capacitive network. These layers divide the applied voltage gradually along the length of the insulation, resulting in a more uniform electric field distribution and lower field intensity at any given point. This technique helps to prevent partial discharges and extend the service life of the insulation system.

In a high-voltage current transformer (CT), the outermost aluminum foil layer is connected to the tank ground. If the CT is equipped with an F-terminal, this means that the second-to-last aluminum layer is brought out and made externally accessible, as illustrated in the figure below.

This design creates a capacitive voltage divider, similar to what is found in high-voltage bushings. The divider consists of:

• C1: Capacitance from the primary high-voltage conductor to the F-terminal.
• C2: Capacitance from the F-terminal to ground.

The capacitance C1 is composed of multiple series-connected capacitive layers depending on the voltage class of the CT. These layers form a graded insulation system to manage the electric field.

If an overvoltage event occurs and the dielectric strength of any of the series insulation layers is exceeded, partial short circuits may develop. This leads to a reduction in the number of effective series capacitors, thereby increasing the total C1 capacitance.

Simultaneously, if the insulation is deteriorating due to aging or contamination (e.g., carbonization of oil-impregnated paper), the dissipation factor (tan δ) of the system will also increase.

Therefore, by measuring the capacitance and tan delta of C1, one can effectively assess the condition of the CT’s internal insulation system.

For more comprehensive diagnostics—or to specifically evaluate the condition of the F-terminal—the capacitance and tan delta of C2 can also be measured. It is important to note that:

• The value of C2 is approximately equal to one of the individual capacitors in the C1 series stack.
• Therefore, C2 is significantly larger than the total C1 capacitance.

These measurements help to detect issues such as layer-to-layer shorting, moisture ingress, or dielectric degradation in the capacitive divider network.

Dissipation Factor & Capacitance Testing of CTs with F-Terminal

If a CT has an F-terminal, then two types of tests must be performed on it. The first test involves the measurement of C1, and is conducted according to the circuit shown below. The high-voltage output of the test set is connected to the primary terminal of the CT, and the measuring terminal (A) is connected to the F-terminal. The measurement is performed in UST-A mode.

In this configuration, the capacitance (C1) and dissipation factor (tan delta) of the C1 insulation are measured.

The second test on CTs equipped with an F-terminal involves measuring the C2 insulation. This test is typically performed at a low voltage (commonly 500 V), but it is critical to consult the manufacturer’s manual to verify the maximum voltage that can be safely applied to the F-terminal.

In this test setup:

• The HV output of the test set is connected to the F-terminal.
• The measuring terminal (A) is connected to the primary terminal of the CT.
• The measurement mode must be set to GSTg-A.

This configuration allows the measurement of the C2 capacitance and its corresponding dissipation factor.

*Never apply a test voltage higher than the rated value for the F-terminal, as doing so may lead to insulation damage or failure.

To validate the accuracy of the individual C1 and C2 measurements, a third optional test can be performed in GST mode. In this mode:

• The HV output is again applied to the F-terminal.
• The ground connection is made as per GST mode configuration (typically the CT base or tank).
• The measurement returns the total insulation capacitance, i.e., C1 + C2.

By comparing the sum of the separately measured C1 and C2 values with the value obtained in GST mode, the reliability of each individual test can be confirmed.
A significant discrepancy between (C1 + C2) and the total measured in GST mode may indicate internal insulation issues, connection errors, or leakage paths.

Dissipation Factor Testing of CTs without F-Terminal

For current transformers that do not have an F-terminal, the insulation between the primary and ground cannot be separated into C1 and C2 components. Therefore, only a single measurement can be performed, using the GST mode, as shown in the diagram below.

In this configuration:

• The high-voltage (HV) output of the test set is connected to the primary terminal of the CT.
• The ground terminal of the test set is connected directly to the CT tank, which acts as the ground reference for the insulation system.

It is critical that both the test instrument’s ground and the CT’s ground are connected to the same grounding point—specifically, to the CT tank. This ensures that the ground current path is properly controlled and prevents measurement errors due to stray currents or floating ground potential.

In this configuration, the measured capacitance and dissipation factor represent the overall insulation condition of the CT. If there is no access to the F-terminal, this single measurement is the only available way to monitor insulation aging or deterioration.

Practical Guidelines for Reliable Dissipation Factor Measurements

Several important considerations must be observed when performing dissipation factor testing on current transformers (CTs):

• No need to disconnect secondary circuits:
  There is no requirement to disconnect secondary circuits or remove the burden during dissipation factor testing. No current flows through the secondary windings during the test, and no overvoltage is transferred to them. On the contrary, disconnecting them may introduce the risk of forgetting to reconnect or reconnecting to the wrong terminal, which can lead to dangerous situations when the CT is energized.
• Isolation requirements depend on CT type:
  For CTs equipped with an F-terminal, full physical disconnection is not necessary. It is sufficient to de-energize and isolate the CT from the system using disconnector switches and circuit breakers. However, if high induced voltages (e.g., several kilovolts) are observed, the primary terminals should also be disconnected to prevent damage to the test equipment.
  In contrast, for CTs without an F-terminal, the primary connections must be fully disconnected. This is because in GST mode, all leakage paths to ground contribute to the measurement. If the CT remains connected to the system via its primary terminals—even if de-energized—the surface leakage currents from bushings, disconnectors, and circuit breakers may distort the tan delta and capacitance readings.
• What the test actually measures:
  Tan delta testing on CTs only evaluates the condition of the insulation between the primary conductor and ground (or F-terminal). It does not assess the insulation between primary and secondary windings, nor between different secondary cores. Even if such tests are attempted, they usually provide no meaningful diagnostic information.
  To evaluate the insulation condition of the secondary windings, one must perform:
  • Saturation tests at rated frequency
  • Insulation resistance tests, which will be covered in future application notes.
• Temperature dependence:
  As with other high-voltage equipment, the dissipation factor of CTs is temperature-dependent. Temperature correction factors should be obtained from the CT manufacturer to ensure correct evaluation of results.
• Trending and comparison are essential:
  The measured values must always be interpreted in the context of baseline (factory or commissioning) values and compared against historical records from previous maintenance cycles. Only through such trend analysis can developing insulation problems be reliably identified.