VLF Tan Delta Testing

Very low frequency (VLF) tan delta testing is a precise and non-destructive method to provide information on the extent of ageing in cable insulation. The test applies an AC sinusoidal waveform at 0.1 Hz frequency and measures the degree of real power dissipation (or losses) in a dielectric material. Knowledge and understanding of the cable condition can help to reduce unnecessary outages and to assist in condition-based and cost-optimized maintenance programs.

What is tan delta?

For simplistic purposes, a cable can be modelled by an equivalent circuit consisting of 2 elements; a resistor and a capacitor. When voltage (U) is applied to the cable under test the total current (I) will be the contributions of the resistive current (I_R) and capacitive current (I_C). Tan delta is the ratio between the resistive current and capacitive current.

If we consider the resistance of an ideal cable to be infinite, then the total current will be purely capacitive and 90° phase shifted to the applied voltage. However, in a real cable system, losses will allow for resistive currents to flow, which will have an impact on the phase shift of the total current with respect to voltage. As a result, a loss angle (δ) will appear, and the tangent of this will correspond to tan delta, which is a dimensionless unit. This relationship also shows that the decrease in insulation resistance will cause tan delta to increase.

Typical reasons for dielectric losses to occur in cables are the presence of water trees in XLPE cables, moisture in joints and terminations, the presence of partial discharge (PD), corrosion of the metallic shield, and poor contact between metallic shield and insulation shield. Water trees are tree-like structures that form from the electrochemical interaction of the electric field and water ingress within XLPE cable. Their growth is extremely slow, but they act as stress enhancements, which can help to initiate an electrical tree.

What are the advantages of measuring tan delta with VLF vs. power frequency?

One of the main advantages with measuring tan delta with VLF is the size and portability of the equipment because 600x less power and current is required to conduct the testing. Without going into the details of this aspect here, please see a previous blog post titled “The Basics of VLF Testing”.

Another very important advantage is the sensitivity of the measurements that a lower frequency enables. Considering tan delta = (2πfRC)^-1, where f = frequency, R = resistance, and C = capacitance, it can be seen that tan delta is very much frequency dependent. At lower frequencies water tree detection is most responsive and sensitive, which can be clearly seen in the following graph.

Since tan delta is measured using a sinusoidal waveform, the test can be combined with PD diagnostics to give a complete overview of the cable condition assessment. Singular defects cannot be located in the cable by conducting a pure tan delta measurement because tan delta is the average value of dielectric losses for the whole cable system. However, PD measurements can be conducted to locate whether PD sources within the cable are the reason for high tan delta values. It is also important to remember that water trees cannot be located using PD diagnostics unless a water tree transforms into an electrical tree.

A paper discussing simultaneous tan delta and partial discharge testing can be downloaded here.

What parameters are typically measured during a VLF tan delta test?

Tan delta testing is typically conducted at 0.5U₀ increments, in which 6-10 measurements are conducted at each increment. Mean tan delta, tan delta stability (standard deviation), and differential tan delta (tip-up; value between 0.5U₀ and 1.5U₀) are measured. The results from each phase can then be portrayed on a graph and compared to adjacent phases, as seen in the graph below.

An advantage of tan delta testing is that for severely aged cable systems, the applied test voltage can remain at or below the operating voltage U₀ to give useful information of the condition of the cable.

The IEEE Std 400.2-2013: IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz) gives recommended test voltages and testing times for simple VLF withstand tests of different cable systems. The guide also helped to create the tan delta monitored withstand test (TD-MWT), which is a cable withstand test while monitoring tan delta. The test is typically divided into two phases – 1) Voltage Ramp-up Phase and 2) Voltage Hold Phase.

During the hold phase, the parameters mentioned above are measured. The values can be compared to condition assessment criteria, or threshold values, pertaining to different cable types, which are given in the IEEE 400.2 guide. These values are based upon statistical analysis of many measurements conducted during the Cable Diagnostic Focused Initiative by NEETRAC. The hold phase is then conducted at 2U₀ and tan delta is continuously monitored over time to measure the temporal stability.

Depending on the severity of the tan delta results, a test time of 30 to 60 minutes is recommended so that breakdown of the cable results during the test and not when in service. On the other hand, if cables have excellent and stable tan delta values, the test may be ended after 15 minutes because good cables do not need to be subjected to enhanced stresses for long lengths of time. This also has the added benefit that testing time and costs are reduced, as the cable can be returned to service much faster.

Papers of the TD-MWT method using the Frida TD can be found here:

1. VLF-MWT – How to apply the new way of cable condition assessment (with Frida TD datasheet)
2. Combining VLF testing and diagnostics for practical use

How to interpret the results?

Even though the actual tan delta test is very simple and easy to perform, the most challenging aspect is actually knowing what to do with the results. This is where operator experience and historical testing data come very handy.

IEEE 400.2 has listed condition grades of cable insulation according to threshold values for measured tan delta parameters for different cable insulation types. The condition grades are listed as:

• No action required
• Further study advised
• Action required

“No action required” means that the tan delta values are within a certain limit and there is no indication that a potentially severe problem is imminent in the cable system and can be returned to service. It is recommended to retest the cable at a later date in order to conduct a trend analysis; i.e., is there a worsening condition with time?

“Further study advised” means that certain tan delta values have been recorded during testing that would indicate areas of poor insulation condition in the cable system. In order to make a condition assessment, further information of the cable is required. This could include:

• Comparing historical results of the tested cable system. Has tan delta continuously increased over time with each conducted test? At what interval were the tests conducted?
• Conducting a monitored withstand test. Is there a decrease of tan delta with increase of testing time? This could be an indication of moisture within cable system being evaporated with the application of voltage.
• Conducting a PD test to locate sources of PD along the cable. If no PD can be detected, this might be an indication of a heavily water-treed cable section.
• Visual inspection of accessories, such as joints and terminations, where accessible, then replace and remeasure.
• Mark the cable system for a retest in the very near future.

“Action required” means the cable system has extremely high tan delta values indicating a significant problem. Measures to conduct then could be:

• Conducting a simple withstand test at 3U₀ in order to cause a failure. Afterwards repair and retest.
• Conducting a monitored withstand test in order to cause a failure. Afterwards repair and retest.
• Immediate replacement of cable system.

As can be seen, there are many possible subsequent tests that can be conducted depending on the severity of the tan delta results, and there is no clearly defined preventative procedure that should be taken. Most important is to compare measured tan delta results to historical data, either with the threshold values given in IEEE 400.2 or with a historical database created from past testing. An operator will best know the cable system design to develop their own specific threshold criteria and determine the best subsequent procedure to be taken.

The goal of tan delta diagnostic testing is to allow for a simple and time-efficient testing solution in order to realize a condition-based and cost-optimized maintenance plan.

HVT’s Range of VLF Tan Delta Testers:

Frida TD	Viola TD	PHG 80 TD
Voltages up to 26.1 kVRMS Integrated tan delta measuring function – no additional hardware required Weighs only 22 kg (48 lbs) Ideal for testing voltage class cables up to 20 kV Ideal for maintenance testing of 25 kV cable systems	Voltages up to 44 kVRMS Integrated tan delta measuring function – no additional hardware required 2 part design for enhanced portability Ideal for testing voltage class cables up to 35 kV Ideal for maintenance testing of 46 kV cable systems	Voltages up to 57 kVRMS Ideal for testing voltage class cables up to 46 kV Ideal for installation in a test van