What is the Tan Delta Test?
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A pure insulator when is connected across line and earth behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have a capacitive component. There is no resistive component of the current, flowing from line to earth through the insulator as in ideal insulating material, there is zero per cent impurity.
In a pure capacitor, the capacitive electric current leads the applied voltage by 90o.
In practice, the insulator cannot be made 100% pure. Also due to the aging of insulators, the impurities like dirt and moisture enter into them. These impurities provide the conductive path to the current. Consequently, an electric leakage current flowing from line to earth through the insulator has a resistive component.
Hence, it is needless to say that, for a good insulator, this resistive component of the electric leakage current is quite low. In another way, the healthiness of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component. For a good insulator, this ratio would be quite low. This ratio is commonly known as tanδ or tan delta.
Sometimes it is also referred to as the dissipation factor.
Thus, tan δ = IR/ IC
NB: This δ angle is known as the loss angle.
On which instruments Tan Delta testing can be done?
Tan δ testing can be done on various power types of equipment used in substations like transformers, winding, current transformer, potential transformer, transformer bushing, cables, generators. It is performed to assess the quality of insulation and is performed in combination with various test likeTTR, WRM, etc.
Reasons for doing Tan δ testing?
The main purpose of the tan delta test is to make sure of maintaining a secure and reliable functioning of the transformer. The calculation of dissipation factor and capacitance values provides the result of insulation behavior of bushings and in windings too.
Variation in the capacitance value, for instance, indicates partial kind of breakdowns in bushings and automated movement of windings. Insulation deprivation, aging of the equipment, enhancement in the energy levels is transformed into heat. The amount of losses in these is calculated as the dissipation factor.
With the tan delta testing method, one can easily know the dissipation factor and the capacitance values at the required level of frequencies. So, any kind of aging factor can be identified earlier and the corresponding action can be implemented.
As we know that 'Transformers' plays a very crucial part in power, so, first of all, will discuss Tan Delta Testing in Transformers.
Tan Delta Testing Process
The below process explains the method of tan delta testing in a step-by-step manner
The requirements necessary for this test such as cable, potential transformer, bushings, current transformer, and winding on which this testing is conducted has to be initially separated from the system.
The minimal frequency level of test voltage is applied along with the equipment where the insulation to be analyzed.
At first, normal voltage levels are applied. When the tan delta values are as expected at this voltage level, then the applied voltage level is increased by 2 times as of applied voltage.
The values of the tan delta are recorded by the tan delta controller.
To the tan delta calculating component, a loss angle analyzer is connected which compares tan delta values at higher and general voltage levels and delivers accurate results.
It has to be noted that the testing procedure to be carried out at very minimal frequency levels.
It is more recommended to conduct testing at minimal frequency levels, because when the applied voltage level is more, then the capacitive reactance of the insulator device reaches very minimal, therefore the capacitive element of the current reaches more. As the resistive element is practically constant; it is based on the applied voltage level and the insulator's conductivity value.
Whereas at increased frequency level the capacitive current, is more, and then the amplitude of the vector amount of both the capacitive and resistive elements of the current reaches very high. So, the necessary level of power for the tan delta test would become more that seems to be not acceptable. Because of this, the power constraint for dissipation factor analysis, very minimal frequency test voltage is required.
What are the Different Modes of the Tan Delta Test?
When it comes to the tan delta test, there are essentially three modes of power factor testing. Those are
GST Guard ' This calculates the amount of current leakage to the ground. This method eliminates the current leakage through red or blue leads. Whereas in UST, the ground is termed to be guard because grounded edges are not calculated. When the UST method is applied to the device, then the current measurement is only through blue or red leads. The current flow through the ground lead gets automatically bypassed to the AC source and thus excluded from the calculation.
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UST Mode ' This is employed for the calculation of insulation in between ungrounded leads of the equipment. Here the individual portion of isolation has to be separated and analyze it having no other insulation connected to it.
GST Mode ' In this final mode of operation, both the leakage pathways are calculated by the test apparatus. The current, capacitance values, UST, and GST guards, loss in watts need to be equal to the GST test parameters. This provides the entire behavior of the test.
When the summing value of GST Guard and UST is not equal to the GST parameters, then it can be known that there is some crashing in the test set, or might the test terminal are not correctly designed.
On the whole, this is a detailed explanation of the Tan Delta Test. Here, in this article, we are completely aware of what is a tan delta test, its principle, its purpose, its methods, and its testing technique.
Also, know about what are LV to earth test, HV to earth test, and LV-HV tan delta testing methodologies and keep reading KPM Technologies 'TECH BLOG' and subscribe for different technological development in the field of power sector/ testing sector/ etc
The very low frequency (VLF) tan delta test is a precise and non-destructive method to provide information on the extent of aging 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 assist in condition-based and cost-optimized maintenance programs.
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 (IR) and capacitive current (IC). Tan delta is the ratio between the resistive current and capacitive current.
Simplified circuit depicting a cable as 2 elements (resistor and capacitor).
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.
Phase diagram for an ideal cable system (left) and cable system with losses (right).
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.
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.
Frequency dependence of tan delta values.
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.
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.
Typical tan delta graph comparing measurements on 3 phases of a cable system.
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-: 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.
Typical test sequence for a TD-MWT according to IEEE 400.2
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:
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' 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:
'Action required' means the cable system has extremely high tan delta values indicating a significant problem. Measures to conduct then could be:
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.
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
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