Well, we all know that there are extensive applications of transformers across many domains. So, it is more crucial to dig deep into the concept of transformer maintenance which involves oil tests, equipment testing, and many others. More concentration is necessary to perform dissolved gas testing where this analyses the entire electrical condition of the transformer. As transformer oil is utilized in circuit breakers, cables, and switches, one has to test the conditioning of the oil too. This is because oil augments the dielectric properties and hence Tan Delta Test is Used to know the condition of oil in the transformer. This article provides a clear and detailed description of what is Tan Delta Test, its principle, different methods, and various modes
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What is the Tan Delta Test?
Tan Delta which is also termed as Dielectric Dissipation or Loss Angle or Power Factor testing method which is performed for testing of insulating oil to know the quality level of the oil. This kind of testing methodology is carried out at two temperature levels. The results that are obtained from the two tests are compared and then consideration is taken in the quality level of the coil. If the test results are good, the oil is continued in service and when the test results are not as expected, then either replacement or change in oil takes place.
Purpose
The main purpose of the tan delta test is to make sure of maintaining a secure and reliable functioning of the transformer. With the calculation of dissipation factor and capacitance values, it provides the result of insulation behavior of bushings and in windings too.
Variation in the capacitance value, for instance, it 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.
Principle of Tan Delta Test
When a pure insulator has a connection between the earth and the line, then it performs like a capacitor. In an ideal kind of insulator, as the insulating substance functions as a dielectric, which is totally pure, then the passage of current through the material holds only capacitive material. There will be no resistive element for the electric current that is flowing from the line to the earth via insulator as in the insulating component, there will be no presence of impurities. The tan delta test circuit diagram is shown as follows:
In a pure capacitive material, the capacitive current precedes the voltage level by 900. As a general, the insulating material is totally pure, and even because of the aging properties of the components, the contaminations such as moisture and dirt might get added. These contaminations create a conductive path for the current. As a result, leakage current that flows from line to earth via the insulator holds resistive elements.
Therefore, it is pointless to claim that, for a good quality of insulator, this resistive element of leakage current is correspondingly minimal. In the other aspect, the behavior of an insulator might be known by the proportion of the resistive element to that of the capacitive element. For good quality of insulator, this proportion is correspondingly less and this is termed as tanδ or tan delta. In a few cases, this is also expressed as a dissipation factor. With the below-depicted vector diagram, it can be known.
Where the x-axis represents the level of system voltage which is the resistive element of leakage current IR. As this capacitive element of leakage current IC precedes by 900, it is taken across the y-axis.
And now, the whole leakage current is given by IL(IC + IR)
And from the diagram, tanδ is (IR /IC)
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tanδ = (IR /IC)
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.
Predicting the Test Results
These exist mainly two approaches to analyze the situation of the insulation method at the time of tan delta testing. The first is, evaluating the past test results to know the worsening of insulation conditions because of the aging effect. Whereas the second scenario is to verify the insulation behavior directly from tanδ value. Here, there is no necessity of assessing past results with that tanδ test values.
When the insulation results are accurate, then the loss factor values are nearly similar for the entire test voltage values. But, in the case when the insulation results are not accurate, then the tanδ values get increased for a higher level of voltages. The increasing tanδ corresponds that, high resistive current element, happens in insulation. These outcomes might be matched with the outcomes of past tested insulators, to go with the appropriate decision either the equipment has to be substituted or not.
This is the way that how to test the result tan delta testing can be done.
What are the Different Modes of the Tan Delta Test?
When it comes to 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 on the device, then the current measurement is only through blue or red leads. The current flow through ground lead gets automatically bypassed to the AC source and thus excluded from the calculation.
- 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.
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(I know I'm not answering the question directly)
Most "meggers" only go to volts, so not enough to test a 5k cable; what's he actual test voltage? Also, IIRC most cable manufacturers will tell you not to test MV cables with DC anyway, at least for modern insulations (see below). A 1kv megger will tell you that the cable is
bad, but it won't tell you that it's really
good for the working voltage and conditions. The common wisdom is to have NETA-certified company do the tests to prove the cable is good unless you're experienced and have the right test gear.
A good source is https://netaworldjournal.org/accept...sting-medium-voltage-electrical-power-cables/, especially the section "High Potential Testing with Direct Current (DC)" and also https://netaworldjournal.org/change...testing-specifications-in-ansi-neta-mts-/
If you search these forums for "mv cable testing", there are some useful discussions, also at
https://hvinc.com/knowledge-center/ has some papers about how tan delta works if you want to get into the guts of the process
and
https://www.okonite.com/media/wysiwyg/Engineering Technical Center/EHB .pdf
(Okonite has a bunch of info about testing).
(I know I'm not answering the question directly)
Most "meggers" only go to volts, so not enough to test a 5k cable; what's he actual test voltage? Also, IIRC most cable manufacturers will tell you not to test MV cables with DC anyway, at least for modern insulations (see below). A 1kv megger will tell you that the cable is bad, but it won't tell you that it's really good for the working voltage and conditions. The common wisdom is to have NETA-certified company do the tests to prove the cable is good unless you're experienced and have the right test gear.
A good source is https://netaworldjournal.org/accept...sting-medium-voltage-electrical-power-cables/, especially the section "High Potential Testing with Direct Current (DC)" and also https://netaworldjournal.org/change...testing-specifications-in-ansi-neta-mts-/
If you search these forums for "mv cable testing", there are some useful discussions, also at
https://hvinc.com/knowledge-center/ has some papers about how tan delta works if you want to get into the guts of the process
and
https://www.okonite.com/media/wysiwyg/Engineering Technical Center/EHB .pdf
(Okonite has a bunch of info about testing).
Ok, interesting stuff.
I would agree that you didn't quite answer my question.
We have roughly 40 feeder cables on site. These cables have been meggered the past 50 years and like I mentioned, when we start getting to around 150 Megaohms, we know trouble is coming soon.
We do our preventative maintenance every 3 years. So roughly test 13 feeder cables a year.
The cables have been neglected by guy before me. His motto was we will deal with it when the time comes. Well, we had two shorted cables last year. Luckily they had back up feeds, but eventually back up feeds run out.
We have 3 cables that we are replacing right now.
Looking forward to the next 5 years, per the megger testing, it look like we need to replace 5 more additional cables.
During our shutdown in July this year, I have the option of VLF testing on the 13 cables. I can obviously do this the next two years as well.
I guess the real question is this:
Should I spend an extra $15,000 to test these 13 cables per year over the next 3 years? I can sort of fit it in the budget but I would like to know if its REALLY going to give me info.....such as, yes this cable meggered bad, but its really OK per the VLF test.
$15,000 (per year) sounds high to me, but maybe not. Please keep in mind these cables will be de-energized and disconnected for megger test already.
If the megger reads bad, but VLF says cable is good, than the $15,000 investment seems wise. Replacing these underground cables is super expensive.
VLF (Very Low Frequency) tan delta testing provides additional insights into the condition of cable insulation that simple "meggering" (insulation resistance testing) cannot offer. Here's a detailed comparison and explanation of why VLF tan delta testing is beneficial:
### Meggering (Insulation Resistance Testing)
1. **Principle**: Measures the insulation resistance of a cable by applying a DC voltage and recording the resistance value.
2. **Frequency**: DC voltage.
3. **What It Tells You**:
- Indicates the general condition of the insulation.
- Can identify gross failures and significant degradation.
- Provides a snapshot of insulation resistance at a particular moment.
4. **Limitations**:
- Limited in detecting water trees or other aging mechanisms not visible through simple resistance measurement.
- Doesn't provide a detailed analysis of insulation health over time.
### VLF Tan Delta Testing
1. **Principle**: Measures the dissipation factor (tan delta) of the insulation by applying a sinusoidal AC voltage at a very low frequency (typically 0.1 Hz).
2. **Frequency**: Very Low Frequency (0.1 Hz).
3. **What It Tells You**:
- **Dielectric Losses**: Tan delta is a measure of dielectric losses within the insulation, which can indicate the presence of moisture, contaminants, or aging.
- **Insulation Quality**: Provides a quantitative measure of the insulation's dielectric properties and its ability to withstand electrical stress over time.
- **Condition Over Time**: Detects trends in insulation condition, allowing for predictive maintenance rather than reactive maintenance.
4. **Benefits**:
- **Early Detection**: Can identify insulation deterioration at an early stage, even before significant resistance degradation is evident.
- **Comprehensive Analysis**: Offers a more comprehensive understanding of insulation health, including detection of water trees, which are common in older cables submerged in water.
- **Condition Monitoring**: Facilitates ongoing condition monitoring, helping to prioritize cable replacements and prevent unexpected failures.
### Why VLF Tan Delta Testing is Worth the Investment
- **Preventive Maintenance**: By identifying cables that are likely to fail in the near future, VLF tan delta testing enables proactive replacement and maintenance, potentially avoiding costly unplanned outages and damage.
- **Long-term Planning**: Provides data that can be used for long-term asset management, helping to extend the life of cables and optimize maintenance schedules.
- **Cost-effectiveness**: While the initial cost of VLF tan delta testing is higher, it can lead to significant cost savings by preventing failures and optimizing maintenance.
### Conclusion
While "meggering" gives you a basic understanding of the insulation resistance, VLF tan delta testing provides a deeper, more comprehensive analysis of the insulation condition. It helps detect issues that might not be evident through resistance testing alone, especially in an older site with cables susceptible to water damage and aging. Investing in VLF tan delta testing can ultimately save costs by preventing unexpected failures and optimizing maintenance schedules.
Most important, You save money after you invest the VLF test and because of the extending your cable life-time