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)
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.
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, purpose of it, methods, and testing technique. Also know about what are LV to earth test, HV to earth test, and LV-HV tan delta testing methodologies?
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Voltage requirement for Tan-Delta Test
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Voltage requirement for Tan-Delta Test
Voltage requirement for Tan-Delta Test
power(Electrical)
(OP)
11 Aug 06 03:06
We are planning to carry tan-delta test for 16.5kV,260MVA generators & 400/16.5kV generator-transformers. Experts pl. discuss that how much voltage needs to be applied as per standards?
Advanced thanks...
Replies continue below
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RE: Voltage requirement for Tan-Delta Test
Skogsgurra(Electrical)
11 Aug 06 04:13
Tan(delta) is normally done up to rated voltage. I know that Siemens does it up to 1.4 times rated voltage on new machines before letting them out of factory.
Divide rated voltage in five equal parts. Start at 20 % and then 40 % and so on. There is a limit to how much tan(d) may change between steps, I remember something like 0.01/step - or was it 0.005? Normal values are around 0.01 at 20 % but up to 0.03 can be seen without too much worries.
No expert. But been there and seen how it is done.Tan(delta) is normally done up to rated voltage. I know that Siemens does it up to 1.4 times rated voltage on new machines before letting them out of factory.Divide rated voltage in five equal parts. Start at 20 % and then 40 % and so on. There is a limit to how much tan(d) may change between steps, I remember something like 0.01/step - or was it 0.005? Normal values are around 0.01 at 20 % but up to 0.03 can be seen without too much worries.
Gunnar Englund
www.gke.org
RE: Voltage requirement for Tan-Delta Test
power(Electrical)
(OP)
11 Aug 06 09:03
Thank you Mr.Skogsgurra,
The rated voltage is rated Phase voltage or rated line voltage if we need to test between phase to earth (R-Y+B+E)
And pl. give if any standard available,
RE: Voltage requirement for Tan-Delta Test
chris(Electrical)
11 Aug 06 09:46
Power,
I have conducted tan delta measurements on large generators and GSU transformers rated up to 600MVA for over 20 years. In theory, tan delta is independent of voltage. For oil-filled transformers, this is true in practice. However, on solid insulation systems, such as a generator, tan delta can be effected by corona activity in the stator winding. The change in power factor as the voltage is increased is refered to as "tip-up."
I usually perform tan delta measurements on transformes at 10kV maximum, but never exceed rated LN voltage. I have performed tan delta measurements on the same transformer using a small battery power test set (9V) and have acheived nearly the same value (ie: 0.5%). In the USA, tan delta is very commonly performed at 10kV.
I use tan delta during the dryout process of large power transformers, using the 9V test set. Tan delta is very sensitive to the degree of moisture in the windings, thus is an excellant indicator of the dryout progress. Below 1%, "percent tan delta = percent moisture" is a good rule of thumb. Thus, we continue our dryout until the numbers are flat and below our limits.
Chris
RE: Voltage requirement for Tan-Delta Test
power(Electrical)
(OP)
11 Aug 06 09:59
I gone through last year readings for 5.1mW motors, tan delta readings are 6% at line voltage applied to LN. Is this reading o.k ? What is the limitation and what is the significance of tip-up value?
Thanks a lot..
RE: Voltage requirement for Tan-Delta Test
Skogsgurra(Electrical)
11 Aug 06 10:12
"Tip up" means that the curve you get when you do a 20, 40, ... 80, 100 % tan(d) test isn't straight, but bends upward at the end. That indicates the onset of PD.
Six percent is rather high. What is your L-N voltage?"Tip up" means that the curve you get when you do a 20, 40, ... 80, 100 % tan(d) test isn't straight, but bends upward at the end. That indicates the onset of PD.
Gunnar Englund
www.gke.org
RE: Voltage requirement for Tan-Delta Test
power(Electrical)
(OP)
11 Aug 06 10:28
Thank you Mr.skogsgurra
L-N Voltage: V but as per records, 10kV test voltage applied to L-N of the motor windings. This year again we are planning to carry preventive maintenance and tan-delta test. If the reading is high as 6%, shall we need to go for drying the winding?
What is the max Tip-up allowed? If Tip-up is more due PD what precauionary measures can we take to avoid PD in dry type insulation. We are planning to check PD as well at the same time of tan-Delta test.
RE: Voltage requirement for Tan-Delta Test
Skogsgurra(Electrical)
11 Aug 06 10:34
There must be standards for that. I do not have the numbers. It is possible that the high tan(d) is because you overstressed the winding with 10 kV to ground. The tip up gets more pronounced the higher you go. There are other guys here that know those standards by heart. You better wait for them to come in.
Gunnar Englund
www.gke.org
RE: Voltage requirement for Tan-Delta Test
power(Electrical)
(OP)
12 Aug 06 07:16
To reduce Tan-Delta test kit size, we are using Resonance inductor. Is there any deviation of tan-delta value by using the kit with out inductor and with inductor? Which is advisable?
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