What is Tangent Delta?

The measurement of the Tangent Delta (hereinafter Tan Delta) is the ratio of the active and reactive current applied to the test object, due to the application of a high voltage, such as during a dielectric strength test. Tan Delta determines the ratio between the naturally circulating currents, due to the intrinsic capacity of the sample, and those caused by (real) losses of insulation or other factor caused by poor quality and/or deterioration of insulation. The measurement is part of the tests to determine the state and features of the insulators, such as: insulation, PI, DAR and DD. Unlike other tests, Tan Delta is performed in alternating current and therefore can express different factors: test voltage, frequency and intrinsic capacity of the sample. An insulation measurement, for example, always gives the same result, the resistance value between casing and winding. The measurement of Tan Delta, since it also depends on the capacitive component (typical of the test piece) at the same losses, it may give different results depending on the nature of the test piece.

Applications of Tan Delta meter

The result provided by the measurement is useful to determine the quality of the insulators installed in motors, stators, transformers and in general in electrical machines. Other fields of application are the quality control of the cables, and their state of wear or contamination (typically water) that can lead to the degradation of the insulating properties. It can also cause the formation of so-called water trees: humidity infiltrations that retrace the cable internally, leading to insulation losses and other negative consequences. The test, unlike some insulation tests, is applied to new manufactured products. The measured data is considered typical for that particular product at that particular stage of the production process. Finally, the determination of Tan Delta is an important test in the quality control capacitors, both of power and not; in this case the test may also be useful in assessing the foreseeable overheating of an installed component.

Test methods

The typical instrument for the measurement of the Tan Delta is represented by the Shering Bridge: it is an annulment bridge that can provide the value of the impedance (complex) of the specimen. Normally it is done manually varying the value of the components constituting the bridge, searching for a minimum and then getting the value of the impedance, according to the values set for the known components. The Shering bridge is able to give very precise results and recover the losses due to connections and others. There are alternative systems to the Shering Bridge, these are techniques and equipment that typically take advantage of modern technologies, signal discretization capability and in general do not require the use of expensive components or that require manual operation of set-up and preparation. We can mention:

• Shering bridge made with components that lead to a “partial” cancellation and subsequent numerical analysis performed by a PC.
• Vector voltmeter (RLC Bridge).

Risatti uses both technologies: Shering bridge on the E12/TD models, ensuring good noise immunity, ease of use and high accuracy. The new equipment of the E12/TD series are true vector network analyzers, optocoupled; it is possible to obtain high performance, very high operational safety and cancellation of any eddy currents that could otherwise affect the measurement. The typical setup therefore provides:

• The generator must be a good quality transformer, free from partial discharges, with adequate power and equipped with voltage variator.
• The measuring bridge.
• Digitization of signals on the reference resistance and in series on the load.
• Computer and application

The test must be carried out at different voltages, typically increasing voltage, according to a predefined scheme for each type of product, so that the data obtained can be compared with previous data and/or measurements made on other samples of the same type. After applying the test voltage, at each change it will be necessary to wait for a predetermined adjustment time and then the detection will be carried out. The trend will go up, with a smooth tone and not a great slope of the curve: sudden changes, generally indicate the approach of the breaking voltage of the insulation.

Understanding the results

After the test, the most challenging aspect is actually the interpretation of the results. At this time the operator’s experience and historical test data are very useful. IEEE 400.2 listed the cable insulation condition grades according to the measured Tan Delta values. The condition grades are listed as:

• No action required
• Further study recommended
• Action requested

“No action required” means that the TanDelta values are within a certain limit and there is no indication that a potentially serious problem is imminent in the cable system. It’s recommended to repeat the cable test at a later time to conduct a trend analysis: there is a deterioration with time? “Further study recommended” means that during the test some Tan Delta values were recorded indicating areas with poor isolation condition in the system. Further information on the cable is required to assess conditions. This may include:

• Comparison of the historical results of the tested cable system. Is the delta measurement constantly increased over time with each test performed? At what interval were the tests performed?
• Performing a monitored resistance test. Is there a decrease in the measurement of Tan Delta with increased test time? This could be an indication of the humidity within the cable system that is evaporated with the application of the voltage..
• Perform a PD test to detect breakage (cracking) of insulation.
• Visual inspection, joints, terminations etc., where accessible, so if necessary replace and remeasure.
• Prepare the cable system for a new test in the near future.

“Required action” means that the cable system has extremely high Tan Delta values indicating a significant problem. The actions to be taken could be:

• Perform a simple 3 V₀resistance test to cause a failure: repair and try again.
• Performing a monitored leak test in order to cause a failure. Then repair and try again.
• Immediate system replacement/repair.

As you can see, there are many possible subsequent tests that can be conducted depending on the severity of the Tan Delta results. There is no clear-cut preventive procedure that should be adopted. The most important thing is to compare the Tan Delta results measured with the historical data, with the threshold values given in IEEE 400.2 or with a historical database created by past tests. Operators are familiar with the design of the cable system to develop their own specific threshold criteria and determine the next best procedure to be adopted. The objective of the Tan Delta diagnostic tests is to allow a simple and time-efficient test solution in order to realize a maintenance plan based on conditions and costs optimization. The above is taken as an example and refers to the measurement of Tan Delta on cables, a typical application, but not the only one: regarding the interpretation of the results, and possible consecutive actions, however, the same considerations may also apply for different product categories. In the end, in the event of a positive result, the test should be repeated according to the default frequency. In case of non-standard values, or rapid increases in the measurement, arrange for corrective actions or repeat measurements at reduced intervals. In case of failure, immediately proceed to replacement/ repair operations by placing the equipment immediately offline.

Tan Delta E12/TD Instrument

The new Tan Delta Measurement System, characterized by outstanding performance, ease of use and high safety, is designed to allow a safe evaluation of losses in the insulations used for the production of windings, stators, transformers and electrical machines in general.

• Ease of use: the System allows the evaluation of the Tan Delta by analog digital acquisition and subsequent processing carried out by a special app.
  • Connecting toPC: USB 2.
  • Operating System: Windows 10
• Modularity: the System is composed of modules that can be easily transported and connected.
  • Two independent channel acquisition tool, connected to the PC via Fiber Optic link:
    • Battery power, galvanic isolation guaranteed.
    • Two channels for test current measurement.
    • Differential inputs.
  • PC interface unit, Fiberà
    • Quick connectors.
    • High data transfer speed.
    • Digital connection.
    • No analog intervention on the sampled signal.

Fiber connection, maximum length 5 meters extensible, for high performance, high flexibility and also low cost.

• Proprietary App for Capture, Calculation, Display and Storage.
  • One application for all features.
  • Use of high resolution numerical software libraries.
  • Calculation system independent from Measurement Bridges (no Shering bridge needed).
  • No need for expensive reference components for measurement.
  • Data logging, remote monitoring, data storage management.

• Adaptability: the System can be assembled according to the most different measuring requirements, without significant lowering of performance.
  • DUT to ground.
  • DUT released by mass.
  • Standard reference unit, Resistive; optionally Capacitive, both with connection to ground and High voltage, floating.
• Accuracy: the Application allows the absolute evaluation, which is relative.
• Calibration: System calibration can be performed/verified via the Control Application.

Composition of Tan Delta meter

The System consists of an acquisition unit, powered by a rechargeable battery, which can be connected to a PC via a Fiber Optic link. In this way you get:

• Safety, galvanic insulation.
• Insensitivity to eddy currents.
• Accurancy

  External elements can be connected to the system. The acquisition unit is capable of measuring currents on two independent channels, having different scale. The image refers to the standard connection, other types can be made.

• AC Generator: is the power source of the System under examination, it can be of any value, the insulation will be guaranteed by the fiber connection. The use of the Measuring System in applications operating at 20-30 KV is frequent.
• Reference Resistor: is the reference resistor for the internal branch of the system. Risatti offers different models and values in its catalogue.
• DUT: is the object of the Test.
• Tan Delta Probe: is the acquisition unit.
• Fiber Interface: is the Fiber Optic interfaceà PC

Note that, deliberately, no ground connection has been indicated: any electrical node can be connected to the ground. The System uses the Fiber optic connection to realize the desired galvanic isolation towards the PC and avoid the creation of dispersed/eddy currents that could affect the measurement. The Probe, as well as the Reference Resistance and all other parts CAN BE CONNECTED TO POTENTIALLY DANGEROUS AND LETHAL VOLTAGES.

Accessories of Tan Delta meter

A series of accessories are available to complete the offer in terms of performance and measurement verification.

• Kit modifica fondo scala acquisizione.
• Generatore HV da laboratorio: secondo richiesta Cliente.
• Kit SoftWare per monitoraggio remoto.

Technical features of Tan Delta meter

Type:	Vector networks analyzer.
Sensors:	Two: Current, differentials
Full scale:	Static: 30 mA, 300 mA.
Conversion A/D:	50 KS, two canals, in phase; 16 bit real time.
Filter:	Software, selectable order, excludable.
Calculation model:	Complex Impedance Measurements
Accurancy:	32 bit.
Ratio S/N measure:	Better than 82 dB, unweighted.
Supply:	6 Vdc
Connection:	Fiber Optic monodirectional standard.
Reference element:	External resistance, Capacitor.
Protection:	External Surge arresters.