DC STEP HIGH VOLTAGE TEST
(EDA TEST- Environmental Data acquisition)
 

TEST DESCRIPTION

 It is an off-line, DC test that obtains information concerning overall status of the insulation. It consists first in the measure and comparison of the capacitance of the insulation submitted at low AC and DC voltage, and second in the analysis of the current through the insulation during two charge and discharge cycles performed at two different test voltages. In this case, the behavior should be linear when the machine status is good. If this is not the case, we have an indication of degradation. EDA test is a non destructive DC voltage Test. The test consists in the following sequence of data acquiring: •

Environmental Data acquisition:

 Ambient temperature and moisture is obtained. This will be part of the report. If the machine was switched off long enough, this temperature could be used to compensate the parameters read. Moisture is used to have a useful indication of ambient conditions when the test was performed, both for air cooled machines and to know if measurement instrumentation performance is exceeded.

• Capacitances measurement:

The system performs insulation capacitance measurement at low voltage and at two frequencies: DC and 1KHz AC. •

First Test Voltage:

System switches DC test voltage to the insulation while 30 minutes. Currents are plotted and recorded meanwhile to determine several parameters. At the end of the 30 minutes, the system performs a 2 minutes discharge period.

Second Test Voltage:

After complete discharge of the insulation from 1st test voltage, a second test voltage with a value above the 1st is applied to the machine. The procedure is the same. This step is made to check for possible problems in insulation at a high voltage and second to check about linearity of insulation with voltage.

Diagnosis:

The diagnosis management works in two steps: First and as reference of initial values, an electrical engineer performs and EDA test during machine commissioning or after any refurbishment, stating so that the reference values are those of health insulation. After that and during routine maintenance, once finished the test, the inspector can immediately check the numeric values of all the parameters and their comparison with previous test done and the initial reference values and have so that a punctual view of the status of the machine insulation.

Trending:

Predictive maintenance techniques need of trending data that can help us decide about last parameter’s evolution and indicate the future tendency. With this data, inspectors can decide about future maintenance actions on the machine…

2.- TEST PARAMETERS AND INTERPRETATION

EDA system offers a group of parameters that when analyzed together give a very good approach of insulation status. Now we indicate each parameter, what does the parameter indicate and how it changes because of different problems with a simple explanation. The main parameters we analyze are: • Capacitance Ratio (CR). This is the ratio between DC and AC (1KHz) capacitances. This ratio allows detect contamination and moisture problems.

ƒ Contamination:

Contamination extends all over the windings in the end-winding area. This contamination extends the plates of the equivalent geometric capacitor formed between the phases and the magnetic core. This extension is measured in DC, but in AC part of the capacitance is neglected due to the effect of the inductance of the coils, so the bigger the difference, or the greater CR, the greater the contamination degree. ƒ

Moisture:

When moisture is in the surface of the insulation, the effect is similar to contamination. If moisture is inside the insulation, then it is the dielectric constant of the insulation that changes from the value in dry insulation increasing capacitances. ƒ

Aging:

The insulation may degrade simply by the pass of time or by another accelerated process usually due to thermal aging. In both situations, components inside the insulation volatilize and the space is filled by air or another gases. In this condition, the effective distance between layers of the capacitor decrease, and also the effective dielectric constant of the equivalent capacitor decrease. Both situations lead to a low but noticeable annual decrease of the machine capacitances that can be easily detected with data trending. •

 Absorption Index (AI).

 It is a ratio between the currents after 30 and 60 seconds subtracting the leakage current while the 30 minutes charge cycle. It indicates if a problem is internal (low values) or external (high values) to the dielectric. It is based in changes of the ground wall insulation homogeneity due to the presence of contaminants coming from the outside: moisture, oil or others ingresses flowing into the dielectric.

This behavior can be appreciated again from figure 7 because: Cg gets charged before the 30 seconds and in 30 minutes leakage current is most contributed from Ri, so Ri influence is neglected. What remains?, only the effect of the equivalents Ra/Ca and Cwx., If the plot of the current gives a defined corner slope it gives high AI values indicating external contamination, if the slope is slow it means that the insulation has problems in the Ra/Ca component indicating through low AI values internal problems. •

Re absorption Current (RC).

After 30 minutes, the insulation is considered fully charged. Then a short-circuit is made and flows the Re-turned absorption current that was acquired while the absorption period but this time without the influence of the other currents. The value is taken after one minute of the discharge cycle. If there is a bigger heterogeneity inside the dielectric (water, vapor, oil, dust…), it has more areas to store energy as polarized dipoles, leading to bigger repolarization currents. From previous tests, if RC goes up, it is possibly due to the degradation of the insulation (delimitation, increase of voids, etc). It’s possible to detect uncured resin if we compare this parameter with test results from similar machines. •

Time Constant (TC).

It is the product of an Insulation Resistance (IR) value and the AC Capacitance. New machines must show high initial values because of the high IR expected initial values. In a normal life and degradation, both IR and CAC decrease, so throughout the life of the machine TC descends slightly. Therefore, if the value falls faster than expected, it indicates a faster aging, and so an abnormal degradation. It confirms contamination or delamination problems, depending of the case. It must be temperature normalized. •

 Leakage Current Ratio (LCR).

 It’s the ratio between the leakage current (current after the 30 minutes of charge process) measured at the first voltage and at the second voltage. The result is made comparable through the voltage ratio between the two processes. The ideal value is 1,0: This value would mean that the insulation behavior is linear with voltage. If the value is far from the ideal value, it should confirm a problem detected with the other parameters. •

 Insulation Resistance (IR).

 Every machine should be above a minimum value, depending on the machine voltage. The decrement of this value indicates insulation contamination or degradation. After external cleaning it should increase noticeably. As can be seen in figure 7, Ri is made up from surface resistance influenced by contamination and internal resistance that includes internal insulation contamination and possible aging. It requires temperature correction. •

 Polarization Index (PI).

It indicates together with IR different problems like contamination and moisture absorption. If PI tendency is going down from previous tests, the machine has one of the described problems. Otherwise, if PI increases in excess, it can indicate brittle insulation in some older types of insulation

3.- SECOND INSTANCE STATOR INSULATION TEST TECHNIQUES

When EDA test gives evidences of one or more insulation problem, it’s the time to confirm this issue with 2nd instance techniques, before taking the decision of opening the machine. These techniques usually need more expensive or complex equipment and needs better trained people not just for the testing but for the interpretation of the results as well, so they are only performed when the first instance tests recommends a deeper study. These techniques are also non destructive tests and could be:

 • Internal Visual Inspection.

The scope of this inspection will depend on the accessibility to the winding of the machine, performed by expert personnel. This will be the most valuable tool to detect the effects of the different degradation mechanisms, especially if those happen in the end-winding area. •

Power factor or Dissipation factor (tan δ) and tip-up.

Tan delta gives a global indication of losses in the dielectric and can give a clue of the thermal aging status of the machine. Also, this test could give information about end-winding area with UST Test Techniques. •

Capacitance and capacitance tip-up.

 High voltage capacitance deduced from tan δ test also gives clear indication of changes in the insulation system. It monitors moisture and other abnormal conditions. The tip-up indicates how changes the equivalent capacitance with voltage, this says us how changes the amount of PD (Partial Discharges) voids activated by voltage increase. It also monitors moisture ingress and delamination. •

 Partial discharge analysis.

 Monitors degradation of the insulation system. It can be on or off-line. It can also give an indication of the possible location of the discharges inside the dielectric (next to the coil, in the ground wall or in the interface to the core). •

TVA probe.

TVA Probe testing (Tennessee Valley Authority Probe) allows physical location of the partial discharge activity, mainly if it is a slot discharge problem. •

 Surge Test.

 It is intended to test turn-to-turn insulation in form-wound multi turn coils. •

 Hi-pot.

It is a destructive test that should be performed with extreme caution. It must be evaluated the convenience of the performance of this test.

4.- TEST DESCRIPTION

Here we present a study of some real cases to show how the value and the trends of these parameters are reliable and help to detect evolving problems in several situations. The cases are taken from Iberdrola testing files and describe problem detection through different EDA test results and how the described 2nd instance techniques confirm the problems.

ƒ All test performed were validated before being processed. Test system (EDAIII) is intended to eliminate most of human error, but apart from this, many test were invalidated because of very high moisture in ambient or machine. This could cause moisture deposit in machine’s surface, most of failed tests were repeated after the machine was dried and results changed to normal values. ƒ After validation, and although more complex intermediate results could be defined, for this study, tests diagnosis are only classified in four categories, the most usual in the test environment and corresponding to the records on machine maintenance history:

 [1] Healthy machines, suitable for operation.

The insulation is in good condition, there is no serious contamination level. If the unit is old (>30years), the degradation degree is according to normal aging and doesn’t show symptoms of accelerated or abnormal thermal aging.

 [2] Surface contamination problems.

Due to dust, insects, oil or simply moisture or also to a combination of several of these factors. If nothing was done, it could lead to electrical tracking and contamination ingress in the insulation decreasing its useful life. It is most of the times easily reversible with cleaning & drying works. 10

[3] Internal degradation problems.

Usually due to thermal aging. It is irreversible and makes necessary a more intensive trending up to the rewinding of the machine be needed. Insulation exceeds usual aging values; it is losing its properties and can support less stress. The machine is subject to a greater probability of failure in service.

[4] Internal contamination problems.

 This situation is not very usual in epoxy insulation, but sometimes and due to cracks or fissures in insulation, delamination and ingress of moisture, dirty or oil can also flow inside the coil insulation. It is most of the times hardly to reverse. ƒ

 It is obvious that some machines can have (normally) more than one problem at a time. For this study, only one problem is allowed in each diagnose, the usual preference is: First Health machines, Contaminated & Degraded as the last. Take note that both contaminations can change Degraded values and change Degradation diagnosis, so the user should first try to solve this easy to reverse problems in Contaminated machines before evaluating other hardly to solve problems related to Degradation. It is also logic that most of the times that Internal Contamination happens; it will also show External Contamination traces. ƒ

The last step of the investigation was to execute histograms for the different test parameters under study: For each parameter, the first histogram presents data distribution from low voltage / high voltage test. The second histogram presents data classification for this four diagnosis categories.

 

PROCEDURE:

THE STEP VOLTAGE TEST CONSISTS IN THE INSULATION OF HIGH DC VOLTAGE ACROSS THE INSULATION (ALWAYS ALLOWS MACHINE’S OPERATION VOLTAGE) LASTING 30 MINUTE FOLLOWED 30 MINUTE TIME.

 

PROCESS  FOLLOW TO EACH BUSBAR SAME AS BELOW.

1^ST

GENERATOR R-PHASE àGENERATOR EARTH

GENERATOR Y&B-PHASE SHORT à EARTH

2^ND

GENERATOR Y-PHASE àGENERATOR EARTH

GENERATOR R&B-PHASE SHORT à EARTH

3^RD

GENERATOR B-PHASE àGENERATOR EARTH

GENERATOR R&Y-PHASE SHORT à EARTH

 

EQUIPMENT:

MEGGER MIT S20/2 SKV INSULATION TESTER

 

RESULT:

Temp: 38°C

DC Voltage in kV	Leakage Current in µA
	R-Phase	Y-Phase	B-Phase
1.6	1.30	1.75	3.74
3.2	2.87	3.18	7.44
4.8	4.55	4.66	11.2
6.5	6.58	6.42	15.5
8.1	8.85	8.29	20.0