How to fill the data logging tables

7. Data records

7.1 How to fill the data logging tables

7.1.1 Bar or coil factory tests

The first column in sample Table 5 to Table 7 is for the identification number of tested bars or coils.

Column 2 and column 3 are for specifying where the discharge occurs. On the connection side or the opposite connection side? On which side of the bar? For example, the top side in position #1. They also indicate if it occurs at the stress control junction or elsewhere.

7.1.2 Fully assembled stator

The first column in Table 8 is to identify the slot number.

The second column identifies the phase and the parallel circuit based on the winding diagram (e.g., A1, T1-1, E1-T1-1, etc.).

The third column shows the phase-to-ground operating voltage of the bar or coil leg. For simplification, in the case of coils, the same voltage is used for both legs. For voltage calculation, use the following equation:

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

The voltage of one bar is:

where UN is the nominal line-to-line voltage in kV, P is the position of the bar or coil with reference to neutral and

where k=1 for coils and k=2 for bars Nslot is the number of stator slots Nphase is the number of phases

Nparallel is the number of parallel circuits of each stator phase winding

The fourth column is used to indicate if the discharge is detected on the top or bottom bar or coil leg. The fifth column will show if the discharge occurs on the connection end or opposite connection end. The sixth column shows the intensity of the discharge according to a relative scale (weak, intermediate, strong), and the seventh column indicates which type of discharges was observed (e.g., bar-to-bar, stress control junction, lashes, instrumentation cable, foreign object).

Table 5 —Factory test—Roebel bars lap winding (diamond) POWER HOUSE:________________________________________________________

Bar identification Position #1 Position #2

Inspected by:______________________________________ Date:_______________

CE CE

OCE OCE

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Table 6 —Factory test—Roebel bars wave winding POWER HOUSE:________________________________________________________

UNIT:_________ TEST

VOLTAGE:___kV MANUFACTURER:_______________________

Nominal voltage:

__kV

Ambient temperature:

___°C

Relative humidity __%

Atmospheric pressure __kPa

Bar identification Position #1 Position #2

Inspected by:______________________________________ Date:_______________

Table 7 —Factory test—Coils

POWER HOUSE:________________________________________________________

UNIT:_________ TEST

VOLTAGE:___kV MANUFACTURER:_______________________

Nominal voltage:

___kV

Ambient temperature:

_°C

Relative humidity ___%

Atmospheric pressure ___kPa

Coil identification Position #1

Position #2

Inspected by:_______________________________________ Date:_______________

CE OCE CE

OCE

Top leg

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Table 8 —Global VPI stators or in situ tests of completely assembled stator on site POWER HOUSE:______________________________________________

UNIT:________ Nominal voltage: ___kV MANUFACTURER :_______________________

_ Test voltage:

__kV

Ambient temperature:

___°C

Relative humidity __%

Atmospheric pressure ___kPa Global VPI in factory:__ Global VPI in situ:__ Completely assembled stator on site:__

Slot Phase circuit

Operating

voltage Top/Bottom coil leg

or bar

CE / OCE Intensity Type

TOTAL:

Inspected by:_______________________________________ Date:_______________

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Annex A (informative)

Theory of optical emissions from external discharges

As described in IEEE Std 1434, each time a partial discharge (PD) occurs, it is accompanied by a current pulse, radio frequency energy, an acoustic emission, and an optical emission. It results either from an electrical breakdown across a cavity within the insulation or at an air layer on the surface of a coil or bar.

Under certain conditions, the discharge process within the cavities or air gaps may assume a pseudoglow or even a pulseless glow character, but will still give an optical emission. The only type of discharges that can be observed visually or with a UV enhancing instrument (named corona-imaging instrument in the current document) are those that are external to the insulation. For purposes of quality control of bars or coils in the factory, the locations that are susceptible to the occurrence of external discharges include the junction between the stress control coating and the semiconducting slot coating and along the slot coating. On fully assembled stators, only the ends of the stator windings are visually accessible. In addition to the stress control junction, any other locations with small spacing between bars in other phases are areas where light emissions due to external discharges can occur.

Extensive discussions on the physics of electrical breakdown in gasses, and on partial discharge and corona can be found in [B2], [B13], and [B15]. The breakdown process in air occurs when free electrons in the air are accelerated by a local electric field above a critical value. If the electric field exceeds approximately 3 kV/mm in dry air at 100 kPa under room temperature, then some of the electrons will accelerate with enough energy to ionize gas molecules and atoms with which they collide. The resulting positive ion and the two electrons (the original electron plus the secondary electron) will also accelerate in the local field.

Above the breakdown strength value, the number of secondary electrons produced will exceed the number of recombined electrons. Since billions of molecules and atoms may experience the ionizing collisions, the electric field across the air collapses due to the numerous free electrons and positive ions as a result of the increased conductivity of the affected region.

There are two main processes by which light can be emitted in gasses undergoing electrical breakdown. In one process, the electrical breakdown of the air involves the formation of photons having various energies and frequencies. In this breakdown mechanism, many of the collisions between the electrons and molecules or atoms are non-ionizing. When an electron does not acquire sufficient energy between collisions, no secondary electron is ejected and the energy from the impacting electron raises the energy level of the electrons orbiting the atoms. After a certain time in this excited state, the energy level of the atom spontaneously returns to its stable unexcited lower energy or ground state, and the excess energy can be released in the form of a photon. The energy of the photon emitted depends on how much energy was transferred to the molecule or atom from the non-ionizing collision between the electron and the molecule or atom and the type of gas present. The energy of the photon is given by hf, where h is Plank’s constant and f is the frequency of the emitted light. Since the energy of the photon is proportional to the difference between the excited states and the lower, or ground, state of the atom or the atoms of a molecular gas, the frequency of the emitted light can be determined. A similar light emission process occurs when a positive ion (created by an ionizing collision) combines with a free electron. The emitted frequency spectrum due to electroluminescence is typically a line spectrum as opposed to that of a continuous spectrum, which is produced at elevated temperatures (e.g., from a heated filament in an incandescent lamp).

Since there are many types of molecules in air (nitrogen, oxygen, carbon dioxide, etc.), and the range of original collision impact energies is wide, the resulting photons have a wide range of frequencies (wavelengths or colors). A measurement of the electromagnetic spectrum wavelength of light that accompanies electrical breakdown in air shows that the light varies from the visible region (lower visible wavelengths) to the ultraviolet (near UV) range. The UV frequency spectrum is situated between that of the visible violet light and long-wavelength X-rays; however, discharges in air do not contain the entire UV spectrum wavelength since the very far end of the UV spectrum with wavelengths <200 nm is absorbed by the oxygen in the air. A typical discharge spectrum in air is shown in Figure A.1. Normally, when

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

discharges occur in air, the number of photons emitted is greater in the UV frequency range than in the longer visible wavelength range. Thus there tends to be more UV light emitted than light in the visible range (which can be detected by the naked eye). The only portion of the external-discharge activity visible to the human eye appears as a faint violet or bluish light. While most of the light emitted during a discharge event occurs within a burst of a few nanoseconds in length, some light may persist for many milliseconds because it takes time for electrons and ions to recombine. For example, some of the excited molecules (occurring as a result of non-ionizing collisions) may take milliseconds to emit a photon.

Figure A.1—Typical wavelength distribution of discharge in air

The photons emitted by a PD can be detected optically only if the PD is external to the insulation. The photons emitted by PDs within the insulation are strongly absorbed by any surrounding solid electrical insulation. Note also that normal glass strongly absorbs light in the UV range. The light emitted by PDs is usually not strong enough to be observed by the naked eye in full light. Complete darkness is required to improve the eye’s sensitivity. Using a corona-imaging instrument sensitive to the UV radiation in the range of interest (300 nm-450 nm) can also increase sensitivity, and complete darkness is not required with such an instrument. It should be noted that corona discharge wavelength is normally in the range 230 nm to 405 nm and the solar blind bandwidth (240 nm-280 nm) has the least interference with solar UV wave and should be more sensitive to detect corona. Even if UV detection is more sensitive in this range, comparison with the human eye, used as the reference in this document, does not consider emission not detected by the eye.

UV radiation by itself can cause photo-ionization or generation of secondary electrons and thus participate in the overall process of breakdown. However, the UV radiation in itself is not an important contributor to the overall degradation of the insulation system. The leading cause of degradation will be the electro-erosion of insulating materials and the aggressive attack of chemical byproducts generated by the discharges, such as ozone and nitrous oxides [B8] and [B28]. The presence of UV during quality control tests confirms that bars, coils, or complete stator windings will suffer degradation at the affected location right from their time of commissioning. Although this degradation can be slow and take several years before causing any problems, it can be minimized by, among other things, using proper materials and workmanship and having an adequate quality control program to help confirm the absence of external discharges.

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

It is also of interest to note that the silicon carbide particles used for electrical stress control are known to give rise to electroluminescence at higher field [B21], [B22], and [B23]. In such a case, the light emitted arises from electronic excitation in the higher energy levels and de-excitation by emission of photons in the visible range, but with no UV content (effectively from 400 nm-600 nm). Thus, this electroluminescence is visible to the eye, but usually not to the UV camera. This process is not associated with air ionization, and the current recommended practice does not intend to detect it; its typical spectrum is depicted in Figure A.2.

Figure A.2—Typical wavelength distribution of silicon carbide electroluminescence

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Annex B (informative)

Variability of discharge inception and extinction voltages

With the dimensions shown in Figure 1, at atmospheric pressure of 101.3 kPa, a temperature of 22 °C and relative humidity 60%, the DIV measured in the laboratory was about 6.3 kV and the discharge-extinction voltage (DEV) was 5.8 kV. The intrinsic variability of the DIV and DEV from one trial to the next is presented in Table B.1 showing variation from 6.0 kV to 6.5 kV for the DIV in three successive trials in a specific electrode arrangement. Because of this, a plus or minus variation of 1 kV in the determination of the DIV and DEV are not considered unusual. Although the DEV is usually lower than the DIV, depending on the conditions, it can be closer to the DIV than in Table B.1 and in some cases equal. The variability in DIV and DEV comes from a combination of the effect of the rate of increase of the voltage and the statistical time lag for initiatory electron.

Table B.1—Example of DIV and DEV for a needle-plane electrode configuration

DIV (kV) DEV (kV)

Trial Room light Darkness Room light Darkness

Instrument #1 6.3 6.3 5.8 5.8

Instrument #2 6.3 6.3 5.8 5.8

Naked eye --- --- --- ---

Instrument #1 6.0 6.0 5.0 5.0

Instrument #2 6.0 6.0 5.0 5.0

Naked eye --- --- --- ---

Instrument #1 6.0 6.0 5.8 5.8

Instrument #2 6.5 5.9 6.1 5.7

Naked eye N/A 6.6 N/A 6.0

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Annex C (informative)

Example of determination of the maximum voltage for a specific winding diagram

The stator winding of most modern three-phase electrical machines is wound in 60° (electrical) zones, which means that the electrical angle between the axes of neighboring phases is 60°. It is obvious that 60°

zoning improves the winding distribution factor and reduces the voltage at the interface coil to coil in the area of the end-arms. This statement is illustrated in a vector diagram for a 2-pole 48-slot generator with a lap winding two parallel circuits with eight coils per group and two coil legs per slot. The general connection diagram for this machine is illustrated in Figure C.1, where the throw is 1-21 and the line-to-line voltage is 22.0 kV but could be recalculated for any voltage proportionally or relative units could be used. The purpose of this chart is to show that in the case of a 2-pole machine with 60° zone, the instantaneous voltage will be less than line-to-ground (12.7 kV in this example). For better visualization, it is assumed that the electromotive force for one half-coil is 500 V.

The winding diagram of this generator is presented in Figure C.1. The vector diagram for one of two parallels of T1-T4 and T3-T6 at the moment when the north pole crosses the phase coil in slot 36 is presented in Figure C.2. This example shows that:

⎯ The side-by-side voltage difference between opposite phases T1-T4 and T3-T6 is less than the phase-to-ground voltage

⎯ Near to the core in the crossover area, the voltage difference between opposite phases is less than the above side-to-side voltage and respectively less than the phase voltage to ground

⎯ Only in areas at a distance from the core could the voltage difference between opposite phases at crossover be greater than the phase voltage to ground. (See circles in the drawing.) For example, the voltage between the top bar of slot 35 and bottom bar of slot 18 at the crossover close to the connection end is 10.86 kV in Figure C.2 for a phase-to-ground voltage of 7.24 kV.

The same explanation is also applicable to any other type of winding with a 60° zone.

The proper clearance at the design stage and the test voltage during the commission test should be established taking into account the above consideration.

Similar calculations should be made for every other type of winding diagram.

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Figure C.1—Example of stator winding diagram wound in 60° electrical zones

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Figure C.2—The dotted line shows a reverse 180° vector diagram of parallels T1-T4 and T3-T6 when the north pole crosses the phase coil in slot 36 of a 22.0 kV line-to-line stator

winding

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Annex D (informative)

Example of correction factor to apply to the test voltage of a stator model and VPI stator for a machine which will operate at altitudes of more than 1000 m

It is widely recognized that, at pressures close to normal atmospheric pressure, the breakdown voltage of air is inversely proportional to pressure. Since pressure decreases with increasing altitude, a successful external-discharge test performed in the factory close to sea level may not guarantee the absence of discharges for stators to be installed above 1000 m. This is particularly true in the case of the stator model test and VPI stator intended to help confirm that the minimum gap spacing between end-arms is sufficient to eliminate external discharges in operation. A correction factor on the test voltage can be used in the factory to compensate for this effect.

The recommended test voltage is:

where Kd is the air density factor given in IEEE Std 4a-2001 (1.3.5.3) by:

⎟ ⎠

p is atmospheric pressure at the power plant’s altitude

po is reference atmospheric pressure (pressure measured during the test) m is a factor which is set equal to 1.0 for sparkover distances less than 1 meter T is air temperature during normal machine operation (in °C)

To is reference temperature (temperature measured during the test in °C)

Note that the following calculations are in accordance with the definitions in IEEE Std 4a, stating that the standard atmosphere is:

⎯ Temperature 20 °C

⎯ Pressure 101.3 kPa (or 760 mm Hg)

⎯ Absolute humidity 11 g/m³

Since the temperature correction factor is already accounted for in Clause 6, the Equation (D.2) will only be used to correct for air pressure difference with altitude by neglecting the effect of temperature and using a value of m = 1.

In addition, the pressure at any altitude can be calculated according to [B24] by:

β

^g^Rβ

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

h is the altitude (meter) corresponding to pressure P

β is temperature lapse rate, in the troposphere a constant = 0.0065 (K/m) g is earth-surface gravitational acceleration = 9.80665 (m/s²)

R is universal gas constant = 286.9 (J/kg*K) M is molar mass of dry air = 0.028944 (kg/mol) Patm is sea level standard atmospheric pressure (Pa) Tatm is sea level standard temperature (°C)

Equation (D.3) can be simplified to:

[

¹ ²^.²⁵⁵⁷⁷ ¹⁰ ⁵^*^h

]

⁵^.²⁵⁵⁸⁸

p= _atm ∗ − × ⁻

(D.4)

In the case of tests in the factory made at sea level (Po in (D.2) = Patm). In such a condition, if (D.4) is introduced into (D.2) and neglecting temperature correction, the air density factor becomes:

Kd = (1 - 2.25577 10^-5 h)^5.25588

(D.5) Combined into (D.1) this gives the following recommended test voltage:

Vtest = Vnominal line-to-line/ (1 - 2.25577 10^-5 h)^5.25588 )

As an example, this means that the recommended test voltage in the factory (at sea level) for a machine of 13.8 kV (nominal voltage), intended to operate at 2500 m would be:

Vtest = Vnominal line-to-line* Temperature and voltage correction *Altitude correction Vtest = 13.8 kV * 1.15 *1.36 = 21.58 kV

where

Temperature and voltage correction is 1.15. This value defined in Clause 6 compensates for both the temperature difference between factory test and operating temperature and a maximum voltage variation of

± 5% to 10%.

Altitude correction = 1/Kd

IEEE Std 1799-2012

IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

Annex E (informative)

Example of operating-voltage table and bar/coil identification table used during test

When performing the inspection of external discharge activity on a stator winding, it is often necessary to identify which bar/coil belongs to which parallel circuit and its phase. It is thus preferable to get a table before the day of the test showing information such as phase winding, parallel circuit, slot number, position

In document IEEE Std 1799-2012 -IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Windings (Page 32-48)