Partial Discharge Paper

Partial Discharge Testing Decreasing the Field Failures of High Voltage Components

Introduction Test engineers have an understanding of high voltage isolation leakage testing, but Partial Discharge testing is much less understood. We first summarize the more traditional Isolation Leakage testing, and then follow with a comprehensive account of Partial Discharge testing.

Isolation Voltage The initial design of a device with an insulation barrier includes a choice of materials and dimensions to achieve an isolation voltage rating. The isolation rating is the voltage level under specified conditions that the device will withstand without breakdown.

Isolation Voltage Failure Modes Isolation breakdown in devices can occur in several ways. In an insulator electrons are tightly bound to atoms and molecules. With moderate gradient potential some electrons are pulled free of their bonds to be later recaptured in collisions with neighboring atoms or molecules. As the gradient is increased beyond the Intrinsic Dielectric Strength of the material, collisions occur with sufficient strength to free more electrons than are captured, resulting in a disruptive breakdown called Intrinsic Strength Breakdown. In another type of breakdown, a path across the surface of a device can lead to carbonization and possibly even Surface Flashover. Another more complex cause of degradation failure inside the volume of the insulation material is Erosion Breakdown which will be later presented. In High Voltage Leakage testing, the tester applies line frequency AC voltage and monitors the resulting device current to not exceed a certain limit. A good device will have both resistive and capacitive leakage components that are proportional to the instantaneous applied voltage. The resistive component of device impedance is typically in the order of Gigohms. A Leakage failure occurs when this resistance is degraded. Excessive device capacitance could also cause leakage failure, though a physical mechanism to cause this seems improbable.

HTPaper: Partial Discharge Testing

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Partial Discharge Testing Decreasing the Field Failures of High Voltage Components Partial Discharge Partial discharges are the source of Erosion Breakdown which affects the long term life of an insulator. Partial discharges are discharges that do not completely bridge the insulation between the terminals. These discharges are termed “partial” because they occur in areas that occupy a small portion of the electrical path length and are limited in magnitude because they are in series with mostly good insulation (which may eventually degrade). These discharges can occur in insulators that contain gaseous inclusions, cavities, or voids. To understand how and why this occurs the following sections will include Electric Field Theory. Electric Field Basics--Electric Flux Density “D” Gauss’s Theorem states that the Electric Flux Density at a distance r 1 from a concentrated charge Q on a small conductive sphere may be determined by enlarging the sphere until its radius is equal to r2 (as the charge is distributed in equilibrium on the surface). See Figure 1. The magnitude of the Electric Flux Density at r2 is the charge Q divided by the surface area of the enlarged sphere. 

The Electric Flux Density “D” at the surface of a sphere of radius r 1 is charge/area (Coulombs per square meter).



The radius of the sphere may be enlarged to find D at another distance r 2 from the center of the sphere.



Note that D is not dependent on the medium.

Charge +Q

r 2 D

D2=Q /4 r r2 2

r 1

Figure 1. Gauss’s Theorem. Electric Flux density D at a distance r2 from Charge


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Electric Field Intensity “ε” The strength and direction of the Electric Field Intensity “ ε” Any point in an electric field may be measured by the force and direction upon a test charge as shown in Figure 2. Electric Field Intensity “ε” measured in Newtons 1 per Coulomb (dimensionally equivalent to Volts per meter) may be thought of as the force effectiveness of the electric field.

+Qt

ƒt =

ƒ t/ Qt

Figure 2. Electric Field Intensity at a Test Charge Unlike Electric Flux Density “D”, the magnitude of the Electric Field Intensity “ ε” is affected by the medium.

ε

is proportional to the Electric Flux Density but inversely proportional to permittivity of

the medium as follows:

ε=

D/e , where is e the permittivity of the medium. The permittivity of a

vacuum is the basic electrical constant eo which equals 8.85x10 -12 F/m, (Farads per meter). The permittivity e of all other materials is expressed as a dimensionless ratio er times the permittivity eo of a vacuum ( e = er eo). Table 1 below indicates the relative permittivity values for some selected materials.

Medium

Relative Permittivity (er) 1 1.006 1.03 2.7 3.4 3 3

Vacuum Air Styrofoam Polystyrene Plexiglass Amber Rubber 1

Newton is the force required to accelerate a mass of one kilogram at the rate of one meter per second squared


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Partial Discharge Testing Decreasing the Field Failures of High Voltage Components Quartz Formica Ammonia (liquid) Glycerine Distilled water Barium Titanate (BaTiO3) Barium Titanate Stannate

5 6 22 50 81 1,200 20,000

Table 1. Relative Permittivity Values.

Q+

Q-

D ε

D ε

Figure 3. Plots of Electric Field Intensity and Flux Density vs Distance between Two Oppositely Charged Plates in Uniform Medium (Air) In Figure 3. the two charges are assumed to be evenly distributed on the inside surfaces, and end effects are ignored. Because of opposite charges and symmetry, cancelation of the electric field within the volume of each plate will occur such that the fields will be zero across the thickness of the two plates. This causes the charges to be present only on the inside surfaces of the two plates. With the permittivity (e) of air a constant, and since ε=D/e, the ε and D fields will therefore have similar shapes across the gap.


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Partial Discharge Testing Decreasing the Field Failures of High Voltage Components This is not the case in Figure 4., since there are two different dielectrics across the gap. As previously stated, Flux Density is not affected by the medium, and will therefore be the same in both mediums as shown in Figure 4. Notice that the Field Intensity will be twice as great in the air where er = 1 as in the dielectric where er = 2. It is important to note that the smaller permittivity dielectric has the higher Electric Field Intensity. Q+

Q-

D

air

er=1

er=2

D

ε

Figure 4. Electric Flux Density and Field Intensity between Two Oppositely Charged Parallel Plates Separated by Two Types of Dielectrics Figure 5. below illustrates a model for a capacitor (or the passive elements of an optocoupler). A quartz insulator 1.0 mm thick is shown between two conducting plates. The insulator has defects in the form of air voids. If 1.0 kV is applied across the insulator, there would be an Electric Field Intensity of 1.0kV per mm ( ε=volts/distance) across the dielectric. However, the air voids have a relative permittivity (

r = 1)

equal to 1/5 that of the quartz, which would cause a localized

Electric Field Intensity of 5 kV/ mm in the air voids. This causes unequal charge distributions in the insulator. Table 2. shows that the dielectric strength of quartz is 30 kV/mm. The air voids,


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Partial Discharge Testing Decreasing the Field Failures of High Voltage Components however, have a dielectric strength of only 3 kV/mm. This produces sufficient ionization in the voids to cause an arc that shorts out the charges that accumulate on opposite sides of the void. This discharge path is in series with good insulation which limits the bandwidth of the picocoulomb discharge. The effect is similar to discharging a capacitor through a high resistance.

Quartz er = 5 oid, Air er = 1

Figure 5. Insulation Barrier with Voids in an Electrostatic Field When the void discharges, the arcing stops and a slow recharging of the void through good insulation begins again until the critical breakdown voltage of the air in the void is again reached, repeating the process of slow charging followed by fast discharging. Thus the void essentially becomes a relaxation oscillator. This effect is known as Partial Discharge. Table 2. below illustrates the relative dielectric strength of some sample materials. Material

Dielectric Strength (kV per mm)

Air (atmospheric pressure)

3

Oil (Mineral)

15

Impregnated Paper

15

Polystyrene

20

Rubber (hard)

21

Bakelite

25

Glass (plate)

30

Paraffin

30

Quartz (fused)

30

Mica

200 Table 2. Material Dielectric Strength


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When high voltage is applied to a device that produces Partial Discharge it may be observed that the effect starts at a certain voltage level, and once started, the voltage must be reduced to a lower voltage before it ceases. These two voltages are called the Inception and Extinction voltages. This effect is illustrated in Figure 6. In typical AC voltage testing Partial Discharge cycles may occur many times during the positive and negative peaks. In applications, if this happens with sufficient magnitude over time, arcing in the voids will degrade the insulation, even producing tree-like patterns in the dielectric that lead to failure. This effect is called Erosion Breakdown.

Figure 6. below illustrates the repetitive nature of Partial Discharge. Once Inception Voltage is reached the frequency of partial the discharge will increase as the AC voltage is approaches its peak value (frequency change not illustrated).

Inception Voltage Extinction Voltage

Typical Partial Discharge cycles in a void

Extinction Voltage

Inception Voltage

Figure 6. Inception and Extinction of Partial Discharge with AC Voltage Testing


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Partial Discharge Testing Decreasing the Field Failures of High Voltage Components There are a number of Regulatory Agencies governing standards throughout the world: USA UL 1577 Germany VDE 0884 Canada CSA comp. acceptance notice 5 UK BSI    

The standard test adopted by the European Community (CE) for semiconductor manufacturers is VDE 0884. This has also become recognized as an international standard. There are three classes of tests that exist for VDE optocoupler test specifications: Type, Random, and Routine. 1. Type Testing is performed on a specified number of devices that are sent to the testing agency. It includes both destructive and non-destructive tests as well as environmental test and is the most all-encompassing, determining whether products meet design requirements. 2. Random Testing, which is performed on random samples from normal production batches, also does both destructive and non-destructive testing. 3. Routine Testing is non-destructive testing that is performed on all production units. Routing Testing (as well as Type and Random) includes partial discharge testing to verify isolation barrier integrity. Partial discharge measurement per VDE 0884 (June 1992) is used to evaluate the insulation integrity of optocouplers. Partial discharge testing replaces the common dielectric withstand voltage test, because dielectric test voltages may pre-damage the insulation of an optocoupler. Partial discharge testing qualifies an optocoupler for operation at voltage levels that are below the inception level so that no harmful breakdown of the insulation occurs during its normal lifetime. The illustrations below describe the partial discharge test for Type and Sampling (Procedure A—Figure 7.) and for 100% production (Procedure B—Figure 8.) testing in accordance with VDE 0884.


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Partial Discharge Testing Decreasing the Field Failures of High Voltage Components AC Voltage Voltage initial (Maximum V initial (MaximumTransient) Transient)

V pr (1. 5 Viorm)

VIORM (Maximun Rated Contiunous)

t

0

tm

tini (10sec)

( 60sec)

td

Figure 7. Procedure A: (for Type and Random Tests)

tini = (measuring time for device leakage) = 10 seconds tm (measuring time for partial discharge and leakage) = 60 seconds td = (partial discharge detection delay, for PD extinction) = Adjustable 0.0 to 9.9 seconds


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V Vpr =1. 875 (Viorm)

Viorm (Maximum Rated Continuous)

t tm

td

Figure 8. Procedure B (For 100% Production Testing)

tm (measuring time for partial discharge and leakage) = 1.0 second td = (partial discharge detection delay, may be kept at 0.0 sec.). Adjustable 0.0 to 9.9 seconds


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Term Vinitial

Definition Maximum test voltage for the Partial discharge test. It is also the maximum transient over voltage occurring in a rated mains service class. At this initial voltage, partial discharge (but no breakdown) may occur. Vinitial also equals Viotm (transient over voltage) which is listed in the applicable VDE insulated related characteristics section. Preferred values for Vinitial are shown in Table 3 below. This is taken from table 2 of the VDE 0884, June 1992 revision.

Vpr

Partial Discharge test voltage applied to an optocoupler and maintained for a specific time period, tst. During this time, Partial Discharge is measured at a specific time interval, tm. Vpr=1.5 X Viorm for procedure A and Vpr=1.875 X Viorm for Procedure B.

Viorm

Working voltage (maximum service insulation voltage); this is the maximum continuous permitted voltage that may be applied to an optocoupler. This value is specified by VDE to each insulator.

tm

Test time for Partial Discharge equal to 60 seconds for Procedure 1 and 1 second for procedure B.

tini

Time beginning at Vinitial test voltage, which equals 10 seconds.

td

Test voltage initialization time.

Pass/Fail

No leakage failures and no optocoupler to have more than 5 pC Partial

Criteria

Discharge during Partial discharge test time, tm.

Table 3. Definitions of Terms used in VDE Partial Discharge Testing.


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Rated Mains voltage up to and including V rms. & V dc 50

1

2

3

4

Peak

Peak

Peak

Peak

330

500

800

1500

100

500

800

1500

2500

150

800

1500

2500

4000

300

1500

2500

4000

6000

600

2500

4000

6000

8000

1200

4000

6000

8000

12000

Table 4. Preferred Insulation Test Voltages for Service Class (Vinitial)2

Comparative tracking Index (CTI) CTI is a measure of the optocoupler mold material and its relative insulating capability. The surface of the mold material is subjected to an alternating low voltage stress, which produces a small current flow. When the current reaches a predetermined value, the corresponding numerical value of the applied voltage is the CTI value. CTI impacts both external creepage and maximum allowable working voltage for the same value of external creepage distance. Material Group Because the behavior of insulating materials is very complex under various contaminants and voltages, direct correlation between deterioration of the insulating material and formation of conductive paths on the insulation surface is not practical. Correlation between the Comparative Tracking Index (CTI) and ranking performance of insulating materials has been found by empirical observation. Consequently, CTI values can be used to categorize insulation materials: Material Group 1

600

<

CTI

Material Group 2

400

<

CTI < 600

Material Group 3a

175

<

CTI < 400

Material Group 3b

100

<

CTI < 175

In some equipment specifications, material Group is used in conjunction with Pollution Degree, Creepage distance and the Working Voltage Table.

2

Source: VDE 0884, June 1992 revision, Table 2


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HT 9464 High Voltage Isolation Tester Figure 9. shows the High Voltage signal paths that are used by the HT 9464 Tester to obtain device leakage and partial discharge measurements. The primary of the H.V. transformer receives regulated constant voltage AC power from the settings of the front panel variacs V1 and V2 for a duration determined by front panel T1 and T2 test time settings. The secondary of the H. V. transformer applies the test voltage to the H. V. test terminal contacts in a handler. Not shown is the safety test site cover that is interlocked to the main controller to prevent operators from touching the contacts when high voltage is present. Notice that the H. V. path includes Z1 and Z2 in series with the contacts. Both of these consist of a 50Mohm resistor shunted by a 22pF capacitor. These limit the energy to the device contacts for the following reasons: 1. Safety. The maximum 50/60 Hz AC current will be about 100uA. By comparison, perception of alternating current through the human body requires at least 0.5 mA. 2. Less degradation to failed devices that may pass when retested at reduced voltage settings. 3. Less high frequency radiation (that can disturb digital logic in both handler and tester) when shorted or arcing devices occur during testing. HT 9464 Leakage Measurement When a device is placed in the contacts the 50/60 Hz current from the transformer high voltage secondary will pass through Z1, Z2 and Zin (less than one ohm) and through the SIGNAL coax to the leakage current measurement circuit in the Main Controller. The current will return in the coax shield to chassis ground and the H. V. secondary. The measured device peak current will be compared with the Leakage Threshold setting and the tester will produce a Fail Leakage if the measurement is excessive.


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H.V. Box Interlock Switch

H.V. Transformer

High Voltage Box

Z1

50Mohm 1nF 5Kohm

H. V. Test Terminals

.01uF

Z2

Zin Narrowband Amplifier & Window Com arator

DC Power Cable +/- 15 Volts

P. D. Threshold Setting METER Coax

Front Panel Variacs V1 and V2 0 to 120 VAC

SIGNAL Coax

Volt Meter Circuitry Peak Detector 0 to 7.50 KVAC (0 to 10.6 KVpeak)

HT Partial Discharge Calibrator, 0 - 9 PicoCoulombs

Leakage Current Measurement uApeak

Partial Discharge Signal Processing

Main Controller

Figure 9. HT 9464 High Voltage Diagram Showing Signals to Main Controller and Partial Discharge Calibrator HT 9464 Partial Discharge Measurement Because of the high frequency nature of partial discharges that may occur in the device under test, the associated current will take a different path than that of 50/60Hz leakage current. The 1 nanoFarad capacitor now becomes the low impedance high voltage source for partial discharge transient current that travels through the two 22pF shunt capacitors in Z1 and Z2 (and the device under test) to Zin (the first stage of the narrowband amplifier). Passing through Zin, transient current then goes directly through the 0.01 microfarad capacitor (bypassing the leakage measurement circuit) and returns to the bottom of the 1 nanoFarad capacitor. The narrowband amplifier will respond to the high frequency components that get amplified and sent to the comparator for amplitude discrimination. The comparator is referenced to a three digit partial discharge threshold setting that has been previously set with the Partial Discharge Calibrator (usually at 5 picoCoulombs). If a partial discharge is detected by the comparator, the red LED on the side of the HV Box will blink and the signal will be sent to the main controller for processing.


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HT 9464 Partial Discharge Threshold Setting Using the Calibrator The HT PDC700V Partial Discharge Calibrator is capable of producing accurate 0 to 9 picoCoulomb pulses that are used to set the three digit Partial Discharge Threshold adjustment. Normally the single digit thumbswitch on the calibrator is set to 5 picoCoulombs. The calibrator spring contacts (pogo pins) are then placed against the handler contacts and the three digit setting on the side of the H.V. Box is increased until the red LED stops blinking. This usually occurs between “050” and “100” when calibrating the detector to 5 picoCoulombs. Other comments: 1. This procedure requires that the Calibrator has been integrated into the handler so that it can make contact with the H. V. device contacts (while the DC power cable is connected between the Calibrator and the Main controller). The handler contacts should be in the closed position when calibration is performed to simulate normal contact capacitance. A device may or may not be present in the contacts. 2. This procedure is done when no High Voltage is present. This is usually guaranteed by the necessity to raise the interlocked Safety Cover over the handler test site before placing the calibrator in contact with the H.V. contacts. The calibrator will be damaged if it experiences High Voltage. 3. It is also assumed that the handler H. V. Test Site has been characterized to see how high in voltage the empty contacts can go (in the closed position as when testing) before failing partial discharge. The test site must be able to go higher than the voltage at which devices will be expected to pass partial discharge.


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Partial Discharge Testing Decreasing the Field Failures of High Voltage Components Paralleling Multiple Devices in High Voltage Isolation Tester Contacts The question is occasionally asked whether more than one device can be tested in the High Voltage contacts. This is an area that introduces uncertainty into the test due to u nknowns at the test site caused by several test devices in parallel. The particular uncertainties from putting several devices in parallel depend on whether Leakage Testing or the Partial Discharge Testing is considered.

Leakage Testing Case

When devices are put in parallel the Leakage Current is the total from all devices. This means that the operator will have to use a larger Fail Leakage threshold, but he must not use the total number of devices times the max permitted leakage for one device. The problem is that we loose the ability to know about each device when we put several in parallel. For example: Suppose at some test AC Voltage we have devices that are typically 1uA with a specification of 5uA max. Also suppose that three devices are tested in parallel with a resulting leakage of 8uA. The test must reject all three by assuming (1uA + 1uA + 6uA) even though all three parts may be in spec at 2.66uA. This uncertainty becomes greater with more devices in parallel. If a high percentage of the parts are good this may be acceptable. Rejected parts may also be tested one at a time to retrieve good ones.

Another consideration when using multiple devices is the effect of the resultant higher device leakage current upon the tester. With higher device leakage currents, the tester Source Voltage will have to be raised more than normal to obtain the desired Device Voltage. These two voltages typically have a small difference due to a drop across the tester source impedance. Typically the Device Voltage is more than 95% of the tester Source Voltage. With multiple devices and larger leakage currents the actual Device Voltage will be a smaller percentage of the Source Voltage and more dependent upon the device leakage currents. If device leakage current is not fairly constant then the Device Voltage will be difficult to keep at the desired setting.


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Partial Discharge Testing Case

During testing, partial discharges in a dielectric produce small transient voltage spikes at the tester contacts that can be detected if they are large enough to exceed the setting of the Partial Discharge Detector Threshold (on the HV Box). When several devices are placed in parallel the capacitances of all devices will add. This increased capacitance will reduce (filter) the small transient voltage spikes from any failing device so that they are more difficult to detect. This may be compensated for by reducing the Partial Discharge Detector Threshold. This may be done accurately by performing the 5pC calibration of the PD Detector Threshold (using the PDC 700V calibrator) when the actual number of devices to be tested are in the tester contacts. As more devices are in parallel the signal produced by the PDC 700V calibrator will be reduced and the PD Detector Threshold setting for 5pC will be set smaller. This will become impractical when the threshold setting becomes so small that it reaches the noise detection level of the system. It should also be noted that the devices that are used for this calibration should b e typical in terms of capacitance.

The Test Equipment required for Partial Discharge Testing is considerably different from the standard Automatic Test Equipment (ATE) utilized to test analog, digital and mixed signal circuits. The design must be able to generate high voltages (up to 10 kV peak) and unlike Hi-Pot Testers, detect leakage in the sub-µA range. Additionally for Partial Discharge measurement the equipment must have the ability to detect and measure minute charges at the picocoulomb level. For accurate and repeatable Partial Discharge measurements, the equipment must be calibrated against a traceable standard. Additionally for use in production interfacing to a handler is mandatory. Most importantly, the equipment needs VDE certification to allow the user to test to the International VDE Standard.


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HT, Inc. is a privately owned company specializing in Semiconductor Equipment. The HT 9460 and 9464 product lines previously owned by Hewlett Packard, were acquired by HT in 1998. HT is located in the heart of Silicon Valley. For further information: Email: [email protected] Phone: 408-980-9738 Fax: 408-727-0344. www.ht-world.com


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Partial Discharge Paper

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