Partial discharge

Partial discharge (also pre-discharge ) is a term used in high voltage engineering, which is concerned primarily about the form and properties of insulating materials: Kicking in high voltage insulation or along air routes strongly inhomogeneous field profiles, as it can locally come to exceed the material-specific breakdown field strength. In this state, an imperfect electric breakdown the isolation between the electrodes is bridged by discharges only partially. Such partial discharges ( abbreviated as "TE" ) occur primarily in the stress isolation with an AC voltage.

• 4.1 PD measuring systems 4.1.1 Conventional TE extraction 4.1.1.1 measuring circuits
• 4.1.1.2 measuring impedance

• 4.1.2.1 Capacitive Sensors
• 4.1.2.2 directional coupler sensors 4.1.2.2.1 Inductive directional coupler sensors

Formation of partial discharges

Partial discharges occur in media with inhomogeneous field profiles by electron emission of free charge carriers, caused by external influences. The picture shows schematically the formation of a partial discharge in a point-plate electrode arrangement, caused by incident radiation. Shown is an inhomogeneous electric field, in which the molecules or atoms are located. From the outside radiation is incident with the energy. This may be, for example, ultraviolet radiation or other ionizing radiation., A photon of sufficient energy to a molecule or atom, in order to solve an electron from its bond, a free electron is formed ( photoelectric effect). This in turn is drawn on the basis of the prevailing field to the positively charged plate electrode, and can on his way electrons from other molecules or atoms solve ( avalanche effect). Characterized many free charge carriers are generated, forming a conductive channel, in which an electrical discharge ( movement of the carrier to the electrode ) take place. Since this discharge is not sufficient over the entire distance between the electrodes, it is called a partial discharge. Another cause of a partial discharge, the " aspiration " of electrons from an electrode due to strong fields be (field emission).

In technical applications, in particular in high-voltage engineering, one usually tries to prevent the formation of inhomogeneous field profiles (such as field scattering electrodes and corona rings). However, damaged parts can favor inhomogeneous field distribution and thus the formation of partial discharges.

Types

We categorized the following symptoms of partial discharges.

Outer partial discharges ( corona )

Outer partial discharges are discharges on the surfaces of free metal electrodes in the surrounding airspace inside. They preferably occur on sharp-edged parts where the field strength is greatly increased. Commonly known is this phenomenon with the audible and visible corona discharges from high voltage overhead lines.

Even St. Elmo's Fire covered. Outer predischarges can be avoided (eg high voltage cascade ) through round design of all edges, as well as by field controlling rings.

Internal partial discharges

As an internal partial discharges are generally all externally invisible discharge phenomena referred to within insulating media. In the insulating media, it may be solid, liquid or gaseous materials.

Discharges occur where inhomogeneities of the medium are under strong magnetic field influence, for example in the case of gas bubbles, which are located in an insulating liquid, such as oil, or in cast resin. These gas bubbles, consisting of air, carbon dioxide (e.g. in the case of exposure to moisture during the curing of the polyurethane resin ) or oil decomposition gases have a smaller dielectric constant than the surrounding oil, whereby an increase of the field strength occurs. The insulation at the location of the gas bubble are disturbed by the locally lower dielectric strength, which is manifested by partial discharges. Also not correct connections of built-in components in manufactured by resin impregnation or components (switching power supply transformers, high voltage cascade ) lead to partial discharges. Other examples are not encapsulated transformer windings of enameled copper wire in SMPS transformers and loosely wound film capacitors for AC application.

Internal partial discharges, because of the ultraviolet radiation and ionization in the long term even with artificial resins for damage to the surrounding insulating material and must therefore be avoided.

Transformers (especially high-voltage and switching power supply transformers ) are therefore often vacuum - impregnated or potted under vacuum.

Creeping

In creeping phenomenon of partial discharge occurs at the longitudinally arranged for field boundary layer on an insulating material. Even then, the homogeneous profile of the electric field is disturbed, and it can lead to the boundary layer along the "sliding " discharges. Dirt and moisture promote this phenomenon, however, many insulating materials in clean condition along their surfaces a lower dielectric strength than the same air-gap length. Again, these sliding or predischarges lead especially in organic insulating injury, the occurrence of leakage currents and subsequently to the punch. Surface discharges can be avoided by extending the boundary layer and moisture protection (ribs of insulators, impregnation). Another measure is the attachment of metal subdivisions defined potentials ( field control ).

Meaning and Measurement

Partial discharges are generally undesirable, they lead to overhead lines to energy losses, and on or in components to ionisationsbedingten harmful effects. On devices such as electrical engineering, among others, transformers, capacitors, high voltage bushings, insulators, instrument transformers and optocouplers therefore be partial discharge measurements.

Here, the safety aspect is important, because a solid insulation, in which there is in operation to partial discharges, is not long-term reliability. To ensure long-term stability, this is detected in the PD measurement that even with this still existing partial discharge certainly again below a limit value above the highest occurring operating voltage. Thus, the Teilentladungsaussetzspannung (UTA ) is determined, which must be above a threshold value, which was determined based application in the framework of the insulation co-ordination with the customer. For this purpose there are various standards in the field of, among others, the Association for Electrical, Electronic & Information Technologies (VDE ) and the International Electrotechnical Commission (IEC 60270 ).

The measurement of the partial discharges is carried out with typical measurement receivers in the range of about 100 kHz to several MHz. The lower measurement threshold is rarely due to disturbances in shielded test sites less than 1 pC ( pico coulombs). In measuring stations (but usually 50 Hz, with switching power supply transformers in the area whose center frequency) provide oscilloscopes, the test voltage, together with the high frequency partial discharge pulses represent and leave according to their position to one another further interpretations to whether it is, for example, is external or internal TE. A typical PD measurement according to the instructions includes Turn on full operating voltage to the value of 1.8 times the rated voltage, which is kept for a certain time. This tension is still no measurement, it only serves as a " suggestion ". The actual measurement is then performed at 1.3 times rated voltage.

An example of safety-related components are ignition transformers ( control of power semiconductor ) in railway electric drives, all of which pass through a 100 % partial discharge measurement in the final test. Large transformers and other high voltage components, such as in substations, are reviewed regularly with mobile PD measuring systems in order to plan preventive maintenance and replacement measures.

Measurement of partial discharge

In order to measure and assess PD pulses can, they must be coupled out of the device under test and an appropriate evaluation system for signal conditioning, signal processing and for visualization and data protection are supplied. In the market, different PD measuring systems from different manufacturers are available for this purpose, which usually allow single-channel PD measurements in some cases pre-defined fixed frequency bands. These measuring systems are, however, mainly optimized for the Prüffeldeinsatz, and possibly by A. sufficient flexibility to avoid, for example, temporarily occurring under on-site conditions fixed frequency interferers suitable by varying the measurement frequency and measurement bandwidth can. Sensitive PD measurements on site are therefore not usually possible. Similarly, these systems are not suitable for simultaneous PD measurements at different output coupling, since in them i A. is only a single measurement channel is available, the partial discharges can capture at several measuring points only temporal succession (using a Messumstellers ). For long high-voltage cable runs, for example, but a synchronous TE measurement of all standing under test voltage sets to reduce the measurement time, but especially also to minimize the risk of an otherwise not observed unintentional failure is just urgent.

Regardless of the PD measuring system used, the decoupling of the PD pulses plays a crucial role from the specimen. To this end, various sensory concepts and measurement procedures are established that are already optimized or specialized in some cases for certain resources ( eg integrated capacitive sensors for PD measurement on sleeves). Therefore, the following is an overview of the usual Auskoppelverfahren to high-voltage cable systems are given.

PD measuring systems

Conventional TE extraction

According to IEC 60270 ( High-voltage test techniques - Partial discharge measurement), the measurement of partial discharges is carried out at the cable end. The decoupling of the PD pulses is via a measuring impedance ( Auskoppelvierpol ) that converts the damage caused by the local insulation breakdown in the test specimen pulsed charging current of the parallel to the DUT connected coupling capacitor in a charge equivalent voltage signal. This voltage signal is then detected and processed by a TE - measuring system.

Measuring circuits

Several types of connection of DUT, coupling capacitor and impedance measurement (,) are possible. Figure 2 shows the measurement setup with grounded coupling capacitor.

In this case, the candidate is with the measured impedance in series, what with small capacitance value resulting devices under test to a good sensitivity. For a breakdown of the specimen, however, is up to the full test voltage at the measuring impedance so that the downstream measurement technique should be covered by appropriate surge protection devices.

Since an isolated formation, or a separation of the earth connection is not possible with most Hochspannungsprüflingen, the measured impedance in the rule must be introduced into the earthly branch of the coupling capacitor. For this purpose, it must be insulated, the ground connection of the DUT remains. Figure 3 shows this type of connection.

The achievable sensitivity in two variants mentioned the PD measurement is determined largely by the amount of available coupling capacitor, or by the ratio of the Prüflingskapazität. The measurable charge a TE pulse is calculated from the apparent charge according to equation 1

Figure 4 shows the relationship graphically.

Cable systems, owing to their great length an enormous capacitive load point (eg, 400 - kV XLPE diagonal Berlin: about 11.5 km, about 2.2uF; London 400 kV XLPE: 20 km, 4, 4 uF; Augsburg 110- kV XLPE: about 3.8 km, about 700 nF ). With the current available capacity values ​​of on-site coupling capacitors suitable for the required voltage levels to a significant reduction in sensitivity would thus inevitably arise. A PD measurement with coupling capacitor is therefore not useful. As an alternative, single-phase PD measurements on cable systems using a neighboring phase ( or both neighboring phases, cf ) is possible as a coupling capacitor. At this point, the demand for TE freedom of the coupling capacitor is abandoned in favor of a significantly increased sensitivity. When occurring PD signals can be clarified by comparing the measurement results of all three phases, however, which is the phase TE - prone.

Another variation in the simultaneous measurement of two phases of a cable system, the coupling of the TE signal by a bridge circuit ( see Figure 5 ) which allows a high degree of common mode rejection.

This measuring principle is based on the assumption that the measurable signals from occurring TE errors from the device under test are temporally correlated to stimuli from acting as a coupling capacitor adjacent phase, whereas glitches (mainly coupled to the cable ends corona disturbers of parallel energized systems ) through both run involved in the measurement phase and at the same time ( and the same polarity ) occur at the measuring location. By using a Ferritübertragers at the TE coupling these common mode interference sources can be effectively suppressed. With this method it is assumed that the test voltage source can provide enough power for several phases on site.

Measuring impedance

Measuring the impedance is usually constructed as a passive band-pass analog filter. Depth frequency components, notably the area near the test or working frequency are highly suppressed to protect the connected measuring technology. In a broad undamped range (< 1 MHz) is then the decoupling of the PD pulses. In this outcoupling additionally tightly integrated additional filters ( bandstop filters ) may be provided, the well-known narrow-band frequency-locked interferers, mainly amplitude modulated radio stations, suppress. Here it should be ensured that consequently reduced by reducing the usable frequency spectrum and the energy coupled out of the TE pulse. In mobile PD measuring systems is also assumed that are not useful by regional differences in the terrestrial broadcasting frequencies permanently implemented bandstop filters. Here, suppression of these interferers may be performed by programmable software filters on the side of the TE - measuring system, for example, by adaptive filtering algorithms.

Another purpose of the band pass filtering of the measurement impedance is a quasi- integration of the measurement signal in the time domain for determining the pulse charge. This is calculated by the Fourier spectral energy of any current pulse according to equation 2:

It is known that the integral of current over time, the desired charge pulse (see Equation 3).

So mathematically equivalent to the spectral signal component at DC (f = 0 Hz) to the sought charge value. However, as described above, test voltage frequencies in the range of (and below ) can be suppressed during the extraction, this frequency component is not available for further evaluation. Assuming that the profile of the frequency spectrum to a characteristic cut-off frequency is nearly constant, a correct determination can be above the charge from 0 Hz by a band-pass range of the constant amplitude in the measurement of the frequency spectrum of the TE - pulse. The narrow-band bandpass measurement also allows for knowledge of the current noise spectrum around the targeted selection of a frequency range for PD measurement, which is largely free from rigid frequency interferers.

Non-conventional field coupling

As described in the previous chapter, the classical extraction of PD pulses on the cable ends by means of coupling capacitor and impedance measurement often does not lead to the required measurement sensitivities of some Picocoulomb. An alternative to this conventional galvanic coupling, the PD detection by field coupling dar. In this method, the recorded electric and magnetic field components generated by the TE - pulses by means of appropriate field sensors and converted into a measurable voltage signals.

Field sensors generally operate in a frequency range above 1 MHz, and are not therefore IEC compliant. In addition, they are not calibrated (output signal in mV instead in pC ) than in the classical sense. Through a variety of successful measurements, especially under disturbed on-site conditions, these sensors, however, have already been proven. In upcoming standards or standard adjustments are field sensors and their calibration, therefore, be taken into account, but not in the current redesign of IEC 60060-3 for standardization of the on-site inspection and metrology. Furthermore, it is in the test for high voltage cable systems common for agreements between the customer and the auditors will amend or replace applicable standards. The use of field sensors is already available and in many cases the only useful method for signal detection in TEMessungen extended cable systems.

A reasonable location for field sensors, the area around the power cable accessories is mentioned. Firstly, the field sensor can be implemented in the clothing assembly on site with little additional effort. Often even the integration of field sensors in sets already in their production at the factory, so that spot, no additional steps are necessary. Secondly, the sensor with its mounting in close proximity to the set near the potential PD fault location is placed, since the high voltage cable ( XLPE) has already been studied in the cable plant on TE freedom, so that usually only components that are assembled on site be ( joints and terminations ), come as PD defects in question.

Another positive effect on the TE coupling by field sensors is the störunterdrückende effect of the specimen itself due to the large cable capacitance of the DUT acts as a low pass filter and attenuates so in the relevant frequency range above 1 MHz external glitches so far as that of the field sensors in the joints can be detected only with a greatly reduced amplitude. The monitoring range of field sensors can be limited to the close vicinity of the sets.

Capacitive sensors

The detection of the electric field component of a TE - pulse is accomplished by capacitive sensors. The sensor electrode can be realized as a conductive strip in the form of a cylinder jacket to the core ( CCS, coaxial cable sensor, see Figure 6). The sensor electrode acts together with the outer cable shield as a capacitance. This results in a capacitive voltage divider composed of lead, and the sensor that enables the extraction of the pulse-like signals from the power cord or the clothing.

This embodiment of the capacitive sensor must be mounted locally. Consequently, also the open cable shield and protective cable jacket shall be made by the sensor assembly again and their proper condition be detected. As an additional vulnerability and the measuring line is mentioned, which leads the potential sensor for the measurement of the cable to the outside. This inevitably pierces the cable sheath and must be adequately protected against possible water entry.

Structurally mature are capacitive field sensors that are implemented directly in this already in the production of complete sets. Here, the existing field-controlling deflectors can be used as a capacitive sensor surface. Here, the semi-conductive and thus frequency-dependent character of the Deflektorwerkstoffes is exploited. During the deflector for the power-frequency fields takes the field controlling function within the sleeve design, can be taken from a shunt resistor for the metrological detection by TE caused high-frequency fields at this.

Since in most cases, only a single capacitive sensor each sleeve is realized, can not be an exact location of a fault by TE - term analysis within the sleeve. A CM accurate fault location is due to the limited to about 20 MHz, the upper frequency limit of the sensor is very limited, even with two sensors.

In the typical capacitive sensors frequency range of about 2 MHz to 20 MHz high frequency pulses during their propagation in the cable are already so heavily damped that the decrease in Impulsamplituten from origination to the point of measurement and the pulse deformation A., a clear distinction of the pulse origin allow (see Figure 7).

So also corona interferer example, can be clearly distinguished from TE out of the sleeve. Regardless enables the precise detection of the absolute time of the determination of the direction of pulse propagation and thus also a reliable differentiation of impulse origin.

For the reasons mentioned above, it is not possible to calibrate the capacitive sensor by an injection of a reference charge on the accessible end of the cable locally. The calibration pulse would have the cable through to the sensor through several 100 meters in the first sleeve and there would be strongly attenuated. The time required for a quantitative evaluation of TE calibration must therefore take place in addition to a built-up collar with short cables in the laboratory. The sensitivity of the sensor is only by the system cable sleeve dependent (eg geometry, conductivity of the conductive layer ). With two existing identical sensors on one sleeve cross- calibration is also conceivable. Here one of the sensors acting as a condenser for supplying the calibration signal while the other sensor is used as a decoupling capacitor. According to Equation 4, the half value of the calculated coupling loss corresponds to the attenuation value of a single sensor due to the symmetry of the sensors.

Also occurring signal losses due to partial reflections within the sleeve construction must be taken into account.

Directional coupler sensors

A directional coupler is a known component of the antenna technology, which allows forward and returning signals coupled out separately. The switching behavior of the directional coupler sensors based on a superposition of inductive and capacitive coupling, the ratio can be adjusted. In an ideal directional coupler both couplings are exactly the same size. Figure 8 shows the principle of constructive and destructive superposition signal.

A signal on line 1 ( shown in Figure 8 by the directed flow arrow, green ) is on line 2, both a common-mode -like inductive coupling component ( blue ), as well as a push-pull shaped capacitive coupling component ( red ) results in which two respectively on the superimpose measuring resistors and lead to the described output signals.

Figure 9b: TE sensor, self-made

Figure 9c: TE sensor, self-made

The directional coupler sensor is characterized by a clear indication of the pulse source direction. A striking on the directional coupler sensor signal is always at the origin side facing the direction of the Richtkopplerausgänge (coupling path) measurable ( constructive superposition of inductive and capacitive signal component ), while at the other output (blocking path) ideally no output appears (destructive superposition ). Under ideal directional couplers there is a complete extinction of the signals in the reverse path. In practice, real directional coupler sensors reach a switching ratio ( ratio of signal blocking path to coupling path ) of the order of 1:10. Down to a signal ratio of 1:2, however, a reliable statement about the direction of origin of the PD signals is usually unproblematic.

The directional coupler sensors are usually mounted directly to the thereof did not affect outer conductive layer of the cable inside the sleeve housing. Figure 9 shows a simple sensor for TE coupling, which was nachtrlich attached to a cable sleeve.

By logically combining the four output signals of the two directional coupler sensors on a sleeve, a clear classification of signals is " from the left ", " coming from the right " and " TE from the socket " is possible. For maximum decision confidence, ie large directivity, the directional coupler sensor for each cable should be unique in its geometry specifically matched because the mechanical and electrical properties of the cable, for example, the thickness of the insulation and the conductivity of the conductive layers go in the directivity.

In inductively tuned directional coupler sensors outweighs inductive coupling. The pulse travel direction is determined in the inductive coupler adapted in contrast to the hitherto considered the directional coupler sensor of the polarity of the output signals of two sensors. External disturbances are coupled with opposite polarity. Signals with the origin between the two sensors, eg, TE out of the sleeve are, however, coupled with the same polarity and are therefore clearly recognizable (see Figure 10)

The operation of inductively tuned directional coupler sensors is derived from the basic principle of a directional coupler, wherein the capacitive coupling is absent. The sensor and the inner conductor of the power cable forming a system of two coupled lines, which have a common inductance. The inductively tuned directional coupler sensor has only one output signal per sensor. The necessary at the directional coupler sensors second output is omitted, since it contains redundant information. For monitoring a sensor of a sleeve according to the left and right of the sleeve is mounted.

A particular advantage of the directional coupler inductive sensors is that they do not need to be specifically tailored for each cable in its geometry as opposed to " normal" directional coupler sensors, and that the evaluation needs to evaluate only two signals to a socket. In addition, the required bandwidth of the transmitter - depending on the required sensitivity which is at full bandwidth of directional couplers - are significantly reduced without the decision certainty about the TE - origin, namely TE from the sleeve or by external influences will. This compares with the reasonable for practical use in many cases disadvantage that with the inductive directional coupler sensor the direction of origin of external noise can no longer be differentiated.

Inductive Sensors

Inductive sensors utilize the magnetic field component of a TE - pulse, and may be mounted externally to the casing of the power cable. By appropriate shielding measures is to ensure that no electric field components superimposed on the measurement signal. A common embodiment of an inductive sensor is the Rogowski coil. by virtue of their regular geometric properties and shielding against the electric field components for decoupling TEImpulsen in power cables is advantageous.

Rogowski coils are characterized by a large measurement bandwidth and by a wide and linear frequency response. Due to the use of extended line elements as the secondary winding of the resultant together with the main current path of the transformer, however, must be taken into account may traveling wave effects with the use of these sensors.

Application

Useful applications of partial discharges see ionizer and corona treatment.

In certain types of lasers, nitrogen partial discharges are used to vorzuionisieren the discharge gap, so that the main discharge is homogenous.

In igniters for high -pressure gas discharge lamps and flash lamps for partial discharges support the ignition by ionize the filling gas in the most pointed electrodes.