THEORY
- What is a Partial Discharge
A Partial Discharge is a movement of charge within an insulator but which does not bridge the conductors. These discharges can occur within the insulation, on the surface of the insulation or in the fluid surrounding the insulation.Partial discharges occur where there is a change in the otherwise spatially homogenous insulation. Defects within the insulation, interfaces between different types of insulation will all result in partial discharges provided the voltage is sufficient. - Properties of Partial Discharges
Partial Discharges are characterised by the following properties:- Requires high electric fields
Breakdown occurs in regions of high electric field where the local field exceeds teh local breakdown strength of the insulation. The amplified fields are caused by differences in dielectric constants of insulating materials or by sharp edges in the conductors. It is therefore less likely to experience discharge activity at lower voltages. - Voltage dependent – initiation and extinction occur at specific voltages
The local breakdown due to the amplified field will not occur until the local breakdown voltage is exceeded at some point in the energising waveform. Once this is exceeded then the PD will continue until the energising waveform falls below the minimum voltage required to sustain the discharges. The rms values of these energising voltages are used as the measured value of the PD Inception and Extinction voltages. The relative values of these voltages is an indication of the type of discharge being emasured. - Waveform dependence – discharges occur in relation to the energising waveform depending on the type of discharge. This is evident from the physics of the breakdown. The distribution of the PD events can be accumulated over a number of cycles and the dispersion pattern that results can be used to identify the discharge and provide evidence as to its location.
Equally the lack of a pattern would provide strong evidence that the signal being measured in unlikely to be a discharge event.
- Unipolar event
The discharge event involves a movement of some charge from one position in the insulation to another. The direction of movement is in one direction only and therefore the resulting pulse will be unidirectional. The propagation path and transducer used may alter the characteristics but this will be evident with the calibration pulses.
Breakdown is modelled as a gaussian pulse and the frequency content of the pulse is dependent upon the rise-time and the pulse duration. This will vary for different types and locations of PD. - Short duration event with quick rise and decay times
The discharge event is a partial breakdown and the rise-times of the pd pulse are high (in the nano-seconds range). The rise times are dependent on the medium that the breakdown occurs in.
The PD pulse is of short duration, explained by the self-extinguishing physics of the breakdown.
The unipolar and short-duration, high rise-and decay-time properties of the PD pulse determine its frequency content which can be useful in identifying the source of the measured signal.
The propagation path between the PD source and the transducer will filter the frequency content and the shape of the pulse. - Repetitive event
PD will happen repeatedly, many times over each cycle. - Electromagnetic emissions
- Acoustic emmissions
Acoustic emissions in the audible and ultra-sonic bands are common. These are easy to detect when the discharge area is exposed and accessible but less likely to be detected when the source is inacessible or indirecttly accessible in which case echos can cause additional uncertainities. - Local heating
Thermal scanning may reveal localised heating. Thermal scanning requires line of sight to the potential sites and thus can be more limited even that the acoustic measurements. Examples are switchgear panels where safe removal of the panels is not advised.
- Requires high electric fields
- How do we detect Partial Discharge
Partial discharges are not measured directly but rather their emissions are measured in order to give an indication of the - Importance of rated voltage
The rated voltage of the device under test impacts onm the design of the insulation system but also on the test voltages and on the energies of the discharges which is proporional to the square of the voltage. - Damage, breakdown, failure
- Failures, failure rates, reliability
The success of a test - Age of the equipment
Different tests are appropriate and more/less successful depending on the age of the asset.
- Hardware that is used
- Calibrators
These are instruments designed to inject a known charge across the conductors of the insulation. The PD measurement system measures the effect of the injected charge through the propagation path and the transducers and a conversion constant can be calculated that converts the unknown measured value from the known injected value. - Different technologies
A number of different classes of equipment are available that exploit the different emissions from PD sources in order to detect, locate and measure the discharges. These are:- Acoustic
- Optic
- ultra-violet
- infra-red
- Radiated electromagnetic
- Conducted electromagnetic
- Thermal
- Almost exclusively the equipment is ultra-wideband digital
- Transducers
Transducers are devices attached to (or remote from) the device under test that convert the PD emissions into a measurable electrical quantity. - Quadripoles
- Data aquisition
- Network interfacing
- Synchronisation
- Calibrators
- Software
- List of Standards
- IEC 60270 unpacked
This is the most often quoted standard for partial discharge measurement.
- Different kinds of Measurements
PD Measurements can be performed using a variety of techniques and equipment. As with most procedures there is a trade-off between ease of performance (and/or analysis) and effort/resource/cost of the measurement. Some tests are simple to perform, give simple outputs that require no analysis or interpretation and some measurements are more complex to perform but give more detailed but sometimes more ambiguous infiormation. I consider that there are teh following levels of PD investigation that can be performed. Each level requires more effort but reveals more information allowing the asset owner to make better decisions.
HF scanning
Acoustic scanning
PD scanning - Sensitivity determination
Prior to the start of any good PD measurement, the environmental influences need to be quantified in order to quantify the sensitivity of teh measurement. - Calibration or reference measurement
A calibration can be performed in order to calibrate parts of the measurement system. This is not a true calibration of the partial discharges but can be considered more as a refernce measurement against which future measurements can be compared. Performing a calibration - Detection
The first actual measurement task in any PD measurement process is to attempt to detect the presence of any discharges. Equipment is used to scan the environment for evidence of PD acivity. If the measurement is online then the entire process is - Measurement
- Evidentary basis
The collection of evidence to support claims of PD activity (or the absence of PD activity) is, in my opinion, central to the result. A measurement that concludes with a lighted led of different colours is meaningless - Types of measurements
- Online vs offline testing
Voltage distribution, loading - Spot tests vs monitoring
- Test voltages and waveforms
- Online vs offline testing
- Conditioning
The insulation can be conditioned by prior actions and this needs to be reported in case the conditioning impacts on the measurement. The temperature of the components and any prior application of voltage and or loading are typical conditioning factors. - Environmental
It is important to record the ambient conditions
- Partial discharge used
- as a type test
PD Testing can be used to test new components for their resilience to PD. Design flaws in the insulation system may produce discharges and these can be eliminated with re-design. - as a manufacturing test
PD testing is useful for detecting insulation defects that are produced in the manufacturing process. The PD Measurement can normally be performed in a controlled electromagnetic environment which means that very sensitive measurements can be performed. This high sensitivity is required when the time-to fail is short for even small discharge levels which may be due to the insulating material intolerace and/or the high energy of the discharges due to the high field strengths. - as an installation test
PD testing is useful as an installation test to test for any defects that may be introduced by the handling of the material or the re-work of the material for insulation. Typically cable joints and terminations are installed on site and PD testing is useful to detect any flaws. - as a field/maintenance test
- as an end-of-life assessment
- as a type test
- Partial Discharge measurements on
- Switchgear/Circuit Breakers
- Switchboards
- Transformers
- Cable systems
- Insulators and conductors
- Motors and Generators
- Substations
- Other devices
- Context
- Analysis of Data
- Severity analysis
- Uncertainties, Limitations and Ambiguities
- Conclusions and Courses of action