Non-Intrusive Measurement of Switchgear Condition
In Issue 1 2009 of Transmission & Distribution, we described how condition assessment achieves measurable business benefits for network operators. In this article, we describe some of the new technology which can be employed to measure the condition of high voltage equipment while it is still in service.
By Charles Williamson, EA Technology Australia Owners and operators of high voltage networks are facing ever-increasing challenges to maximise utilisation of existing equipment and reduce operational costs whilst maintaining and improving system performance. Yet, they all have the same problem – an ageing infrastructure of equipment that has been installed and in operation for 20, 30 and in most cases 40 years.
In recent years, owners and operators of electrical networks have become increasingly interested in condition monitoring of electrical plant to prevent failures, reduce maintenance and extend plant life. Implementation of an effective switchgear testing, inspection, and maintenance program is essential. Even switchgear labelled “maintenance free” requires periodic testing and maintenance.
However, getting plant offline for testing and maintenance is becoming increasingly difficult, due to higher network loads and network configuration limitations. One proven method of condition monitoring of HV switchgear involves the use of non-intrusive partial discharge test equipment, to gain the maximum condition information before an outage decision is taken.
Causes of switChgear failure Partial Discharge (PD) activity is a major cause of long term insulation degradation in HV switchgear. EA Technology’s experience of working with switchgear operators for more than 30 years indicates that PD activity is a factor in around 85% of disruptive substation failures.
Mechanical issues of the switching equipment are often addressed by manufacturer prescribed maintenance and mechanical related condition is easier to check. However, insulating parts, surface or internal degradation, especially if triggered by environmental or electrical stresses, may become critical to equipment operating life when ageing phenomena start, and standard maintenance procedures are not likely to prevent or detect them.
The majority of degradation processes affecting insulating components, such as bushings, cable terminations, current and voltage transformers etc, are associated with PD activity, defined as a localised electrical discharge in an insulation system that does not completely bridge the electrodes. As insulation ages and degrades over time, PD activity typically increases as the defect evolves. Therefore, PD activity measurement can be used as a powerful diagnostic tool to non-intrusively assess equipment condition, to locate the defect source, and to target selective intervention to remove the ageing component prior to complete failure.
How is PD Measured?
Partial discharges may occur in gas, liquid and solid insulation during a temporary over-voltage, a high voltage test, or under transient voltage conditions during operation. If these partial discharges are sustained due to poor materials, design, and/or foreign inclusions in the insulation, degradation and possible failure of the insulation structure will occur.
There are two types of partial discharge in high voltage insulation: surface partial discharge and internal partial discharge.
Surface partial Discharge
When surface partial discharge is present, tracking occurs across the surface of the insulation, which is exacerbated by airborne contamination and moisture. Surface partial discharge leads to erosion of the insulation and tracking phenomena as illustrated below in Figure 1.
Figure 1. Surface Partial Discharge
Surface discharges are best detected using an ultrasonic detection instrument with an uninterrupted air path between the discharge site and the instrument to allow the sound pressure waves to be detected externally. Surface discharges tend to occur between the particles of a contaminant, producing heat, light, smoke, sound, electromagnetic radiation and ozone and nitrogen gasses.
In the early stages, the high frequency sound waves generated by the partial discharge activity are readily detected ultrasonically in the 40kHz range. Often moisture combines with NOx gases to produce nitric acid which attacks the surrounding metalwork and leads to severe corrosion of the equipment. Insulation surfaces affected by such an acid attack produce an ideal surface for tracking to occur, leading to the creepage distance of the insulator being compromised.
Tracking is the result of carbonisation of the surface of insulation, brought on in the early stages by the breakdown of contaminants in the presence of high humidity.
Internal Partial Discharge
Internal partial discharge occurs within the bulk of insulation material and is caused by age, poor materials or design, or poor quality manufacturing processes. The current transformer illustrated in Figure 2 was known to be exhibiting internal partial discharge. When it was removed and sectioned, the damage was seen to be at the top and is illustrated in the figure on the right.
Figure 2. Internal Partial Discharge
Within all insulation material, however manufactured, microscopic voids or cracks are present. Under service voltage, these voids charge up and discharge within the 50/60Hz cycle, like small capacitors. Eventually, because the breakdown strength of air is less than that of the surrounding insulation, the air in the void breaks down with a (very small) arc and a “partial” discharge occurs.
These arcs produce heat, light, smoke, sound and electromagnetic radiation but only the electromagnetic waves are detectable externally (see Figure 3).
This discharge action also erodes the voids, and as they get bigger, the discharge energy dissipated with each discharge increases in magnitude. Carbonisation of the inner surface of the void occurs, which progressively makes the void conductive and increases the electrical stress on adjacent non-conductive voids. If enough conductive voids are present, the insulation fails, even under normal working voltage, but particularly following short-time over-voltages caused, for example, by switching operations.
Transient Earth Voltages Electromagnetic waves propagate away from a partial discharge site in all directions. These high frequency voltage pulses (between 0.1mV and a few volts) escape through joints in the metalwork and pass from the inner to the outer metal surface of the equipment and then down to ground at the speed of light. The voltage pulse will stay on the surface of the metalwork, as their high frequency leads to a skin effect.
These pulses were first observed at EA Technology in 1974 by Dr John Reeves and were termed Transient Earth Voltages (TEVs), because they only last for a very short time and travel down to earth. It was found after extensive trials that these TEV signals are directly proportional to the magnitude of any active partial discharge and the condition of the insulation for switchgear of the same type and model, measured at the same point. This produced a very powerful comparative field-based technique for non-invasively checking the condition of switches of the same type and manufacture, whilst the equipment is live and still in service.
Interpretation of TEV Measurements For each type of activity the pulses shown have two distinct features, the size (or magnitude) of the individual pulse and the number of pulses per cycle. As partial discharge activity increases over time, the magnitude and number of pulses per cycle increases. The method of interpreting the TEV readings relies on being able to combine two aspects in the manner shown in Figure 4.
Figure 4. Analysis of Transient Earth Voltages (TEV’s) A low number and low magnitude of pulses means that there is little risk of insulation failure. Either a low number of high magnitude pulses or a high number of low magnitude pulses means that the risk of insulation failure is increasing and more regular testing should be undertaken. Finally, a high number of high magnitude pulses means that the risk of failure may be unacceptable and early intervention is necessary.
EA Technology began partial discharge measurements of indoor metalclad switchgear using TEV detection instruments in 1983, and has assembled a database of partial discharge survey results with over 100,000 entries, covering many manufacturers and types of high-voltage switchgear. Survey TEV readings (in dBmV) are compared with the database results for comparable equipment (of the same voltage level, insulation type and design). If the level of partial discharge is within the top 5%, then it is deemed to be in the ‘red’ quadrant of Figure 4. If the readings are in the top 25% of the database values, then the equipment is in the ‘amber’ quadrant, and otherwise in the ‘green’ quadrant.
PD Instruments PD testing is well-proven over many years for checking the condition of insulation systems in MV/HV assets. The PD Locator™ quantifies the level of PD activity using a single TEV-sensing probe. With an additional TEV probe, it can locate the source of PD activity accurately, using time of flight measurement of pulses. The PD Monitor™ uses multiple TEV probes, networked to a central server, to monitor PD activity over time in hundreds of assets simultaneously, and includes software which automatically analyses and interprets activity in the form of condition reports.
Both of these original EA Technology instruments have been used for many years to collect the data for the PD database, and continue to be benchmark instruments for online assessment of switchgear in service. However, it is only quite recently that more lightweight, portable and effective monitoring tools have been developed to realise the full potential of PD measurement, deployed in the hands of the network operator as a strategic tool for asset maintenance and management.
New PD developments incorporate dual sensors into single instruments: an ultrasonic sensor principally to measure surface PD activity, plus a TEV sensor to measure internal PD activity. This has produced a whole family of new instruments, each of which has a specific role in the detection, location, measurement and monitoring of PD activity.
EA Technology’s UltraTEV Detector™ is the world’s first dual-sensor handheld PD detection tool, which in 2007 won the Queen’s Award for Industry: Innovation – one of Britain’s top industrial awards. It is now standard issue for substation use with every electricity network operator in the UK, and has been widely adopted by operators worldwide.
The UltraTEV Detector™ has been compared to an Apple iPod because of its ease of use, minimum training required and user friendly interface, beneath which lies a great deal of technological development. A single button turns it on, and a simple traffic-light system shows the operator instantly if PD activity is at one of three levels: green indicates OK, amber indicates that the asset require further investigation, and red shows that it requires immediate intervention and/or that PD activity is at a dangerous level.
Figure 5. The latest handheld instruments measure PD activity as both ultrasonic and TEV emissions Whilst the basic UltraTEV Detector™ is invaluable for taking ‘first pass’ PD readings quickly with minimal operator training, it has spawned a range of even more sophisticated dual-sensor instruments of increasing versatility. The UltraTEV Plus+™ (Figure 5) presents ultrasonic emissions as numerical decibel values and audible signals, which engineers can listen to via headphones, with the option of a directional dish microphone for measuring PD activity in overhead assets. Measurement of internal PD activity in the form of TEV signals is presented as numerical values: and a continuous PD measurement mode incorporates maximum level, pulse count per cycle and severity level.
Ultrasonic measurement is the most valuable technique for surveying overhead line assets. The UltraMET™ is a new addition to the instrument range, with a parabolic concentrator probe for the detection of ultrasonic emissions from overhead line hardware, outdoor cable terminations and insulators.
Dual sensors are also built into multiple sensing nodes of the UltraTEV Alarm™ system (Figure 6), which provides permanent or semi-permanent monitoring for PD activity in multiple switchboard panels through to whole substations. When PD activity reaches critical levels, the UltraTEV Alarm™ automatically generates alerts via SMS or email. This has now been augmented by the PD Monitor GIS™, a system specifically designed to monitor the condition of high voltage gas-insulated switchgear by measuring internal partial discharges in the UHF range.
Conclusion
At its most basic level, the ability to detect and measure PD activity in live assets is an extremely valuable technique for spotting faults in the early stages of their development, before they become failures and outages. It is also a uniquely effective way of ensuring the safety of personnel, to the extent that it is now standard practice for many engineers to check for PD activity every time before they enter a substation.
However, it is rapidly becoming apparent that the greatest commercial value of PD measurement is the ability to use accurate data on the condition of assets to fundamentally change the way they are managed. Instead of basing asset maintenance and replacement on time schedules, assets can be managed more efficiently on the basis of their actual condition. This results in less needless invasive maintenance, less downtime, better targeted outages, potentially longer asset life, greater safety and lower operating costs. The availability of data on the condition of each asset also leads to more cost-effective decisions for asset replacement and refurbishment.
As the experience of SP Powergrid in Singapore showed in our article last issue, the bottom line is that condition measurement pays rich dividends.
EA Technology T&D Magazine, Issue 2 2009 www.powertrans.com.au