The effectiveness and safety of power distribution networks substantially depend on the correct operation of electrical equipment, including medium-voltage (MV) switchgear. Partial discharge (PD) is the most well-known hazard to the reliability of these systems.
PDs, or localised dielectric breakdowns, are triggered by factors like high voltage, moisture, insulation faults, or even mechanical damage to the insulating material. They not only jeopardise the electrical supply but also gravely affect the employees who work there. All these call for the need to detect and monitor PD activity at the earliest.
The ultrasonic frequency spectrum of PD is usually in the range of 20 kHz to 200 kHz, above the human audible range. Ultrasonic testing is one of the most sophisticated techniques for detecting PD.
Ultrasonic PD detection sensors are generally piezoelectric transducers. They convert the ultrasonic energy into electrical energy, which can then be processed and analysed to detect PD activity. Ultrasonic PD detection can be performed in a variety of ways, such as:
- Acoustic imaging: Acoustic imaging can be used to precisely locate the source of PD activity. This method creates a visual representation of the ultrasonic energy released from the switchgear enclosure.
- Handheld scanning: A handheld ultrasonic PD detector is simple and comparatively inexpensive. This is used to scan the exterior of the switchgear enclosure. Yet, if the enclosure is large or complex, it can be difficult to locate the source of PD.
- Permanently mounted sensors: Permanently mounted ultrasonic PD detectors can be either installed inside or outside of the switchgear enclosure. While this method is more expensive and complex to implement, it provides continuous monitoring of PD activity.
A non-destructive partial discharge testing, or NDT, method called ultrasonic PD detection makes it easy to find and keep an eye on PD activity in electrical devices, especially MV switchgear. The method uses ultrasonics to capture high-frequency sound waves from PD discharges. However, before non-intrusive ultrasonic PD detection in MV switchgear can be properly incorporated, certain significant problems must be addressed.
The Science of Ultrasonic PD Detection
Physical discharges, also known as PD discharges, arise whenever a small insulation area in a high-voltage environment cannot resist the undesired breakdown and electrical stress. This stress can be due to a range of reasons, such as design defects, contamination through particles, a detrimental substation environment, and spaces in the solid due to manufacturing defects. If your asset is old and has gone through deterioration, the reason can be a combination of all.
PD releases high-frequency sound waves. These sound waves are used for the partial discharge detection and analysis of ultrasonic impulses. The practice is beneficial as it allows the identification of PD activity without having access to electrical equipment components. Since MV switchgear typically has restricted access and is vulnerable to incursions of various types that result in significant expenses and delays, this is especially important.
While ultrasonic PD detection is beneficial, there are some significant deterrents related to its use in MC switchgear that need to be carefully considered. In most cases, the MV switchgear is placed inside a metallic enclosure. The enclosure walls could possibly be made of materials such as steel, which can cause attenuation via enclosure walls.
Understanding Sound Wave Propagation at The Steel/Air Interface
Characteristic impedance (Z), also known as natural impedance, defines how sound waves behave when transitioning between different media. It is defined as Z=p x c. Here, p represents the density of the medium, and c represents the velocity of sound in the medium.
(Video can be embedded here.)
In our context,
ρ (density of air) = 1.225 kg/m³
c (speed of sound in air) = 343 m/s
ρ (density of steel) = 7,800 kg/m³
c (speed of sound in steel) = 5,900 m/s
Based on the values,
Z₁ (characteristic impedance of air) = 414 Rayl
Z₂ (characteristic impedance of steel) = 40,300,000 Rayl
The contrast between the characteristic impedances of air and steel plays a crucial role in the behaviour of sound waves as they transition from one medium to another. The majority of the energy is reflected when waves travel from one medium with a lower characteristic impedance (air) to another with a higher characteristic impedance (steel). This can be quantified using the reflection coefficient formula, that is:
Ri = Z₂ – Z₁²Z₂ + Z₁²
In the case of sound waves travelling from lower characteristics impedance (air) to higher characteristics impedance (steel), the reflection coefficient is high to 0.99998. This means nearly 99.998% of the sound energy is reflected back into the switchgear enclosure.
Non-Intrusive Ultrasonic PD Detection
A non intrusive ultrasonic PD detection method is recommended as it eases PD detection without opening or de-energisation of the switchgear. Below are some ways to overcome the challenges of non intrusive ultrasonic PD detection in enclosed switchgear.
- Use a parabolic dish: A parabolic dish focuses the ultrasonic energy onto the detector, which significantly increases the detection range, even though the switchgear enclosure is in the way.
- Use a structure-borne ultrasonic sensor: A structure-borne ultrasonic sensor detects PD activity by measuring the ultrasonic vibrations transmitted over the metal structure of the switchgear.
- Use a high-sensitivity ultrasonic detector: Here, PD activity is detected by using a high-sensitivity ultrasonic detector. The advanced signal processing techniques amplify and filter the ultrasonic signal.
Benefits of Non-Intrusive Ultrasonic PD Detection
- Non-Destructive Method: Ultrasonic PD detection does not damage the switchgear or its components, as it doesn’t require switchgear to be opened. This is critical in MV switchgear designed for longevity.
- Cost-Effective: Non-intrusive ultrasonic PD detection is relatively inexpensive compared to other methods, and it reduces the need for invasive procedures, which can save money.
- Efficiency: The method is quick and straightforward, enabling rapid PD detection and monitoring, which is essential in a world where operational efficiency is a priority.
- Accessibility to Remote Areas: Non-intrusive ultrasonic PD detection can reach areas within the switchgear enclosure that may be difficult or impossible to reach with other PD detection methods.
Challenges of Non-Intrusive Ultrasonic PD Detection
While we discuss the benefits and drawbacks of non-intrusive ultrasonic PD monitoring and detection, we also have to identify the challenges. Here are some challenges that need to be addressed for the successful implementation of switchgear.
- Noise Mitigation: It is important to employ advanced noise filtering techniques and cutting-edge technology to mitigate the impact of background noise. Also, consider scheduling ultrasonic PD detection during a low noise period can improve the accuracy of the results.
- Optimising Design Gaps and Air Vents: Manufacturers, engineers, and maintenance professionals can collectively work towards optimising the vents and gaps in the switchgear. This can help improve the effectiveness of ultrasonic PD detection.
- Frequency Selection: Considering the specifics of switchgear, including the materials of the enclosure wall and thickness, can help in selecting the right frequency. This is highly important as lower frequencies can penetrate thicker walls but could be subject to more interference from background noise.
- PD Source Location: Consider placing PD sources closer to gaps and air vents in the switchgear when possible. This can help in successful detection.
An effective instrument for the early identification and surveillance of PD activity in MV switchgear is non-intrusive ultrasonic PD detection. Employing it in this scenario is made difficult by a few factors, including the high reflection coefficient caused by the steel/air interface and possible interference from surrounding noise. However, this approach has numerous benefits, like non-destructive testing, low cost, high effectiveness, and portability to remote areas.
By understanding the challenges and drawbacks of this method suggested by Ocean-me, the leading Engineering Company in Oman, maintenance professionals can look for ways to overcome the issues and improve the safety and reliability of MV switchgear.