TEV & ultrasonic PD Sensor Comparison

TEV & ultrasonic in MV Switchgear

Partial discharge (PD) detection is important for assessing the condition of medium voltage switchgear, which is essential for reliable electrical power distribution. Transient Earth Voltage (TEV) and ultrasonic partial discharge sensors are widely used for their effectiveness in identifying defects.

1. TEV Partial Discharge Sensors

Partial discharge activity inside metal clad high voltage plant induces small voltage impulses called Transient Earth Voltages on the surface of the metal cladding. These TEV pulses travel around the cladding surface to the outside of the gear, where they can be picked up using capacitive coupled transducers.

2. Ultrasonic Partial Discharge Sensors

Surface tracking can seriously damage high voltage insulating surfaces in a way that will ultimately lead to flashover and complete failure of the insulator. This discharge activity creates acoustic emission that can be detected using a high frequency sensor. The magnitude of the acoustic emission is indicative of the degree and severity of the discharge activity.

Partial Discharge in MV Switchgear

Partial discharge activity in MV switchgear can broadly be grouped into 2 categories: internal PD and surface discharge.

Internal PD is partial discharge that occurs in electrically stressed gas filled cavities in solid insulation, for instance voids, delaminations and cracks.

Internal PD is most easily detected by TEV PD sensors

Surface discharge is partial discharge that occurs on the surface of insulators as a result of excessive electrical stress that may be caused by a combination of contamination and moisture or high humidity on the insulator surface.

Surface discharge is most easily detected by ultrasonic sensors

Internal PD

TEV detection of internal PD

Discharges occur in internal defects as soon as the electrical stress in the gas in the cavity exceeds the breakdown strength of the gas. The conductor, cavity and the switchgear earth effectively form a capacitive divider. When the gas in the cavity ionises, it effectively briefly becomes a conductor inducing a current flow between the conductor and earth. This current pulse propagates across the switchgear panels towards earth. Being very high frequency transients, pulses travel according to the surface effect and as they pass a capacitively coupled TEV sensor on the switchgear surface, they induce a pulse that can be measured and recorded.

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Ultrasonic detection of internal PD

Partial discharges in cavities in solid insulation generate ultrasonic acoustic energy that radiates away from the source through the insulation. However there are two reasons why this acoustic energy is difficult to detect in practice:

1.?????? The amplitude and intensity of ultrasonic waves reduce as they pass through solid media due to absorption and scattering. This attenuation is generally very high in most electrically insulating materials as the base polymeric materials have high coefficients of attenuation and fillers used for mechanical integrity have a scattering effect. In practice it means that acoustic energy is rapidly diminished as it passes through the insulation. ?

2.?????? The Characteristic Acoustic Impedance[1] difference between any solid insulation and air is so large that the vast majority of incident sound waves are reflected back into the insulation with very little passing into the air where it could be detected by a sensor in the switchgear. ?

Surface PD

Ultrasonic detection of surface discharge

When insulator surfaces become contaminated by a combination of dirt and moisture, the insulating properties of the surface are compromised and under high electrical stress conditions small leakage currents can flow. This heats the surface and through evaporation causes interruption in the moisture film. Large potential differences are generated over the gaps in the moisture film and small sparks, or discharges, can bridge the gaps. Heat from the sparks causes carbonisation of the insulation and leads to the formation of permanent carbon tracks on the surface. The discharges generate very high frequency sound waves that propagate through the air away from the source, and this energy can be detected by ultrasonic sensors mounted within the switchgear where there is a clear air path between the discharge activity and the sensor.

TEV detection of surface discharge

As with internal PD, a small current does flow between the conductor and earth each time a discharge occurs however the current is usually significantly lower making detection by TEV sensors very difficult. There are two key reasons for this difference:

1.?????? The discharge energy that is likely to cause progressive damage leading to eventual breakdown over the surface of an insulator is much lower than that which will lead to internal breakdown of a solid insulator.

2.?????? For a given discharge energy, the current flowing to earth is less in a surface discharge than it is with PD in a cavity as the voltage drop at the point of discharge is very often higher. This is because the distribution of the voltage across a contaminated surface is a function of surface conductivity which can be highly variable, whereas the voltage across a cavity in solid insulation is a function of, and limited by, the relative permittivities of the insulator and the gas in the cavity.

Conclusion

Partial discharge (PD) activity in MV switchgear can manifest in two primary forms: internal PD and surface discharge, each with distinct characteristics and detection challenges. Internal PD, which occurs in gas-filled cavities within solid insulation, is best detected using TEV sensors, as the high-frequency transients generated by the discharges are easily captured by these devices. However, detecting internal PD via ultrasonic sensors is more difficult due to the attenuation and reflection of acoustic energy within the solid insulation. On the other hand, surface discharge, caused by contamination and moisture on insulator surfaces, produces detectable ultrasonic emissions, making it more amenable to detection by ultrasonic sensors. TEV detection of surface discharge is less effective due to the lower current and voltage distribution involved, which makes capturing the discharge energy more challenging.

Ultimately, understanding the different detection methods and the nature of each type of PD is crucial for effectively monitoring and diagnosing the condition of MV switchgear, allowing for early identification of potential faults and minimizing the risk of equipment failure.

Author: Dr Colin Smith , Managing Director, IPEC

[1] https://en.wikipedia.org/wiki/Acoustic_impedance#Characteristic_acoustic_impedance_2

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