Tracking Down Tev Switchgear Signals Using Time Of Flight Techniques
Transient earth voltage or TEV is a term coined by a pioneering British scientist in the 1970s. If the signals it identifies were first discovered in the United States, they would have more likely been called transient ground voltage (TGV) as engineers in Europe typically refer to our ground as earth.
TEV signals are generated by partial discharge activity, which is essentially a partial insulation failure. TEV signals are high-frequency voltage transients or impulses on the ground. They also have the unique feature of being synchronous – more about this later. In the case of switchgear assemblies, these signals propagate or ride along the external surface of the grounded switchgear enclosure. As shown in Figure 1, their magnitudes are greater on the enclosure panels that are closest to the discharge source.
This is how it works: Partial insulation failure creates a spark or partial breakdown that emits a radio frequency signal – think of the flawed insulation area as a small radio transmitter within the switchgear. For outdoor or open structure insulation, these radiated signals can produce signals that interfere with AM radio receivers, sometimes causing that sudden annoying static on a vehicle's radio as it is driven under an overhead distribution line. Partial discharge activity occurring inside of switchgear or other electrical assemblies acts in the same manner. However, the grounded enclosure functions similar to a leaky Faraday cage. It attenuates the signal enough that it is not readily detected with a typical antenna but can still be detected with a TEV sensor. Basically, the TEV signal is trapped inside the enclosure and adheres to its grounded surface but propagates to the outside switchgear enclosure surface though the edges of the metal via the skin effect.
TEV sensors consist of a simple insulated conductive plate that, when placed against the switchgear enclosure, forms a capacitor within the enclosure itself, acting as the other plate for the capacitor. The high-frequency TEV signals are captured and then processed and analyzed by the instrument's electronics.
Ok, what about the synchronous characteristics referred to earlier? In simple terms, at zero volts there is not enough voltage present for anything to breakdown. But as the ac sine wave rises towards the positive or negative voltage peak, a critical voltage (partial discharge inception voltage) is reached, which creates a localized partial breakdown. This localized partial breakdown or partial discharge is what creates the signals described earlier. Typically, these signals then persist through the sine wave until they extinguish at a critical voltage known as the partial discharge extinction voltage.
The same sequence then repeats itself through the opposite voltage polarity peak, resulting in a pattern of voltage transients that are synchronous or in phase with the ac sine wave. Figure 2 shows such a typical PD pattern.
Although one small spark itself only causes negligible insulation damage, thousands of these sparks occurring over days, weeks, and even years will eventually cause cumulative irreversible damage to the insulation until complete catastrophic failure occurs. This process has sometimes been described in literature as electrical rust but perhaps could better be described as death by a thousand razor cuts.
Before jumping into the Time of Flight location method, it is worth of mentioning that just as the TEV signals can reach and travel along the outside of a leaky switchgear enclosure, they cannot escape well-sealed and well-shielded equipment such as oil-filled transformers. For these types of equipment, the PD signals can escape from the inside of the equipment enclosure through an external ground connection, where signal decoupling can be accomplished using split-core, high-frequency current transformers.
The time of time of flight method for locating TEV signal origination, and thus PD source location, involves using dual capacitive sensors as shown in Figure 3. Sophisticated instrumentation is used to time the arrival of the TEV signals at one sensor in comparison to the arrival time at the other sensor. As shown in Figure 4, a bar graph display indicates which sensor is closest to the discharge. By moving the magnetic capacitive probes around the switchgear at different locations, the PD source can be pinpointed with considerable accuracy.
As technology continues to advance and greater field experience is accumulated, partial discharge techniques and instrumentation will advance accordingly. Present PD instruments can now isolate and ignore undesirable noise from PD signals, and new advancements have led to better methods for pinpointing PD sources. These advancements have also led to the discovery, identification, and repair of insulation defects, which have significantly enhanced reliability for electrical equipment not only in the U.S. but internationally as well.
