Advancements in the Industry: Capacitive Coupling: Measurement Quality and Safety Issues

Felix Feustel, OMICRON electronics CAP Corner, Spring 2022 CAP Corner

A basic distinction can be made between system safety and product safety. Product safety has improved constantly over the past few years, if not decades. One reason for this is that various standards have been established or improved recently. Usually, norms or standards reflect the state-of-the-art in modern techniques.

Using state-of-the-art measurement equipment that meets quality standards for safety minimizes the risk of accidents. Some available devices have software as well as hardware mechanisms. For example, one hardware mechanism can detect when a unit is powered from an insufficiently earthed socket or when the unit is not yet earthed via the earthing screw. This device can provide so-called prechecks that carry out a plausibility check at a much lower voltage. The advantage of such a precheck is that the device under test is not subjected to the full test voltage if something has not yet been connected or has been connected incorrectly.

Safety and the quality of the results go hand in hand and are very dependent on the surroundings. Considering environmental conditions for preparing, setting up, and troubleshooting the measurement is essential for ensuring that it’s successful and safe.

Dissipation factor measurement demonstrates how environmental conditions can influence measurement results. This measurement is dangerous because of high test voltages and the associated risks, as well as the many factors that can influence it externally. The dissipation factor measurement is highly dependent on:

  • Temperature and humidity
  • Guard resonances
  • Proper preparation of measurement device (bushings cleaned, properly disconnected from other parts, non-short-circuited windings, presence of a screen electrode)
  • Quality of the earthing

ENVIRONMENTAL CONDITIONS INFLUENCE TEST PREPARATION

Environmental conditions are heavily dependent on preparing a test object properly. When a test object has been separated improperly, the connection itself (additional capacitance) and the effect of electrical interference from active conductors in the neighbourhood will have a negative impact on the quality of the measurement. It follows that all electrical connections going to and from the transformer bushings should be completely disconnected, as the dielectric measurement will measure all parts connected to the device under test as well as induced voltages. Furthermore, all windings and the neutral connection (if available) should be shorted. Figure 1 shows how improperly separating the test object and an active line with an electrical coupling to the test object could influence the measurement. The level of interference depends on other factors, for example, how far away the live line is from the test object and whether or not another line is running parallel to it.

Figure 1: Interference Created by Improper Separation

ENVIRONMENTAL CONDITIONS INFLUENCE SAFETY

In addition to the measurement quality reduction mentioned earlier, environmental conditions can also influence critical safety factors. An individual measurement device can be considered safe as several safety-related features and checks are built into it. External interference is very much dependent on the measurement setup itself and the environment near the device under test. Induced voltages can apply a voltage to the test object or the test setup, which could be dangerous.

Therefore, in addition to complying with all safety rules, preparing the test object properly is essential for performing the individual tests. Considering all these factors results in a very safe, time-optimized, and qualitatively good measurement. In addition, placing barriers between the danger zone and the safe zone and having visual and audible indicators can show the current status of the measurement device (active = under power, inactive = safe).

As mentioned earlier, accurate separation of the device under test from other equipment such as cables, transmission lines, busbar, surge arrestors, voltage, and current transformers is crucial. This is essential to the quality of the measurement results and very important for safety. For example, a short line connected to a transformer can be capacitively charged via the electrical coupling of an adjacent energized system. This charge can pose a danger to the tester while troubleshooting, re-wiring, or preparing the measurement setup. Capacitive coupling occurs between conductors that have different potentials. As a result of the potential difference, there is an electric field between the conductors, which we’ve displayed in a circuit diagram with stray capacitance as seen in Figure 2.

Figure 2: Diagram of Stray Capacitance

RE and CE represent the parallel connected internal resistances and the stray capacitance of the live line (system I) and the asset with or without the short lines connected (system II). CI/II reveals the stray capacitance between system I and system II. Possible countermeasures for reducing the capacitive coupling between both systems include:

  • Reduce CI/II, for example, by using the shortest possible parallel cable runs (i.e., wire-wrap wiring), increasing the distance between the conductors, shielding of system II (see Figure 3).
  • Reduce the size of RE, i.e., low-resistance circuit technology. For example, the high-voltage and low-voltage sides are short-circuited during dissipation factor measurements. If no measurement is made, this connection should be earthed so that the capacitive coupling is eliminated.

The electrical coupling between two conductors can be expressed with a coupling matrix. The diameter and the distance between the conductors play an important role (Figure 3).

Figure 3: Electrical Coupling between Conductors

For simplicity’s sake, an approximate formula was used to calculate the capacitance between two conductors. The two conductors are 8 m (26 ft) apart, and the live line is 400 kV. The radius of both conductors was assumed to be 3 cm (1.18 in).

[1]

The capacitance value with the given geometrical properties for both systems results in C’= 4.43 pF/m (1.35 pF/ft). The touch current in system II at 50 Hz is due to this capacitive coupling of approximately 0.32 mA/m (0.1 mA/ft).

CONCLUSION

Environmental conditions should be known or kept in mind during testing. Furthermore, the test object should be accurately separated from other system components in order to achieve good quality measurement results and maximum safety during the measurements. In terms of safety, the previous example is exemplary. It follows that the short-circuited high-voltage or low-voltage winding does not result in large touch currents during a dissipation factor test as the length of this short-circuited line is not critical. However, connected line segments on the transformer could lead to a drastic increase in the capacitive coupling as the length and consequently the touch current increases, making it dangerous. Another factor influencing capacitive coupling is the distance between the systems and the diameter of the systems.

REFERENCES

[1] Zinke, Otto, Brunswig, Heinrich. High Frequency Technology 1: High Frequency Filters, Cables, Antennas. Springer Textbook, ISbN 3540580700.

Felix Feustel has been an application engineer at OMICRON electronics GmbH in Klaus, Vorarlberg, since 2013. As a product owner, he is involved in the development and application of testing solutions for current and voltage transformers. He studied industrial engineering with a specialization in electrical power engineering at the Rheinisch-Westfälische Technische Hochschule Aachen (Germany), where he received his BSc in 2011 and his MSc in 2013.