Monitoring Quality

With the magnitude and complexity of equipment connected to the grid expected to increase in the next few years, power quality (PQ) disturbances are likely to go up proportionally. Therefore, there is a need to verify if real PQ improvements have been made, for which effective measurement of the supply quality is crucial. It also helps track the sources of interruptions and enables PQ comparisons at various locations.

Measurement tools

Installing measurement equipment can help managers determine whether disturbances are coming from the supply side or are being generated at the consumers’ end. Some of these tools are multimeters, PQ data loggers, oscilloscopes, disturbance analysers, spectrum analysers, harmonic analysers and PQ analysers.

Multimeters are the basic equipment to measure voltage and/or current. They measure overloading of circuits, under- and over-voltage issues, and imbalances between circuits. PQ data loggers are electronic devices that record data over time or in relation to location either with a built-in current/voltage sensor or via external current transformer/potential transformer. Increasingly, but not entirely, they are based on a digital processor (or computer). They are generally small, battery powered, portable, and equipped with a microprocessor, internal memory for data storage and sensors. Oscilloscopes are valuable when performing real-time measurements. By looking at the voltage and current waveforms, an observer can look for distortions and detect any major variations in signals.

Disturbance analysers typically measure a wide variety of system disturbances from very short duration transient voltages to long duration outages or under-voltages. Spectrum analysers and harmonic analysers are online monitoring equipment with the capability to sample waveforms and perform harmonic calculations. They represent the statistical nature of harmonic distortion levels with changing load conditions. PQ analysers are used to monitor and analyse power supply lines for disturbance, which can disrupt the reliable delivery of power or cause damage to equipment plugged into the grid.

Utility initiatives

In an effort to establish baseline data about PQ parameters in the Indian power system, Power Grid Corporation of India Limited had conducted field measurements at different voltage levels, that is extra high voltage level (765 kV, 400 kV, 220 kV, etc.), sub-transmission level (132 kV, 33 kV), low tension (LT) level (415 V) and at the end-consumer level on various appliances, such as fan, light, laptop and UPS. These measurements were taken to assess PQ at the grid level across all the five regions in the country, covering all the states and union territories. The measurements were carried out in 175 cities/ towns at various voltage levels covering more than 500 feeders/points. The PQ parameters were collected from all the feeders and buses of the selected substation and measurements were carried out for about 6-24 hours on different feeders. Simultaneous measurements of various PQ parameters were carried out using a portable three-phase PQ analysers. It was observed that high content of voltage harmonics was present at 65 cities/towns, for up to 4 per cent of the time. The transmission system voltages and current were found to be rich in 5th and 7th harmonics whereas the LT level was found to have high content of 3rd harmonics. The voltage imbalance exceeding permissible limits (for short durations) was observed at 79 cities/towns during field measurements.

Many instances of voltage sag/dip were also observed in the transmission network. Higher value of flicker that gives an impression of instability in the visual sensations was observed mainly in the LT supply in almost all the cities/towns across the country. Meanwhile, at the end-consumer level, PQ parameters measured on commonly used appliances used in offices and homes show their non-linear nature, which, in turn, reflects in the form of high content of current harmonics. It has been observed that these appliances draw current, rich in odd harmonics such as 3rd, 5th, 7th, and so on, in a diminishing order of magnitude. Further, high content of harmonics was also observed in the current/voltage of the supply feeder of offices and apartments (at 415 V) along with large values of neutral current. Tata Power Delhi Distribution Limited (TPDDL) initiated an intelligent PQ monitoring project in association with Metrum, a Swedish product development and solutions manufacturing company focusing on power monitoring solutions, under the India-Sweden Innovations Accelerator Programme. The intelligent PQ management solution is facilitating TPDDL in monitoring PQ issues at the network voltage level of 66 kV/11 kV/415 V. Also, the necessary software at server systems is deployed for checking of PQ parameters such as power factor, harmonics, etc., and for monitoring them through web-based systems while integrating with other systems.

Conclusion

The effects of bad quality power may not be visible immediately, but they can be severe in terms of production and revenue loss. In addition, as utilities and consumers are increasingly moving towards energy efficiency, it is important to optimise electricity use for any given load and PQ management plays a crucial role in it.

With inputs from the Forum of Regulators’ report, “Power Quality of Electricity Supply to the Consumers”

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