Power quality and reliability are two important aspects of an electrical power supply system. Therefore, the effective measurement of supply quality is crucial to track the sources of interruptions and enable the comparison of power quality (PQ) at various locations.
Tools of measurement
Measurement equipment can help managers/supervisors determine if disturbances in power flow are coming from the supply side or the consumer end. Some of these tools are multi-meters, PQ data loggers, oscilloscopes, disturbance analysers, spectrum and harmonic analysers, and PQ analysers.
Multi-meters are used to measure voltage and current. They can measure circuit overloading and imbalance, under and over-voltage, etc. PQ data loggers are electronic devices that record data on the basis of time and location either with a built-in current/voltage sensor or an external current transformer/potential transformer. Most data loggers are now based on a digital processor, or a computer. They are small, battery powered, portable and equipped with a microprocessor, internal memory and sensors. Oscilloscopes are useful for real-time measurements. Through voltage and current waveforms, an observer can detect distortions and variations in signals.
Disturbance analysers measure a wide range of system disturbances from very short duration transient voltages to long-duration outages or under-voltages. Spectrum analysers and harmonics analysers are online monitoring equipment with the capability to sample waveforms and perform harmonics calculations. They statistically represent the harmonics distortion levels and changing load conditions. PQ analysers are used to continuously monitor power supply lines to keep a check on disturbances, which can disrupt the reliable delivery of power or cause damage to the equipment plugged into the grid supply.
PQ evaluation techniques
The evaluation of PQ issues is another important aspect of ensuring the quality of power supply. PQ evaluation techniques can be classified into two parts namely evaluation of PQ issues affecting utilities and evaluation of PQ issues affecting the end users. The evaluation techniques help in optimising the operation of electrical installations. They help save energy, reduce energy bills, make users aware of costs and ensure good power quality.
The evaluation of PQ issues affecting utilities involves analysing PQ disturbances and correlating them with equipment abnormalities/failures such as transformer/cable overheating, protection equipment malfunctioning, cable joint arcing, overhead lines coming in contact with trees, and lightning. This involves the use of data analytics and advanced image processing algorithms to identify and rectify such failures in real time, allowing much shorter mean time to repair and turnaround time for utilities. The technique of evaluating PQ issues affecting the end users includes identifying and locating the equipment subject to disturbance, determining the time and date when the problem occurred, any correlating it with particular meteorological conditions (strong winds, rain, storm), and applying corrective measures using data such as type of load and age of network components. Other evaluation techniques include monitoring of installation (wiring, grounding, circuit breakers and fuses) to detect and record the source of the problem.
PQ monitoring techniques
PQ monitoring is the process of gathering, analysing, and interpreting raw measurement data into useful information. Some of the PQ monitoring techniques are compliance monitoring, system performance monitoring, site characterisation, troubleshooting monitoring, and monitoring for advanced application and studies. Compliance monitoring is undertaken to verify adherence to the regulatory requirements as per a given standard for a specific site. System performance monitoring is carried out at sites with the help of monitoring units scattered throughout the network. The main purpose is assessment of average power quality for a specified region for asset management, strategic planning and determination of long-term trends for the system operator. Site characterisation is specific monitoring of a site for measuring pre-connection PQ levels for a specific customer, specifying constraints on new customers and verifying performance of existing customers. Troubleshooting monitoring is mostly aimed at analysing network phenomena and investigating equipment damage for an existing PQ issue based on a customer complaint. It may be followed by compliance monitoring if PQ limits are breached.
Utility initiatives
In an effort to establish base line data on 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 (765 kV, 400 kV, 220 kV), sub-transmission (132 kV, 33 kV), low tension (415 V), and at end consumer level on various appliances such as fan, light, laptop and UPS. With these measurements, it attempted to assess power quality at the grid level across all five regions in the country, covering all the states and union territories. The measurements were carried out in 175 cities and towns at various voltage levels covering more than 500 feeders and points. The data for different power quality parameters was collected from all the feeders and buses of the selected substation and measurements were carried out for 6-24 hours on different feeders of the substation.
The various power quality parameters were measured simultaneously using a portable three-phase power quality analyser. A high level of voltage harmonics was recorded in 65 cities/towns. The transmission system voltages and current were high in fifth and seventh harmonics, whereas the LT level was found to have a high level of third harmonics. Voltage imbalances exceeding the permissible limit (for short durations) were observed at 79 cities/towns during the field measurements. Many instances of voltage sag were also observed in the transmission network.
A higher value of flicker, which gives an impression of instability in visual sensations, was observed mainly in LT supply in almost all cities/town across the country – At the consumer end, power quality parameters were measured on appliances used in offices and homes. It showed their non-linear nature, which resulted in 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 the diminishing order of magnitude. Further, high content of harmonics was observed in the current/voltage of the supply feeders at offices and apartments (at 415 V) along with large values of neutral current.
Meanwhile, Tata Power Delhi Distribution (TPDDL) recently initiated an intelligent power quality 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 power quality management solution helps TPDDL monitor power quality issues at the network voltage level of 66 kV/11 kV/415 V. Also, necessary software at server systems is deployed for monitoring power quality parameters through web-based systems.
Conclusion
Today, consumers across the domestic, commercial and industrial categories have installed sophisticated electrical and electronic equipment that require uninterrupted, reliable and “good quality” power. Meanwhile, utilities are faced with challenges relating to grid integration of renewables and decentralised power, which cause voltage fluctuations and harmonics currents, and result in “bad power” supply if left unchecked. 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 power quality management plays a crucial role in it.
Based on the Power Quality of Electricity Supply to the Consumers report by the Forum of Regulators