• • • TEST & MEASUREMENT • • •
Harmonic currents in distribution systems can cause harmonic distortion, low power factor, and additional losses as well as overheating in the electrical equipment, leading to a reduction in the lifetime of equipment and increases in cooling costs. Nonlinear single-phase loads served by these substation transformers deform the current’s waveform. The unbalance of nonlinear loads leads to additional losses on power transformers, additional load of neutrals, unexpected operation of low power circuit breakers, and incorrect measurement of electricity consumed. Figure 3 illustrates the effect of these linear loads.
Electricity generation by wind and photovoltaic (PV) solar systems injected into the grid cause several power quality problems as well. On the wind generation side, wind intermittency creates harmonics and short-duration voltage variations. The inverters in PV solar systems create noise that can produce voltage transients, distorted harmonics, and radio frequency noise because of the high speed switching commonly used to increase the efficiency of the energy harvested.
Factories Power quality problems caused by power supply variations and voltage disturbances, cost approximately $119billion (U.S.) per year for industrial facilities in the United States, as per an Electric Power Research Institute (EPRI) report. Additionally, 25 EU states suffer an equivalent of $160billion (U.S.) in financial losses per year due to different PQ issues, according to the European Copper Institute.
These figures are linked to subsequent downtime and production losses as well as the equivalent of intellectual productivity losses. Degradation of power quality is usually caused by intermittent loads and load variations from arc furnaces and industrial motors. Such disturbances give rise to surges, dips, harmonic distortions, interruptions, flicker, and signalling voltages. To detect and record these disturbances inside a factory installation, it is necessary to have power quality monitoring equipment in several points throughout the electric installation or, even better, have it at the load level. With the arrival of new Industry 4.0 technologies, power quality monitoring at the load can be addressed by industrial panel meters or sub-meters to have a comprehensive view of the quality of the power delivered to each load.
Figure 3: The impact of current harmonics generated by a nonlinear load
EV chargers
EV chargers can face multiple power quality challenges, both in power sent to and from the grid (see Figure 4). From a power distribution company perspective, power electronics-based converters used in EV chargers inject harmonics and inter-harmonics. Chargers with improperly designed power converters can inject direct currents (DC). Additionally, fast EV chargers introduce rapid voltage changes and voltage flicker into the grid. From the EV charger side, faults in transmission or distribution systems lead to voltage dips or interruption of supply voltage to the charger. Reduction of voltage from the EV charger tolerance limits will lead to activation of under-voltage protection and disconnection from the grid (which leads to a very bad user experience).
Data centres Presently, most business activities depend on data centres in one way or another to provide email, data storage, cloud services, etc. Data centres demand a high level of clean, reliable, and uninterrupted electricity supply. PQ monitoring excellence helps managers prevent costly outages and helps manage equipment maintenance, or replacement, required due to issues on the power supply units (PSU). The integration of uninterruptable power supply (UPS) systems into rack power distribution units (PDUs) represents another reason to add PQ monitoring to IT racks inside the data centre. This integration can provide visibility to power issues at a power socket level.
UPS system failure, including UPS and batteries, is the primary cause of unplanned data centre outages according to a report made by Emerson Network Power.
Around a third of all reported outages cost companies nearly $250,000. UPS systems are used on every data centre to ensure clean and uninterrupted power. These systems isolate and mitigate most of the power problems from the utility side, but they do not protect against issues generated by the PSU of IT equipment itself. IT equipment PSUs are nonlinear loads that can introduce harmonic distortion in addition to other problems caused by equipment such as those that can result in high density cooling systems with variable frequency speed-controlled fans. Apart from these issues, PSUs also face interferences that come in multiple forms such as voltage transients and surges, voltage swells, sags, and spikes, imbalance or fluctuations, frequency variation, and poor facility grounding.
Figure 4: Power quality issues for EV chargers
electricalengineeringmagazine.co.uk
Power quality standards defined Power quality standards specify measurable limits to the electricity magnitudes as to how
far they can deviate from a nominal specified value. Different standards apply to different components of the electricity system. Specifically, the International Electrotechnical Commission (IEC) defines the methods for measurement and the interpretation of results of PQ parameters of alternating current (AC) power systems in the IEC 61000-4-30 standard. The PQ parameters are declared for fundamental frequencies of 50Hz and 60Hz. This standard also establishes two classes for measurement devices: Class A and Class S.
• Class A defines the highest level of accuracy and precision for the measurements of PQ parameters and is used for instruments requiring very precise measurements for contractual matters and dispute resolution. It is also applicable to the devices that need to verify compliance of the standard.
• Class S is used for power quality assessment, statistical analysis applications, and diagnostics of power quality problems with low uncertainty. The instrument in this class can report a limited subset of the parameters defined by the standard. The measurements made with Class S instruments can be done on several sites on a network, on complete locations or even on single pieces of equipment.
Figure 5: IEC power quality standards
It is important to note that the standard defines the measurement methods, establishes a guide for the interpretation of the results, and specifies the performance of the power quality meter. It does not give guidelines on the design for the instrument itself.
The IEC 61000-4-30 standard defines the following PQ parameters for Class A and Class S measurement devices:
• Power frequency
• Magnitude of the supply voltage and current • Flicker
• Supply voltage dips and swells • Voltage interruptions • Supply voltage unbalance
• Voltage and current harmonics and inter- harmonics
• Rapid voltage change • Under-deviation and over-deviation • Mains signalling voltage on the supply voltage
ELECTRICAL ENGINEERING • MARCH 2024 19
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