Issues in Power Quality

Power quality generally refers to a series of boundary conditions that allow electrical systems connected to the network to operate in the expected way without causing significant performance or life losses. Thus, the operation of power systems outside these borders has a direct impact on the overall economic performance of the whole system. The disturbances responsible for this degradation are classified as conducted low-frequency, radiated low-frequency, conducted high-frequency, radiated high-frequency, electrostatic discharge, and nuclear electromagnetic phenomena [1].

Power quality deterioration can cause problems with or the shutdown of processes and equipment. The consequences range from excessive energy costs to complete work stoppages. The interdependence of different systems increases the complexity of power quality issues. Some of the problems lie inside the facility: installation—inadequate grounding, inadvertent routing, or undersized distribution; operation—equipment operated outside the design parameters; mitigation—inadequate protection or lack of power factor correction; maintenance—deteriorating cable insulation or ground connection [2]. Even the equipment that is perfectly installed and maintained in a perfectly designed facility can cause problems in terms of power quality with age.

The effects of power quality problems can consist of supply voltage waveform distortion, deviation from its nominal value, or a complete rupture. The problems of power quality can last from milliseconds to hours [3]. Various power electronics, such as domestic, industrial, and office equipment, connected to power supplies may have non-linear load characteristics that lead to poor power quality. Equipment such as photocopiers, computers, printers, etc., can cause electrical disturbances that can destroy certain sensitive equipment. When connected to the same source of supply, in some cases, they may cause malfunction. Industrial motors powered by electronic converters produce electrical disturbances. When disturbances occur, the quality of electricity is poor and production losses occur, with resulting financial losses. The main effects of voltage failure include early equipment failures, cost-effectiveness loss in rotating machines, equipment failures in information technology, loss of data or stability, process interruptions, failures of measuring and control devices, etc.

Electromagnetic disturbances that directly affect the electrical network, and, thus, the power quality of consumers, are of low frequency and include the following: frequency and voltage variations in the electrical voltage; voltage dips and interruptions; harmonic distortions; flickers; asymmetries; transients; DC components on AC voltage; signal voltage disturbances; low-frequency voltages [4,5,6].

There are many engineering solutions used to reduce the impact of power quality problems, and this is a very active area of innovation and development. The articles presented in this Special Issue, “Power Quality Analysis and Experiments”, briefly discuss issues of power quality and their solutions in low- and medium-voltage applications [7].

Most electrical consumers are single-phase and deforming (contain switch mode power supplies), and they are found especially in domestic, residential, and industrial fields. It is a challenge to balance single-phase consumers in three-phase circuits with the possibility of obtaining a high-power factor at the level of low-voltage power substations [8].

Power quality can be evaluated through characteristic indices. In [9], a new methodology is proposed for determining the indices of voltage variation in a short period of time; to implement the new method, real data from more distribution system companies are used. This analysis is useful for introducing short-term voltage variation indices.

High-power industrial applications include offshore oil and gas platforms, where current distortion is high and power factors are low. To improve these power quality indices, the use of active power filters (made with silicon and silicon carbide) was proposed in [10]; compensation strategies were presented, at two different voltage levels, to identify the best solutions for improving the deformation regime and increasing the power factor.

Also, active power filters have medium-voltage applications for improving power quality. The decision tree method (as an optimal solution) for choosing and dimensioning active power filters was proposed to minimize energy losses and investment costs in [11].

Residual current devices from low-voltage networks have, as their main application, preventing electric shocks, with direct contact considered an additional protection. Power electronics that use the pulse width modulation technique can determine ground fault currents that are not detected via residual current devices. The limitations of the standards in force are identified and proposals for their improvement are made in [12].

Capacitor banks can be used in low-voltage power substation networks, in addition for improving the power factor and creating passive power filters to reduce the deforming regime. Because the capacitors are very demanding in such applications, special attention must be paid to the dielectric to increase their lifetime [13]. Also, it is important to choose the proper configuration and the values of passive filter components to obtain the optimal performance of the passive filter [14].

Non-linear autoregressive-type recurrent artificial neural networks with external input control can estimate harmonic behavior in photovoltaic systems [15]. Dynamic control and harmonic distortion reduction can, thus, be evaluated.

The continuous insurance of electricity supply is a field of wide interest in power quality. It can be used for uninterruptible power sources powered by vanadium redox flow batteries to provide long-term electricity in critical installations [16].

Matrix converters have applications in hybrid power filters because they have low total harmonic distortion for the fundamental voltage. They can be controlled via a fuzzy system or PI regulator in terms of compensation speed, accuracy, total harmonic distortion of supply current, and overall integrity [17].

The power factor regulators installed in low-voltage power substations that measure the current on one phase and the voltage on the other phases have important limitations for the conditions of distorting currents and imbalances in three-phase networks [18]. An analysis of a low-cost system for the automatic regulation of the power factor, with a reduction in transients and an increase in the lives of contacts, can be used in low-voltage power substations with capacitor banks controlled by one three-phase solid-state relay; such as system is presented in [19].

Voltage dips and short-term voltage interruptions have major implications both in low- and medium-voltage networks. In [20], an analysis of these indices was carried out for several companies from different industries to evaluate the understanding of these power quality issues.

Short-term voltage dips are usually caused by failure, high-speed electrical large loads, or intermittently loose electrical connections in the power line. Voltage fluctuations are usually related to system problems, but they also occur when heavy loads are switched on or a large motor is initiated [1].

Voltage fluctuations are frequent and widespread, and they can be seen as the first power quality phenomenon affecting the industry. Interruptions are the result of electrical system failures, equipment failures, and control errors. The duration of interruption due to equipment failures or disconnected connections may be irregular. The most common problem associated with short-duration RMS variations is equipment shutdown. Short-term interruptions can lead to process shutdowns that require hours of restarting. In many facilities, if the equipment travels, the effects on the process are the same for short-term variations and long-term phenomena [21]. Short-term dips cause many process interruptions. In addition, many control and emergency switch circuits use relays and contactors that are very sensitive to voltage dips. The common solution to this problem is to provide a constant voltage. Momentary and temporary interruptions almost always cause the equipment to stop operating and may lead to the failure of the inductive motors.

Voltage problems and the production of a balanced current are the two main areas in which power quality problems occur. Dips and swells, voltage transients, power interruptions and voltage imbalances can be monitored, analyzed, and compared with the device operation history to determine the cause and severity of the problem of power quality. The same can be undertaken with different harmonic currents of a system. It is also important to note that the power quality problems are often inter-related. Power quality problems must be addressed through a whole-plant approach, without losing focus on how they affect individual loads; sometimes fixing one energy quality problem can exacerbate another problem. By using three-phase power quality analyzers, we can identify the root causes of power quality problems and correct both symptoms and overall problems [22].

Voltage spikes and swells cause damage to electronic components, the flammability of insulation materials, excessive screen brightness, the damage or interruption of sensitive equipment, data processing errors or data losses, and electromagnetic interference [8]. Harmonics causes power consumption and leads to inefficient electricity use and unintentional equipment malfunctions. It affects the smooth operation of industrial machines and causes production interruption. In hospitals, it can lead to loss of life. It affects the data processing activities of information technology equipment, such as losing transactions in real time, etc.

The overheating of wiring (in particular, neutral conductors in three-phase systems) and equipment can be caused by low power quality. When communications cables are in parallel with power cables, the harmonic frequency interferes with communication signals, resulting in incorrect signals. Harmonics can cause the protective relay to be incorrectly operated.

The economic costs of power quality problems are high, especially in industry. Costs include production losses, damaged equipment, salaries, and restart costs. These costs can be quantified as additional money that the customer wishes to pay to avoid this inconvenience [23].

The business risks posed by power quality issues are real and even low-tech industries are exposed to serious financial losses. On the other hand, prevention is relatively cheap, with solutions ranging from simple good practice design techniques to installing widely available support equipment. Energy quality problems cost the about EUR 10 billion per year, whereas preventive measures cost less than 5% of that figure. Understanding the nature of a problem and assessing how it affects the business, as well as the risk associated with it, is vital [24]. The reliability and coherence of electricity supply is important for many industrial and service activities. If power quality is low, the company suffers. It is surprising and disturbing that companies often do not recognize the causes of low reliability, even though cost-effective solutions are available to them [25].

In the future, the main approaches used in power quality analysis should be related to development of new types of batteries for use with uninterruptible power supplies, the use of new types of controls for hybrid and active power filters, studies on limiting passive filters operations, the realization of switched-mode power supplies that reduce current distortion, the use of new statistical methods for power quality analysis, the judicious design of electrical installations in deforming regimes (especially for neutral conductor), the analysis of deforming regime effects on switching and protection equipment and on electrical insulation, methods for reducing flicker and transient regimes for high-power consumers, and the realization of load balancing methods in three-phase systems.