Oliver Lanz, Livingston
The world is becoming increasingly reliant on consumption of huge quantities of power in order to satisfy a wide range of needs (with the European Union alone accountable for 2,858TW-hours annually). In many cases it now comes via decentralized renewable energy sources, such as wind, hydro-electric, geothermal and solar - which are not continuously available, but intermittent in nature. This leads to certain supply difficulties. At the same time migration to complex ‘smart grid’ technology also sets new challenges in terms of maintaining a steady stream of electricity that proves acceptable to the customer base, especially for sites possessing particularly large consumption profiles. Voltage surges/sags and loss of service continuity are symptomatic. The need for in-depth, methodical analysis of power quality to combat this has now become critical throughout the entire supply hierarchy.
Numerous different elements make up the infrastructure responsible for generating/distributing electricity. These include generators, transformers, switchgear and cabling. How well each of these elements works within the system must be evaluated on a regular basis to ensure the grid operates with a high degree of accuracy (see Figure 1). Through power quality analysis, engineering professionals can scrutinize the efficiency of complex systems and uncover evidence of possible energy wastage - whether it is due to poorly implemented system design or because of individual components parts becoming worn out over time. It is then possible for them to implement more efficient, better-balanced systems and, in the process, significantly reduce their overheads.
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Figure 1: System infrastructure must be evaluated on a regular basis to ensure the grid operates with a high degree of accuracy
Motivation for Power Quality Analysis
In modern three-phase AC electrical supply networks the voltage and current signals should ideally be very close approximations of pure sine waves. Also there should not be any phase shift witnessed between the voltage and current. Considerable losses will ensue if optimized conditions are not kept too - detrimentally effecting the system’s on-going operation. The efficiency levels reached will be below what is theoretically achievable and the system‘s life expectancy will be unnecessarily shortened. Both of these dynamics have obvious financial penalties associated with them.
Through thorough checking of the quality of power passing through the system, it is possible to assess the operational integrity of deployed equipment and identify where potential problems could arise in the future. The time between servicing can subsequently be increased and any monetary loss constituted by system downtime mitigated. Alternatively improvements in operational efficiency made to a system by altering its composure (replacing parts, etc.) can be assessed.
There can be a multitude of underlying reasons why analysis procedures of this kind are embarked upon. In addition to utility companies looking to control/deal with anything that poses a negative effect on their profitability, countless other organizations are now starting to rely on it. Electrical systems in hospitals, factories, university campuses, industrial processing plants, apartment buildings, and offices need to habitually make use of it. Deployed infrastructure will be in place for several decades. This infrastructure is exposed to operational stresses constantly and, as it ages, there will be deterioration in its functional effectiveness. Often located in outdoor environments, it is left vulnerable to harsh weather conditions, as well as extreme temperatures - leading to increased likelihood of failure.
The stakes can be high should the power supplied exhibit poor quality characteristics. If the outcome of this is, for example, a hospital having an unreliable supply to its critical patient care equipment or conversely a manufacturing facility’s production lines being brought to a halt, the costs involved (either to human life or brand reputation) can be incredibly severe. Utility company’s clients can have service level agreements put in place outlining the minimum specifications for the quality of the electrical power they are supplied with. Performing in-house analysis can serve as a way of ensuring that agreed service levels continue to be maintained by the utility.
Industry standards have been introduced, through which the precise testing techniques required for fully effective power quality analysis can be defined. Currently, the higher more profile of these are IEC61000-4-30, IEC61000-4-15 and IEC61000-4-7 and EN 50160. They provide a common framework which means there are no inconsistencies in the conclusions that test engineers come to, based on the same captured data, because of differences in the test procedures followed. EN 50160 is based on voltage measurements, while the derivatives of IEC 61000 add the dimension of current into the equation.
Major Power Quality Issues & Sources
A variety of different phenomena exist which can be responsible for power quality issues emerging in electrical generation/distribution systems. Among them are:
Voltage Imbalances:This results from uneven loading of the phases in a three-phase electrical system. It can cause motor overheating and thereby shorten the lifespan of equipment. Imbalance problems can be straightened out by redistribution of loads onto different phases, so that there is an equal apportioned current drawn on each one. They can also be tackled by having special transformers installed.
Harmonic Distortions:This occurs when non-linear loads alter the current signal so that it is no longer sinusoidal - with harmonic current flow through components in the system ensuing. As a consequence there can be an overheating of these components - the tripping of circuit breakers/protection relays and a marked heightening in the likelihood of system failures arising. Problems of a harmonic nature can be handled through implementation of better thought out designs. Sadly the impact of harmonics is rarely considered that early on, more often they are only attended to as an afterthought. If signs of distortion are witnessed within a system once it has already been deployed, it will be necessary for harmonic filtering (either passive or active) to be retrofitted.
Neutral Currents:Due to the imbalance and harmonics already discussed, current can be caused to flow in neutral conductors, which are not designed for such things. This current can contribute to energy losses coming from resistance in the cabling. Once again it can be remedied through use of either special transformers or filtering.
Voltage Transients:These can be sudden spikes in the signal brought about by electrostatic discharges. They can alternatively be oscillatory, instigated by a change in the steady-state condition of the current/voltage signal. If they are present, then the utilization of high performance surge protection mechanisms will be called for.
Load Switching:When a load is activated, the current drawn can potentially set off a notable drop in the voltage level observed. It can have serious functional implications for the system over time, with malfunctions in other parts of the system transpiring.
Reactive Power Losses:These come from a phase shift between voltage and current. Reactive energy hence flows back and forth through the system and expensive power compensation systems will be necessitated.
Selecting Effective Analyzing Tools
The scale of data involved in modern power analysis means that highly advanced pieces of test equipment are now essential. Early analysis hardware was just single-phase. This was because the principal technology lacked the capacity to cope with the fast Fourier transforms required by multi-phase operation. Thanks to incorporation of next generation semiconductor devices into the latest test kit, three-phase analysis is now becoming commonplace. The highly sophisticated analyzers currently found on the market integrate a broader range of features, with the ability to acquire voltage, current and frequency signals simultaneously at high speeds. They permit energy-auditing staff to carry out examination of longer waveforms and a greater number of parameters. A far better understanding of the electrical system can accordingly be grasped without any need for apportioning excessive human resources or unnecessary time being taken up.
It is vital that the efficiency of any system involved in the generation, conversion or conveyance of electrical energy is constantly kept at the highest performance levels. As the number of non-linear loads now appearing on utility’s distribution networks continues to rise and overall complexity increases, the power quality characteristics of the electricity supplied are threatened. As we have seen, unacceptable power quality can stem from load imbalances within the system due to badly implemented design, harmonic distortions, reactive loads, excessive resistance in the wiring, currents flowing through neutral conductors and an array of other factors - all leading to significant losses. Provided that each of these is fully addressed, a highly efficient and reliable system for supplying electricity can be achieved.
The practice of power quality analysis has now become a key aspect of modern-day electrical network management. Engineers, through the employment of progressively refined analysis techniques and state-of-the-art test hardware, are able to gain total visibility. They can determine where areas of concern are situated and thereafter commence corrective actions to eradicate them. By utilizing equipment that is compliant with recognized standards they can be safe in the knowledge that models from different manufacturers will give consistent results.