Harmonic voltage and current distortion
Equipment which uses power electronic switching techniques to control intake power is a common source of current distortion. Examples include variable speed drives, uninterruptible power supplies and switch mode power supplies. The dominant frequency components tend to be of low orders with 3rd, 5th ,7th, 11th and 13th being common. The presence of these components in current and voltage waveforms intro- duces additional losses in the conductors and magnetic circuits of cables, transformers, motors and generators. The frequency components can also excite resonance in Power Factor Correction (PFC) capacitors.
The following mitigating measures can be applied:
● specification of frequency content in current loading and applied voltage on purchase specifications
● use of K-factor rated transformers
● use of detuned PFC banks
● use of double-rated neutrals (or higher) in systems with high 3rd harmonic loading
● use of passive or active filters to reduce harmonic content
Voltage and current waveform distortion is also a consequence of distorting loads. Mitigating measures for waveform distortion include:
● specification of waveform distortion tolerance requirements in purchase specifications
● use of equipment which does not rely on phase measurement by point-on-wave such as zero-crossing detection
● use of rms rather than peak sensing
● use of 3-phase rather than 1-phase sensing for devices such as AVRs
● use of permanent magnet generators with generator AVRs
● use of generators with 2/3 pitch windings to help minimize harmonic impedance and thus minimize voltage distortion
Low frequency harmonic distortion is also often accompanied by high frequency non-harmonic components. These are produced by rapid switching and are a feature of virtually all power electronic systems. For this reason, it is important that earthing and bonding be applied in order to minimize impedance. Stranded conductors of large cross-sectional area installed with parallel paths in a mesh arrangement are examples of good practice.
Voltage dip immunity in end-user equipment continues to be a major quality issue in Europe and is only gradually improving with the implementation of IEC and CENELEC standards. As a result, equipment malfunction due to transient voltage dips continues to lead to complaints about adverse power quality. In some regions of Europe it may even be perceived that power quality is reducing due to lack of voltage dip immunity alone. The reality is more likely to be that such perception is influenced by an increase in the numbers of susceptible equipment, or changes in the application of equipment, despite the fact that dip immunity complies with present day EMC standards. However, the IEC and CENELEC standards offer a framework for specification and testing that could eventually benefit both the electricity supply industry and end users.
The common cause of voltage dips is system faults within public electricity distribution networks. Severe voltage dips will also occur during fault clearance on end-user distribution networks. Thus, although it is common to use circuit protection to isolate on-site faults, the widespread disturbing effect of the accompanying voltage dip is often overlooked. References 14E and 14F discuss voltage mitigation measures some of which are briefly referred.
Mitigating measures to be applied are:
● specification of dip-tolerant equipment at the time of purchase
● use of mitigation measures based on FACTS techniques, such as the Dynamic Voltage Restorer (DVR). This can be applied at the PCC to support the supply voltage during network faults. It is expensive but can offer an effective solution.
● use of constant-voltage transformers to improve voltage regulation without the need for energy storage
● application of battery, diesel rotary or flywheel storage to provide energy ride-through for critical plant
● use of mechanically latched rather than electrically held relays and contactors on critical plant
Monitoring is essential when tackling equipment malfunction which is thought to be power quality related. It is also a good policy to benchmark the quality of any new supply arrangements that support important business processes so that the relevance of power quality conditions, or changing conditions, can be assessed at a later time. There are a large number of power quality monitors available (reference 14G) and their selection should target the particular power quality parameters of importance. It must be decided whether data logging is required, or a more simple detection of power quality parameters that exceed predefined thresholds. Some monitors offer the opportunity to achieve both functions simultaneously and then to examine retrospectively with regard to thresholds or standards. Large data storage capability using hard disk drives or static memory has made the monitoring activity much easier to apply but can lead to an enormous amount of data to inspect.
Power quality monitors are packaged in portable and fixed patterns. Fixed pattern versions are becoming more common and offer the opportunity to collect data and observe trends throughout the life of the installation. Permanently installed monitors can also be internet connected to allow the data to be downloaded and to allow expert opinion to be called upon for analysis.
Any power quality survey should be carefully planned with clear objectives. The monitor and its sensors (such as current transformers) must be selected to provide the appropriate operating range and response bandwidth. Also the physical constraints of conductor diameter and distances, together with safe working, must be considered when planning how the power quality monitor is to be applied.
Time correlation is a powerful tool in relating power quality disturbances to process problems. Therefore, it is necessary to establish a time log, in which the operators can enter the time and details of events. It is not unusual to discover power quality characteristics that were unknown but bear no relationship to the problem being investigated.