Power Quality Background
A perfect electrical supply would deliver a constant frequency sinusoidal voltage waveform with unvarying magnitude. In daily work, power supplies deviate from this ideal situation because system loads vary and phenomena such as transients and outages may occur. If the power quality of the network is good, loads connected to it will run satisfactorily and efficiently. Insufficient power quality, however, can lead to failures of machinery, electric control systems or computers connected to the electrical supply network.
Power quality measurements characterize the degree to which a practical supply resembles the ideal situation, in terms of harmonic pollution, reactive power and load imbalance.
This entails measurements on the supply including frequency, interruptions, flickering, harmonic and interharmonic voltage, voltage variations like dips, temporary overvoltage or rapid changes and voltage unbalance. The EN 50160 standard defines the supply voltage characteristics of the electricity by public distribution systems for these measurements. The standard also includes compliance limits (see table).
Measuring power quality and finding a bugbear in the network which is messing with the power are considered a highly paid job. Every electrical network and its problems with harmonics, transients, or disturbances are unique and need careful planning, setting the stage, measuring, and finally understanding where the problem is. That’s what power-quality engineers do.
Power Quality Measurements
Before taking the measurements, it is advisable to carry out an inspection to examine the system. Important information can be gathered from a “sensory” point of view, by sight (state of the machinery, presence of evidence of short circuits, overloads, overheating, presence of pollution of possibly conductive dust, etc), by ear (humming in the cabinet, and their trend as a function of active loads), and by “feel” (cabinet temperature, humidity).
At this point, the picture should be clear and it will be necessary to think about:
The choice is fundamental and strategic, since the measurements must provide the useful data on which to reflect to solve the problem. Measuring should not be overdone, risking complicating the problem with a volume of data to be analysed and managed that can become unnecessarily embarrassing.
Where: establish in which points of the system the measurements are to be performed: at the terminals of the disturbed loads, at the terminals of the disturbing loads, just downstream of the MV/LV transformer, etc. In systems with complex topology, the choice of where to take the measurement is fundamental; making a mistake in the measurement point means having to “throw away” all the measurements previously taken and starting again from scratch.
When: at full load (or during start-up of the main loads) if the maximum current and voltage harmonic distortion and/or the minimum voltage is to be measured. With minimum load (e.g. evening hours) if possible voltage variations beyond the nominal value are to be evaluated. In the case of problems encountered during certain processes or system states, measurements should be conducted reproducing these conditions.
How long: measurements aimed at identifying max THDI and max THDV can take less than an hour, if carried out when the system is at maximum load. On the other hand, measurements aimed at voltage or power profiling should be conducted for at least one full week (CEI EN 50160). Statistically based investigations (e.g., voltage gap counts and their duration and depth characteristics) need a continuous measurement of at least a few months.
Which measurements to take and with which instruments: this depends on numerous factors; it is not possible to generalise. It is a case-by-case assessment; the value added lies in the skill of the electrical measurement experts. We usually use Fluke to finish this job.
YT Power Quality Solutions
Active Power Filter: YTPQC-APF
Static Var Generator:YTPQC-SVG
Active Load Balancer:YTPQC-ALB
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