Poor power quality is responsible for many different electrical problems. In addition to the obvious accidental causes like power cables being damaged by digging work, or bird strikes on overhead lines, every load on the electrical grid will have some impact on the power that is delivered. Some power quality problems are very complex, but there are also many common causes says John Outram, managing director of Outram Research
A purely resistive load, for example an oven, will tend to pull down the local voltage due to the current flowing in the power lines that have a finite impedance; capacitive and inductive loads can cause the current to lead or lag the voltage, degrading the power factor; motors and pumps have large inrush currents, potentially superimposing transients onto the voltage; and modern compact fluorescent lighting may only draw current at the peak of the voltage waveform, causing harmonic distortion of the supply voltage.
In most cases the impact of loads on the power supply is not problematic, but in extreme cases the effect on other devices on the grid may be serious. The mechanisms by which the utility companies try to accommodate all of these influences may themselves add transients and momentary variations, further complicating the situation. The effect of poor power quality can range from unnecessarily lost power in transmission lines caused by low power factor (out-of-phase current and voltage), to flickering lights, and power wasted in motors and transformers due to the presence of harmonics.
Taking motors as an example, when power is wasted it will generally be dissipated as heat, increasing wear and shortening life. Harmonics may also cause vibrations in motors which can increase noise as well as be a potential source of mechanical failure.
Some causes of power quality problems are almost impossible to control; vulnerability to accidental causes such as those mentioned above is a function of our infrastructure and they should be expected occasionally. However, with the right equipment and approach, power quality issues can be identified and resolved, and although complex situations may require experienced power quality professionals, engineers can identify straightforward issues by following six simple steps.
Step 1 - Gather Information
If you don't measure it, you can't manage it! However, before any measurement survey is made, think about the best approach. How is the problem presenting itself? Are complaints widespread? Are there any common threads? What about the infrastructure or the installation itself? Is it old; is there any corrosion, leaking oil? If so, could the distribution impedance have been compromised?
Most problems are local or self-inflicted. One of the best sources of information is the operator of equipment affected. Asking the operator when the problem happens, whether other things go on at that time and what he/she thinks is causing it, can provide excellent clues to the cause of the problem.
This stage should also be used to prepare for the survey. What are the local loads? How many points need to be monitored? The more information available, the better the monitoring can be targeted.
Step 2 - Produce a Harmonic Profile
Harmonics on the line tend to lead to long-term problems - motors and transformers overheating or other failures that do not happen instantaneously, although they can also cause rapid equipment failure. Typically these measurements would be taken over a period of at least a week, as most power quality issues have either daily or weekly periodicity (for example, happening the same time every day, or happening during the week but not at weekends). Understanding the periodicity can give important clues as to the cause of the problem; for example a car breaker's yard is unlikely to be causing problems that happen during the night!
Harmonics are evaluated continuously and averaged over a period of time. Measuring the harmonics does not require as high a sample rate as transients because the lower harmonics will tend to contain the most energy. Of particular interest in 3 phase systems are the 3rd, 6th and 9th harmonics, as these will not generate balanced current flow but reinforce each other, and therefore can cause high currents to flow in the neutral.
The harmonic direction - the phase angle of the current with respect to the voltage waveform of the harmonics, can also be a helpful clue to the cause of the problem. In this case however, it is important to make sure the analyser records harmonic direction correctly from typical data, rather than inferring it from a non-typical waveform capture, which may only be captured due to a momentary transient, notch, or ring.
Step 3 - Look for transients
Short-term transients such as spikes, dips and sags can cause immediate failures - for example blown light bulbs, PLCs resetting or computers dropping internet connections and worse, when they lead to partial process failures, where part of a production line is affected causing back-up, overflow, perhaps spillage and general loss of control. They can also be responsible for less catastrophic though highly irritating problems such as flickering of light bulbs.
Although some transients can be slow, others can be very fast. Monitoring such sub-cycle transients requires high-speed waveform capture, and power quality analysers such as the Outram Research PM7000, offer sampling speeds in excess of 1MSPS.
However, with high sampling rates it is not possible to record all the data, and so some way needs to be found of identifying the waveforms to be retained. Most power quality meters offer a threshold approach, which may mean an iterative process of setting different thresholds until the right amount of data is captured. The best systems offer a data management technology, which retains the ‘worst' waveforms over whatever is the monitoring period chosen. This avoids the need to set thresholds, simplifies setup, and significantly improves the likelihood of capturing useful data first time.
Step 4 - Compare the current and the voltage
Having monitored transients and harmonics, the engineer will have a good idea of what the problem is electrically, but may still have no idea of the cause. By monitoring the current and voltage together with good time resolution the cause of the problem can often be identified quickly. If the current and voltage rise or fall together, the problem is likely to be outside of the system being monitored, whereas if they move in opposite directions, the problem is likely to be inside.
Consider monitoring the supply at the point of entering a building - if the current spikes up and the voltage spikes downward at the same time, it will probably be because a load within the building has drawn more current, pulling down the voltage due to impedance in the transmission lines, whereas if they both move together, the fall in voltage is likely to be caused by an external load, and the local current falls in sympathy. The situation is not always simple because some modern electronic equipment, particularly those using switch-mode power supplies, can display negative resistance. Investigating the cause of a positive or negative link between current or voltage movement should add to the understanding.
Many power quality analysers offer a fixed interval for current and voltage measurements, an approach that might cause critical information to be lost. Figure 1 shows how variable sampling intervals - in this case Outram's adaptive store technology - can identify rapid changes whilst still making effective use of the analyser's memory.
Step 5 - Undertake some detective work
At this stage we should know whether the problem is within our building or not. If we are causing the problem, then all that is required is a step by step approach to identify the culprit. This can be done by moving the power quality analyser on to monitor different loads within the building or simply by turning things off while monitoring until the cause is identified. Sometimes problems can be revealed by their physical effects; hot spots on connections or excessive humming of transformers are typical examples.
If the problem is external, a little more detective work is required and you may need to involve the Utility Company. Some frequent causes include pumping stations, compressors, car breakers yards and welding shops, although causes can range from fixed installations or steelworks and other heavy industry to mobile equipment such as cranes.
If the source of the problem is still not obvious, then it is useful to measure the power quality at the substation to isolate the cause. Sometimes the premises next door to the cause may suffer serious power quality issues, but the impedance in the line will mean that nothing is visible at the substation.
Most power quality issues will require the engineer to repeat these steps in an iterative process - for example repeating the steps at different points in the electrical supply network to try to identify the cause geographically.
Step 6 - Confirm the diagnosis
Once the engineer has identified what he/she thinks is the cause of the problem, it is useful to see if there is any other corroborating evidence or even any contra-indications - particularly if the remedial action is likely to be expensive or unpopular!
Usually measurements will also be taken after the problem has been resolved to ensure that no lingering effects exist.
Quality of power is becoming an increasingly important issue. Utilities are penalising companies for poor power factor as the power wasted can be considerable and costly to the Power Company. Modern loads, such as electronic power supplies and compact fluorescent lights, are more and more introducing significant harmonic distortion that not only causes power to be wasted, but can ultimately shorten the life of motors, transformers and other valuable equipment.
By following a systematic step-by-step process and using the right equipment, an engineer can troubleshoot his plant for simple power quality problems. Complex issues may require more specialised expertise using instruments capable of distinguishing and recording unpredictable events, enabling unexpected or previously unencountered power quality issues to be identified.
The consequences of poor power quality include increased electricity consumption and equipment and process failure. With the ever-increasing focus on efficient operations, reducing energy costs and cutting carbon dioxide emissions, power quality issues must not be ignored.