Absolute Power: Tackling issues with power quality
Ever since people started to use electricity, there has been an ongoing discussion on the subject of power quality. The only difference between the early days of electrical use and now is the degree of "dirty power" present in the electrical system. At the time of Nikola Tesla, the pioneering inventor of more than 700 electrical devices who lived from 1856 to 1943, the power received from the 13 generators at Niagara Falls was predominantly a pure sinusoidal waveform. Nowadays, the situation has changed dramatically.
With advancements in electronic technology and increased power demand, the voltage and current distortion on a perfect sinusoidal waveform has become electrical enemy number one. Impure power can come in the form of voltage fluctuations, sags, swells and transients or steady-state distortion. The most recent — and the best-camouflaged — effect on the system or the connected equipment results from harmonic distortion.
What are harmonics?
Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency. For example, if the fundamental frequency is 60 Hz, then the second harmonic is 120 Hz, the third is 180 Hz, etc. Waveforms may include odd and even harmonics in addition to the fundamental frequency.
Because most non-linear loads generate odd-number harmonics, even-numbered harmonics are less likely to occur and are usually associated with equipment’s burned out components, such as diodes.
Current harmonics generated at non-linear loads can produce load voltage distortion at other parts of the circuit. Furthermore, harmonics can be divided into positive, negative and zero sequence harmonics.
Harmonics are the inevitable by-products of modern electronics. Today’s office buildings and manufacturing facilities house a wide assortment of equipment that draws electrical current in short pulses. While this improves the equipment’s operating efficiency, it can cause harmonics in the load current. They are especially prevalent wherever there are large numbers of personal computers, adjustable speed drives and other types of equipment.
Because this equipment is designed to draw current only during a controlled portion of the incoming voltage waveform, the distorted current wave shapes from these pulses can cause harmonic currents to flow back into other parts of the power system. The results can include overheated transformers and neutral units and tripped circuit breakers.
Looking at the waveform will highlight the problem. Whereas a normal 60 cycle power line voltage appears on an oscilloscope as a near sine wave, the waveform is distorted when harmonics are present. These are described as non-sinusoidal waves. Because the voltage and current waveforms are no longer simply related, they are termed "non-linear".
Finding the problem is relatively easy once you understand the nature of harmonics and where to find them. The following is a brief background on harmonics, and the procedures and test equipment that can be used to help you find them.
Effects of harmonic currents
Symptoms of harmonics usually show up in the power distribution equipment that supports non-linear loads. There are two basic types of non-linear loads: single-phase and three-phase. Single-phase, non-linear loads are prevalent in offices, while three-phase loads are widespread in industrial plants.
Each component of the power distribution system manifests the effects of harmonics a little differently. Yet all are subject to damage and inefficient performance. Here are some examples of components within the system, and the effects that harmonics can have on them:
– Neutral conductors: In a three-phase, four-wire system, neutral conductors can be severely affected by non-linear loads connected to the 120V branch circuits. Dangers can include overheated conductors and higher than normal voltage drops between the neutral conductor and ground.
– Circuit breakers: A peak sensing electronic trip circuit breaker won’t always respond properly to harmonic currents. Since the peak of the harmonic current is usually higher than normal, this type of circuit breaker may trip prematurely at a low current. If the peak is lower than normal, the breaker may fail to trip when it should.
– Bus bars and connecting lugs: These can become overloaded when the neutral conductors are overloaded with harmonics.
-Electrical panels: Panels that are designed to carry 60 Hz currents can become mechanically resonant to the magnetic fields generated by higher frequency harmonic currents. The panel vibrates and emits a buzzing sound at the harmonic frequencies.
-Telecommunications: Harmonics in the neutral conductor commonly cause inductive interference that can be heard on a phone line. This is often the first indication of a harmonics problem.
– Transformers: Single-phase non-linear loads connected to the receptacles can produce harmonics that add up in the neutral current. When this neutral current reaches the transformer it can cause overheating and transformer failures. Higher frequency harmonic currents can also cause increased core loss resulting in more heating.
– Generators: Standby generators can be especially vulnerable to harmonics. In addition to overheating, harmonics can cause interference and instability in the generator’s control circuits.
The diagnostic challenge:
The best approach to troubleshooting harmonics is to begin with what is known as a "harmonic survey". This will give you a good idea whether or not you have a harmonics problem and where it is located. Some very basic guidelines for a harmonic survey are:
– Conduct a load inventory during a walking tour of the facility to look at types of equipment being used;
-Locate the transformers feeding non-linear loads and check for excessive heating;
– Use a true-rms meter (see "The Right tools" below for a definition of true-rms) to check transformer currents. Tests include: measuring and recording the transformer secondary currents in each phase and in the neutral; calculating the kVA delivered to the load and comparing it to nameplate rating; comparing the measured neutral current to the value predicted from the imbalance of the phase currents; measuring neutral current frequency.
– Survey the sub-panels that feed harmonic loads and measure the current in each branch neutral;
– Measure the neutral-to-ground voltage at the receptacle.
The right tools
Having the proper tools is critical to diagnosing harmonics problems. The type of equipment you use varies with the complexities of the measurements you need.
To determine whether you have a harmonics problem, you need to measure the true-rms value and the instantaneous peak value of the wave shape. For this you will need a handheld digital multimeter or clamp meter with true-rms.
True-rms refers to the root-mean-square, or equivalent heating value of a current or voltage wave shape. "True" distinguishes the measurement form those taken by "average responding" meters. Most low cost portable amp clamp meters fit the average-responding description. These instruments give correct readings for pure sine waves only, and will typically read low (in some cases up to 50 percent low) when confronted with a distorted current waveform.
True-rms meters give correct readings for any wave shape within the instrument’s crest factor and bandwidth specifications. Using a true-rms meter with a "peak" function allows you to easily calculate the crest factor and detect harmonics. Once you have determined that harmonics are present, you can make a more in-depth analysis of the situation with a harmonic analyzer.
In the event of harmonics, there are a number of remedies that can be applied, from load balancing and adding filters to derating transformers.
Although finding harmonics can be relatively easily, performing an in-depth analysis and applying solutions can be much more complex procedures. At this point it is always best to call a power-quality expert to analyze the problem and design a plan tailored to your specific situation.
Denise Deveau is a writer with Hand, Willingham, Bricker. She compiled this feature on behalf of Fluke Electronics.
A practical guide to power harmonics troubleshooting
So, you want to conduct a power harmonics analysis on your plant? Well, as author Denise Deveau suggest in our feature story, it’s best to call in the experts to help you with the survey.
But if you’re looking for some basic tips and an example of a successful analysis in practice, Dr. Ali Mihirig of Delta, B.C. has put together a research paper that will likely be of help. Entitled "Harmonic study analysis guidelines for industrial power systems", the paper sets out the step-by-step basic guideline of the practice of harmonic analysis, goes into detail about how these analyses should be performed and what specific elements of a power supply should be examined, and offers a practical method for system modelling that can be applied to any plant. (You can view the full text of the paper at www.amse.net/c99am.htm.)
"Harmonic study analysis must be conducted in the engineering design stage of all industrial systems that include harmonic producing equipment alongside with load flow and short circuit studies," writes Mihirig. "The interaction between load flow and harmonic study should lead to the best system configuration design, optimal operating conditions and proper size and location of power factor correction capacitors. When medium voltage capacitor banks are considered, it is also important to conduct transient analysis study to assess the possibility of switching problems."
Mihirig continues with a look at how plants can install power factor correction capacitors, distributed capacitors, centralised capacitors and the advantages and disadvantages of each power-efficiency monitoring system.
The section on system modelling concludes the paper,with a look at harmonic sources, motor loads, transformers, transmission lines and cables, and other areas where harmonics come into play. Mihrig has also included a list of sources for further reading.
For more information, you can contact Dr. Ali Mihirig of AM Power Systems by phone at 604-572-3725 or via email at firstname.lastname@example.org