On radial utility distribution feeders and industrial plant power systems, the main tendency is for the harmonic currents to flow from the harmonic-producing load to the power system source. This is illustrated in Figure 1. The impedance of the power system is normally the lowest impedance seen by the harmonic currents.
Thus, the bulk of the current flows into the source.
This general tendency of harmonic current flows can be used to locate sources of harmonics. Using a power quality monitor capable of reporting the harmonic content of the current, simply measure the harmonic currents in each branch starting at the beginning of the circuit and trace the harmonics to the source.
For example, adding a capacitor to the previous circuit as shown in Figure 2 may draw a large amount of harmonic current into that portion of the circuit. In such a situation, following the path of the harmonic current will lead to a capacitor bank instead of the actual harmonic source.
Thus, it is generally necessary to temporarily disconnect all capacitors to reliably locate the sources of harmonics.
It is usually straightforward to differentiate harmonic currents due to actual sources from harmonic currents that are strictly due to resonance involving a capacitor bank. A resonance current typically has only one dominant harmonic riding on top of the fundamental sine wave. Note that harmonic sources produce more than one single harmonic frequency.
Waveforms of these harmonic sources have somewhat arbitrary waveshapes depending on the distorting phenomena, but they contain several harmonics in significant quantities. A single, large, significant harmonic nearly always signifies resonance.
Simply measure the current in the capacitors. If it contains a very large amount of one harmonic other than the fundamental, it is likely that the capacitor is participating in a resonant circuit within the power system. Always check the capacitor currents first in any installations where harmonic problems are suspected.
Another method to locate harmonic sources is by correlating the time variations of the voltage distortion with specific customer and load characteristics. Patterns from the harmonic distortion measurements can be compared to particular types of loads, such as arc furnaces, mill drives, and mass transits which appear intermittently.
Correlating the time from the measurements and the actual operation time can identify the harmonic source.
Where to control harmonics
The strategies for mitigating harmonic distortion problems differ somewhat by location. The following techniques are ways for controlling harmonic distortion on both the utility distribution feeder and end-user power system.
The X/R ratio of a utility distribution feeder is generally low. Therefore, the magnification of harmonics by resonance with feeder banks is usually minor in comparison to what might be found inside an industrial facility. Utility distribution engineers are accustomed to placing feeder banks where they are needed without concern about harmonics.
However, voltage distortion from the resonance of feeder banks may exceed limits in a few cases and require mitigation.
To change the flow of zero-sequence harmonic currents, changes are made to the neutral connection of wye-connected banks.
To block the flow, the neutral is allowed to float. In other cases, it is more advantageous to aid the flow by putting a reactor in the neutral to convert the bank into a tuned resonant shunt for a zero-sequence harmonic.
Harmonic problems on distribution feeders often exist only at light load! The voltage rises, causing the distribution transformers to produce more harmonic currents and there is less load to damp out resonance. Switching the capacitors off at this time frequently solves the problem.
Should harmonic currents from widely dispersed sources require filtering on distribution feeders, the general idea is to distribute a few filters toward the ends of the feeder. While this is not done frequently, the number of feeder filter installations is growing.
Figure 1 above shows one example of a filter installed on an overhead distribution feeder. This shortens the average path for the harmonic currents, reducing the opportunity for telephone interference and reducing the harmonic voltage drop in the lines.
The filters appear as nearly a short circuit to at least one harmonic component. This keeps the voltage distortion on the feeder to a minimum. With the ends of the feeder “nailed down” by filters with respect to the voltage distortion, it is more difficult for the voltage distortion to rise above limits elsewhere.
Harmonic flow studies should always be performed when large capacitor banks are installed in distribution substations. One cannot count on system losses to damp out resonance at this point on the system, and magnification by resonance can be severe.
When harmonic problems arise in an end-user facility, the first step is to determine if the main cause is resonance with power factor capacitors in the facility. When it is, first attempt a simple solution by using a different capacitor size.
With automatic power factor controllers, it may be possible to select a control scheme that avoids the configuration that causes problems. In other cases, there will be so many capacitors switched at random with loads that it will be impossible to avoid resonant conditions. Filtering will be necessary.
IMPORTANT! Installation of filters on end-user low-voltage systems is generally more practical and economical than on utility distribution systems.
The criteria for filter installation are more easily met, and filtering equipment is more readily available on the market. When the magnitude of harmonic currents injected by loads is excessive, industrial users should also investigate means of reducing harmonics by using different transformer connections and line chokes.
In office buildings, zigzag transformers and triplen harmonic filters can reduce the impact of triplen harmonic currents on neutral circuits. Studies should be performed on all capacitors installed on the main bus in industrial systems. At this location, there are insufficient line losses to dampen resonance.
Thus, when resonance coincides with a harmonic frequency that is a strong component in the load current, the resulting voltage distortion is often severe. Resonance problems are often less severe when capacitors are located out on the plant floor on motors and in motor control centers.
This assumes that the cables are sufficiently long to introduce enough resistance into the circuit to dampen the resonance. In plants with short cables, it may not be possible to achieve significant harmonic reduction benefit.
Reference // Electrical power systems quality by Roger C. Dugan, Mark F. McGranaghan, Surya Santoso and H. Wayne Beaty (Purchase hardcopy from Amazon)