Harmonic Distortions – Effects, Measurements and Analysis
- Does the level of the current exceeds the limit on the conductors?
- Do overcurrent circuit breakers trip and you do not know why?
- Do electric motor overheat and have excessive vibrations?
What are the harmonic frequencies?
The distortion of the power signal in the power network from the shape of the ideal sinusoid indicates a certain harmonic content in this signal. Figure 1 shows three waveforms from a three-phase network with a high degree of harmonic content. The shape of the waveform of each phase is far from the ideal sinusoid.
Theory says that each distorted signal can be decomposed into a set of harmonics being a multiples of the fundamental frequency. For a power network with fundamental frequency of f1 = 50 Hz harmonics are all component signals with frequencies being multiples of 50 Hz, such as f2 = 100 Hz, f3 = 150 Hz, etc.
Four examples of the negative impact of harmonics:
1. Temperature of the conductor outside the limit.
The skin effect is the cause of excessive heating of the conductor wire with high harmonic content in the current signal. For the frequency of the 50 Hz, the skin effect is small, but above 250 Hz, which corresponds to the 5th harmonic, the effect is significant.
The third current harmonic and its multiples does not cancel in a three-phase four-wire network. The lack of the third harmonic cancellation effect results in the addition of each phase harmonic currents in the neutral conductor. In this case, a current which will flow can significantly exceed the thermal limit of the neutral conductor. This can lead to overheating and, as a result, melting or even igniting the insulation of the cable and flammable materials in the vicinity.
2. Undesirable shutdown of the installation.
Protection devices in electrical networks, such as overcurrent breakers with bimetallic element, are designed to operate after exceeding a certain threshold temperature. Harmonic signals causing excessive heating of conductors may contribute to incorrect operation of such protections designed to work properly for 50 Hz. In the case of an increased level of harmonic frequencies, overcurrent circuit breakers may be triggered. The circuit breaker will trigger prematurely even if the RMS value of current and duration of the load state seems to be in the range of the device.
3. Excessive heating of the transformer
Higher harmonic frequencies are the cause of the occurrence of increased losses and as a result of increased heating of the transformers. Losses in the transformer include losses in the core and in the windings. Increased losses in the core are associated with eddy currents and are proportional to the square of the frequency. However, increased losses in the windings are associated with increased thermal losses in the conductor. The above two effects will cause an additional increase in the temperature of the transformer, which may lead to its overheating and in extreme cases to destruction.
Tip: Contactless temperature measurement can be made with the Sonel KT thermal imaging camera.
4. Shortened engine life
In motors, the deformed voltage can cause excessive vibrations and increased thermal losses through the creation of eddy currents in the same way as in transformers. Additional thermal losses and vibrations also arise due to field generation with the frequency of harmonic signals in the stator.
Each harmonic frequency rotates the motor at a different speed. In addition, the rotation of the motor in the nominal direction, provided by the manufacturer, is caused by positive sequence harmonics i.e. 1st, 4th, 7th, etc. In turn, the rotation in the opposite direction is caused by negative sequence harmonics of the order of 2nd, 5th, 8th, etc. This situation can be compared to driving a car, during which the driver simultaneously accelerates and brakes.
Therefore, the presence of harmonics at levels above the threshold for a given engine will contribute to undesirable vibrations and excessive heating and as a result shorten the life of the engine.
A few simple steps to perform diagnostics and measurement of harmonics:
- Connect any PQM Sonel series analyzer according to the manufacturer’s recommendation and set the measurement mode of harmonic amplitudes, THD, TID and TDD parameters.
- During the measurement, check the level of THD, TID and TDD parameters and then the level of each of the harmonics for current and voltage.
- If the level of the parameter exceeds the threshold of EN 50160, IEEE 519 or other related standards, also indicated by the manufacturer, take action.
Analysis of harmonic measurement results.
- Read the measured values for the THD, TID and TDD parameters and compare with the limit values.
- Display the set of harmonic amplitudes values as a bar graph, in the form of percentage value relative to the fundamental frequency.
- Compare with the limits of harmonic amplitudes for given requirements, eg EN50160.