Harmonics is a very important power quality problem in power lines. We need to know about it more to solve this power quality problem. Only a few research paper exists in literature about that. So in this forum I hope all the researcher/ professor will contribute something to understand the characteristics, problems description, methods how to solve, a better understanding about this important issue.
Well, first you have to distinguish between voltage and current harmonics. Non-linear loads (rectifiers, inverters, converters in general) will generate current harmonics on the power grid side (harmonics at the output of an inverter are a different beast...).
So, let's say you have a Variable Speed Drive (VSD) for an induction motor. This will have a rectifier at the input stage, so it will produce odd harmonics in your power line. Depending on the Z=R+jX of the power line, a voltage drop will occur accross the power line for each harmonic frequency, which will cause the occurence of voltage harmonics. If the VSD is connected to an infinite bus (theoretical) no voltage harmonics can occur. So, the amount of voltage harmonics will vary according to the line impedance and the current harmonics.
Some of the harmonics (the 3rd harmonic, but not in all cases) can be mitigated by using transformers in Delta configuration, which is why usually a distribution transformer is required to have at least one winding in Delta.
Additionally, power harmonics can be mitigated by using frequency traps (Resonant Harmonic Filters or Harmonic Blocking Compensators) or by installing inductors (chokes) in series with the non-linear load.
When it comes to the harmonics produced at the output of a VSD, one has to keep in mind that current harmonics are minimal (current will be almost a pure sine wave), but voltage harmonics will be present at high frequencies (kHz order) due the PWM. In this case, care has to be taken regarding the cable connecting the VSD to the motor, in order to avoid EMC problems.
Also, an EMC filter is often used at the power grid side for the same reason (to mitigate high frequency harmonic content, which would otherwise pollute the grid).
Although 3rd harmonics are frequently the highest, I have seems cases where very high order harmonics (63rd harmonic, for ex., are also troublesome. Hence the need to use a network analyzer to understand the characteristics of all domains - time domain and frequency domain.
- Badges
- Science topic