What are harmonics?
Harmonic is a component of a periodic wave having a frequency that is an integral multiple of the fundamental power line frequency of 50 Hz. For example, 150 Hx (3 x 50 Hz) and 250 Hz (5 x 50 Hz) is a 3rd and 5th order harmonic of the fundamental frequency. The result is harmonic distortion of the power waveform when the fundamental with 3rd and 5th harmonic are combined. Harmonics are a steady-state phenomenon and should not be confused with short-term phenomena that last less than a few cycles.
Consequences of high harmonic distortion level?
Harmonics can cause several problems such as
How are harmonics generated?
Harmonics are generated by nonlinear loads. A nonlinear load is a circuit element that draws current in a nonsinusoidal manner. Until recently, most factory loads were primarily linear, with current waveform closely matching the sinusoidal voltage waveform and changing in proportion to the load. More recently, however, factory loads with major nonlinear components have increased dramatically
Examples of Linear and Nonlinear Devices
- Some motors
- Incandescent lighting
- Heating loads
- DC drives
- Variable frequency drives
- Programmable controllers
- Induction furnaces
- Solid-state uninterruptible power supplies (UPSs)
- Arc furnaces
- Arc welders
Nonlinear devices that cause the most problems can generally be grouped into two categories—electronic power converters and arcing devices.
Transients, electrical disturbances, overvoltage surges, and undervoltage sags in the supplied voltage are not harmonics. Some of these short-term disturbances in voltage or current can be mitigated by transient voltage surge suppressors, line reactors, or isolation transformers.
However, these devices usually have little, if any, effect on harmonic currents or voltages. The level of voltage or current harmonic distortion existing at any one point on a power system can be expressed in terms of the total harmonic distortion (THD) of the current or voltage waveform. The
THD (for a voltage waveform) is given by the following formula:
IEEE Standard 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, represents the most recent effort to establish a standard level of acceptable harmonic distortion levels on a power system.
Note: The current distortion limits are dependent upon the size of the customer’s load relative to the available short-circuit capacity of the utility (stiffness). In this way, customers whose loads potentially have more effect on the utility system and neighboring customers are held to the tighter limits.
UK G5/4, published in March 2001 seeks to limit harmonic voltage distortion levels on public networks at the time of connection of new non-linear loads to ensure compatability of all connected equipment. G5/4 defines planning levels of 5% and introduces compatibility levels of 8% for individual harmonics and THD over the voltage range from 400V to 400kV.
Hybrid Var Compensator combine the technical advantages of AHF, ALB and SVG with the cost-effectiveness of conventional contactor or thyristor switched detuned filter capacitor banks to form an economical stepless real-time compensator with a single controller compared to a full SVG solutions.
Using single-phase loads on a three-phase electric power system (connected phase-to-phase or phase-to-neutral) results in unbalanced load conditions in the system. Unbalanced load currents result in unbalanced voltages and affect other loads connected at the point of common coupling. Unbalanced load conditions also cause excessive neutral current, resulting in overheating motors and transformers, power losses and lower system efficiencies. Load balancing is necessary to improve the power quality and efficiency of the system.
SVGs deliver real-time inductive or capacitive reactive power compensation. Rapid response time provides stable and accurate power factor correction without the drawbacks of conventional solutions like capacitor banks and reactor banks.
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