What is harmonic distortion?
Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60Hz (50Hz for European power and 60Hz for American power). For example, if the fundamental power frequency is 60 Hz, then the 2nd harmonic is 120 Hz; the 3rd is 180 Hz, etc. In modern test equipment today harmonics can be measured up to the 63rd harmonic. When harmonic frequencies are prevalent, electrical power panels and transformers become mechanically resonant to the magnetic fields generated by higher frequency harmonics. When this happens, the power panel or transformer vibrates and emits a buzzing sound for the different harmonic frequencies.
What causes harmonic distortion?
Harmonics are caused by and are the by-product of modern electronic equipment such as personal or notebook computers, laser printers, fax machines, telephone systems, stereos, radios, TVs, adjustable speed drives and variable frequency drives, battery chargers, UPS, and any other equipment powered by switched-mode power supply (SMPS) equipment. The above-mentioned electronic SMPS equipment is also referred to as non-linear loads. This type of non-linear loads or SMPS equipment generates the very harmonics they’re sensitive to and that originate right within your building or facility. SMPS equipment typically forms a large portion of the electrical non-linear load in most electrical distribution systems. There are basically two types of non-linear loads: single-phase and three-phase. Single-phase non-linear loads are prevalent in modern office buildings while three-phase non-linear loads are widespread in factories and industrial plants.
What does harmonic distortion do to the power at my facility?
In today’s environment, all computer systems use SMPS that convert utility AC voltage to regulated low voltage DC for internal electronics. These non-linear power supplies draw current in high amplitude short pulses. These current pulses create significant distortion in the electrical current and voltage wave shape. This is referred to as a harmonic distortion and is measured in Total Harmonic Distortion (THD). The distortion travels back into the power source and can affect other equipment connected to the same source.
Example: To give an understanding of this, consider a water piping system. Have you ever taken a shower when someone turns on the cold water at the sink? You experience the effect of a pressure drop to the cold water, reducing the flow of cold water. The end result is you get burned! Now imagine that someone at a sink alternately turns on and off the cold and hot water. You would effectively be hit with alternating cold and hot water! Therefore, the performance and function of the shower is reduced by other systems. This illustration is similar to an electrical distribution system with non-linear loads generating harmonics. Any SMPS equipment will create continuous distortion of the power source that stresses the facility’s electrical distribution system and power equipment.
What other effects besides distorting the shape of the voltage and current sinusoids, do harmonics cause?
Since non-linear loads produce harmonic currents with frequencies considerably higher than the power system fundamental frequency, these currents encounter much higher impedances as they propagate through the power system than does the fundamental frequency current. This is due to “Skin Effect” which is the tendency for higher frequency currents to flow near the surface of the conductor.
Since little of the high-frequency current penetrates far beneath the surface of the conductor, less cross-sectional area is used by the current. As the effective cross section of the conductor is reduced, the effective resistance of the conductor is increased. The higher resistance encountered by the harmonic currents will produce a significant heating of the conductor, since heat produced — or power lost — in a conductor is I2R, where “I” is the current flowing through the conductor. This increased heating effect is often noticed in two particular parts of the power system: neutral conductors and transformer windings.
Typical problems, together with before mentioned overheating in neutral conductors, transformers, or induction motors, include:
- Malfunctioning of microprocessor-based equipment.
- Deterioration or failure of power factor correction capacitors.
- Erratic operation of breakers and relays.
- Pronounced magnetic fields near transformers and switchgear.
- Large load currents in the neutral wires of a 3 phase system. Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires. Since only the phase wires are protected by circuit breakers of fuses, this can result in a potential fire hazard.
- Overheating of standard electrical supply transformers which shortens the life of a transformer and will eventually destroy it. When a transformer fails, the cost of lost productivity during the emergency repair far exceeds the replacement cost of the transformer itself.
- High voltage distortion exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment” and manufacturer’s equipment specifications.
- High current distortion and excessive current draw on branch circuits exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment” and manufacturer’s equipment specifications.
- High neutral-to-ground voltage often greater than 2 volts exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment.”
- High voltage and current distortions exceeding IEEE Std. 519-1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.”
- Poor power factor conditions that result in monthly utility penalty fees for major users (factories, manufacturing, and industrial) with a power factor less than 0.9.
- Resonance that produces over-current surges. In comparison, this is equivalent to continuous audio feedback through a PA system. This results in destroyed capacitors and their fuses and damaged surge suppressors which will cause an electrical system shutdown.
- False tripping of branch circuit breakers.