[vc_row][vc_column width=”2/3″][vc_column_text]Harmonics are a distortion of the normal electrical current waveform, generally transmitted by nonlinear loads. Switch-mode power supplies (SMPS), variable speed motors and drives, photocopiers, personal computers, laser printers, fax machines, battery chargers and UPSs are examples of nonlinear loads. Single-phase non-linear loads are prevalent in modern office buildings, while three-phase, non-linear loads are widespread in factories and industrial plants.
A large portion of the non-linear electrical load on most electrical distribution systems comes from SMPS equipment. For example, 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 that create significant distortion in the electrical current and voltage wave shape—harmonic distortion, measured as total harmonic distortion (THD). The distortion travels back into the power source and can affect other equipment
connected to the same source.
Most power systems can accommodate a certain level of harmonic currents but will experience problems when harmonics become a significant component of the overall load. As these higher frequency harmonic currents flow through the power system, they can cause communication errors, overheating and hardware damage, such as:
- Overheating of electrical distribution equipment, cables, transformers, standby generators, etc.
- High voltages and circulating currents caused by harmonic resonance
- Equipment malfunctions due to excessive voltage distortion
- Increased internal energy losses in connected equipment, causing component failure and shortened life span
- False tripping of branch circuit breakers
- Metering errors
- Fires in wiring and distribution systems
- Generator failures
- Crest factors and related problems
- Lower system power factor, resulting in penalties on monthly utility bills
- Overloading Neutral Conductors
The three-phase system consists of three individual phase conductors and a neutral conductor. If all the phase conductors carry the same current, the phase currents tend to cancel one another out provided there is a balanced load. This balanced load makes it possible to reduce the size of the neutral conductor. Unfortunately, switched mode power supplies used in computers have a very high third-harmonic current. While harmonic currents cancel out on the neutral wire, the third harmonic current is additive in the neutral. In buildings with a large number of installed personal computers, the neutral wire can carry much higher currents than the wire was designed to accommodate, creating a potential fire hazard.
- Overheating Transformers and Increased Associated Losses
For transformers feeding harmonic-producing loads, the eddy current loss in the windings is the most dominant loss component in the transformer. This eddy current loss increases proportionate to the square of the product’s harmonic current and its corresponding frequency. The total transformer loss to a fully loaded transformer supplying to a nonlinear load is twice as high as for an equivalent linear load. This causes excessive transformer heating and degrades the insulation materials in the transformer, which eventually leads to transformer failure.
- Nuisance Tripping of Circuit Breakers
All circuits containing capacitance and inductance have one or more resonant frequencies. When any of the resonant frequencies correspond to the harmonic frequency produced by nonlinear loads, harmonic resonance can occur. Voltage and current during resonant frequency can be highly distorted. This distortion can cause nuisance tripping in an electrical power system, which can ultimately result in production losses.
Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency. If the fundamental power frequency is 60 Hz, then the 2nd harmonic is 120 Hz, the 3rd is 180 Hz, etc. (see Figure 1). 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. Harmonic frequencies from the 3rd to the 25th are the most common range of frequencies measured in electrical distribution systems.
All periodic waves can be generated with sine waves of various frequencies. The Fourier theorem breaks down a periodic wave into its component frequencies. The total harmonic distortion (THD) of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental. It provides an indication of the degree to which a voltage or current signal is distorted.
Solutions to compensate for and reduce harmonics
While standards to limit the generation of harmonic currents are under consideration, harmonic control today relies primarily on remedial techniques. There are several approaches that can be taken to compensate for or reduce harmonics in the power system, with varying degrees of effectiveness and efficiency.
Oversize the neutral wiring
In modern facilities, the neutral wiring should always be specified to be the same capacity as the power wiring, or larger—even though electrical codes may permit under-sizing the neutral wire. An appropriate design to support a load of many personal computers, such as a call center, would specify the neutral wiring to exceed the phase wire capacity by about 200 percent. Particular attention should be paid to wiring in office cubicles. Note that this approach protects the building wiring, but it does not help protect the transformers.
Use separate neutral conductors.
On three-phase branch circuits, instead of installing a multi-wire branch circuit sharing a neutral conductor, run separate neutral conductors for each phase conductor. This increases the capacity and ability of the branch circuits to handle harmonic loads. This approach successfully eliminates the addition of the harmonic currents on the branch circuit neutrals, but the panelboard neutral bus and feeder neutral conductor must still be considered. Use DC power supplies, which are not affected by harmonics.
In the typical data center, the power distribution system converts 480-volt AC utility power through a transformer that steps it down to 208-volt AC power that feeds racks of servers. One or more power supplies within each server convert this AC input into DC voltage appropriate for the unit’s internal components.
These internal power supplies are not energy efficient, and they generate substantial heat, which puts a costly burden on the room’s air conditioning system. Heat dissipation also limits the number of servers that can be housed in a data center
Use K-rated transformers in power distribution components.
A standard transformer is not designed for high harmonic currents produced by non-linear loads. It will overheat and fail prematurely when connected to these loads. When harmonics were introduced into electrical systems at levels that showed detrimental effects (circa 1980), the industry responded by developing the K-rated transformer.
K-rated transformers are not used to handle harmonics, but they can handle the heat generated by harmonic currents and are very efficient when used under their K-factor value. K-factor ratings range between 1 and 50. A standard transformer designed for linear loads is said to have a K-factor of 1. The higher the K-factor, the more heat from harmonic currents the transformer is able to handle. Making the right selection of K-factor is very important, because it affects cost and safety.
Harmonic currents can have a significant impact on electrical distribution systems and the facilities they feed. It is important to consider the impact of harmonics when contemplating additions or changes to a system. In addition, identifying the size and location of non-linear loads should be an important part of any maintenance, troubleshooting and repair program.
References
- Treating Harmonics in electrical distribution systems, Victor A. Ramos Jr.
- Application notes from Controlled power company.
- Mirus Harmonics and Harmonic Mitigating Transformer
- An introduction to Power System Harmonics by Power System Engineering
- Application notes from Energy Vortex
[/vc_column_text][/vc_column][vc_column width=”1/3″][/vc_column][/vc_row][vc_row][vc_column width=”2/3″][vc_column_text]AUTHORS
1.Bunty B. Bommera
2.Dakshata U. Kamble[/vc_column_text][/vc_column][vc_column width=”1/3″][/vc_column][/vc_row]