Harmonic Mitigation transformers are much superior than K rated transformer

The use of electronic equipment has continued to proliferate in both offices and industrial plants. These electronic devices are powered by either switching power supplies or a rectifier circuit. Examples of these devices include computers, fax machines, copiers, printers, cash registers, UPS’s and solid-state ballasts. They all contribute to the distortion of the current waveform and the generation of harmonics.

Harmonics, in an electrical system, are currents created by non-linear loads that generate non-sinusoidal (non-linear) current waveforms. These current and voltage waveforms operate on frequencies that are in multiples of the fundamental 50 hertz frequency. That is, the fundamental frequency is at 50 hertz, the 2nd harmonic is at 100 hertz frequency (50x 2), the 3rd at 150 hertz, and so forth.

Harmonics are principally the by-product of switch-mode power supply technology where AC is rectified to DC, and back again. In the process, a capacitor is charged in the first half-cycle and then discharged in the next half-cycle in supplying current to the load. This cycle is repeated. This action of recharging causes AC current to flow only during a portion of the AC voltage wave in abrupt pulses. These abrupt pulses distort the fundamental wave shape causing distortion to the various harmonic frequencies. Non-Linear Loads Today, non-linear loads make up the majority of all electrical demand. Rectified input, switching power supplies and electronic lighting ballasts are the most common single phase non-linear loads. Harmonic currents and voltages produced by single phase, non-linear loads which are connected phase-to-neutral in a three phase four wire system, are third order, zero sequence harmonics (the third harmonic and its odd multiples - 3rd, 9th, 15th, 21st, etc., phasors displaced by zero degrees). These third order, zero sequence harmonic currents do not cancel, but add up arithmetically on the neutral bus, creating a primary source of excessive neutral current. 

K-factor transformers are designed to reduce the heating effects of harmonic currents created by loads like those in the table below. The K-factor rating is an index of the transformer's ability to withstand harmonic content while operating within the temperature limits of its insulating system.

The electrical industry’s first response to these four problems was to double the ampacity (current carrying capacity) of the neutral conductors so that they would not burn-up. This is becoming a standard design practice in office buildings. The second response was to beef-up the distribution transformer so that it would not fail due to the higher heat losses caused by the harmonic currents flowing through it. These transformers are now known as k-factor rated transformers. The k-factor is a mathematical formula which predicts that the eddy current losses in a transformer will be increased in direct proportion to the sum of the products of each harmonic current amplitude squared multiplied by its harmonic number squared. 

Doubling the neutral and using a k-factor rated transformer will solve the electrical safety half of the harmonics problem. Unfortunately, these two steps do nothing to solve problem 3, poor power factor, nor problem 4, lowered computer system reliability.

On the Other hand, Harmonic Mitigating transformers are superior to the above transformers in that they reduce voltage distortion (flat-topping) and power losses due to current harmonics created by single-phase, non-linear loads such as computer equipment. Secondary windings are arranged to cancel zero sequence fluxes and eliminate primary winding circulating currents. They treat zero sequence harmonics (3rd, 9th and 15th) within the secondary windings and 5th and 7th harmonics upstream with appropriate phase shifting. Our dual output, phase shifting Harmonic Mitigating Transformers provide extremely low output voltage distortion and input current distortion even under severe non-linear load conditions (data centers, internet service providers, telecom sites, call centers, broadcasting studios, etc.). Combining zero sequence flux cancellation with phase shifting treats 3rd, 5th, 7th, 9th, 15th, 17th and 19th harmonics within its secondary windings.

BENEFITS:

● Allows the loads to operate the way they were designed.

● Minimizes the impact of the harmonic currents and frequencies.

● Maintains a high energy efficiency, even when feeding non-linear loads.

● Reduces electrical consumption.

● Reduces pollution as a result of its higher energy efficiency.

● Doesn't allow the triplens to circulate in the delta winding of the transformer, which results in an increase in usable transformer capacity.

APPLICATIONS:

● Where there is high computer, printer and communication equipment usage, and stable operation is necessary.

● Medical facilities including hospitals.

● Data centers.

● Office buildings & Schools.

CONCLUSION:

With their ability to treat both power quality and overheating harmonic concerns, Harmonic Mitigating Transformers are much superior than K-rated cousins. Power distribution systems designed with HMTs are capable of servicing any level of non-linear loading without suffering negative consequences. This should justify more widespread use of the HMT in applications where high concentrations of non-linear loads are found. Continued use of K-rated transformers, on the other hand, will lead to distribution systems that are incapable of being fully utilized without distorting the voltage waveform above recommended maximum levels. 

SOURCES:

PICTURE - HAMMOND POWER SOLUTIONS