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Harmonics have become an integral part of modern electric systems.  Harmonics are caused by non-linear loads.  Most electronic devices will fall in this category.  Harmonics cause a wide range of adverse effects ranging from false operation of relays to overheating.  Excess heating is a principal effect of harmonics.  The high frequency of the harmonics cause increased heat losses in generators, conducting cables and motors. 

Power sources such as transformers need to be operated at reduced capacity when connected to harmonics-causing loads.  For instance, a 100 kVA transformer may have to be operated at 60 kVA. 

The K rating is a factor that is developed to indicate the amount of harmonics that the load can generate.  The K rating is extremely useful when designing electric systems and sizing components.

The formula for K rating is




 
 Where Ih is the total harmonic current of particular harmonic.  Ih is expressed as p.u. basis

The k rating of loads helps decide on the specifications of the power source such as transformers and also the size and capacity of the conductors.  The amount of harmonics drawn by a load determines its k rating.  Thus loads such as resistance heaters, transformers and motors with less harmonics have a k rating of 1.  Welders and induction heaters have a k rating of 4 as they generate a substantial amount of harmonics. 

Loads which have higher k rating are connected to transformers having a corresponding k rating.  Thus a transformer of 100kVA with a k rating of 4 will deliver 100kVA as long as the k rating of the load is 4.  At higher k ratings, the transformer may have to be derated.

Transformers with higher k ratings are made of thicker conductors and are designed to withstand higher eddy currents.  This leads to the transformers having bigger size.  These transformers also occupy higher space.  The neutral conductors in these transformers are thicker to permit the circulation of the triplen harmonics through them.

Transformers can get heated due to a variety of reasons.  Excessive heating of the transformer increases transformer losses, weakens the insulation and may result in reduced transformer life.  The principal reasons for transformer overheating are

Overload

Excessive loading can cause overheating.  Transformers are rated in kVA.  This kVA rating is dependent on temperature.  Some transformers have two kVA ratings.  One rating is for below a specific ambient temperature, say 45 degrees while another rating is above the ambient temperature.

Excess current in the neutral of the transformer

Excess current in the neutral of the transformer is usually caused by high zero sequence harmonic components.  This can be resolved by proper grounding usually through a zig zag transformer (See article).

Problems in the Cooling system

Malfunctioning of the cooling equipment such as blockages in the cooling oil circuit.  Failure of the cooling fans to operate in the case of Forced Air cooling systems.

High Harmonic content in the power supply

High harmonic loads can also cause heating in the transformer.  These loads need to be identified and suitable remedial measures such as harmonic filters can be implemented.

Sustained Overvoltages

Overvoltages which exist for a long period of time can overexcite the transformer and cause overheating.

Piezoelectric power technology--using pressure generated by people walking to produce electricity--has been used in small applications before. Rotterdam has a human-powered dance club, English supermarket Sainsbury's features people-powered checkout lines, and a Tokyo subway station runs turnstiles and displays with piezoelectric power. But Toulouse, France, recently became the first city to stick the pressure-sensitive modules on the sidewalk so that residents can generate power just by walking down the street.

The city is using the same modules found at Rotterdam's people-powered Club Watt. The technology, developed by Dutch company Sustainable Dance Club, features embedded microsensors that generate electricity when pressure is exerted by passersby. According to the UK Guardian, Toulouse's experiment marks the first time that the SDC modules have been used on the street.

So far, Toulouse has installed a trial section of eight modules that produce 50 to 60 watts, or enough to power a street lamp. The city hasn't decided whether to expand the sidewalk program, but Rotterdam is plowing ahead with a pilot scheme to stick SDC modules in a soccer stadium. Soon enough, we might all produce energy just by power-walking to work.

courtesy : www.fastcompany.com

DC alone is used because we need to measure only the resistance of the winding insulation.  If we use AC to conduct the insulation test, we will be measuring impedance instead of the resistance as the insulation  has a substantial capacitive reactance.

The health of the winding is best indicated by its resistance and not its impedance.  This is why only DC is used to measure the insulation resistance.