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Understanding Current & Voltage Harmonics – 2

How do Current & Voltage Harmonics Affect the System?

Current harmonics increase the rms current flowing in the circuit and thereby, increase the power losses. Current harmonics affect the entire distribution all the way down to the loads. They may cause increased eddy current and hysteresis losses in motors and transformers resulting in overheating, overloading in neutral conductors, nuisance tripping of circuit breakers, over-stressing of power factor correction capacitors, interference with communication, etc. They can even lead to overheating and saturation of reactors.

Voltage harmonics affect the entire system irrespective of the type of load. They affect sensitive equipment throughout the facility like those that work on the zero-voltage crossing as they introduce voltage distortions.

Understanding IEEE 519 Guidelines

The purpose of harmonic limits in a system is to limit the harmonic injection from individual customers to the grid so that they do not cause unacceptable voltage distortion in the grid. IEEE 519 specifies the harmonic limits on Total Demand Distortion (TDD) and not Total Harmonic Distortion (THD). TDD represents the amount of harmonics with respect to the maximum load current over a considerable period of time (not the maximum demand current), whereas, THD represents the harmonics content with respect to the actual load current at the time of measurement.

It is important to note here that a small load current may have a high THD value but may not be a significant threat to the system as the magnitude of harmonics is quite low. This is quite common during light load conditions.

TDD limits are based on the ratio of the system’s short circuit current to load current (I_SC/I_L). This is used to differentiate a system and its impact on voltage distortion of the entire power system. The short circuit capacity is a measure of the impedance of the system. The higher the system impedance, the lower will be the short circuit capacity and vice versa.

The Guidelines IEEE 519-2014 at PCC Level are as under:

Systems with higher I_sc /I_Lhave smaller impedances and thus contribute less to the overall voltage distortion of the power system to which they are connected. Thus, the TDD limits become less stringent for systems with higher I_sc /I_Lvalues. In other words, the higher the rating of the transformer used for the same amount of load, the higher will be the allowable current distortion limits.

The limits on voltage are set at 5% for total harmonic distortion and 3% fundamental for any single harmonic at PCC level. Harmonics levels above this may lead to erratic functioning of equipment. In critical applications like hospitals and airports, the limits are more stringent (less than 3% V_THD) THD as the erroneous operation may have severe consequences. As discussed already, the harmonic voltage will be higher downstream in the system.

Solutions for Current & Voltage Harmonics

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