2 replies to this topic

[kens]

Due to a change in the local distribution networks pricing regime many of our customers are now requiring PFC units, we have been measuring voltage distortion on the supplies but have been asked by equipment suppliers to measure current distortion as well, I understand the relationship between the voltage distortion and capacitors but I don't understand the effect that current distortion would have. Does someone have a simple explanation that I can use for me and my customers.

 

Thanks 

[mariomaggi]

Hi, kens,

simply speaking, in case of high current harmonic content, the harmonic currents are flowing 

mainly between non-linear loads and PFC units. In certain situations the Ithd should be very high, while Vthd should be very low; in this case PFC units are at risk.

Regards,

Mario

[marke]

Hi Ken

 

Yes, the topic of Harmonics locally is becoming a bit of a hot one at present.

 

THDv influences the level of harmonic current that flows through capacitive loads such as PF correction, lead/lag ballasts etc and this in turn affects the life of the capacitors.A poor THDv will also increase the slip losses dissipated in the rotors of induction motors and is possibly the cause of a number of premature motor failures, although often not yet identified as such.

 

Harmonic currents (THDi) cause the the THDv, being the product of the harmonic currents and the the supply impedance, so a bad THDi will increase the local THDv.

Additionally, the RMS value of the current is increased by the presence of the harmonics. The shape factor of the distorted current waveform causes an increase in losses in the supply components in the same way that reactive current does. Hence, we tend to talk about true power factor as being made up of displacement power factor and also distortion power factor.

If you measure power factor by measuring KW/KVA, this will give the true power factor, not just the displacement power factor. Smart meters have the ability to determine power factor this way rather than the angle between the current and voltage zero crossings as used in the traditional meters.

 

Come down to Canterbury, and in rural areas, we have THDv in excess of 10% due to the high levels of THDi.

By not monitoring the level of THDi being added to the network, the THDv can creep up without being noticed.

If the THDi is limited, the THDv will also be limited.

 

I expect that we will see a positive move towards actively penalizing distortion power factor in the same way that they discourage displacement power factor.

Problems : Increased supply losses and potentially interference with the operation of other equipment.

 

Best regards,

Mark.