Fig. below shows a distribution system radial primary feeder from the Rio Salado

Question

Fig. below shows a distribution system radial primary feeder from the Rio Salado substation. Line reactances at 60 Hz are shown in per unit on a 10 MVA base. For simplicity, unity power factor is assumed at the loads. Fifth harmonic currents are measured at the University and ASU – W loads. The high side (69 kV) of the Rio Salado substation is assumed to be an infinite bus and is assumed to be a perfect sinusoidal voltage. You are asked to find
(A) whether the loads shown individually comply with the IEEE 519 standard for maximum harmonic current and
(B) whether the voltages at the three indicated 14.8 kV buses comply with the IEEE 519 maximum harmonic voltage limits. Note: this is a three phase system. You may have to make some reasonable estimates in your assessment.

Rio Salado Substation 10 MVA, 3.7%X, 69 kV/ 14.8 kv, wye / delta 0 +j0.0008 1 MVA, 3% fifth harmonic, pf= 1.00 University 0 +j0.0011 ASU - W 1 MVA, 1% fifth harmonic, pf 1.00

Explanation / Answer

a) For a given amount of harmonic current flow, the resulting voltage distortion will be relatively small (excepting harmonic resonance situations). Therefore, when someone mentions harmonic levels well in excess of 5% he is probably talking about current harmonics.100 A of 60 Hz current combined with 20 A of 5th harmonic current (300 Hz) adds up to 102 ARMS, not 120 A. Unless harmonics are very high, the RMS current is likely to be very close to the 60 Hz fundamental current.

According to IEEE 519, harmonic voltage distortion on power systems 69 kV and below is limited to 5.0% total harmonic distortion (THD) with each individual harmonic limited to 3%. The current harmonic limits vary based on the short circuit strength of the system they are being injected into. Essentially, the more the system is able to handle harmonic currents, the more the customer is allowed to inject.

b)

Bear in mind that dealing with harmonics may still be required, whether or not the goal is to meet IEEE 519 standards. In low-voltage systems (600 V or less), capacitors are typically the lowest impedance at harmonic frequencies, and experience very high RMS currents and increased heat which causes them to fail.

High-voltage systems can have up to 2.0% THD where the cause is an HVDC terminal whose effects will have attenuated at points in the network where future users may be connected.

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