How to Solve Neutral Overload Problems

Nov 1, 2001 12:00 PM, By Larry Ray, Square D

When a plant engineer must replace bus duct at a battery charging facility, the issue of overloaded neutrals comes front and center.

The plant engineer at an automobile battery-charging facility needed to replace the aging 600V bus duct in his company's plant, but he was conflicted over the bus duct vendors' claims that he should use a 200% neutral conductor rating in the proposed bus duct. He knew battery chargers produce significant amounts of harmonic currents but wondered if it was necessary to pay a premium price for specifying 200% neutrals.

As if the neutral issue weren't enough, the subject of harmonic currents led to a question of transformer loading: What are the effects of these harmonic currents on the 2,500kVA transformer serving the battery-charging circuits? Fig. 1 (in original article) shows a 4-cycle, captured wave from a circuit monitor located at the 2,500kVA transformer. As the engineer suspected, both voltage and current were severely distorted.

The engineer's concern about the effects of harmonics on the proposed bus duct was justified, but the maximum neutral current at his plant did not exceed the phase current. In fact, no commercially available loads exist that contain enough triplen, or multiples of three times the fundamental frequency, harmonic currents to necessitate more than 100% neutrals at the 480V level or above. The neutral overloading issue is limited to 208/120V switch-mode power supply loads, such as personal computers. Fig. 2 (in original article) shows the average phase current and resulting neutral current as measured at the 2,500kVA transformer. Note the neutral current does not exceed the average phase current.

The Table (in original article) illustrates the neutral current makeup. The fundamental neutral current (H1) is small compared to the fundamental phase currents. The fundamental current on the neutral represents the unbalance of the fundamental component on the phase conductors. The fifth harmonic neutral current (H5) is also small in comparison to its phase currents, but the third harmonic component (H3) adds on a shared neutral. In fact, the predominant component on the neutral is the third harmonic (Fig. 3 in original article).

To accomodate the neutral currents, the engineer decided to purchase new bus ducts with a neutral ampacity of 100%. At 600V, battery-charging circuits do not inject enough triplen currents to cause excessive neutral loads. Shared neutrals serving 208/120V circuits, however, can be subjected to currents that exceed the phase current. While it's good practice to maintain well-balanced 3-phase circuits, you cannot solve a neutral problem by changing the current balance among conductors. Better balance may increase the neutral current for severely distorted phase currents.

Dealing with the transformer loading issue was more difficult. As a stopgap, the plant engineer closed a bus tie circuit breaker on the double-ended substation (to share harmonic loading among two transformers). Although this temporarily reduced the overloading concern, you should avoid this practice because it can lead to worse problems. The failure of the transformer or bus duct can cost millions of dollars in damaged product and lost sales.

What should he have done? If you are concerned about neutral overloading on a 208/120V circuit, you have a few options. By running a separate neutral conductor for each phase wire, you can increase the current-carrying capacity of the neutral and eliminate the shared neutrals. Replacing a shared neutral with one rated for 200% of the phase current is another way you can increase the capacity of the neutral. The neutral buses on panelboards and transformers may need to be increased as well, but keep in mind that only 4-wire, 208/120V circuits are subject to neutral currents that exceed phase currents.

Although it's easy to verify the current levels on your neutral with power monitoring, the transformer loading issue is more complicated. IEEE/ANSI Standard C57.110-1986 offers a method for calculating the effect that harmonic currents above 5% have on transformer life. The standard has been used to de-rate a standard transformer based on the amount of harmonic current it must transmit, but de-rating a standard transformer is not recommended as a permanent solution.

Installing power transformers designed to transmit more than 5% current THD is one way to deal with harmonic currents in power transformers. K-factor transformers are manufactured for this purpose. The k-factor rating is related to the harmonic content of the expected load and calculated from the harmonic current measured in the load.

K-rated transformers typically have additional coil capacity to reduce eddy-current losses. They are typically wound 480V delta primary and 208/120V secondary, with an electrostatic shield and a reduced core flux to compensate for harmonic voltage distortion. Because these transformers usually serve computer loads, they also contain double-size neutral terminals to accommodate extra neutral conductors. The higher the k-rating, the more harmonic current the transformer can transmit; k-4 or k-13 ratings are adequate for most circuits.

Larry A. Ray is a manager for Square D's Systems Engineering group, Raleigh, N.C.

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