7/27/2019 Comp,Ups Application

1/3

POWER CONTROL EQUIMENTS DESIGN MANUAL

NEUTRAL BUS FOR COMPUTER & UPS APPLICATIONS

In 3 Phase Power systems Neutral current is the result of imbalance of thePhase currents. When the 3 phase currents are balanced, the vector sum ofthe 3 currents is zero. Hence the neutral current is zero. This is true for

purely sinusoidal currents where the 3 phase currents are equal and arehaving a phase difference of 120 electrical degrees.

In most 3 phase power systems supplying single phase loads there will besome phase current unbalance and some neutral current. It is a common

practice to design the neutral wire for 50% of the Phase current. This is forproviding a path for the neutral current. This is considered adequate in theworst case of unbalanced currents, provided they are free from harmonics.

There are conditions where even perfectly balanced single phase loads canresult in significant neutral current. Nonlinear loads such as rectifiers and

power supplies have phase currents, which are not sinusoidal. The vectorsum of balanced nonsinusoidal 3-phase currents does not necessarily equalto zero. For example balanced square wave currents will result in significantneutral current.

In three-phase circuits the triplen harmonic neutral currents (3, 6,9etc) addinstead of cancel.

Computer loads are generally nonlinear loads. The currents of typicalbalanced currents contain triplen harmonics. The neutral currents are nolonger zero, even if the phase currents are balanced. The currents of typical

balanced line-to-line connected power supplies contain no triplen harmonics.However, the typical currents of balanced line to neutral connected powersupplies are very rich in triplens.

Recent trends in computer systems have increased the likelihood ofsignificant neutral currents. Single-phase power supplies are more in use

than three phase power supplies. SMPS units, which are more efficient andless expensive, are more in use. These when operated between Phase andneutral without a step down Transformer result in more triplen harmoniccurrents. The transformer used with power supplies reduces the triplenharmonic currents because of its series inductance and because it allowsconnection to the line-to-line voltages.

7/27/2019 Comp,Ups Application

2/3

POWER CONTROL EQUIMENTS DESIGN MANUAL

Another source of triplen harmonics is the usage of DC motors. These alsorequire nonlinear input currents for usages such as Fans, tape drives and discdrives. DC motors require power supplies with similar characteristics asother computer power supplies.

The table below shows the extent of Harmonic components in a typicalcomputer application.

Harmonic h Line to Line Power

supply

Line to Neutral Power

Supply

1 0.82 0.653 - 0.525 0.49 0.42

7 0.287 0.299 - 0.1311 0.074 0.1213 0.033 0.098

Three Phasecurrent= Ih2

1.00 1.00

Neutral current=3 *(I32+ I92 )

0.0 1.61

Ratio of Neutral current

to Total Phase current

0.0 1.61

As per the table for balanced line to neutral power supplies the neutralcurrents would be 1.61 times the phase currents. Under worst conditionswith rectifier conduction angles OF 60 the neutral current could be1.73 times the phase current.

Recommended design practices:

All 3 Phase 4 wire, power feeder or branch circuits for computer systemsshould be designed to accommodate neutral currents unto 1.73 times the

phase current. The wiring ampacities should be calculated considering theneutral to be current carrying conductor.

All neutral components, including neutral terminals and neutral bus bars,should be sized for additional neutral current. It may not be possible while

7/27/2019 Comp,Ups Application

3/3

POWER CONTROL EQUIMENTS DESIGN MANUAL

building the Panel to allow such large nonstandard values for the neutralelements. In such cases the maximum loading on the phases to be limited to

prevent overloading the neutral components. For example if a 500 ampsPanel is to be made with a 500 amps neutral accessory, assuming the worstcase of neutral current being 1.73 Phase current, the loading on the Panel

board to be limited to 500/1.73=300 amps (approx).

If this is not acceptable to the Purchaser then the entire neutral system has tobe designed for 1.73x 500=850 amps (approx).

Example:

To choose bus bars for a 500 KVA/415 V, UPS system:

Phase current: 700 amps,

Al Bus bar Size chosen: 50x10: CCC= 754 amps

Neutral Busbar to be designed for: 700x1.73 = 1210 Amps

Size to be chosen: 2x (50x10): CCC = 1203 Amps.