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Goals and Symptoms
An incorrect wiring connection or 3-phase load unbalance issue may cause resultant current flowing on neutral conductor. Apart from these possibilities, high 3rd harmonic current present on each phase may also cause high neutral current.
Causes and Fixes
Harmonics overload the neutral wiring
3-phase building wiring consists of 3 hot (or phase) conductors, a ground conductor, and a neutral conductor. Single phase loads are connected between the different hot conductors and the neutral conductor. Therefore, the neutral conductor serves as the "common" return for all of the single phase load currents. It is a property of three phase power systems that if each of the three hot conductors has a nearly equivalent load, that the neutral current will be nearly zero due to the fact that each phase current is "out of phase" with the other. In other words, the load currents "cancel out" in the neutral wire. In North America, sometimes the building wiring design takes advantage of the cancellation and the neutral wire is sized smaller than the hot wires. Unfortunately, the harmonic currents created by computers cause the operation of this system to change. Computers generate a substantial amount of 3rd harmonic current. Due to the mathematical phase properties, third harmonic currents add instead of cancel out on the neutral wire (see Fig.1). Therefore, in a building with a large number of Personal Computers installed, the neutral wire can carry much higher currents than it was designed for. In fact, the harmonic current alone in the neutral wire can be larger than the full rated current of the power wiring. This is the most critical problem relating to harmonics and PCs. Note that the data above shows that while it is unlikely for the neutral current to exceed the phase current, the neutral current can reach the phase current value in a PC environment. For this reason it is essential that neutral undersizing never be used in an office environment.
Fig.1 Phase relation between fundamental voltage (upper) and 3rd harmonic current (lower)
There are a number of approaches to avoiding harmonic problems. These include:
Specifying EQUIPMENT that does not create harmonics
Active harmonic filters: It uses power electronics to monitor the nonlinear load and dynamically correct every harmonic order. By injecting a compensating current into the load, the sine wave is restored, and distortion is reduced. The high speed process cancels high frequency output current, while it ultimately determines the precise value of injected load current. Active harmonic filter is designed to operate at levels that continuously adapt to rapid load fluctuations. With its efficient operation and small physical size, it is ideal for a wide variety of industrial and commercial environments.
(Removed Image URL: /PubResEXPORT.nsf/2b87ee90be777fc085257c28006ee4ef/66dd624e949b7303c12575c400108c98/fl_block_5/0.1AD6?OpenElement&FieldElemFormat=gif)Date Created: May 26, 2009
Last Revised: May 27, 2009
Original Author: TL
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