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Severe line-current unbalance can cause VFD nuisance tripping and 3rd harmonic currents, which are unusual for phase-to-phase nonlinear loads.

Are you having problems with a "temperamental" variable frequency drive (VFD)? Is it nuisance-tripping, showing all the signs of circuit overload, even though measurements show otherwise? The culprit may be unbalanced phase currents.

What happens is the overload protection trips when the VFD line currents become very unbalanced because there's excessive current in one or two phases. This happens even though the average current of the three phases is well below the VFD's current rating.

According to a paper from the Electric Power Research Institute's (EPRI's) Power Electronics Application Center (PEAC) (Application No. 1 dated September 1995), line-voltage unbalances usually cause these line-current unbalances. In fact, current unbalance can be 20 times as high as the voltage unbalance. Unevenly distributed single-phase loads can cause the latter across a 3-phase power system. In other words, instead of balancing phase-to-neutral loads evenly across all three phases, more are distributed onto one or two phases. The same goes for phase-to-phase single-phase loads. Instead of balancing the loads on an A-B, B-C, A-C basis, they are distributed unevenly on two pair of phases. Voltage unbalance can also result when using some types of transformer connections such as the "open-delta" connection, where two transformers are used to make a three-phase system.

There are a couple of symptoms to look for The first symptom in most cases is nuisance tripping of upstream breakers due to unbalance current. Many new digital motor control protection relays use a current unbalance trip, and in some cases, the trip limit can be set as low as 5%. The line-current unbalance also increases current harmonic distortion, which can overload building wiring and transformers. You may see excessive 3rd harmonics (normally associated with phase-to-neutral nonlinear loads) as one of the resulting consequences of line-current imbalance on a VFD (a phase-to-phase nonlinear load). Another may be low power factor.

Of course, the "robustness" of the unbalanced power system serving the drive affects the amount of unbalance. According to PEAC, if the system is very strong and has a large transformer and high available fault current, the unbalance will be greater than with a weaker system.

Table 1 on page 16 lists line-to-line voltages (first three columns), calculated voltage unbalance (fourth column), line currents (next three columns), and calculated current unba- lance (last column) for a typical 5-hp VFD for voltage unbalance conditions.

Table 2 on page 16 shows the current waveforms for the same unbalanced conditions noted in Table 1. As you can see, the current of one or two phases changes, as the current unbalance increases, from a double-pulse waveform, which is characteristic of a VFD, to a single-pulse waveform.

The 3rd harmonic component also increases as the current unbalance increases, which is unusual for phase-to-phase non-linear load.

So, how do you go about reducing this problem? First, you measure the VFD's line current during normal operation, just to verify if, in fact, there's an overcurrent in any phase. If the measured value on any phase is not greater than the VFD rated line current and the reason for tripping is due to an upstream current unbalance relay, then increase the trip setting of the relay to a maximum of 30%. One of the benefits of VFDs is that the line side voltage unbalance or the current unbalance does not reflect on the motor side; therefore, you can relax the unbalance trip setting without any concern for damage to the motor connected to the VFD.

Second, if there is a significantly higher phase current (higher than expected for the VFD load), then calculate the amount of voltage unbalance at the VFD power panel. (According to PEAC, greater than 2A per hp load or greater than 15% unbalance is beyond normal expectations for VFD.)

Third, if the voltage is unbalanced by more than 2%, then try rebalancing the line voltages by redistributing as evenly as possible all single-phase loads across all three phases, or relocating unbalanced loads to different power panels, or correcting all overloads within the building.

Let's try a sample problem Let's say you measure VsubAB at 448V, VsubBC at 465V, and VsubAC at 450V. Next, you do some simple arithmetic calculations and insert the results into the following equation: %Unbalance = ((maximum deviation from average) divided by (average of all 3 phase-to-phase voltages)) times 100.

Obviously, the average voltage is the sum of all three voltages divided by 3, which, in this case, is 454V. The maximum deviation is 465V minus 454V, or 11V. Plugging these values into the above equation yields 2.4%. Based on this, you need to do some single-phase load relocation work.

What happens if load relocation doesn't work? You may have to install an AC line reactor at the problem VFD. It will reduce line currents and harmonic distortion as well. You should size the reactor to carry the VFD's full load current, and it should have an impedance rating anywhere from 2% to 6%. (These ratings describe the expected rms voltage drop across the reactor at rated current.)

Choosing the best rating between 2% and 6% can be a problem. If you choose a higher rating, the higher voltage drop will give you the greatest amount of current balance but you'll sacrifice the responsiveness of your VFD. PEAC suggests starting with a 3% impedance.

There are other benefits to ridding your system of line-current unbalance. According to PEAC, balancing 3-phase voltages can reduce system losses and prevent costly shutdowns. Besides reducing current unbalance and harmonic distortion, adding reactors also can increase power factor when unbalanced 3-phase voltages persist. And, they help protect VFDs from motor-starting and capacitor-switching transients.

Suggested Reading Practical Guide to Quality Power for Sensitive Equipment, Second Edition. Order #6670; Electronic Drives, Order #6113. To order, call 1-800-543-7771.