In Figure 2, Phase B draws the most current and highest peaks for each cycle, followed by Phase C and Phase A. This means that Phase B will carry a higher percentage of the load and consequently be subject to greater heating.
This “rabbit ear” $1 also displays a condition known as zero conduction. This means that, between the two pulses on a phase, a point occurs when no current is being conducted. The $1 is clearly not sinusoidal, and the variance of this current waveform to the sinusoidal voltage waveform is called harmonic distortion. Since this is reflected to the utility grid back through the transformers, it can cause disruption to other devices connected to the same feed. This harmonic distortion also represents inefficiency in power delivery, which forces the utility to push more power down the line to make up for the losses caused by the inefficiency.
This is why utility companies require the conduction of a harmonic study prior to the installation of large VFD systems (generally greater than 50 horsepower), which will represent a high percentage of the load on the supply. Smaller VFD installations may not require a study or mitigation because the distortion produced will be a small percentage of the overall load on the system.
Most mitigation techniques are designed for either a balanced, three-phase or a single-phase supply. Even though the open delta is unbalanced, it still draws current on all three phases, so it does not behave like a single-phase input.
One technique to mitigate or reduce harmonic distortion is to add impedance somewhere in the circuit to slow the rise and fall of the current. Two common types of devices are input line reactors or DC link chokes. A line reactor is a set of three coils, one for each phase, placed in front of the VFD’s diode bridge. The DC link choke is placed in the DC circuit of the drive just after the diode bridge. In the system represented in Figure 2, a DC link choke was available to be placed in the circuit so the effect on current distortion could be observed.
In Figure 3, the voltage waveform for each phase has been added and the time per division increased. This allows users to see the current pulse relationship to the voltage sine wave. Even though current draw is not sinusoidal, the current pulses phase to phase are much closer to equal. Phase A and Phase B have eliminated the zero conduction time, but it is still present during Phase C. Overall, the voltage waveform in Figure 3 looks clean.
Figure 3. Figure 2's voltage waveforms after a DC link choke was placed in the circuit
The main reason for installing an open-delta supply is to reduce the cost of power installation. It is always preferable to have a supply with balanced impedance phase to phase whether the intended load is a VFD or an across-the-line motor.
To ensure a long VFD service life, understanding the type of supply power being fed to the VFD and taking appropriate countermeasures are important.
While this case study is too small to draw broad conclusions about all open-delta installations, in this particular topology, the effects of the unbalanced impedance phase to phase on the VFD are seen. The DC link choke still has a positive effect in mitigating harmonics on this open-delta system and additionally benefit the VFD by bringing the load phase to phase closer to balance.
End users, installers and salespeople should be aware of the power supply topology before selecting any equipment to be supplied power on an open-delta supply. The imbalance may require that a VFD or across-the-line motor be derated to handle the phase-to-phase imbalance. Consult the manufacturer for recommendations.