Open-Delta Systems & VFDs
To understand the effects that this configuration will have on the VFD, end users should begin by looking at the basic schematic diagrams of the two systems. Figure 1 shows a balanced and unbalanced system. This is not the only possible configuration of either a balanced or unbalanced system. Figure 1 simply shows two common configurations found in general service. The open delta configuration shown in Figure 1 also has two different size transformers, with the large 50-kilovolt-ampere (kVA) unit being center-tapped to ground so that it can be used for 240-volt, single-phase loads.
The voltage measurement to ground being unequal is not an indication of an unbalanced system because a center-tapped, closed delta would also generate these levels and still represent balanced impedance phase to phase. Unbalanced line-to-line systems will create unbalanced voltage and current phase to phase that is capable of producing excessive heating in standard three-phase motors run across the line. However, when a VFD is placed on this supply, the effects of the imbalance will cause the diode bridge to draw current unevenly from the three legs.
Figure 1. Balanced (top) and unbalanced (bottom) systems
Because the VFD rectifies the incoming supply and inverts it to the motor, it will effectively isolate the motor from the imbalance. The same imbalance that can cause heating in the motor is now transferred to the VFD. This can cause excessive heating of the conductors and any switch gear in front of the drive. It can also cause excessive heating in the diode bridge, as the diodes on the high leg will switch on first and stay on longer than the other legs.
The diode bridge of the VFD will draw current from the utility in pulses and not follow the incoming voltage sine wave, which is the cause of harmonic distortion that gets reflected back to the utility. This is the normal function of the diode bridge. However, on an unbalanced, three-phase input, not only will the diode bridge draw current in pulses, the pulses will not be balanced line to line. This will cause uneven heating of the diode bridge as well as all conductors carrying current to the VFD.
Figure 2 displays the actual current traces of an open-delta system such as the one shown in Figure 1. The current shown in Figure 2 supplies a VFD running a submersible pump for irrigation water. In the top part, all the current traces are on the same zero reference line, and seeing the imbalance of the current pulses is easy. In the bottom part, the same traces are separated so that each one can be seen separately, which allows users to see the imbalance not only phase to phase but also pulse to pulse within the same phase. While the “rabbit ear” $1 is common for a six-pulse diode bridge, the imbalance in current draw is not.
Figure 2. Current traces of an open-delta system running a submersible pump for irrigation
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.