Delta Connections
Calculation of Capacitor Unbalance
(Article # GES4233)
By Neal S. Ciurro
The industrial market has become aware of the money that can be saved with the use of power-factor
correction capacitors and has given rise to more delta-connected capacitor banks. Customers showing concern for
identifying failures within the banks find that the biggest problem is in finding an easy method of determining the
current differences in unbalanced conditions so that relay and/or alarms can be used. This problem can be reduced
considerably with mathematics. First, consider the balanced delta connection of Figure 1. Using a simple and standard
connection, let the system be 7200 volts, delta, with 2400 kvar. This means 800 kvar per phase.
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| Figure 1 |
Converting kvar into microfarads, we have:
Using the parameters set up:
From here, the capacitive reactance can be determined.
Where:
V = phase to phase voltage
C = capacitance in microfarads
Xc = capacitive reactance
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Figure 2. Hypothetical circuit |
Based on the balanced load, we know the phase voltages are 120° apart. We also know that current
leads the voltage by 90° degrees. This is shown in Figure 3.
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| Figure 3. Phasor diagram of the circuit. |
From Figures 2 and 3, we have:
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