Voltage Regulators
Cooper Power Systems
Reference Information
Connecting Single-Phase Voltage
Regulators for Parallel Operation
R225-60-9
In any connection of parallel devices there is the possibility of circulating currents that may develop. Due to the nature of
voltage regulators and the changing voltage at their output terminals, there is a voltage difference between the two units
connected in parallel. The method described below is designed to limit the circulating the current to avoid overloading of
the equipment, voltage regulators and transformers, connected in the circulating loop.
Negative Reactance Method:
The current that will circulate in the loop will be largely reactive current. By using the reactive line drop compensation
circuit of the control, the control will respond to a raising circulating current to either raise or lower its tap position to limit
the circulating current. This method has been described in greater detail in other papers in the industry for many years.
The limitations to this method are that the loop impedance must be relatively high, a minimum of 20%. This loop impedance is the combination of the equipment in the loop, which will likely include two transformers. So if each transformer has
impedance of 10%, this method will work properly. Too little impedance will result in higher than acceptable loop current
and possibly overload the equipment in the loop. The use of reactors may be used to increase the loop impedance.
Another limitation to this method is that since the line drop compensation is being used to limit the circulating current, only
the resistive component is available for the control to maintain correct voltage at the load center. If the resistive component
is to be used while the negative reactance method of paralleling is used, the resistive settings of all of the regulators being
paralleled should be set to the same value. The advantages to this method are that there is no interconnection between
the regulators so they can be located relatively far apart. Also more than two units can be paralleled together, if required
using this method.
To set the current controls offered by Cooper Power Systems, the system connection configuration does not change. So if
the units are connected in wye (star), function code 41 is set to 0. To configure the control for the negative reactance, set
function code 5 to some large negative number. The control allows a setting of up to negative 24 volts.
November 1999 • Supersedes August 1997
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Cooper Power Systems, Inc.