The present invention relates to a plug-in neutral regulator and, more particularly, a plug-in neutral regulator for a three-phase 4-wire system to control only unbalanced power rather than using a fully rated inverter leg.
A pulse-width modulator (PWM) inverter is widely used to produce three-phase power from a direct current (DC) source to feed alternating current (AC) motors or AC loads. This inverter is a three-phase three-wire AC source to the load. For applications where both single phase and three phase loads are connected to the output of the inverter, a three-phase four-wire system is required, and a delta-wye transformer, an auto-transformer or zig-zag transformer is typically used to create the fourth line (i.e., neutral), as shown in
Referring
As can be seen, there is a need for an inverter/converter system that can produce a three-phase four-wire output without the need for a bulky four-leg inverter and without the need for separate module, such as a delta-wye transformer.
In one aspect of the present invention, a plug-in neutral module comprises a first input from a positive direct current (DC) bus; a second input from a negative DC bus; and a neutral line output from the plug-in neutral module.
In another aspect of the present invention, an inverter system comprises an inverter for converting a positive direct current (DC) voltage and a negative DC voltage to a three-phase three-wire alternating current (AC) voltage; and a neutral module connected to a positive DC bus and a negative DC bus, the neutral module providing a neutral line, thereby providing a three-phase four-wire AC current voltage to various loads.
In a further aspect of the present invention, a three-phase four-wire alternating current (AC) power supply system comprises a positive direct current (DC) bus; a negative DC bus; an inverter receiving a DC signal from the positive and negative DC busses, the inverter outputting three-phase three-wire AC power; a DC neutral point created by a mid-point of two split capacitors connected between the positive DC bus and the negative DC bus (split-capacitor for the DC bus of plug-in module is necessary, but optional for the three-phase inverter); and a separate, plug-in neutral module connected to the positive DC bus, the negative DC bus and the DC neutral point, wherein the neutral module maintains a voltage balance across each of the two split capacitors for an unbalanced load, and wherein the neutral module provides a neutral line to give a three-phase four-wire AC power supply.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.
The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.
Various inventive features are described below that can each be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above or may only address one of the problems discussed above. Further, one or more of the problems discussed above may not be fully addressed by any of the features described below.
Broadly, the present invention provides a neutral line regulator as a plug-in module that can be easily used as an upgrade to an existing retrofit system, providing capability to supply power to unbalanced/nonlinear loads. The neutral line regulator according to embodiments of the present invention may be designed for controlling only the unbalanced power rather than using a fully rated inverter leg. Since this plug-in module may be separate from the main inverter and may operate at a lower power, the switching frequency could be higher than the main inverter. Thus, the size and weight can be significantly reduced. In addition, the plug-in regulator of an embodiment of the present invention may maintain voltage balance between the center-tapped DC link capacitors for non-linear, unbalanced loads, as described in greater detail below. Moreover, the plug-in module according to embodiments of the present invention may be used as a retrofit module, replacing, for example, delta-wye transformers to reduce overall weight and volume. Because the neutral line regulator is a separate plug-in module, no coordination is needed between the plug-in module and the existing inverter and/or transformer. For a well-balanced load, the neutral line will not carry any fundamental current. The fourth leg is not required in this case. The neutral line is required only to regulate power for the unbalanced loads. Embodiments of the present invention may also be used to provide an effective solution to eliminate unbalanced DC capacitor voltages at the mid-point of the DC bus due to manufacturing tolerances, inconsistency in switching device characteristics or non-linear/unbalanced three-phase loads which are supplied with a three-wire AC system.
Referring to
Referring now to
For an unbalanced and/or a non-linear inverter load condition, the power delivered through the positive DC bus 14a and neutral-line 22 would not be the same as the power delivered through negative DC bus 14b and neutral-line 22 resulting in unbalanced capacitor voltages. Voltage across one capacitor 24 may drift toward the full DC bus voltage while the voltage across the other capacitor 26 may drop close to zero.
At the 100 millisecond (ms) time point in
Referring to
Referring to
For the purpose of illustration, referring now to
The voltage outer loop 42 may compare the voltages of the two capacitors 24, 26 (see
The current inner loop 44 may help ensure fast, dynamic and response and protection. Current inner loop 44 may generate the gating pattern for the switching devices ‘IGBTs’ (or switching device of similar configuration with bidirectional control of power) 34 and 36 of the plug in neutral regulator 30. This control may incorporate an inner current loop 44 to regulate the neutral terminal for the unbalanced load at desired power which is different from control described in prior art.
At about the 125 ms time point, the neutral regulator 30 may be enabled, as shown in the bottom graph 96. The switching IGBTs or switching device of similar configuration with bidirectional control of power 34, 36 may create an inductor current signal 56 as shown in the third graph 94. Within less than 5 ms, the capacitor voltages are close to the midpoint voltage as shown in graph 92. Moreover, the distortion in the output phase neutral voltages, as shown in graph 90, is dissipated.
Those skilled in the art would appreciate that the concepts described for the standalone plug-in module of an embodiment of the present invention are completely different from those described in the prior art. The scope of the present invention can be expanded to cover many other applications where a split DC link capacitor with similar configuration is required. For example, the concepts presented in the present invention could also be advantageously used not in a limiting sense for achieving:
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.