Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-02-18
2001-03-13
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S131000
Reexamination Certificate
active
06201720
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to power conversion apparatus and methods, and more particularly, to apparatus and methods for producing polyphase alternating current (AC) power from a direct current (DC) source in apparatus such as uninterruptible power supplies (UPS). UPSs are power conversion devices that are commonly used to provide conditioned, reliable power for computer networks, telecommunications networks, medical equipment and the like. UPSs are widely used with computers and telecommunications devices, including but not limited to personal computers, workstations, mini computers, network servers, routers, switches, disk arrays and mainframe computers, to insure that valuable data is not lost and that the device can continue to operate notwithstanding temporary loss of an AC utility source. UPSs typically provide power to such electronic equipment from a secondary source, such as a battery, in the event that a primary alternating current (AC) utility source drops out (blackout) or fails to provide a proper voltage (brownout).
Referring to
FIG. 1
, a conventional pulse width modulated (PWM) polyphase (e.g., three-phase) UPS generates a polyphase AC output at a load
20
from a DC voltage V
DC
generated between positive and negative voltage rails by a DC voltage generating circuit
10
. Depending on the configuration of the UPS, the DC voltage V
DC
may be generated in a number of different ways. For example, the DC voltage V
DC
may be directly generated by battery or, as shown, by DC/DC conversion of a lower battery voltage V
B
generated by a battery
12
. In some configurations, the DC voltage V
DC
may be generated from an AC source, or from a combination of an AC source and a DC source. The DC voltage V
DC
is inverted by a polyphase PWM inverter circuit
30
, which typically includes a plurality of switches (e.g., transistors) that are selectively operated to synthesize AC voltage waveforms from the DC voltage V
DC
. Additional filtering may be provided to reduce harmonics generated by the switching operations.
In order to improve control of such PWM inverter circuits, so-called “space vector modulation” (SVM) techniques have recently been introduced. SVM techniques are commonly used in polyphase induction motor control applications, and typically involve transforming a “reference” voltage vector to a vector in a “d-q” or “space vector” coordinate domain, which is in turn used to generate a PWM switching pattern. In a closed loop motor control application, for example, the reference voltage may be a control voltage derived from an error voltage generated by a speed or voltage control loop. The vector produced by the space vector (d-q) transformation is mapped to various logical values of switching signals applied to the switching devices of a PWM inverter circuit based on its orientation in the d-q plane. A variety of mappings may be utilized, as described, for example, in the article by Hava et al. entitled “Carrier-Based PWM-VSI Overmodulation Strategies: Analysis, Comparison and Design,”
IEEE Trans. on Power Electronics
, Vol. 13, No. 4, pp. 674-689 (July 1998).
UPS's are often used to serve unbalanced polyphase loads, particularly in large-scale operations in which a single high-capacity UPS may be used to provide power to a variety of single-phase loads connected to individual phases. Although loads may be configured with a view toward minimizing phase imbalance, practical constraints in load location and use often result in significant load imbalances. Accordingly, it is generally desirable to compensate for imbalances in the loads presented to the phases of a UPS. Unfortunately, however, conventional polyphase UPS control schemes often provide poor regulation of individual phases.
SUMMARY OF THE INVENTION
According to embodiments of the present invention, UPSs, power conversion apparatus and power conversion methods are provided which may improve phase regulation of polyphase AC voltages in the presence of load imbalances. According to one embodiment of the present invention, a UPS includes a DC voltage generating circuit that produces a DC voltage. A space vector domain controlled inverter couples the DC voltage generating circuit to an AC load, and produces a polyphase AC output voltage at the load by sensing a plurality of voltages at the load, transforming the plurality of sensed voltages according to a space vector (d-q) transformation to produce an output space vector, generating an error space vector from the output space vector and a reference space vector, and selectively coupling the DC voltage generating circuit to the load responsive to the error space vector. In this manner, a voltage control loop is closed in the d-q domain, which may improve per-phase regulation of the polyphase AC output. Preferably, compensation is applied to the error space vector to produce a command space vector that compensates for poles or other transfer characteristics of the forward path of the voltage control loop. In an embodiment of the present invention, the reference space vector is generated by an outer voltage amplitude loop that compares a sum of the plurality of output voltages at the load to a reference voltage amplitude, generating an error amplitude voltage which is compensated and then transformed according to a space vector transformation to produce the reference space vector that serves as the set point for the inner, space vector domain control loop.
In particular, according to one embodiment of the present invention, an uninterruptible power supply (UPS) for supplying a polyphase AC output at a load includes a DC voltage generator that produces a DC voltage. A voltage sensor is configured to connect to the load and detects a plurality of output voltages at the load. A space vector domain controlled inverter is coupled to the voltage sensor and to the DC voltage generator and configured to connect to the load. The space vector domain controller inverter transforms the detected plurality of output voltages according to a space vector transformation to generate an output space vector corresponding to the detected plurality of output voltages. The space vector domain controlled inverter generates an error space vector from the output space vector and a reference space vector, and selectively couples the DC voltage generator to the load responsive to the error space vector.
In another embodiment of the present invention, the space vector domain controlled inverter comprises a space vector domain controller that transforms the detected plurality of output voltages according to a space vector transformation to estimate an output space vector corresponding to the detected plurality of output voltages. The space vector domain controller generates an error space vector from the output space vector and a reference space vector, and that generates a switching command signal responsive to the error space vector. A polyphase inverter circuit is coupled to the space vector domain controller and to the DC voltage generator and configured to connect to the load, and selectively couples the DC voltage generator to the load responsive to the generated switching command signal. Preferably, the space vector domain controller compensates the error space vector to generate a command space vector and generates the switching command signal responsive to the command space vector.
An output filter may be configured to couple the polyphase inverter circuit to the load. In one embodiment of the present invention, the output filter includes a transformer having first winding connected to the polyphase inverter circuit and a second winding configured to connect to the load. The voltage sensor detects a plurality of voltages produced at the second winding of the transformer. The space vector domain controller transforms the detected plurality of output voltages produced at the second winding of the transformer according to a space vector transformation that compensates for at least one of a phase and magnitude transformation imposed by the transf
Akbari Edward
Chavez Miguel E.
Tassitino, Jr. Frederick
Tracy John G.
Berhane Adolf Deneke
Myers Bigel & Sibley & Sajovec
Powerware Corporation
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