Voltage controller for minimizing the utility power provided...

Details Voltage controller for minimizing the utility power provided... Voltage controller for minimizing the utility power provided...

2000-08-02

2001-11-13

Sterrett, Jeffrey (Department: 2838)

Electricity: power supply or regulation systems

For reactive power control

Using converter

Reexamination Certificate

active

06316920

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to voltage controllers for power systems, and more particularly to a power system voltage controller used at a load-site.
Electrical power generation is often performed at generation sites distant from the consumers of electrical power. The electrical power is transmitted from the generation sites to the consumers by feeder distribution networks. Local electrical generator units are sometimes used at load sites to augment utility supplied electrical power. Local electrical generators may be used to provide electrical power at a cost cheaper than that supplied by the utility. Large consumers of electricity, such as manufacturing plants and the like, sometimes find it economical to produce some or all of the electrical power they require from local electrical generators, such as micro turbines, rather than to purchase the power from a utility.
For ideal economic local generation of power, the local electrical generator operates at capacity, and all of the power generated by the local electrical generator is used by the load. Further, it is desirable to limit the real power supplied by the utility in some locations to zero. In addition, the reactive capability of the local micro turbine power generation system can be used to control the local voltage, especially at the end of high impedance feeders.
Control of the inverter to achieve this is difficult, however. Due to changes in the load the requirements placed on the inverter to operate micro turbines and maintain efficiency may change dynamically, both as to real and reactive power requirements.
A simplified single line diagram of a local electrical generator coupled to a load in parallel with a feeder distribution system is illustrated in FIG.
1
. Although
FIG. 1
includes features of the present invention,
FIG. 1
is also useful in describing the background of the present invention. As illustrated in
FIG. 1
, a utility
11
is connected via a source impedance
13
to a local power generation and distribution system
15
. The local power generation and distribution system comprises a local energy source
19
, whose output power is modified to be compatible with the utility voltage and frequency by an inverter
21
, coupled to a load
17
. The inverter is, for example, a pulse width modulated (PWM) inverter.
An output filter
23
reduces the harmonic content produced by the inverter. The filter is connected to the load and utility by an additional impedance
25
of the distribution system. The inverter is controlled by a control system
27
that senses voltages and current and regulates the inverter to perform required functioning.
An example power regulator system is illustrated in FIG.
2
. In the system of
FIG. 2
, a local power source and associated inverter (indicated together)
351
are coupled to a transmission line
325
at a load site. The power source and associated inverter provide power to a load
327
. Coupled to the connection between the power source and associate inverter and the load is a filter including a capacitor
329
. The local power source is therefore connected in parallel to the utility.
The power regulator system of
FIG. 2
includes an inverter current regulator (
311
and
323
). The current regulator provides a signal to the local power source and associated inverter for use in the control of the power source and associated inverter. In the system illustrated in
FIG. 2
, an inverter current output vector i
inv
of the inverter is regulated to a desired value.
The current regulator is a vector control system based upon a Park-vector, or space-vector, representation of all three-phase electrical quantities. The use of Park-vectors facilitate transformation of control signals from sinusoidal values in a stationary frame to largely DC level signals in a synchronous frame. Methods of transforming signals from one reference frame to another is well known by those familiar with the art. Park vectors are described in, for example, Transient Phenomena in Electrical Machines by P.K. Kovacs, published by Elsevier (1984), the disclosure of which is incorporated herein by reference.
Accordingly, the inverter current output vector i
inv
is determined. As the inverter current output vector i
inv
is measured in the stationary reference frame, a capacitor voltage vector v
cap
is also determined for use in transforming the inverter current output vector to the synchronous frame. In order to reduce ac signal components in the synchronous frame signal, the capacitor voltage is filtered to reduce harmonics and other noise at frequencies other than those about the fundamental system frequency. Therefore, a rotational reference frame is extracted from the filtered capacitor voltage vector to form a unit vector for transformation to the synchronous frame in an extraction unit
331
. The unit vector is provided to a transformation unit
332
, as is the inverter current output vector i
inv
. The transformation unit
332
outputs a vector i
k
, which is comprised of essentially DC signals of a real component and a reactive component, representing the inverter current vector in the synchronous frame. The vector i
k
, therefore, is the inverter current output vector in the synchronous frame.
The vector i
k
is compared with a command reference vector i
ikcmd
at a summer
323
. Generally the command reference signal i
ikcmd
is empirically determined, and is changed only infrequently. As it is often desirable to provide as much real power from a local power source generator to the load as possible, the real power component is generally set to a maximum, which is a value of one power unit (p.u.) in a normalized system. The reactive component of the command reference signal i
ikcmd
is generally set to 0.
The output of the summer
323
is provided to a controller
311
. The controller
311
, in the prior art, amplifies the output of the summer, and provides a voltage vector command v
ik
in the synchronous frame. The voltage vector command provided by the controller is transformed to the stationary frame by a transformation unit
333
, again based upon a unit vector provided by the extraction unit
331
. The output
313
of the transformation unit is provided to the local power source and associated inverter
351
to control inverter operation.
The control system of
FIG. 2
, as described above, is well known to those skilled in the art. Such a control system supplies constant real power to the utility. The system of
FIG. 2
, however, does not optimize provision of reactive power to the system, and does not adaptively modify local power supply output based on changes in real power requirements. Further, in the system of
FIG. 2
the filter may introduce unwanted power variation, particularly about resonant frequencies of the capacitor.
SUMMARY OF THE INVENTION
The present invention provides a load site voltage regulation control system. The load site regulation control system uses the real power generator capabilities of a micro turbine system to minimize utility power supplied and uses any excess KVA generator capability to reduce the reactive power supplied by the utility to the load.
Further, the described control system acts in a preferential manner to maximize real power generation by the micro turbine generator system, then using excess reactive power generation capabilities to reduce utility reactive power supplied. Thus, the micro turbine generation system is operated approximate its maximum efficiency.
Many of the attendant features of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings in which like reference numerals indicate like parts throughout.


REFERENCES:
patent: 4845418 (1989-07-01), Conner
patent: 5041959 (1991-08-01), Walker
patent: 5329221 (1994-07-01), Schavder
patent: 5329222 (1994-07-01), Gyugyi et al.
patent: 5343139 (1994-08-01), Gyugyi et al.
patent: 5428283 (1995-06-01), Kalman et al.

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