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...

1999-12-11

2001-12-11

Sterrett, Jeffrey (Department: 2838)

Electricity: power supply or regulation systems

For reactive power control

Using converter

Reexamination Certificate

active

06329798

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. Each consumer expects that the electrical power delivered over the feeder distribution networks will be at a stable voltage, and electrical machinery used by the consumers is likewise designed to operate at stable voltages. The voltage at the consumer end of the feeder distribution networks, however, is generally not constants For example, as large loads arc placed on the system, such as due to the operation of heavy industrial machinery, the voltage at the load site may vary.
Voltage regulation devices are therefore often used to perform voltage regulation at load sites. These voltage regulation devices are often in the form of tap changers. Tap changers operate by providing a series of connections at slightly different voltages. As the voltage at the load site changes, a mechanical switch modifies a mechanical connection to contact a tap at either a higher or a lower voltage. Tap changers, however, are relatively costly, and arc subject to mechanical failure. In order to avoid excessive wear on the tap changers and to prolong the service life of tap changers, shunt capacitors may also be used to regulate load site voltage. Shunt capacitors, however, may also be costly and are also subject to failure.
It is also often desirable to reduce power supplied by the utility, both real and reactive power, by providing locally generated power. Accordingly, sometimes local power generation units are used at the load site to supplement power supplied by the utility, as well as to regulate voltage at the load site.
An example power regulator system is illustrated in FIG.
3
. In the system of
FIG. 3
, a local power source and associated inverter (indicated together)
351
are coupled to a transmission line at a load site. The power source and associated inverter provide power to the load. Coupled to the connection between the power source and associate inverter and the load is a filter including a capacitor (not shown). The local power source is therefore connected in parallel to the utility (not shown).
The power regulator system of
FIG. 3
includes a 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. 3
, a current vector 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 determined 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
363
. 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 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 to control inverter operation.
The control system of
FIG. 3
, as described above, is well known to those skilled in the art. Such a control system reduces real power required to be supplied by a utility, as well as providing voltage regulation at a load site. The system of
FIG. 3
, 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. 3
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. In one embodiment the invention comprises a power controller adapted for load site voltage regulation for a load site operating in parallel with a distant power source. The load site includes a load site power source. The power controller comprises a real power controller receiving an indication of real power supplied from the distant power source and a reference real power signal. The real power controller outputs a commanded real power signal. The power controller also comprises a reactive power controller receiving an indication of reactive power supplied from the distant power source and a reference reactive power signal. The reactive power controller outputs a commanded reactive power signal. Further, the power controller comprises a regulator controller receiving the commanded real power signal and the commanded reactive power signal, as well as an indication of power at the load. The regulator controller outputs a regulated control signal for use in load site voltage regulation.
In one embodiment the reactive power controller comprises a first controller section and a second controller section. The first controller section forming, a reactive power error signal, an indication of which is provided to the second controller section for use in forming the commanded reactive power signal.
In another embodiment, the invention comprises a method for regulating voltage at a load site connected to a utility power source over transmission lines. The load site includes a power generator unit operatively coupled to an inverter operatively coupled to a load by a path including a filter. In this embodiment the method comprises determining an indication of load voltage as well as determining an indication of commanded current provided by the utility power source to the load. The method further comprises determining an indication o

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