Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
2001-01-26
2004-04-20
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S018000, C361S100000, C361S115000
Reexamination Certificate
active
06724596
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to hysteretic current control of inverter circuits, and more particularly to a hysteretic current control method for an inverter using variable dead time between actuation of complimentary inverter switches.
BACKGROUND OF THE INVENTION
Increasingly, businesses, hospitals, utilities, and even consumers are relying on electronic and computerized equipment to conduct their daily activities. Indeed, as we progress through the new economy and the information age, the amount of reliance and the required sophistication of the electronic equipment used will only increase. Unfortunately, such increased use and sophistication of the electronic equipment brings an increased demand for reliable, quality electric power without which operations may be disrupted and critical data may be lost.
Despite the advances in the sophistication and availability of electronic and computerized equipment, the availability and reliability of high quality electric power in the quantities demanded by the growing economy has not kept pace. Indeed, while many utilities believe that rolling brown-outs provide an adequate solution to their inability to supply the electric power required by their customers, the impact that such brown-outs has on a business' productivity and profitability is, quite simply, unacceptable.
In addition to the utilities' inability to reliably supply the amount of electric power required, the quality of the power that is supplied often is so poor so as to affect the operation of the modem sophisticated electronic and computer equipment. Voltage sags and spikes are relatively common on the utility power lines, particularly during periods of factory shift changes in industrialized areas. Other power quality problems may be introduced by natural causes such as lightning induced voltage spikes, voltage droops caused by accidental contact with power distribution equipment by animals, tree limbs, etc. Oftentimes, these power quality perturbations have a more detrimental effect on the electronic and computerized equipment than complete power losses because the operating characteristics of the components of such equipment varies. That is, some portions of the electronic equipment may cease operating before other portions shut down, possibly resulting in erroneous operation, corrupted data, etc.
To overcome these problems, uninterruptible power supplies, line conditioners, and sophisticated power supplies have been developed. These devices and systems typically use electronic switching components to construct a regulated, high quality output voltage despite the perturbations of the input AC power from the utility. Indeed, the uninterruptible power supplies have the ability to continue to supply a high quality output voltage to the electronic and computerized equipment even during a complete loss of input AC electric power from the utility. Various well understood circuit arrangements and topologies provide this functionality.
One such well-known topology, known as a double conversion half-bridge inverter, is illustrated in simplified single line schematic form in FIG.
1
. The power inputs to this inverter topology typically include the AC utility voltage
10
and, in the case of an uninterruptible power supply, a battery or other electric power storage device
12
. Of course, one skilled in the art will recognize that either of these sources
10
,
12
may be removed as an input without an effect on the inverter's ability to generate an output AC voltage at its output terminals
14
. In operation, a positive and a negative voltage are developed on the positive rail
16
and the negative rail
18
of the inverter respectively. The positive
20
and negative
22
bus capacitors provide the energy storage for development of the output voltage waveform during operation of the inverter. This output voltage waveform is constructed by alternately opening and closing the electronic switching elements
24
,
26
as is well known in the art. Output filtering of the voltage waveform so constructed is accomplished through the output inductor
28
and filter capacitor
30
.
While there are many control and command methodologies that may be used with an inverter to construct an AC output voltage waveform, most methodologies use both voltage and current control loops to ensure stable, reliable performance for the equipment coupled thereto. As with the overall methodology, various methods are well known in the art for providing both the voltage and current control and regulation. One such current control method made popular by its ease of implementation and inherent stability is a variable frequency current control topology known as hysteretic control. In simplified functional block diagrammatic form, a controller using hysteretic current control is illustrated in FIG.
2
. As may be seen from this simplified block diagram, the basic inverter controller
32
receives the voltage feedback signal
34
from the inverter
36
and generates an output current command
38
to the hysteretic control block
40
. The hysteretic control block also receives a current feedback signal
42
from the inverter
36
. Within the hysteretic control
40
, the current command signal
38
creates a current control band having a maximum current limit
38
a
and a minimum current control limit
38
b
as illustrated in FIG.
3
. The actual current
42
supplied by the inverter
36
is monitored against these control limits
38
a
,
38
b
. The hysteretic control
40
then generates an output PWM signal
44
to the gate drive circuitry
46
to control the switching elements
24
,
26
(see
FIG. 1
) of the inverter
36
.
As will be recognized by those skilled in the art, the hysteretic current control provides a method of controlling the output current of the inverter where the instantaneous output current is allowed to vary within the current control band &Dgr;I defined by the maximum current limit
38
a
, and the minimum current limit
38
b
. This hysteretic control
40
operates to turn on the upper switching element
24
(see
FIG. 1
) when the current
42
is below the maximum current limit
38
a
. As the current
42
rises to this maximum limit
38
a
, the hysteretic control
40
operates to turn off the switching element
24
and to turn on the switching element
26
. This causes the instantaneous current
42
to fall. Once the current
42
reaches the minimum current limit
38
b
, the hysteretic control
40
operates to turn off the lower switching element
26
and to turn on the upper switching element
24
to again cause the instantaneous current
42
to rise. This alternative switching of the switching elements
24
,
26
continues to maintain the instantaneous current
42
within the upper and lower current control limits
38
a
,
38
b
in a known manner.
The time T
1
between these events defines the switching frequency of the hysteretic control. This time T
1
will typically vary as the output voltage rises and falls over the AC cycle. Near the peak of the AC output voltage cycle, the actual inverter current
42
will rise rapidly and fall very slowly, and therefore the frequency of switching will be greatly decreased. Near the zero cross of the AC outlet voltage waveform, the current will rise and fall relatively rapidly, therefore increasing the switching frequency commanded by the hysteretic control. A typical variation in the output switching frequency resulting from the hysteretic control
40
may be in the range of 10:1 in frequency when going from the zero cross to peak AC. Unfortunately, at the higher switching rates, the pure hysteretic control results in excessive heating of the inverter switches due to the inherent switching losses that occur each time the switching devices are transitioned. At the lower switching rates, excessive harmonic distortion may occur if the rate drops too low.
There exists, therefore, a need in the art to provide a control mechanism that allows the use of hysteretic current control, but that better
Jackson Stephen W.
Leydig Voit & Mayer Ltd
Powerware Corporation
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