Electrical transmission or interconnection systems – Plural supply circuits or sources
Reexamination Certificate
2002-08-01
2003-11-11
Toatley, Jr., Gregory J. (Department: 2836)
Electrical transmission or interconnection systems
Plural supply circuits or sources
C307S009100, C307S071000, C191S004000
Reexamination Certificate
active
06646360
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a system, method and apparatus for connecting a low-voltage high-current DC source (P
L
) in series with a high voltage low-current DC source (P
D
) when additional power is required by the load to operate at maximum power (P
d,max
). The invention enables the series connection to be made under full power by using the second source P
L
to commutate the load current and allow the first source P
D
to be reconfigured from series to parallel operation, doubling the current rating of the first source P
D
.
In a preferred embodiment, the invention doubles the current rating of the low-current high-voltage source P
D
by separating it into two series sources and reconfiguring the sources to operate in parallel. The output voltage of P
D
in parallel mode is only half its series value but this is compensated for by the addition of the series connected high-current low-voltage source P
L
. One feature of this invention is that it enables the second source P
L
to be connected in series with the first source P
D
seamlessly while under full power and without appreciably dropping the flow of power to the load or raising the voltage of the load. Further, this transition to a series connection can be done while operating within the rating constraints of the first and second sources P
D
, P
L
such that the power transfer is transparent to the load. As such, no anomalies in power (current or voltage interruptions, spikes, oscillations, drop-off, perturbations, etc.) are sensed by the load. Anomalies are avoided by using the second source P
L
to naturally commutate the load current. The resulting circuit topology allows the load current to be increased to its limit with both sources contributing power at their rated limits and the load power to be increased to its maximum, P
d,max
, where P
d,max
=P
D
+P
L
. A further feature of this invention is the control strategy used to control the source P
D
and effect the transitions between modes by means of switches (S
1
, S
2
, S
3
) connected to the outputs of rectifiers
1
and
2
.
BACKGROUND OF THE INVENTION
A Problem Solved by this Invention
FIG. 1
illustrates a conventional diesel operation system
100
. In system
100
, an electrical load
105
is supplied by a diesel engine
110
driving a three phase, single-winding alternator
120
connected to a diode rectifier
130
. The alternator/rectifier combination
120
and
130
is constrained by current and voltage ratings based on the rated engine power at full rpm. In this case, the load is generally operated at constant DC voltage with power varying in proportion to the DC input current. In some cases, the DC voltage may be reduced when the load requires lower voltage. In addition, the alternator may consist of two windings with two rectifiers and means to effect series or parallel connections of these rectifiers in a manner known to a person familiar with the technology, allowing the alternator and rectifier to be smaller for a given load. This technique is not suitable for the problem at hand.
It is desirable to operate the load at higher power by connecting a second high-current power source in series with the output of the rectifier. The second source, which is constrained at about half the rated load voltage, must be connected in series while the load is operating at full diesel power and, furthermore, must not appreciably raise the load voltage.
The problem, therefore, is to find an economically viable circuit topology and control strategy to connect a low-voltage high-current power source P
L
in series with a high-voltage low-current power source P
D
that is operating an electrical load or loads. The circuit must double the current rating of P
D
, not increase the load voltage, and allow for smooth connection of the second source P
L
at times when increased load current is required for higher power operation.
Prior Approaches to Supply More Current to the Load at Rated Voltage
FIG. 2
illustrates a conventional parallel line connection system
200
. In system
200
, a diesel engine
210
drives an alternator
220
connected to a diode rectifier
230
. Additional current is available to operate a load
205
at higher power if an external DC source
215
is connected in parallel with the output of the rectifier
230
. When the voltage of external source
215
is greater than the output of rectifier
230
, the rectifier diodes become reverse biased, the load current transfers from alternator
220
to external source
215
and, as a result, the alternator/rectifier current decreases to zero. This type of parallel line connection allows an external source to supply the additional current and power at the rated voltage of the load. The connection can be made while the load is operating at full diesel power but the voltage of the external source must equal the required voltage of the load. Details of effecting smooth connect and disconnect transitions are familiar to those with knowledge of the art.
FIG. 3
illustrates a conventional series line connection system
300
. In system
300
, a diesel engine
310
drives an alternator
320
connected to a diode rectifier
330
. If the voltage of an external source
315
is lower than the rated load voltage, source
315
can be connected in series with rectifier
330
to provide additional power while maintaining the required load voltage. In this type of series line connection, alternator
320
is operated at a reduced voltage so that the resulting load voltage remains at its rated valued. Rectifier
330
and external source
315
each carry the full load current but they contribute power proportional to their respective voltages. This mode is also referred to herein as Diesel Boost Operation since the voltage of external power source
315
is boosted by the diesel/alternator/rectifier combination to supply the load at its rated voltage and with higher power.
Disadvantages of These Approaches
The use of a parallel line connection as in system
200
described with reference to
FIG. 2
is limited to cases where the voltage of the external source is within the allowable operating voltage of the load.
The use of a series line connection as in system
300
described with reference to
FIG. 3
has two serious problems. The first is that the alternator and rectifier must be oversized to handle the increased load current even though they operate at less than rated voltage while in series mode. For example, if the load power is doubled during Diesel Boost Operation and the external source supplies half the load voltage, then the alternator and rectifier must carry twice their rated current at half their rated voltage. While the alternator's output power remains essentially the same as in system
100
described with reference to
FIG. 1
, the losses due to the high currents are prohibitive and this mode of operation is only possible for a very short time. It may be noted that prior systems typically use two windings and two rectifiers with series and parallel operating modes to avoid oversizing the components. But prior systems do not seek to provide more power by means of one (or more) additional sources connected in series.
The second problem is that there is considerable difficulty in switching from Diesel Operation (system
100
) to Series Line Operation (system
300
) without shutting off or disrupting uniform power to the load. The required load transfer must be rapid and not cause any abrupt change in load current or voltage. This requires the complicated steps of transferring the load current over a temporary commutation path, interrupting the rectifier (and alternator) output current, reducing the rectifier (and alternator) output voltage, connecting the external voltage source in series with the rectifier output, and then increasing the current to its former level to complete the commutation process. Only after this transition is complete can the current be increased to provide the desired higher load power. There is no economical method to known in prior systems
Rios Roberto J.
Siemens Energy & Automation
Toatley , Jr. Gregory J.
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