Electric power conversion systems – Frequency conversion without intermediate conversion to d.c. – By induction-type converter
Reexamination Certificate
2000-10-03
2002-03-12
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Frequency conversion without intermediate conversion to d.c.
By induction-type converter
C323S348000
Reexamination Certificate
active
06356472
ABSTRACT:
BACKGROUND
1. Field of Invention
This invention pertains to control of electrical power transmission, and particularly to transmission of power between electrical systems.
2. Related Art and Other Considerations
Some electrical transformers, for example tap-changing transformers such as variacs, merely vary voltage. Other transformers, known as stationary phase shifting transformers, can divert power and move power through a torque angle.
Mere voltage-varying transformers and stationary phase shifting transformers may be adequate for interconnecting two electrical systems operating at the same electrical frequency, or for transmission within a utility company. However, such transformers are incapable of interfacing two electrical systems operating a differing frequency (e.g, inter-utility transfers of electricity).
There exist a number of areas in the world where interconnections between power systems require an asynchronous link. For some of these areas the power systems have different nominal frequencies (e.g , 60 Hz and 50 Hz). Even for interconnections in other systems having the same nominal frequency, there is no practical means of establishing a synchronous link having enough strength to permit stable operation in an interconnected mode.
The prevailing technology for accomplishing an asynchronous interconnection between power systems is high voltage direct current (HVDC) conversion.
FIG. 8
is a one-line diagram schematically illustrating a prior art HVDC interconnection system
820
.
FIG. 8
shows interconnection system
820
connecting a first or supply system
822
(shown as AC Power System #1) and a second or receiver system
824
(shown as AC Power System #2). AC Power System #1 is connected to interconnection system
820
by lines
826
for supplying, in the illustrated example, a three-phase input signal of frequency F
1
(F
1
being the frequency of supply system
822
). Interconnection system
820
is connected by lines
828
to receiver system
824
, with lines
828
carrying a three-phase output signal of frequency F
2
from interconnection system
820
to receiver system
824
.
HVDC interconnection system
820
of
FIG. 8
includes a back-to-back DC link
830
situated between bus bars
832
and
834
. Bus bar
832
is connected to supply lines
826
and to reactive compensation bus
842
. Bus bar
834
is similarly connected to lines
828
and to reactive compensation bus
844
.
Each side of back-to-back DC link
830
includes two transformers (e.g., transformers YY and Y&Dgr; on the first system side; transformers YY and &Dgr;Y on the second system side) and a
12
pulse converter group. As illustrated in
FIG. 8
, the
12
pulse converter group for the first side of link
830
includes two six pulse converter groups
850
and
852
; the
12
pulse converter group for the second side of link
830
includes two six pulse converter groups
860
and
862
. As a three phase group is illustrated, each converter group includes six thyristors connected in a manner understood by the man skilled in the art. Smoothing filter
864
is connected between converter groups
850
and
860
.
Also shown in
FIG. 8
are reactive power supply systems
870
and
880
connected to reactive compensation buses
842
and
844
, respectively. Reactive power supply system
870
includes a shunt reactor
871
connected to bus
842
by switch
872
, as well as a plurality of filter branches
873
A,
873
B,
873
C connected to bus
842
by switches
874
A,
874
B, and
874
C, respectively. Similarly, reactive power supply system
880
includes a shunt reactor
881
connected to bus
844
by switch
882
, as well as a filter branches
883
A,
883
B,
883
C connected to bus
844
by switches
884
A,
884
B, and
884
C, respectively. Although three such filter branches
873
A-
873
C and
883
A-
883
C have been illustrated, it should be understood that a greater number of filter branches may reside in each reactive power supply system
870
,
880
.
For any given HVDC installation, reactive power supply systems such as systems
870
and
880
are difficult to design and are expensive. Moreover, there are a large number of switched elements that have to be carefully coordinated with a given power level. Various constraints are simultaneously imposed, such as keeping harmonic performance below a requisite level (i.e., harmonic performance index) and yet maintaining reactive power between limits, all the while essentially constantly switching the filters in systems
870
and
880
as power changes. Concerning such restraints, see (for example) Larsen and Miller, “Specification of AC Filters for HVDC Systems”, IEEE T&D Conference, New Orleans, April 1989.
Thus, HVDC is complicated due e.g., to the need to closely coordinate harmonic filtering, controls, and reactive compensation. Moreover, HVDC has performance limits when the AC power system on either side has low capacity compared to the HVDC power rating. Further, HVDC undesirably requires significant space, due to the large number of high-voltage switches and filter banks.
Prior art rotary converters utilize a two-step conversion, having both a fully-rated generator and a fully-rated motor on the same shaft. Rotary converters have been utilized to convert power from AC to DC or from DC to AC. However, such rotary converters do not convert directly from AC to AC at differing frequencies. Moreover, rotary converters run continuously at one predetermined speed (at hundreds or thousands of RPMs), acting as motors that actually run themselves. Prior art rotary converters accordingly cannot address the problem of interconnecting two electrical systems that are random walking in their differing frequency distributions.
In a totally different field of technical endeavor, the literature describes a differential “Selsyn”-type drive utilized for speed control of motors. See Puchstein, Llody, and Conrad,
Alternating
-
Current Machines
, 3rd Edition, John Wiley & Sons, Inc., New York, pp. 425-428, particularly FIG. 275 on page 428, and Kron,
Equivalent Circuits of Electric Machinery
, John Wiley & Sons, Inc., New York, pp. 150-163, particularly FIG. 9.5
a
on page 156. The literature cites the differential Selsyn drive only in the context of speed control of motors, i.e., motor speed control via relative speed adjustment between a motor and generator. Moreover, the differential Selsyn drive has a low bandwidth and makes no effort to dampen rotor oscillations.
SUMMARY
An electrical interconnection system comprises a rotary transformer and a control system. The control system adjusts an angular position of the rotary transformer so that measured power transferred from a first electrical system to a second electrical system matches an inputted order power. The rotary transformer comprises a rotor assembly and a stator, with the control system adjusting a time integral of rotor speed over time.
The control system includes a first control unit and a second control unit. The first control unit compares the input order power to the measured power to generate a requested angular velocity signal The second control unit compares the requested angular velocity signal to a measured angular velocity signal of the rotary transformer to generate a converter drive signal, thereby controlling the angular positioning of the rotor assembly relative to the stator.
The rotary transformer comprises a rotor connected to the first electrical system and a stator connected to the second electrical system. A torque control unit or actuator rotates the rotor in response to the drive signal generated by the control system.
The bandwidth of the control system is such to dampen oscillations (natural oscillations of the rotor including its reaction to the transmission network into which it is integrated). The bandwidth of the first (slow) control unit is chosen to be below the lowest natural mode of oscillation; the bandwidth of the second (fast) control unit is chosen to be above the highest natural mode of oscillation. As used herein, the bandwidth of a control unit o
Larsen Einar V.
Runkle Mark A.
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