COMPENSATION FOR POWER TRANSFER
SYSTEMS USING ROTARY TRANSFORMER
This application is related to simultaneously-filed United States
Patent Application Serial No. 08/ , (attorney docket 17GE-5721)
entitled "POWER FLOW CONTROL AND POWER RECOVERY WITH
ROTARY TRANSFORMER", and is a continuation-in-part of United
States Patent Application Serial No. 08/814,374 filed March 11, 1997 by
Larsen et al. and entitled "POWER FLOW CONTROL WITH ROTARY
TRANSFORMERS", which in turn is a continuation-in-part of abandoned
United States Patent Application Serial No. 08/550,941 filed October 31,
1995 by Runkle et al. and entitled " INTERCONNECTION SYSTEM FOR
TRANSFERRING POWER BETWEEN ELECTRICAL SYSTEMS ",
which in turn is a continuation-in-part of abandoned United States Patent
Application Serial No. 08/426,201 filed April 21, 1995 by Mark A. Runkle
and entitled "INTERCONNECTION SYSTEM FOR ELECTRICAL
SYSTEMS HAVING DIFFERING ELECTRICAL CHARACTERISTIC",
all of which are incorporated herein by reference.

TECHNICAL FIELD
This invention pertains to the transmission of power between
electrical systems or areas, and particularly to power flow apparatus and
method for controlling the transmission of power.
BACKGROUND
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, 60Hz and
50 Hz). The prevailing technology for accomplishing an asynchronous
interconnection between power systems is high voltage direct current
(HVDC) conversion. HVDC conversion 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.
Rauhut has disclosed a rotary transformer for coupling multi-phase
systems having a small frequency difference. See, for example, United
States Patent 3,471,708 to Rauhut wherein a non-synchronous rotary
machine has stator windings connected to a first three-phase power system
grid and rotor windings connected to a second three-power system grid. If
the frequency of one system is different from that of the second system, a
torque is exerted on the rotor in one direction or the other so as to cause
rotation of the rotor at a rotational rate equal to the different between the
network frequencies.
A closed loop angular positioning control system which operates a
rotary transformer for transferring power from a first electrical system to a
second electrical system is disclosed in United States Patent Application
Serial No. 08/825,502 filed March 31, 1997 by Runkle et al. entitled "
INTERCONNECTION SYSTEM FOR TRANSFERRING POWER
BETWEEN ELECTRICAL SYSTEMS", which is incorporated herein by
reference. Further, a power flow controller including two rotary
transformers connected together and controlled to provide an effective
phase shift and voltage magnitude ratio between two electrical areas is
disclosed in United States Patent Application Serial No. 08/814,374 filed
March 11, 1997 by Larsen et al. entitled "POWER FLOW CONTROL
WITH ROTARY TRANSFORMERS", also incorporated herein by

reference.
DISCLOSURE OF THE INVENTION
In an interconnection system for transferring power between a first
grid operating at a first electrical frequency and a second grid operating at a
second electrical frequency which includes a rotary transformer,
compensation subsystems are provided. A shunt compensation circuit,
which regulates voltage by adjusting reactive current injected in shunt into
the interconnection system, is connected between the first grid and the
rotary transformer. The shunt compensation system is situated on a lower
voltage point at one side of the rotary transformer and provides
simultaneous regulation for both sides of the rotary transformer. A series
compensation circuit, which regulates e.g., reactive power flow through the
rotary transformer, is preferably connected to a step-down transformer
situated between the first grid and the rotary transformer.
Various embodiments of shunt compensation circuits are provided.
In differing embodiments, either fixed and switched capacitors can be used,
or static VAR compensation devices.
Several embodiments of series compensation circuits are also
provided. In one embodiment, the series compensation circuit includes a
simple capacitor. In another embodiment, the series compensation circuit
includes a dual rotary phase shifting network. In yet another embodiment,
the series compensation circuit includes an inverter.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The foregoing and other objects, features, and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments as illustrated in the accompanying
drawings in which reference characters refer to the same parts throughout
the various views. The drawings are not necessarily to scale, emphasis
instead being placed upon illustrating the principles of the invention.
Fig. 1 is schematic view of an electrical power interconnection
system according to an embodiment of the invention, showing preferred
connections of a shunt compensation circuit and a series compensation
circuit.
Fig. 2 is a partial schematic, partial perspective view of rotary
*
transformer according to an embodiment of the invention.
Fig. 3 A is a schematic view of a first embodiment of a shunt
compensation circuit usable in the electrical power interconnection system
of Fig. 1.
Fig. 3B is a schematic view of a second embodiment of a shunt
compensation circuit usable in the electrical power interconnection system
of Fig. 1.
Fig. 4A is a schematic view of a first embodiment of a series

compensation circuit usable in the electrical power interconnection system
of Fig. 1.
Fig. 4B is a schematic view of a second embodiment of a series
compensation circuit usable in the electrical power interconnection system
of Fig. 1.
Fig. 4C is a schematic view of a third embodiment of a series
compensation circuit usable in the electrical power interconnection system
of Fig. 1.
BEST MODE FOR CARRYING OUT THE INVENTION
In the following description, for purposes of explanation and not
limitation, specific details are set forth such as particular architectures,
interfaces, techniques, etc. in order to provide a thorough understanding of
the present invention. However, it will be apparent to those skilled in the
art that the present invention may be practiced in other embodiments that
depart from these specific details. In other instances, detailed descriptions
of well known devices, circuits, and methods are omitted so as not to
obscure the description of the present invention with unnecessary detail.
Fig. 1 shows an electrical power interconnection system 100
connected between a first grid or AC electrical power system (area 1) 22
and a second grid or electrical AC power system 24 (area 2). Power
interconnection system 100 is connected to grid 22 by 3-phase line 26 and

to grid 24 by 3-phase line 28. Electrical power interconnection system 100
can be situated, for example, at a substation as represented by broken line
101 shown in Fig. 1.
Power interconnection system 100 includes a rotary transformer,
known herein as variable frequency transformer (VFT) 102. Step-down
transformer 30 is provided on line 26, whereby a voltage VT1 on line
segment 26(1) and applied to primary coil 30P is stepped down on
secondary coil 3OS to voltage VVFTI for application on line segment 26(2) to
first terminal 103(1) of variable frequency rotary transformer 102. Step-up
transformer 32 is provided on line 28, whereby the voltage VVFT2 emanating
from second terminal 103(2) of variable frequency rotary transformer 102
on line 28(1) and applied to primary coil 32P is stepped up at secondary
coil 32S to voltage VT2 for application on line segment 28(2) to grid 24. In
the illustrated example, step-down transformer 30 is a 100 MVA step-down
(GSU) transformer, and step-up transformer 32 is a 100 MVA step-up
(GSU) transformer. As just one example, the voltages mentioned above
can have the following illustrative examples: VTI = 230 kv; VVTFI = is kv; VVTF2
= 18 kV; and VT2 = 230 kV .
As shown in more detail below with reference to Fig. 2, at its first
terminal 103(1) the variable frequency transformer 102 is connected by 3-
phase lines RA, RB, RC (included in line 26) ultimately to the first AC
Power system (grid 22). At its second terminal 103(2), the variable
frequency transformer 102 is connected by by 3-phase lines SA, SB, and
SC (included in line 28) to the second AC Power System (grid 24).

The first grid 22 and the second grid 24 may have and likely do have
a differing electrical characteristic, such as differing electrical frequency.
In the particular example illustrated, grid 22 operates at 60Hz and grid 24
operates at 50Hz. It should be understood that while these frequencies are
common, the principles of the present invention can be applied when one or
both of the grids 22, 24 operate at other frequencies.
Power interconnection system 100 further includes both shunt
compensation subsystem, also known as shunt compensation circuit 40, and
series compensation subsystem, also known as series compensation circuit
50. Shunt compensation circuit 40 serves to regulate voltage in power
interconnection system 100 by adjusting the reactive current injected in
shunt with system 100. Differing embodiments of shunt compensation
circuit 40 are hereinafter described with respect to Fig. 3A and Fig. 3B.
Series compensation circuit 50 serves to regulate reactive power
flow through substation 101, or to regulate real power flow in a bandwidth
faster than variable frequency transformer 102 can achieve due to inherent
inertia of variable frequency transformer 102. Differing embodiments of
series compensation circuit 50 are hereinafter described with respect to Fig.
4A, Fig. 4B, and Fig. 4C.
A discussion of variable frequency rotary transformer 102 now
precedes elaboration of embodiments of shunt compensation circuit 40 and
series compensation circuit 50. In particular, and with reference to Fig. 2,

variable frequency rotary transformer 102 includes both a rotary
transformer assembly 105 and a torque control unit 106 (also known as the
rotor drive motor or drive motor). Rotary transformer assembly 105
includes both a rotor subassembly 110 and a stator 112. Rotor subassembly
110 includes a rotatable shaft 113, collector rings 114 (also known as slip
rings), and rotor cage section 116. Three-phase lines RA, RB, RC leading
from first AC Power System 22 are connected to collector rings 114; three-
phase lines SA, SB, and SC leading to second AC Power System 24 are
connected to stator 112.
As shown in Fig. 2 and understood by the man skilled in the art, in
the illustrated embodiment rotary transformer assembly 105 is wound with
sixty degree phase belts, with rotor windings being labeled as RA+, RC-,
RB+, RA-, RC+, and RB- and stator windings labeled as SA+, SC-, SB+,
SA-, SC+, and SB-. It should be understood that the invention is not
limited to a sixty degree phase belt-wound system, rather the principles of
the invention are applicable for rotary transformer assemblies of phase two
and greater.
Rotor assembly 110 is rotatable about its axis RX in both clockwise
direction CW and counter-clockwise direction CCW. Rotation of rotor
assembly 110 is effected by rotor drive section 106, also known as the drive
motor. Rotor drive section 106 is shown symbolically in Fig. 2 as a
cylindrical section mounted on rotor assembly 110. Thus, rotor drive
section 106 of Fig. 2 generally depicts various alternative and different
types of drive mechanisms for causing rotation of rotor assembly 110. In

some embodiments, rotor drive section 106 includes an actuator and some
type of linkage (e.g., gearing) which interfaces with rotor assembly 110.
For example, in one embodiment rotor drive section 106 comprises a worm
gear drive arrangement. In other embodiments, rotor drive section 106
comprises an actuator such as a stepper motor acting through a radial (e.g,
spur) gear, a direct drive arrangement, a hydraulic actuator turning a gear
on rotor assembly 110, or a pneumatic actuator turning a gear on rotor
assembly 110. Thus, any suitable drive mechanism may be employed for
rotor drive section 106. Further understanding of variable frequency rotary
transformer 102 is provided by various related applications already
incorporated herein by reference.
As shown in Fig. 1, shunt compensation circuit 40 is preferably
connected between line segment 26(2) and ground, and is preferably
situated between secondary coil 3 OS of step-down transformer 30 and first
terminal 103(1) of variable frequency rotary transformer 102. Such being
the case, a first side of shunt compensation circuit 40 has voltage VVFTI,
while a second side of shunt compensation circuit 40 is at ground.
Shunt compensation circuit 40 can include fixed and switched
capacitors. For example, the illustrative embodiment of shunt
compensation circuit 40A shown in Fig. 3 A includes a plurality of
capacitors 200(1), 200(2), and 200(3), each connected between line
segment 26(2) and ground. Of these, capacitor 200(3) is a fixed capacitor
and capacitors 200(1) and 200(2) are switched capacitors, as indicated by
switches 202(1) and 202(2). It should be understood that a greater or lesser

number of capacitors may comprise other embodiments of shunt
compensation circuit 40A, and that the mixture of fixed and switched
capacitances in such embodiments may vary.
Shunt compensation circuit 40 can also include static VAR
compensation devices, typically in combination with capacitors and
inductors. For example, Fig. 3B shows an illustrative embodiment of shunt
compensation system 40B. Shunt compensation system 40B of Fig. 3B
includes three lines 210(1), 210(2), and 210(3) connected in parallel
between line segment 26(2) and ground. Line 210(1) includes the series
connection of capacitor 212(1) and inductor 214(1). Line 210(2) includes
the series connection of static VAR compensation device 216(2) and
inductor 214(2). Line 210(3) includes the series connection of static VAR
compensation device 216(3), capacitor 212(3), and inductor 214(3). It
should be understood that a greater or lesser number of lines 210 can
comprise other embodiments of shunt compensation circuit 40B, and that
differing combinations of elements can be included on such lines.
Shunt compensation circuit 40 may also be located in system 100
other than as shown in Fig. 1. For example, shunt compensation circuit 40
can be connected between line segment 28(1) and ground, and situated
between variable frequency rotary transformer 102 and step-up transformer
32, although such location is presently deemed less desirable. The
particular positioning of shunt compensation circuit 40 as shown in Fig. 1
advantageously connects on a lower voltage point, while providing voltage
regulation on lines 26 and 28 simultaneously.

As shown in Fig. 1, series compensation circuit 50 is preferably
connected between primary coil 3 OP of step-down transformer 30 and
ground. As further shown in Fig. 1, a first side of series compensation
circuit 50 has voltage VNi, a second side of compensation circuit 50 being .
at ground.
Series compensation circuit 50 can take various forms, three
examples of which are illustrated in Fig. 4A, Fig. 4B, and Fig. 4C. Fig. 4A
shows series compensation circuit 50A as comprising a capacitor 300
connected between primary coil .3 OP of transformer 30 and ground.
Fig. 4B shows series compensation circuit 50B as comprising two
variable frequency rotary transformers 302(1) and 302(2) connected
between primary coil 3 OP of transformer 30 and ground. The two variable
frequency rotary transformers 302(1) and 302(2) have rotor windings
connected together in series as indicated by line 304. The two stator
windings are connected together to a source of auxiliary power. The
source of auxiliary power can be taken from the terminals of variable
frequency transformer 102. No real power is drawn from such source,
only reactive power. The connection and cooperation of the two variable
frequency rotary transformers 302(1) and 302(2) is understood with
reference to United States Patent Application Serial No. 08/814,374 filed
March 11, 1997 by Larsen et al. and entitled "POWER FLOW CONTROL
WITH ROTARY TRANSFORMERS", already incorporated herein by
reference.

Fig. 4C shows series compensation circuit 50C which utilizes
inverter technology. In particular, series compensation circuit 50C
comprises a voltage-source inverter 350 with a source converter 354,
arranged in a configuration well established in the field of motor speed
control systems. The source bridge serves to maintain the dc link
voltage, exchanging power as needed from an auxiliary source. This
auxiliary source can be drawn from the terminals of the variable
frequency transformer 102.
Series compensation circuit 50 regulates reactive power flow
through substation 101, and regulates real power flow in a bandwidth faster
than the rotor of variable frequency transformer 102 can achieve due to its
inherent inertia. Reactive flow is regulated by adjusting the magnitude of
voltage on one side of variable frequency transformer 102 with respect to
the other side thereof. Real power is regulated by injecting a voltage in
quadrature with the transmission voltage.
Series compensation circuit. 50 can, in other embodiments, be
connected other than as shown in Fig. 1. For example/in an embodiment
currently deemed less preferable, series compensation circuit 50 can be
connected to secondary coil 3 OS of step-down transformer 30. However,
the higher current and lower voltage encountered in such position generally
adds additional complexity and costs to series compensation circuit 50.

While the invention has been described in connection with what is
presently considered to be the most practical and preferred embodiment, it
is to be understood that the invention is not to be limited to the disclosed
embodiment, but on the contrary, is intended to cover various modifications
and equivalent arrangements included within the spirit and scope of the
appended claims.

WE CLAIM
1. A system for transferring power between a first grid and a second grid,
the first grid operating at a first electrical frequency and the second grid
operating at a second electrical frequency, the system comprising:
a rotary transformer having a first winding and a second winding, the
rotary transformer further having a rotatable shaft and a drive motor for
rotationally driving the rotatable shaft;
a step-down transformer having a primary coil and a secondary coil, the
first winding of the rotary transformer being connected to the first grid
through the step-down transformer; and
a series compensation circuit connected to one of the primary coil and the
secondary coil of the step-down transformer.
2. The system as claimed in claim 1, wherein the series compensation circuit
is connected between the primary coil of the step-down transformer and
ground.
3. The system as claimed in claim 1, wherein the series compensation circuit
comprises a capacitor connected to one of the primary coil and the
secondary coil of the step-down transformer.
4. The system as claimed in claim 1, wherein the series compensation circuit
comprises a pair of variable frequency rotary transformers connected to
one of the primary coil and the secondary coil of the step-down
transformer.

5. The system as claimed in claim 1, wherein the series compensation circuit
comprises a voltage-source inverter with a source converter.
6. The system as claimed in claim 1, comprising a shunt compensation circuit
connected to a coil of the step-down transformer.
7. The system as claimed in claim 6, wherein the shunt compensation circuit
is connected to a terminal of the rotary transformer.
8. The system as claimed in claim 1, further comprising:
a step-up transformer connected between the rotary transformer and the
second grid.
9. The system as claimed in claim 1 or 8, further comprising a shunt
compensation circuit connected to a coil of one of the step-down
transformer and the step-up transformer.
10.The system as claimed in claim, wherein the shunt compensation circuit is
connected to a coil of the step-down transformer.


a series compensation circuit connected to one of the primary coil
and the secondary coil of the step-down transformer; and
a step-up transformer connected between the rotary transformer and
the second grid, and
a shunt compensation circuit connected between (1) a coil of one of
the step-down transformer and the step-up transformer and (2) electrical
ground.
10. The apparatus of claim 9, wherein the series compensation
circuit is connected to the primary coil of the step-down transformer.
11. The apparatus of claim 9, wherein the shunt compensation
circuit is connected to a terminal of the rotary transformer.
12. A system for transferring power between a first grid and a
second grid, the first grid operating at a first electrical frequency and the
second grid operating at a second electrical frequency, the system
comprising:
a rotary transformer having a first winding and a second winding,
the rotary transformer further having a rotatable shaft and a drive motor for
rotationally driving the rotatable shaft;
a step-down transformer having a primary coil and a secondary coil,
the first winding of the rotary transformer being connected to the first grid
through the step-down transformer;
a shunt compensation circuit connected to a coil of the step-down
transformer.

13. The apparatus of claim 12, wherein the shunt compensation
circuit is connected to a terminal of the rotary transformer.
14. A system for transferring power between a first grid and a
second grid, the first grid operating at a first electrical frequency and the
second grid operating at a second electrical frequency, the system
comprising:
a rotary transformer having a first winding and a second winding,
the rotary transformer further having a rotatable shaft and a drive motor for
rotationally driving the rotatable shaft;
a step-down transformer having a primary coil and a secondary coil,
the first winding of the rotary transformer being connected to the first grid
through the step-down transformer;
a step-up transformer connected between the rotary transformer and
the second grid, and
a shunt compensation circuit connected to a coil of one of the step-
down transformer and the step-up transformer.
15. A system for transferring power between a first grid and a
second grid, the first grid operating at a first electrical frequency and the
second grid operating at a second electrical frequency, the system
comprising:
a rotary transformer having a first winding and a second winding,
the rotary transformer further having a rotatable shaft and a drive motor for
rotationally driving the rotatable shaft;

a step-down transformer having a primary coil and a secondary coil,
the first winding of the rotary transformer being connected to the first grid
through the step-down transformer;
a step-up transformer connected between the rotary transformer and
the second grid, and
a compensation circuit connected to a coil of one of the step-down
transformer and the step-up transformer.
16. The apparatus of claim 15, further comprising a series
compensation circuit connected to one of a primary coil and a secondary
coil of the step-down transformer.
17. The apparatus of claim 16, wherein the series compensation
circuit comprises a capacitor connected to one of the primary coil and the
secondary coil of the step-down transformer.
18. The apparatus of claim 16, wherein the series compensation
circuit comprises a pair of variable frequency rotary transformers connected
to one of the primary coil and the secondary coil of the step-down
transformer.
19. The apparatus of claim 16, wherein the series compensation
circuit comprises a voltage-source inverter with a source converter.

20. The apparatus of claim 15, wherein the shunt compensation
circuit is connected to a coil of one of the step-down transformer and the
step-up transformer.
21. A system for transferring power between a first grid and a
second grid, the first grid operating at a first electrical frequency and the
second grid operating at a second electrical frequency, the system
comprising:
a rotary transformer having a first winding and a second winding,
the rotary transformer further having a rotatable shaft and a drive motor for
rotationally driving the rotatable shaft;
one of a step-down transformer and a step-up transformer connected
to the rotary transformer; and
a compensation circuit connected to a coil of one of the step-down
transformer and the step-up transformer.

ABSTRACT OF THE DISCLOSURE

In an interconnection system (100) for transferring power between a
first grid (22) operating at a first electrical frequency and voltage and a
second grid (24) operating at a second electrical frequency and voltage,
compensation circuits including a shunt compensation circuit (40) and a
series compensation circuit (50) are provided for use in conjunction with a
rotary transformer (102). The shunt compensation circuit (40) regulates
voltage by adjusting reactive current injected in shunt, and is preferably
connected between the first grid (22) and the rotary transformer (102). The
series compensation circuit (50), which regulates e.g., reactive power flow
through the rotary transformer (102), is preferably connected to a
transformer (30) which interfaces the first grid (22) to the rotary
transformer (102).