TRANSMISSION AND DISTRIBUTION
Variable frequency transformer for
asynchronous power transfer
by Einar Larsen, Richard Piwko and Donald McLaren, GE Energy
A new power transmission technology has been developed. The variable frequency transformer (VFT) is a controllable, bidirectional transmission
device that can transfer power between asynchronous networks. Functionally, the VFT is similar to a back-to-back HVDC converter.
The core technology of the VFT is a rotary
transformer with three-phase windings on
both rotor and stator. A motor and drive
system are used to adjust the rotational
position of the rotor relative to the stator,
thereby controlling the magnitude and
direction of the power flowing through the
VFT [1]. The world’s first VFT was recently
installed and commissioned in HydroQuebec’s Langlois substation, where it will
be used to exchange up to 100 MW of
power between the asynchronous power
grids of Quebec (Canada) and New York
(USA) [2].
General VFT concept and core
components
The variable frequency transformer (VFT) is
essentially a continuously variable phase
shifting transformer that can operate at
an adjustable phase angle. The core
technology of the VFT is a rotary transformer
with three-phase windings on both rotor
and stator (Fig. 1). The collector system
conducts current between the three-phase
rotor winding and its stationary buswork.
One power grid is connected to the rotor
side of the VFT and another power grid is
connected to the stator side of the VFT.
Fig. 1: Core components of the VFT.
Power flow is proportional to the angle of
the rotary transformer, as with any other
AC power circuit. The impedance of the
rotary transformer and AC grid determine
the magnitude of phase shift required for
a given power transfer.
Power transfer through the rotary transformer
is a function of the torque applied to the
rotor. If torque is applied in one direction,
then power flows from the stator winding
to the rotor winding. If torque is applied in
the opposite direction, then power flows
from the rotor winding to the stator winding.
Power flow is proportional to the magnitude
and direction of the torque applied. If no
torque is applied, then no power flows
through the rotary transformer. Regardless
of power flow, the rotor inherently orients
itself to follow the phase angle difference
imposed by the two asynchronous systems,
and will rotate continuously if the grids are
at different frequencies.
Torque is applied to the rotor by a drive
motor, which is controlled by the variable
speed drive system. When a VFT is used
to interconnect two power grids of the
same frequency, its normal operating
Fig. 2: One-line diagram of langlois VFT.
speed is zero. Therefore, the motor and
drive system is designed to continuously
produce torque while at zero speed
(standstill). However, if the power grid on
one side experiences a disturbance that
causes a frequency excursion, the VFT
will rotate at a speed proportional to the
difference in frequency between the two
power grids. During this operation the load
flow is maintained. The VFT is designed
to continuously regulate power flow with
drifting frequencies on both grids.
A closed loop power regulator maintains
power transfer equal to an operator
setpoint. The regulator compares measured
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power with the setpoint, and adjusts motor
torque as a function of power error. The
power regulator is fast enough to respond
to network disturbances and maintain
stable power transfer.
Reactive power flow through the VFT
follows conventional AC-circuit rules. It is
determined by the series impedance of
the rotary transformer and the difference
in magnitude of voltages on the two
sides. Unlike power-electronic alternatives,
the VFT produces no harmonics and
cannot cause undesirable interactions
with neighboring generators or other
equipment on the grid.
TRANSMISSION AND DISTRIBUTION
Langlois 100 MW VFT station
Hydro-Quebec’s Langlois substation is
located southwest of Montreal, Quebec,
at the electrical inter face between
the Quebec and USA power grids. A
100 MW VFT was installed at Langlois to
enable power transfer between the two
asynchronous power grids.
Fig. 2 shows a simplified one-line diagram
of the Langlois VFT, which is comprised of
the following:
l
One 100 MW, 17 kV rotary transformer
l
One 3000 HP DC motor and variable
speed drive system
l
Three 25 MVAr switched shunt capacitor
banks
l
Two 120/17 kV conventional generator
step-up transformers.
The Langlois VFT station has been designed
to be expandable, accommodating
another 100 MW rotary transformer and its
auxiliary equipment. The yard has space
for transformers, capacitor banks, and
switchgear associated with the second
VFT unit.
Fig. 3: Physical layout of a typical 200 MW VFT station with two units.
VFT Station layout
Fig. 3 shows a typical physical layout for
a 200 MW VFT station, with two 100 MW
VFT units. The rotar y transformer, drive
motor, collector, and ventilation system
are located in the large section of the
building. Control and auxiliary equipment
are located in the smaller wing of the
building.
VFT operation and control features
From an operational perspective, a VFT
is ver y similar to a back-to-back HVDC
converter station. The VFT has automated
sequences for energisation, starting,
and stopping. When starting, the VFT
automatically nulls the phase angle
across the synchronising switch, closes the
breaker, and engages the power regulator
at zero MW. The operator then enters a
desired power order (MW) and ramp rate
(MW/minute).
Power regulation is the normal mode of
operation. The VFT uses a closed-loop
power regulator to maintain constant power
transfer at a level equal to the operator
order. The power order may be modified
by other control functions, including
governor, isochronous governor, powerswing damping, and power runback.
Fig. 4: VFT control and protection system.
Governor
The governor adjusts VFT power flow on
a droop characteristic when frequency
on either side exceeds a deadband.
This function is designed to assist one of
the interconnected power grids during
a major disturbance involving significant
generation/load imbalance. If frequency
falls below the deadband threshold, the
VFT will increase power import (or reduce
export) to assist in returning grid frequency
to the normal range. The VFT is designed
Fig. 5: VFT power circuit model for short-circuit and power flow analysis.
to operate with one side isolated. If the
local grid on one side of the Langlois VFT
becomes isolated from the rest of the
network, the VFT will continue to operate
regardless of whether the isolated system
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has local generation. If there is no local
generation, the VFT will automatically
feed all the necessar y power up to its
full rating. If there is local generation, the
VFT will make up the difference between
TRANSMISSION AND DISTRIBUTION
local generation and local load, and
share frequency governing with the local
generator. VFT also has an isochronous
governor that will regulate the frequency
of the isolated network to 60 Hz, when
engaged by the operator.
Power-swing damping
This function adds damping to inter-area
electromechanical oscillations, normally in
the range of 0,2 Hz to 1 Hz. This function
is installed but disengaged at Langlois,
as system conditions do not require it at
this time.
Power runback
This function quickly steps VFT power to
a preset level. It is externally triggered
following major network events (e.g., loss of
a critical line or generator). The VFT control
system is designed to accommodate up
to four runbacks with separate triggers and
runback levels, but only one is presently
used at Langlois.
Like any other transformer, the VFT has
l e a ka g e r e a c t a n c e t h a t c o n s u m e s
reactive power as a function of current
passing through it. Shunt capacitor banks
are switched on and off to compensate
for the reactive power consumption of
the VFT and the adjacent transmission
network. The reactive power controller has
three modes:
Power schedule mode: The capacitor
banks are switched as a function of VFT
power transfer, with appropriate hysteresis
to prevent hunting. This mode includes
a voltage supervision function that takes
precedence if the bus voltage falls outside
of an acceptable range.
Voltage mode: The capacitor banks are
switched to maintain the bus voltage within
an operator-settable range.
Manual mode: The capacitor banks are
switched on and off by the operator.
VFT control and protection system
The control system for the Langlois VFT is
comprised of digital processors arranged
in a modular configuration (Fig 4). A VFT unit
is controlled by the unit VFT control (UVC),
which contains automated sequencing
functions (start/stop, synchronisation, etc.)
power regulator, governor, reactive power
control, power runback, and a variety
of monitoring functions. The UVC also
includes a local manual operator panel,
which is a backup to the higher-level
operator interface system.
A VFT unit is protected by redundant unit
protection systems, each comprised of
about ten standard protective relays.
Protective functions are typical of AC
s u b s t a t i o n s a n d g e n e r a t i n g p l a n t s,
including ground fault, negative sequence,
differential, over-current, over-voltage,
breaker failure, capacitor protections,
and synchronisation-check. The UVC and
Fig. 6: VFT model for dynamic simulations.
Parameter
Typical value (based on VFT rating)
XVFT
12%
XmagVFT
5,6 pu (i.e., magnetizing current = 18%)
XT1, XT2
10%
B1, B2
20% to 80% total, depending on transmission grid needs
H
26 pu-sec
Table 1: Typical VFT data for preliminary planning studies.
unit protections are essentially identical
for any VFT unit.
Redundant bus and line protections are
specific to each VFT installation. The
protections cover the interconnections
between the VFT equipment and the local
grid. At Langlois, these protections cover
a section of the Langlois bus on one side
of the VFT and a transmission line to Les
Cedres substation on the other side of
the VFT.
The main VFT control (MVC) is primarily a
data concentrator and communications
interface. It contains high-level functions
for the entire VFT station, SCADA interface
to enable unmanned operation, and
substation automation and data
concentration from the digital relays, UVC
processors, and other intelligent electronic
devices (IEDs). The MVC’s primary purposes
are to support the operator interface,
SCADA interface, and to coordinate multiunit VFTs.
The human-machine interface or operator
inter face (HMI) uses a GE D200 data
concentrator coupled with PowerLink
Advantage software for the graphical
operator inter face. Operator screens
include one-lines with several levels
of detail, unit control, station control,
temperature, ventilation, communication
diagram, active alarm, historical alarm
(sequence of event recorder), and
trending. The local operator HMI has
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Fig. 7: VFT step response.
dual flat panel color screens. A remote
operator HMI with similar features is located
in another building within the Langlois
substation.
This overall control system design enables
separation of control functions by priority
within the overall control hierarchy (i.e.,
higher priority functions are implemented
at lower levels within the hierarchy). It
also supports expandability to several VFT
units within a substation sharing the same
operator interface.
VFT models for system planning studies
For power flow analysis, the VFT is represented
TRANSMISSION AND DISTRIBUTION
as a phase angle regulator. The limits
of phase angle can be set as large as
needed to obtain the desired power flow.
The series reactances of the system and the
level of reactive compensation determine
maximum achievable power flow. Fig. 5
illustrates the power circuit to be included
between two high-voltage buses. Reactive
compensation is represented as switched
shunt capacitors.
For short-circuit calculations, the impedance
is the only information required. The
contribution to a bus will be on the order of
150% to 250% of the VFT rating, depending
upon the strength of the transmission grid on
the opposite side. The step-up transformers
are similar to those used on generators, with
the high-voltage side grounded-wye and a
delta winding on the machine side.
A VFT model for dynamic simulations
is shown in Fig. 6. The power circuit
representation is identical with that for the
power flow model. For dynamic events, the
phase angle of the VFT varies as a function
of rotor inertia dynamics and the torque
applied by the drive motor.
The VFT control system measures power,
shaft speed, and several other signals.
The power regulator, in conjunction with
the governor, power swing damping
control, and other active functions within
the VFT control system, develops a torque
command, which the drive system applies
to the VFT rotor. The difference between the
drive motor torque (T d) and the electrical
torque on the rotary transformer windings
(Te) produces an accelerating torque. The
rotor inertia equations calculate speed of
the rotor and phase angle of the rotary
transformer. Typical parameters for the
power flow and dynamic VFT models are
indicated in Table 1.
VFT dynamic response
Fig. 8: VFT response to fault in AC network.
Dynamic performance of the VFT has been
analysed and verified by a combination
of digital computer simulations, realtime simulator tests using the actual VFT
control system, and staged tests at the
Langlois VFT station. A few examples of VFT
performance are presented here.
Fig. 7 illustrates the response of the VFT to
steps in power order. The red trace shows
the torque command stepping from zero,
to 1 pu (100 MW), to –1 pu, and back to
zero. The blue traces show the VFT actual
power transfer and the angle of the rotary
transformer. This response was recorded on
the real-time VFT simulator. Step tests were
also performed at the Langlois VFT station
with similar results.
Fig. 8 illustrates the response of the VFT to a
fault in the AC network. The voltage on the
machine terminals remains above zero,
due to contribution from the unfaulted
side. The large inertia holds the rotor
relatively stationary during the fault, and
after recovery the control readjusts the
position to meet the desired power flow.
Fig. 9 illustrates the Langlois VFT power
transfer between the asynchronous Quebec
Fig. 9: VFT power ramps during asynchronous operation with the New York and
Quebec power networks.
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TRANSMISSION AND DISTRIBUTION
and New York power grids. Note that the
time scale covers 70 minutes. The top trace
shows VFT power in MW as the operator
ramped power from zero to +100 MW to
– 1 0 0 M W a n d b a c k t o z e r o. U n l i ke
conventional HVDC transmission that cannot
operate below about 10% power, the
VFT’s ramp through zero MW is smooth.
Also note that power transfer is smooth
despite frequency variances in the two
grids – including the trip of a large unit in the
Quebec grid during the measuring period.
Fig. 10 shows the response of the VFT
governor in a condition where one side
of the VFT has a small isolated grid.
Initially, two generators at the nearby
Hydro Plant were connected to one side
of the VFT, together with a small amount
of local load. The generating plant was
producing a total of 45 MW. Generated
power not consumed in the island
was transmitted through the VFT to the
Hydro-Quebec grid. The plot shows the
VFT power transfer and the frequency of
the isolated grid for an event where the
35 MW generator was tripped off line,
leaving only 10 MW of generation. VFT
power dropped instantly, and frequency
of the isolated grid declined at a rate of
1,5 Hz/sec. The VFT governor engaged
when the frequency dropped below
59,4 Hz. A steady state operating point
was reached with VFT power at 10 MW
and the frequency at about 59,2 Hz, per
Fig. 10: VFT response to generator trip on islanded grid.
the deadband and droop characteristic
of the VFT governor.
Conclusions
Although the VFT concept is new, VFT
equipment is comprised of well-established
hydrogenerator, motor, and variablespeed drive technology. The world’s first
VFT for bulk system power transfer between
large asynchronous power grids has
been successfully installed and tested
at Hydro-Quebec’s Langlois substation in
Canada. The VFT is a viable alternative
to back-to-back HVDC converters for the
interconnection of asynchronous power
grids.
References
[1] Development of a 100 MW Variable Frequency
Transformer, by P Doyon, D McLaren, M White,
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Y Li, P Truman, E Larsen, C Wegner, E Pratico,
R Piwko, presented at Canada Power, Toronto,
Ontario, Canada, September 28-30, 2004.
[2] First VFT Application and Commissioning,
by M Dusseault, J-M.Gagnon, D Galibois,
M Granger, D McNabb, D Nadeau, J Primeau,
S Fiset, E Larsen, G Drobniak, I McIntyre,
E Pratico, C Wegner, presented at Canada
Power, Toronto, Ontario, Canada, September
28-30, 2004.
Acknowledgements
Development of the VFT was a major
team effort, including many individuals
and business units working in excellent
cooperation between GE and HydroQuébec TransÉnergie.
Contact Mark Digby, GE Energy,
Tel 021 506-6045,
mark.digby@ge.com v