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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.
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 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 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.
Langlois 100 MW VFT station
Hydro-Quebec’s Langlois substation is
located southwest of Montreal, Quebec,
at the electrical interface 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.
VFT Station layout
Fig. 3 shows a typical physical layout for
a 200 MW VFT station, with two 100 MW
VFT units. The rotary 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 very 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.
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 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 has local generation. If there is no local
generation, the VFT will automatically
feed all the necessary power up to its
full rating. If there is local generation, the
VFT will make up the difference between 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
leakage reactance that consumes
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
substations and generating plants,
including ground fault, negative sequence,
differential, over-current, over-voltage,
breaker failure, capacitor protections,
and synchronisation-check. The UVC and 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
s u b s t a t i o n a u t o m a t i o n a n d d a t a
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
interface (HMI) uses a GE D200 data
concentrator coupled with PowerLink
Advantage software for the graphical
operator interface. 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 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.