INTRODUCTION
The variable frequency transformer (VFT) is essentially a continuously variable phase shifting transformer that can operate at an adjustable phase angle. A direct application of a VFT is as a phase shifting transformer connecting two power systems operating at the same frequency and controlling the power flow. A phase shifting transformer (conventional or in conjunction with power electronic devices) is used to control real power flow along a transmission line and have their respective drawbacks.
The VFT, when used as a phase shifting transformer, overcomes all these difficulties.
The versatility of the VFT, however, lies in connecting two systems that are asynchronous. The conventional method of connecting two such asynchronous systems is to use back - to - back high voltage direct current (HVAC) connection. The technical performance of these two methods is comparable. In [3], such an application of the VFT has been reported.
VARIABLE FREQUENCY TRANSFORMER OVERVIEW
The core technology of the VFT is the rotary transformer (also known as “Rankle Machine” within GE) with three phase windings on both rotor and the stator. A three phase’s collector system conducts current between the three phase rotor winding and its stationary bus duct. The two separate electrical networks are connected to the stator and rotor respectively. Electrical power is exchanged between the two networks by the magnetic coupling through the air gap. A drive motor and a variable speed drive system is used to apply torque to the rotor of the transformer and adjust the rotational position of the rotor relative to the stator, thereby controlling the magnitude and direction of the power flow through the VFT. Fig 1 shows the core components of the VFT.
Fig. 2 illustrates a conceptual system diagram of the VFT. Conventional transformers are used to match the transmission voltage to the machine voltage. Shunt capacitors are used to compensate for the reactive magnetizing currents. As with any other AC power circuit, the real power flow through the rotary transformer is proportional to the phase angle difference between the stator and the rotor. The impedance of the rotary transformer and AC grid determine the magnitude of phase shift required for a given power transfer. Reactive power flow through the VFT is determined by the series impedance of the rotary transformer and the difference in magnitude of voltages on the two sides
1 VFT Concept and 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 (see Figure 1). The collector system conducts current between the three-phase rotor winding and its stationary busywork. 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.
MECHANICAL DESIGN OVERVIEW
The mechanical aspects of the machine were tailored to the vertical arrangement of the VFT rotary system. It is composed of three main components –
(a) Rotating transformer
(b) Drive motor and
(c) Collector.
The various components are shown in Fig. 1 and in Fig. 4. The three phase collector is at the top of the rotary system. The collector comprises of conventional carbon brush technology on copper slip rings. The collector rings are connected to the rotor windings via a three phase’s bus that runs through the hollow shaft. Fig. 5 shows the site assembly of a typical collector system. The drive motor is a conventional dc motor. The rotating components, since they have very little self cooling capability because of the low rotational speed, are force air cooled. The inertia of the total rotary system is rather large. Typically, in per unit on a 100 MVA base, it has an equivalent H factor of about 26 up sec. This large inertia helps to maintain stability during grid disturbances.
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 set point. The regulator compares measured power with the set point, 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.
4. 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 energization, starting, and stopping. When starting, the VFT automatically nulls the phase angle across the synchronizing 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, power-swing damping, and power runback. These are described below:
Governor –
The governor adjusts VFT power flow on a droop characteristic when frequency on either side exceeds a dead band. 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 dead band 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 Lang Lois 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.
5. VFT Control and Protection System
The control system for the Langlois VFT is comprised of digital processors arranged in a modular configuration (see Figure 4). A VFT unit is controlled by the unit VFT control (UVC), which contains automated sequencing functions (start/stop, synchronization, 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 synchronization-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 substation automation and data concentration from the digital relays, UVC processors, and other intelligent electronic devices (IED’s). The MVC’s primary purposes are to support the operator interface, SCADA interface, and to coordinate multi-unit VFTs
The human-machine interface or operator interface (HMI) uses a GE D200 data concentrator coupled with Power Link 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
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-wyes and a delta winding on the machine side.
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-wyes and a delta winding on the machine side.
A VFT model for dynamic simulations is shown in Figure 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 (Td) 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.
VFT BASIC THEORY
Power Transfer Through The VFT Can Be Approximated As
PVFT = Vs VR sin{as –(θr+θrm)}
X sr
PVFT = PXMAX sin θnet ------------------------------- 1
where,
PVFT = Power through VFT from stator to rotor,
Vs = Voltage magnitude on stator terminal,
Vr = Voltage magnitude on rotor terminal,
Xsr = Total reactance between stator and rotor terminals,
θs = Phase angle of ac voltage on stator, with respect to a reference phasor,
θr = Phase angle of ac voltage on rotor, with respect to a reference phasor,
θrm = Phase angle of Runkle Machine rotor with respect to stator,
θnet = θs – {θr +.θrm } , where the phasor relationships are indicated in Fig 6 and
PXMAX = VsVr ,
Xsr
maximum theoretical power transfer possible through the VFT in either direction
which occurs when the net angle θnet is near 90◦( π/2 radians) in either direction.
For stable operation, the angle θnet must have an absolute value significantly less than (π/2 radians), which means that power transfer will be limited to some fraction of the maximum theoretical level given by (1). Within this range, the power transfer follows a monotonic and nearly linear relationship to the net
angle, which can be approximated by:
PVFT = PXMAX θnet .----------------------------------- (2)
The power flow equations below are based on an ideal Runkle Machine, with negligible leakage reactance and magnetizing current. Further, for clarity, only real power flow is addressed. Fig. 7 illustrates a VFT system connected between two power systems, with a third power system providing a power sink or source for the torque control drive system
The power flow directions shown in Fig. 7 are based on generator convention with positive sign indicating power flowing out of the machine windings and into the shaft through the drive system. The actual power flow direction may be either positive or negative depending on the operating condition. Power balance requires that the electrical power flowing out of the stator must flow into the combined electrical path on the rotor and the mechanical path to the drive system:
Ps = PD-Pr ------------------------------------------------ (3)
where,
Ps = electrical power out of stator windings
Pr = electrical power out of rotor windings
PD = mechanical power to the torque control drive system, eventually appearing as electrical power exchanged with the power system to which the drive system is connected.
Since the Runkle Machine behaves like a transformer, ampere turns
must balance between rotor and stator:
Ns * I s =-Nr * I r ------------------------ (4)
where,
Ns = number of turns on stator winding,
Nr = number of turns on rotor winding,
Is = current out of the stator winding and
Ir = current out of the rotor winding.
Both the stator and rotor windings link the same magnetic flux. However, the frequency differs so the voltage will also differ by the same ratio:
Vs = Ns * f s *Y a and --------------------(5)
Vr = Nr * f r * Y a , -----------------------(6) (or),
Vr / Nr = Vs / Ns * f r / f s ----------------(7)
where,
Vr = voltage on rotor winding,
Vs = voltage on stator winding,
fs = electrical frequency on stator winding (Hz),
fr = electrical frequency on rotor winding (Hz) and
Ψa = airgap flux.
The nature of the machine is such that in steady state, the rotor speed is proportional to the difference in the electrical frequency on the stator and rotor windings,
f rm = f s - f r , and -------------------(8)
ωrm = f rm* 120/NP------------------ (9)
where,
frm = rotor mechanical speed, in electrical frequency units (Hz),
NP = number of poles in Runkle Machine and
ωrm = rotor mechanical speed in rpm.
Combining the above relationships gives the power exchanged with the drive system as
PD = Ps.Pr
Vs*I s.Vr*I r
Vs*I s- {Nr*Vs /Ns* f r / f s }*.{Ns*I s /Nr .}
Vs*I s*{1- f r / f s }
PD = Ps* {1- f r / f s} -----------------. (10)
The torque produced by the drive system (TD) is
T D = PD/ f rm
Vs*I s*[. f s- f r ./ f s]/. f s- f r .
Vs*I s / f s
Ns*f s*Y a*I s/ f s
TD=Ns*I s*Ya . ---------------------------(11)
It should be noted that the drive system torque TD is independent of rotational speed, being only proportional to stator current and air gap flux. Since the machine will operate near constant flux, this means that torque is proportional only to stator current. Hence, if the stator frequency is constant, then the torque is proportional to through power
TECHNICAL HIGHLIGHTS
The advantages of VFT over other competing technologies are:
a. It provides a simple and controlled path between electrical grids, permitting power exchanges that couldn't previously be accomplished owing to technical constraints such as asynchronous boundaries or congested systems.
b. A VFT substation has a smaller footprint; a complete 100 MW VFT substation should occupy a space of about 30 m x 80 m as compared to a similar conventional HVdc site requiring typically 2 to 3 times that space.
c. Unlike power electronic alternatives, the VFT produces negligible harmonics and cannot cause undesirable interactions with neighboring generators or other equipment on the grid.
d. The first VFT prototype was validated and expected performances were demonstrated in service leading to confidence in future application studies. The behavior obtained during commissioning tests demonstrated that this new technology is an effective way of transferring power between asynchronous systems.
CONCLUSION
The VFT has proven itself to be a viable alternative to backtobackHVdc converters for the hafting transformers.
Future application of the VFT would be to utilize the dual ability to control the phase as well as to compensate for the frequency variation. This could be used to operate pumps or hydro turbines closer to their maximum efficiency conditions. It could also be used to stabilize or absorb load swings in a power system, which would permit operation with a lower spinning reserve.
REFERENCES
o INDUCTION MOTOR –IEEE TRANSACTION
o VARIABLE FREQUENCY DRIVE
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