The variable frequency transformer (VFT) operates as a 100-MW converter, making it possible to transfer power across asynchronous system boundaries. With the installation of this VFT, Hydro-Québec TransÉnergie has increased interconnection capacity.

Hydro-Québec TransÉnergie wanted to provide value to its customers with the use of this technology, and the VFT seemed to fit the transmission provider's requirements to control power flow. Even more attractive was the simple design of individual components, based on established and widely used rotating machinery technology, ensuring long-term maintainability and lower maintenance costs.

Unlike traditional HVDC technology, the VFT provides “plug and play” capability to provide controlled transmission flow solutions. The VFT's minimal impact, with respect to harmonics, control interactions and effect on nearby generators, allows the installation and operation to be decoupled from other grid issues. The simple design eliminates the need for high-voltage filters and allows for a more compact substation design. The VFT is capable of continuous operation, controlling flows up to 100 MW in any direction.

Hydro-Québec TransÉnergie, after thorough analysis, awarded a contract to GE Energy in June 2001 to install the new VFT technology at Langlois Substation, located about 60 km (37 miles) southwest of Montreal, Canada. This 315/120-kV substation was selected because of its strategic location on the Hydro-Québec TransÉnergie power system (Fig. 3).

The Details

Langlois Substation was designed to be expandable and to accommodate another 100-MW VFT, if required. The substation yard has space for additional transformers, capacitor banks and switchgear. Also, considering the proximity of existing or future 230-kV interconnection lines, provision has been made for future conversion from 120 kV to 230 kV.

The VFT provides a parallel path to some hydro power plants. This allows the operator to choose between hydro units and the VFT to meet power schedules. Because the VFT can be operated at any power level from +100 to -100 MW and can be ramped smoothly through zero power level, it simplifies power scheduling.

Some of the aspects operators consider while setting up power transfers include:

• Use the VFT as a fine-tuning device to minimize scheduling errors and keep hydro units near optimal efficiency. It has no minimum operating power level.

• Use the VFT to make hourly changes in scheduling instead of switching units. This aspect is important because it is easier and faster to change the power order on the VFT than to switch and synchronize units. Hourly scheduling requires many switching operations. The VFT allows much more flexibility to anticipate hourly scheduling and limit switching operations.

• Use the VFT during peak conditions to keep more units on the Hydro-Québec TransÉnergie system. This is beneficial because the network benefits from the units feeding directly into the load centers.

The Design

The VFT is designed to supply 100 MW of transfer capability with a power factor of 0.9 to allow for a certain quantity of reactive power to flow through the installation. As power transfer increases, the VFT is required to supply enough reactive power to compensate for its own reactive power consumption. An additional 15 MVAR also is required for network voltage support. Individual capacitor banks are limited to 25 MVAR to minimize voltage variations during switching operations.

The Langlois VFT comprises:

• One 3750-hp dc motor and variable speed drive system.

• One 100-MW, 17-kV rotary transformer.

• Two 120/17-kV conventional generator step-up transformers.

VFT Operational and Control Features

From an operational perspective, a VFT is similar to a back-to-back HVDC converter station. The VFT has automated sequences for energizing, starting and stopping. When starting, it automatically nulls the phase angle across the synchronizing switch, closes the breaker and engages the power regulator at 0 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.

Voltage regulation is provided by switched capacitors within the VFT facility. These normally switch according to power command, but an optional closed-loop voltage-regulating mode can be selected by the operator.

Protection and Control System

The control system for the VFT comprises digital processors arranged in a modular configuration (Fig. 5). A VFT unit is controlled by the unit VFT control (UVC), which contains automated sequencing functions (such as start/stop and synchronization) 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 comprising about 10 standard protective relays. Protective functions are typical of ac substations and generating plants, including ground fault, negative sequence, differential, overcurrent, overvoltage, 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 Substation, these protections cover a section of the Langlois bus on one side of the VFT and a 120-kV transmission line on the other side.

The main VFT control (MVC) is primarily a data concentrator and communications interface. It contains high-level functions for the entire VFT station, a 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 and SCADA interfaces and to coordinate multi-unit VFTs. The human-machine interface (HMI) or operator interface uses a GE Energy 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 at Langlois Substation. This overall control system design enables separation of control functions by priority within the overall control hierarchy (such as higher priority functions that are implemented at lower levels within the hierarchy). It also supports expandability to several VFT units sharing the same operator interface within a substation.

Staged Field Tests

Dynamic performance of the VFT has been analyzed and verified by a combination of digital computer simulations, real-time simulator tests using the actual VFT control system and staged tests at Langlois Substation.

Figure 6 illustrates the response of the VFT to a fault on the ac network as determined on a real-time simulator. 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.

Figure 7 illustrates an example of asynchronous power transfer at Langlois VFT. 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 to -100 MW and back to zero. Unlike conventional HVDC transmission, which cannot operate below about 10% power, the VFT's ramp through 0 MW is smooth. Also note that power transfer is smooth despite frequency variances in the two grids, including the trip of a large unit on the Quebec grid during the measuring period.

Figure 8 shows the response of the VFT governor under conditions where one side of the VFT has a small isolated grid. Initially, two generating units at the nearby hydro plant were connected to one side of the VFT, together with a small amount of local load. The 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 unit was tripped off line, leaving only 10 MW of generation. VFT power dropped instantly, and the 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 the deadband and droop characteristic of the VFT governor.

VFT Performance Meets Expectations

The first VFT application has been successfully commissioned on the Hydro-Québec TransÉnergie system. This milestone represents the fruit of much effort over many years from the development of the new VFT technology to implementation in a power system.

Hydro-Québec TransÉnergie has addressed key performance requirements for the addition of a VFT to a power system. Extensive simulations both in stability and with a real-time simulator demonstrate that the VFT and its control system meet specified requirements and provide a stable system response. The results of commissioning tests have shown this new technology is effective in transferring power between asynchronous systems. Expected on-site performance should lead to confidence in future applications.

Robert Gauthier works in the marketing department of Hydro-Québec TransÉnergie, the Hydro-Québec transmission division. He is Hydro-Québec contact for the Langlois VFT project and also is responsible for the commercial aspect of technology innovation for Hydro-Québec TransÉnergie. Gauthier graduated as an electrical engineer from École Polytechnique in 1980 and began his career at Hydro-Québec in 1981.
Gauthier.Robert@hydro.qc.ca

About the Variable Frequency Transformer

The variable frequency transformer (VFT) is 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.

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 the other to the stator side. 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/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 to 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 the 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.