Variable frequency transformer for asynchronous power transfer

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 Hydro- Quebec 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 Fig. 1: Core components of the VFT. Fig. 2: One-line diagram of langlois VFT. 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. energize - June 2009 - Page 34

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: One 100 MW, 17 kv rotary transformer One 3000 HP DC motor and variable speed drive system Three 25 MVAr switched shunt capacitor banks 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 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 Fig. 4: VFT control and protection system. 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 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 energize - June 2009 - Page 35

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: 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. 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 energize - June 2009 - Page 36 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

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 (T e ) 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 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. 8: VFT response to fault in AC network. Fig. 9: VFT power ramps during asynchronous operation with the New York and Quebec power networks. energize - June 2009 - Page 37

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 100 MW and back to zero. Unlike 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 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, Fig. 10: VFT response to generator trip on islanded grid. 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 Hydro- Québec TransÉnergie. Contact Mark Digby, GE Energy, Tel 021 506-6045, mark.digby@ge.com v energize - June 2009 - Page 38