Variable-frequency transformer

Abstract: A new power transmission technology has been developed The variable frequency transformer (VFT) is a controllable, bi-directional 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 The world's first VFT was recently installed 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) This paper describes the VFT technology and provides an overview of the VFT equipment installed at Langlois substation Results of commissioning tests are also included

Abstract: This paper compares performance of a back-to-back HVDC system with series compensation external to the converter transformers, and a variable frequency transformer for power flow control feeding or supplying a weak AC network. The steady state and dynamic simulations show that both technologies are able to control power flow accurately. The variable frequency transformer consumes less reactive power than a back-to-back HVDC system, provides faster initial transient recovery and better natural damping capability. Back-to-back HVDC converters, however, provide smoother and faster recovery to pre-disturbance level for the same system faults. A back-to-back HVDC system also provides smoother and faster response to a controlled power change.

Abstract: Voltage instability caused by integrating large-scale wind generation into a weak system is a well-known challenge. The ERCOT 2012 Long Term System Assessment report indicated that a voltage stability limit will constrain wind power delivery to the rest of the ERCOT system if the northwestern-most portion of the ERCOT Panhandle CREZ system becomes over-subscribed. To identify the system upgrades necessary to accommodate future wind generation, ERCOT transmission planning performed the Panhandle study. In this study, the ability of several compensation devices to improve voltage stability was investigated, including the static var compensator (SVC), the synchronous condenser and the variable frequency transformer. This paper presents both the static and dynamic voltage stability issues associated with the integration of large-scale wind plants in weak grids.

Abstract: Proper use of phase-controlled switching technology by controlling the making or breaking of the circuit breaker (CB) at predetermined phase angles of current or voltage can improve the service life of the CB, avoid relay protection device from mal-operation, and reduce the impact on the power grid when power equipment or load is energized or taken out. This paper proposes a novel phase-controlled switching method to address the influence of residual flux for three-phase wye winding unloaded transformers with three limbs, such as large capacitor variable frequency transformer or mine explosion-proof transformer. The optimal operation phase angle for CBs is firstly obtained via theoretical analysis. A model based on the alternative transients program/the electromagnetic transients program (ATP/EMTP) is then built to simulate the transient procedure of a power transformer during breaking and making. The simulated results confirm that the proposed switching strategy can reduce the excitation inrush current effectively when the unloaded transformers are energized. The experimental results reported in this paper are used to validate the accuracy and effectiveness of the phase-controlled switching strategy.