Grid integrated renewable DG systems: A review of power quality challenges and state-of-the-art mitigation techniques

This paper presents a novel control scheme using a variable frequency transformer (VFT) of 100 MW to effectively reduce power fluctuations of an equivalent 80-MW aggregated doubly-fed induction generator (DFIG)-based offshore wind farm (OWF) connected to an onshore 120-kV utility grid. The q-d axis equivalent-circuit model is employed to establish the mathematical models for the VFT and the OWF to derive the complete dynamic equations of the studied system under three-phase balanced conditions. A frequency-domain approach based on a linearized system model using eigen techniques and a time-domain scheme based on a nonlinear system model subject to disturbance conditions are both performed to examine the effectiveness of the proposed control scheme. It can be concluded from the simulation results that the proposed VFT is effective to smooth the fluctuating active power of the OWF injected into the power grid while the damping of the studied OWF can also be improved.

Reduction of Power Fluctuations of a Large-Scale Grid-Connected Offshore Wind Farm Using a Variable Frequency Transformer

The drive for higher reliability has motivated many utilities to move toward a more meshed system. Two control areas are often connected together with tie-lines. Power flow through the tie-lines connecting two control areas is difficult to control. This lack of controllability of power flow is one of the major issues in the modern grid. It causes asymmetric stress on the grid assets. This makes some grid assets more vulnerable to failure than others, and therefore, decreases the overall system reliability. Presently utilities can achieve very limited power flow control using devices like load tap-changing transformers and phase-shifting transformers. Controllable network transformers (CNTs) were introduced as a simple, low-cost solution to the power flow problem. This paper develops a theoretical analysis for the operation of CNT in a meshed network. It also shows the various possible applications of the CNT. Experimental validation of the working principle of a small-scale prototype CNT is also provided.

Power flow control in networks using controllable network transformers

Variable frequency transformer (VFT) is a controllable bidirectional transmission device that can transfer power between asynchronous networks. The construction of VFT is similar to conventional asynchronous machines, where the two separate electrical networks are connected to the stator and rotor respectively. Electrical power is exchanged between the two networks by magnetic coupling through the air gap of the VFT. This paper describes the basic concept of the VFT and presents an overview of the mechanical and electromagnetic design of a unit having four poles and rated at 100 MW, 17 kV/17 kV, 60 Hz.

Variable Frequency Transformer—Concept and Electromagnetic Design Evaluation

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.

Evaluation of the Performance of Back-to-Back HVDC Converter and Variable Frequency Transformer for Power Flow Control in a Weak Interconnection

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

Variable frequency transformer - a new alternative for asynchronous power transfer

A variable frequency transformer (VFT) is being installed at AEP's Laredo substation, where it will be used to exchange up to 100 MW of power between the asynchronous power grids of ERCOT (Texas) and CFE (Mexico). 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 installed at Hydro-Quebec's Langlois substation. Some additional control features have been included in the Laredo VFT design, related to tie flow regulation, power runback, reactive power control, and black start capability. This paper describes the VFT application at Laredo and includes simulation tests of performance during severe grid disturbances.

R.J. Piwko

Papers

Variable frequency transformer - a new alternative for asynchronous power transfer

VFT Operational Overview - The Laredo Project