S. Upadhyay

Bio: S. Upadhyay is an academic researcher from GE Energy Infrastructure. The author has contributed to research in topics: Quadrature booster & Variable-frequency transformer. The author has an hindex of 2, co-authored 2 publications receiving 63 citations.

Variable Frequency Transformer—Concept and Electromagnetic Design Evaluation

Abstract: 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 - an overview

Abstract: Variable frequency transformer (VFT) is a controllable bi-directional transmission device that can transfer power between asynchronous networks. The construction of VFT is similar to conventional asynchronous machines. Electrical power is exchanged between the two networks by magnetic coupling through the air gap of the VFT. This paper gives an overview of the VFT.

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

Abstract: 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.

A Comprehensive Review of Power Flow Controllers in Interconnected Power System Networks

Abstract: Energy security is one of the most crucial factor in the development of any nation. Interconnections among different power system networks are made to lower the overall price of power generation as well as enhance the reliability and the security of electric power supply. Different types of interconnection technologies are employed, such as AC interconnections, DC interconnections, synchronous interconnections, and asynchronous interconnections. It is necessary to control the power flow between the interconnected electric power networks. The power flow controllers are used to (i) enhance the operational flexibility and controllability of the electric power system networks, (ii) improve the system stability and (iii) accomplish better utilization of existing power transmission systems. These controllers can be built using power electronic devices, electromechanical devices or the hybrid of these devices. In this paper, control techniques for power system networks are discussed. It includes both centralized and decentralized control techniques for power system networks. This paper also presents a comprehensive review of HVDC interconnections, asynchronous AC interconnections, synchronous AC interconnections and different types of power flow controllers used in these interconnections. Moreover, some important and multivariable flexible AC transmission system (FACTS) devices such as UPFC and IPFC are also discussed with their merits and limitations. Finally, a new asynchronous AC link called flexible asynchronous AC link (FASAL) system is also described in detail. At last, a summary of the comparative analysis of power system link and power flow controllers is given based on recent publications. More than 400 research articles and papers on the topic of power transfer control are covered in this review and appended for a quick reference.

Application of Transformer-Less UPFC for Interconnecting Two Synchronous AC Grids With Large Phase Difference

Abstract: In this paper, the application of an innovative transformer-less unified power flow controller (UPFC) for interconnecting two synchronous ac grids with large phase difference is presented. The proposed transformer-less UPFC is based on two cascaded multilevel inverters. As is well known, the real power flow between two generators is mainly determined by their phase difference. If two grids with large phase difference are initially separate from each other, once connected, there will be huge current flowing through the transmission line and will, thus, damage the generators or other supplementary equipments. Therefore, to connect two synchronous ac grids with each other without using an extra device is impossible. For decades, researchers have been investigating different approaches to this problem but still difficult to conquer, especially for real hardware implementation. An effective solution using the transformer-less UPFC is demonstrated in this paper. The transformer-less UPFC can realize grid interconnection, independent active and reactive power control, dc-link voltage balance control, etc. Furthermore, a 1-pu equipment can compensate system with phase difference as large as 30°. Experimental results based on the 13.8-kV/ 2-MVA transformer-less UPFC prototype are shown to validate the theoretical analysis.

A Hierarchical Control Strategy With Fault Ride-Through Capability for Variable Frequency Transformer

Abstract: A variable frequency transformer (VFT) is being considered as a new alternative to the classical back-to-back high voltage direct current (HVDC) link for interconnection of two asynchronous networks. The VFT is a retrospective form of frequency converter using the wound rotor induction machine (WRIM), which converts the constant frequency input into a variable frequency output. The prime objective of VFT is to achieve controlled bidirectional power transfer between the two asynchronous networks. This paper presents a detailed working principle of VFT technology and proposes a new hierarchical control strategy for establishing the VFT connection with two power systems to achieve bidirectional power transfer between them. Also, to restrict the grid fault propagation from one side of the VFT to the other side, a series dynamic braking resistor based fault ride-through (FRT) scheme is proposed. The performance of the VFT during the synchronization process, steady-state, dynamic, and the grid fault conditions is evaluated using the real-time hardware in-loop (HIL) system. The plant is simulated in real time using OPAL-RT real-time simulator while the control algorithm is implemented in digital signal processor to carry out HIL study. All the important results supporting the effectiveness of the proposed control strategy and FRT scheme are discussed.