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

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.

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

Increasing societal concern for global warming and energy security have led many of the US states to adopt policies like Renewable Portfolio Standards (RPS). Introduction of these environmental policies in the energy market are expected to have significant impact on grid operation as well transmission investment. The energy delivery system has to be upgraded significantly in order to make it capable of handling these changes. It has been claimed that upgrading the present transmission system to a Smart Grid would facilitate the integration of renewable resources. Smart Wires are distributed, low cost, autonomous smart assets that are capable of controlling power flow in a meshed transmission network. This paper studies the benefit of Smart Grid technologies like Smart Wires in reducing transmission investment that is required to implement policies like the RPS.

Reducing transmission investment to meet Renewable Portfolio Standards Using Smart Wires

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.

D. Jacobson

Papers

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