Ulas Karaagac

Coordinated Control of Wind Energy Conversion Systems for Mitigating Subsynchronous Interaction in DFIG-Based Wind Farms

The paper presents methods for mitigating subsynchronous interaction (SSI) between doubly fed induction generator (DFIG) based wind farms and series capacitor compensated transmission systems. SSI damping is achieved by introducing a supplemental control signal in the reactive power control loop of the grid side converter of DFIG and full-scale frequency converter wind turbines, as well as in the reactive power control loop of the HVDC onshore multimodule converter (MMC) of offshore wind farms. This paper also investigates the impact of the phase imbalance series capacitive compensation concept that was introduced in the 1990s as a subsynchronous resonance countermeasure on SSI damping. The validity and effectiveness of the proposed methods are demonstrated on a test benchmark through time domain simulation studies using the ElectroMagnetic Transient Program (EMTP-RV).

Multiphase Load-Flow Solution for Large-Scale Distribution Systems Using MANA

The unbalanced nature of distribution systems requires a multiphase load-flow solution capable of handling arbitrary network topologies and providing accurate results. The need for detailed analysis of secondary grid systems found in dense urban areas and the modeling of distribution networks including the subtransmission level requires using highly efficient and large-scale system-capable methods. In this paper, three different load-flow solution algorithms are presented using the modified-augmented-nodal-analysis formulation. The load-flow solution algorithms are compared for the IEEE 8500-node distribution test feeder using a proposed regulator tap control strategy.

Synchronous Machine Modeling Precision and Efficiency in Electromagnetic Transients

This paper presents various synchronous machine models implemented in the computation of electromagnetic transients. This paper proposes new models for achieving better computational efficiency while maintaining precision. In addition to simple infinite bus analysis, the machine models are also compared for a more sophisticated and practical case study. Precision analysis includes the surrounding network accuracy constraints.

An Efficient Synchronous Machine Model for Electromagnetic Transients

This paper proposes a new synchronous machine model based on the application of Parks transformation to the discretized equations of the phase-domain model. The proposed approach maintains the precision of the phase-domain model and eliminates its computational inefficiencies through a constant admittance matrix. The model includes magnetic saturation. Precision analysis is performed within the accuracy constraints of the surrounding network.

Offshore Wind Farm Modeling Accuracy and Efficiency in MMC-Based Multiterminal HVDC Connection

The large number of switching elements in the modular multilevel converter (MMC) is a challenging problem when modeling the MMC-HVDC systems for the computation of electromagnetic transients. The modeling complexity increases even further when a multiterminal (MT) MMC-HVDC system is used to integrate offshore wind farms (OWFs) with power-electronics-based wind energy converters, such as doubly fed induction generators (DFIGs). This paper compares modeling accuracy and computational performances for various combinations of MMC and OWF models. Onshore and offshore ac fault simulations are performed for an OWF system composed of DFIG-type wind turbines and connected to a practical ac grid through an MT MMC-HVDC system. The OWF system model includes the detailed representation of the offshore collector grid and the associated overcurrent protection. The offshore MMC controls include an offshore fault current limiter and fast OWF power generation reduction-based fault-ride-through function.

Short-circuit current contribution of converter interfaced wind turbines and the impact on system protection

Traditional short-circuit modeling techniques and the associated models existing in commercially available packages are not accurate enough and do not accurately represent the behavior of converter interfaced renewable energy resources during short-circuit events. To address those issues, detailed EMT-type, time domain models of Type III and Type IV wind turbines have been developed as part of this work. Those models can be used to achieve an improved understanding of the way in which these devices affect system protection and of improved short-circuit models for system studies. The potential impact of renewable energy resources on relay misoperation and protection coordination is discussed, given that depending on the type of the wind turbines and the associated controls, the short circuit current contribution of a wind park might be the same for different locations of the fault. Finally, presently available modeling techniques of renewables in frequency domain are presented and compared. Their disadvantages are discussed along with an approach for improved modeling.

An Efficient Voltage-Behind-Reactance Formulation-Based Synchronous Machine Model for Electromagnetic Transients

This paper proposes a new synchronous machine model based on the voltage-behind-reactance (VBR) formulation. The proposed approach maintains the accuracy of the VBR model and eliminates its computational inefficiencies. The new model also includes an iterative solution option for achieving higher accuracy. The proposed model is tested and compared to other modeling approaches using practical power system test cases.

Simulation of Startup Sequence of an Offshore Wind Farm With MMC-HVDC Grid Connection

This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs.

An Accurate Type III Wind Turbine Generator Short Circuit Model for Protection Applications

The integration of renewables into power systems introduces several technical challenges including the development of appropriate models for system protection studies and accurate evaluation of short circuit contributions. Converter interfaced wind turbines (WTs) produce significantly different current waveform signatures compared to the traditional synchronous or asynchronous generators. This paper proposes a new phasor domain modeling approach for Type III WTs with doubly-fed induction generators (DFIGs) using the concept of control based equivalent circuits. The proposed model accounts for the impact of negative sequence quantities on WT control response to achieve accurate simulation of unbalanced faults.

Sub synchronous resonance dapming in DFIG-based wind farms using optimal control

An optimal controller for damping of sub synchronous resonance (SSR) due to a series compensated line connected to a DFIG-based wind farm is presented in this paper. The design procedure includes system modeling, analysis of sub synchronous phenomena, linearization of system about an operating point and controller design. The proposed controller is designed based on LQR method and the eigenvalue analysis is utilized to describe the behavior of system. IEEE SSR first benchmark used to show the effectiveness of the proposed controller.