Ani M. Gole

Negative Sequence Current Control in Wind Power Plants With VSC-HVDC Connection

Large offshore wind power plants may have multi-MW wind turbine generators (WTG) equipped with full-scale converters (FSC) and voltage source converter (VSC) based high voltage direct-current (HVDC) transmission for grid connection. The power electronic converters in the WTG-FSC and the VSC-HVDC allow fast current control in the offshore grid. This paper presents a method of controlling the negative sequence current injection into the offshore grid from the VSC-HVDC as well as WTG-FSCs. This would minimize the power oscillations and hence reduce the dc voltage overshoots in the VSC-HVDC system as well as in the WTG-FSCs; especially when the offshore grid is unbalanced due to asymmetric faults. The formulation for negative sequence current injection is mathematically derived and then implemented in electromagnetic transients (EMT) simulation model. The simulated results show that the negative sequence current control mitigates the power oscillations and therefore limits the dc voltage excursions in the VSC-HVDC system during the asymmetric faults.

Damping Performance Analysis of IPFC and UPFC Controllers Using Validated Small-Signal Models

The paper discusses the dynamic behavior of two different flexible ac transmission system devices; the interline power-flow controller (IPFC) and the unified power-flow controller (UPFC) in a benchmark system. The small-signal model of the interline power-flow controller is developed and validated using detailed electromagnetic transients simulation. Using this validated model, the damping capabilities of the IPFC and the UPFC are compared and rationalized. From a small-signal dynamics point of view, it is shown that the series branches of these devices essentially segment the network creating a new structure. This structure change may be used to effectively improve system damping without requiring the design of a tuned feedback controller. The IPFCs two series branches in contrast to the UPFCs single series branch permit more opportunities for network segmentation. Hence, the IPFC has greater potential for improving the systems dynamic performance.

Improved Coherency-Based Wide-Band Equivalents for Real-Time Digital Simulators

This paper introduces an approach which enables very large power systems to be modeled on real-time electro-magnetic transients (EMT) digital simulators. This is achieved using an improved wide-band multi-port equivalent, which reduces a large power network into a small manageable equivalent model that preserves wide-band behaviors. The low-frequency or electromechanical transients are captured with a transient stability analysis (TSA) type electromechanical equivalent derived using coherency-based reduction techniques. The high-frequency behavior is accurately captured by placing in parallel with the TSA equivalent, a passive frequency dependant network equivalent (FDNE). The validity of the proposed technique is demonstrated by comparing the approach with detailed electromagnetic simulations of a modified version of the New England 39-bus test system that includes an HVDC infeed. The power of the method is demonstrated by the real-time electromagnetic transient simulation of a large 2300-bus 139-generator system.

Harmonic performance analysis of an OPWM-controlled STATCOM in network applications

The method of selective harmonic elimination is often used as an efficient means for obtaining harmonic-reduced waveforms from voltage sourced converters (VSC). However, the switching angles required for its implementation are usually pre-calculated on the basis of a fixed dc bus voltage. In practice, there are harmonics present in the dc source because of the finite dc bus capacitance and other realistic operating conditions, which can potentially introduce additional noncharacteristic harmonics. This paper examines the harmonic performance of a VSC-based static synchronous compensator as a function of design and operating parameters. These include the value of dc capacitor, the strength of the connected ac network and ac system unbalance. The results are generated using theoretical analysis for special cases and numerical simulation using electromagnetic transients simulation for the more general case.

Interline power flow controller (IPFC) steady state operation

This paper investigates the steady state operation of the interline power flow controller (IPFC). A mathematical model of the IPFC is presented and the model is used to investigate the flexibility of power flow control, in the presence of operating constraints of the IPFC. Some case studies are presented to illustrate the analysis and the possibility of using improved control strategies is discussed.

Advanced screening techniques for Sub-Synchronous Interaction in wind farms

This paper aims to outline comprehensive screening guidelines associated with the Sub Synchronous Control Interaction (SSCI) and Sub Synchronous Torsional Interaction (SSTI) for wind power plants. Specific guidelines and methodologies to identify the critical system conditions to be assessed for screening analysis are presented. The application of these techniques on a portion ERCOT grid model with series compensated lines has been demonstrated. Current injection based frequency scans on the system and turbine side are utilized. Based on these scans a guideline to identify the specific candidates for further investigation using Electro Magnetic Transient (EMT)-type simulation is proposed. The results of the screening study are further corroborated by the EMT based simulation case studies. An electrical damping analysis based screening technique for the analysis of SSTI has been presented. The SSTI screening studies have been further corroborated by means of EMT simulations.

Compensating for Interface Equipment Limitations to Improve Simulation Accuracy of Real-Time Power Hardware In Loop Simulation

This paper presents an improved algorithm for power hardware-in-loop (PHIL) simulation that takes the errors introduced by the interface equipment into account. Through modeling and analysis of a PHIL simulation circuit, which is composed of a voltage-source converter and a simple network, the impact of the bandwidth of the interface amplifier and equipment on the PHIL simulation is examined. Based on the analysis, an improved algorithm is proposed that uses additional interface filters (implemented in hardware and/or software) rather than the use of previously attempted compensation techniques. More stable and accurate results can be obtained by using the new algorithm. The validity of the proposed algorithm is verified through a software case study and hardware case studies.

Stability Analysis of Converter-Connected Battery Energy Storage Systems in the Grid

This paper analyzes the stability of a battery energy storage system (BESS) connected to the grid using a power-electronic interface. It is shown that the internal resistance and internal voltage of the battery affect system stability. Variations in these parameters may occur due to aging and changes in the state-of-charge (SoC). Using average-value modeling, this problem is framed into a nonlinear system formulation and the region of stability as a function of the internal resistance and the internal voltage of the battery is determined. This paper also extends its results in determining the configuration of a battery pack in terms of the number of battery cells in series and parallel to prevent instability while meeting demand power requirements. The findings are useful both in the design and operation stages of large-scale battery storage systems in the grid.

Influence of the MIIF index on operation of multi-infeed HVDC systems

The Multi-infeed Interaction Factor (MIIF) is an important index for quantifying the interaction between the converters of a Multi-infeed (MI) HVDC transmission system. Using an analytical formulation of power flow, the impact of ac and dc side system parameters on the MIIF is analyzed. The analysis is conducted using a two-infeed HVDC test system derived from the CIGRE HVDC Benchmark model. Results show that the ac grid effective short ratios at the MI converters, length of the tie line connecting the converter ac buses, and the power flow distribution in the ac network are the main factors affecting the MIIF value. The magnitude of the voltage drop at the converter bus does not influence the MIIF value.

General methodology for analysis of sub-synchronous interaction in wind power plants

Summary form only given. This paper presents a general methodology for analysis of sub-synchronous interaction in wind power plants. These include appropriate frequency scanning method for the assessment of the sub-synchronous control interaction, and calculation of the electrical damping provided by the wind turbine generator for investigation of the sub-synchronous torsional interaction. A general formulation of both methods applicable to any given wind turbine and network is presented. A dynamic frequency scanning method for the turbine side is developed which takes account of the turbine non-linearities and its active behavior. Various aspects that need to be considered when injecting a voltage or current signal into the system for dynamic frequency scanning are discussed in detail. The veracity of these methods is confirmed against electromagnetic transient analyses. The application of these tools and techniques is demonstrated on a practical power system comprising type 3 wind turbines and series compensated lines.