Rajiv K. Varma

Bio: Rajiv K. Varma is an academic researcher from University of Western Ontario. The author has contributed to research in topics: Photovoltaic system & Electric power system. The author has an hindex of 31, co-authored 150 publications receiving 4995 citations. Previous affiliations of Rajiv K. Varma include Indian Institutes of Technology & Indian Institute of Technology Kanpur.

Novel application of a PV solar plant as STATCOM during night and day in a distribution utility network

Abstract: PV solar farms produce power during the day and are completely idle in the nights. This paper presents a novel utilization of a PV solar plant as STATCOM in the night for load reactive power compensation and voltage control. This STATCOM functionality will also be available to a substantial degree during the daytime with the inverter capacity remaining after real power production. The local distribution company London Hydro is implementing this new technology, for the first time in Canada, on a 10 kW solar system to be installed on the rooftop of their headquarters building. The PV solar plant inverter controller will be designed, developed and tested in the university Lab and will then be installed in field by Spring 2011. This paper presents the EMTDC/PSCAD simulation studies of the performance of the 10 kW PV solar plant as a STATCOM. The benefits of employing such a new control of the PV solar system as a Flexible AC Transmission System (FACTS) device are presented.

Fault Current Management Using Inverter-Based Distributed Generators in Smart Grids

Abstract: This paper presents a novel fault current management (FCM) technique for radial distribution systems with embedded inverter-based distributed generators (IB-DGs). At the point of connection to a power system, many distributed generators (DGs) require power electronic (PE) interfaces, which are normally idle during faults. The proposed FCM method employs these PE interfaces for control of the fault current. For this purpose, operation of IB-DGs is modified to FCM mode at the moment of fault and new current references are applied. Of the two controllable parameters of the IB-DG output current-current magnitude and current phase angle-the current phase angle is chosen as the means of controlling the fault current magnitude. The reference current phase angle is calculated based on the relation between the fault current elements and their phase angles. As a result of this novel operation, IB-DGs with larger capacity can be connected at different locations of the system without affecting the fault current magnitude. Also, implementing this technique in smart grids is economically proven, since the asset of power system which have been designed for normal operation are employed to manage the fault current magnitude. Moreover, possibilities of synchronization problems are reduced by keeping IB-DGs connected to the system at all the time. The evaluation of the proposed FCM technique using the standard IEEE 33-bus distribution system demonstrates the effectiveness of the proposed method for managing the fault current magnitude.

Impact of wind turbine generators on network resonance and harmonic distortion

Abstract: Distributed generators (DGs) are being increasingly connected at medium voltage level distribution networks. It is likely that different types of DGs, i.e. wind turbine generators (WTGs), photovoltaic solar systems, etc, may be connected on the same feeder. Most of the modern DGs are equipped with power electronic converters at their terminals, which act as sources of harmonic injection into the network. Any distribution feeder system will have its own network resonant frequencies. In this paper, the impact of WTG connections on resonant modes of system impedance and total harmonic distortion in the network are discussed in detail. First, a representative study system is devised where the impact of different elements of power system on system resonance frequencies is analyzed. Then the influence of system background harmonics on harmonic distortion is shown for the study system as well as for one of the actual Hydro One distribution feeder systems. It is shown that several scenarios are possible when these resonant frequencies align with harmonic frequencies that are likely to be injected by other power electronic based system equipment including DGs, thus causing unacceptable total harmonic distortion. The understanding will be useful to utilities for avoiding network resonance based harmonic amplification in distribution systems where wind farms and/or inverter based generation systems are integrated.

Enhancement of Solar Farm Connectivity With Smart PV Inverter PV-STATCOM

Abstract: This paper presents an innovative smart PV inverter control as STATCOM (PV-STATCOM) for obviating the need for a physically connected STATCOM in a distribution network for controlling steady-state voltage and temporary over voltages (TOVs) resulting from unsymmetrical faults. Two 10-MW PV solar systems are already connected in the distribution feeder of a utility in Ontario, Canada. A STATCOM is installed to prevent the steady-state voltage and TOV issues arising from the connection of a third 10-MW PV solar farm at same bus. It is demonstrated from PSCAD electromagnetic transient studies that if the proposed PV-STATCOM control is implemented on the incoming third 10-MW PV solar farm, all the above voltage issues are mitigated satisfactorily as required by the utility grid code. This proposed smart inverter PV-STATCOM control therefore eliminates the need for the physical STATCOM, saving an enormous cost for utilities dealing with voltage rise and TOV issues with grid-connected PV systems. Such a control can effectively increase the distributed generator hosting capacity of distribution feeders at more than an order of magnitude lower cost under similar network conditions. Moreover, this novel grid support functionality can open new revenue making opportunities for PV solar farms.

Mitigation of subsynchronous oscillations in a series compensated wind farm with static var compensator

Abstract: Subsynchronous resonance (SSR) is a potential problem in a series compensated power system. A wind farm employing self-excited induction generator (SEIG) connected to the grid through a series compensated transmission line may experience the same problem. This paper presents a study of subsynchronous oscillations resulting from torsional interactions as well as induction generator self-excitation effects in such a wind energy conversion system (WECS). It is shown that these two phenomena may cause system instability if proper preventive measures are not taken. A static var compensator (SVC) with a voltage controller has been employed at the induction generator terminal to damp the subsynchronous oscillations. It is also found that an auxiliary subsynchronous damping controller (SSDC) improves the damping of torsional oscillations. Extensive time domain simulations have been carried out using EMTDC/PSCAD to validate the performance of the SVC in preventing SSR.

VSC-Based HVDC Power Transmission Systems: An Overview

Abstract: The ever increasing progress of high-voltage high-power fully controlled semiconductor technology continues to have a significant impact on the development of advanced power electronic apparatus used to support optimized operations and efficient management of electrical grids, which, in many cases, are fully or partially deregulated networks. Developments advance both the HVDC power transmission and the flexible ac transmission system technologies. In this paper, an overview of the recent advances in the area of voltage-source converter (VSC) HVdc technology is provided. Selected key multilevel converter topologies are presented. Control and modeling methods are discussed. A list of VSC-based HVdc installations worldwide is included. It is confirmed that the continuous development of power electronics presents cost-effective opportunities for the utilities to exploit, and HVdc remains a key technology. In particular, VSC-HVdc can address not only conventional network issues such as bulk power transmission, asynchronous network interconnections, back-to-back ac system linking, and voltage/stability support to mention a few, but also niche markets such as the integration of large-scale renewable energy sources with the grid and most recently large onshore/offshore wind farms.

Energy function analysis for power system stability

Abstract: (1990). ENERGY FUNCTION ANALYSIS FOR POWER SYSTEM STABILITY. Electric Machines & Power Systems: Vol. 18, No. 2, pp. 209-210.

Thyristor-Based Facts Controllers for Electrical Transmission Systems

Abstract: 1. Introduction. 1.1 Background. 1.2 Electrical Transmission Networks. 1.3 Conventional Control Mechanisms. 1.4 Flexible ac Transmission Systems (FACTS). 1.5 Emerging Transmission Networks. 2. Reactor--Power Control in Electrical Power Transmission Systems. 2.1 Reacrive Power. 2.2 Uncompensated Transmission Lines. 2.3 Passive Compensation. 2.4 Summary. 3. Principles of Conventional Reactive--Power Compensators. 3.1 Introduction. 3.2 Synchronous Condensers. 3.3 The Saturated Reactor (SR). 3.4 The Thyristor--Controlled Reactor (TCR). 3.5 The Thyristor--Controlled Transformer (TCT). 3.6 The Fixed Capacitor--Thyristor--Controlled Reactor (FC--TCR). 3.7 The Mechanically Switched Capacitor--Thristor--Controlled Reactor (MSC--TCR). 3.8 The Thyristor--Switched capacitor and Reactor. 3.9 The Thyristor--Switched capacitor--Thyristor--Controlled Reactor (TSC--TCR). 3.10 A Comparison of Different SVCs. 3.11 Summary. 4. SVC Control Components and Models. 4.1 Introduction 4.2 Measurement Systems. 4.3 The Voltage Regulator. 4.4 Gate--Pulse Generation. 4.5 The Synchronizing System. 4.6 Additional Control and Protection Functions. 4.7 Modeling of SVC for Power--System Studies. 4.8 Summary. 5. Conceepts of SVC Voltage Control. 5.1 Introduction 5.2 Voltage Control. 5.3 Effect of Network Resonances on the Controller Response. 5.4 The 2nd Harmonic Interaction Between the SVC and ac Network. 5.5 Application of the SVC to Series--Compensated ac Systems. 5.6 3rd Harmonic Distortion. 5.7 Voltage--Controlled Design Studies. 5.8 Summary. 6. Applications. 6.1 Introduction. 6.2 Increase in Steady--State Power--Transfer Capacity. 6.3 Enhancement of Transient Stability. 6.4 Augmentation of Power--System Damping. 6.5 SVC Mitigation of Subsychronous Resonance (SSR). 6.6 Prevention of Voltage Instability. 6.7 Improvement of HVDC Link Performance. 6.8 Summary. 7. The Thyristor--Controlled SeriesCapacitor (TCSC). 7.1 Series Compensation. 7.2 The TCSC Controller. 7.3 Operation of the TCSC. 7.4 The TSSC. 7.5 Analysis of the TCSC. 7.6 Capability Characteristics. 7.7 Harmonic Performance. 7.8 Losses. 7.9 Response of the TCSC. 7.10 Modeling of the TCSC. 7.11 Summary. 8. TCSC Applications. 8.1 Introduction. 8.2 Open--Loop Control. 8.3 Closed--Loop Control. 8.4 Improvement of the System--Stability Limit. 8.5 Enhancement of System Damping. 8.6 Subsynchronous Resonanace (SSR) Mitigation. 8.7 Voltage--Collapse Prevention. 8.8 TCSC Installations. 8.9 Summary. 9. Coordination of FACTS Controllers. 9.1 Introduction 9.2 Controller Interactions. 9.3 SVC--SVC Interaction. 9.4 SVC--HVDC Interaction. 9.5 SVC--TCSC Interaction. 9.6 TCSC--TCSC Interaction. 9.7 Performance Criteria for Damping--Controller Design. 9.8 Coordination of Multiple Controllers Using Linear--Control Techniques. 9.9 Coordination of Multiple Controllers using Nonlinear--Control Techniques. 9.10 Summary. 10. Emerging FACTS Controllers. 10.1 Introduction. 10.2 The STATCOM. 10.3 THE SSSC. 10.4 The UPFC. 10.5 Comparative Evaluation of Different FACTS Controllers. 10.6 Future Direction of FACTS Technology. 10.7 Summary. Appendix A. Design of an SVC Voltage Regulator. A.1 Study System. A.2 Method of System Gain. A.3 Elgen Value Analysis. A.4 Simulator Studies. A.5 A Comparison of Physical Simulator results With Analytical and Digital Simulator Results Using Linearized Models. Appendix B. Transient--Stability Enhancement in a Midpoint SVC--Compensated SMIB System. Appendix C. Approximate Multimodal decomposition Method for the Design of FACTS Controllers. C.1 Introduction. C.2 Modal Analysis of the ith Swing Mode, C.3 Implications of Different Transfer Functions. C.4 Design of the Damping Controller. Appendix D. FACTS Terms and Definitions. Index.

Modular Multilevel Converters for HVDC Applications: Review on Converter Cells and Functionalities

Abstract: In this paper, the principle of modularity is used to derive the different multilevel voltage and current source converter topologies. The paper is primarily focused on high-power applications and specifically on high-voltage dc systems. The derived converter cells are treated as building blocks and are contributing to the modularity of the system. By combining the different building blocks, i.e., the converter cells, a variety of voltage and current source modular multilevel converter topologies are derived and thoroughly discussed. Furthermore, by applying the modularity principle at the system level, various types of high-power converters are introduced. The modularity of the multilevel converters is studied in depth, and the challenges as well as the opportunities for high-power applications are illustrated.

Damping controller design for power system oscillations using global signals

Abstract: This paper describes a new power system stabilizer (PSS) design for damping power system oscillations focusing on interarea modes. The input to the PSS consists of two signals. The first signal is mainly to damp the local mode in the area where PSS is located using the generator rotor speed as an input signal. The second is an additional global signal for damping interarea modes. Two global signals are suggested; the tie-line active power and speed difference signals. The choice of PSS location, input signals and tuning is based on modal analysis and frequency response information. These two signals can also be used to enhance damping of interarea modes using SVC located in the middle of the transmission circuit connecting the two oscillating groups. The effectiveness and robustness of the new design are tested on a 19-generator system having characteristics and structure similar to the Western North American grid.