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.

VSC-Based HVDC Power Transmission Systems: An Overview

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

Energy function analysis for power system stability

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.

Thyristor-Based Facts Controllers for Electrical Transmission Systems

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.

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

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.

Damping controller design for power system oscillations using global signals

This paper proposes a technique to estimate geomagnetically induced currents (GICs) in a transformer by measuring its absorbed reactive power. GIC is induced in electrical transmission lines by changes in the earth's magnetic field typically caused by solar magnetic disturbances, and flows into transformers through neutral grounding connections. An assessment of GIC from readily available reactive power measurements is an attractive alternative to the installation of dedicated GIC monitoring equipment on every transformer of an high-voltage transmission network. This technique is verified with electromagnetic transients software simulations and shows good agreement with the historical records captured in Hydro One's GIC detection network during the May 15, 2005 geomagnetic disturbance.

Determination of Geomagnetically Induced Current Flow in a Transformer From Reactive Power Absorption

This paper presents a new control of PV solar system as a STATCOM, termed PV-STATCOM, for faster post-fault voltage recovery, thereby avoiding the voltage instability of a neighboring critical induction motor load. Distributed Generators (DGs), such as PV solar systems, are typically required to disconnect upon detection of fault according to the current grid code of DG interconnection. Although, the preferred industry trend is to allow them to remain connected with Low Voltage Ride Through (LVRT) capabilities. In this paper, a case study is presented where an induction motor close to a PV solar system becomes unstable due to slow post fault voltage recovery. A fast fault detection control strategy is utilized in the PV solar system to disconnect the PV solar panels and convert the PV solar system inverter into a STATCOM, thereby providing rapid reactive power support to stabilize the voltage. This novel control strategy of utilizing the PV solar farm temporarily as a STATCOM after the fault prevents a critical induction motor from becoming unstable.

Novel control of a PV solar system as STATCOM (PV-STATCOM) for preventing instability of induction motor load

Integration of large wind farms may get constrained owing to the available transfer capacity of existing transmission networks. This transmission capacity may be enhanced by incorporating series compensation. However, series capacitors are known to cause subsynchronous resonance (SSR) oscillations in synchronous generators. In this study, the potential of SSR is investigated with series-compensated lines connected to wind farms based on single-cage self-excited induction generators. A small-signal mathematical model is developed for the prediction of SSR oscillations in such a wind farm for a study system similar to the IEEE First SSR benchmark system. Eigenvalue analysis is performed through MATLAB at various operating points. This is validated by detailed time-domain simulation through electromagnetic transient simulation software EMTDC/PSCAD for a three-phase-to-ground (LLLG) fault at the remote end of the compensated line. An equivalent circuit analysis is performed to examine the impact of a similar fault at the terminals of the wind farm. It is shown from detailed non-linear simulation that even at a realistic level of series compensation a three-phase fault at generator terminals for low levels of wind farm power generation may subject the generator shafts to potentially dangerous magnitudes.

Subsynchronous resonance in single-cage self-excited-induction-generator-based wind farm connected to series-compensated lines

Photovoltaic solar farms generate power only during daytime and remain completely idle during the nights. This paper presents a novel technology of utilizing Photovoltaic (PV) Solar Farms in the nighttime. New controls are developed for the solar farm inverters to operate as STATCOM - a Flexible AC Transmission System (FACTS) Controller, using the entire inverter capacity in the night for accomplishing various power system objectives, such as voltage regulation, improvement of power transfer capacity, load compensation, etc. During the daytime, the same objectives can be achieved to a substantial degree with the inverter capacity remaining after real power generation. This technology is termed PV-STATCOM. Two novel applications of solar farms operating as PV-STATCOM in realistic transmission and distribution systems are presented in this paper. These are : i) Improving power transfer capacity of transmission lines.; and ii) Increasing the connectivity of neighbouring wind farms.

Novel nighttime application of PV solar farms as STATCOM (PV-STATCOM)

This paper presents one of the first studies of the harmonic impact of a significantly large photovoltaic (PV) solar farm of 20 MW in a utility distribution system. This solar farm is a constituent of the 80-MW PV solar farm in Sarnia, Ontario, which is so far the largest solar farm in Canada. The utility network is modeled in detail using the commercial grade PSCAD/EMTDC software, which is validated through load flow studies conducted by CYME software and correlated with SCADA measurements. The validated network model is used for network resonance study and harmonics impact analysis of the solar farms under different network conditions. The harmonics data instrumented for several months were provided by the transmission utility at the two solar farm units and at the main feeder substation. These data were utilized for extensive harmonic impact studies with widely different short-circuit levels and network resonance conditions. This paper presents the detailed procedure adopted for performing such harmonic impact studies. It is concluded that this large solar farm may not cause any substantial voltage distortion on the distribution network during steady-state operating conditions. However, recommendations are made for utilities to perform such studies to ensure the safe operation of critical loads.

Rajiv K. Varma

Papers

Determination of Geomagnetically Induced Current Flow in a Transformer From Reactive Power Absorption

Novel control of a PV solar system as STATCOM (PV-STATCOM) for preventing instability of induction motor load

Subsynchronous resonance in single-cage self-excited-induction-generator-based wind farm connected to series-compensated lines

Novel nighttime application of PV solar farms as STATCOM (PV-STATCOM)

Harmonic Impact of a 20-MW PV Solar Farm on a Utility Distribution Network