Modular multilevel converters have several attractive features such as a modular structure, the capability of transformer-less operation, easy scalability in terms of voltage and current, low expense for redundancy and fault tolerant operation, high availability, utilization of standard components, and excellent quality of the output waveforms. These features have increased the interest of industry and research in this topology, resulting in the development of new circuit configurations, converter models, control schemes, and modulation strategies. This paper presents a review of the latest achievements of modular multilevel converters regarding the mentioned research topics, new applications, and future trends.

Circuit topologies, modeling, control schemes, and applications of modular multilevel converters

(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

Modular multilevel converter (MMC) is considered as a promising topology for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. This paper presents a new control strategy for MMC-HVDC under unbalanced grid conditions. First, a new inner loop current control strategy based on nonideal proportional resonant (PR) controllers in stationary αβ frame is designed, which is more concise compared to the existing dual sequence current control scheme. Second, an analytical expression for circulating current is obtained which shows that the circulating currents will be asymmetric under unbalanced grid conditions and can be decomposed into positive-, negative-, and zero-sequence component. In order to suppress all these components, a new circulating current suppressing strategy is analyzed and designed also based on nonideal PR controllers. Application of nonideal PR controllers makes the control system well adapt to the fluctuation of grid frequency. The effectiveness of the proposed control strategy is verified through a simulation case of a 251-level MMC-HVDC transmission system using real-time digital simulator.

Circulating Current Suppressing Strategy for MMC-HVDC Based on Nonideal Proportional Resonant Controllers Under Unbalanced Grid Conditions

Consideration of transient stability constraints in optimal power flow (OPF) problems is increasingly important because modern power systems tend to operate closer to stability boundaries due to the rapid increase of electricity demand and the deregulation of electricity markets. Transient stability constrained OPF (TSCOPF) is however a nonlinear optimization problem with both algebraic and differential equations, which is difficult to be solved even for small power systems. This paper develops a robust and efficient method for solving TSCOPF problems based on differential evolution (DE), which is a new branch of evolutionary algorithms with strong ability in searching global optimal solutions of highly nonlinear and nonconvex problems. Due to the flexible properties of DE mechanism, the hybrid method for transient stability assessment, which combines time-domain simulation and transient energy function method, can be employed in DE so that the detailed dynamic models of the system can be incorporated. To reduce the computational burden, several strategies are proposed for the initialization, assessment and selection of solution individuals in evolution process of DE. Numerical tests on the WSCC three-generator, nine-bus system and New England ten-generator, 39-bus system have demonstrated the robustness and effectiveness of the proposed approach. Finally, in order to deal with the large-scale system and speed up the computation, DE is parallelized and implemented on a Beowulf PC-cluster. The effectiveness of the parallel DE approach is demonstrated by simulations on the 17-generator, 162-bus system.

Application of Differential Evolution Algorithm for Transient Stability Constrained Optimal Power Flow

The unified power flow controller (UPFC) is a solid-state controller which can be used to control active and reactive power flows in a power transmission line. In this paper, the authors propose a control strategy for UPFC in which they control real power flow through the line, while regulating magnitudes of the voltages at its two ports. They design a controller for this purpose which uses only local measurements. The control strategy is evaluated using digital simulation for a case study.

/pdf/control-design-and-simulation-of-unified-power-flow-3sf9n5wbes.pdf

Control design and simulation of unified power flow controller

Modal controllability, observability, and transfer function zeros play an important role in the selection of candidate locations and feedback signals for flexible ac transmission systems (FACTS)-based power swing damping controllers. This paper investigates location of zeros of the transfer function corresponding to FACTS devices and certain local feedback signals. Signals synthesized from locally measured signals are derived that can give good pole-zero separation. It is also shown that relative modal controllability of FACTS controllers at various locations can be directly inferred from relative modal observability in certain signals at those locations. These signals are identified for various FACTS devices. Case studies using detailed model are used to validate the results.

Power oscillation damping using FACTS devices: modal controllability, observability in local signals, and location of transfer function zeros

Design of reactive current and voltage controller of static condenser

Power system stabilisers (PSS) are added to excitation systems to enhance the damping of low-frequency oscillations. The design of PSS for a single machine connected to an infinite bus using periodic output feedback is proposed. The non-linear model of a machine is linearised at different operating points and 16 linear plant models are obtained. For each of these plants an output injection gain is obtained using the LQR technique. A robust periodic output feedback gain which realises these output gains is obtained using an LMI approach. This robust periodic output control is applied to a non-linear plant model of the machine at different operating (equilibrium) points. This method does not require the complete set of states of the system for feedback and is easily implementable.

Design of power system stabiliser for single-machine system using robust periodic output feedback controller

This paper presents a new approach for determining the effectiveness of preventive generation rescheduling and shunt/series compensation in improving transient stability using trajectory sensitivity (TS) method. It is shown that the effectiveness of changes in parameter values can be judged by evaluating the TS of certain variables with respect to clearing time. Therefore, the need to evaluate a host of sensitivities with respect to many parameters is obviated. This heuristic is motivated by the fact that for simple models and small changes, controllability of trajectories using certain parameters can be inferred from observability in certain output signals. An example of such a parameter and signal pair is shunt susceptance at a bus and the square of voltage at that bus, respectively. Case studies are carried out on a 10-machine 39-bus system with detailed models and 17-machine system to validate this method.

Anil Kulkarni

Papers

Control design and simulation of unified power flow controller

Power oscillation damping using FACTS devices: modal controllability, observability in local signals, and location of transfer function zeros

Design of reactive current and voltage controller of static condenser

Design of power system stabiliser for single-machine system using robust periodic output feedback controller

Determination of effectiveness of transient stability controls using reduced number of trajectory sensitivity computations