Showing papers on "AC power published in 2008"

Power system dynamics : stability and control

Abstract: About The Authors. Preface. Acknowledgements. List of Symbols. PART I: INTRODUCTION TO POWER SYSTEMS. 1 Introduction . 1.1 Stability and Control of a Dynamic System. 1.2 Classification of Power System Dynamics. 1.3 Two Pairs of Important Quantities: Reactive Power/Voltage and Real Power/Frequency. 1.4 Stability of Power System. 1.5 Security of Power System. 1.6 Brief Historical Overview. 2. Power System Components. 2.1 Structure of the Electrical Power System. 2.2 Generating Units. 2.3 Substations. 2.4 Transmission and Distribution Network. 2.5 Protection. 2.6 Wide Area Measurement Systems. 3. The Power System in the Steady-State. 3.1. Transmission Lines. 3.2. Transformers. 3.3. Synchronous Generators. 3.4. Power System Loads. 3.5. Network Equations. 3.6. Power Flows in Transmission Networks. PART II: INTRODUCTION TO POWER SYSTEM DYNAMICS. 4. Electromagnetic Phenomena. 4.1. Fundamentals. 4.2. Three-Phase Short-Circuit on a Synchronous Generator. 4.3. Phase-to-Phase Short-Circuit. 4.4. Synchronization. 4.5. Short Circuit in a Network and its Clearing. 5. Electromechanical Dynamics - Small Disturbances. 5.1. Swing Equation. 5.2. Damping Power. 5.3. Equilibrium Points. 5.4. Steady-State Stability of Unregulated System. 5.5. Steady-State Stability of the Regulated System. 6. Electromechanical Dynamics - Large Disturbances. 6.1. Transient Stability. 6.2. Swings in Multi-Machine Systems. 6.3. Direct Method for Stability Assessment. 6.4. Synchronization. 6.5. Asynchronous Operation and Resynchronization. 6.6 Out-Of-Step Protection Systems. 6.7. Torsional Oscillations in the Drive Shaft. 7. Wind Power. 7.1 Wind Turbines. 7.2 Induction Machine Equivalent Circuit. 7.3 Induction Generator Coupled to the Grid. 7.4 Induction Generators with Slightly Increased Speed Range Via External Rotor Resistance. 7.5 Induction Generators with Significantly Increased Speed Range: DFIGs. 7.6 Fully Rated Converter Systems: Wide Speed Control. 7.7 Peak Power Tracking Of Variable Speed Wind Turbines. 7.8 Connections of Wind Farms. 7.9 Fault Behaviour of Induction Generators. 7.10 Influence of Wind Generators on Power System Stability. 8. Voltage Stability. 8.1. Network Feasibility. 8.2. Stability Criteria. 8.3. Critical Load Demand and Voltage Collapse. 8.4. Static Analysis. 8.5. Dynamic Analysis. 8.6. Prevention of Voltage Collapse. 8.7. Self-Excitation of a Generator Operating on a Capacitive Load. 9. Frequency Stability and Control. 9.1. Automatic Generation Control. 9.2. Stage I - Rotor Swings in the Generators. 9.3. Stage II - Frequency Drop. 9.4. Stage III - Primary Control. 9.5. STAGE IV - Secondary Control. 9.6. FACTS Devices in Tie-Lines. 10. Stability Enhancement. 10.1. Power System Stabilizers. 10.2. Fast Valving. 10.3. Braking Resistors. 10.4. Generator Tripping. 10.5. Shunt FACTS Devices. 10.6. Series Compensators. 10.7. Unified Power Flow Controller . PART III: ADVANCED TOPICS IN POWER SYSTEM DYNAMICS. 11. Advanced Power System Modelling. 11.1 Synchronous Generator. 11.2. Excitation Systems. 11.3. Turbines and Turbine Governors. 11.4. FACTS Devices. 12. Steady-State Stability of Multi-Machine System. 12.1. Mathematical Background. 12.2. Steady-State Stability of Unregulated System. 12.3. Steady-State Stability of The Regulated System. 13. Power System Dynamic Simulation. 13.1. Numerical Integration Methods. 13.2. The Partitioned-Solution. 13.3. The Simultaneous Solution Methods. 13.4. Comparison Between the Methods. 14. Power System Model Reduction - Equivalents. 14.1. Types of Equivalents. 14.2. Network Transformation. 14.3. Aggregation of Generating Units. 14.4. Equivalent Model of External Subsystem. 14.5. Coherency Recognition. 14.6. Properties of Coherency-Based Equivalents. Appendix. References. Index.

Adaptive Decentralized Droop Controller to Preserve Power Sharing Stability of Paralleled Inverters in Distributed Generation Microgrids

Abstract: This paper addresses the low-frequency relative stability problem in paralleled inverter-based distributed generation (DG) units in microgrids. In the sense of the small-signal dynamics of a microgrid, it can be shown that as the demanded power of each inverter changes, the low-frequency modes of the power sharing dynamics drift to new locations and the relative stability is remarkably affected, and eventually, instability can be yielded. To preserve the power sharing stability, an adaptive decentralized droop controller of paralleled inverter-based DG units is presented in this paper. The proposed power sharing strategy is based on the static droop characteristics combined with an adaptive transient droop function. Unlike conventional droop controllers, which yield 1-DOF tunable controller, the proposed droop controller yields 2-DOF tunable controller. Subsequently, the dynamic performance of the power sharing mechanism can be adjusted, without affecting the static droop gain, to damp the oscillatory modes of the power sharing controller. To account for the power modes immigration at different loading conditions, the transient droop gains are adaptively scheduled via small-signal analysis of the power sharing mechanism along the loading trajectory of each DG unit to yield the desired transient and steady-state response. The gain adaptation scheme utilizes the filtered active and reactive powers as indices; therefore, a stable and smooth power injection performance can be obtained at different loading conditions. The adaptive nature of the proposed controller ensures active damping of power oscillations at different operating conditions, and yields a stable and robust performance of the paralleled inverter system.

Eliminate Reactive Power and Increase System Efficiency of Isolated Bidirectional Dual-Active-Bridge DC–DC Converters Using Novel Dual-Phase-Shift Control

Abstract: This paper proposes a novel dual-phase-shift (DPS) control strategy for a dual-active-bridge isolated bidirectional DC-DC converter. The proposed DPS control consists of a phase shift between the primary and secondary voltages of the isolation transformer, and a phase shift between the gate signals of the diagonal switches of each H-bridge. Simulation on a 600-V/5-kW prototype shows that the DPS control has excellent dynamic and static performance compared to the traditional phase-shift control (single phase shift). In this paper, the concept of ldquoreactive powerrdquo is defined, and the corresponding equations are derived for isolated bidirectional DC-DC converters. It is shown that the reactive power in traditional phase-shift control is inherent, and is the main factor contributing to large peak current and large system loss. The DPS control can eliminate reactive power in isolated bidirectional DC-DC converters. In addition, the DPS control can decrease the peak inrush current and steady-state current, improve system efficiency, increase system power capability (by 33%), and minimize the output capacitance as compared to the traditional phase-shift control. The soft-switching range and the influence of short-time-scale factors, such as deadband and system-level safe operation area, are also discussed in detail. Under certain operation conditions, deadband compensation can be implemented easily in the DPS control without a current sensor.

Control of Distributed Uninterruptible Power Supply Systems

Abstract: In the last years, the use of distributed uninterruptible power supply (UPS) systems has been growing into the market, becoming an alternative to large conventional UPS systems. In addition, with the increasing interest in renewable energy integration and distributed generation, distributed UPS systems can be a suitable solution for storage energy in micro grids. This paper depicts the most important control schemes for the parallel operation of UPS systems. Active load-sharing techniques and droop control approaches are described. The recent improvements and variants of these control techniques are presented.

Distributed Reactive Power Generation Control for Voltage Rise Mitigation in Distribution Networks

Abstract: This paper addresses the problem of voltage rise mitigation in distribution networks with distributed generation. A distributed automatic control approach is proposed to alleviate the voltage rise caused by active power injection. The objective of the proposed approach is not to control bus voltage but to guarantee that generator injections alone do not cause significant voltage rise: a solution in which distribution network operators (DNOs) are kept to their traditional task of voltage regulation for load demand. The approach is discussed in the perspective of effectiveness and adequacy. Its consequences to DNO control effort are evaluated. Illustration is provided for a single feeder with stochastic generation and transformer on-load tap-changing voltage regulation.

Temporary Primary Frequency Control Support by Variable Speed Wind Turbines— Potential and Applications

Abstract: This paper quantifies the capability of providing a short-term excess active power support of a commercial multi-megawatt variable speed wind turbine (VSWT) and generalizes the findings by considering different wind turbine (WT) physical parameters in a wider range from the example case. The paper also identifies some possible applications of it, in particular, in a hydro dominated system. To be able to quantify the system characteristic, a delay model of the studied hydro system is developed. Due to the fact that the initial power surge of a hydro turbine is opposite to that desired, the short-term extra active power support from a wind farm (WF) could be beneficial for a hydro dominated system in arresting the initial frequency fall, which corresponds to an improvement in the system temporary minimum frequency (TMF). The improvements in the TMF are calculated by using both the developed delay model and a detailed model, and the results show good agreement. It is shown that the WT under consideration can provide a 0.1 pu extra active power support for 10 s quite easily which is twice the Hydro-Quebec requirement.

Control and Power Management of Converter Fed Microgrids

Abstract: This paper presents a voltage-power droop/frequency-reactive power boost (VPD/FQB) control scheme that allows multiple voltage source converters (VSCs) to operate in parallel in a VSC fed microgrid Each current controlled VSC in such a microgrid has its own VPD/FQB controller that sets its current references to regulate the voltage and frequency of a common microgrid bus By drooping the voltage reference of each controller against its real power output, multiple VPD/FQB controllers jointly regulate the microgrid voltage while sharing a common load power in proportion to a predetermined ratio Similarly, by boosting the frequency reference of each controller against its reactive power output, multiple VPD/FQB controllers jointly regulate the microgrid frequency while sharing the reactive load in proportion to a predetermined ratio The proposed control scheme can also operate in grid connected mode Experimental results are provided to validate the VPD/FQB control scheme

Direct Power Control of an AFE Using Predictive Control

Abstract: This paper presents a new control scheme for an active front-end rectifier using model-based predictive control. The control strategy minimizes a cost function, which represents the desired behavior of the converter. Future values of currents and power are predicted using a discrete-time model. The active and reactive powers are directly controlled by selecting the optimal switching state. The main advantages of this method are that there is no need of linear current controllers, coordinates transformations or modulators. The rectifier operates with sinusoidal input currents and unity power factor. Simulation and experimental results are presented to verify the performance of the proposed power control scheme.

A Method for Placement of DG Units in Distribution Networks

Abstract: In this paper, a method for placement of distributed generation (DG) units in distribution networks has been presented. This method is based on the analysis of power flow continuation and determination of most sensitive buses to voltage collapse. This method is executed on a typical 34-bus test system and yields efficiency in improvement of voltage profile and reduction of power losses; it also may permit an increase in power transfer capacity, maximum loading, and voltage stability margin.

Prospects of multilevel VSC technologies for power transmission

Abstract: Deregulation and privatization are posing new challenges to high voltage transmission and distributions systems. System components are loaded up to their thermal limits, and power trading with fast varying load patterns is leading to an increasing congestion. In addition to this, the dramatic global climate developments call for changes in the way electricity is supplied. Innovative solutions with HVDC (High Voltage Direct Current) and FACTS (Flexible AC Transmission Systems) have the potential to cope with the new challenges. New power electronic technologies with self-commutated converters provide advanced technical features, such as independent control of active and reactive power, the capability to supply weak or passive networks and less space requirements. In many applications, the VSC (Voltage-Sourced Converter) has become a standard for self-commutated converters and will be increasingly more used in transmission and distribution systems in the future. This kind of converter uses power semiconductors with turn-off capability.

Shunt Active-Power-Filter Topology Based on Parallel Interleaved Inverters

Abstract: In this paper, an interleaved active-power-filter concept with reduced size of passive components is discussed. The topology is composed of two pulsewidth-modulation interleaved voltage-source inverters connected together on the ac line and sharing the same dc-link capacitor. The advantages of the proposed approach are as follows: 1. significant reduction in the linkage inductors' size by decreasing the line-current ripple due to the interleaving; 2. reduction of the switching stress in the dc-link capacitor, due to the shared connection; and 3. more accurate compensation for high-power applications, because the power sharing allows one to use a higher switching frequency in each inverter. This paper analyzes the design of the passive components and gives a practical and low-cost solution for the minimization of the circulation currents between the inverters, by using common-mode coils. Several simulation results are discussed, and experimental results with a three-phase 10-kVA 400-V unit are obtained to validate the theoretical analysis.

Frequency Response Capability of Full Converter Wind Turbine Generators in Comparison to Conventional Generation

Abstract: With an increased number of wind turbine generators (WTGs) connected to an electricity network the system operator may request that they participate in frequency control in the event of a sudden unbalancing of power generated and consumed on the system. In this paper the frequency response capability of the full converter variable speed wind turbine generator (FCWTG) with permanent magnet synchronous generator (PMSG) is investigated. A control scheme is developed that improves the frequency control performance, illustrating the importance of the initial active power output of the FCWTG. A method of carefully ending the frequency support of a wind farm is proposed and simulated. The resulting frequency control performance compares favorably to that of a conventional synchronous generator plant.

Model Reference Adaptive Controller-Based Rotor Resistance and Speed Estimation Techniques for Vector Controlled Induction Motor Drive Utilizing Reactive Power

Abstract: In this paper, a detailed study on the model reference adaptive controller (MRAC) utilizing the reactive power is presented for the online estimation of rotor resistance to maintain proper flux orientation in an indirect vector controlled induction motor drive. Selection of reactive power as the functional candidate in the MRAC automatically makes the system immune to the variation of stator resistance. Moreover, the unique formation of the MRAC with the instantaneous and steady-state reactive power completely eliminates the requirement of any flux estimation in the process of computation. Thus, the method is less sensitive to integrator-related problems like drift and saturation (requiring no integration). This also makes the estimation at or near zero speed quite accurate. Adding flux estimators to the MRAC, a speed sensorless scheme is developed. Simulation and experimental results have been presented to confirm the effectiveness of the technique.

Individual Voltage Balancing Strategy for PWM Cascaded H-Bridge Converter-Based STATCOM

Abstract: This paper presents a new control method for cascaded connected H-bridge converter-based static compensators. These converters have classically been commutated at fundamental line frequencies, but the evolution of power semiconductors has allowed the increase of switching frequencies and power ratings of these devices, permitting the use of pulsewidth modulation techniques. This paper mainly focuses on dc-bus voltage balancing problems and proposes a new control technique (individual voltage balancing strategy), which solves these balancing problems, maintaining the delivered reactive power equally distributed among all the H-bridges of the converter.

Instantaneous Reactive Power Theory Applied to Active Power Filter Compensation: Different Approaches, Assessment, and Experimental Results

Abstract: In this paper, the five main formulations of the instantaneous reactive power theory have been chosen to study nonlinear load compensation. They are p-q original theory, d-q transformation, modified or cross-product formulation, p-q-r reference frame, and vectorial theory. The obtention of the compensation current according to each formulation has been established. Next, the behavior of an active power filter (APF) that is implemented with those different control algorithms has been studied. On one hand, a simulation platform with control, APF, and load has been built to test them. Results obtained in an unbalanced and nonsinusoidal three-phase four-wire system have been compared by means of the most adequate indexes. On the other hand, the APF control strategies have been implemented in an experimental platform constituted by a 20-kVA power inverter and a 400-MHz digital signal processing controller board. The final analysis shows that, in general, the five theories present a different behavior, which depends on supply voltage, with respect to distortion. However, all of them widely decrease the waveform distortion. Moreover, a more general compensation objective is possible. It obtains balanced and sinusoidal source current in any conditions of the supply voltage.

Coordinated Control of DFIG's Rotor and Grid Side Converters During Network Unbalance

Abstract: This paper proposes a coordinated control of the rotor side converters (RSCs) and grid side converters (GSCs) of doubly-fed induction generator (DFIG) based wind generation systems under unbalanced voltage conditions. System behaviors and operations of the RSC and GSC under unbalanced voltage are illustrated. To provide enhanced operation, the RSC is controlled to eliminate the torque oscillations at double supply frequency under unbalanced stator supply. The oscillation of the stator output active power is then cancelled by the active power output from the GSC, to ensure constant active power output from the overall DFIG generation system. To provide the required positive and negative sequence currents control for the RSC and GSC, a current control strategy containing a main controller and an auxiliary controller is analyzed. The main controller is implemented in the positive (dq)+ frame without involving positive/negative sequence decomposition whereas the auxiliary controller is implemented in the negative sequence (dq)- frame with negative sequence current extracted. Simulation results using EMTDC/PSCAD are presented for a 2 MW DFIG wind generation system to validate the proposed control scheme and to show the enhanced system operation during unbalanced voltage supply.

Mitigation of Subsynchronous Resonance in a Series-Compensated Wind Farm Using FACTS Controllers

Abstract: The rapid growth of wind power systems worldwide will likely see the integration of large wind farms with electrical networks that are series compensated for ensuring stable transmission of bulk power. This may potentially lead to subsynchronous-resonance (SSR) issues. Although SSR is a well-understood phenomenon that can be mitigated with flexible ac transmission system (FACTS) devices, scant information is available on the SSR problem in a series-compensated wind farm. This paper reports the potential occurrence and mitigation of SSR caused by an induction-generator (IG) effect as well as torsional interactions, in a series-compensated wind farm. SSR suppression is achieved as an additional advantage of FACTS controllers which may already be installed in the power system for achieving other objectives. In this study, a wind farm employing a self-excited induction generator is connected to the grid through a series-compensated line. A static var compensator (SVC) with a simple voltage regulator is first employed at the IG terminal in addition to the fixed shunt capacitor for dynamic reactive power support. The same SVC is shown to effectively damp SSR when equipped with an SSR damping controller. Also, a thyristor-controlled series capacitor (TCSC) that is actually installed to increase the power transfer capability of the transmission line is also shown to damp subsynchronous oscillations when provided with closed-loop current control. While both FACTS controllers-the SVC and TCSC-can effectively mitigate SSR, the performance of TCSC is shown to be superior. Extensive simulations have been carried out using EMTDC/PSCAD to validate the performance of SVC and TCSC in damping SSR.

Distribution System Voltage Performance Analysis for High-Penetration PV

Abstract: Distributed generation can have an impact on distribution feeder voltage regulation, and distributed solar photovoltaics (PV) are no exception As the penetration level of solar PV rises over the coming decades, reverse power flow on the distribution feeder will happen more frequently and the associated voltage rise might lead to violations of voltage boundaries defined by ANSI C841 The severity of possible voltage problems depends on the relative size and location of distributed PV generation and loads, distribution feeder topology, and method of voltage regulation In this paper, an illustrative distribution system feeder is assumed, and various case studies are conducted The performance of the commonly used distribution voltage regulation methods under reverse power flow are investigated and presented Voltage performance of the feeder, and the flow of active and reactive power are studied under different loading assumptions, and different assumptions of PV inverters' participation The paper also explores the system performance using coordinated controls of inverters and utility equipment

Virtual-Flux-Based Predictive Direct Power Control of AC/DC Converters With Online Inductance Estimation

Abstract: This paper presents an improved predictive direct power control (P-DPC) algorithm for grid-connected three-phase voltage source converters without AC-side voltage sensors. The new algorithm is based on virtual-flux (VF) estimation and operates with constant switching frequency. Predictive controller selects in every sampling period appropriate voltage vector sequence and calculates duty cycles in order to minimize instantaneous active and reactive power errors. The theoretical principles of this algorithm are discussed, and selected experimental measurements and scope graphs that illustrate the operation and performance of the system are presented. It is shown that VF-P-DPC algorithm exhibits several advantages, particularly sinusoidal-grid-current low harmonic distortion even when grid voltage is distorted. In addition, the algorithm provides high dynamics at low switching frequency of 2 kHz.

Voltage and Reactive Power Control in Systems With Synchronous Machine-Based Distributed Generation

Abstract: This paper investigates voltage and reactive power control in distribution systems and how the presence of synchronous machine-based distributed generation (DG) affects the control. A proper coordination among the onload tap changer (OLTC), substation switched capacitors and feeder-switched capacitors in order to obtain optimum voltage and reactive power control is proposed. It is assumed that there is no communication link between the OLTC and the capacitors, a normal case in distribution system operation these days. The results indicate that the proposed method decreases the number of OLTC operations, losses, and voltage fluctuations in distribution systems, with and without DG present. The power-flow reversal due to the DG is shown not to interfere with the effectiveness of the OLTC operation. Further, it is also shown that as long as the available capacitors are enough to compensate the reactive power demand, the DG operation mode does not give a significant effect to the distribution system losses. However, DG operating at a constant voltage is beneficial for a significant reduction of OLTC operation and voltage fluctuation in the distribution system.

AC vs. DC distribution: A loss comparison

Abstract: Environmentally friendly technologies such as photovoltaics and fuel cells are DC sources. In the current power infrastructure, this necessitates converting the power supplied by these devices into AC for transmission and distribution which adds losses and complexity. The amount of DC loads in our buildings is ever-increasing with computers, monitors, and other electronics entering our workplaces and homes. This forces another conversion of the AC power to DC, adding further losses and complexity. This paper proposes the use of a DC distribution system. In this study, an equivalent AC and DC distribution system are compared in terms of efficiency.

Intelligent wireless power charging system

Abstract: A system and methodology for intelligent power management of wirelessly networked devices. The system provides for reliable wireless communication via a wireless power charging method and, a method to maintain power capacity of batteries in a wireless device. The batteries are charged via an RF harvesting unit embedded inside the wireless device. An intelligent wireless power charging system further comprises at least two batteries and at least two RF adaptor devices coupled to an AC power line. The first adaptor is set for data communication while the second adaptor is used to transmit the power. In addition, when a first battery is in use during active mode, the second battery is subjected to wireless charging.

A New Control Philosophy for a Unified Power Quality Conditioner (UPQC) to Coordinate Load-Reactive Power Demand Between Shunt and Series Inverters

Abstract: This paper presents a novel philosophy to compensate the load-reactive power demand through a three-phase unified power quality conditioner (UPQC). Most of the UPQC-based applications show the dependency on shunt inverter for load-reactive power compensation, whereas the series inverter is always looked as controlled voltage source to handle all voltage-related problems. This paper proposes a new functionality of UPQC in which both the shunt and series APFs supply the load-reactive power demand. This feature not only helps to share the load-reactive power demand, but also helps to reduce the shunt APF rating, and hence, the overall cost of UPQC. This results in better utilization of the existing series inverter. The theory and complete mathematical analysis termed as ldquopower angle control (PAC)rdquo is presented. The simulation results based on MATLAB/Simulink are discussed in detail to support the concept developed in the paper. The proposed approach is also validated through experimental study.

Cascaded Multilevel Inverter With Regeneration Capability and Reduced Number of Switches

Abstract: Multilevel converters are a very interesting alternative for medium and high power drives. One of the more flexible topologies of this type is the cascaded multicell converter. This paper proposes the use of a single-phase reduced cell suitable for cascaded multilevel converters. This cell uses a reduced single-phase active rectifier at the input and an H-bridge inverter at the output side. This topology presents a very good performance, effectively controlling the waveform of the input current and of the output voltage and allowing operation in the motoring and regenerative mode. The results presented in this paper confirm that this medium voltage inverter effectively eliminates low frequency input current harmonics at the primary side of the transformer and operates without problems in regenerative mode.

A Modular Strategy for Control and Voltage Balancing of Cascaded H-Bridge Rectifiers

Abstract: In this paper, a new strategy for voltage balancing of distinct dc buses in cascaded H-bridge rectifiers is presented. This method ensures that the dc bus capacitor voltages converge to the reference value, even when the loads attached to them are extracting different amounts of power. The proposed method can be used for an arbitrary number of series H-bridges, different voltage levels, and different power levels in unidirectional or bidirectional rectifiers. To reduce the current harmonics and distortion, the input current is programmed to be sinusoidal and in phase with the input voltage; however, it is possible to adjust the input power factor to control both the active and reactive powers. In the proposed approach, both the low frequency (stepped modulation) and high frequency [pulse-width modulation (PWM)] switching methods are utilized to improve the performance of the rectifier. Using theoretical analysis, the acceptable load power limits for a rectifier with N-H-bridge cells are derived. The validity of the proposed method is verified by simulation and experimental results.

Wind farms as reactive power ancillary service providers - technical and economic issues

Abstract: This paper examines the possibility of providing reactive power support to the grid from wind farms (WFs) as a part of the ancillary service provisions. Detailed analysis of the WF capability curve is carried out considering maximum hourly variation of wind power from the forecasted value. Different cost components are identified, and subsequently, a generalized reactive power cost model is developed for wind turbine generators that can help the independent system operator (ISO) in managing the system and the grid efficiently. Apart from the fixed cost and the cost of loss components, a new method is proposed to calculate the opportunity cost component for a WF considering hourly wind variations. The Cigre 32-bus test system is used to demonstrate a case study showing the implementation of the developed model in short-term system operations. A finding is that higher wind speed prediction errors (a site with high degree of wind fluctuations) may lead to increased payments to the WFs for this service, mainly due to the increased lost opportunity cost (LOC) component. In a demonstrated case, it is found that 2340 $/h is paid to the WF as the LOC payment only, when the wind prediction error is 0.5 per unit (p.u.), whereas 54 $/h is the expected total payment to the WF when the prediction error is 0.2 p.u. for its reactive power service.

Predictive Current Control of an Induction Machine Fed by a Matrix Converter With Reactive Power Control

Abstract: A different approach to perform the control of an induction machine fed by a matrix converter (MC) is presented in this paper. The proposed technique is based on predictive control and effectively controls input and output variables to the power converter, as expected from an MC. The method allows the use of all valid switching states, including rotating vectors that are not considered in most control techniques, as space vector modulation or direct torque control for induction machines fed by MCs. Experimental results show the excellent performance of the proposed approach, with low-distortion input currents, adjustable power factor, sinusoidal output currents with smooth frequency transitions, and good speed control in motoring and regeneration conditions, even working under an unbalanced power supply. The implementation and comprehension of the method should be considered simple compared to other control strategies with similar features. The high computational effort required should not be a problem considering recent progresses in digital signal processors-and even less in years to come.

Understanding of tuning techniques of converter controllers for VSC-HVDC

Abstract: A mathematical model of a voltage source converter is presented in the synchronous reference frame for investigating VSC-HVDC for transferring wind power through a long distance. This model is used to analyze voltage and current control loops for the VSC and study their dynamics. Vector control is used for decoupled control of active and reactive power and the transfer functions are derived for the control loops. In investigating the operating conditions for HVDC systems, the tuning of controllers is one of the critical stages of the design of control loops. Three tuning techniques are discussed in the paper and analytical expressions are derived for calculating the parameters of the current and voltage controllers. The tuning criteria are discussed and simulations are used to test the performance of such tuning techniques.

A Model-Based Direct Power Control for Three-Phase Power Converters

Abstract: The direct power control (DPC) technique has been widely used as a control strategy for three-phase power rectifiers due to its simplicity and good performance. DPC uses the instantaneous active and reactive power to control the power converter. The controller design has been proposed as a direct control with a lookup table and, in recent works, as an indirect control with an inner control loop with proportional-plus-integral controllers for the instantaneous active and reactive power errors. In this paper, a model-based DPC for three-phase power converters is designed, obtaining expressions for the input control signal, which allow the design of an adaptive control law that minimizes the errors introduced by parameter uncertainties as the smoothing inductor value or the grid frequency. A controller design process, a stability study of the system, and experimental results for a synchronous three-phase power rectifier prototype are presented to validate the proposed controller.

Solving multi-objective optimal power flow using differential evolution

Abstract: A differential evolution approach to solve optimal power flow problem with multiple and competing objectives is presented. Two sub-problems of optimal power flow namely active power dispatch and reactive power dispatch are considered. The problem is formulated as a nonlinear constrained true multi-objective optimisation problem with competing objectives. Constrain-domination approach have been used to handle inequality constraints, which eliminates the use of penalty factors. The performance of the proposed approach was tested on standard IEEE 30-bus system and is compared with a conventional method. The result demonstrates the capability of the proposed approach to generate diverse and well-distributed Pareto-optimal solutions.