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In the following years power levels of 20 kW to 50 kW will be demanded by the telecommunication satellites. It is not possible to keep 0.5 kW modules with up to 100 modules in parallel and, therefore, higher power modules must be used. A higher bus voltage seems also necessary and any voltage below 150 V and above 100 V could meet the needs. In addition, new optimized topologies that process only a part of the total power delivery can be employed to improve the efficiency. In this article, design guidelines and experimental results of a high efficiency battery discharge regulator (BDR) module based on parallel power processing are given.

With the rapid increase in distributed generation (DG), the issue of voltage regulation in the distribution network becomes more significant, and centralized voltage control (or active network management) is one of the proposed methods. Alternative work on intelligent distributed voltage and reactive power control of DG has also demonstrated benefits in terms of the minimization of voltage variation and violations as well as the ability to connect larger generators to the distribution network. This paper uses optimal power flow to compare the two methods and shows that intelligent distributed voltage and reactive power control of the DG gives similar results to those obtained by centralized management in terms of the potential for connecting increased capacities within existing networks. Index Terms--Dispersed storage and generation, optimal power flow, power distribution planning, power generation control, voltage control.

This two-part paper surveys the frequency and voltage control ancillary services in power systems from various parts of the world. In this second part, essential economic features that must be taken into account when designing markets for ancillary services are first discussed. The methods adopted in eight different systems (Australia, France, Germany, Great Britain, New Zealand, PJM, Spain, and Sweden) for trading frequency and voltage control ancillary services are then compared. Lastly, new indicators for comparing volumes and costs of ancillary services across systems are proposed and calculated for the systems included in this survey. The companion paper surveys the technical features of these ancillary services. Index Terms--Ancillary services, cost, frequency control, market, reactive power, system services, voltage control, volume.

This paper illustrates how a multiagent system (MAS)-based scheme can be developed for a control/optimization problem. The prototype problem considered is the dispatching of distributed generators on a distribution feeder to provide voltage support. The popular control net protocol (CNP) for MAS has been adopted to facilitate distributed control. This paper illustrates that characterization of the optimal solution is necessary in order to develop a protocol for the MAS to implement. This paper also shows that MAS facilitates a model-free control procedure, as it can monitor the local sensitivities. Test results, based on simulations on a prototype feeder, show that the proposed MAS-based control scheme is very effective in obtaining the solution for the prototype problem. The proposed method needs fast communication among the distributed generators (DGs) in order to assure fast response during emergency conditions. Communication requirements have also been identified in this paper. Index Terms--Distributed control, distributed generation (DG), optimization, power distribution, voltage regulation.

Starting with the Computer Business Equipment Manufacturers' Association acceptability curves, a procedure for quantifying the voltage quality is proposed. It is based on using the reciprocal of the permissible time duration for each value of the customer voltage. Since the magnitude of the voltage at any network bus changes with time depending on the load curve, the daily average value of the above suggested voltage quality index will provide a suitable figure of merit for the quality assessment. The analysis starts with the derivation of an analytical expression of the voltage quality index, denoted as voltage penalty function, in terms of the voltage. Then, relations are presented for the dependence of this index on the network as well as of the load parameters. Special interest is given to the load factor, the load power factor, its peak value, and the short-circuit level of the power network. The impact of eventual shunt compensation (and its level) on the voltage quality is discussed for both permanently connected capacitors, and for those which are switched only for a certain duration. In the latter case, the effect of the capacitor switching time point is demonstrated. The compensation level, the time point of application, and its duration are the three parameters for which a detailed sensitivity analysis and optimization for the least voltage daily average penalty are conducted. The applicability of the suggested procedure to nonrepeated events in distribution networks is also addressed. Index Terms--Electrical equipment, power-supply quality, voltage sag effect assessment, voltage sags.

Reactive power control through the static compensator (STATCOM) has gained wide attentions due to its outstanding performance. But for a STATCOM with traditional control strategies, unbalanced utility voltages will greatly affect the performance of the STATCOM and, in severe cases, may even cause the shutdown of the STATCOM for overcurrent protection. This paper proposes novel control strategies to ensure normal operation of STATCOM in three-phase three-wire systems when severe system voltage unbalance occurs and, furthermore, to mitigate voltage unbalance at the point of common coupling (PCC) whether it is caused by the utility or load. The control laws, operation principles, compensation characteristics, and operating point optimization of the proposed control strategies are analyzed and compared in detail. The power flow in the STATCOM and corresponding dc-side voltage-control schemes for these control strategies are also introduced. It is shown that the proposed voltage-controlled current source (VCCS) strategy and modified voltage-controlled voltage source strategy are valid for voltage unbalance compensation, especially the VCCS strategy for low rating under the same performance. Finally, the simulation and experimental investigations were carried out with these three parameter-optimized control strategies, respectively, for light and heavy load conditions. The results verified that the current passing through the STATCOM is under control and the voltage at the PCC is more balanced than before compensation. The compensation performance of the STATCOM is thus proven satisfactory. Index Terms--Control strategy, point of common coupling (PCC), static compensator (STATCOM), voltage-controlled current source (VCCS), voltage-controlled voltage source (VCVS), voltage unbalance.

Voltage dips and interruptions may cause major economic damage, not only can they create considerable loss of production (manufacturing) or data (ICT), there is also loss of market, the loss of client trust, comfort, etc. that incite energy consumers to implement certain forms of protection for their systems. However, the selection of the most cost-effective mitigation method is difficult because of the wide range in available protection devices and the mostly unknown and variable interruption cost. In this paper, the theoretical selection method is compared with the currently used selection methods in industry, showing a discrepancy between theory and practice. Using an alternative selection method, an optimal mitigation method can be found. This paper describes this method and its use by means of a practical example. The case examined is the protection of a 250 kVA installation, sensitive to voltage dips (ICT load). The proposed method makes a cost-benefit analysis of several proposed mitigation solutions resulting in an overview giving the optimal solution for a certain interruption cost interval. It enables to clearly interpret the investment costs and to compare completely different mitigation methods, only using data available to an industrial customer. Index Terms--Mitigation, power quality, technoeconomic assessment, voltage dips and interruptions.

A systematic procedure for the design of a new controller for an electronic tap changer has been described in Part I of this paper. The objective of this paper is to introduce quick measurement techniques of voltage amplitude and a phase angle for the controller and recommend the most convenient technique. One major assumption in the design of the controller in Part I of the paper was that a quick technique was available for detecting the amplitude and phase angle of the sinusoidal variables. In order to validate the design procedure, a designed prototype electronic tap changer is built and tested. Index Terms--Controller design, electronic tap changer, voltage regulation

Controller of a conventional tap changer must be modified when an electronic tap changer is used. A new controller design is generally preferred for an electronic tap changer in order to take full advantage of the tap changer. The first part of this two-part paper presents a systematic design procedure for the controller of an electronic tap changer. Performance of a tap changer equipped with the designed controller is simulated. Index Terms--Controller design, electronic tap changer, voltage regulation.

Wind-driven induction generators are popular due to their robust construction and performance. They are typically connected to a distribution system through a feeder and may cater to a local load of comparable size. Wind characteristics are unpredictable, sometimes causing large bursts of power flow. During such bursts, the feeder voltage regulation is problematic, possibly leading to a collapse. A robust collapse indicator, with the least possible local measurements, located at the sending or receiving end of the feeder, is desirable to operate protective equipment. This paper presents an implementation of such a dynamic voltage collapse indicator. Simulation results are included to demonstrate the versatility of the proposed method. Index Terms--Dynamic voltage collapse, voltage collapse indicator, wind generator.

Single-event transients are investigated for two voltage regulator circuits that are widely used in space. A circuit-level model is developed that can be used to determine how transients are affected by different circuit application conditions. Internal protection circuits--which are affected by load as well as internal thermal effects--can also be triggered from heavy ions, causing dropouts or shutdown ranging from milliseconds to seconds. Although conventional output transients can be reduced by adding load capacitance, that approach is ineffective for dropouts from protection circuitry. Index Terms--Radiation testing, single-event transient, voltage regulator.

SET-induced oscillations were measured on a stable linear-regulator that uses the LM124 op-amp in its regulation block. Simulations showed that single events act as stimuli that initiate a large-signal oscillation in the LM124 differential amplifier and compensation capacitor path. Laser testing validated the analysis and confirmed that transistors connected to the compensation capacitor are the most vulnerable. Index Terms--Linear voltage regulator, macro-model, Single Event Oscillation.

Total ionizing dose effects in a low-dropout voltage regulator are explained based on experimental data and circuit simulations. Transistor gain degradation is shown to be the dominant cause of the circuit degradation at lower dose rates. In addition, collector-to-emitter leakage current in one of the NPN transistors of the bandgap reference part of the circuit is responsible for increasing the postirradiation output voltage at high dose rates. Parametric changes in the bandgap, differential amplifier, and output pass transistor circuit blocks are identified that are responsible for various aspects of the observed circuit degradation. The different annealing characteristics of oxide-trap and interface-trap charge are responsible for the complex postirradiation recovery of the output voltage. Index Terms--Enhanced low-dose-rate sensitivity, IC radiation response, linear bipolar ICs, modeling and simulation, radiation-induced leakage, voltage regulator.

The concept of hybrid clamped is proposed in multilevel-inverter topologies, and a hybrid-clamped multilevel-inverter topology comprising active and passive clamping devices is presented in this paper. In this topology, the dc-link capacitor voltages can be balanced without additional circuitry or separated dc voltage sources, regardless of load characteristics. It can be used in real and reactive power conversion applications. The topology structure, operating principle, and self-voltage balancing ability are analyzed. In addition, the validity is confirmed by simulations and experiments based on a five-level inverter. Finally, the functions of different clamping devices are compared. Index Terms--Hybrid clamped, multilevel inverter, neutral-point voltage balancing, topology.

This letter presents a current control technique for fast transient-current response in a sensorless vector-controlled permanent magnet synchronous motor. Pontryagin's maximum principle is employed to derive the time-optimal voltage solution. Based on this principle, the technique can guarantee the current vector transition from one operating point to another with minimum time. Compared to a proportional-integral controller, the current response by using the time-optimal controller is much faster, even under a limited dc bus voltage. Experiments are carried out to substantiate the proposed technique. Index Terms--Fast-transient, current control, permanent magnet synchronous motor (PMSM), sensorless vector control, time-optimal control.

The transient-induced resonance on the output voltage of a power supply can be eliminated if the output filter of the power supply is designed to appear as a constant resistor to the load. A switched active filter is studied as a means to provide this constant resistor over those narrow frequency bands where the resistor cannot be realized by passive means. Design-oriented modeling of the hybrid filter is presented, and is supported by a high-current, high-frequency simulation example, and a low-current, low-frequency experiment. Index Terms--Active filter, adaptive voltage positioning (AVP), extended adaptive voltage positioning (EAVP), hybrid filter, microprocessor power delivery, switched active filter, transient regulation.

Inductance-capacitor-inductance (LCL)-filters installed at converter outputs offer higher harmonic attenuation than L-filters, but careful design is required to damp LCL resonance, which can cause poorly damped oscillations and even instability. A new topology is presented for a discrete-time current controller which damps this resonance, combining deadbeat current control with optimal state-feedback pole assignment. By separating the state feedback gains into deadbeat and damping feedback loops, transient overcurrent protection is realizable while preserving the desired pole locations. Moreover, the controller is shown to be robust to parameter uncertainty in the grid inductance. Experimental tests verify that fast well-damped transient response and overcurrent protection is possible at low switching frequencies relative to the resonant frequency. Index Terms--Electromagnetic interference (EMI), inductance-capacitor-inductance (LCL)-filter, voltage source converters (VSCs).

This paper begins with a review of different current sensing methods for the voltage regulator (VR) to achieve adaptive voltage position (AVP). These methods are either inaccurate or with high power consumption for current sensing. Next, a new input-side current sensing method for AVP is proposed. The basic idea of the proposed method is to sense the average input current, which results in low power dissipation for current sensing. Meanwhile, a compensation circuit is proposed to eliminate current sensing errors due to input and output voltage variations. As a result, both low power dissipation and good current sensing accuracy can be achieved by applying the proposed method. The transient and tolerance analysis of this method are provided and a design guideline is illustrated and experimentally verified. Index Terms--Adaptive voltage position (AVP), voltage regulator (VR).

By utilizing a differential tunable active inductor for the LC-tank, a wide tuning-range CMOS voltage-controlled oscillator (VCO) is presented. In the proposed circuit topology, the coarse frequency tuning is achieved by the tunable active inductor, while the fine tuning is controlled by the varactor. Using a 0.18-[micro]m CMOS process, a prototype VCO is implemented for demonstration. The fabricated circuit provides an output frequency from 500 MHz to 3.0 GHz, resulting in a tuning range of 143% at radio frequencies. The measured phase noise is from -101 to -118 dBc/Hz at a 1-MHz offset within the entire frequency range. Due to the absence of the spiral inductors, the fully integrated VCO occupies an active area of 150 x 300 [micro][m.sup.2]. Index Terms--Differential active inductors, frequency tuning range, miniaturization, multistandard transceivers, phase noise, voltage-controlled oscillators (VCOs), wideband.

This paper investigates the use of the constraint reactive implicit coupling (CRIC) method for the engine of a continuation power flow program. Full Newton continuation power flow methods are robust and accurate but are computationally expensive. Fast decoupled methods provide accurate results and require less computational time, but their performance worsens at heavy loading conditions, where the system active/reactive power decoupling characteristics are lost. This paper makes use of the CRIC method, which preserves the decoupled power flow solution structure but better models the active/reactive coupling. Effective stopping criteria are proposed for the continuation method, which helps to speed up computation. Such stopping criteria are also applied for tracing QV curves for some practical Brazilian power systems, with all operating limits considered. Index Terms--Constraint reactive implicit coupling (CRIC) method, continuation methods, Newton method, power flow, QV curves, stopping criteria, voltage collapse.

In this paper, a single phase inductance-capacitance-inductance (LCL) output stage for grid coupled inverters is designed and built. An accurate model and observer of the output filter and the distorted grid voltage are implemented. The paper deals with the construction of a 14-state model, and the feedback control loop to obtain adequate closed loop response. Simulations indicate a good performance of the controller, with a total harmonic current distortion (THD) below 1%. Experimental results confirm simulations, and illustrate the correct operation of the Kalman observer to estimate the distorted grid voltage (THD 3%). The observer only uses the inverter current measurement as input. The output filter effectively reduces the pulsewidth modulation harmonics in the grid current. Index Terms--Active front end, active power filter, harmonics, inverter current control, inductance-capacitance-inductance (LCL) output stage design, low grid current distortion, power quality, voltage source inverter (VSI).

The dynamic voltage restorer (DVR) as a means of series compensation for mitigating the effect of voltage sags has become established as a preferred approach for improving power quality at sensitive load locations. Meanwhile, the cascaded multi-level type of power converter topology has also become a work-horse topology in high power applications. This paper presents the detailed design of a closed loop regulator to maintain the load voltage within acceptable levels in a DVR using transformer coupled H-bridge converters. The paper presents system operation and controller design approaches, verified using computer simulations, and a laboratory scale experimental prototype. Index Terms--Dynamic voltage restorer (DVR), H-bridge converter, voltage sags.

This paper presents an ac-dc converter topology for realization of power factor correction (PFC) voltage regulators for applications where the mains frequency is high and a low input current harmonic is required, e.g., in aircraft power systems. The proposed converter represents a minimal configuration consisting of two basic converters, which can be systematically derived from a previously proposed general synthesis procedure for rectifierless ac-dc converters. The proposed PFC converter has incorporated a control method which drastically reduces the circulating power and hence raises the efficiency to a level comparable to existing PFC converters. The proposed PFC converter can completely eliminate any crossover distortion, which can be significant for supply systems having a high mains frequency. In addition, the proposed converter allows bidirectional energy flow ensuring all inductors work in continuous conduction mode hence eliminating the distortion due to the abrupt change of dynamic response when the operating mode changes. Analysis and design of the power and control circuits will be given and discussed. An experimental system will be presented for verification purposes. Index Terms--AC-DC converter, airborne application, low distortion, power factor correction (PFC), rectifierless converter.

This paper is the first part of a work aimed at predicting the voltage sag performance of distribution networks using digital simulation and an EMTP-like tool. The paper discusses modeling guidelines for representing power distribution components in voltage sag simulations and details the approaches selected in this work. Different models for representing some components can be used to obtain voltage sag characteristics with similar accuracy. Test results are included to illustrate the performance of the developed models and support the selection of different modeling approaches. Index Terms--Modeling, power distribution, power quality, simulation, voltage sags.

The static var compensator (SVC) has fast dynamic characteristics that can support effective system voltage following disturbances. Keeping reactive power reserve in an SVC during steady-state operation is always needed to provide reactive power requirements during system dynamics. This paper presents a new SVC control strategy with two stages of regulation slopes and two voltage regulation controls, which are fixed-voltage reference control and variable (floating) voltage reference control. The aim of this control is to limit the reactive power output from the SVC to the desired value during the steady-state voltage range and compensate the reactive power requirement from the upstream networks via the coordination with the under-load tap changer (ULTC). However, when the voltage deviates the steady-state voltage range, the SVC will react to support the system voltage using the SVC reactive power reserve. When a disturbance results in a new operating point, with a steady-state reactive-power output, the variable voltage reference control effectively changes the SVC output slowly and returns it within the steady-state margin. Index Terms--Coordinated control, static var compensator (SVC), under-load tap changer (ULTC) SVC, variable reference control.

A novel approach has been presented in this paper to calculate voltage flicker components precisely by using instantaneous voltage vectors. After the voltage waveform of a phase is recorded, the smart discrete Fourier transform can be used to obtain the system frequency and magnitude. Then, the other two phases are assumed perfectly sinusoidal to construct a virtual three-phase system. The instantaneous voltage vectors are calculated from the virtual three-phase voltages. Finally, the fast Fourier transform is used to obtain the voltage flicker components from instantaneous voltage vectors. The flicker components of the other two phases can be calculated by repeating the procedure. The flicker values of three phases are calculated individually and separately. The effects of jump-sampling, harmonics, power frequency shifting, and sampling rates are investigated. The calculation ability of this approach is compared with the traditional indirect demodulation method. Some given waveforms and field measured waveforms of arc furnace loads with voltage flicker disturbances are used to show the goodness of this approach. From the results, this approach could calculate flicker components accurately with short calculation time by using small size data. It also avoids the frequency leakage effect. Index Terms--Indirect demodulation method, instantaneous voltage vector, power quality, smart discrete Fourier transform, voltage flicker.

Voltage fluctuations caused by rapidly changing loads, such as arc furnaces, can propagate to different parts of a power system. Although the flicker level at its origin can be high, levels that are measured at other sites are subject to attenuation, a process that is influenced by fault levels, transformer impedances, line impedances, and composition of the connected loads. This paper presents the methodology, measurement results, and data analysis in relation to synchronized flicker measurements carried out in a high-voltage (HV)/medium-voltage (MV) power system which contains an arc furnace supplied by a dedicated feeder connected to the HV busbar. The flicker transfer coefficients derived from measurement results clearly indicate that flicker transfer from the arc furnace site to the upstream HV busbar is governed by the fault levels at the two locations. However, the transfer of flicker from the upstream HV busbar to other downstream busbars is dependent on the downstream load composition. These flicker transfer coefficients are vital in the application of methodologies described in many reports and standards in relation to establishing planning levels at various voltage levels and in the allocation of flicker emission to customers. Index Terms--Flicker, flicker measurement, flicker transfer coefficient.

This paper proposes a novel configuration of a unified power quality conditioner (UPQC) which can be connected to the distribution system without series injection transformers. The operation of the proposed system was analyzed through simulations with PSCAD/EMTDC and experimental works with a scaled hardware model, assuming that the UPQC is connected with the 22.9-kV distribution line. The proposed UPQC has the ultimate capability of improving power quality at the point of installation on power distribution systems. It has flexibility in expanding the operation voltage by increasing the number of H-bridge modules. Index Terms--dynamic voltage restorer (DVR), H-bridge, PSCAD/EMTDC, static synchronous series compensator (SSSC), Static var Compensator (STATCOM), unified power-flow controller (UPFC), unified power-quality conditioner (UPQC).

Differences between the tap-changing process in conventional mechanical and electronic tap-changers are briefly reviewed. A model of the controller of a mechanical tap-changer system is then presented. Modifications to this controller are then proposed for adaptation to an electronic tap-changer. A general structure of an electronic tap-changer and the simulation of the modified controller are then presented. A number of experimental results shows the performance of the modified controller. Index Terms--Controller, design, electronic tap-changer, voltage regulation.

A new simple balanced circuitry using four printed circuit board trace capacitors effectively detects the cold-cathode fluorescent lamp current and the transformer voltage of the inverter without grounding either side of the lamp. The inverter employing this circuit is free from the problems caused by grounding one side of the lamp, and it provides a stable and wide dimming control as well as an open-lamp voltage regulation with a better efficacy. Index Terms--Cold-cathode tube, current sensing, printed circuit board (PCB) capacitors, resonant inverters, voltage sensing.

The paper investigates the dynamic operation of novel control scheme for both Static Synchronous Compensator (STATCOM) and Static Synchronous Series Compensator (SSSC) based on a new full model comprising a 48-pulse Gate Turn-Off thyristor voltage source converter for combined reactive power compensation and voltage stabilization of the electric grid network. The complete digital simulation of the STATCOM and SSSC within the power system is performed in the MATLAB/Simulink environment using the Power System Blockset (PSB). The STATCOM scheme and the electric grid network are modeled by specific electric blocks from the power system blockset, while the control system is modeled using Simulink. Two novel controllers for the STATCOM and SSSC are presented in this paper based on a decoupled current control strategy. The performance of both STATCOM and SSSC schemes connected to the 230-kV grid are evaluated. The proposed novel control schemes for the STATCOM and SSSC are fully validated by digital simulation. Index Terms--48-pulse Gate Turn-Off (GTO) thyristor model STATCOM, novel decoupled control strategy, reactive compensation, Static Synchronous Series Compensator (SSSC), voltage stabilization.

In this paper, tour different system topologies for dynamic voltage restorers (DVRs) are analyzed and tested, with particular focus on the methods used to acquire the necessary energy during a voltage sag. Comparisons are made between two topologies that can be realized with a minimum amount of energy storage, with energy taken from the grid during the voltage sag, and two topologies that take energy from stored energy devices during the voltage sag. Experimental tests using a 10-kVA DVR show that the no-energy storage concept is feasible, but an improved performance can be achieved for certain voltage sags using stored energy topologies. The results of this comparison rank the no-storage topology with a passive shunt converter on the load side first, followed by the stored energy topology with a constant dc-link voltage. Index Terms--Control, dynamic voltage restorer (DVR), medium-voltage-source converter, power quality, voltage dips.

The utilization of dynamic voltage restorers to protect sensitive loads against short-term voltage disturbances has been proven to be a competitive solution in medium- and high-power applications. For low-power applications further cost reductions are required to make the technology even more competitive, although just a few numbers of studies have already been conducted in this area. This paper discusses the design and operation of a three-phase dynamic voltage compensator, including control strategies for the converters. The proposed topology and control strategies are evaluated through simulation studies based on measured data, selected by the IEEE Power Quality Task Force for the characterization of power quality events and device performance evaluation, along with disturbances recorded in a steel plant, in Minas Gerais State, Brazil. An experimental implementation of the compensator is developed to allow a better evaluation of specific and critical operating modes, such as the forced commutation of the bypass thyristors and inverter operation. Index Terms--Component-reduced converters, power quality, voltage sag compensators.

This paper proposes a step-by-step procedure for designing the LCL filter of a front-end three-phase active rectifier. The primary goal is to reduce the switching frequency ripple at a reasonable cost, while at the same time achieving a high-performance front-end rectifier (as characterized by a rapid dynamic response and good stability margin). An example LCL filter design is reported and a filter has been built and tested using the values obtained from this design. The experimental results demonstrate the performance of the design procedure both for the LCL filter and for the rectifier controller. The system is stable and the grid current harmonic content is low both in the low- and high-frequency ranges. Moreover, the good agreement that was obtained between simulation and experimental results validates the proposed approach. Hence, the design procedure and the simulation model provide a powerful tool to design an LCL-filter-based active rectifier while avoiding trial-and-error procedures that can result in having to build several filter prototypes. Index Terms--Cascade control, LCL filter, rectifier, stability, voltage-source converter (VSC).

A voltage-source back-to-back system for the purpose of achieving power flow control and/or line-frequency change in transmission systems has the attractive features of reliable and continuous operation, even during power line faults. However, an overvoltage appearing across the dc link during the faults should be limited to as low a value as possible because it does affect the power device ratings. This paper proposes a dc-link voltage regulator for effectively suppressing the overvoltage during power line faults. This regulator is characterized by compensating for a time delay inherent in each current controller, and for a power flow imbalance occurring during power line faults. The validity of the proposed regulator is confirmed by theory and computer simulation. Index Terms--Back-to-back (BTB) system, dc-link voltage regulation, line-frequency change, power flow control, power line faults.