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An approach for the control of a voltage-sourced converter-interfaced distributed energy resource microgrid environment with multiple energy sources is analyzed and experimentally validated. The control approach is designed to operate in grid-connected and islanded modes of operation, as well as provide a smooth transition between the two modes. Additional features including islanding detection with positive feedback and dynamic overcurrent limiting are also evaluated. Validation is achieved through the results obtained from a scaled down prototype system with further results from the time-domain simulation of a medium-voltage microgrid.

This paper presents supervisory control of a dynamic flow controller (DFC) based on the discrete-event systems (DES) theory. A DFC can be considered as a flexible AC transmission system controller and includes a mechanically-switched phase-shifting transformer, a multimodule thyristor-switched capacitor, a multimodule thyristor-switched reactor, and a mechanically switched capacitor. Owing to the inherent discrete switching nature of a DFC, its components are modeled as finite automata; then, a DES supervisory control is designed to implement the control logic of the DFC system in different modes of operation (i.e., ldquoautomaticrdquo and ldquoauto/manualrdquo). It is shown that the specifications are controllable and the synthesized supervisors are nonblocking in both modes and the modular supervisors nonconflict in auto/manual mode.

Without storage provision, a wind energy conversion system (WECS) does not have fault ride-through capability for most temporary faults on the utility feeder. This paper proposes a hybrid valve switching and control strategy for a voltage-sourced converter (VSC) used for interfacing a WECS to the utility grid. The hybrid control of the VSC ensures continuous operation of the system in the presence of temporary single line to ground faults on the utility feeder without the need for a storage provision. The fast acting hybrid control also limits reactive fault current contribution by the converter, and therefore, avoids problems associated with overcurrent protection of the feeder. The hybrid valve switching and control of the VSC consists of: 1) sinusoidal pulse width modulation (SPWM) based valve switching and current-controlled voltage-source operation of the VSC during normal system operating conditions and 2) hysteresis space vector modulation (HSVM) based switching together with controlled current-source operation of the VSC during temporary fault conditions. The hybrid control of the VSC isolates the WECS from the grid side disturbances to ensure uninterrupted operation of the unit. Simulation studies of the grid-interactive WECS in PSCAD/EMTDC confirm the validity of the proposed hybrid control scheme.

This paper presents a new voltage control strategy for an electronically-interfaced distribution generation (DG) unit that utilizes a voltage-sourced converter (VSC) as the interface medium. The control strategy is based on the concept of voltage-controlled VSC (VC-VSC) rather than the conventional current-controlled VSC (CC-VSC). The proposed VC-VSC 1. enables operation of a DG unit in both grid-connected and islanded (autonomous) modes, 2. provides current-limit capability for the VSC during faults, 3. inherently provides an islanding detection method without non-detection zone, 4. provides smooth transition capability between grid-connected and autonomous modes, and 5. can accommodate ride-through capability requirements under a grid-connected mode. This paper also investigates performance of the proposed VC-VSC strategy based on an eigenanalysis in MATLAB, and time-domain simulations in the PSCAD/EMTDC environment.

This paper addresses real and reactive power management strategies of electronically interfaced distributed generation (DG) units in the context of a multiple-DG microgrid system. The emphasis is primarily on electronically interfaced DG (EI-DG) units. DG controls and power management strategies are based on locally measured signals without communications. Based on the reactive power controls adopted, three power management strategies are identified and investigated. These strategies are based on 1) voltage-droop characteristic, 2) voltage regulation, and 3) load reactive power compensation. The real power of each DG unit is controlled based on a frequency-droop characteristic and a complimentary frequency restoration strategy. A systematic approach to develop a small-signal dynamic model of a multiple-DG microgrid, including real and reactive power management strategies, is also presented. The microgrid eigen structure, based on the developed model, is used to 1) investigate the microgrid dynamic behavior, 2) select control parameters of DG units, and 3) incorporate power management strategies in the DG controllers. The model is also used to investigate sensitivity of the design to changes of parameters and operating point and to optimize performance of the microgrid system. The results are used to discuss applications of the proposed power management strategies under various microgrid operating conditions

This paper presents a detailed model for simulation of a Superconducting Magnetic Energy Storage (SMES) system. SMES technology has the potential to bring real power storage characteristic to the utility transmission and distribution systems. The principle of SMES system operation is reviewed in this paper. To understand transient and dynamic performance of a SMES system, a detailed SMES system benchmark model is given with extensive simulation results. This system is demonstrated using an electromagnetic transient program-PSCAD/EMTDC.

This paper investigates the use of an AC-AC matrix converter, as an alternative to AC-DC-AC converter system, to interface a high-speed micro-turbine generator (MTG) to a utility grid as a distributed generation unit. A new switching strategy and a control mechanism for the converter are presented. Based on time-domain simulations in the PSCAD/EMTDC environment, the dynamic behavior of a micro-turbine generation system, including the dynamic models of the micro-turbine (thermodynamics), generator, matrix converter, converter control and micro-turbine control, is studied. The studies conclude that a matrix converter is technically a viable option to interface a MTG to a utility grid.

A recently proposed high power switch-mode rectifier topology offers advantages over the conventional line-commutated thyristor rectifiers in terms of size and cost. This paper presents modeling and analysis of the high power switch-mode rectifier configuration, operated under a newly developed digital control strategy. The developed strategy provides a fast transient response to disturbances and achieves equal current sharing between the buck converter modules which are paralleled within the switch-mode rectifier. A continuous-time state-space model of the switch-mode rectifier and analyses of both the steady-state and the transient operation modes of the digitally controlled rectifier are presented. The continuous-time model is used as a basis for time-domain simulations of the rectifier's behavior in the MATLAB/Simulink environment. The validity of the presented model is demonstrated based on comparison between the simulation results and the corresponding experimental results. The experimental results are obtained from a 2-kW scaled-down laboratory setup of the rectifier. The stability of the digitally controlled rectifier system is also examined based on an overall small-signal sampled-data model of the rectifier system including the controls.

A benchmark system is presented for digital time-domain simulation of a shunt connected active power filter. The active power filter consists of a 3-phase voltage source converter operating under pulse-width modulation. The converter is controlled using a digital deadbeat current regulator. Harmonic prediction is employed to allow accurate harmonic compensation despite controller latencies. A PSCAD/EMTDC simulation model is developed based on a laboratory scale active power filter. By employing an experimental system as a basis, a pragmatic control structure, which accounts for all implementation constraints, is assured. Experimental measurements clearly validate the accuracy of the simulation model.

A novel method of extraction and measurement of individual harmonics of a signal with time-varying frequency is presented. The proposed method is based on a nonlinear, adaptive mechanism. Compared with the well-established techniques such as DFT, the proposed method offers (i) higher degree of accuracy, (ii) structural/performance robustness, and (iii) frequency-adaptivity. The structural simplicity of the algorithm renders it suitable for both software and hardware implementations. The limitation of the proposed method as compared with DFT-based methods is its slower transient response. Based on simulation studies, performance of the method is presented and its accuracy and response time are compared with a DFT-based method.

This paper presents a new synchronization method which employs an enhanced phase-locked loop (EPLL) system. The operational concept of the EPLL is novel and based on a nonlinear dynamical system. As compared with the existing synchronization methods, the introduced EPLL-based synchronization method provides higher degree of immunity and insensitivity to noise, harmonics and other types of pollutions that exist in the signal used as the basis of synchronization. The salient feature of the EPLL-based synchronization method over conventional synchronization methods is its frequency adaptivity which permits satisfactory operation when the centre frequency of the base signal varies. The proposed EPLL-based method of synchronization is also capable of coping with the unbalanced system scenarios. Structural simplicity of the EPLL-based method greatly simplifies its implementation in digital software and/or hardware environments as an integral part of a digital control platform for power electronic converters. The primary application of the proposed synchronization method is for the distributed generation units, e.g., wind generation systems, which utilize power electronic converters as an integral part of their systems.

A signal processing system for extraction of harmonic and reactive current components is introduced and its performance is evaluated. The extraction system is adopted as part of the control system of a single-phase active power filter (APF) to provide the required signals for harmonic filtering and reactive power compensation. Performance of the overall system is evaluated based on digital time-domain simulation studies. The APF control system including the signal processing algorithms are implemented in Matlab/Simulink Fixed-Point Blockset to accommodate bit-length limitation which is a crucial factor in digital implementation. The power system including the APF, load and the supply system are simulated with the PSCAD/EMTDC software to which the Matlab-based control model is interfaced. The simulation results indicate that the signal processing unit can provide the required signals for APF to perform filtering/compensation within the transient period of 2 to 3 cycles.

A new method for estimation of power frequency and its rate of change is presented. Unlike conventional methods which are based on the concept of linearization, the proposed scheme accommodates the inherent nonlinearity of the frequency estimation problem. This makes the method capable of providing a fast and accurate estimate of the frequency when its deviation from the nominal value is incremental or large. The estimator is based on a newly developed quadrature phase-locked loop (QPLL) concept. The method is highly immune to noise and distortions. The estimator performance is robust with respect to the parameters of its structure. Structural simplicity and performance robustness are other salient features of the method.

Conventionally, control system of a static transfer switch (STS) is designed based on the assumption that the STS terminal voltages are in-phase. This paper investigates the impact of phase difference of the STS terminal voltages on the STS transfer time and cross current. The phase difference is assumed to be a result of the difference of the feeder impedances. The paper shows that phase differences of even up to 4.5/spl deg/, which are accompanied by up to 5% voltage drop at the STS terminal, can noticeably increase the transfer time and the cross current magnitude. The studies are conducted on the IEEE STS-l benchmark system using the PSCAD/EMTDC software.

This paper deals with the calculation of a sparse network equivalent (SNE) for the analysis of electromagnetic transients in large systems. The main feature of the new approach is the enforcement of sparsity, stability, passivity, and accuracy at specific frequencies of the equivalent. The procedure is based on time-domain fitting with quadratic programming to enforce the constraints. The SNE can be interfaced with transient calculation programs directly in time domain. Results are presented in the companion paper.

This paper deals with the calculation of a sparse network equivalent (SNE) for the analysis of electromagnetic transients in large systems. The methodology and the mathematical details are presented in the companion paper . This sequel presents application examples to demonstrate the accuracy and computational efficiency of the SNE. Equivalents for networks composed of 12, 50, and 100 transmission lines are derived and the results are discussed.

A novel method of frequency estimation, for power system applications such as control and protection, is proposed and its performance is evaluated. The proposed frequency estimator can measure small as well as large deviations from the nominal point. It closely follows step, ramp and oscillatory variations of the frequency over time. Other significant features of the proposed algorithm are: (a) structural simplicity, which renders it suitable for hardware/software implementation; (b) performance robustness in the presence of DC offset and harmonic components; (c) noise immunity; (d) performance robustness with respect to external disturbances such as commutation notches and switching transients; and (e) flexibility of control over speed and accuracy. Rate of change of frequency is also directly provided by the estimator which is a requirement in some system protection algorithms. In a highly polluted environment, the proposed estimator can be set to measure the frequency in few cycles of the signal and with a steady-state error which is limited to 0.02 Hz.

High power switch-mode rectifiers offer advantages over conventionally used line-commutated thyristor rectifiers in terms of size and cost. This paper describes operation of the high power switch-mode rectifier based on an original control strategy which is digitally implemented. The rectifier is operated in a constant output current mode. The control strategy provides a fast transient response to disturbances and achieves equal current sharing between the buck converter modules which are paralleled within the switch-mode rectifier. Equal current sharing is maintained in steady-state as well as in transient operation. The paper demonstrates that the transient response to load disturbances achievable with the proposed control strategy is superior to the response obtained with the conventional control strategy in terms of the output current overshoot and the settling time. The control algorithm is digitally implemented on a DSP/FPGA based digital control platform and its performance is verified on a 2 kW scaled-down experimental setup of the switch-mode rectifier.

A benchmark system for studying the sinusoidal pulse-width-modulated distribution-static synchronous compensator (D-STATCOM) is proposed. A simple multifunction controller is provided to offer DC bus voltage control, power factor correction, and AC voltage control. Simulation results are presented and compared to experimental results obtained from a 10 kVA laboratory D-STATCOM. Excellent agreement between simulation and experimental results is obtained, validating the simulation output.

The ability to predict or confirm ferroresonance depends primarily on the correctness of the transformer model used by the computer simulation. Ferroresonance is introduced and a general modeling approach is given. An overview of available literature and contributors to this area is provided. A simple ease of ferroresonance in a single phase transformer is used to illustrate this "phenomenon". Three phase transformer core structures are discussed. Ferroresonance in three phase grounded-wye distribution systems is described and illustrated with waveform data obtained from laboratory simulations. Representation of the study zone is discussed, modeling techniques are presented, and implementation suggestions are made. Three case studies are presented. Transformer representation is critical to performing a valid simulation. The direction of ongoing research is discussed, and the reader is advised to monitor the literature for ongoing rapid improvements in transformer modeling techniques.

This paper introduces a new online voltage disturbance detection approach based on the wavelet transform. The proposed approach: (1) identifies voltage disturbances; and (2) discriminates the type of event which has resulted in the voltage disturbance, e.g. either a fault or a capacitor-switching incident. The proposed approach is: (1) significantly faster; and (2) more precise in discriminating the type of transient event than conventional voltage-based disturbance detection approaches. The feasibility of the proposed disturbance detection approach is demonstrated based on digital time-domain simulation of a power distribution system using the PSCAD/EMTDC software package.

This paper presents an alternate approach for modeling of steady state performance of large turboalternators including the effects of magnetic saturation. To avoid the superposition limitations of the conventional d-q approach, a sectoral model of a pole of the machine is developed and solved iteratively. The computational effort is orders of magnitude less than for a finite element solution. Results from the proposed model are compared with those from a conventional d-q model and with measurements made for a wide range of load conditions on a 588-MVA turbogenerator.

With the proliferation of static switching circuits in power systems, there is a need to understand not only their steady-state behaviour but their dynamic interaction with the network as well. Analysis is difficult due to the nonlinear nature of switching processes associated with such devices. Linear, time-invariant (LTI) dynamic models are needed for standard analytical tools such as eigenanalysis for the study of inter-area as well as higher frequency modes of the system. To this end, LTI small-signal dynamic models are developed for high power static switching circuits in a synchronous rotating dq-reference frame. Two conventional devices are chosen for study: a thyristor controlled series capacitor (TCSC) and a HVDC power converter. The proposed models are shown to represent the dynamic response of the static switching circuits to nominal changes in operating point. In particular, the damping characterizing the dynamic behaviour of the TCSC is shown to vary significantly with operating point. This accounts for the observed responses arising from field tests and simulations but not accounted for by the quasi-static model. On the other hand, the damping characterizing the dynamic behaviour of the HVDC converter is shown to vary little with operating point. The linearized models are validated by digital time-domain simulation of the nonlinear static switching circuits.

This paper reports a set of digital time-domain simulation studies conducted on TEHMP161A Coupling Capacitor Voltage Transformer (CCVT) of Haefely-Trench. The Electro-Magnetic Transients Program (EMTP) is used to develop the CCVT model and conduct the transient studies. The accuracy of the CCVT model is verified through comparison of the EMTP simulation results with those obtained from test results. The investigations demonstrate that the developed model can accurately predict CCVT transient response, e.g. the phenomenon of ferroresonance. The model is developed (1) to determine impact of transients on CCVT response, (2) to design, optimize and compare protective and ferroresonance suppressor devices of CCVT, and (3) to predict CCVT transient response on power system monitoring and protection schemes.

This paper presents a new approach for the dynamic control of FACTS apparatus, such as the STATCOM and UPFC, which utilize voltage source inverters (VSI) as their main building block. The control concept is based on a linearization of the dq inverter model. Feedforward techniques which are traditionally used for the approximate decoupling of d and q-axis control are discarded, in favour of a high gain full state feedback approach which assigns both closed loop system poles and, more importantly, their associated eigenvectors. Experimental validation of the approach is carried out on a laboratory STATCOM setup. Due to the nonlinear nature of the VSI equations and the uncertainty of AC system parameters, actual closed loop system dynamics can stray quite dramatically from those desired. Root locus analysis is therefore performed to investigate the small signal system dynamic behaviour. The loci demonstrate that the effect of system nonlinearity on the closed loop poles is virtually eliminated by the proposed control. The effect of AC system parameter variations is also shown to be minimal.

This is the fourth supplement to "A bibliography for the Study of Subsynchronous Resonance Between Rotating Machines and Power Systems". This paper includes the published papers in the area of subsynchronous resonance during the time span of February 1990 to end of January 1996.

Conventional subsynchronous resonance (SSR) study methods such as eigenanalysis require a linear dynamic model of each device. FACTS devices such as thyristor-controlled series capacitors (TCSC) are difficult to model due to their nonlinear switching behaviour. Linearizing the TCSC steady-state average model is inadequate as this does not model the passive damping associated with the open-loop operation of a TCSC. The proposed approach exploits the fact that the thyristor controlled reactor associated with the TCSC is switched in a regular pattern. A linear model is obtained by linearizing the half-period map associated with sampling the TCSC capacitor voltage twice every cycle. Such an approach models the passive damping which varies with the steady-state conduction angle of the TCSC. It is shown that passive damping has a significant effect on the modal damping of the torsional modes associated with subsynchronous resonance phenomena.

The first part of this three-part report explains the objectives of the task force and the scope of its report (1995). The first part also provides modelling guidelines for the investigation of small-signal torsional oscillations, large-signal shaft transient stresses, turbine-blade vibrations, and fast bus transfer based on the use of digital computer time-domain simulation methods. This paper provides modelling guidelines for the analysis of controller interactions, harmonic interactions, and resonance phenomena in power systems. Sample test systems and typical time-domain simulation results for each phenomenon are also provided.

This paper provides comprehensive development procedures and final forms of mathematical models of a unified power flow controller (UPFC) for steady-state, transient stability and eigenvalue studies. Based on the developed models, the impacts of control strategy, parameters and location of UPFC on power system operating conditions are discussed. The accuracy of the developed models is verified through comparing the study results with those obtained from detailed time-domain simulation using the Electromagnetic Transients Program (EMTP).

In this paper, the noncharacteristic harmonics of an HVDC power converter station are calculated based on the use of digital time-domain simulation methods. An enhanced version of the Electromagnetic Transients Program (EMTP) is used for the studies. The noncharacteristic harmonics of interest are: (1) the DC side triplen harmonics; and (2) the AC side second harmonic. Impacts of loading conditions, neutral filter, and converter firing angle on the DC side triplen harmonics are discussed. The effects of AC side network parameters, static VAr compensator (SVC), transformer half-cycle saturation, and geomagnetically induced currents (GIG) on the AC side second harmonic instability are also presented. This paper concludes that the digital time-domain simulation method provides significant flexibility for accurate prediction of: (1) generation mechanism; and (2) adverse impacts of HVDC noncharacteristic harmonics.

A.A. Edris (see ibid., vol.5, no.1, p.219-36, 1990) described a new concept for mitigation of the phenomenon of subsynchronous resonance (SSR) based on asymmetrical series capacitor compensation at SSR frequencies. The studies reported by A.A. Edris are based on a digital time-domain simulation technique. This paper provides a quantitative evaluation of the concept using a novel eigen analysis approach. The eigen analysis approach represents the mathematical models of power system components in the three-phase basis, and can evaluate the impacts of asymmetry and imbalance on the system dynamics in the subsynchronous frequency range. The study results restate the technical feasibility of the proposed SSR countermeasure. This paper opens the avenue for examination of active power filter topologies to introduce artificial asymmetry at SSR frequencies to counteract torsional oscillations. >

This paper deals primarily with the application of standstill frequency response (SSFR) models to subsynchronous resonance (SSR) studies involving large turbogenerators. It has been already recognized that models derived from SSFR testing intrinsically define the so-called subsubtransient frequency domain as well as the transient and subtransient domains. The application of a variety of SSFR models has been described previously only for one machine. Another turbogenerator possessing markedly different damping characteristics is compared with the one referred to above. Comparisons are also made with an IEEE "Benchmark" model. Comments regarding the damping performance of each of these three machines in the SSR range are discussed as well as the effect of rotor and field on stator differential leakage fluxes. Proposals are made for consideration by the IEEE Power System Engineering and Electric Machinery Committees involved in benchmark SSR stability programs or in related generator modeling standards. >

This paper describes the results of a comprehensive investigation on: (1) small-signal dynamic interactions of voltage control loops of multiple static VAR compensators (SVCs); (2) the phenomena of SVC-network interactions; and (3) torsional interactions with SVCs. This paper is concerned with small-signal dynamics in the high frequency (HF) range, i.e. above 4 Hz. The studies are conducted on various power system configurations, based on the use of a newly developed eigen analysis software tool. The eigen analysis tool provides a systematic approach to include the dynamic mathematical model of the transmission network in the overall system model, which is necessary for HF dynamic studies. The investigations identify undesirable small-signal oscillations associated with SVC-SVC and/or SVC-network interactions, and provide fundamental information for design/coordination of SVCs. >

This paper examines various functions that can be assigned to a static phase shifter (SPS) to enhance the performance of a power system during steady-state conditions, small-signal dynamics, and large-signal dynamics. The investigations are conducted on four test systems which exhibit typical power system operational problems, e.g. torsional oscillations, inter-area oscillations, transient instability, transient mechanical torques and loop-flow phenomenon. An AC-DC power flow program, an eigenvalue analysis program, an AC-DC transient stability program, and the EMTP are used as the study tools. The investigations show that depending upon the type and the location of an SPS, it may be utilized for mitigation of small-signal oscillations and/or enhancement of transient stability, in addition to steady-state power flow regulation. The studies also reveal that most dynamic characteristics of an SPS can be achieved by augmenting an existing conventional (mechanical) phase-angle regulator with a relatively small size static power converter. >

This paper elaborates on a new approach introduced in an earlier paper for the steady-state and transient analysis of synchronous machines and demonstrates the validity and accuracy of this model for system studies. The approach is based on developing an equivalent flux linkage circuit model from the dimensions and material properties of the machine. The model provides inherent flexibility for representation of magnetic saturation, deep bar effects, and saliency. The paper includes a number of example applications in which the analytical results are compared with lest results and with those obtained from conventional analysis based on Park's two axes machine model. >

This paper investigates the application of a GTO voltage source inverter in a two-terminal HVDC link, which is fed at the sending end by a line-commutated rectifier. This type of HVDC link may be applied when power transfer is predominantly unidirectional, especially to a weak AC system. The investigations are based on analytical studies and digital time-domain simulations with the Electromagnetic Transient program for DC systems (EMTDC). Control method and protection requirements are studied, together with dynamic behavior of the system following disturbances, e.g. DC fault, AC fault, start-up etc. The studies are aimed at exhibiting the technical feasibility of the proposed HVDC scheme. Need for further studies is pointed out when necessary. >

This paper presents part of the investigations carried out under EPRI contract RP2473-42, "Applications of static phase shifters for FACTS". Based on feasible semiconductor switches and converter topologies for high power applications, different static phase shifter (SPS) circuit configurations are identified and their operational principles are explained. SPS configurations are compared and their relative merits and/or limitations are highlighted. Two of the introduced SPS configurations provide two new layouts for retrofitting existing mechanical phase-angle regulators. A comparison paper examines technical performances of the SPS's for different applications. >

The standard EMTP (Electromagnetic Transients Program) is based on time domain calculations as these are eminently appropriate for simulation purposes. An approach to direct frequency-domain representation of an external system of any size or complexity is presented. The requirements for the external system are that it must be linear and time invariant, and it should include a transmission lines connecting it to the study zone and providing a travel delay tau . It is represented in the phase domain and does not have to be symmetrical. At the connecting bus between the study zone and the external system, Fourier transforms provide the conversion from the time domain to the frequency domain and vice versa. The accuracy and simplicity of direct frequency-domain modeling is preserved since no approximations or fitting are necessary, even for complex, three-phase frequency-dependent components. >

The authors investigate the impact of an inverter station on the torsional dynamics of a turbine-generator set which is located at the rectifier side of a parallel HVDC-AC power system. The stiffness of an AC bus is described in terms of its effective short-circuit ratio (ESCR). The studies show that depending on the stiffness of the inverter AC bus and electrical coupling between the inverter and rectifier AC buses, the inverter closed loop control can have a noticeable contribution to the instability of torsional dynamics. This adverse torsional impact is observed in the study results even when the inverter AC bus is fairly strong (ESCR larger than 5.0). >

The authors investigate the impact of synchronous machine parameters and second-order and third-order models on the phenomenon of torsional dynamics. The results were compared with those obtained from the conventional (2d, 1q) model. For each model, the effect of rotor-field differential leakage flux on the damping of the torsional oscillations is studied. Numerical values of the elements corresponding to each model were obtained based on a parameter fitting process to a set of stand-still frequency response (SSFR) test results. An eigen analysis method was used for the studies. The eigen analysis results were verified by digital time-domain simulation, using the electromagnetic transients program (EMTP). >

The author presents a method for reducing transient torsional stresses of steam turbine-generators in series-capacitor-compensated power systems, which are equipped with ZnO varistor schemes. The fundamental concept is to reduce the coupling between the electrical network and the shaft system with respect to the troublesome torsional frequency. The electrical network and the rotating shaft system are coupled through the generator air-gap torque. This coupling can be attenuated by unbalancing the network with respect to the phenomenon of torsional oscillations. This is achieved by setting the protective voltage levels of the three phases of a three-phase series capacitor at unequal values. The technical feasibility of the proposed method is demonstrated on a test system, using the BPA Electromagnetic Transients Program (EMTP) as the study tool. The impact of the proposed countermeasure on the transient torques of the system, as a result of different disturbances, is demonstrated. The limitations of the proposed method are discussed. >

The authors extend the analysis of torsional oscillations into the nonlinear range and investigate the Hopf bifurcation phenomenon of the torsional dynamics of a turbine generator in a series capacitor compensated power system. The transition of growing torsional oscillations into nonlinear oscillations with limit cycles is demonstrated. Based on physical reasoning, a set of approximate nonlinear differential equations which explain the fundamentals of nonlinear torsional dynamics and the associated bifurcation phenomenon is developed. The results from the nonlinear equations are verified by time-domain simulation studies, using the Electromagnetic Transients Program (EMTP). The studies indicate that because of the Hopf bifurcation phenomenon, the range of instability (growing oscillations) of torsional oscillations is noticeably narrower than the one which is predicted by an eigenanalysis method. >

A report is presented on the theoretical background and the methodology of development of a software tool for offline coordination of power system controllers in either a high-frequency (5-55 Hz) or a low-frequency (0.2-2.0 Hz) range. The controllers considered for coordination are: voltage regulators, power system stabilizers (PSSs), speed governors, main and auxiliary controllers of HVDC converters, and main and auxiliary controllers of static VAr compensators (SVCs). The software package calculates the eigenvalues associated with a prespecified number of least-stable oscillatory modes of the system using a modified version of the S matrix method, and it adjusts the controller parameters to increase a damping of the least-stable mode, using a gradient optimization technique. The software tool is referred to as EICO (EIgenvalue program of COordination of controllers). EICO is coded in Fortran. The application of EICO is demonstrated by coordinating the controllers of a power system with 371 state variables and 140 controller parameters, in the high-frequency range. EICO results are verified by detailed digital simulation of the system, using the electromagnetic transients program (EMTP). The study results demonstrate the technical advantages of the coordinated controllers as compared with the noncoordinated scheme. >

This paper presents a systematic approach for the design and analysis of a supervisory hybrid control scheme for a micro grid system using hybrid control techniques. A generic micro grid configuration is assumed. The approach is elaborated with a specific micro grid configuration containing a self-excited induction machine based wind energy conversion system. By definition a micro grid operates in both grid-connected and in isolated modes. In each mode of operation there could be different combinations of the available energy sources in the system that are catering to the load demand. A hybrid control scheme which utilizes different control mechanisms for optimal control of a system under different operating conditions and in different operating states, presents an attractive paradigm for the control design of such a system. By partitioning a micro grid into different modules along suitable axis, the complexity of a Multiple Input Multiple Output (MIMO) control problem of the system can be significantly reduced. The control of the different modules of a micro grid system can then be tackled using the well established linear control theory which could then be combined using suitable transition, load and power management strategies to achieve optimal control of the micro grid system in all its desirable operating states. Supervisory hybrid control of a wind energy conversion and storage system is presented to illustrate the supervisory hybrid control design and analysis philosophy outlined in this paper.

Two novel power flow controller topologies are proposed for flexible AC transmission systems (FACTS). The first one consists of a shunt connected source of reactive power, and two series connected voltage-sourced converters - one on each side of the shunt device. The second topology is a dual of the first; it is based on two shunt connected current-sourced converters around a series connected reactive element. In both cases the converters can exchange active power through a common DC circuit. Both topologies make combined use of passive components and converters and can therefore be regarded as hybrid. Employing hybrid topologies enables use of converters to enhance the functionality of existing equipment in a power system. The paper demonstrates that, by using appropriate converter additions and control, the functionality of switched shunt or series capacitors can be enhanced to generalized power flow control - a functionality commonly associated with the UPFC. Since existing equipment is fully utilized, the hybrid topology requires considerably lower total converter ratings compared to the UPFC.

This paper investigates the nonlinearity of unified power flow controller (UPFC) on the power system load flow problem. Since the various UPFC parameters equations are nonlinear, a form of vector presentation in complex power plane is used to visualize the effect of different control parameters of UPFC on line power transfer. This visualization helps to identify the effectiveness of each control parameters on power control region.

This paper presents a unified power signal processor (PSP) for use in various applications in power systems. The introduced PSP is capable of providing a large number of signals and pieces of information which are frequently required for control, protection, status evaluation, and power quality monitoring of power systems. The PSP receives a set of locally measured three-phase voltage and current signals and provides their fundamental components, amplitudes, phase angles, frequency, harmonics, instantaneous and stationary symmetrical components, active and reactive currents and powers, power factor, and the total harmonic distortion. Simplicity and integrity of its structure as well as its robustness with respect to internal parameters and external disturbances and noise render the proposed scheme very attractive for practical implementations.