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This paper presents a new grid connected induction generator system having open-end stator windings with a series compensator (voltage source inverter (VSI) connected in series) for reactive power compensation. The proposed scheme consists of a two-level capacitor fed voltage source inverter connected to one side of the windings of the induction generator while the other end of the windings are connected to the grid. A current oriented control scheme is developed to control the operation of the VSI to ensure that it compensates the reactive power required by the induction generator. Thus the induction generator system can supply active power to the grid without drawing reactive power from it for excitation. Unlike the shunt compensated induction generator systems, the proposed scheme does not require an interfacing inductor for connecting to the grid thereby reducing the hardware footprint. The proposed scheme is validated by simulation using PLECS software and the simulation results are presented in the paper.

This paper proposes a novel damping controller for subsynchronous resonance (SSR) mitigation using existing 11-kV variable frequency drive interfaced auxiliary power plant loads. The input of the proposed auxiliary damping controller is the turbine output power $(P_{t})$ . This is a standard signal that is monitored in power plant control rooms and is available locally without the need for additional measurement and/or communication infrastructure. The auxiliary damping controller (ADC) adds an auxiliary speed signal to the existing speed reference in the closed-loop motor drive control in response to any torsional range oscillations seen in $P_{t}$ , via a feedback compensator. The ADC extracts damping by exploiting SSR load interactions and is tuned using a residue-based pole placement technique. The ADC performance is evaluated for both torsional interaction and torque amplification types of SSR in the IEEE First Benchmark and IEEE 68-bus networks. The results show that the ADC is effective in providing positive damping to mitigate unstable SSR oscillations under a range of operating conditions. The proposed solution is a simple yet effective means of providing local control of SSR with minimal additional cost.

This paper proposes a new fault ride-through (FRT) configuration for a fixed-speed induction generator-based–wind turbine (FSIG–WT). It utilizes a nine-switch converter (NSC) to enable shunt and series compensation in response to the system dynamics and grid faults. The steady-state control strategy and transient-management scheme are developed to ensure proper performance in steady-state, dynamic operation as well as enhanced FRT capability of the FSIG–WT. In addition to that, the developed control strategy tackles the balanced and unbalanced operation of the electric grid with an enhanced positive- and negative-sequence compensation control scheme. The recently reported grid synchronization scheme, namely, the frequency-locked loop (FLL) is implemented to achieve fast phase detection and transient response. Consequently, the performance of the FLL has been compared with the multiple reference–frame–phase-locked loop (MRF–PLL) to demonstrate its superior performance for enhancing transient response. The proposed topology, FLL, and control strategies are verified through a comprehensive simulation study to demonstrate the enhancement of the transient response and FRT capability of FSIG-WT in adherence to grid-code requirements.

This paper proposes a novel busbar protection method based on the polarity comparison of superimposed current. Analysis shows that all lines connected to the faulted busbar have the same polarities for the superimposed currents in case of an internal fault to busbar; but for a fault occurring on any one of these lines, the polarity on the faulted line is opposite of those on healthy lines. According to this important characteristic, the busbar protection criterion can be established. To improve the reliability of the busbar protection method, waveforms of superimposed currents are integrated within a short time after the fault. Moreover, a new phase-mode transformation matrix is proposed to realize that single aerial modulus can reflect all fault types. A real 500-kV substation busbar model was built in PSCAD to evaluate the performance of busbar protection. Simulation results demonstrate that the proposed protection method has good adaptability, fast operation speed, and high reliability. In addition, its performance is rarely influenced by the fault initial conditions, series compensation, and CT saturation, etc.

High penetration of distributed generation (DG) has led to frequent voltage fluctuations in the distribution network. This paper describes the design of a partially rated, power-electronic-assisted onload tap-changing (OLTC) autotransformer. Positive and negative compensation of the grid voltage can be achieved on feeders that have high distributed generation and/or loading. A novel design of taps comprised of several no-load switches and a single semiconductor-mechanical hybrid switch have been proposed, that requires reduced voltage rating and a number of switches. In steady state, the mechanical switch in the hybrid switch conducts the load current resulting in low steady-state losses. During the tap change process, the OLTC uses semiconductor switches, namely insulated-gate bipolar transistor /metal-oxide semiconductor field-effect transistor, thus achieving arc-free tap change and long lifetime of switches. The OLTC system has been customized for both low-voltage and medium-voltage three-phase distribution networks. An open-delta configuration for the medium-voltage application has been proposed that requires only two OLTC units to control all three line voltages. Simulations are carried out to verify the steady-state and transient operation of the proposed OLTC.

This paper presents a damping control to mitigate sub-synchronous interactions (SSI) in doubly-fed induction generator (DFIG) wind turbines connected to series-compensated lines. This issue has gained attention due to the recent SSI phenomena reported in DFIG wind farms located near series capacitors. Two approaches which add a supplementary damping control signal are compared: one of them, integrated to the grid-side converter, and the other one, to the rotor-side converter. The SSI damping controls are designed using a multi-input multi-output state-space methodology. This allows to easily tune a high performance controller using several measurements and control inputs. Smalland large-signal stability analyses, robustness aspects, impact of the supplementary controls on the system modes, and influence of different operating conditions on the SSI are also discussed. The obtained results show that the supplementary control is able to properly damp the sub-synchronous oscillations of DFIG wind turbines by updating the existing DFIG control systems without the inclusion of expensive additional damping devices, and reducing the risk of wind generation tripping. Fil: Leon, Enrique Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto de Investigación En Ingeniería Eléctrica; Argentina Fil: Solsona, Jorge Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto de Investigación En Ingeniería Eléctrica; Argentina. Universidad Nacional del Sur. Departamento de Ingenieria Electrica y de Computadoras; Argentina

A simple and robust procedure with good transient response is proposed for d-q voltage control in conjunction with harmonics reduction in the output of stand-alone single-phase inverters. The distortion in the load voltage waveform is mainly due to the voltage drop in the filter-inductor caused by the harmonic components of the current. For extracting the in-phase and quadrature signals from harmonic-rich periodic waveforms, a composite observer has been developed, which is faster and more accurate than a simple observer. Separate observers have been provided for the voltage and current signals. Feed forward compensation has been obtained by using the quadrature components derived from the composite current observer. Further, an Inverter could be modeled as a feed back control system with the fundamental component as the desired output and the harmonics as the noise creeping into the output. The well-known control strategy of using a large feed back around the noise signal can be employed to reduce its effect at the output, exhibiting low total harmonic distortion under non-linear loads. The controllers can be easily modeled in the discrete-time domain and the ideas can be directly implemented digitally for single-phase inverters controlled by FPGA or DSP chips. � 2010 IEEE.

Self-excited induction generators (SEIGs) are increasingly being used in small-capacity isolated applications for harnessing both conventional and renewable energy resources. These SEIGs suffer from poor voltage regulation even when driven by constant speed prime movers or fixed head hydro turbines. The suitability of these SEIGs to regulate the terminal voltage is a key factor in deciding its use in various applications. This paper deals with the voltage-regulating (VR) schemes for SEIGs, which are found scattered in the literature, and summarizes their operational aspects and relative suitability. These VR schemes are classified on the basis of shunt and series compensation as classical schemes, schemes based on solid-state switching devices, and schemes based on self-commutating solid-state switching device converters. The recommendations drawn from this study are helpful in developing a new cost-effective regulating scheme of SEIGs for different industrial applications.

Operation of a distributed-generation (DG) source, producing sinusoidal output voltage and supplying nonlinear loads is characterized by a nonsinusoidal voltage at the point of common coupling (PCC). The situation gets particularly aggravated during the islanding mode, when the grid power is not available. This deteriorates the performance of other loads connected in parallel. A scheme based on nonsinusoidal pulsewidth-modulated control of a stand-alone inverter-based DG source is proposed to regulate and produce a sinusoidal voltage at the PCC. It is demonstrated that without any current sensor or compensating device, the individual harmonic components and THD of the supply voltage at the PCC can be reduced to the desired level. Simulations have been performed to study the viability of the proposed scheme and all of the results are presented.

This paper presents an orthogonal-core type series compensator for a power flow control in an electric power system. The orthogonal-core type series compensator is constructed with the variable inductors using the orthogonal-cores and capacitors. For reduction of harmonic currents, the orthogonal-core utilizes wedge gaps and third windings. The orthogonal-core type series compensator is able to control the active power and its flow direction.

This paper discusses the merits of an analytical method for determining the location and amount of series compensation to increase the steady state power transfer capability in the power system based primarily on cascading line overload considerations. Steady state transmission limits define the boundaries of a power system operating point. This paper presents tools to identify these boundaries and analytically estimates the change in the boundary limits due to the application of series and shunt compensation. The approach identifies critical lines that can initiate cascading line outages and identifies the optimal location and amount for series compensation. The methodology can also be used to select sites for devices that can extend the availability of power in flowgates.

Thyristor-controlled series compensation (TCSC) is expected to be applied in transmission systems to achieve a number of benefits. To ensure best use of this new controller, planning engineers require analysis capability tailored to the specific performance characteristics of the TCSC. This paper presents a model for TCSC which is applicable for typical transient and oscillatory stability studies. Also included is a discussion on relevant information to extend the modeling detail of the TCSC for use with long-term stability analysis. >

The design of controllers sited in the transmission network for damping interarea power oscillations requires several types of analytical tools and field verification methods. Probably the most important aspect of such control design is the selection of proper feedback measurements from the network. This paper describes concepts which provide design engineers with the insight to control performance and the understanding needed to ensure the secure operation of the bulk transmission system. Specific attention is directed to procedures for selecting feedback signals. >

A study of the potential for control interaction between a proposed thyristor controlled series compensation (TCSC) and an existing static VAr compensator (SVC) is presented. The results indicate that a control interaction exists between the voltage-input power swing damping control (PSDC) of the TCSC, the series compensated AC system resonances, and the SVC controls. The addition of recommended filtering and synthesized angle-input PSDC showed improved performance. >

This paper presents a systematic design procedure based on the H/sub /spl infin// optimization technique for tuning robust damping controllers of power system thyristor controlled series compensation (TCSC) for a wide range of operating conditions. A new mixed model-matching robustness formulation is used in the design. The model-matching part requires a nominal damping controller, and the robustness part allows for the tuning of the controller. The formulation simplifies the selection of weights for tuning the controller. The procedure is used to design TCSC controllers to enhance the local mode damping in a single-machine power system and the inter-area mode damping in a 2-area 4-machine power system. >

A new type of fault current limiter (FCL) is proposed based on the triggered vacuum switch (TVS). The TVS-based FCL (TFCL) is mainly composed of a capacitor, a current-limiting reactor connected with the capacitor in series, and a TVS connected with the capacitor in parallel. With TVS at the "off" or "on" state, the whole TFCL behaves as conventional series compensation or fault current limitation, respectively. Compared with other types of FCLs, such as superconductivity, thyristor/GTO-based FCL, TFCL is distinguished by its characteristics, such as high capacity, loss free, and low price. The digital simulation and prototype experiment based on the LC resonant test circuit show that it is feasible to develop TFCL.

The authors summarize papers that were presented at a panel session sponsored by working group 15.05.08 on DC economics and operating strategies. Typical cost figures are provided for line and terminal equipment. There are detailed discussions and extensive references on many other factors that are usually included in such comparisons. The reliability and capacity comparisons between AC and DC alternatives are discussed regarding a case study done by Ontario Hydro. The conversion of an AC power corridor to DC to increase the power-transfer capability is of increasing interest because new transmission corridors are very difficult to obtain. The economic perspective on the conversion of a power corridor is treated in some detail. Power-transfer capacity can also be increased by adding series and shunt compensation as discussed. The impact of each alternative on the interconnected power-system reliability and security is considered. System dynamic performance considerations showed how one experienced planner compared the assumed performance of an existing HVDC transmission system with an assumed AC alternative in response to some specific contingencies and specific system conditions. >

An important problem in the field of multivariable control systems is the design of suitable feedback and series compensation elements which, when added to a given system, result in an over-all system with improved characteristics. This improvement may apply to the behavior of the controlled variables, to the disturbance response of the system, or to both simultaneously.