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The paper describes a control method, to be used in conjunction with wide-area measurement system (WAMS), to damp the very low frequency interarea modes of very large power systems from only a limited number of power stations. It requires knowledge of the eigenvectors of the targeted modes only. The other modes are not destabilized. Results from numerical simulations and eigenvalue evaluations are given. Index Terms--Damping slow oscillation modes, Eigen analysis, Eigen properties, interarea oscillations, power system stability, wide-area measurement system.

This paper presents an invention, the rotor position phase lock loop (PLL), which enables maximum power point (MPPT) tracking of wind by doubly-fed induction generators without needing a tachometer, an absolute position encoder, or an anemometer. The rotor position PLL is parameter variation insensitive, requiring only an estimate of the magnetization inductance for it to operate. It is also insensitive to noise in the electrical measurements. Proof of concept is by: 1) digital simulations and 2) experimental testing of a laboratory prototype. Index Terms--Decoupled P-Q control, doubly-fed induction generator (DFIG), maximum power point tracking, wind turbine generator (WTG).

As wind power penetration increases and fossil plants are retired, it is feared that there will be insufficient kinetic energy (KE) from the plants to support the system frequency. This paper shows the fear is groundless because the high inertias (H [congruent to] 4 seconds) of wind turbine-generators (WTGs) can be integrated to provide frequency support during generation outage. Index Terms--Frequency control, stability, wind power generation.

In this paper, minute-to-minute wind power variations are decomposed into slow, fast, and ramp components to assess the influence of each component on power system operation. Using detailed, long-term simulation models, this paper confirms that most power systems can absorb the impacts of wind power variations with little difficulty. Yet, since ramps in wind power and system demand could coincide, systems with limited ramping capabilities are at risk. It is shown that extending simulation models to include load dynamics and automatic generation control (AGC) time delays do not alter these conclusions. This paper also discusses wind penetration approaches and control area performance measures, linking the latter to the placement of the wind farms within the interconnection. Index Terms--AGC model, automatic generation control (AGC), long-term dynamic model, penetration limit, system dynamic response, wind events, wind farm model, wind power.

'Harmonic transfer through converters' has caused waveform distortion in multiterminal dc systems based on a voltage-source converter (VSC-MTDC). The harmonic transfer can be by the positive and negative sequences or by the zero sequence. A comprehensive solution consists of decoupling the ac side from the dc side of the VSC. Such decoupling has other benefits in VSC-MTDC because disturbances on the ac side of any VSC do not propagate to other VSCs (except the VSC assigned to function as the dc Voltage Regulator). Decoupling also allows a VSC to be planned and designed independently of the other VSCs of the MTDC. Index Terms--Current tracking control, deadbeat, decoupling, harmonic transfer through converters, high voltage direct current (HVDC), multiterminal, multiterminal dc system (MTDC), resonance, uncharacteristic harmonics, voltage source converter (VSC), waveform distortion.

Wind power fluctuations cause frequency deviation from the 60-Hz standard. Using the composite transfer function G (fw) of a small system of thermal power plants, it is estimated that power fluctuation of 5% of the total thermal plant capacity can be tolerated without exceeding 1% frequency deviation. The technology to filter out the power fluctuations by wind turbine generators for increasing wind power penetration already exists. However, perfect filtering sacrifices as much as 27.6% of the wind power that can otherwise be utilized. The paper presents a method of quantifying wind penetration based on the amount of fluctuating power that can be filtered by wind turbine generators and thermal plants. For optimal wind power acquisition, the penetration level is conservatively estimated to be 50%. Index Terms--Automatic generation control (AGC), Bode diagrams, filter, frequency fluctuation, power fluctuation, speed governors, steam turbines, thermal power plants, transfer function, wind farms, wind power penetration.

The nonlinear control method, associated with the books of A. Isidori, H. Nijmeijer, and A. J. van der Shaft, has so far been applied to the STATCOM (system order, N = 3). There have been two impediments to progress in applying the method: 1) difficulty in synthesizing the nonlinear transformation which is the key to the method; 2) the advanced mathematical language which makes the aforesaid books generally inaccessible. This paper describes the successful application of the nonlinear control method to control systems based on back-to-back pulsewidth modulation voltage-source converter. This paper is written in a tutorial form to explain how the nonlinear control method can be applied without using the advanced mathematical language. The SSSC and the UPFC (system order, N = 5) are chosen as illustrative examples. Index Terms--Flexible ac transmission system, nonlinear control, static synchronous series compensator, unified power flow controller, voltage-source converter.

A VSC-MTDC (multi-terminal dc) system consists of Voltage-Source Converters (VSCs) connected to a dc network at their dc terminals. The MTDC is most vulnerable to a dc fault which paralyses all the VSCs until the dc fault is cleared. As dc circuit breakers are expensive, this paper proposes a solution based on extinguishing the dc fault current by opening all the ac-circuit breakers (ac-CBs) which the VSCs are already equipped with on the ac-sides. However, it is necessary to identify which dc line is the faulted line (in case it is a permanent fault) so that it can be isolated by fast dc switches (which are much more economical than the dc circuit breakers), prior to restoring the MTDC system by re-closing all the ac-CBs. This paper presents the handshaking method, which locates and isolates the faulted dc line and restores the MTDC without telecommunication. Index Terms--DC choppers, dc circuit breakers, dc fault, distributed generation, fault detection, fuel cell, HVDC, LVDC, micro-turbines, multi-terminal direct current (MTDC), photo voltaic, protection, voltage-source converters (VSCs), wind farms.

This paper shows that the capability of voltage-source converters (VSCs) to control the real power output of wind turbine generators (WTGs) can be applied to smooth power fluctuations due to wind turbulence. The paper calls attention to instability that can arise from: 1) the wind turbine itself and 2) the doubly fed induction generator (DFIG). The paper presents [G.sub.P/V] (s) to characterize the dynamic behavior of WTGs. Index Terms--Doubly fed induction generator (DFIG), decoupled P-Q control, filtering, frequency fluctuation, moment of inertia, smoothing, voltage-source converters (VSCs), wind farm, wind turbine generators (WTGs), wind power fluctuation.

Departing from aerodynamics (micromodel), the macromodel begins with the power output of a single wind turbine generator (WTG). The N units of WTGs in a wind farm are characterized by the time delays it takes wind, at average velocity, to traverse the distances separating them. Predictions from simulations are in agreement with recorded wind farm data. Smoothing of high-frequency power components is by a factor close to [N.sub.-1/2]. Smoothing of low-frequency harmonic power components is small because the wind farm is limited in size. A theory, based on Fourier analysis, is presented to explain how the macromodel simultaneously copes with: 1) the high-frequency components of wind velocity (which have poor correlation) even for short distances and 2) the low-frequency components (which have some correlation). Index Terms--Correlated, frequency fluctuation, penetration, smoothing, spatial filtering, wind farm modeling, wind farms, wind turbine generators (WTGs), uncorrelated.

This paper presents the results of a study on the frequency deviation of the utility grid due to wind power fluctuations. The deviation is estimated by a deterministic method based on the transfer functions of system components. As the grid frequency is regulated, the deviation can limit high wind power penetration. The research identifies speed governors as one key component in enabling high wind penetration and there is need to factor their increased wear and tear into the ancillary services cost. It also raises the question as to whether the existing standards set for frequency deviation, which have to be complied at the points of connection of wind energy providers, are adequate to promote high wind penetration. Index Terms--Frequency fluctuation, governor, inertia, steam valve, transfer function, turbine, wind farms, wind power fluctuation.

Subtransmission and distribution systems in large cities are often served by underground cable systems. Because of the compactness and the reduced cost of dc cable transmission lines, a Multi-Terminal HVDC system based on IGBT Voltage Source Converters (VSCs) can be an attractive alternative to an ac cable system. This paper draws attention to the added value which comes from the capability of Multi-Terminal HVDC (VSC-M-HVDC) system to ensure uninterrupted quality power to sensitive loads. The value of Premium Quality Power Park obtainable from a VSC-M-HVDC system should enhance its attractiveness. Index Terms--HVDC, multi-terminal, power quality, underground cables, uninterrupted power supply, voltage dip, voltage-source converter, voltage swell.

This paper first presents a general theory predicting harmonic components in naturally sampled pulse-width modulation (PWM) output signals, so that any input signal with a Fourier series representation may be handled. This theory provides a basis for the mathematical analysis of PWM systems, such as converters transmitting a main signal and a broadband of feedback or feed forward signals, or Class D amplifiers. Ultimately, the results of this theory are applied to two concrete problems, and conditions similar to the Nyquist theorem or Carson's rule for FM modulation are derived for the cases of recovery of an input signal consisting of a main signal and a bandwidth of small feedback or feedforward signals and a bandlimited signal consisting of several harmonics of comparable magnitude. A rule of thumb, and conservative estimate of the bandwidth in both cases is one third of the carrier frequency ([[omega].sub.upper] < (1/3)[[omega].sub.c]). This research has been based on the premise that the results obtained will provide valuable insight into the general behavior of PWM systems, and provide a supporting theory for concrete systems utilizing signals of this kind, such as PWM converters whose purpose is to ensure a dominant sinusoidal waveform along with a broadband frequency channel for small feedback signals. Index Terms--Bandwidth limit, fast Fourier transform (FFT), Nyquist theorem, power amplifier, pulse-width modulation (PWM).

This paper shows that the addition of dielectric capacitors lowers the cost of series compensation by static synchronous series compensator (SSSC). Normally the capacitor resonates with the line inductance and the L-C electrical resonance can interact with the torsional resonances of the multiple-inertia elastic shaft of steam turbine-generators to give rise to subsynchronous resonance (SSR) instability. The SSSC has two functions: (1) to provide some series capacitive reactance compensation; (2) to damp out incipient unstable SSR modes. Digital simulation using HYPERSIM shows that for an overall degree of capacitive compensation of 0.7 pu, the SSSC component is only about 1/3 of the capacitive Mvar. Based on the transient stability limit, the transmissibility of the line is increased by a factor of 2.23. Index Terms--Damping controller, series capacitive reactance compensation, static synchronous series compensator (SSSC), subsynchronous resonance (SSR), transient stability limit, voltage-source converter.

Commercial installations of voltage-source converter-high voltage direct current (VSC-HVDC) are now in service as asynchronous link and as underwater/underground dc transmission. The research reported in this paper is directed to providing VSC-HVDC with synchronizing power and damping power so that after a transient, the steady-state is reached in minimal time. Damping power is achieved by injecting incremental power which is proportional to the frequency deviation measured by a phase lock loop. Synchronizing power is achieved by: 1) introducing a delay in the power measurement and 2) by incorporating a static synchronous series compensation in the VSC-station. The effectiveness of both the synchronizing and damping power is demonstrated through digital simulations of a turbine-generator system which is connected to a VSC-HVDC Station. It is shown that with the synchronizing power and damping power, recovery from the fault disturbance is very much faster. Index Terms--High voltage dc (HVDC) transmission, static synchronous series compensation (SSSC), synchronizing power, voltage-source converter (VSC), voltage-source converter-high voltage direct current (VSCHVDC).

Security requires that a system continues to function even when any one subsystem fails. A multiterminal HVDC system consists of N voltage-source converters (VSCs) exchanging power through a dc network. When any one converter is lost, before the surviving (N-1) converters have time to re-establish a new power balance, the excess dc power can produce voltage spikes which are destructive to the power electronic switches. The paper shows that the advanced dc voltage controller (ADCVC), which is a higher hierarchical controller, can meet the targeted voltage margin. Index Terms--DC transmission, HVDC, over-voltage, voltage margin, voltage spike, voltage-source converter, VSC.

This paper addresses the exploitation of the offshore wind energy. A good example is in the shallow coastal waters around the Baltic Sea where there is high potential for wind turbines, located 10-30 km from the shore. The underwater transmission of power to shore has to be by cables. Therefore, dc transmission is required to avoid the large capacitive reactance currents of ac cables. This paper describes the optimal acquisition and aggregation of wind power by multiterminal high-voltage direct current based on sinusoidal pulse-width-modulated, three-phase voltage-source converters connected at their ac terminals to the wind turbine generators. Index Terms--Multiterminal high-voltage direct current (HVDC), sinusoidal pulse width modulation, voltage-source converters, wind farm, wind power generation, wind turbine generator.

Optimal wind-power acquisition requires automatic tracking of the optimum wind-turbine speed for the prevailing wind velocity. As the wind velocity keeps changing with time so the wind-turbine must keep adjusting its speed. In a wind-farm, the wind velocities depend on the locations of the wind-turbines, each of which has its optimal turbine speed at any given time. With an eye to costs, the wind-farm of this paper is conceived as operating with cheap induction generators driven by variable speed wind-turbines, without the expense of speed governors. This paper shows that the voltage-source converters of low voltage direct current (LVDC) transmission Systems (which are now commercially available) can be tailored as speed-sensorless drives of the wind-turbine induction generators, while meeting the objective of optimal wind-power acquisition. The LVDC system aggregates the power of many wind-turbine induction-generator units into a dc grid. Then a dc voltage regulator inverts the collected power into a three-phase ac electric utility grid. Index Terms--AC, three-phace, LVDC, voltage-source converters.

This paper offers a novel topology for the implementation of a shunt STATCOM based on a grouping of four 3-level, diode-clamped converters, switching at line frequency and coupled to the ac system through wye/wye and wye/delta phase-shifting transformers. Advantages include direct voltage control through the gating pattern and reduced size of the dc capacitor. The dominant harmonic on the ac side is the 11th and on the dc side the 12th. The proposed STATECOM can meet typical harmonic distortion requirements. Keywords: STATCOM, advanced static var compensator, FACTS, wye/wye/delta, 3-level converter.

This paper describes a STATCOM based on diode-clamped multilevel converters. The current rating is increased by parallel connections of multiple converter modules. The gate-turn-off thyristor (GTO) switching angles of the fundamental switching strategy are exploited in the design to accomplish three objectives: 1) low total harmonic distortion (THD) in voltages and currents; 2) equal sharing of the currents in the individual modules; and 3) direct control of ac voltage magnitude. Three proportional-integral (P-I) feedback loops are employed for the purposes of: 1) regulating the total dc voltages and ensuring the quadrature relationship of the ac currents and voltages; 2) regulating the reactive power drawn by the STATCOM; and 3) equalizing the dc capacitor voltages in all the levels. Index Terms- FACTS, multilevel converters, multimodules, STATCOM

This paper shows that sinusoidal pulsewidth modulation (SPWM) flexible ac transmission system (FACTS) controllers can be synthesized by employing the phase-shifted triangle carrier technique to multiconverters at a switching rate of only three pulses. The options of parallel, series, and series/parallel module connections are examined for current equalization. This paper presents simulation and experimental results. Index Terms- Current equalization, FACTS, multiconverters, phase-shifted, SPWM, THD.

Series var compensators based on force-commutated static power converters are proving to be a viable alternative to shunt compensators as a means of enhancing power transmission and distribution capability. This paper proposes a converter structure, based on a multilevel cascade of single-phase converters, which can be coupled to the transmission system without transformers. The individual converters are switching at very low frequencies, resulting in high efficiency, and are fed from independent self-controlled dc capacitor buses. Advantages of the proposed series compensator include low injected voltage harmonic distortion and fast response to changes in the compensation level. Pattern generation options are presented. Gating and control schemes are discussed. Simulation results are verified experimentally. Index Terms - Multilevel converter, pulsewidth modulation converter, reactive power, series compensation.

The voltage-source, GTO-HVdc stations require capacitor banks at their dc terminals for two purposes: (i) filtering of the GTO switching noise and (ii) voltage support. This paper examines the feasibility of exploiting the capacitance of submarine and/or underground cables, thus offsetting some of the cost. It is shown that the capacitance of cable length as short as 10 km can significantly reduce the Total Harmonic Distortion (THD) of the dc voltage from 7.76% to 2.26%. However, the THD of the dc current is of the order of 18%. An inductor of 20mH (0.106 p.u.) is sufficient to reduce the current THD to 7%. The cable is modeled as L-R-C [Pi] section representation of the transmission line (1-section per km). The results are obtained by digital simulations (Matlab/Simulink). Keywords: GTO, HVdc, cable transmission line, submarine, voltage-source converters,

This paper reports on research aimed at achieving computer control of power electronic converters for high-power applications. Hitherto, in motor-drives applications, the 4-Timer Method has been adequate in approximating the sinusoidal pulsewidth modulation (SPWM) strategy for microprocessor control. This paper shows that the ideas of the 4-Timer Method can be extended, with modifications, to the multimodule arrays of converters which will be required to meet high megavoltampere ratings. Simulation and experimental results have shown that the resultant 7-Timer Method can implement the linear SPWM strategy for multiconverters. Index Terms - Digital signal processor, field programmable gate array, flexible ac transmission systems, 4-Timer, phase-shifted multiconverters, 7-Timer, sinusoidal pulsewidth modulation.

The acronym UPFC covers Ggugyi's unified power flow controller and Akagi and Fujita's unified power flow conditioner. This paper describes one method of meeting high megavoltampere specifications by using multiple modules of two-level three-phase converter modules controlled by the phase-shifted triangle carrier technique of the sinusoidal pulsewidth modulation (SPWM) strategy. Two conceptual designs are presented. The first is the gate-turn-off (GTO)-SPWM-UPFC at the lowest switching rate, [f.sub.c]/[f.sub.m] = 3. to minimize GTO switching loss. The design is based on applying the fundamentals of SPWM theory to overcome unbalances arising from sideband harmonic interferences. The second is the insulated gate bipolar transistor (IGBT)-SPWM-UPFC, at [f.sub.c]/[f.sub.m] = 12 or higher, which SPWM theory assures that direct paralleling of the modules is free of current unbalance. A laboratory UPFC based on six modules of three-phase 5-kVA size IGBT converters operating at the baseline condition of dual unity displacement factor at the sending end and the receiving end has been implemented. Index Terms - Flexible ac transmission systems, phase shifted, gate-turn-off-SPWM-UPFC, insulated gate bipolar transistor-SPWM-UPFC, multimodular, sinusoidal pulsewidth modulation, unified power flow conditioner, unified power flow controller.

Pulse-width-modulated (PWM) phase shifters allow the smooth control of power flow in a transmission line. This paper analyzes a PWM quadrature booster phase shifter based on a multimodule ac controller structure to attain high voltage levels and improve the harmonic spectrum. The modules are based on a three-phase PWM ac controller topology that employs only four force-commutated switches and is controlled by duty-cycle variation. The PWM technique is carrier based, and the individual modules are gated through phase-shifted triangular carriers. As a result, harmonic cancellation takes place in the input current and output voltage. Low-order harmonics are therefore eliminated and the amplitude of remaining components reduced. An additional operating mode with negligible harmonic generation and step-like control is identified. The feasibility and advantages of the proposed phase shifter are demonstrated by means of a power system simulator. Index Terms - FACTS, harmonic cancelation, multimodule converters, phase shifter, PWM ac controller.

This paper shows that the voltage-source matrix converter has all the features of an asynchronous intertie, namely: (1) frequency changing capability, (2) 360 [degrees] phase shift range, and (3) bi-directional real power transfer. In addition, it has the capabilities for: (4) series line reactance compensation using a complex conjugate property, and (5) static VAR compensation. Series reactance compensation gives rise to high synchronization power which improves transient stability. The matrix converter also has the flexibility to control both the transmitted real power and, independently, the reactive power flows at the sending-end and receiving-end of the transmission line. Keywords: AC transmission, FACTS, power flow controller, line compensation, phase shifter, asynchronous link, GTO, matrix converter.

Because of the high MVA ratings, it would be expensive to provide independent, equal, regulated de voltage sources to power the multilevel converters which are presently proposed for STATCOMs. De voltage sources can be derived from the dc link capacitances which are charged by the rectified ac power. This paper addresses two control issues: (1) regulation of the dc capacitance voltages and (2) their equalization. Equalization is necessary (1) to ensure the even sharing of voltage stresses in the gate-turn-off thyristors (GTOs), and (ii) to prevent the degradation of total harmonic distortion (THD) factors, as all harmonic elimination strategies depend on equal voltages in their assumptions. The strategies considered are: (a) the Fundamental Frequency Method, (b) the Sinusoidal Pulse Width Modulation Method. Digital Simulations are used to confirm the feasibility of the control methods. Keywords: GTO inverter, advanced static var compensator, multilevel, fundamental frequency method, pulse width modulation

The conventional structures used for phase shifters employ quadrature voltage injection controlled by means of on-load tap changers that require considerable maintenance. Line-commutated thyristor structures have been proposed to replace tap changers, but problems related to filter requirements or the number of switches have limited their utilization. This paper proposes a pulse width modulation (PWM) quadrature-booster phase shifter based on a force-commutated ac controller. It offers features such as fast dynamic response, continuous variation of the phase angle with low harmonic injection, and it requires a simple power structure and can be controlled by adjusting the duty cycle of the switches. The operating principles of the proposed phase shifter are analyzed and their feasibility is demonstrated through digital simulation and experimental implementation. Index Terms - FACTS, phase shifter, PWM

This paper presents a static phase-shifter based on a parallel connection of modules of a non-conventional PWM ac-ac converter. Conduction losses are kept low by using only one active power conversion stage where the average number of switches through which the current flows is small. Filtering losses are also expected to be low because of the high order and small magnitude of the harmonics generated by switching. This is obtained by operating the modules with symmetrically phase shifted carriers to provide harmonic cancellation. Thus, even with the switches operating at low frequencies to keep the switching losses low, a high equivalent switching frequency is achieved. The performance of the phase-shifter for damping power oscillations in open loop is also evaluated.

The Pulse Width Modulation (PWM) technique has been proposed for the force-commutated Shunt and Series VAR Controllers and Unified Power Flow Controllers in Flexible AC Transmission Systems. The PWM converters can be operated as linear amplifiers of constant gain so that treasure trove of linear control system theory can be brought to bear more easily when applying feedback controls. For example, pole-placement and active filtering have been successfully applied in laboratory models. This paper is written as a tutorial describing the stages of signal processing: modulation, amplification and demodulation, without reference to power electronics since the solid-state switches are modelled as ON-OFF switches. Keywords: Pulse width modulation (PWM), inverter, converter, switch-mode linear amplifier, Flexible AC Transmission System, FACTS.

This paper describes how the different methods of eliminating the power disturbances: (1) outages by shunt UPS action, (2) low frequency harmonic disturbances by the 'virtual short circuit' principle, (3) swells/sags by the inductive/capacitive reactance principle, (4) high frequency disturbance by passive filtering, can all be integrated into a single strategy based on Reference Voltage Tracking Control. The Control Law has been derived from first principles. Digital Simulation results are presented to prove that the strategy fulfils the multi-functional requirements. The PWM Converter which implements the Control Law is modelled as an ideal linear amplifier. keywords: Tracking control, active filter, VAR compensation, multi-variable control, phase lock loop, custom power, harmonic voltage distortion.

This paper shows that by transforming both sides of the matrix converter to the d-q-o frames of their respective frequencies, there is significant simplification in the transformation matrix which enables the network of one side to be integrated to the other. Thus, it is possible to design for operation with field vector control on the motor side and unity displacement factor on the supply side. Laboratory experimental and digital simulation results are presented to substantiate the theory. The matrix converter used in the test is based on three modules of voltage-source SPWM converters to implement the nine switches and it has the innovative feature that the SVC voltages (for displacement factor improvement) can be derived from the zero-sequence voltages.

The peak line-to-line inverter ac voltage, in general, cannot exceed the supply dc voltage and transformers have to be used when voltage step-up is required. This paper describes how the necessity of using a Tri-Level PWM strategy in the Current Source Inverter can be turned into an advantage by producing a voltage boost so that transformers are no longer required. During the frequent occurrence of the 'shoot-through' states, which is the characteristic of the Tri-Level PWM-controlled inverter, magnetic energy builds up in the dc link inductor. At the end of these periods, the [L.sub.dc]d[I.sub.dc]/dt voltage across the inductor augments the inverter output voltage in the same way as in the basic boost type dc/dc converter. The paper describes the circuitry required to ensure linear control. Analytical, simulation, and experimental results are given. Applications are found in all instances where the ac voltage is higher than the available supply dc voltage.