Search

Modular multilevel cascade converters (MMCCs) are becoming attractive solutions as high-voltage Static Synchronous Compensators (STATCOMs) for power plants in renewable energy generation, in order to satisfy the strict grid codes under both normal and grid fault conditions. This paper investigates the performances of four potentially used configurations of the MMCC family for the STATCOM in large-scale offshore wind power plants, with special focus on asymmetrical low-voltage ride through capability under grid faults. The specifications and the sizing of components of each type of practical 80-MVar/33-kV-scaled MMCC-STATCOM are carefully designed and compared. The total cost and volume are compared based on the total power semiconductor chip area and the total energy stored in the passive components. Asymmetrical reactive power delivering operation of the MMCC family considering the dc-link capacitor voltage-balancing method is solved mathematically in order to quantitatively understand the performance limitations and behaviors. The electrothermal stress of the power modules used in each type of the MMCC for a practical 80-MVar/33-kV-scaled STATCOM is analyzed. The asymmetrical reactive power capability of the MMCC solutions is compared under different scenarios of grid faults, while considering the device temperature limits as well as voltage saturation. It is found that the MMCC configuration with double-star bridge cells becomes the most attractive circuit configuration for the STATCOM application based on the obtained results. [ABSTRACT FROM AUTHOR]

This paper presents a voltage-controlled distribution static synchronous compensator (DSTATCOM)-based voltage regulator for low-voltage distribution grids. The voltage regulator is designed to temporarily meet the grid code, postponing unplanned investments while a definitive solution could be planned to solve regulation issues. The power stage is composed of a three-phase four-wire voltage-source inverter and a second-order low-pass filter. The control strategy has three output voltage loops with active damping and two dc bus voltage loops. In addition, two loops are included to the proposed control strategy: the concept of minimum power point tracking (mPPT) and the frequency loop. The mPPT allows the voltage regulator to operate at the minimum power point, avoiding the circulation of unnecessary reactive compensation. The frequency loop allows the voltage regulator to be independent of the grid voltage information, especially the grid angle, using only the information available at the point-of-common coupling. Experimental results show the regulation capacity, the features of the mPPT algorithm for linear and nonlinear loads, and the frequency stability. [ABSTRACT FROM PUBLISHER]

This paper presents implementation of distribution static compensator (DSTATCOM) using kernel incremental metalearning algorithm (KIMEL) control algorithm. It demonstrates the functions of DSTATCOM such as reactive power compensation, harmonics elimination, and load balancing under linear and nonlinear loads with its self-supporting dc-bus voltage. The proposed control algorithm based on KIMEL is applied for extraction of fundamental active and reactive power components of load currents that are required for estimation of reference supply currents. These reference supply currents are used for gating pulses generation using indirect current control principle. A DSTATCOM with features of self-supporting dc bus (capacitor) is developed using DSP (dSPACE-1104), and the response of proposed control algorithm is studied under various operating conditions and found satisfactory. [ABSTRACT FROM AUTHOR]

This paper deals with coupling transformer core saturation problem in cascaded multilevel converter (CMC) based transmission-type static synchronous compensator (T-STATCOM) systems. The effects of nonlinear characteristics of the coupling transformer and CMCs on the core saturation phenomenon are investigated, and the associated mechanisms are described. In order to prevent the transformer core from being saturated, dc current components in the coupling transformer secondary lines are eliminated by modulating one or more than one voltage steps of staircase CMC voltage waveforms via the electronic gating circuitry. The proposed dc current elimination method is implemented on a 154-kV, ±50-MVAr CMC-based T-STATCOM system, which is operating either in interarea oscillation damping or terminal voltage regulation modes at any time in a 400/154-kV transformer substation since March 2010. The effectiveness of recommended dc current elimination method is verified in the field. [ABSTRACT FROM AUTHOR]

This paper presents a new multilevel selective harmonic elimination pulse-width modulation (MSHE-PWM) technique for transformerless static synchronous compensator (STATCOM) system employing cascaded H-bridge inverter (CHI) configuration. The proposed MSHE-PWM method optimizes both the dc-voltage levels and the switching angles, enabling more harmonics to be eliminated without affecting the structure of the inverter circuit. The method provides constant switching angles and linear pattern of dc-voltage levels over the modulation index range. This in turns eliminates the tedious steps required for manipulating the offline calculated switching angles and therefore, easing the implementation of the MSHE-PWM for dynamic systems. Although the method relies on the availability of the variable dc-voltage levels which can be obtained by various topologies, however, the rapid growth and development in the field of power semiconductor devices led to produce high-efficiency dc-dc converters with a relatively high-voltage capacity and for simplicity, a buck dc-dc converter is considered in this paper. Current and voltage closed loop controllers are implemented for both the STATCOM and the buck converter to meet the reactive power demand at different loading conditions. The technique is further compared with an equivalent conventional carrier-based pulse-width modulation to illustrate its enhanced characteristics. The effectiveness and the theoretical analysis of the proposed approach are verified through both simulation and experimental studies. [ABSTRACT FROM AUTHOR]

Voltage-sourced converter (VSC)-based static synchronous compensators (STATCOMs) are used for voltage regulation in transmission and distribution systems. Unlike PWM-controlled STATCOMs, angle-controlled STATCOMs are switched at line frequency to limit the system losses. In recent years, angle-controlled STATCOMs have been deployed by utilities for the purpose of transmission system voltage regulation, voltage stability improvement, and increasing operational functionality. Despite the superior feature on voltage waveform quality and efficiency, the practical angle-controlled STATCOMs suffer from the over-current (and trips) and possible saturation of the interfacing transformers caused by negative sequence current during unbalanced conditions and faults in the utility. This paper specifically proposes a control structure to improve the angle-controlled STATCOMs performance under unbalanced conditions and faults. The main improvement is a substantial decrease in the negative sequence current and dc-link voltage oscillations under power system faults by the proposed control. This eliminates the need to redesign the STACOM power components to operate under fault current and dc-link voltage oscillations. The proposed control structure is designed based on adding appropriate oscillations to the conventional angle-controller output that is the control angle by which the VSC voltage vector leads/lags the line voltage vector. Since this control structure uses two angles for controlling the VSC output voltage, it is called dual angle control (DAC). PSCAD/EMTDC and experimental results verify the validity of the proposed control structure under unbalanced system conditions and faults. The experiments were conducted on a transient network analyzer, a unique hardware-based flexible ac transmission system simulator which was designed to study system faults and transients for a 2 × 100 MVA STATCOM field installation. [ABSTRACT FROM AUTHOR]

This paper presents a novel dc capacitor voltage balance control method for cascade multilevel static synchronous compensator (STATCOM) and a general analytical method for balance control strategy. Considering that the imbalance of dc capacitor voltage is caused by the inconsistency of active power absorbed and consumed by chain, a balance control strategy based on active voltage vector superposition is proposed, in which an active voltage component is superposed to chain's output voltage to change its absorbed active power. A general analytical method based on vector analysis is also presented, by which the performance of balance control strategy can be analyzed, including stability and regulation capacity. To find out the most appropriate balance control strategy, a comparison still based on vector analysis among the proposed and other two commonly used methods is provided, from which it can be known that the proposed balance control strategy has the advantage of good stability and strong regulation capacity, and simulations are performed to prove it. The effectiveness of proposed control scheme has been verified by experimental results based on a three-phase 36-chain cascade multilevel STATCOM laboratory prototype. [ABSTRACT FROM AUTHOR]