Multilevel Static Synchronous Compensator Research Articles

Optimal Switching-Sequence-Based Model Predictive Control for a Hybrid Multilevel STATCOM

In this article, we introduce a model predictive control with optimal switching sequence (OSS-MPC) for a multilevel static synchronous compensator (STATCOM). The studied STATCOM is based on a hybrid multilevel inverter named HC-FNPC formed by the cascaded connection of H-bridge cells with a full-bridge neutral point clamped cell (FNPC). In this article, the optimal switching sequences are derived for HC-FNPC STATCOM by considering the impact of each switching sequence on the voltage of capacitors. Then, the control objectives of hybrid multilevel STATCOM, i.e., ac current control and capacitors’ voltages balancing, are formulated in terms of OSS-MPC for the first time. In contrast to the conventional MPC, this method provides a fixed-switching frequency. Also, by a combination of optimal switching sequences in MPC, the regulation of capacitors’ voltages with unequal values is realized. The control system's performance is verified on a nine-level (9L) 2.4 Mvar/11 kV HC-FNPC STATCOM in MATLAB/Simulink. Also, the experimental results are provided on a 9L 1.25 kvar/220 V scaled-down hardware prototype.

Voltage regulation of a remote microgrid bus with a modular multilevel STATCOM

This paper presents a strategy to control the voltage at a remote bus of a distribution network by using a modular multilevel static synchronous compensator. The mathematical models presented in this paper are based on the transfer impedance of the nodal admittance matrix of the system. They allow building a voltage control loop in the dq-reference frame to regulate the voltage of a remote bus of the power system. As the mains’ impedance may vary during the system operation, the ac voltage control loop is designed considering the worst case to ensure the correct operation of the voltage control loop. It is also shown how the variation of the mains’ inductance impacts the voltage control of the remote bus. Moreover, it is shown that the use of a PI controller provides a better margin phase for the remote bus voltage control loop. A static synchronous compensator based on the modular multilevel converter, with 28 submodules per leg, is modelled in an electromagnetic transient program and connected to a medium-voltage distribution network based on the IEEE 14 bus system. Lastly, an heuristic algorithm is used to regulate the reference signal of the ac voltage control loop so that the PCC voltage does not exceed its voltage limit. It is shown that this algorithm can reconcile the regulation of the remote bus voltage and the PCC voltage limit.

Reactive Power Compensation and Harmonic Mitigation in Single-Phase Distribution System using Multilevel Converter Based D-STATCOM

This paper presents a study of a single-phase cascaded H-bridge multilevel converter-based Distribution Static Synchronous Compensator (D-STATCOM) with active harmonic filtering capability. For the proper operation of the proposed D-STATCOM, a constant voltage is maintained at its DC bus link using a proportional and integral (PI) controller. Based on instantaneous current errors, gate pulses for controlling duty cycles of the D-STATCOM operation are generated using the hysteresis current control (HCC) technique. The proposed D-STATCOM was modeled and simulated in MATLAB. Simulation results show that the proposed method effectively compensates for reactive power and mitigates current harmonics. The proposed method offers structural simplicity and efficiency without complex calculations.

Wear-out failure analysis of modular multilevel converter-based STATCOM: The role of the modulation strategy and IGBT blocking voltage

The modular multilevel converter (MMC) is an attractive option for medium- and high-voltage static synchronous compensator (STATCOM) application. For this application, three aspects are relevant. Firstly, reliability and efficiency are directly related to the operation costs of this converter. Secondly, either high IGBT blocking voltage capability, featuring few number of submodules (SMs), or low-voltage IGBT modules, with a larger number of SMs, can be employed in an MMC-STATCOM. Lastly, either modulation strategies with fixed or variable switching frequency can be employed. Nevertheless, the literature does not present works evaluating these three aspects of an MMC-STATCOM at the same time. Therefore, this paper analyzes the impact of modulation strategies and number of SMs on wear-out failure and efficiency of an MMC-STATCOM. The effect of all SM components is considered. Two distinct modulation strategies are selected: phase-shifted pulse-width modulation (PS-PWM), with fixed switching frequency, and nearest-level control with cell tolerance band algorithm (NLC-CTB), with variable switching frequency. Four commercially available semiconductor blocking voltages are considered to evaluate the converter with different number of SMs. The results demonstrate that the number of SMs and the modulation strategy affect the MMC-STATCOM performance. Indeed, the converter presents different reliability at system-level and energy losses per year, depending on the IGBT blocking voltage and modulation strategy. The design with higher IGBT blocking voltage is the most affected.

Novel multilevel STATCOM for power system stability enhancement on DFIG-based wind farms

This work proposes the evaluation of the multilevel topology based on three-phase cells with a Single DC-link Cascaded H Bridge converter (SDC-CHB) with Model Predictive Control (MPC) as Static Synchronous Compensator (STATCOM) used to support DFIG wind farm operation. The well-known Hydro-Quebec system containing a 9 MW DFIG-based wind farm was chosen as a test bench to evaluate the proposed SDC-CHB steady and transient dynamic response to contribute to the DFIG fault ride-through. Simulation results demonstrate that the proposed topology enhances the power system stability and power quality, improving the voltage profile, on average, by 10% during symmetrical and asymmetrical faults. Furthermore, the SDC-CHB and MMC-CHB parametric study is presented proving the proposed topology robustness. Both topologies present similar steady and transient dynamic responses, corroborating that the proposed multilevel converter contributes to the DFIG fault ride-through with an enhanced topology when compared with classical multilevel converters.

Experimental evaluation of negative-sequence voltage compensation in distribution networks by a modular multilevel static synchronous compensator

This paper presents a negative-sequence voltage compensation strategy of a distribution network by using a static synchronous compensator based on modular multilevel converter. Mathematical relations of the static compensator are used to derive small-signal linearised models, in the synchronous reference frame, and to design voltage and current controllers to compensate for unbalanced voltages at the point of common coupling. Moreover, a comprehensive model of the DC bus voltage with a control loop implementation are presented. Furthermore, a notch filter is connected in the feedback path of the DC voltage control loop to eliminate the oscillation in the DC voltage. Digital simulations results, obtained with a 10 MVA / 24 kV MMC-DSTATCOM with 14 submodules per arm, will be used to validate the small-signal linearised mathematical models, and the performance of the designed controllers, during steady-state and faulty grid conditions. In addition experimental results obtained with a small-scale MMC prototype, with 03 submodules per arm, will be presented to demonstrate the feasibility of the compensation strategy proposed here.

An effective PSO based cascaded H-bridge multilevel inverter STATCOM with reduced number of materials count

Medium voltage and high power multilevel inverters are nowadays widely used for reactive power compensation applications. Multilevel inverters based Static Synchronous Compensator (STATCOM) devices makes the power system more feasible due to the benefits offered by the multilevel inverters such as output voltage with lower harmonics, high modularity, and high-power quality in the output. This paper presented an effective Particle Swarm Optimization (PSO) algorithm based cascaded H-bridge multilevel inverter STATCOM with reduced number of materials count. The materials used for the semiconductor devices are the Insulated Gate Bipolar Transistors (IGBTs). These IGBTs are fired using the optimized switching angles, which further leads to minimum total harmonic distortion. PSO technique is being used for the calculation of optimized switching angles. Further, the performance of PSO based multilevel inverter with STATCOM is investigated by calculating the harmonics reduction and total harmonic distortion.

Enhancing the Performance of Pre-Charging Strategy of a Modular Multilevel Static Synchronous Compensator

This paper presents an improvement in an energizing strategy for a three-phase four-wire modular multilevel converter connected to an electric network as a static synchronous compensator. The new procedure comprises three stages. During the first stage, two thyristors control the current drained from the grid in order to charge the capacitors of the submodules of the positive and negative arms of two phases of the converter. After that, the energy stored in these capacitors is shared with the others submodule capacitors by firing the semiconductor switches of the converter in a strategic way. In addition of limiting the current drained from the grid during all the energizing procedure, the proposed strategy does not produce overvoltage at the converter dc terminals. The results of digital simulations are presented to validate the mathematical analysis and to demonstrate the performance of the proposed energizing strategy.

Enhancement of cascaded multi-level VSC STATCOM performance using ANN in the presence of faults

A system can be disturbed in terms of stability when connected to a number of loads at the distribution ends or when subjected to faults. To reverse such systems to a stable state, FACTS devices such as static synchronous compensator (STATCOM) are used. In this paper, a cascaded multi-level voltage source converter (VSC) STATCOM was designed and implemented with a novel space vector pulse width modulation (SVPWM) scheme. Artificial neural network (ANN) controller was used instead of instead of Proportional-Integral (PI) controller in the proposed scheme to improve the response time (RT) and performance of STATCOM in terms of power factor (PF) and voltage amplitude during periods of voltage sag. During the implementation, two fault sources (single-line-to-ground (SLG) and line-to- line (LL) faults) were used to create voltage sag. STATCOM was subjected to performance evaluation in the presence of these disturbances via MATLAB simulation in IEEE 3-bus system. The outcome of the simulation studies showed the ANN controller to perform better than PI as it was able to rapidly recover voltage value (<1 cycle) with unity PF.

Robust optmized control of multi levels STATCOM

<p>Reactive power compensation is an essential part of a power system and the static synchronous compensator (STATCOM) plays an important role in controlling the reactive power flow over the transmission line. The basic building block of the STATCOM is a voltage source inverter (VSI) that generates a synchronous sinusoidal voltage and because of the high MVA ratings, it would be expensive to provide independent, equal, regulated dc voltage sources to power the multilevel converters which are presently proposed for STATCOMs. Dc voltage sources can be derived from the dc link capacitances which are charged by the rectified ac power. In this paper a new stronger control combined of nonlinear control based Lyapunov’s theorem and Ant Colony Algorithm (ACA) to maintain stability of multilevel STATCOM and the utility.</p>

Zero-sequence voltage compensation of a distribution network through a four-wire modular multilevel static synchronous compensator

This paper presents a zero-sequence voltage compensation strategy for a three-phase four-wire modular multilevel static synchronous compensator used to regulate the network voltages. The control strategy is based on a synchronous reference frame mathematical modelling used to control the positive and zero-sequence voltages and currents of a MMC static synchronous compensator. The static compensator is controlled to synthesize AC currents in such a way to restore the positive-sequence voltage to its rated value and to mitigate the zero-sequence voltage at the point of common coupling. Digital simulations are presented to validate the static synchronous compensator mathematical analysis and to demonstrate the performance of the proposed control strategy.

Vehicle-Grid System Stability Analysis Based on Norm Criterion and Suppression of Low-Frequency Oscillation With MMC-STATCOM

In recent years, low-frequency oscillation (LFO) events sometimes occur between the traction network and the electric motive units (EMUs) in vehicle-grid systems of a high-speed railway when multiple EMUs access the traction network. Some measures have been taken from the vehicle side to suppress LFO by changing the control parameters or control strategies of the converters. This paper presents a proposed scheme of suppressing LFO by applying a static synchronous compensator (STATCOM) to the traction network. The stability of the vehicle-grid system is first analyzed using the norm criterion. Next, for unstable vehicle-grid systems, the approach of utilizing a modular multilevel converter STATCOM (MMC-STATCOM) connected to the grid is proposed to suppress the LFO. Further, when the MMC-STATCOM is connected to the grid, the parameter sensitivity analysis is implemented in detail; the analysis results indicate that the MMC-STATCOM-based approach operates well in suppressing LFO, regardless of the change of the system parameters. The simulation results validate the stability analysis and the efficient suppression of LFO for the vehicle-grid system.

Study of Cascaded H-Bridge Converter Control Strategies and their Impact on Switching Harmonics

The Cascaded H-Bridge (CHB) multilevel static synchronous compensator (STATCOM) has been widely accepted in the industry [1]-[8]. An outstanding advantage of the CHB multilevel topology is their modular structure. This enables higher-level STATCOM to be easily implemented through series connection of more cells. since the pulse-width modulation (PWM) signals of each cell are generated all by the central controller instead of their own controller, the cell does not look like an “independent module”. To achieve “true” modular implementation, a distributed control system (DCS) for the CHB multilevel STATCOM should be developed.

Modelling and Analysis of Real Time Power System With Cascaded Multilevel STATCOM Using Fuzzy Controller

Electric Power is a major concern that has generated more importance in electrical utilities and consumers. To assure a reliable and quality power system, all the bus are expected to maintain steady acceptable voltage before and after being subjected to any disturbance. Among the FACTs devices a static synchronous compensator(STATCOM) is analyzed to provide a reliable and feasible solution. The STATCOM has been developed using cascaded multilevel converter. A FUZZY based controller for Cascaded Multilevel STATCOM is developed which produces improved performance.This paper presents a comprehensive analysis of CMC based STATCOM with fuzzy controller in real time power system concerning the problem of variable loading and losses. A real time substation with 20 bus are analyzed with power flow analysis with and without STATCOM. Without the connection of STATCOM, a observable changes in the system voltage, line flows and losses are in system behaviour. But this influences the quality of grid and hence the customers are affected badly, indicating the need of power flow analysis and location of STATCOM on the system to improve the voltage profile. It is demonstrated that the described model works to an excellent level in improving the substation performance.

Hybrid Communication Topology and Protocol for Distributed-Controlled Cascaded H-Bridge Multilevel STATCOM

Distributed control scheme is preferred to the multiple-modules-based system because of the simple structure, low maintenance, and high scalability. This paper proposes a hybrid communication topology and a route-switching communication protocol to implement the distributed control applied in the cascaded H-bridge (CHB) multilevel static synchronous compensator (STATCOM). The hybrid topology consists of multiple paralleled synchronous communication branches and each branch has very small transmission delay, which makes it possible to apply the low-speed communication network, such as 5 Mb/s optical fiber, to reduce the cost and improve the reliability. The route-switching communication protocol promises the global fault response of the STATCOM as quick as 10 μs by switching from the long normal data to the short fault signal when the module fault occurs. Experiments based on the CHB 13-level STATCOM prototype verify the effectiveness, flexibility, and reliability of the proposed topology and protocol.

Performance investigation of nine-level cascaded H-bridge inverter-based STATCOM for mitigation of various power quality problems

This paper deals with the design and implementation of cascaded H-bridge multilevel voltage source converter-based static synchronous compensator (STATCOM) for simultaneous active and reactive power compensation, voltage stability and damping of power oscillations. The main objective is to provide active power support and maintain the voltage stability by compensating the reactive power in the power system. In this paper, the multi pulse inverter-based STATCOM has been replaced by multilevel inverter-based STATCOM to overcome the drawbacks of the former. Cascaded H-bridge (CHB) MLI for implementation of STATCOM has been used as CHB MLI is considered the best for STATCOM applications due to its modular structure and requirement of lesser number of components. MLI-based STATCOM is used to provide simultaneous reactive power and active power compensation established by using super-capacitor. The super-capacitors are connected with each DC link capacitor of CHB MLI by using the DC-DC buck boost converter. The STATCOM is also used to provide voltage stability during the power quality problems such as voltage sag and voltage swell, it is established by injecting or absorbing reactive power. The STATCOM is also used for damping the power oscillations by controlled injection of reactive power hence enhancing transient stability. Nine-level cascaded H-bridge MLI-based STATCOM is designed for simultaneous reactive power and active power compensation, voltage stability and damping the power oscillations. Exhaustive simulations have been carried out using MATLAB/Simulink and SimPowerSystems software. The simulation results are presented to prove the effectiveness of MLI-based STATCOM for simultaneous active and reactive power compensation, voltage stability and damping of power oscillation.

Reduced-Capacitance Thin-Film H-Bridge Multilevel STATCOM Control Utilizing an Analytic Filtering Scheme

In this paper, an analytic capacitor voltage filtering scheme for a cascaded H-bridge (CHB) multilevel converter-based static synchronous compensator (STATCOM) is proposed. The scheme is able to filter out a low-frequency ripple of any magnitude and introduces a negligible delay to the control system. Therefore, it facilitates using high-reliability film capacitors with low capacitance instead of the conventional electrolytic capacitors. The proposed analytic filtering scheme is implemented and verified on a single-phase seven-level CHB multilevel converter-based STATCOM. The parameters' sensitivity evaluation is provided to demonstrate practicability of the proposed scheme. Simulation and experimental results are provided to compare the performance of the proposed filtering scheme with the performance of the most commonly used second-order filter-based approach to demonstrate its effectiveness.

Advanced compensation mode for cascade multilevel static synchronous compensator under unbalanced voltage

An improved control scheme for delta-connected cascade multilevel static synchronous compensator (STATCOM) operating under unbalanced conditions is proposed. The individual phase instantaneous current control is suitable for delta-connected cascade multilevel STATCOM which can be equivalent to three single-phase inverters. To compensate the system imbalance, the reactive control mode and the voltage control mode are studied, and the discrete current loop controller has been designed. Accordingly, the cascade multilevel STATCOM can maintain normal operation under unbalanced voltage by using the individual phase instantaneous current control strategy while the unbalanced system can be effectively compensated by the two modified compensation modes. The proposed control scheme has been verified by simulation and experimental results based on a three-phase 36-chain cascade multilevel STATCOM laboratory prototype are shown.

Chopper Controller Based DC Voltage Control Strategy for Cascaded Multilevel STATCOM

The superiority of CMI (Cascaded Multilevel Inverter) is unparalleled in high power and high voltage STATCOM (Static Synchronous Compensator). However, the parameters and operating conditions of each individual power unit composing the cascaded STATCOM differ from unit to unit, causing unit voltage disequilibrium on the DC side. This phenomenon seriously impairs the operation performance of STATCOM, and thus maintaining the DC voltage balance and stability becomes critical for cascaded STATCOM. This paper analyzes the case of voltage disequilibrium, combines the operation characteristics of the cascaded STATCOM, and proposes a new DC voltage control scheme with the advantages of good control performance and stability. This hierarchical control method uses software to achieve the total active power control and also uses chopper controllers to enable that the imbalance power can flow among the capacitors in order to keep DC capacitor voltages balance. The operating principle of the chopper controllers is analyzed and the implementation is presented. The major advantages of the proposed control strategy are that the number of PI regulators has been decreased remarkably and accordingly the blindness of system design and debugging also reduces obviously. The simulation reveals that the proposed control scheme can achieve the satisfactory control goals.

Eliminating DC Current Injection in Current-Transformer-Sensed STATCOMs

Grid-connected power electronics converters/inverters usually have certain amount of dc component at their ac terminal. They are likely to inject unwanted direct current into the power grid, unless a line-frequency transformer is employed. This study investigates this dc injection problem for static synchronous compensator (STATCOM) and connects the dc injection with current transformer (CT), a current sensing element widely used in high-voltage converters. By introducing CT's model into the STATCOM's model, this study found that using CT for current feedback control can cause large dc injection, if no extra means is taken. Expressions of the dc injection for different controllers were derived. Then, a dc injection elimination method was proposed. This method eliminates the dc component of the output current by building an indirect dc feedback loop; therefore, it can prevent injecting the direct current even when it cannot be sensed. Experimental results taken on a 25-level cascaded multilevel STATCOM prototype verifies the analysis and the proposed method.