Cascaded H-bridge Static Synchronous Compensator Research Articles

Sensorless Capacitor Voltage Detection Method of Cascaded H-Bridge STATCOM Based on Special Switching Mode Identification

The cascaded H-bridge (CHB) static synchronous compensator (STATCOM) has been widely used in wind farm and conflux station. As the grid-connected voltage increases, the number of cascaded CHB proportionately increases. In order to achieve the capacitor voltage balance, it is necessary to independently detect the capacitor voltage of all H-bridge cells, which raises the cost of capacitor voltage detection. To solve this problem, the mapping relationship is studied between the converter output voltage and capacitor voltages by analyzing carrier phase-shifted sinusoidal pulse width modulation technique (CPS-SPWM), and special switching mode is firstly found. Based on the above finding, without changing the conventional CHB topology, an improved sensorless detection method is proposed to solve the problems of detection error, large computation burden and the application limitation in existing sensorless detection methods. Compared with conventional sensor detection, this method not only saves DC voltage sensors and data transmission channels but also suppress the third harmonics in the converter current. Finally, the effectiveness of the detection method applied on CHB STATCOM is proved by simulation and experiment.

Neural Network Model-Predictive Control for CHB Converters With FPGA Implementation

Finite Control Set Model Predictive Control appears an interesting and effective control technique for Cascaded H-Bridge converters but, because of its computational complexity, becomes impractical when the number of levels of the converter increases. This paper proposes a Neural Network-based approach capable of overcoming the computational burden of conventional predictive control algorithms. The proposed control is then applied to a Cascaded H-Bridge Static Synchronous Compensator using FPGA and tested via hardware in the loop. Results and analysis demonstrate that optimal control of multilevel converters with many levels can be obtained with low computational effort.

Cascaded H-bridge Based STATCOM With Improved Ride Through Capability of Submodule Failures

This article investigates the operation of cascaded H-bridge (CHB) based static synchronous compensator (STATCOM) after failure in its submodules (SMs). Once a switch fails, the corresponding faulty SM is bypassed. In the post-fault operation, a zero-sequence voltage (ZSV) is injected to the converter phases to generate the maximum possible line-to-line voltage. Using a clamping technique, the fundamental component of injected ZSV (FZSV) is minimized according to the operating point of CHB STATCOM. But, FZSV leads to unequal flow of active power into the converter phases. In the proposed method, negative-sequence current is controlled to manage the active power flow within CHB phases. By doing this, the effect of FZSV on active power of CHB phases is compensated and the voltages of capacitors are kept balanced. This method ensures the continuous operation of CHB STATCOM with the minimum over-design and guarantees the voltage balance of capacitors after a failure. The presented simulation and experimental results validate the feasibility and effectiveness of the proposed method.

A Circuit Topology of Cascaded H-Bridge STATCOM to Operate With Multiple Faulty Bypassed Cells

In the present work, a circuit topology of cascaded H-bridge (CHB) converter for static synchronous compensator (STATCOM) application is proposed, which can operate with multiple faulty bypassed cells. The topology uses one redundant three-phase cell connected in series with the CHB converter. After the faulty cells are bypassed in CHB, the redundant cell is inserted into the power circuit of the CHB. The aim of the proposed topology is to tolerate multiple faulty bypassed cells in a single leg or multiple legs of the CHB converter and maintain prefault rated line voltage and reactive power capability of the converter during postfault conditions. A novel geometrical approach is proposed for output voltage generation for the healthy cells and the redundant cell during postfault operation, which ensures that the capacitor voltages are balanced for all the remaining healthy cells in CHB and the redundant cell. Additionally, the voltage stress on the remaining healthy cells does not increase during postfault condition. The proposed fault tolerant topology of CHB STATCOM is verified for single and multiple faulty bypassed cells using simulation and a downscaled experimental prototype.

Zero-Sequence Voltage Injection Method for DC Capacitor Voltage Balancing of Wye-Connected CHB Converter under Unbalanced Grid and Load Conditions

Leg capacitor energy balancing control is one of the crucial issues for stable operation of a cascaded H-bridge (CHB) converter. Because this topology inherently consists of numerous submodule cells with DC capacitors, the cell voltages and leg capacitor energy instantaneously fluctuate depending on operation sequence of the CHB converter. In general, a wye-connected CHB-converter-based static synchronous compensator (STATCOM) utilizes a zero-sequence voltage component for leg capacitor energy balancing. In this paper, to improve the dynamics of leg energy balancing control, a feedforward calculation method of the zero-sequence voltage injection is proposed. The feedforward term can be instantaneously calculated by using the information from the measured leg voltages and leg currents, and the method ensures successful regulation of the leg energy balance even under unbalanced grid and load conditions. Moreover, the verification of the proposed method is supported by the mathematical vector theorems. A 50MVA full-scale wye-connected CHB–STATCOM system simulation was performed to verify the proposed feedforward calculation method considering unbalanced grid as well as unbalanced load conditions.

Coordinated Control Strategy of PMSG and Cascaded H-Bridge STATCOM in Dispersed Wind Farm for Suppressing Unbalanced Grid Voltage

This article proposes an unbalanced voltage compensation method for a dispersed wind farm (DWF) by cooperating permanent magnetic synchronous generators (PMSGs) with a star-connected cascaded H-bridges (CHB) static synchronous compensator (STATCOM). The topology of a typical DWF is given and the mathematical models of CHB-STATCOM and PMSG inverter are established to show that the unbalanced grid voltage can be suppressed by controlling their virtual negative-sequence output admittance (NSOA). The relationship between maximum phase voltage and dc voltage corresponding to zero-sequence voltage injection is obtained to quantitatively analyze the negative-sequence current injection ability of the CHB-STATCOM. Moreover, the phase current limit is introduced to address the over-current problem, and a virtual NSOA method for sharing unbalanced voltage is proposed. An allocation coefficient calculation method is also proposed in consideration of both remaining capacity and voltage unbalanced factor (VUF) for different PMSGs. Furthermore, a coordinated control strategy of PMSGs and CHB-STATCOM for unbalanced voltage suppression is proposed. The effectiveness of the proposed coordinated control strategy is verified by simulation on a 50 MW DWF with a ±5 MVar star-connected CHB-STATCOM.

An Improved Fault-Tolerant Control Scheme for Cascaded H-Bridge STATCOM With Higher Attainable Balanced Line-to-Line Voltages

Fault-tolerant operation ability is of great importance for stable operation of cascaded H-bridge (CHB) converters, under open-circuit (OC) or short-circuit (SC) switch failures in submodule (SM). In this article, an improved fault-tolerant control strategy is proposed for CHB-based static synchronous compensator (STATCOM) under SM faults. First of all, compared with the conventional fault-tolerant method of directly bypassing the faulty SMs, the proposed fault-tolerant method takes advantage of the healthy switches of the faulty SMs, where they are still able to generate either positive or negative voltage level. As a result, more output voltage levels can be generated, and it raises the attainable balanced line-to-line voltage, especially when different fault types exist at the same time. Then, based on the specific condition of OC fault or SC fault, when the output voltage reference of the faulty phase reaches its limit, the references of the other two healthy phases are redistributed to generate the desired line-to-line voltage. With the reconfiguration of modulation waves, the attainable balanced line-to-line voltage can be further improved. In addition, the proposed fault-tolerant method possesses the ability of cluster voltage balancing, which is an important issue for the STATCOM application. Simulation and experimental results validate the effectiveness of the proposed fault-tolerant method.

Impedance Modeling and Stability Analysis in Vehicle-Grid System with CHB-STATCOM

Recently, the low-frequency oscillation (LFO) frequently happens in the vehicle-grid system of high-speed railways when electric multiple units (EMUs) are put into operation. As a possible solution, a single-phase cascaded H-bridge static synchronous compensator (CHB-STATCOM) directly connected to the traction network is applied to dynamically compensate the reactive power of vehicle-grid system. This paper focuses on the impedance modeling and stability analysis in the vehicle-grid system with CHB-STATCOM. Firstly, the vehicle-grid system is simplified to a single-phase rectifier with large grid inductance. The accurate dq -frame impedance of the rectifier system with CHB-STATCOM is derived in detail. Secondly, through the generalized Nyquist stability criterion (GNSC), the influence of CHB-STATCOM and its different control parameters on the stability of rectifier system is revealed. Thirdly, the excellent performance of CHB-STATCOM in suppressing the LFO and the accuracy of theoretical analysis are verified via time-domain simulations. Finally, to further verify the effectiveness of CHB-STATCOM in real applications, a dSPACE based controller hardware-in-the-loop (HIL) simulation platform is established, which includes the physical control circuits of eight EMUs. The experimental results show that the CHB-STATCOM has a good dynamic compensation effect and can efficiently suppress the LFO.

An Enhanced Static Compensator With DC-Link Voltage Shaping Method

This paper proposes a dc-link voltage shaping method for a cascaded H-bridge (CHB) static synchronous compensator (STATCOM). A recently proposed low capacitance-STATCOM system, which intentionally operates with large voltage ripples on the dc-link capacitors, offers the advantage of superior harmonic performance. However, the capacitor voltage ripples decrease when operating at below-rated reactive current, which removes the aforementioned harmonic performance benefit of this system. In addition, such harmonic performance benefit is only achieved in capacitive operation mode. In order to expand the operating region where advantageous voltage ripples are present, a new voltage shaping method is proposed. The proposed method uses a CHB multilevel converter coupled with simple dc–dc converters to shape voltage ripples on the dc-link capacitor regardless of its operating point. This is achieved by controlling the average current of the dc–dc converter using model predictive control. The concept is best applied to an enhanced STATCOM, or E-STATCOM, application in which the STATCOM is integrated with either a renewable energy source or energy storage system. The operational principle of the proposed approach is validated via simulation and experimental results on a five-level single-phase CHB STATCOM.

Third‐order harmonic currents suppression method based on selective component compensation for cascadedH‐bridge STATCOM under asymmetric grid

To achieve low cost and compactness, small dc capacitance in dc-link is preferred in the cascaded H-bridge (CHB) static synchronous compensator (STATCOM). However, small dc capacitance will definitely result in high dc-link ripples among three clusters, and thus cause severe third-order harmonic currents under the asymmetric grid. In this study, to mitigate the third-order harmonic currents, a selective component compensation approach for the CHB STATCOM is proposed. To study the effect of the dc-link ripples on the phase currents, sequence component decomposition is employed to derive the relationship between the dc-link ripples and the converter voltages. The relationship reveals that the converter voltages contain positive, negative, and zero sequence components in both fundamental and third-order under the asymmetric grid, while only the positive and negative sequence components in third-order converter voltages result in third-order harmonic currents due to the three-wire system. Therefore, the third-order harmonic currents suppression method is proposed by only compensating the positive and negative components in third-order converter voltages. A 380 V/15 kVA STATCOM prototype is built to verify the proposed control method.

Analysis of unbalanced clustered voltage and control strategy of clustered voltage balancing for cascaded H-bridge STATCOM

To explore the clustered voltage balancing mechanism of the cascaded H-bridge static synchronous compensator (STATCOM), this paper analyzes the causes of unbalanced clustered voltage. The negative-sequence current caused by the compensation of unbalanced reactive power or detection and control errors and the zero-sequence voltage caused by voltage drift of the STATCOM neutral point contribute to unbalanced clustered voltage. On this basis, this paper proposes a control strategy to inject negative-sequence current and zero-sequence voltage simultaneously. The injection of negative-sequence current may cause current asymmetry in the grid, and the zero-sequence injection has a relatively limited balancing ability in the clustered voltages. The proposed control strategy can not only generate a faster balancing response than the traditional zero-sequence voltage injection method, but also lower the extent of current asymmetry compared with the traditional negative-sequence current injection method. Then, the negative-sequence current and zero-sequence voltage injection are further transformed into the dq frame to establish a unified frame. The effectiveness of the proposed control strategy is verified by the simulation and experimental results.

A diode-clamped cascaded H-bridge STATCOM for voltage balancing of individual capacitors

This paper firstly proposes a novel topology for static synchronous compensator (STATCOM) based on diode-clamped cascaded H-bridge (DCCH) circuit. The proposed topology guarantees the voltages across each capacitor in the same phase are clamped and equal to each other through the diode-clamped circuits. The balancing process through the diode-clamped circuits is bidirectional and it is realized by diode-clamped circuits only. The amount of calculation is thus reduced and the complexity of algorithm is also lowered. The parameter of clamping inductor has a great impact on voltage balancing effect. So, this paper further proposes the parameter selection principle of inductor in diode-clamped circuit. Compared with traditional software individual voltage balancing strategy, the balancing speed of the proposed topology is improved dramatically and the balancing effect is also strengthened. Simulation and experimental results verify the parameter selection method of clamping inductor and validate the effectiveness of proposed topology in Var generation and voltage balancing of individual capacitors.

DC Capacitor Voltage Balancing Control for Delta-Connected Cascaded H-Bridge STATCOM Considering Unbalanced Grid and Load Conditions

In this paper, a comprehensive control scheme for a delta-connected cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM) is presented, especially focusing on improving dynamic performance by novel feedforward control method. The method can conspicuously improve the dynamics of zero-sequence circulating current regulation of delta-connected CHB STATCOM especially under grid fault condition as well as load unbalance without excessive dc cell capacitor voltage fluctuation. The validity of the proposed method is demonstrated by mathematical proof. Finally, the full-scaled simulation results and the down-scaled experimental results verify that stable operation is guaranteed for both emulated grid and load unbalance conditions.

A Simplified Branch and Bound Approach for Model Predictive Control of Multilevel Cascaded H-Bridge STATCOM

Applying model predictive control (MPC) to cascaded H-bridge multilevel converters, while improving the dynamic performance, suffers from high computation burden due to substantial voltage vectors and switching combinations. In this paper, a simplified branch and bound approach is proposed to reduce the computation of the MPC for a cascaded H-bridge Static Synchronous Compensator (STATCOM). The underlying optimization problem of the current predictive control is a two-variable integer quadratic programming problem. With the proposed approach, the global optimal voltage vector can be selected within linear time instead of polynomial time. This is achieved by selecting the possible values of one variable as the branch, and enumerating the optimal integer results of each branch. The execution time of the proposed approach and conventional methods are compared to demonstrate the effectiveness of the proposed approach. Simulation and experimental results for a seven-level cascaded H-bridge STATCOM system are presented to verify the usability and reliability of the proposed approach.