H-bridge STATCOM Research Articles

DC-Link Capacitance Design for Cascaded H-Bridge STATCOM With Sum of Squares Formulation

DC-Link Capacitance Design for Cascaded H-Bridge STATCOM With Sum of Squares Formulation

Minimum Voltage Operation of Cascaded H-Bridge StatComs Using Discontinuous Modulation

Minimum Voltage Operation of Cascaded H-Bridge StatComs Using Discontinuous Modulation

Maximum Reactive Power Generation Method Based on Limitation of Output Capacity for Star-Connected Cascaded H-Bridge STATCOM Under Voltage Sag

Star-connected cascaded H-bridge (SCHB) STATCOM is being widely used in medium and high voltage to support grid by injecting positive and negative sequence current. Under balanced conditions, the rated reactive power is usually utilized to calculate the required reactive current reference based on instantaneous power theory. However, under grid voltage sag, the rated reactive power changes due to the introduced negative sequence current and extra zero sequence voltage for cluster voltage balancing, which may increase overcurrent and overmodulation risk. Therefore, how to derive the maximum compensable reactive power is required for SCHB STATCOM under the grid voltage sag. In this paper, the quantitative relationship between the imbalanced degree of grid voltages and reactive power reference is fully explored. On this basis, the maximum reactive power is calculated under voltage sags by considering current and voltage limitation. In this calculation, a new geometrical analysis method is proposed, which makes the calculation speed faster for online calculation. Therefore, a new grid support control strategy is proposed for the SCHB STATCOM under grid voltage sag, which can effectively support the grid with the maximum output reactive power. Finally, the proposed method is verified by experimental results on a 400V/7.5kVar SCHB STATCOM prototype.

An Online Monitoring Method for Capacitor Condition of Cascaded H-Bridge STATCOM Based on Sensorless Capacitor Voltage Detection

An Online Monitoring Method for Capacitor Condition of Cascaded H-Bridge STATCOM Based on Sensorless Capacitor Voltage Detection

A Coordinated Control Technique for a Faulty Cascaded H-Bridge STATCOM Under Unbalanced Grid Conditions

A Coordinated Control Technique for a Faulty Cascaded H-Bridge STATCOM Under Unbalanced Grid Conditions

Predictive Discontinuous Modulation Strategy With Embedded Inter-Phase Cluster Voltage Control for Cascaded H-Bridge StatComs

Predictive Discontinuous Modulation Strategy With Embedded Inter-Phase Cluster Voltage Control for Cascaded H-Bridge StatComs

Discretized Discontinuous Modulation Strategy for Cascaded H-Bridge StatCom

This paper proposes a discontinuous modulation (DM) scheme for cascaded H-bridge (CHB) static compensators (StatComs) with star configuration. The proposed DM scheme guarantees zero steady-state active power in each converter phase-arm, even in the presence of severe grid imbalance conditions, thus remaining decoupled from the inter-phase capacitor voltage balancing controller. Moreover, the proposed DM scheme exploits the possibility of clamping to the zero-voltage level, thus providing a more flexible discontinuous operation than conventional DM methods. This involves lower zero-sequence-voltage injection requirements and lower switching losses. The proposed modulation scheme is experimentally corroborated and its performance under different grid voltage conditions is compared with traditional modulation schemes.

Delta-CHB Statcom with Reduced Capacitance Through Third Harmonic Injection

This work investigates the third harmonic circulating current injection in delta-connected Cascaded H-Bridge (Delta-CHB) STATCOM, aiming the cell capacitance volume reduction. Firstly, analytical expressions for capacitor voltage ripple are derived without and with third harmonic injection. These expressions are used to obtain the third harmonic current waveform which minimizes the sum of capacitor voltages ripple in the whole operation range. These expressions are validated in an experimental setup based on 620 VA/220 V cell. Although this approach minimizes the ripple, an increase in the semiconductors current effort is expected. This paper quantifies the increase through analytical expressions and proposes an improved third-harmonic injection scheme which injects the third harmonic in a range of operating points. The performance of the proposed strategy is evaluated for a 17 MVA/13.8 kV Delta-CHB STATCOM with 24 cells per cluster. The results indicate a reduction of 17.2% in the theoretical cell capacitance. A physical realization of cell capacitor reveals that the proposed scheme, in the best scenario, reduces the volume in 25%. Finally, if the same power module and heatsink are employed, the proposed scheme leads to an increase in junction temperature of 13◦C for IGBT and diode.

A Five Level Modified Cascaded H-Bridge Inverter STATCOM for Power Quality Improvement

Multilevel converters have received serious attention on account of their capability of high voltage operation, high efficiency, and low electromagnetic interference. It has many advantages compared to conventional two-level inverters such as high dc-link voltages, reduced harmonic distortion, fewer voltage stresses, and low electromagnetic interferences. The multilevel converters have been used for STATCOM widely as it can improve the power rating of the compensator to make it suitable for medium or high-voltage high power applications. While deploying multilevel STATCOMs, designer’s role is to reduce the number of switching devices since, the total switching losses are proportional to the number of switching devices. The reduction in the count of switching devices also reduces the size and cost. In this paper, a five-level modified cascaded H-bridge inverter STATCOM is proposed for mitigation of harmonics. Modified Five-level CHB configuration is the most suitable as with lesser number of switches, give better performance resulting in a compact system. The PQ theory-based controller is developed for control of STATCOM operation. MATLAB simulation results are presented to demonstrate mitigation of harmonics.

Maximizing the Post-fault Output Voltages of Cascaded H-Bridge STATCOM with Reserved Cell

Maximizing the Post-fault Output Voltages of Cascaded H-Bridge STATCOM with Reserved Cell

Operation of the Low-Capacitance Cascaded H-Bridge StatCom Under Grid Voltage Swells

Grid voltage transients and, in particular, grid voltage swells pose control challenges for static compensators (StatComs). This article presents an efficient control strategy for changing the dc value of capacitor voltages, which is especially useful for operating a low-capacitance StatCom (LC-StatCom) under grid voltage swells. In the proposed strategy, the capacitor voltages are changed as a function of the grid voltage magnitude in each phase. This article details all the required control stages for implementing this control strategy. The effectiveness of this strategy to deal with grid voltage swells is compared with other strategies both in conventional StatComs and LC-StatComs using a simulated 36-MVA 11-level cascaded H-bridge converter in MATLAB Simulink. The comparison is based on current transient performance and an analytical model of switching losses. Finally, the feasibility of this strategy is confirmed by experiments on a 1 kVA prototype.

Startup Control to Eliminate Inrush Current for Star-Connected Cascaded H-Bridge STATCOM

Soft startup is essential to the operation of the star-connected cascaded H-bridge (CHB) STATCOM. The conventional startup stage involves only uncontrolled rectifier process in which the dc voltages are charged by inserting startup resistors to limit the inrush current. Nevertheless, another inrush current is induced at the beginning of normal operation due to the low charged dc voltages during the startup stage. In this article, to eliminate this inrush current, a novel startup control is proposed for further charging the dc voltages after the uncontrolled rectifier process. During this controlled charging process, the startup resistors are not bypassed so as to smoothen the dc voltages and limit the inrush current. In this scenario, the overall dc voltage control with constant charging current is proposed by establishing a relationship between the overall dc voltage and the positive sequence modulation index. Besides, the cluster dc voltage balancing control based on the negative sequence voltage injection is proposed by redistributing the active power of the STATCOM. Moreover, the charging current is designed to reduce the active power of the startup resistors and increase the charging speed. Due to the flexibility of the startup control, it can be applied to any voltage-source-based ac/dc grid-connected converter. Finally, the performance of the proposed control is verified by the simulation and experimental results.

Constrained Control of Low-Capacitance Delta Cascaded H-Bridge StatComs: A Model Predictive Control Approach

This article presents a constrained model predictive control (MPC) strategy for a three-phase low-capacitance static compensator (LC-StatCom) with delta configuration. The controller consists of an outer loop that provides dynamic references for the active grid current component and the circulating current; whereas, the inner predictive loop minimizes the quadratic error of state variables subject to operating limit conditions, thus providing optimal control signals that produce a fast response while guaranteeing the prescribed safe operating limit conditions. The article also proposes an enhanced discrete-time model, which uses intersample values, that improves the accuracy of the model predictions when the sampling rate is not high enough to assume constant state variables. As a result, it facilitates the implementation of MPC on less powerful processors compared to the conventional modeling. The proposed control is especially suited for the LC-StatComs since it incorporates analytical computation of desired steady-state trajectories, which takes into account the induced oscillation on the capacitor voltages that are inherent to the LC-StatComs. The proposed approach has been verified by simulation and experimentally with a laboratory prototype.

Low-capacitance cascaded H-bridge STATCOM for full power operation

Low-capacitance cascaded H-bridge STATCOM for full power operation

Flexible Nonsinusoidal Zero Sequence Voltage Injection Method to Extend Negative Sequence Current Compensation Range for Star-Connected CHB STATCOM

DC voltage balance based on zero sequence voltage is commonly used for star-connected cascaded H-Bridge (SCHB) STATCOM to compensate negative sequence current. However, zero sequence voltage injection brings about side-effect of increasing STATCOM output voltage and thus the Negative sequence Current Compensation Range (NCCR) is significantly limited to avoid overmodulation. To extend the NCCR, improving the dc voltage utilization by injecting third harmonic component is a typical method. Compared with third harmonic injection (THI), square wave features lower peak-to-average ratio. Unfortunately, the capability of square wave injection (SWI) for the NCCR extension has not been well explored. In this article, to further extend NCCR, not only the SWI is employed for SCHB STATCOM, but also it is compared with THI in term of NCCR by establishing the relationship between the maximum STATCOM voltage and negative sequence current. The comparison results reveal that the THI and SWI both have their own strengths and weaknesses in NCCR, which highly depends on the amplitudes and phases of positive and negative sequence currents. Furthermore, the criteria associated with the amplitudes and phases of the positive and negative sequence currents are derived to determine the harmonic injection method with larger NCCR by using symbolic-graphic combination method and MATLAB tools. As a result, a new harmonic injection method is proposed by selecting the THI or SWI with larger NCCR flexibly according to the amplitudes and phases of positive and negative sequence currents. Finally, the proposed method is verified by experimental results on a 400 V/7.5 kVar SCHB STATCOM prototype.

Enhancing Inductive Operation of Low-Capacitance Cascaded H-Bridge StatComs Using Optimal Third-Harmonic Circulating Current

The cascaded H-bridge (CHB) low-capacitance static compensator (LC-StatCom) has a limited operating range in the inductive region compared to a conventional StatCom due to the inherent large oscillations on the capacitor voltages. This limitation constitutes a great drawback of CHB LC-StatComs. This article presents an effective hardware-free solution to overcome this drawback. The proposed solution is based on the injection of an optimal third-harmonic circulating current to suitably shape the capacitor voltages aiming at avoiding overmodulation. Considering all the effects of the third-harmonic circulating current, the article proposes a reference design that guarantees a prescribed safe operating voltage limit, thus increasing the operational range of the LC-StatCom. The proposed approach takes into account the inherent induced oscillation on the capacitor voltages. This approach has been verified by simulation and experimentally with a laboratory prototype, corroborating the excellent performance that agrees with the theoretical analysis.

Design of Cascaded H Bridge STATCOM with Distributed Control

Aiming at the problem of low redundancy in the structure of the cascaded H-bridge STATCOM. This paper studies and designs a cascaded H-bridge STATCOM device based on STM32 distributed control. The article introduces the basic topology of the device and the realization principle of the control system. The turn-on and turn-off time of the MOSFET in each H-bridge is calculated by using the law of minimum harmonics, realize the output of the fitted sine wave. A simulation model of 12H bridge cascade is built in the EMTP power system electromagnetic transient analysis simulation software. The simulation results show the correctness and reliability of the design.

Fuzzy Logic based H-bridge STATCOM Controller to compensate Unbalanced Voltages

In this paper a new method four-leg topology is suggested for shunt compensation, the H-bridge multilevel converters based on the half-bridge converters, to achieve higher performance as a STATCOM in a distorted and unbalanced medium-voltage large-current system. Further, an extended Fuzzy based STATCOM is proposed in order to manage more accurate compensation for high-power applications. Both proposals can be controlled for various purposes such as reactive power and unbalance compensation, voltage regulation, and harmonic cancellation. One interesting application for the Fuzzy logic based STATCOM could be the improvement in power quality and performance of the electrified railway traction power supply system. Both the H-bridge based STATCOM along with their proposed control strategies were simulated.

Strategies of voltage controllers in delta-connected H-bridge STATCOMs

This paper presents strategies for a voltage controller in which droop control and var reserve control (an auxiliary controller) are included in delta-connected H-bridge STATCOM. To supply the linear property and load sharing effect, the droop control has been applied to a voltage controller. In addition, the var reserve control has been chosen to enlarge the output margin depending on the dead band range. According to the installation environment, the two auxiliary controllers are chosen. However, the combination control of two auxiliary inputs has not been determined. In this paper, three different control strategies (combination methods) are proposed, and the trajectory of equilibrium point is identified. The three combination methods are implemented from HILS tests.

Cluster Voltage Regulation Strategy to Eliminate Negative-Sequence Currents Under Unbalanced Grid for Star-Connected Cascaded H-Bridge STATCOM

Under unbalanced grid, three cluster voltages are typically controlled balanced and unchanged for the star-connected cascaded H-bridge static synchronous compensator (CHB STATCOM), however, to deal with the unbalanced grid voltages, the modulation voltages become unbalanced, which would result in fewer steps of the staircase voltages and unbalanced double-line-frequency ripples across the cluster voltages. Thus, the harmonic performance of the currents is deteriorated. In addition, the negative sequence currents are always injected for active power redistribution, which degrades the power quality. To improve the current performance under the unbalanced grid, three cluster voltages are regulated to different values and three-phase modulation voltages can maintain balance. Based on this concept, three cluster voltages can be derived by decomposing the positive- and negative-sequence components from the converter voltages to meet the balance of three-phase currents. To ensure the cluster voltages stable, the zero-sequence voltage is further extracted to analyze the active power distribution. It is interesting to find that the total active power in each phase induced by both the negative and zero-sequence voltages is equal to zero, which reveals the CHB STATCOM has the natural zero-active-power feature. To keep three unequal cluster voltages within a certain boundary, the relationship between the unbalanced grid voltages and the cluster voltages is established, based on which the algorithm of limiting the cluster voltages is proposed. The theoretical analysis and the proposed control scheme are verified by the simulation and experimental results.