Vector PI Controller Research Articles

Zero-sequence controller requirements and comparison for a delta-CHB STATCOM under unbalanced operation

Delta-configured Cascaded H-Bridge (DCHB) topology is a suitable alternative for Static Synchronous Compensator (STATCOM) applications. However, under unbalanced voltage and/or current conditions, zero-sequence current needs to be injected to guarantee dc-link capacitor voltage balancing. The control loop of this zero-sequence current is analyzed in this paper, in order to determine the requirements that the implemented controller must fulfill. Considering these requirements, appropriate transient response and stability margin indicators are defined to quantify and evaluate the performance of different controllers that could be employed —Proportional-Resonant controller (PR), PR controller with delay compensation (PRd), and Vector Proportional-Integral controller (VPI) are proposed and analyzed for this application in this paper. Based on the defined indicators, the VPI is the most appropriate among the studied control techniques. Experimental results validate the analytical model of the controllers and their performance.

Vector Proportional Integral Control with Delay Compensation for Stator Excited Doubly Salient Synchronous Motors

This paper proposes a novel current control technique to solve the problem that the special doubly salient structure of the studied stator excitation doubly salient synchronous motor (SEDSSM) causes the motor back EMF to cause large harmonic current and torque ripple. Based on the space vector pulse width modulation sinusoidal drive method, a vector proportional-integral controller with delay compensation (VPIC) is introduced into the current loop to replace the traditional PI controller to control the motor. The SEDSSM stator current under the current loop control mode of conventional PI controller is analyzed, and it is found that its waveform is obviously affected and disturbed by the third harmonic current, which will cause torque pulsation. Therefore, additional resonance items are considered to be introduced into the controller to suppress its harmonic. At the same time, considering that the digital delay in the application of the digital system will lead to an unstable state of the control system under the vector proportional-integral controller, the vector proportional-integral controller with delay compensation is introduced for this purpose. Experimental data tested on a 12/8 SEDSSM prototype show that the VPIC controller can significantly reduce stator current ripple and motor torque ripple compared to the existing PI controller.

Multifunctional inverter based on virtual synchronous machine implemented in synchronous reference frame

A new multifunctional inverter control structure is proposed in this paper by embedding the harmonic and unbalance regulators into the virtual synchronous machine (VSM). While including the harmonic and unbalance regulators, inverters can provide support to the grid not only as VSMs, but also as active power filters (APFs) and unbalance conditioners. Different from the traditional multifunctional inverter in the stationary frame, the proposed method is implemented in the synchronous reference frame using the vector proportional-integral controllers without high pass filter for APF. Moreover, with VSM for power control and frequency tracking, harmonic and unbalance regulators can use the phase tracked by VSM so that the adaptive control is achieved regardless of the frequency variations. As a result, the simplified control structure with more operation modes is available without phase-locked loop (PLL) and unbalance regulators can reuse the voltage/current control of positive sequence. Furthermore, control decoupling condition of different functions is analysed to ensure the independent design of each function. Finally, the proposed structure is verified by MATLAB/Simulink and experiments.

Design and Analysis of Vector Proportional–Integral Current Controller forLC-Coupling Hybrid Active Power Filter With Minimum DC-Link Voltage

In this article, a vector proportional-integral (VPI) current controller for the LC-coupling hybrid active power (LC-HAPF) with ideally zero steady-state error, optimal transient response, and good robustness is proposed. First, the transfer function of the proposed VPI current controller is introduced, and its parameter design for obtaining optimal transient response is discussed. Second, a detailed robustness analysis of the VPI current controller, including the system parameter variation, distorted grid voltage, and grid frequency variation conditions, is proposed. Third, the digital implementation method of the VPI controller and an overall control block diagram for the LC-HAPF with the proposed VPI current controller are presented. Moreover, a comparative study and analysis between the VPI current controller and the other representative current controllers under the minimum dc-link voltage is discussed in which the VPI current controller obtains the best performances under the minimum dc-link voltage operation. The simulation and experimental results are provided to verify the advantages and effectiveness of the proposed VPI current controller for a three-phase four-wire LC-HAPF in comparison with the other representative current controllers under different dc-link voltage conditions. Finally, the robustness under different grid conditions and unbalanced compensation capability of the proposed VPI-controlled LC-HAPF are also verified by the experimental results.

An Improved Control Strategy of Three-Phase PWM Rectifiers Under Input Voltage Distortions and DC-Offset Measurement Errors

This paper proposes an enhanced current control strategy to reject the impact of dc-offset measurement error and the distortion in the supply voltage on the dc output voltage and the input current performance of three-phase pulse-width modulation rectifiers. The proposed current controller designed in the synchronous reference frame is developed with a proportional integral (PI) and two vector PI (VPI) controllers. The PI controller regulates the fundamental current to follow its reference generated from outer dc-link voltage control, meanwhile two VPI controllers tuned at the fundamental grid frequency ( $\omega _{s}$ ) and $6\omega _{s}$ are employed to eliminate the dc and harmonic components in the input current. Compared with the standard resonant controllers, the proposed VPI controller offers some advantages such as simpler and straightforward tuning process for controller gains and more robust control system against control delay time and frequency variations. Thanks to the effectiveness of the VPI controllers, the dc output voltage is controlled at a stable manner without any ac ripple, and the three-phase input currents are controlled to be balanced, sinusoidal, and extremely low dc component despite the presence of the dc-offset measurement error and distorted supply voltage conditions. Various experimental tests are provided to verify the effectiveness of the proposed control algorithm.

Control Strategy to Eliminate Impact of Voltage Measurement Errors on Grid Current Performance of Three-Phase Grid-Connected Inverters

This study proposes an advanced current control strategy for three-phase grid-connected inverters to reject the impact of the dc offsets and scaling errors in the grid voltage measurement on the grid current performance. The proposed current controller designed in the synchronous ( d-q ) reference frame is developed with a proportional integral (PI) plus three vector PI controllers. The PI controller regulates the fundamental current to follow its reference, meanwhile, three vector PI controllers tuned at the fundamental grid frequency ( ${\omega _s}$ ), $2{\omega _s}$ , $6{\omega _s}$ are employed to eliminate the dc, unbalance, harmonic components in the grid current. As a result, the three-phase grid currents are controlled to be balanced, sinusoidal, and extremely low dc component despite the presence of the dc offset and scaling errors in the grid voltage measurement and distorted grid voltage conditions. The main advantage of the proposed control scheme is that it is developed without the need of additional hardware circuit, dc extraction, and harmonic detection scheme so that it can be integrated into the existing grid-connected inverter system without extra cost. The effectiveness of the suggested solution is verified by experimental results under various grid voltage conditions and the grid voltage measurement errors.

Arm Current Control Strategy for MMC-HVDC Under Unbalanced Conditions

Circulating current can be decomposed into double line-frequency positive-, negative- and zero-sequence components under unbalanced voltage conditions. First, in order to decrease peak/rms value of arm current, double line-frequency circulating current should be eliminated. Second, ac-side current that consists of positive- and negative-sequence components under unbalanced voltage conditions should be regulated to track the reference signal. Furthermore, fundamental frequency circulating current caused by asymmetry of the upper and lower arm should be also removed. To effectively control ac-side current and eliminate all circulating currents, an arm current control strategy based on a decoupler and proportional-integral controller plus the vector proportional integral (VPI) controller is proposed in this paper. The proposed control strategy works well under balanced and unbalanced voltage conditions as well as asymmetry of the upper and lower arm. In addition, this paper proposes a design procedure for the VPI controller to reduce the uncertainty of this process. A three-phase modular multilevel converter simulation model is set up on PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.

Power Quality Improvement in Modified Solid State Transformer System Using Statcom

Power quality is a major issue of loads in the distribution system used in industrial and domestical appliances. In this work, we proposed a modified solid state transformer system using statcom to improve the power quality of loads in the distribution system. The proposed model is used to eliminate voltage sag, and swell. The matrix converter is adopted in the proposed design of modified solid state transformer to reduce the power loss. The control strategy of matrix converter is done by sinusoidal pulse width modulation techniques. In addition to this, harmonics are reduced by using vector proportional integral controller. The simulation is done in MATLAB/SIMULINK software, the several case studies are carried out and the simulation results shows that the proposed system has better voltage regulation than that of conventional system.

Standalone Hybrid Wind Solar Power Generation System Using Multilevel Converter with Less Number of Switches

In this paper a cascaded multilevel converter with reduced number of switches is proposed for standalone hybrid solar wind power generation system to improve the waveform quality of the output voltage and current. Renewable energy sources such as solar and wind power generation systems play important role in energy generation. However, the output power of solar are usually fluctuant due to the uncertainty and intermittence of solar and wind energy, which requires a large capacity of energy storage to satisfy the load demand when the system works in grid connected mode. Storage systems are used to compensate or absorb the difference between load demand and generation. This paper also presents improvement of power system at different variable load and wind profile conditions. The main advantage of the proposed technique is the closed loop control system which has a vector PI controller which ensures optimal power delivery from solar is matched with the wind energy. The system is employed on a multilevel converter, DC-DC boost converter, photovoltaic system, PMSG and Battery storage system. The performance of the proposed system has been evaluated by Matlab/Simulink. The proposed system can not only improve the power supply reliability, but also reduces the cost of the system.

Improvement of unified power quality conditioner performance with enhanced resonant control strategy

This study proposes an enhanced resonant control strategy for the unified power quality conditioner (UPQC) to simultaneously tackle voltage sags, unbalance and distortions on the supply side as well as current harmonics on the load side. The proposed control strategy is developed in both the series and shunt active power filters (APFs) of the UPQC. In the series APF, a proportional-resonant controller and a resonant controller are employed to mitigate voltage sags, unbalance and distortions, whereas a proportional-integral (PI) controller and three vector PI controllers are used in the shunt APF to compensate harmonic currents. The performance of the proposed UPQC control scheme is significantly improved compared with the conventional control strategy owing to the superiority of the resonant controllers. In addition, voltage sag/harmonic and current harmonic detectors are not required in the proposed control scheme, which helps to simplify the control strategy and to improve the control accuracy. The proposed control strategy is theoretically analysed and its feasibility is validated through experiments.

Transient response evaluation of stationary‐frame resonant current controllers for grid‐connected applications

This study deepens on the transient response analysis of the so-called vector proportional–integral (VPI) controllers and compares them with the popular proportional–resonant (PR) controllers for grid-connected applications. The employed methodology is based on the study of the error signal roots: both reference tracking and disturbance rejection abilities are considered for proper gain tuning. This study proves that PR controllers lead to shorter settling times than VPI controllers. A three-phase voltage source converter prototype has been implemented. Experimental results comparing the transient behaviour of VPI and PR controllers in different conditions are provided: a + 90° phase-angle jump in the current reference and a ‘type C’ voltage sag at the point of common coupling.

Improved DPC Strategy of Grid-connected Inverters under Unbalanced and Harmonic Grid Conditions

This paper presents an improved direct power control (DPC) strategy for grid-connected voltage source inverter (VSI) under unbalanced and harmonic grid voltage conditions. Based on the mathematic model of VSI with the negative sequence, 5th and 7th harmonic voltage components consideration, a PI controller is used in the proposed DPC strategy to achieve the average output power regulation. Furthermore, vector PI controller with the resonant frequency tuned at the two times and six times grid fundamental frequency is adopted to regulate both negative and harmonic components, and then two alternative targets of the balanced/sinusoidal current and smooth active/reactive output power can be achieved. Finally, simulation results based on MATLAB validate the availability of the proposed DPC strategy.

An Advanced Current Control Strategy for Three-Phase Shunt Active Power Filters

This paper proposes an advanced control strategy to enhance performance of shunt active power filter (APF). The proposed control scheme requires only two current sensors at the supply side and does not need a harmonic detector. In order to make the supply currents sinusoidal, an effective harmonic compensation method is developed with the aid of a conventional proportional-integral (PI) and vector PI controllers. The absence of the harmonic detector not only simplifies the control scheme but also significantly improves the accuracy of the APF, since the control performance is no longer affected by the performance of the harmonic tracking process. Furthermore, the total cost to implement the proposed APF becomes lower, owing to the minimized current sensors and the use of a four-switch three-phase inverter. Despite the simplified hardware, the performance of the APF is improved significantly compared to the traditional control scheme, thanks to the effectiveness of the proposed compensation scheme. The proposed control scheme is theoretically analyzed, and a 1.5-kVA APF is built in the laboratory to validate the feasibility of the proposed control strategy.

Improvement of dynamic performance of doubly fed induction generator-based wind turbine power system under an unbalanced grid voltage condition

With the current increase in wind power penetration into the energy market, control and operation of wind turbine generators becomes a major research topic. Wind turbine based on doubly fed induction generator (DFIG), which is sensitive to grid disturbances, is widely used. Under an unbalanced grid voltage condition, oscillations of the DFIG's electromagnetic torque and instantaneous stator powers strongly affect the dynamic performance of the DFIG. In this study, a new configuration based on vector proportional'integral (VPI) controller is proposed to eliminate such oscillations. This new configuration is employed in the rotor side converter (RSC) of the DFIG. With the proposed VPI control strategy, decomposition of sequential components and mathematical complexity are reduced. Compared with the conventional field-oriented control based on the standard single PI controller, the VPI controller can successfully eliminate torque and stator power oscillations. The effectiveness of the proposed control strategy is validated through simulation results obtained on a 1.5'MW DFIG-based wind turbine system model built in MATLAB/Simulink.

Analysis and Design of Resonant Current Controllers for Voltage-Source Converters by Means of Nyquist Diagrams and Sensitivity Function

The following two types of resonant controllers are mainly employed to obtain high performance in voltage-source converters: 1) proportional + resonant (PR) and 2) vector proportional + integral (VPI). The analysis and design of PR controllers is usually performed by Bode diagrams and phase-margin criterion. However, this approach presents some limitations when resonant frequencies are higher than the crossover frequency defined by the proportional gain. This condition occurs in selective harmonic control and applications with high reference frequency with respect to the switching frequency, e.g., high-power converters with a low switching frequency. In such cases, additional 0-dB crossings (phase margins) appear; therefore, the usual methods for simple systems are no longer valid. In addition, VPI controllers always present multiple 0-dB crossings in their frequency response. In this paper, the proximity to the instability of PR and VPI controllers is evaluated and optimized through Nyquist diagrams. A systematic method is proposed to obtain the highest stability and avoidance of closed-loop anomalous peaks: it is achieved by the minimization of the inverse of the Nyquist trajectory distance to the critical point, i.e., the sensitivity function. Finally, several experimental tests, including an active power filter that operates at a low switching frequency and compensates harmonics up to the Nyquist frequency, validate the theoretical approach.