Grid Current Measurement Research Articles

State-Space Control for LCL Filters: Converter Versus Grid Current Measurement

This article deals with discrete-time state-space current control of three-phase converters equipped with an LCL filter. Either the converter or grid current is measured, and the unknown states are estimated using a reduced-order observer. The stability and dynamic performance of the control designs based on these two current measurement options are compared by means of analysis and experiments at different sampling frequencies and under varying grid conditions, ranging from strong to very weak. Equal reference-tracking performance under nominal conditions is used as a basis for comparison between these two options. If a strong grid is assumed in the control tuning, the controller based on the grid current measurement (GCM) is found to be more robust against varying grid conditions in a wide range of sampling frequencies than the controller based on the converter current measurement (CCM). The CCM leads to better dynamic performance as compared to the GCM if the resonance frequency of the system falls below the critical resonance frequency.

Cost-Effective DC Current Suppression for Single-Phase Grid-Connected PV Inverter

Due to the disparity of power modules, asymmetry of driving pulses and measurement errors of sensors, dc currents may be injected to grid-connected photovoltaic (PV) inverters. The dc current injection may cause magnetic saturation of the power transformers. To solve this issue, this article thus proposes an effective current control strategy and compensation method, which does not require any extra sensor and hardware circuit. First, the root-cause of dc current injection is comprehensively analyzed. Subsequently, a proportional-integral-resonant (PIR) controller is proposed to eliminate the dc component caused by disparity of power modules, asymmetry of driving pulses and measurement errors of grid voltage. The injected dc current caused by grid current measurement error is estimated from the line-frequency ripple of the dc-link voltage, and then it is suppressed by a feedback compensation controller. In addition, the dc current rejection capability is evaluated, and the proposed method is benchmarked with the virtual capacitor-based method. Finally, experimental tests are performed on a 1.2-kW single-phase PV inverter to verify the effectiveness of the proposal.

A Low-Cost Phase-Angle Compensation Method for the Indirect Matrix Converters Operating at the Unity Grid Power Factor

Unity grid power factor is essential for the indirect matrix converter (IMC) applications due to the grid-friendly requirement. The existing closed-loop grid power factor control methods usually consist of complex structures, resulting in high requirements both in hardware and software aspects. In this paper, a low-cost phase-angle compensation method is proposed to reduce the costs. Based on the modeling for the IMC, the influences from the system parameters on the grid power factor angle are analyzed. Then, a virtual adaptive resistance model is denoted and calculated with the least mean square algorithm to generate a virtual voltage signal, which is consequently utilized to generate the phase compensation angle by a newly designed proportional-integral closed loop to regulate the grid power factor angle. Compared to the typical existing method, the proposed low-cost phase-angle compensation method requires only one grid current measurement in hardware and reduces 18.9% computational burden in software, while the advantages being adaptive to the system parameters and suitable for a wide load-range are reserved. Simulation and experimental research for the proposed method is carried out to validate its steady-state and dynamic-state compensation effectiveness under various load conditions.

Optimum PR Control Applied to <italic>LCL</italic> Filters With Low Resonance Frequency

A control strategy for LCL grid-connected voltage source inverters is proposed. Using the injected grid current measurement exclusively, the proposal allows the use of the proportional plus resonant regulator optimum design, regardless of the filter resonance frequency. Simulation and experimental results that demonstrate the validity and effectiveness of the proposal for different LCL filter resonance frequency values are presented. Also its superiority compared with a control method recently proposed in the literature is shown.

FPGA-Based Fault-Tolerant Space Vector-Hysteresis Current Control for Three-Phase Grid-Connected Converter

This paper investigates the field-programmable gate array (FPGA) implementation of a fault-tolerant space vector-hysteresis current control (SV-HCC) for three-phase grid-connected converter. The proposed control ensures continuous operation despite of an open-circuit fault in one grid current measurement circuit (caused by defected cables, lines or connectors at the output of the current measurement circuit). It is based on hardware redundancy of current measurement circuits for fault detection and isolation. The faulty measured current is identified through the computation of residuals with well-defined thresholds. Thanks to the high computation capabilities of FPGAs, the implemented fault-tolerant SV-HCC is executed with a very high sampling frequency and very low execution time. As a consequence, the defined theoretical thresholds remain available for the experimental tests. Several experimental results, carried out on FPGA-based prototyping platform, are given in order to illustrate effectiveness and reliability of the proposed fault-tolerant SV-HCC.

Grid impedance estimation based hybrid islanding detection method for AC microgrids

This paper focuses on a hybrid islanding detection algorithm for parallel-inverters-based microgrids. The proposed algorithm is implemented on the unit ensuring the control of the intelligent bypass switch connecting or disconnecting the microgrid from the utility. This method employs a grid impedance estimation technique that uses resonance excitation once grid fault occurs. It includes two stages: the first one is to detect grid impedance variations resulting from a grid fault; and the second one is to excite the resonance by using a virtual resistance in order to extract the grid impedance parameters. Grid impedance variations detection algorithm is based on grid current measurements temporal redundancies with fast current acquisition period. Once the grid impedance variation is detected, the excitation resonance is performed by injecting a resonance frequency in only one inverter control to avoid interactions with other units. The selected inverter will be the one closest to the controllable distributed generation system or to a healthy grid side in case of meshed microgrid with multiple-grid connections. The detection algorithm is applied to quickly detect the resonance phenomena, so that the resonance excitation is canceled and the resistive and inductive grid impedance parts are estimated. Simulation results are carried out to illustrate the effectiveness of the proposed method.

Development of Hard-wired Instrumentation and Control for the Neutral Beam Test Facility at KAERI

Since the start of the KSTAR (Korea Superconducting Tokamak Advanced Research) project, Instrumentation and Control (I&C) of the Neutral Beam Test Facility (NB-TF) has been striving to answer diverse requests arising from various facets during the project's development and construction phases. Hard-wired electrical circuits have been designed, tested, fabricated, and finally installed to the relevant parts of the system. In relation to the vacuum system I&C, controlling functions for the rotary pumps, a Roots pump, two turbomolecular pumps, and four cryosorption pumps have been constructed. I&C for the ion source operation are the temperature and flow rate signal monitoring, Langmuir probe signal measurements, gradient grid current measurements, and arc detector circuit. For the huge power system to be monitored or safely operated, many temperature measurement functions have also been implemented for the beam line components like the neutralizer, bending magnet, ion dump, and calorimeter. Nearly all of the control and probe signals between the NB test stand and the control room were made to be transmitted through the optical cables. Failures of coolant flow or beam line vacuum pressure were made to be safely blocked from influencing the system by an appropriate interlock circuit that will shut down the extraction voltage application to the system or prevent damages to the vacuum components. Preliminary estimation of the beam power through the calorimetric measurement shows that 87.9% of the total power of the 60kV/18A beam with 200 seconds duration is absorbed by the calorimeter surface. Most of these I&C results would be highly appropriate for the construction of the main NBI facility for the KSTAR national fusion research project.

Grid-current measurements with a D-C analog computer

INCREASED USE of amplifiers at frequencies of 0 to 20 cycles per second has focused attention on the problem of designing direct-coupled amplifiers. Grid current, in particular, is a problem, inasmuch as it causes a change in tube bias that cannot be distinguished from a signal. For this reason, direct-coupled amplifiers are designed to operate in the region where grid current is normally considered negligible. Operation in this region, however, is not in itself a solution because even these small currents are significant in direct-coupled amplifiers. Since grid-current characteristics for this region are not included in handbooks, the designer must solve the new problem of measuring these currents.