Ride-through Operation Research Articles

Low-Voltage Ride-Through Operation of Power Converters in Grid-Interactive Microgrids by Using Negative-Sequence Droop Control

Due to the increasing penetration level of microgrids (MGs), it becomes a critical issue for MGs to help sustaining power system stability. Therefore, ancillary services, such as the low-voltage ride-through (LVRT) capability should be incorporated in MGs in order to guarantee stable operation of the utility grid during grid faults. In this paper, a LVRT control strategy based on positive/negative sequence droop control is proposed for grid-interactive MGs to ride-through voltage sags with not only inductive/resistive, but also complex line impedance. By using the proposed control strategy, MGs can support the grid voltage, make profits, and also ride-through the voltage dip during the whole fault period. A two-layer hierarchical control strategy is proposed in this paper. The primary controller consists of voltage and current inner loops, a conventional droop control and a virtual impedance loop, while the secondary controller is based on a positive/negative sequence droop scheme which is able to coordinate the power injection during voltage sags. Experimental results are obtained to verify the effectiveness of the proposed control strategy.

A Transformer Inrush Reduction Technique for Low-Voltage Ride-Through Operation of Renewable Converters

The low-voltage ride-through (LVRT) capability is a critical requirement of grid-connected converters of renewable energy resources to uphold the stability of the power system. The step-up transformer that connects the power converter to the grid is exposed to sudden voltage changes throughout the LVRT, and the resulting magnetic flux deviation often leads to severe inrush current when the grid fault clears. The inrush current exerts significant mechanical forces in the transformer and damages its insulation. This paper proposes a control method of the power converter to reduce the risk of inrush current by mitigating the magnetic flux deviation during the LVRT operation. The proposed method also manages the output current of the power converter within the capacity of semiconductor devices to reduce their stress while performing the proposed flux compensation and other LVRT operations. The reliability of the power converter and the transformer can be improved by the proposed control method.

Response of wind power park modules in distribution systems to transmission network faults during reverse power flows

Penetration levels of distributed wind power park modules (WPPMs) reach such high levels in parts of the world, for example, Germany, that reverse power flows (RPFs) from distribution to transmission level occur regularly in certain areas of the power system. This study compares the impact of normal and RPFs on the network fault response of WPPMs in distribution systems that are prone to fault-induced delayed voltage recovery issues for a fault in the transmission system. The study contributes to the ongoing discussion on grid connection requirements for WPPMs and raises awareness of a prolonged low-voltage ride-through (LVRT) operation of WPPMs when these are set to ride-through faults in ‘blocking mode’, during which the set-points for the active and reactive currents are set to zero. For RPFs such LVRT operation causes a substantial sustained active power deficit in power system areas with high penetration of distributed WPPMs and potentially leads to a stability risk.

A Hybrid Modular Multilevel Converter With Novel Three-Level Cells for DC Fault Blocking Capability

A novel hybrid, modular multilevel converter is presented that utilizes a combination of half-bridge and novel three-level cells where the three-level cells utilize a clamp circuit which, under dc side faults, is capable of blocking fault current thereby avoiding overcurrents in the freewheel diodes. This dc fault blocking capability is demonstrated through simulation and is shown to be as good as the modular multilevel converter which utilizes full-bridge cells but with the added benefits of: lower conduction losses; fewer diode and semiconductor switching devices, and; fewer shoot-through modes. The semiconductor count and conduction loss of the proposed converter are reduced to around 66.5% and 72% of that of modular multilevel converter based on the full-bridge cells respectively, yielding lower semiconductor cost and improved efficiency. Dc fault ride-through operation is realized without exposing the semiconductors to significant fault currents and overvoltages due to the full dc fault blocking capability of the converter.

A Voltage Detection Method for the Voltage Ride-Through Operation of Renewable Energy Generation Systems Under Grid Voltage Distortion Conditions

This paper proposes a new voltage detection method to assist the low-voltage ride-through operation of grid-tied converters. In particular, it consists of one synchronous reference frame phase-locked-loop (SRF-PLL) and two voltage detection modules. The fast voltage detection module tracks maximum points or zero points of three-phase grid voltage waveforms to calculate the phase voltage amplitude. The accurate voltage detection module operated in parallel assumes an additional cascaded delayed signal cancellation block to filter out the possible low-order voltage harmonics and then employs the same operational principle as the fast voltage detection module to precisely derive the phase voltage amplitude. Coupling with the SRF-PLL, where the grid voltage amplitude before voltage sag or surge can be precisely calculated, the voltage variation ratios can then be derived in sequence. A progressive logic algorithm is designed to determine the converter control commands by fully considering the different voltage variation ratios derived at different time points. Doing so, the proposed method can assist the grid-tied converter to satisfy the grid voltage ride-through operation requirements under various grid voltage distortion conditions. Finally, validity and accuracy of the proposed method were verified by simulation and experimental results.

Fault Analysis for Distribution Networks With Current-Controlled Three-Phase Inverter-Interfaced Distributed Generators

The fault current of a three-phase inverter-interfaced distributed generator (IIDG) depends on the control strategy employed during grid faults. A new short-circuit current calculation method for IIDGs is developed in this study according to the current control and reactive power support during the low-voltage ride-through operation period. To investigate how the ride-through time relates to the short-circuit current of IIDGs, short-circuit responses under different fault conditions are compared. The short-circuit current of a multi-IIDG distribution network is determined with the proposed technique to consider ride-through time and sequence current control. The correctness of the proposed method is verified by simulation.

A Low Component Count Series Voltage Compensation Scheme for DFIG WTs to Enhance Fault Ride-Through Capability

Fault ride-through (FRT) operation has been a challenge for the doubly fed induction generator (DFIG) based wind turbines (WTs) as the stator winding is directly connected to the grid. Additionally, several grid codes have been established for the grid interconnection of WTs, which demand the WT to stay connected and provide the predefined reactive current support to the grid during FRT operation. The series voltage compensation (SeVC) based FRT schemes for DFIG WTs outperforms all others in terms of smooth transient performance. However, such FRT schemes require an additional voltage-source converter (VSC) and a bulkier series transformer to provide the SeVC. This paper proposes a low component count SeVC scheme that is applicable to both individual WTs and wind parks to cope up with the recent grid codes requirements. The proposed configuration eliminates the need of a series transformer and an additional VSC for the SeVC operation with the use of three additional IGBTs/switches. Furthermore, a coordinated control strategy is proposed to control the WT that enhance its FRT capabilities. The proposed low component count FRT scheme and coordinated control strategy are validated using the detailed mathematical modeling and simulation of 1.5 MW DFIG WT.

Reduced junction temperature control during low‐voltage ride‐through for single‐phase photovoltaic inverters

Future photovoltaic (PV) inverters are expected to comply with more stringent grid codes and reliability requirements, especially when a high penetration degree is reached, and also to lower the cost of energy. A junction temperature control concept is proposed in this study for the switching devices in a single-phase PV inverter in order to reduce the junction temperature stress, and thus to achieve improved reliability of a PV inverter. The thermal stresses of the switching devices are analysed during low-voltage ride-through operation with different levels of reactive power injection, allowing an optimal design of the proposed control scheme with controlled mean junction temperature and reduced junction temperature swings. The effectiveness of the control method in terms of both thermal performance and electrical performance is validated by the simulations and experiments, respectively. Both test results show that single-phase PV inverters with the proposed control approach not only can support the grid voltage recovery in low-voltage ride-through operation but also can improve the overall reliability with a reduced junction temperature.

Modulation Methods for Neutral-Point-Clamped Wind Power Converter Achieving Loss and Thermal Redistribution Under Low-Voltage Ride-Through

The three-level neutral-point (NP)-clamped (3L-NPC) converter is a promising multilevel topology in the application of megawatt wind power generation systems. However, the growing requirements by grid codes may impose high stress and even give reliability problem to this converter topology. This paper investigates the loss and thermal performances of a 10-MW 3L-NPC wind power inverter undergoing low-voltage ride-through (LVRT) operation. A series of new space vector modulation methods is then proposed to relocate the thermal loading among the power switching devices. It is concluded that, with the proposed modulation methods, the thermal distribution in the 3L-NPC wind power inverter undergoing LVRT becomes more equal, and the junction temperature of the most stressed devices can be also relieved. Also, the control ability of the dc-bus NP potential, which is one of the crucial considerations for the 3L-NPC converter, is even more improved by the proposed modulation methods.

A Novel Grid Synchronization PLL Method Based on Adaptive Low-Pass Notch Filter for Grid-Connected PCS

The amount of distributed energy resources (DERs) has increased constantly worldwide. The power ratings of DERs have become considerably high, as required by the new grid code requirement. To follow the grid code and optimize the function of grid-connected inverters based on DERs, a phase-locked loop (PLL) is essential for detecting the grid phase angle accurately when the grid voltage is polluted by harmonics and imbalance. This paper proposes a novel low-pass notch filter PLL (LPN-PLL) control strategy to synchronize with the true phase angle of the grid instead of using a conventional synchronous reference frame PLL (SRF-PLL), which requires a d-q-axis transformation of three-phase voltage and a proportional-integral controller. The proposed LPN-PLL is an upgraded version of the PLL method using the fast Fourier transform concept (FFT-PLL) which is robust to the harmonics and imbalance of the grid voltage. The proposed PLL algorithm was compared with conventional SRF-PLL and FFT-PLL and was implemented digitally using a digital signal processor TMS320F28335. A 10-kW three-phase grid-connected inverter was set, and a verification experiment was performed, showing the high performance and robustness of the proposal under low-voltage ride-through operation.

A sensitivity based approach to assess the impacts of integration of variable speed wind farms on the transient stability of power systems

The impact of large penetration of wind power on the transient stability of multi-machine power systems is discussed in this paper. Variable speed wind farms with doubly fed induction generators (DFIG) are considered. A new index based on the sensitivity of the DFIG terminal voltage is used to assess the transient stability margin of power systems, both for constant wind speed condition and during a sudden change in wind speed. Suitable locations for connecting the DFIG to an existing power system are identified with the help of this index. The effect of increase in wind power penetration on the transient stability condition of the existing power system is studied. The proposed index is also used to select the optimum crowbar resistance during fault ride-through operation.

Coordinated Control of DFIG's RSC and GSC Under Generalized Unbalanced and Distorted Grid Voltage Conditions

This paper proposes a coordinated control of the rotor-side converter (RSC) and grid-side converter (GSC) of a doubly fed induction generator (DFIG)-based wind-turbine generation system under generalized unbalanced and/or distorted grid voltage conditions. The system behaviors of the RSC and GSC during supply imbalance and distortion are investigated. To enhance the fault ride-through operation capability, the RSC is properly controlled to eliminate the torque oscillations, whereas the GSC is carefully designed to ensure constant active power output from the overall DFIG generation system. To achieve simultaneous regulation of the positive-/negative-sequence currents and fifth-/seventh-order harmonic currents for both the RSC and GSC, a novel current controller, consisting of a conventional proportional-integral regulator and a dual-frequency resonant compensator, is proposed and implemented in the positive (dq)+ reference frame. The simulation and experiment studies demonstrate the correctness of the developed model and the effectiveness of the suggested control strategy for DFIG-based wind-turbine systems under such adverse grid conditions.

Low-Voltage Ride-Through Capability of a Single-Stage Single-Phase Photovoltaic System Connected to the Low-Voltage Grid

The progressive growing of single-phase photovoltaic (PV) systems makes the Distribution System Operators (DSOs) update or revise the existing grid codes in order to guarantee the availability, quality, and reliability of the electrical system. It is expected that the future PV systems connected to the low-voltage grid will be more active with functionalities of low-voltage ride-through (LVRT) and the grid support capability, which is not the case today. In this paper, the operation principle is demonstrated for a single-phase grid-connected PV system in a low-voltage ride-through operation in order to map future challenges. The system is verified by simulations and experiments. Test results show that the proposed power control method is effective and the single-phase PV inverters connected to low-voltage networks are ready to provide grid support and ride-through voltage fault capability with a satisfactory performance based on the grid requirements for three-phase renewable energy systems.

Control of an Off-Shore Synchronous Generator Based Wind Farm with Uncontrolled Rectifier HVDC Connection

Large off-shore wind farms based on synchronous generators can be connected to the mainland grid using modern wind turbines allow for tight control of terminal voltage. Therefore, an integrated design of wind turbine and HVDC link control can lead to a substantial simplification of the off-shore rectifier. This paper introduces an integrated control algorithm for line commutated thyristor (LCT) based HVDC transmission line whereby the off-shore rectifier tap changer is eliminated and a cheaper and more efficient uncontrolled rectifier is used instead of a LCT-based rectifier. The presented control strategy allows for off-shore ac-grid voltage and frequency control and shows good performance during steady state and transient operation, including on-shore fault ride-through operation.

Experimental Investigation on Ride through Capability of Closed Loop Controlled Matrix Converter Fed Cage Drive

Three phase matrix converters have received considerable attention in recent years because they may become a good alternative to voltage-source inverter pulse widthmodulation (VSI-PWM) converters. In fact, the matrix converter provides bidirectional power flow, sinusoidal input/output waveforms, and controllable input power factor. Furthermore, the matrix converter allows a compact design due to the lack of dc-link capacitors for energy storage. The matrix converters are more sensitive to input power disturbances than conventional PWM voltage source inverters due to the absence of dc-link. With the rapid increase of adjustable speed drives (ASDs) in commercial and industrial facilities, the susceptibility of ASDs under power disturbances such as sags, swells, transients and short term power interruption (STPI) has become more important issue. A matrix converter laboratory prototype of 250 VA, 230 V capacity has been developed with the embedded PWM VSI part, to investigate the ride through capability. The ride through approach has been shown to maintain the rotor flux and keep synchronization between the motor and matrix converter during momentary power interruption. It has been shown that by regenerating the mechanical energy stored in load inertia and transferring it to dc-link capacitor within the embedded PWM VSI part, successful ride-through operation is accomplished. The duration of the ride-through operation depends on initial motor shaft speed level, load torque type and load inertia and so the approach is more effective for applications with sufficient load inertia. The ride-through capability has been achieved by the minimal addition of hardware and software into the matrix converter.Energy efficient data transfer is the key factor for the design of energy efficient wireless sensor network (WSN). In this paper, an energy efficient cooperative MIMO (C-MIMO) technique is proposed in the WSN where fixed constellation size is considered in transmitting both the local and long-haul communication. A selection criterion is used based on the channel conditions where a selected number of sensors in a cluster is used to form a multiple input multiple output (MIMO) structure wirelessly connected with multiple antennas of a data gathering node (DGN). The selected approach is tested on the correlated data scenario. Experimental results show that the selected C-MIMO structure outperforms the unselected C-MIMO in terms of total energy consumption and they show energy efficient performance over existing one when they are compared with SISO structure. Energy models are evaluated for this scenario while energy consumption and efficiency are compared for different combination of cluster sizes and selected number of sensors. Comparisons are shown for different fixed number of constellation sizes. The effect of correlation coefficient and intersensor distance are analyzed. Later, a delay analysis is shown considering optimal constellation size.

Experimental evaluation of ride-through capabilities for a matrix converter under short power interruptions

The matrix converters, which are direct power electronic converters, are able to provide important benefits such as bidirectional power flow, sinusoidal input currents with adjustable displacement angle, and a great potential for size reduction. Still, two major disadvantages exist: a lower than unity voltage transfer ratio and high sensitivity to power grid disturbances. Many solutions to provide continuous operation of adjustable speed drives (ASDs) during power grid disturbances have been proposed, but they are all applied to DC-link ASD. In this paper, a new solution to provide limited ride-through operation is presented with a matrix converter using a scalar controlled induction motor for a duration of hundreds of milliseconds, without any hardware modification. During the ride-through operation, the drive is not able to develop torque or to control the motor flux. By recovering the necessary power to feed the control hardware of the matrix converter, it is able to keep the ASD operating. When normal grid conditions are reestablished, the matrix converter is able to accelerate the motor from nonzero speed and flux by initializing the modulator with the estimated frequency and the initial angle of the reference output voltage vector. The maximum duration of the ride-through operation depends on the initial motor flux, speed level, rotor time constant, load torque, and inertia. This method is verified on a laboratory setup with a matrix converter.

Limited ride-through capabilities for direct frequency converters

Many solutions to provide continuous operation of adjustable speed drives (ASDs) during power grid disturbances have been proposed, but they are all applied to DC-link ASD. In this paper a new solution to provide limited ride-through operation of a scalar controlled direct frequency converter (DFC) for a duration of hundreds of milliseconds, without any hardware modification, is presented. During the ride-through operation, the drive is not capable of developing torque or to control the motor flux. By recovering the necessary power to feed the control hardware, the DFC is able to keep the ASD operating. When normal grid conditions are re-established, the DFC is also able to accelerate the motor from nonzero speed and flux by initializing the modulator with the estimated frequency and initial voltage vector angle. The duration of the ride-through operation depends on the initial motor flux, speed level, rotor time constant, load torque and inertia.