Unbalanced Ac Grid Conditions Research Articles

An MMC based HVDC system with optimized AC fault ride-through capability and enhanced circulating current suppression control

Modular multilevel converter (MMC) is a proven technology for HVDC applications due to its salient features such as modularity and excellent power quality. To ensure best possible grid support, recent grid codes require incorporating fault ride-through (FRT) strategies so that HVDC converter stations remain connected and maintain reliable operation under various symmetrical and asymmetrical AC faults. In this paper, a communication-free enhanced fault ride-through technique without the need of DC chopper has been proposed. The proposed FRT strategy ensures quick post fault recovery operation and can effectively manage DC link and capacitor voltages within safe limits. Along with proposed FRT strategy, in order to avoid high circulating current (CC) inside an MMC, this paper has proposed an optimal circulating current control approach based on proportional resonant and PI controllers in an abc reference frame. The suggested technique lowers the ripple in capacitor voltages while reducing the magnitude of the CC. Under both balanced and unbalanced ac grid conditions, the ripple in the dc link voltage is also reduced without the use of dual synchronous reference frame or any additional controllers. Simulation results confirm the effectiveness of the proposed FRT and CC suppression techniques for a 580-kV, 850-MW MMC-based HVDC system.

An Optimized Circulating Current Control Method Based on PR and PI Controller for MMC Applications

Modular multilevel converter (MMC) is an excellent topology for medium- and high-voltage applications due to its advantages over other multilevel converters. However, the control algorithm design needs meticulous attention as each submodule (SM) capacitor voltage is balanced around their reference. Any voltage inequality between the SM voltage causes the second harmonic-dominated circulating current inside the MMC. The circulating current increases the rms value of the arm currents, component ratings, and the ripple in the capacitor voltages unless it is appropriately controlled. This article proposes an optimized closed-loop circulating current control method based on PR and PI controllers in abc reference frame to prevent high circulating current inside an MMC. The proposed method suppresses the magnitude of circulating current while reducing the ripple in capacitor voltages. The ripple in the dc link voltage is also reduced without any supplementary controller under balanced and unbalanced ac grid conditions. Thus, the proposed circulating current control method reduces the conduction losses and component ratings, while the converter's efficiency and reliability increase. The method's verification is tested on a point-to-point connected MMC-based high voltage direct current system in a real-time simulator with Xilinx FPGA-based MMC emulator and arm controller.

Analysis and experimental verification of improved two-level converter behaviors under unbalanced AC conditions

An improved two-level converter (I2LC) represents a practical design trade-off between the conventional two-level converter (C2LC) and modular multilevel converter (MMC), in which the MMC like AC and DC fault responses and relative simplicity of C2LC power circuit and control are prioritized. Therefore, this paper presents theoretical analysis, which helps to develop broader understandings of the I2LC, and identification of the needed countermeasures to ensure correct operation during balanced and unbalanced grid conditions. The correctness of the findings and observations drawn from the presented analysis has been validated with the aid of scaled-down simulations and experimentations of the I2LC. Unlike the C2LC, it has been revealed that the I2LC can offer two concurrent operational objectives at system level during unbalanced AC grid conditions, such as balanced output currents and ripple-free DC link current. Further investigation conducted on the performance of the I2LC based two-terminal DC link during asymmetrical AC faults corroborates the theoretical analysis and scaled-down simulation and experimental studies.

DC-link voltage compensation with fuzzy-based MMC to mitigate the low-order circulating current

ABSTRACT In this paper, a novel fuzzy controller-based technique is used to control the harmonics of the circulating currents in Pulse width modulation (PWM) based modular multilevel converters (MMC). This technique is based on average model circuit of MMC, which recognises the cross-relation in between the sub-module and system-level elements using dependent sources. In addition to improving the damping of the system, the main superiority of this technique is that it maintains the natural balancing property of DC bus system in a PWM-based MMC by combining upper and lower arms together, instead of considering separately like in existing schemes. Fuzzy controller is most suitable controller for the decision-making. Fuzzy gives more accuracy, for the nonlinear loads or heavy industrial applications. Fuzzy controller allows the reduction of the harmonics of the circulating current in the dc-link system and works effectively during the unbalanced AC grid conditions also. Therefore, it is suitable for multi-frequency AC systems. The accuracy of the proposed method is verified using simulation results which include the arm currents, common mode (circulating) currents and DC-link voltages along with AC grid voltage and currents.

Performance Improvement of AC-DC Power Converters under Unbalanced Conditions

In this paper, a single synchronous reference-frame based control method is proposed to improve the performance of AC-DC interlink converters feeding DC loads under unbalanced AC grid conditions. Unbalanced grid voltages cause undesirable double frequency (2ω0) oscillations on the DC link voltage in AC-DC converters. In the medium/high power applications, low switching frequency, and the oscillatory power of the input filter considerably degrade the functionality of the existing methods for removing 2ω0 ripples from DC link voltage. In this paper, an analytical equation for the terminal active power is derived considering the input filter inductance. Accordingly, suitable current references are proposed to eliminate the undesirable 2ω0 ripples from DC-link voltage considering low switching frequency. It is shown that the filter inductance adds a non-linear term to the active power equation which complicates the current reference calculation. Accordingly, a real-time recursive method is proposed to solve the equations and find the current references. To evaluate the performance of the proposed method, different grid unbalanced conditions, including asymmetrical short circuit faults, are applied to a test system in the PSCAD/EMTDC environment. Furthermore, the functionality of the proposed method is compared with that of the existing method for unbalanced conditions in the literature.

Capacitor Voltage Reduction in Modular Multilevel Converters Under Grid Voltages Unbalances

Negative- and zero-sequence internal currents of a modular multilevel converter (MMC) under unbalanced ac-grid conditions can increase the peak voltage of the submodule (SM) capacitors, which should be kept below a maximum voltage limit. This paper investigates the impact of offset pulsewidth modulation (OPWM) and zero-sequence voltage injection on reducing peak SM capacitor voltages of the MMC during unbalanced ac-grid conditions. It is demonstrated that OPWM extends the operating range of the MMC for a given set of restrictions, as it leads to a reduction of capacitor voltage in every phase by utilizing all the SMs in the arms. Zero-sequence voltage injection further reduces the maximum SM capacitor voltage among three phases under certain conditions. Simulation results from an 800-MVA MMC-based high-voltage direct current transmission system demonstrate the effect of OPWM in the operation of the converters and the efficacy of the method compared to standard modulation methods.

Operation and Control Methods of Modular Multilevel Converters in Unbalanced AC Grids: A Review

High-voltage direct current (HVdc) transmission systems based on modular multilevel converters (MMCs) are a promising solution for efficient bulk power transmission over long distances. As in grid-connected converters, the occurrence of grid-side faults is rather common, leading to imbalances and distortions of the ac-side voltages. Therefore, operation and control of MMC-HVdc systems under grid imbalances become of great significance in order to satisfy grid codes and reliability requirements of the system. The aim of this paper is to provide a comprehensive review of operation and control methods applied to MMC-HVdc systems with a specific focus on unbalanced ac-grid conditions. The methods are classified based on their main control targets that include ac-side power control, control of the circulating current, and dc-side power ripple suppression control. Special attention is given to the comparison of the different control methods and specific requirements under certain operating conditions that include grid, load, or internal imbalances.

Robust Control Design of MMC-HVDC Systems Using Multivariable Optimal Guaranteed Cost Approach

The modular multilevel converter (MMC) represents the important technological innovation that emerged among the diverse available topologies of voltage source converter and is avowedly the most suitable solution for converters in high voltage direct current (HVDC) transmission and multiterminal direct current grids. Special focus is given through this paper to the dynamic performance of an MMC-based, point-to-point HVDC system. Using an optimal guaranteed cost control theory, a robust control approach is designed in order to reject the impact of the unmodeled uncertainty, mainly in the ac side of the MMC. For this aim, a small-signal state-space linear model is derived for the control design of an advanced local controller of each MMC station. Furthermore, a new optimal guaranteed cost controller is proposed based on the convex optimization problem using linear matrix inequality optimization approach. The proposed strategy leads to regulate simultaneously the ac grid and differential currents as well as total stored energy per phase in abc frame. To ensure the energy balancing between upper and lower arm per phase, an outer control loop is used in order to control the energy difference per phase between upper and lower arms of MMC. For the MMC linked to HVDC system, the active power reference is generated through an outer classical dc voltage controller. This combined control strategy between classic and advanced robust regulation methods allows exploiting the advantages of both control methods. Effectiveness of the proposed optimal robust control strategy for point-to-point MMC-HVDC system is evaluated across accurate and skillful simulation study under MATLAB/SimPowerSystem environment. The simulation results convince satisfactory dynamics responses of a two-terminal MMC-HVDC system based on the robust control approach under various operating conditions, even under unbalanced ac grid conditions (e.g., asymmetrical fault).

Impact of Circulating Current Control in Capacitor Voltage Ripples of Modular Multilevel Converters Under Grid Imbalances

The negative-sequence current of a modular multilevel converter (MMC) under unbalanced ac grid conditions increases the voltage ripples of the submodule capacitors. This paper investigates the impact of circulating current control in capacitor voltage ripples under different unbalanced ac grid conditions, while considering multiple control objectives and independent control of active and reactive power. It is demonstrated that higher order harmonic injection in the circulating current through a reference calculation based on instantaneous information improves operation of the MMC reducing capacitor voltage ripples at the cost of increased peak arm current and dc current oscillations. The efficacy of the circulating current control is validated through real-time digital simulations of an 800 MVA benchmark MMC high-voltage direct current transmission system.

Investigation of MMC-HVDC operating region by circulating current control under grid imbalances

The maximum-allowable voltage/energy ripple and peak arm current define the operating limits of a modular multilevel converter (MMC). Furthermore, under unbalanced ac-grid conditions, the control targets of a power electronics converter with regards to balanced active power, reactive power or grid currents affect the power capabilities and operating region of the MMC. The objective of this paper is to investigate and quantify the operating region of an MMC under unbalanced ac-grid conditions and the impact of circulating current control and system control targets on the limits of the MMC operating region. It is demonstrated that higher order harmonic injection in the circulating current through a reference calculation based on instantaneous information extends the operating region of the MMC under grid imbalances, reducing capacitor voltage ripples compared to a direct elimination of the second order harmonic regardless of the converter control target. The efficacy of both circulating current control methods is validated through real-time digital simulations of an 800MVA benchmark MMC-based high-voltage dc (MMC-HVDC) transmission system.

Parallel Operation of Bidirectional Interfacing Converters in a Hybrid AC/DC Microgrid Under Unbalanced Grid Voltage Conditions

Today, interests on hybrid ac/dc microgrids, which contain the advantages of both ac and dc microgrids, are growing rapidly. In the hybrid ac/dc microgrid, the parallel-operated ac/dc bidirectional interfacing converters (IFCs) are increasingly used for large capacity renewable energy sources or as the interlinking converters between the ac and dc subsystems. When unbalanced grid faults occur, the active power transferred by the parallel-operated IFCs must be kept constant and oscillation-free to stabilize the dc bus voltage. However, under conventional control strategies in unbalanced grid conditions, the active power transfer capability of IFCs is affected due to the converters’ current rating limitations. Moreover, unbalanced voltage adverse effects on IFCs (such as output power oscillations, dc-link ripples, and output current enhancement) could be amplified by the number of parallel converters. Therefore, this paper investigates parallel operation of IFCs in hybrid ac/dc microgrids under unbalanced ac grid conditions and proposes a novel control strategy to enhance the active power transfer capability with zero active power oscillation. The proposed control strategy employs a new current sharing method which introduces adjustable current reference coefficients for parallel IFCs. In the proposed control strategy, only one IFC, named as redundant IFC, needs to be designed and installed with higher current rating to ensure the constant and oscillation-free output active power of parallel IFCs. Simulation and experimental results verify the feasibility and effectiveness of the proposed control strategy.

Characteristic analysis and fault-tolerant control of modular multilevel converters under sub-module faults

This paper analyzes the operation characteristics of the modular multilevel converter when the arms are asymmetrical with different sub-module number caused by sub-module faults. The research reveals the problems caused by asymmetrical arms, including the asymmetrical capacitor voltages, the asymmetrical direct current (dc) components of the arm currents, the asymmetrical circulating currents, the dc current fluctuation, and the voltage offset at the dc or alternating current (ac) side. Then, it is demonstrated by the theoretical analysis that the series of problems can be solved by rebalancing the fundamental frequency components of the upper arm voltage and the lower arm voltage. Therefore, a fault-tolerant control strategy is proposed by introducing a fundamental frequency resonant controller to the traditional second-order frequency proportional resonant circulating current controller. Besides, the research shows that the coordination of the proposed control strategy and the negative sequence current suppressing control can keep the output voltages of the three phases balanced. A 201-level modular multilevel converter–high-voltage direct current time-domain simulation model is established based on PSCAD/EMTDC, and the simulation studies confirm the efficiency of the proposed controller under balanced and unbalanced ac grid conditions. Copyright © 2015 John Wiley & Sons, Ltd.