Modular Multilevel Converter Based High Voltage Direct Current System Research Articles

Optimal Energy Utilization of MMC-HVDC System Integrating Offshore Wind Farms for Onshore Weak Grid Inertia Support

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems have the potential to support grid frequency by utilizing the electrostatic energy stored in sub-module capacitors. However, existing energy control schemes cannot effectively use the MMC capacitor energy to provide sufficient inertia support. Additionally, those control schemes are based on grid following control, suffering from stability issues when the AC grid short circuit ratio (SCR) decreases. Thus, a grid-forming control scheme capable of active energy management for MMC-HVDC systems integrating offshore wind farms (OWFs) is developed, achieving flexible energy regulation and stable operation under weak grid conditions. To improve inertia support performance, an optimal two-stage energy utilization method is further proposed to overcome the drawbacks of existing grid-forming methods. Finally, comparative simulations are performed in a 2-area 4-machine weak grid to verify the effectiveness and advantages of the proposed method.

Dual Grid-Forming Control With Energy Regulation Capability of MMC-HVDC System Integrating Offshore Wind Farms and Weak Grids

To integrate weak onshore grids, some grid forming (GFM) control schemes taking advantage of DC voltage synchronization have been proposed for modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems. In these control schemes, the sub-module (SM) capacitor energy is only used for grid synchronization. Since the SM capacitor is an energy storage device, it has the potential to actively regulate the stored energy. To fully exploit the energy management capability of MMC, a dual grid forming control scheme with energy regulation capability is presented in this paper. The proposed control scheme enables grid forming for both onshore and offshore MMC stations. Meanwhile, the capacitor energy of MMCs can be flexibly regulated to provide frequency support for the onshore grid. To verify the effectiveness of the proposed control scheme, case studies are first carried out in a point-to-point offshore wind farms (OWFs) integrated HVDC system. In addition, the frequency support performance is evaluated in a 3-area 4-machine grid integrated with two OWFs-HVDC systems. Finally, a single-terminal three-phase MMC prototype is developed to test the voltage decoupling and energy management capabilities of the proposed control strategy.

An Enhanced AC Fault Ride through Scheme for Offshore Wind-Based MMC-HVDC System

This study presents an improved, communication-free Fault Ride-Through (FRT) strategy for type-3 and type-4 wind turbine integrated modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems in offshore wind power plants (OWPPs). The research aims to enhance the reliability and resilience of OWPPs by ensuring their connection with AC grids remains intact during and after faults. Simulation results conducted on a 580 kV, 850 MW MMC-HVDC system using PSCAD/EMTDC software v.4.6.2 demonstrate quick post-fault recovery operation and the ability to effectively manage DC link and capacitor voltages within safe limits. Furthermore, the circulating current (CC) and capacitor voltage ripple (CVR) remain within acceptable limits, ensuring safe and reliable operation. The study’s major conclusion is that the proposed FRT strategy effectively mitigates the adverse effects of short circuit faults, such as a rapid rise in DC-link voltage, on the performance of the MMC-HVDC system. By promptly suppressing DC-link overvoltage, the proposed FRT scheme prevents compromising the safe operation of various power electronics equipment. These findings highlight the significance of FRT capability in OWPPs and emphasize the practical applicability of the proposed strategy in enhancing the reliability of offshore wind power generation.

Multi-Stage Sequential Network Energy Control for offshore AC asymmetric fault ride-through of MMC-HVDC system integrated offshore wind farms

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) is the preferred solution to the problem of long-distance transmission from large-scale offshore wind farms (OWFs). When an offshore AC asymmetric fault occurs, OWFs and MMC-HVDC will face the challenge of fault ride-through with power electronic converters on both sides of the fault location. In this paper, a Multi-stage Sequential Network Energy Control (MSNEC) is proposed for offshore AC asymmetric fault ride-through (FRT). Firstly, the constraint relationship among the sequence networks under AC asymmetric faults is revealed. Secondly, the energy control potential of WFMMC is fully explored from the perspective of energy transformation and transfer. Energy control reduces the fault phase current amplitude by putting in submodules at the fault phase. Additional sub-modules are put in the non-faulted phase to realize series voltage dividing effect to actively prevent non-faulted phase fault overvoltage. The MSNEC considers the FRT requirements for multiple stages of fault occurrence, fault steady state, and fault recovery. It combines sequential network control, energy control, and voltage support of the converter station. It realizes AC asymmetric FRT from four aspects: power equivalence, negative sequence suppression, energy control, and voltage stabilization. Finally, a simulation model of OWFs connected to the onshore grid via MMC-HVDC is established in PSCAD/EMTDC. Simulation results verify MSNEC not only improves the AC asymmetric FRT capability of the OWFs and WFMMC, but also promotes the efficient consumption of renewable energy and improves the economics of the OWFs grid-connected system.

A practical impedance modeling method of MMC-HVDC transmission system for medium- and high-frequency resonance analysis

The modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission system can arouse resonance that threatens the safety and stability of the power grid. In this paper, the impedance model and resonance mechanism of an MMC-HVDC transmission system are investigated at medium and high frequencies. First, an MMC-HVDC transmission system equivalent circuit model is proposed based on the partitioning impedance modeling method. Second, the concept of normalized sensitivity index is proposed to quantify the influential effects of crucial factors on the MMC equivalent impedance, including the control gains, control link delay, and circuit parameters; subsequently, the influencing factors are graded, and reduced-order impedance models are proposed for different situations. Then, the resonances in the MMC AC- and DC-side systems are analyzed considering both active and passive resonance, i.e., with or without a harmonic source, to systematically reveal the resonance mechanism of the MMC-HVDC system. Finally, a two-terminal MMC-HVDC simulation model is built to verify the proposed model and resonance analysis.

Analysis and research on the influence of multiple factors on the steady operation area of large-capacity MMC-HVDC transmission system

Aiming at the MMC-HVDC (modular multilevel converter based high voltage direct current), this paper studies the influence of multiple factors such as the AC system and converter characteristic limits on the steady-state operating range of the MMC-HVDC transmission system. First, a mathematical model based on the steady-state operation of the MMC-HVDC transmission system is established, and the AC-side power quality requirements and stability limiting factors of the MMC-HVDC system are analyzed and summarized, as well as the constraints of multiple limiting factors on the converter side. Based on this, the power operation range of MMC-HVDC transmission project under different conditions is obtained. Finally, combined with the actual project, the effects of modulation ratio, equivalent reactance between the AC outlets of the converter and AC system equivalent impedance on the steady-state power operating range are analyzed and studied separately.

Impact of control system on the DC impedance characteristics of low-medium frequency bands and stability of modular multilevel converter based HVDC system

The resonance in modular multilevel converter based high voltage direct current (MMC-HVDC) system poses a potential threat to the safe and stable operation of the power grid. The mechanism of the resonance phenomenon is complicated, which is closely related to the impedance characteristics of MMC system. By establishing the DC impedance model of MMC-HVDC system at both ends, the influence of parameters of control system in converter stations on the DC impedance characteristics of MMC system was analyzed, revealing the main influencing frequency bands and effect of each control parameter. The DC impedance characteristics of low and medium frequency bands could be shaped in a targeted manner by adjusting the control parameters, the stability margin of the system will be improved as well. The correctness of the results was verified by time domain simulation. The analysis results of this article can provide reference for the selection of the parameters of control system and the improvement of stability margin of MMC-HVDC system.

Coordinated Low Voltage Ride-Through of MMC-HVDC Transmission System and Wind Farm With Distributed Braking Resistors

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission is becoming a trend in offshore wind-farm integration. However, the large DC-side energy dissipation equipment, which is utilized to dissipate surplus wind power under grid fault conditions, will largely increase the cost of MMC-HVDC systems. To reduce the cost, a novel low voltage ride-through (LVRT) strategy is proposed in this paper. When a grid fault occurs and the DC voltage exceeds the limit, the sending end converter is controlled to reduce the AC voltage of the wind farm. The LVRT of the wind generators will be activated, and the output active power of the wind farm is reduced. With this coordination, the DC-side energy dissipation equipment only needs to dissipate surplus power in the early stage of the grid fault before the output active power of the wind farm drops. Therefore, the heat generated by braking resistors can be significantly reduced. On this basis, the braking resistors can be distributed into the submodules of the receiving end converter (REC) station. The LVRT problem can be solved without building an individual energy dissipation station. Using the proposed coordination strategy, the construction cost of the MMC-HVDC system with offshore wind farm integration can be significantly reduced.

Comparative Analysis of Three Types of SFCL Considering Current-Limiting Requirement of MMC-HVDC System

DC fault protection is the bottleneck that restricts the development of modular multilevel converter based high voltage direct current (MMC-HVDC) system. Superconducting fault current limiters (SFCL) is an ideal protection device that can significantly limit short circuit current and enhance the fault tolerance ability of MMC-HVDC system. At present, several topologies of SFCLs have been proposed, but there is no systematical study of their current-limiting characteristics in MMC-HVDC. This paper studies the development process of DC short-circuit faults in MMC-HVDC system, and analyzes its demand for fault current limiting. Then, resistive type, saturated iron core type and hybrid type SFCL are modeled and their limiting effect of critical fault current under DC pole-to-pole fault is tested. By comparison, the current-limiting characteristics of three types of SFCL are concluded. Results show that resistive SFCL has best performance in steady state, while saturated iron core SFCL has best performance in transient state. The current-limiting characteristic of hybrid type SFCL is between the other two types.

A New Analytical Approach for Fast and Accurate Dynamic Simulation of MMC-HVDC System

The Speed of execution along with acceptable accuracy of the simulation is crucial for Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) systems in offshore applications. The high number of power electronic components in the MMC-HVDC system poses a challenge to solve them using the existing methods, based on average models, with required speed and accuracy. This paper presents a method to simulate all the submodules of the MMC-HVDC system, which enables to study of the dynamics in the subsea cable, submodules, and at the system level. MMC-HVDC system in a standard three-phase CIGRE benchmark configuration is modeled to validate the analytical method. It is compared with PSCAD simulations (based on Dommel’s method) and validated both in steady state and in transient conditions of the MMC-HVDC system. The results indicate that the analytical method is accurate, and the execution time is lower compared to PSCAD based on Dommel’s method.

Accurate estimation on AC short-circuit current for grid-connected MMC-HVDC system considering FRT and various control modes

The modular multilevel converter based high voltage direct current (MMC-HVDC) system aggravates the short-circuit current of the AC system. Accurate fault current estimation for the grid-connected MMC-HVDC system under various control modes is essential for equipment selection and protection systems design. Therefore, this paper proposes a universal and accurate algorithm to estimate the AC short-circuit current in MMC-based hybrid AC/DC grids. First, the MMC AC-side fault current response characteristics in different control modes of the MMC are analyzed. Subsequently, the transient and steady-state fault current expressions considering various fault ride-through (FRT) strategies are presented. Further, based on the hybrid parameter matrix, the equivalent multiport network for the MMC-based AC/DC transmission system is established. Then, the fault current of the MMC is iteratively calculated. Finally, the effectiveness and adaptability of the proposed method is validated, and the influences of the AC system parameters for the fault current are studied.

Transient fault identification method for bipolar short‐circuit fault on MMC‐HVDC overhead lines based on hybrid HVDC breaker

A bipolar short-circuit transient fault identification method for overhead transmission lines in modular multi-level converter-based high voltage direct current (MMC-HVDC) grid based on the hybrid HVDC breaker is proposed. The circuit breakers on both sides of the positive and negative overhead transmission lines will trip after a bipolar short-circuit fault in the MMC-HVDC grid. In order to identify whether the fault is a transient fault, the isolating switch of the DC breaker on one line side is reclosed first, and the voltage change on this line side in the case of transient and permanent faults is analysed. By comparing the electrical characteristics of the voltage on the line side under transient and permanent faults, a transient fault identification method is proposed, and the calculation method of the setting value of the criterion is obtained. Finally, by building the MMC-HVDC system model for simulation in power systems computer aided design, the accuracy and effectiveness of the proposed method for transient fault identification are verified.

Modular multilevel converter predictive control strategy based on energy balance

In MMC-HVDC (modular multilevel converter-based high-voltage direct current) applications, conventional control methods have defects such as complicated control and difficulty in controlling the internal energy of the converter. To ensure the safe and stable operation of an MMC-HVDC system, the problem of uneven internal energy distribution and increased fluctuations in the modular multilevel converter under asymmetrical network voltage conditions must be addressed. This paper has designed, a novel model predictive control (MPC) for MMC-HVDC applications. Through the proposed strategy, the switching states of all the MMC units can be optimized, which eliminates the circulating currents and achieves a voltage balance of the capacitor by redundant switching states. Moreover, an energy control circuit is established to adjust the DC bus power distribution in the MMC three-phase bridge arm. Thus, the symmetrical ac-side current can be realized, and the MMC internal energy imbalance caused by the transient process of the system can be avoided. Finally, the proposed novel predictive control strategy is tested via a case study. The obtained simulation and experimental results verify the effectiveness of the proposed control strategy.

Coordinated control of MMC‐HVDC system with offshore wind farm for providing emulated inertia support

With the continuous increase of renewable energy generation and high voltage direct current (HVDC) power transmission, the system inertia is declining, thus resulting in greater risks of frequency instability. To solve this issue, a dc-link inertia control is firstly proposed for a point-to-point modular multilevel converter based HVDC (MMC-HVDC) link, which enables the MMC-HVDC system to provide emulated inertia support by employing the electrostatic energy stored in the dc capacitors. Moreover, in order to maintain reasonable submodule capacitor size as well as provide more emulated inertia support, a virtual capacitor control for MMC-HVDC connected an offshore wind farm (OWF) is proposed. With the virtual capacitor control, the OWF is able to provide a large capacity virtual capacitor in dc side of MMC-HVDC link and supply desired emulated inertia support for onshore power system by utilising the offshore wind turbines’ rotor rotational kinetic energy. Furthermore, a procedure for selecting the appropriate virtual capacitor control parameters is proposed. Finally, the performance of the proposed coordinated control is researched in PSCAD/EMTDC, and the simulation results show that the onshore power system inertia is effectively improved with the coordinated control.

Transient current estimation of MMC-HVDC system under grid-converter phase difference

When a modular multilevel converter based high voltage direct current (MMC-HVDC) system operates under grid faults, there would be a phase difference between the grid voltage and the converter output. It can be caused by grid voltage phase shifting and the integration effect of the phase-locked loop (PLL) in the converter. The system may exhibit undesirable transient process with expanded current which is harmful for the converter devices. In this paper, a transient current estimation method is proposed for the scenario of converter-grid phase difference caused by the frequency shifting of the grid side. A piecewise phase calculation model is established to determine the transient phase difference of the system. Then, according to the maximum phase difference calculated by the model, the system stability is assessed and the maximum value of system transient current can be estimated after comparing the maximum phase difference with the critical phase difference and π. Finally, the current estimation results are verified by detailed circuit simulations of a 21-level MMC-HVDC system built in PSCAD.

Minimum Short Circuit Ratio Requirement for MMC-HVDC Systems Based on Small-Signal Stability Analysis

This paper determines the minimum short circuit ratio (SCR) requirement for a modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. Firstly, a simplified model of MMC is introduced; the MMC is represented by its AC and DC side equivalent circuit. Next, by linearizing the MMC subsystem and the DC network subsystem, the deduction of the small-signal models of MMC subsystem, the small-signal model of the DC network and MMC-HVDC are carried out successively. Thirdly, the procedure for determining the minimum SCR requirement of MMC-HVDC is described. Finally, case studies are performed on a two-terminal MMC-HVDC system under four typical control schemes. The results show that the restraint factors for the rectifier MMC is predominantly the voltage safety limit constraint, and the restraint factors for the inverter MMC are mainly the phase locked loop (PLL) or the outer reactive power controller. It is suggested that the minimum SCR requirement for the sending and the receiving systems should be 2.0 and 1.5 in the planning stage.

Enhanced Fault Current-Limiting Circuit Design for a DC Fault in a Modular Multilevel Converter-Based High-Voltage Direct Current System

The main weakness of the half-bridge modular multilevel converter-based high-voltage direct current (MMC-HVDC) system lies in its immature solution to extremely high current under direct current (DC) line fault. The development of the direct current circuit breaker (DCCB) remains constrained in terms of interruption capacity and operation speed. Therefore, it is essential to limit fault current in the MMC-HVDC system. An enhanced fault current-limiting circuit (EFCLC) is proposed on the basis of fault current study to restrict fault current under DC pole-to-pole fault. Specifically, the EFCLC consists of fault current-limiting inductance L F C L and energy dissipation resistance R F C L in parallel with surge arrestor. L F C L reduces the fault current rising speed, together with arm inductance and smoothing reactor. However, in contrast to arm inductance and smoothing reactor, L F C L will be bypassed via parallel-connected thyristors after blocking converter to prevent the effect on fault interruption speed. R F C L shares the stress on energy absorption device (metal oxide arrester) to facilitate fault interruption. The DCCB requirement in interruption capacity and breaking speed can be satisfied effortlessly through the EFCLC. The working principle and parameter determination of the EFCLC are presented in detail, and its effectiveness is verified by simulation in RT-LAB and MATLAB software platforms.

Measurement and Analysis of Radiated Disturbance Characteristics of ±320 kV Modular Multilevel Converter System

The radiated electromagnetic disturbance was evaluated by measuring the electric field intensity in a bandwidth of 10 kHz-2.75 GHz near the valve hall, direct-current (DC) area, reactor room, and transformers in a ±320-kV modular multilevel converter-based high-voltage, direct-current (MMC-HVDC) converter station. In addition, the distribution of electric field intensity was analyzed in various regions in the converter station. According to the law of electric fields attenuation with distance, the radiated electric field intensities at the position 200 m away from the converter station was calculated by using the electric fields measured in the converter station. The results show that the levels of radiated disturbance inside and outside the converter station meet the standards' requirements. The amplitude spectra of the electric field intensity in a traditional HVDC system and MMC-HVDC system were compared. Finally, the mechanism for radiated electromagnetic interference of the MMC-HVDC converter system was analyzed from the aspects of disturbance source characteristics, wall shielding and radiated structure.

Voltage Polarity Reversing-Based DC Short Circuit FRT Strategy for Symmetrical Bipolar FBSM-MMC HVDC System

Fault ride-through (FRT) control and protection are of vital importance for a modular multilevel converter-based high-voltage direct current (MMC-HVDC) system when dc-side pole-to-pole (PTP) or pole-to-ground (PTG) fault occurs. In this paper, a FRT strategy by reversing dc voltage polarity is proposed for symmetrical bipolar full-bridge submodule-based MMC-HVDC system. It is suitable for both PTP and PTG faults. By adopting this method, the dc voltage stress in PTG fault scenario and dc current stress in PTP fault scenario are eliminated. Meanwhile, the capability of transmitting rated active power is maintained during faults, which can minimize the impact of dc-side faults on ac grid. In addition, based on a deduced dc-side equivalent circuit, the dc line current oscillation phenomenon during FRT transient process is analyzed. An active damping control method is presented to suppress the transient oscillation. Finally, validity of the proposed FRT strategy is verified by PSCAD/EMTDC simulations.

Modeling on Transient Characteristic and Suppression Control of DC Fault in MMC-HVDC System

This paper focuses on analyzing the transient characteristic of DC transmission line fault in the modular multilevel converter based high voltage direct current (MMC-HVDC) system. Equivalent circuit models of DC transmission line pole-to-ground fault, pole-to-pole fault and line breaking fault are established respectively in this paper. The mechanism of DC faults is analyzed and mathematical expressions of fault voltage and fault current are derived. The transient processes of these faults are analyzed based on the equivalent circuit models. The suppression control for DC fault based on virtual impedance is studied to reduce the over current level. Simulation results with a 2-terminal MMC-HVDC model on the RT-LAB platform verify the modeling and analysis on transient characteristic and the effect of suppression control of DC transmission line fault.