DC Voltage Control Strategy Research Articles

Coordinated DC voltage control strategy for the receiving end hybrid LCC-VSC system

The receiving end hybrid line commutated converter (LCC) — voltage source converter (VSC) system has the obstacle that when an AC fault occurs at the sending LCC side, the DC voltage of the rectifier side will fall, resulting in the power transmission reduction of the whole system. Particularly, when the rectifier side DC voltage falls below the inverter side DC voltage, the power transmission is interrupted, posing a huge threat to the stability of the power grid at both the sending and receiving ends. To tackle these problems, a coordinated DC voltage control strategy for the receiving end is proposed. By controlling the DC voltage of the receiving VSC, the proposed method can improve the DC system power transmission capability when voltage drops at the sending side. The proposed control strategy is simple but practical, which is good for the stable and secure operation of both the sending and the receiving power system.

Output DC Voltage Control Strategy for Switched Reluctance Generator

This paper presents a proposed generated voltage control for a Switched Reluctance Generator (SRG). The strategy developed uses a PI controller to vary the magnetization angle of the machine phases, acting in the top switches opening angle (θoff) of the converter. The strategy is used together with the freewheel intermediate step, between the steps of magnetization and demagnetization of the phases. The main vantage is decrease the input power of the excitation source and to get the better use of mecanic energy to increase the phase current of the generator, maximizing the electric energy production. Is used the classic half-bridge converter without the need to alteration in your project, changing only the switching strategy. A nonlinear model of the SRG and the control strategy were implemented in MatLab/Simulink. The experimental platform was developed to validate the proposal presented.

Distributed collaborative optimization DC voltage control strategy for VSC–MTDC system with renewable energy integration

Distributed collaborative optimization DC voltage control strategy for VSC–MTDC system with renewable energy integration

Research on Coordinated DC voltage Control Strategy of DC Microgrid based on Photovoltaic Power Generation System

DC microgrid system is a flexible and commonly used renewable power generation system. Among them, the energy storage system (ESS) can reduce the waste light of the photovoltaic (PV) power generation system and provide stable power output for the power grid. However, a reasonable control strategy is needed to realize the energy balance of PV, grid-connected converter and ESS, so as to ensure the safe and stable operation of DC microgrid. Therefore, this paper proposes a DC voltage coordination control strategy without communication. The control strategy can automatically coordinate the working state of each converter of DC microgrid, which according to the change of DC voltage and the ESS’s state of charge (SOC), At the same time, the inertia of DC voltage in each working state is analyzed, and the stability of DC microgrid is improved through the backup inertia support of ESS. Finally, the effectiveness of the proposed method is verified by simulation and experiments.

Improvement of voltage adaptability for decentralised doubly‐fed induction generator wind power system based on nine‐switch converter

This paper proposes a method to improve the voltage adaptability of doubly fed induction generator (DFIG) wind turbines in a decentralised scenario by incorporating a nine-switch converter (NSC). With the proposed scheme, the DFIG can realise low voltage ride-through (LVRT) and continuous operation under the grid voltage harmonic, drop, and rise. The topology of the NSC-DFIG system is introduced and is used to solve the circuit model. Control strategies for voltage compensation and current compensation are based on the circuit model. The corresponding voltage compensation scheme and the NSC DC voltage control scheme were designed according to the specific capacity configuration of the NSC in this study. To reduce the DC-side voltage of the NSC, a third harmonic-injection modulation method and a dynamic modulation ratio allocation strategy were incorporated. Simulations under four power grid voltage conditions were used to validate the scheme. This method can improve the voltage adaptability for decentralised DFIG wind power systems, which can enable their successful interactions with the power grid.

Integration Control of Renewable Energy/Hydrogen Energy System Based on Flexible DC Interconnection

Wind/solar complementary for hydrogen production and hydrogen energy utilization have become one of the important directions for clean energy transformation and new economic growth point. This paper proposes a renewable energy/hydrogen energy system based on flexible DC interconnection and its coordinated control technology covering AC/DC voltage source converters and photovoltaic units, DC hydrogen production loads and energy storage devices. First, the typical structure of the renewable energy/ hydrogen energy system with low-voltage flexible DC interconnection is proposed, and the local control strategy of each unit is analysed, including the constant DC voltage control strategy and the constant active power control strategy. Then, coordinated control methods of renewable energy/hydrogen energy under different system operation states are proposed. Finally, a corresponding system simulation model and test platform are built, and then used to simulate and verify the proposed coordinated control method.

Implementation of DC voltage controllers on enhancing the stability of multi-terminal DC grids

Because of the increasing penetration of intermittent green energy resources like offshore wind farms, solar photovoltaic, the multi-terminal DC grid using VSC technology is considered a promising solution for interconnecting these future energies. To improve the stability of the multi-terminal direct current (MTDC) network, DC voltage control strategies based on voltage margin and voltage droop technique have been developed and investigated in this article. These two control strategies are implemented in the proposed model, a ±400 kV meshed multi-terminal MTDC network based on VSC technology with four terminals during the outage converter. The simulation results include the comparison and analysis of both techniques under the outage converter equipped with constant DC voltage control, then the outage converter equipped with constant active power control. The simulation results confirm that the DC voltage droop technique has a better dynamic performance of power sharing and DC voltage regulation.

A VSG-Based Coordinated DC Voltage Control for VSC-HVDC to Participate in Frequency Regulation

In this paper, a coordinated DC voltage control strategy is proposed based on the VSG (virtual synchronous generator) method for the VSC-HVDC transmission system to participate in the frequency regulation of the connected weak grid. The voltage and power control capability of the VSC-HVDC is explored to attenuate the rate of change of frequency and to diminish the deviation of frequency. This is realized by the coordinated control of DC voltages at both the sending and the receiving ends with the VSG method. A small-signal model is established to investigate the dynamics of the control system. A tuning method for the selection of control parameters is also discussed in detail. The validity and superiority of the proposed control strategy are tested in the scenarios of sudden load changes and short circuit faults.

Coordinated DC voltage control strategy of hybrid HVDC with multi‐infeed MMC inverters considering commutation failures

Coordinated DC voltage control strategy of hybrid HVDC with multi‐infeed MMC inverters considering commutation failures

DC Voltage Control and OLTC Control Strategy for MMC-Based HVDC System Under SM Failure

The Modular multilevel converters (MMCs) have excellent performance when operated with high modulation index such as the case of gird-connected converters in high voltage direct current (HVDC) system. Reliable and fault tolerant operation (FTO) of the system is very important in HVDC. A novel FTO algorithm under sub module failure without using any additional redundant SMs is proposed. In the proposed method, the direct current bus voltage control and on-load tap changer (OLTC) control strategy are combined for interruption free operation of the system without any additional voltage stress on switching devices and performance degradation of the system during fault. The analytical details of the normal as well as FTO operation are given. The proposed FTO strategy is verified through simulation and also experimentally using a prototype. The simulation and experimental results are presented.

Research on Voltage Control of Multi Terminal Flexible Medium Voltage DC Distribution Network

In order to solve the problem that the original control strategy cannot meet the voltage quality requirements of the system under various operation modes due to the expansion of DC distribution network scale, this paper studies the voltage control strategy of a ring type medium voltage DC distribution network. Firstly, the traditional control method is improved, the droop control is added to the margin control as the slave station control method. At the same time, the DC transformer connected with the energy storage is used as the backup voltage regulating station in case of power shortage to meet the voltage quality requirements of the system in normal operation and fault. Then, a DC voltage control strategy based on depth first search is proposed to solve the problem of voltage deviation caused by system fault, so that the distribution network can automatically adjust the control according to the real-time structure in case of fault and improve the voltage quality. Finally, the proposed control strategy is simulated in PSCAD / EMTDC to verify the effectiveness of the control strategy.

Optimal Voltage Regulation and Power Sharing in Traction Power Systems With Reversible Converters

This paper proposes an efficient dc voltage control scheme in rail transit traction power systems with reversible converters for dc voltage regulation and power sharing. The proposed dc voltage control scheme is executed in a hierarchical structure based on the optimal power flow. Unlike conversional constant dc voltage control applied in traction power systems, this scheme allows the reversible converters operating in both rectifier and inverter regions and considers regenerative braking power sharing among multiple traction substations. The steady-state equivalent models of traction power systems including reversible converters are established, and the ac/dc sequential power flow algorithm based on Newton–Raphson method is presented. Finally, a multi-trains dynamic operation simulation is carried out with Matlab. The simulation results prove the validity of proposed scheme and indicate that the system energy saving effect in traction power systems with reversible converters is obvious with proposed scheme.

Highly reliable DC voltage control strategy for modular multilevel converter based‐UPFC

The DC bus voltage of the unified power flow controller (UPFC) is closely related to the active power balance between the series and the shunt sides. In this study, a highly reliable modular multilevel converter (MMC)-UPFC DC bus voltage control strategy is proposed to ensure the continuous and stable operation of the MMC-UPFC. The DC voltage coordinated control strategy of dual converters in the shunt side, which combines the constant DC voltage control, droop control and margin control, is based on the topology configuration of the main converter and the standby converter. The DC voltage coordinated control strategy is adopted for different control targets under conditions of small-range adjustment of active power flow, wide-range adjustment, and the shunt-side converter failure, to achieve reliable and stable operation of UPFC under different operating conditions. Finally, a simulation model was established under the MATLAB/Simulink environment to verify the feasibility of this control strategy.

DC Voltage Control Strategy of Three-Terminal Medium-Voltage Power Electronic Transformer-Based Soft Normally Open Points

The power electronic transformer (PET) or solid-state transformer (SST) is a key technique in the future distribution grid. One of PET's future possible application is the soft normally open points (SNOP) to handle the power and voltage challenge brought by the renewable energy sources. In this paper, a PET-based three-terminal medium-voltage SNOP topology is proposed. The topology consists of three cascade H-bridge (CHB) stages and a multiactive bridge (MAB) stage. The dc voltage control strategy for the proposed PET-SNOP is studied. Two strategies - the CHB+CHB+CHB strategy and CHB+MAB strategies are developed from the existing strategies but they both have some drawbacks. In order to overcome the drawbacks, a novel dc voltage control strategy-CHB×3+MAB strategy is proposed, which combines the advantages and avoids the drawbacks of these two strategies, moreover, it also brings more flexibility and robustness to the dc voltage control. The effectiveness of the proposed PET-SNOP topology as well as the proposed CHB×3+MAB dc voltage control strategy is verified by both simulation and experiment results.

Start Control for the MMC-HVDC

This paper designs the control strategies and start procedure of MMC-HVDC. The start procedure consists of three stages, uncontrollable charging stage, controllable charging stage and power-raising stage. The current circuit is analysed and the equivalent circuit is obtained in uncontrolled charging stage. The computational formula of current-limit resistor is obtained in the uncontrollable stage. Two different control strategies to further charge the capacitors to the rated voltage in controllable charging stage are designed. Strategy one makes use of DC voltage control and balancing strategy of capacitor voltage while Strategy two divides the capacitors of one arm into two groups. In power-raising stage, to avoid the impact of power step, slope control is used in power outer loop. Finally, a simulation model is established in the PSCAD/EMTDC. The simulation results show that the effectiveness and rationality of the designed start procedure and control strategies

A coordinated DC voltage control strategy for cascaded solid state transformer with star configuration

Cascaded solid-state transformer (SST) can be directly connected to the high-voltage distribution grid, and it has broad applications in the future smart grid, traction locomotive, and renewable energy integration. However, the reliability of the cascaded SST is seriously influenced by the DC voltage and power imbalance issues with this configuration. This paper presents a coordinated DC voltage control strategy for cascaded SST with star configuration, in which the average DC voltage and the balancing DC voltage are controlled separately without current sensors in dual active bridge (DAB) modules. This strategy not only reduces the cost and the complexity of the control system but also realizes the unbalanced reactive compensation to some extent. Simulation and experimental results verify the feasibility and validity of the proposed method

Review of the DC voltage coordinated control strategies for multi‐terminal VSC‐MVDC distribution network

The multi-terminal voltage sourced converter medium-voltage DC (VSC-MVDC) distribution network has high prospects for future development, hence the proposed alternative for the AC distribution in commercial and industrial applications. However, a number of obstacles stand on its way in realising its immense potential in the modern power system. This study presents a review and focuses on the complexity of controlling DC voltage/power flow in the multi-terminal DC (MTDC) grids and the possible DC voltage coordinated control approaches aimed at identifying the suitable control strategies for the proposed MVDC distribution grids. Several centralised and distributed DC voltage control strategies for MTDC have been considered. In conclusion, the droop-based control strategies have been identified as reliable, flexible, and expandable with high dynamic performance and minimal communication needs in MTDC grids. These strategies are suitable and adaptable for stable and secure operation of the proposed multi-terminal MVDC distribution network. The study recommends a comparative study of the key DC voltage coordination control features of the droop-based control strategies using the multi-terminal VSC-MVDC distribution network dynamic model for future studies.

Hybrid Modulated Model Predictive Control in a Modular Multilevel Converter for Multi-Terminal Direct Current Systems

In this paper a hybrid modulated model predictive control (HM2PC) strategy for modular-multilevel-converter (MMC) multi-terminal direct current (MTDC) systems is proposed for supplying power to passive networks or weak AC systems, with the control objectives of maintaining the DC voltage, voltage stability and power balance of the proposed system. The proposed strategy preserves the desired characteristics of conventional model predictive control method based on finite control set (FCS-MPC) methods, but deals with high switching frequency, circulating current and steady-state error in a superior way by introducing the calculation of the optimal output voltage level in each bridge arm and the specific duty cycle in each Sub-Module (SM), both of which are well-suited for the control of the MMC system. In addition, an improved multi-point DC voltage control strategy based on active power balanced control is proposed for an MMC-MTDC system supplying power to passive networks or weak AC systems, with the control objective of coordinating the power balance between different stations. An MMC-HVDC simulation model including four stations has been established on MATLAB/Simulink (r2014b MathWorks, Natick, MA, USA). Simulations were performed to validate the feasibility of the proposed control strategy under both steady and transient states. The simulation results prove that the strategy can suppress oscillations in the MMC-MTDC system caused by AC side faults, and that the system can continue functioning if any one of the converters are tripped from the MMC-MTDC network.

A new fault-ride-through strategy for MTDC networks incorporating wind farms and modular multi-level converters

This paper presents a DC voltage control strategy for enhancing the fault-ride-through (FRT) capability of wind farms comprising of fully rated converter permanent magnet synchronous generators (FRC-PMSGs) connected to multi-terminal high voltage direct current (MT-HVDC) grids through modular multi-level converters (MMCs). The proposed FRT strategy is implemented on a master controller located in the offshore AC substation of each wind farm. The underlying issue addressed via the scheme relates to over-voltages in the HVDC links when the power transfer is disrupted due to faults occurring in the AC onshore grid. The corresponding Matlab/Simulink® model has been validated using transient simulation, while the practical feasibility of the controller is demonstrated utilising Opal-RT© real-time hardware platform.

DC voltage control strategy of chain star STATCOM with second‐order harmonic suppression

Chain static synchronous compensators (STATCOMs) with star structure produce second-order harmonic fluctuations on the DC side. This effect is often ignored and only the average voltage value is taken into consideration when describing this system for control purpose. In this study, the unbalance of per-phase DC voltages caused by active power absorbed and consumed by the chain is analysed. The steady-state expressions regarding the average capacitor voltage balancing between phase clusters as well as the suppression of second-order harmonic fluctuation are obtained, respectively, based on the injection of fundamental and third-order harmonic zero-sequence voltages. Although the expressions are helpful to understand the essence of DC capacitor voltage control, this method has complicated calculation process and the calculation results may be beyond the modulation limit. Therefore, it is not conducive in real-time control. For this reason, a new control strategy based on the instantaneous zero-sequence voltage injection is proposed. The evaluation function with the transient variables is established to get the instantaneous value of the zero-sequence voltage including fundamental and third-order harmonic components. The fundamental and third-order harmonic zero-sequence voltages are automatically allocated to fully use the remaining modulation capacity. Finally, simulation and experimental results are provided, which verifies the correctness of the theoretical analysis and the high performance of the proposed method.