Flexible DC System Research Articles

Research on Fault Identification of Hybrid Multi-Feed High-Voltage Direct Current System Based on Line Commutated Converter and Voltage Source Converter

With the rapid development of voltage source converter (VSC) and line commutated converter (LCC) technology and the relative concentration of power and load, the inverter station of the flexible DC system is fed into the same AC bus with the conventional DC rectifier station, and the high-voltage direct current (HVDC) parallel hybrid feed system is formed in structure. As the electrical distance between the converter stations is very close, when a fault occurs in the near area, the current on the AC wiring on the VSC side will fluctuate greatly, resulting in the misoperation of the AC wiring protection. For this reason, this paper proposes a fault identification method based on VSC/LCC hybrid multi-fed HVDC system, which discriminates the fault and outputs the protection signal according to the protection criterion, and logically judges the combination of the output protection signal to identify the fault type. The simulation results show that the method can identify all kinds of faults of hybrid multi-feed DC system and solve the problem of protection misoperation of the hybrid multi-feed DC system.

Frequency Stability Control Strategy for Voltage Source Converter-Based Multi-Terminal DC Transmission System

The voltage source converter-based multi-terminal DC transmission (VSC-MTDC) system can use additional frequency control to respond to the frequency change of faulty AC system. However, the control coefficient of traditional additional frequency control is mostly fixed, and the control flexibility is insufficient, so it cannot be adjusted adaptively according to the frequency change of the system. Therefore, a frequency control strategy of the VSC-MTDC system based on fuzzy logic control is proposed. Based on the DC voltage slope controller, this strategy introduces an additional frequency controller based on fuzzy logic control, takes the frequency deviation and frequency change rate as the additional controller input, and dynamically adjusts the control quantity through the fuzzy logic control link to realize the adaptive adjustment of the VSC-MTDC system to the AC system’s frequency. Finally, a three-terminal flexible HVDC system is built on the PSCAD/EMTDC simulation platform for simulation verification. The results show that the proposed control strategy can effectively use the flexible DC system to support the frequency of the AC system and significantly improve the frequency stability of the faulty AC system.

Research on coordinated control strategy of photovoltaic energy storage system

In this paper, the modular design is adopted to study the control strategy of photovoltaic system, energy storage system and flexible DC system, so as to achieve the design and control strategy research of the whole system of “photovoltaic + energy storage + DC + flexible DC”. This realizes the flexibility and diversity of networking. Due to space reasons, this article focuses on the detailed explanation of the photovoltaic energy storage system control strategy, including the maximum power tracking control strategy of photovoltaic power generation, photovoltaic power generation boost chopper circuit control strategy, photovoltaic power generation DC/AC converter control strategy, PCS device control strategy, PCS virtual impedance control algorithm, etc. Simulation experiments are carried out according to the specific case setting parameters. The simulation results prove that the proposed flexible DC system coordinated control strategy can ensure grid frequency stability and grid voltage stability, and improve the consumption capacity of distributed new energy.

Frequency Coordinated Control of Wind Power Flexible Direct System Based on Voltage Sourced Converter Based Multi-Terminal High Voltage Direct Current

Background: With the continuous penetration of renewable energy in modern power systems, the problem of excessive frequency deviation of power systems, which is caused by the integration of wind power into electricity grids, needs to be solved urgently. Objective: Make up for frequency fluctuations when AC systems fail, improve the frequency characteristics of main system networks, and reasonably distribute unbalanced power. Methods: According to the principle of active power distribution, a frequency division coordinated control strategy that uses low-pass filter control to divide power fluctuations into highfrequency fluctuations and low-frequency fluctuations is proposed. High-frequency fluctuations are attached to the wind farm side, and low-frequency fluctuations are attached to the DC system to realize frequency division control. Result: The simulation results showed that the coordination of the frequency division control of the wind farm and flexible DC system could effectively reduce the main grid’s frequency fluctuations, which are caused by the AC system failure, and improve the stable operation ability of the AC and DC systems. Conclusion: The frequency division control can overcome the unbalanced power distribution caused by traditional droop control, reduce the frequency deviation of the AC system, realize frequency autonomy, and enhance system robustness.

Research on the Fault-Transient Characteristics of a DC Power System Considering the Cooperative Action of a Flexible Current-Limiting Device and a Circuit Breaker

As an effective carrier of a new energy collection, the DC power grid has low inertia and weak damping characteristics, making it essential to limit fault current and isolate the DC system. To quickly and effectively suppress fault current, a flexible current-limiting device (FCLD) is proposed, which can realize transient fault self-recovery without circuit breaker action and permanent and quick isolation of a fault. It improves the operational ability of the DC system under an asymmetric condition. First, a rectifier provides a set-slope current to each cascade inductor, so the voltage of the inductor can be clamped. Second, a controlled current source (CCS) is applied to generate inverse flux to prevent the inductor from magnetic saturation. The protection action time of the DC circuit breaker is reformulated. Finally, by considering the synergistic action of the current-limiting device, the circuit breaker, and the transient characteristics of the DC grid fault, the protection scheme of the multi-terminal flexible DC system can be formulated. To verify the validity of the proposed flexible current-limiting device, a multi-terminal flexible DC simulation platform is established, and the faults of DC lines are simulated and analyzed.

Development and application of AC section power loss protection device

—The so-called AC section power loss refers to the loss of all AC power supplies of the system on this side. After the AC section at the receiving end of the converter station loses power, it will lead to serious AC overvoltage, and the energy of the arrester will increase sharply at the same time, which will seriously affect the safety of the equipment. In view of this situation, this paper studies the action mechanism of AC section power loss protection, and gives the design scheme of AC section power loss protection device. The developed protection device has been verified by RTDS simulation test, passed the third-party detection test of a detection and Research Institute, and is running on site. The field operation is good. The device is not only suitable for conventional DC system and flexible DC system, but also for some occasions where regional stability needs to be considered.

Study on the oscillation transmission paths in direct‐drive wind farm transmitted via flexible dc system based on dynamic energy flow

Concerning the sub-synchronous oscillation induced by the interaction among wind power converter, flexible DC converter and multiple control links, a method to study the oscillation transmission path based on dynamic energy flow is proposed. First, the system is divided into several subsystems according to different control links, and the dynamic energy model of each subsystem is built by using the first integration method. Then, the energy flows induced by different control links are determined according to the corresponding oscillation components. On this basis, the intersection of energy flows in each dynamic energy model is defined as an energy converging point. Thus, the energy flows between different energy converging points are obtained, and the oscillation transmission path of the interconnected system between different subsystems is determined. Finally, combining the parameters of a real direct-drive wind farm transmitted via flexible DC system, a test system is built in RT-LAB for simulation verification. The results show that the proposed method can describe the oscillation transmission paths between different control links quantitatively, and reveal the control links that make against the stability of system. Besides, studying the energy distribution in the oscillation transmission paths can contribute to further research on the control parameter setting and energy suppression, and lay the theoretical foundation for the design of supplementary compensation branches.

A pilot protection scheme for flexible HVDC transmission lines based on modulus power

Existing protection methods for Flexible HVDC transmission lines easily fail to detect the fault location fast and reliably. Concerning this problem, a modulus power based pilot protection scheme is proposed. By analyzing the dynamic switching process of sub-modules in flexible DC converter station, the expression of converter station DC voltage taking into account the influence of AC system is derived. On this basis, the modulus voltage and modulus current are calculated by phase-modulus transformation, and the modulus power calculation model containing DC components alone is built. Then, the expressions of modulus power in different fault cases are derived, and a protection criterion based on the polarity difference characteristics of modulus power is established. Finally, the RT-LAB is used to build the model of a flexible DC system for simulation verification. The results of simulation tests verify that, the proposed protection scheme can detect the fault line and fault pole fast and accurately, with easy threshold setting and low demand on communication synchronization. Besides, it is unaffected by the fault resistance and the specific boundary conditions of current-limiting reactor, etc.

Impedance Modeling and Stability Analysis of MMC Flexible DC System at the DC Side

Aiming at the stability of modular multi-level converter (MMC) on the dc side, this paper adopts harmonic linearization method to establish impedance model of MMC dc side. This method ignores the higher-order components of voltage and current, and considers the influence of the control loops to guarantee high accuracy at the same time. The high-frequency band of impedance presents inductive characteristics, and the mid- and the low-frequency band is related to the control loop. The parameters of the current loop have a greater influence on the impedance characteristics of the mid-frequency band while the voltage loop parameters have a greater influence on the low-frequency band. For the back-to-back transmission system based on MMC, the voltage at the dc side has obvious oscillation when the impedance mismatch or the stability margin is small. The risk of dc side voltage instability can be eliminated by optimizing the controller parameters and phase angle margin can be improved.

Research on the Coordinated control Strategy of fault crossing capability of offshore wind power multi-terminal flexible DC system

To improve the ride-through capability (RTC) of the multi-terminal offshore wind power flexible DC system in response to faults, this paper is based on the basic structure and control strategy of the voltage source converter (VSC) station of the multi-terminal offshore wind power flexible DC transmission system (MTDC), and aims at the AC grid connected to the system with three-phase short circuits and wind turbine output changes According to the situation, the mechanism of voltage drop caused by the fault is analyzed, and the DC chopper energy bleeder circuit is connected to the DC side to cooperate with the comprehensive control scheme for fault ride-through of multi-terminal offshore wind power flexible DC collection system. Through theoretical analysis and building a simulation model, the design of the control strategy of the flexible collection system and the simulation study of fault ride-through is carried out on the Shandong Peninsula Beihai Wind Farm. The results show that: the VSC station controller of the offshore wind power multi-terminal system can make the multi-terminal system wind power distribution and consumption reasonable, and realize the stable operation of the system.

Design of transformer protection scheme for DC system

Back-to-back flexible DC system can be used for interconnection of asynchronous power grids. In view of the different characteristics and operation modes of connected transformer equipment in back-to-back flexible DC system, this paper puts forward some special problems that need to be considered in the design of connected transformer protection, and puts forward a complete configuration scheme of connected transformer protection in view of its particularity. Finally, we establishes a real-time digital simulation model to verify the validity of the model using actual engineering parameters. The simulation results show that the theoretical analysis is correct and the proposed protection scheme can effectively guide the engineering application.

Modeling and Analysis of Short-Circuit Fault Current for MMC-HVDC Grid with DC Circuit Breaker

DC line fault is one of the important factors restricting the development of flexible DC system. DC circuit breaker is an effective means to realize the short circuit fault isolation on DC side of flexible DC system. Therefore, it is necessary to add DCCB into the short-circuit current calculation and modeling of MMC-HVDC power grid for comprehensive analysis. Firstly, the external characteristics of the hybrid HVDC circuit breaker and the modular multilevel converter (MMC) are modelled. Then, the modeling of short-circuit fault current in different stages was analysed. The analysis method of short circuit fault current of four-terminal MMC system with DC circuit breaker is obtained. Finally, the simulation results and the calculation results are compared and results show the effectiveness of the short-circuit calculation method proposed by this paper.

Short-Circuit Fault Current Calculation Method for the Multi-Terminal DC Grid Considering the DC Circuit Breaker

The analysis and calculation of the short-circuit fault current in the DC grid is of great significance to the design and configuration of the converter station and DC circuit breaker parameters. The existing flexible DC system not only includes the modular multilevel converter (MMC) converter, but also needs power equipment such as the fault current limiter and DC circuit breaker. Therefore, the system modeling and short circuit calculation of the multi-terminal DC system after adding the DC circuit breaker are also of great significance for the design of DC power system parameters and the grid troubleshooting ability. In this paper, firstly, the parameters of the four-terminal DC system of the modular multilevel converter (MMC) are simplified, and the analytical solution of the short circuit fault current of the multi-terminal DC system is given. Then, the external characteristics of the cascaded hybrid DC circuit breaker are modeled. Based on the equivalent circuit of the fault current in different stages, the short circuit calculation method of four-terminal MMC system with DC circuit breaker is obtained. This method can effectively describe the overall trend of fault current and provide the basis for the configuration of DC line protection settings and DC circuit breaker related parameters.

Modeling and Simulation of ±10 kV DC Transmission System based on Mechanical DC Circuit Breaker

In order to simulate the interruption process of mechanical DC circuit breaker in actual DC power grid and obtain various stress characteristics of DC circuit breaker in actual interruption process, it is necessary to establish a stable and effective flexible DC system model with good dynamic performance. In this paper, the controller models of system level, converter station level and converter valve level are established for the 10kV flexible DC system, and then the steady-state characteristics and dynamic characteristics of power step of the system are simulated. After analyzing the variation of voltage, current and power and dynamic response speed of converter station, the simulation results show that DC voltage and active power of the system operate stably according to the reference value and the fluctuation error of DC voltage is less than 1%. The internal and external loop controllers all track the dynamic changes of their respective command values rapidly within 0.5s. The control performance of the flexible DC system model can meet the requirement of the interruption simulation of DC circuit breakers.

Active control-based protection for a flexible DC system of a PV power plant

A flexible DC collection system for a photovoltaic (PV) power plant contains a large number of feeders. When fault occurs, the fault current rapidly increases, causing electronic devices to block to protect themselves. This blockage presents a challenge to the protection because of extremely short data windows. To address this difficulty, a protective method based on active control is proposed. The fault current control principle is analyzed and derived so that DC/DC converters can provide a low-amplitude and controllable post-fault stable current signal. The system protection algorithm is designed based on identifying the direction of the generated current signal. Simulation results indicate that the fault section is accurately distinguished and that the proposed method performs efficiently against transition resistance and noise. A highly reliable and complete protective action can be completed within 5 ms after a fault occurs, with grounding impedance, noise, and distributed capacitance considered. The proposed approach provides a solution for future DC system protection.

Improved Voltage Control Strategy for Multi-terminal Flexible DC System

This paper presents an improved Voltage Droop control strategy, which updates the reference of DC link voltage by adding the difference between reference and actual voltage, which also adopts a dynamic P-V droop coefficient to improve the DC voltage level. By applying this method, we can eliminate the steady-state deviation of DC link voltage caused by ignoring the power loss in converter. In this paper, in order to demonstrate the reduction of deviation of DC link voltage, an improved control strategy is implemented for a three-terminal flexible DC power system modelled in PSCAD/EMTDC.

A DC fault handling method of the MMC-based DC system

The flexible DC system based on the modular multilevel converter (MMC) has a good development prospect. However, the DC fault handling is still a technical difficulty since the large capacity DC circuit breaker technology and its engineering application are not yet mature. Therefore, the converter-based fault isolation method is widely discussed. One typical idea of the converter-based isolation methods is to create a symmetrical short circuit at the AC system artificially after a DC fault, so the ac-side source could be separated from the DC fault point and the DC fault current can decay to a very small value naturally, which is represented by the double-thyristor switch scheme (DTSS). This paper analyzed the feasibility of the converter-based fault isolation method, including the isolating speed, influence on the ac-system protection and restart of the system. After that, the improved DC fault handling method by introducing damping modules was proposed, which could accelerate the DC fault isolating speed, avoid the maloperation of the ac-system protection and guarantee the fast restart of the system. Finally, the simulation cases based on the PSCAD/EMTDC are carried out to verify the superiority of the proposed scheme.

The Model and Parameters Based on the Operation Mode of a 500kV Multi-terminal Flexible DC Power Grid

The operating conditions of 500kV Zhangjiakou-Beijing Demonstration Project at all operation modes were studied regarding a multi-terminal flexible DC system with DC switches and a true bipolar wiring method. Based on the actual AC-DC hybrid method, this paper concluded 34 kinds of N-1 operation modes for keeping the system stable, simulated the multi-level converter valve (MMC) bank by using RTDS small-step elements, adopted an ideal voltage balancing method for the treatment of sub-units and established a MCC system simulating structure in compliance with the site. The operation modes of the system were simulated by selecting the parameters which are consistent with the line, using such parameters as the actual configurations and parameters of key elements such as the converter transformer, filter, bridge arm reactor, converter, etc. adopted in the project, and establishing a flexible DC control system with a layered structure. According to the result of the simulation, the corresponding modes of the system were provided and references were given for the feasibility study and implementation of the project.

Analysis of pole‐to‐pole fault characteristics of flexible DC system and design of current‐limiting device

Analysis of pole‐to‐pole fault characteristics of flexible DC system and design of current‐limiting device

Closed-Loop Distribution Network by a Midvoltage Flexible HTS DC System

A scheme of closed-loop distribution network by a midvoltage flexible high-temperature superconducting (HTS) dc system is proposed and analyzed. This midvoltage (10-kV) scheme is compared, in terms of capital costs, with that of the closed-loop distribution network by a high-voltage (110-kV) flexible conventional dc system. Based on the HTS cables that are made of the first-generation Bi2223 tapes with a length of 2.5 km, the total investment cost of the 10-kV flexible superconducting dc system with a power capacity of 42 MVA is estimated about 1% lower than that of the 110-kV flexible conventional dc system with the same power capacity, owing to the substantial cost reduction of the voltage-source converter stations with the reduced voltage level. According to the “N - 1” principle of distribution networks, with the interconnecting flexible HTS dc system of 42 MVA, the total power to supply loads of the connected substations can be increased by the same power capacity of the interconnecting system. The simulation results of a three-phase short-circuit fault reveal that the voltage recovery time of the critical loads in the closed-loop distribution network can be reduced to less than 0.1 s, which is much faster than that in open-loop distribution networks.